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Types of Gas Carriers with reference to nature of cargo and its protection in case of accident as categorized in IGC Code:

The two main types of liquefied gas carriers are:

1. LPG (Liquefied Petroleum Gas) Carriers, and
2. LNG (Liquefied Natural Gas) Carriers.

To understand the design characteristics of these two types of ships, we first need to know a few notable details about the composition and properties of LPG and LNG.

Liquefied Petroleum Gas (LPG): Petroleum hydrocarbon products such as Propane and Butane, and mixtures of both have been categorised by the oil industry as LPG. It is widely used in domestic and industrial purposes today. The most important property of LPG is that it is suitable for being pressurised into liquid form and transported. But there are conditions related to pressure and temperature that need to be maintained for the above to be carried out without posing threat to life, environment, and cargo.

At least one of the following conditions need to be complied with, for transportation of LPG:

• The gas should be pressurised at ambient temperature.
• The gas should be fully refrigerated at its boiling point. Boiling point of LPG rangers from -30 degree Celsius to -48 degree celsius. This condition is called fully-refrigerated condition.
• The gas must be semi-refrigerated to a reduced temperature and pressurised.

We will see, at a later stage, how the above conditions affect the design of different types of LPG tankers.

Other gases such as ammonia, ethylene and propylene are also transported in liquefied form in LPG carriers. Ethylene, however, has a lower boiling point (-140 degree celsius) than other LPGs. Hence it must be carried in semi-refrigerated or fully-refrigerated conditions.

Liquefied Natural Gas (LNG): Natural gas from which impurities like sulphur and carbon-dioxide have been removed, is called Liquefied Natural Gas. After removal of impurities, it is cooled to its boiling point (-165 degree Celsius), at or almost at atmospheric pressure. Note here, that unlike LPG, LNG is cooled to low temperatures but not pressurised much above atmospheric pressure. This is what makes the design of LNG carriers slightly different from LPG carriers. LNG, at this condition is transported as liquid methane.

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Design of Different Types of Gas Carriers:

In this article, we will understand the general arrangement, and other design details of gas carriers as and when we look into the different types of vessels based on their functionality and type of cargo being carried. The most important feature of gas carriers is the cargo containment system. It is according to this criteria that LPG carriers are categorized into types.

Integral Tanks: These are the tanks that form a primary structural part of the ship and are influenced by the loads coming onto the hull structure. They are mainly used for cases when LPG is to be carried at conditions close to atmospheric condition, for example – Butane. That is because, in this case, there are no requirements for expansion or contraction of the tank structure.

Independent Tanks: These tanks are self-supporting in nature, and they do not form an integral part of the hull structure. Hence, they do not contribute to the overall strength of the hull girder.

According to IGC Code, Chapter 4, independent tanks are categorized into three types:

Type ‘A’ Tanks: These tanks are designed using the traditional method of ship structural design. LPG at near-atmospheric conditions or LNG can be carried in these tanks. The design pressure of Type A tanks is less than 700 mbar. The following figures show the general arrangement of a liquid methane carrier with Type ‘A’ tanks.

The general arrangement of an LPG ship is almost same as that of an oil carrier, with the cargo tanks spread over a certain length forward and abaft the midship, the machinery and superstructure at the aft. A forecastle is fitted at the bow so as to prevent green waters on deck. Ballast water cannot be carried in the cargo tanks, hence spaces for ballast are provided by incorporating double hull spaces (note the double hull in the midship section), bilge and upper wing tanks.

The most notable and distinguishing feature of Type ‘A’ tanks is that the IGC Code specifies that Type ‘A’ tanks must have a secondary barrier to contain any leakage for at least 15 days. The secondary barrier must be a complete barrier of such capacity that it is sufficient to contain the entire tank volume at any heel angle. Often, this secondary barrier comprises of the spaces in the ship’s hull as shown in the figure below.

One important question that could arise, here, is that the tank in the midship section view seems to be an integral part of the hull. Why then, is this type of tanks categorised under Independent Tanks? To find the answer we need to have a closer look at how the tank is installed in the hull.

The above figure shows how the aluminum tank structure is not integrated to the inner hull of the methane carrier by means of any metal contact. The inner hull plating and aluminum tank plating are separated by layers consisting of timber, glass fibre, and balsa panels for insulation from external temperatures. The balsa panels are held together by plywood on both faces which are sealed using PVC foam seals. An inert space of 2 or 3 mm separates the inner glass fibre layer from the aluminum tank plate. This space is provided for insulation and also allows expansion and contraction of the tank structure. This type of non-welded integration makes this tank structurally independent in nature.

Type ‘B’ Tanks: The concept behind the design of such tanks is to have such a structure in which a crack can be detected long before the actual failure. This allows a time margin before the actual failure occurs. The methods used for design of such tanks include determination of stress levels at various temperatures and pressures by first principle analyses, determination of fatigue life of tank structure, and study of crack propagation characteristics. This enhanced design of such tanks requires on a partial barrier, that we will look into, soon.

The most common arrangement of Type ‘B’ tank is Kvaerner-Moss Spherical Tank, as shown below in Figure 4.

The tank structure is spherical in shape, and it is so positioned in the ship’s hull that only half or a greater portion of the sphere is under the main deck level. The outer surface of the tank plating is provided with external insulation, and the portion of the tank above the main deck level is protected by a weather protective layer. A vertical tubular support is led from the top of the tank to the bottom, which houses the piping and the access rungs.

As evident from the layout, any leakage in the tank would cause the spill to accumulate on the drip tray below the tank. The drip pan and the equatorial region of the tank are equipped with temperature sensors to detect the presence of LNG. This acts as a partial secondary barrier for the tank.

LNG is usually carried in this type of tanks. A flexible foundation allows free expansion and contraction according to thermal conditions, and such dimensional changes do not interact with the primary hull structure, as shown in Figure 5.

The following are the advantages of Kvaerner-Moss Spherical tanks:

1. It enables space between the inner and outer hull (see Figure 4.) and this can be used for ballast and provided protection to cargo in case of side-ward collision damages.
2. The spherical shape allows even distribution of stress, therefore reducing the risk of fracture or failure.
3. Since ‘Leak before Failure’ concept is used in the design, it presumes and ensures that the primary barrier (tank shell) will fail progressively and not catastrophically. This allows crack generation to occur before it propagates and causes ultimate failure.

Type ‘C’ Tanks: These tanks are designed as cryogenic pressure vessels, using conventional pressure vessel codes, and the dominant design criteria is the vapour pressure. The design pressure for these tanks is in ranges above 2000 mbar. The most common shapes for these tanks are cylindrical and bi-lobe. Though Type ‘C’ tanks are used in both, LPG and LNG carriers, it is the dominant design in LNG carriers.

The following figures show the arrangements of cylindrical and bilobe tank arrangements in midship view. The cylinders can be either vertically or horizontally mounted, depending on the dimensions and spatial constraints of the ship. Note, in Figure 6, that the space between the two cylinders is rendered useless. Due to this, the use of cylindrical tanks is a poor use of the hull volume. In order to circumvent this, the pressure vessels are made to intersect, or bilobe tanks are used (Figure 7).

These types of tanks do not require a secondary barrier. Rather, to detect the leakage of cargo from the tanks, the hold space (refer to Figure 6) is filled with inert gas or dry air. Sensors placed in the hold space can detect the change in composition of the inert gas or dry air due to fuel vapour, and leakages can hence be detected and prevented. Bilobe tanks at the forward end of the ship are tapered at the end.

Membrane Tanks: Unlike independent tanks, membrane tanks are non-self-supporting structures. Their primary barrier consists of a thin layer of membrane (0.7 to 1.5 mm thick). The membrane is supported to the inner hull structure through an insulation that can range upto 10 mm thickness as per IMO IGC Code. Due to their non-self-supporting nature, the inner hull bears the loads imparted onto the tank. This way, the expansions and contractions due to thermal fluctuations are compensated by not allowing the stress to be taken up by the membrane itself. Membrane tanks are primarily used for LNG cargo.

Often, there are two layers (primary and secondary) of insulation and membranes placed alternatively. The most common types of membtane tanks are the ones designed and developed by two French companies Technigaz and Gaz Transport. The Tehnigaz system makes use of a stainless steel system that is constructed with corrugated sheets in such a way that one sheet is free to expand or contract independent of the adjacent sheet. The Gaz Transport system uses Invar as the primary and secondary membranes. Invar has low coefficient of thermal expansion, which makes corrugations unnecessary. The insulation is usually made of materials like Reinforced Polyurethane. In GTT membrane tanks, the primary membrane is made of Corrugated SUS 304, and the secondary membrane is made of Glued Triplex. Figure 8 illustrates the anatomy of twin-membrane tanks.

Some of the advantages of membrane tanks are as follows:

• They are generally of smaller gross tonnage, that is the space occupied within the hull is lower for a given cargo volume.
• Due to the above reason, maximum space in the hold can be used for cargo containment.
• Since the height of tanks above the main deck is significantly lesser compared to the cases of Moss tanks, membrane tanks provide allow visibility from the navigational bridge. This also allows a lower wheelhouse. This can be compared in Figures 10 and 11.

LPG Containment Systems: Unlike LNG, LPG cargo requires storage at conditions that are different from atmospheric conditions. The LPG containment systems are classified into three types, and each LPG carrier is designed according to any one of them.

Fully Pressurized Tanks: Propane, Butane and Anhydrous ammonia are carried in fully pressurized tanks. The capacity of these tanks is usually less than 2000 cubic meters. They are usually uninsulated cylindrical pressure vessels that are arranged partly below main deck level. Since these are Type C tanks, they often prevent complete utilization of under deck volume.

Semi Pressurized or Semi Refrigerated Tanks: Though the cargo carried by semi-pressurized ships are same as that of fully-pressurized ships, the volume of semi-pressurized ships is about 5000 cubic meters. These use Independent Type C tanks, and are constructed with ordinary grades of steel. The outer surface of these tanks are insulated, and refrigeration or reliquefication plants are installed on these ships to maintain the working pressure of the cargo. The most ommon types of tanks used for this purpose are cylindrical and bi-lobe type.

Fully Refrigerated Tanks: Fully Refrigerated gas carriers have a capacity of 10,000 to 1,00,000 cubic meters. The ships in the smaller size range are used to carry multiple types of cargo, whereas the larger ones are designed for a single type of cargo to be transported on a permanent route. The tanks used for this purpose is usually Type ‘A’ prismatic tanks that are sloped at the top end to reduce free surface effect, and sloped at the bottom to suit the shape of the bilge structure. They are usually divided longitudinally by a liquid-tight bulkhead, in order to reduce free surface effects further. These tanks are constructed with notch ductile steel, in order to be provided with maximum notch toughness at temperatures as low as -48 degrees Celsius, at which cargo like Propane is transported.

The number of gas carriers have increased drastically over the last ten years, owing to the increasing need for alternative fuel. These are usually high speed ships with fine hull-form, which makes it possible for extensive research opportunities to improve on hull efficiencies in order to achieve more power efficiency. A lot of research is also being carried out to design advanced cargo containment systems and concepts of adjoining bunkering systems are being developed by various countries that are opening themselves to extensive use of natural gas. Today, not all shipyards are equipped to design and build specialised ships like LPG and LNG carriers. This leaves a wide scope for designers and shipbuilders to develop skills and infrastructure to specialise in building these ships.

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Methods of carriage of liquefied gases:

Reliquefaction with Intercooling: – Intercooling is used in conjunction with two stage compression. The first stage discharge is desuperheated in the intercooler and returned to the 2^nd stage. The discharge from the 2^nd stage is condensed in the condenser, and thence to the intercooler for further cooling. The condensate is returned to the cargo tank.

Non-condensable gases are separated out in the purge condenser and transferred by the cross over line to the collector and mast, or via the stripping or bottom distribution lines back into the cargo tank. If two or more tanks are being reliquefied simultaneously via one cargo system the distribution of condensate between the tanks is to be controlled manually.

If, during intercooling, there is insufficient condensate in the intercooler drum, additional liquid can be transferred from the cargo tank using the stripping/condensate line and a cargo pump. This is more likely to occur during the early stages of intercooling, or if the 2nd stage compression temperature is too high.

The condenser pressure is to be maintained at approximately 1 bar above the saturation pressure of condensate at the condensate temperature.

Reliquefaction without Intercooling:-  Gas is drawn into the 1st stage of the compressor from the tank, via the surge drum (if fitted), compressed and discharged through the intercooler, but without cooling. The gas is liquefied in the condenser, expanded to tank pressure and returned via the spray or stripping/condensate line.

When several tanks are being cooled simultaneously by one reliquefaction plant, the operation should start with the tank having the highest pressure. Pressure is to be equalised before the tanks are interconnected. Also, when several tanks are being cooled simultaneously a careful watch must be kept on the liquid return to ensure equal filling.

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Cooling down of tanks:

• Prior to loading if returned with insufficient heel.
• After dry-docking, off-hire or during initial commissioning.
• LNG carriers with a typical capacity of, say, 153,000 m³ are loaded at about 12,000 m³/h.
• The volume of liquid LNG loaded displaces an equivalent quantity of vapour in the ship’s empty cargo tanks which is returned to the LNG storage tanks for processing in the site’s fuel gas system.
• This BOG will be available for typically 12 hours in each loading cycle. If the ship’s tanks are warm, loading takes a longer period of time as initially volumes of LNG are vaporised when they contact the warm sides of the LNG tanks, thereby cooling them.
• During loading, more than one LNG storage tank can be used simultaneously to load the carriers.
• Where jetty lines are long, the loading line generates significantly more BOG due to heat ingress from the pumps as a result of the larger duty.
• With relatively short jetty/transfer lines < 1 km, the heat component from LNG pumping is relatively small (typically around 5% of total BOG).
• However, for example where the LNG must move in excess of 7 km the pumping component becomes significantly larger at an estimated 45% of total BOG.

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SIGTTO:

SOCIETY OF INTERNATIONAL GAS TANKERS AND TERMINAL OPERATORS (SIGTTO) was born out of a recognition that an industry specializing in the transport of liquefied gas needed to establish and promote the adoption and implementation of the very highest standards if it was first to win and then to maintain the confidence of the public at large. In acting as a beacon for quality and best practices, SIGTTO and its members have done just that, and that the excellent safety and pollution record of the sea borne gas transport industry to date defines it quite categorically as a highly responsible and effective sector.

By the late 1970s it was clear the international LNG business was set for a period of rapid expansion. A number of involved companies were therefore concerned to agree essential common standards for the industry, to aid its expansion, underpin public confidence and avoid a proliferation of unilaterally defined regulations. This group resolved to establish a body to draw together industry member companies in an effort to establish commonly agreed standards and best practice criteria. Hence the Society was formed and registered as a Bermuda Exempted Company (non-profit making) with limited liability in October 1979.

The Society was granted consultative status at the IMO in 1982. Formed originally with thirteen Members the Society has steadily grown over twenty years to a membership of more than 100 companies; representing virtually the whole of the world’s LNG trades and over half its LPG capacity.

PURPOSE: – The Society is the international body established for the exchange of technical information and experience, between members of the industry, to enhance the safety and operational reliability of gas tankers and terminals. The organization has been organized to encourage safe and responsible operation of liquefied gas tankers and marine terminals handling liquefied gas; to develop advice and guidance for best industry practice among its members and promote criteria for best practice to all who have responsibilities for, or an interest in, the continuing safety of gas tankers and terminals.

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LNG as ship’s fuel:

• LNG as a ship fuel has great potential as fuel of choice for the future
• Clean burning – meets all current and future emission standards
• Existing Technology: Dual fuel engines in use on LNG carriers and coastal vessels/ferries in Europe
• LNG Fuel has lower cost today – about 30% lower than diesel fuel
• Vessel regulatory issues manageable
• LNG can be stored in tanks internal to the hull
• Variable Range Possible – Short Range on LNG (2,500 mi), Long Range with diesel fuel (dual-use)

Liquid Natural Gas (LNG) characteristics:-

• It is liquid only at cryogenic temperatures (-163°C) so it requires special storage tanks, pipe systems, and handling
• It will normally slowly evaporate when stored so a means to deal with boil off gas (BOG) is required – venting to air is not allowed
• When in gas state it can be explosive in an enclosed space at the right mixture with air – ventilation system needed
• It is clean burning so low SOx, Particulate, and NOx emissions can be achieved without add on hardware – emissions are compliant with ECA regulations
• LNG has half the density of diesel fuel so larger storage tanks for the same range are needed – tanks are not at high pressure so can be stored below deck, unlike CNG

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Pre-Arrival Checklist:

The daily operation of a liquefied gas carrier involved potential hazards. It should be noted that cargo pipes, valves and connections and any point of leakage at the gas cargo may be intensely cold. Contact may cause severe cold burns.

Pressure should be carefully reduced and liquid cargo drained from any point of the cargo transfer system, including discharge lines, before any opening up or disconnecting is begun.

Some cargoes such as ammonia have a very pungent, suffocating odour and very small quantities may cause eye irritation and disorientation together with chemical burns. Seafarers should take this into account when moving about the vessel, and especially when climbing ladders and gangways. The means of access to the vessel should be such that it can be closely supervised and is sited as far away from the manifold area as possible. Crew members should be aware of the location of eye wash equipment and safety showers.

Everyone involved in liquefied natural gas trans- portation takes safety very seriously. There are many lives and a great deal of money at stake. Government and industry work together to make sure these ships are designed, maintained, and manned with safety in mind; industry maintains them with oversight by periodic government inspection, and government sets the standards for crew training.

The popular perception of liquefied natural gas is that it is inherently dangerous. While it possesses a set of hazards that need to be managed, when look- ing at the actual incidents involving liquefied natural gas, there are very few that put the surrounding area and public in danger. The rigorous attention to detail, coupled with the constantly emerging tech- nology, should continue to give LNG one of the bet- ter safety records for a hazardous material.

Within 5 days of the ship’s estimated time of berthing, the following checks and tests shall be carried out, and the results recorded. These records are to be made available to the gas terminal upon request.

1. Deck water spray line.
2. Water curtain
3. Gas free condition of hold space.
4. Alarm function of fixed gas detection equipment.
5. Cargo gauging system and alarm set points.
6. Emergency Shutdown System (ESD), all the relevant system shall be tested prior to arrival port and time needed to shut should be confirmed around 25 up to 30 seconds.
7. Operation of cargo system remote control valves and their position indicating systems.
8. Confirm Cargo transfer emergency stops fully operational and date of last test.
9. Confirm tank high level and pressure alarms operational.
10. Confirm that remotely operated manifold valves have been operated through a complete open/closed cycle, functioning and advise valve type(ball, gate, etc)and actual closing time. The corresponding records shall be produced by the master on the ship arrival at berth. Any defects or deficiencies must be reported to the terminal as an addendum to the Pre-Arrival information notice.
11. (11) Deep well cargo pump and booster pump mechanical seals are free of oil leaks.

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ICS data sheets:

• ICS data sheets outlines the main characteristics of individual cargoes, and the action to be taken in an emergency.
• Matters relating solely to maintenance of the purity of individual cargoes and their condition during carriage have not been included.
• It’s something like material safety data sheet of gas cargo.
• With respect to ICS Data Sheet – The IMO Codes require the following information to be available to every ship and for each cargo:
  • The master must only load a cargo which is listed on his certificate of fitness.
  • Data sheets for these cargoes should be on board.
  • The master and all those concerned should use the data sheet and any other relevant information to acquaint themselves with the characteristics of each cargo to be loaded. If the cargo to be loaded is a mixture (e.g LPG), information on the composition of the mixture should be sought; the temperature and pressure readings in the shore tank can be used to verify this information.
  • Special notes should be made of any contaminants that may be present in the cargo, e.g.”water”.
  • Ref: SIGTTO publication for gas carrier  appendix 1

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SEMI-REFRIGERATED OR PRESSURE CARGOES:

Loading:

• This operation follows the general principles very closely. The shore loading hose, and, if available, the vapour return line hose, are connected to the liquid and vapour line connections at the cargo manifold.
• Loading is effected through the liquid line, and pressure relief, etc., through the vapour return line. When the vessel arrives at the loading terminal, her tanks should be:
  • Empty of liquid, but under suitable pressure of vapour from her previous cargo (gassed-up); or gas-free, but under the maximum vacuum possible (usually in the vicinity of 80 per cent.).
  • An increasing number of terminals insist that the vessel’s cargo tanks be inerted before the final vacuum is created prior to loading.
• If the vessel’s cargo tanks are full of vapour at a suitable pressure (gassed up), loading can start at once.
• This being the simpler of the two cases given above, it will be described first.
• As the liquid enters the tank, the vapour trapped in the space above the liquid will be compressed, become supersaturated and condense.
• If a vapour return line has been provided, any excess pressure can be returned ashore.
• If no vapour return line is provided, then the pressure can be relieved in the following ways: firstly, by spraying part of the cargo into the tank, secondly by refrigeration and finally, if these methods fail, by allowing the excess pressure to escape into another tank.
• In this connection, it is not advisable to attempt loading all the, cargo through the sprays because to do so places an unnecessary restriction on the line.
• The sprays should be used only for the purpose of providing a larger surface area upon which the supersaturated vapour condenses.
• In the second case, when the vessel arrives alongside gas-free and under an 80 per cent. vacuum, the first step is to break the vacuum with vapour taken from shore, and raise the pressure within the tanks to a suitable level.
• If a vapour return line is provided, this is simple. If no vapour return line is provided, the cargo tanks can be gassed-up by either using the vaporiser, or by spraying very small quantities of liquid into the tank via the fine spray line in such a manner that the liquid droplets evaporate before they come into contact with the tank’s sides.
• This effectively gasses-up the tank, but if the ship arrived with an 80% vacuum, 20 per cent. of the ship’s capacity will be occupied by incondensibles at atmospheric pressure, the incondensibles being either air or inert gas (usually nitrogen).
• However, under pressure, the physical space occupied by these incondensibles is much less.
• For example, under 3 bars pressure (gauge) they would occupy a quarter of the space they would occupy at atmospheric pressure.
• If the incondensibles remaining in the tanks consist of air, the atmosphere within the tank will be very “over-rich” after gassing-up, but to dispose of the incondensibles by the separator using the reliquefaction process involves putting a gas/air mixture through the compressors which involves a risk, however small, of an explosive mixture being passed through the compressors.
• For this reason, some refineries insist on the cargo tanks being inerted prior to creating the final vacuum before loading, but many loadings have taken place over a long-period without any accident being attributed to this cause, and the danger may be more the theoretical than real.
• The usual loading programme is to load the lower tanks first, and to complete loading in the upper tanks. The loading rate depends upon the diameter of the liquid lines and the number of valves open. When the number of valves opened towards the completion of cargo is reduced, the loading rate should be reduced accordingly.
• Soundings of all tanks should be checked at regular intervals to ascertain the loading rate, and also to ensure that no liquid is entering a tank which has been completed, or not started.
• This is very important because it is an old maxim that it is the unmatched tank which always overflows.
• To load a very warm cargo from fully pressurized storage at ambient temperature into a semi-refrigerated ship, the reliquefaction plant must be started up and run at its maximum capacity to cool the cargo as it comes on board.
• The usual technique is so to adjust the loading rate as to maintain a pressure in the tanks below the safety valve relief setting.
• This can be achieved by maintaining a constant pressure at the loading manifold by frequently adjusting the manifold valve.
• Experience will soon show the best pressure to maintain and it is usually very close to the maximum pressure at which it is intended to load the product.
• The loading manifold pressure gives a much better indication of the valve adjustment needed, than watching the tank pressures, and adjusting the loading rate by guesswork.

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Deep Well Pump:

• Deepwell pump is the pump type that is often used on gas tankers.
• Deepwell pumps are pumps with a long shaft between the driving motor and the pump.
• The shaft goes inside the tank’s discharge pipe from the pump up to the tank dome.
• The discharge pipe is a solid pipe that goes up through the tank and out to the flange on the tank dome to the liquid line.
• The discharge pipe is constructed with several lengths with pipes, and there is a shaft bearing on each flange.
• The bearings are lubricated and cooled down by the liquid that is pumped from the tank.
• It is very important not to run the pump without liquid.
• This may result in damage of bearings and then the shaft.
• The motor that drives the pump is either electric or hydraulic.
• There is a mechanical sealing device between the motor and the discharge pipe in the cargo tank.

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IGC Code:

• The Code which applies to new gas carriers (built after 30th June 1986) is the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk.
• In brief, this Code is known as the IGC Code.
• The IGC Code, under amendments to Safety of Life at Sea Convention (SOLAS), is mandatory for all new ships.
• As proof that a ship complies with the Code, an International Certificate of Fitness for the Carriage of Liquefied Gases in Bulk should be on board.
• In 1993, the IGC Code was amended and the new rules came into effect on 1st July 1994.
• Ships on which construction started on or after 1st October 1994 should apply the amended version of the Code but ships built earlier may comply with previous editions of the IGC Code.

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Main characteristics of the different tank types:

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Differentiate between Integral Tanks & Membrane Tanks:

INTEGRAL TANKS	MEMBRANE TANKS
Integral tanks form a structural part of the ship’s hull and are influenced by the same loads which stress the adjacent hull structure, and in the same manner.	Membrane tanks are not self-supporting tanks; they consist of a thin layer (membrane), Normally not exceeding 1 mm thick, supported through insulation by the adjacent hull structure.
This form of cargo containment is not normally allowed if the cargo temperature is below -10OC.	The rounded parts of the layer are designed to accommodate thermal expansion and contraction, and other types thereof.
This containment system is partly used on some LPG ships dedicated to the carriage of butane.	The semi-membrane design has been developed for carriage of LNG, and the material of construction is 9% nickel steel or aluminium.

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Difference between Fully Refrigerated & Semi – Refrigerated / Semi – Pressurised Gas Carrier:

Fully Refrigerated Gas Carrier	Semi – Refrigerated / Semi – Pressurised Gas Carrier
Shape:- The tankers have prismatic-shaped cargo tanks	Shape:- The tanks are cylindrical in shape and of a thinner construction than the pressurised vessels.
Cargo capacity: – The ships are typically in the range 15,000m3 – 85,000m3, with three common sizes for LPG/Ammonia trades of 30,000m3, 52,000m3 and 80,000m3.	Cargo capacity: – The Ships typically ranges up to 5,000 m3 in size. Their construction is based on carrying propane at a pressure of 8.5 kg/cm2, and a temperature of -10°C.

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Various types of Liquefied Gas Carriers considering Survival Capacity as per IGC code:

IGC Code 2.1.2: Ships subject to the Code shall be designed to one of the following standards:

1. A type 1G ship is a gas carrier intended to transport the products indicated in chapter 19 that require maximum preventive measures to preclude their escape.
2. A type 2G ship is a gas carrier intended to transport the products indicated in chapter 19, that require significant preventive measures to preclude their escape.
3. A type 2PG ship is a gas carrier of 150 m in length or less intended to transport the products indicated in chapter 19 that require significant preventive measures to preclude their escape, and where the products are carried in type C independent tanks designed (see 4.23) for a MARVS of at least 0.7 MPa gauge and a cargo containment system design temperature of -55°C or above. A ship of this description that is over 150 m in length is to be considered a type 2G ship.
4. A type 3G ship is a gas carrier intended to carry the products indicated in chapter 19 that require moderate preventive measures to preclude their escape.

Therefore, a type 1G ship is a gas carrier intended for the transportation of products considered to present the greatest overall hazard and types 2G/2PG and type 3G for products of progressively lesser hazards. Accordingly, a type 1G ship shall survive the most severe standard of damage and its cargo tanks shall be located at the maximum prescribed distance inboard from the shell plating.

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Types of Gas Carriers:

Gas carriers range in capacity from the small pressurised ships of between 500 and 6,000 m³ for the shipment of propane, butane and the chemical gases at ambient temperature up to the fully insulated or refrigerated ships of over 100,000 m³ capacity for the transport of LNG and LPG. Between these two distinct types is a third ship type — the semi-pressurised gas carrier. These very flexible ships are able to carry many cargoes in a fully refrigerated condition at atmospheric pressure or at temperatures corresponding to carriage pressures of between five and nine bar.

Fully Pressurised ships:

• Today, most fully pressurised LPG carriers are fitted with two or three horizontal, cylindrical or spherical cargo tanks and have capacities up to 6,000 m³.
• However, in recent years a number of larger capacity fully-pressurised ships have been built with spherical tanks, most notably a pair of 10,000 m³ ships, each incorporating five spheres, built by a Japanese shipyard in 1987.
• Fully pressurised ships are still being built in numbers and represent a cost-effective, simple way of moving LPG to and from smaller gas terminals.

Semi-pressurised ships:

• These carriers, incorporating tanks either cylindrical, spherical or bi-lobe in shape, are able to load or discharge gas cargoes at both refrigerated and pressurised storage facilities.
• The existing fleet of semi-pressurised ships comprises carriers in the 3,000-15,000 m³ size range, although there is a notable exception — a ship of 30,000 m³ delivered in 1985.
• Ethylene and gas/chemical carriers Ethylene carriers are the most sophisticated of the semi-pressurised tankers and have the ability to carry not only most other liquefied gas cargoes but also ethylene at its atmospheric boiling point of –104°C. The first ethylene carrier was built in 1966 and as of 1995, there were about 100 such ships in service ranging in capacity from 1,000 to 12,000 m³.
• These ships feature cylindrical, insulated, stainless steel cargo tanks able to accommodate cargoes up to a maximum specific gravity of 1.8 at temperatures ranging from a minimum of –104°C to a maximum of +80°C and at a maximum tank pressure of 4 bar.
• The ships can load or discharge at virtually all pressurised and refrigerated terminals, making them the most versatile gas carriers in terms of cargo-handling ability.

Fully Refrigerated Ships:

• The 1960s also saw another major development in gas carrier evolution — the appearance of the first fully refrigerated ship, built to carry liquefied gases at low temperature and atmospheric pressure between terminals equipped with fully refrigerated storage tanks. The first purpose-built, fully refrigerated LPG carrier was constructed by a Japanese shipyard, to a United States design, in 1962.
• The ship had four prismatic-shaped (box-like) cargo tanks fabricated from 31⁄2 per cent nickel steel, allowing the carriage of cargoes at temperatures as low as –48°C, marginally below the atmospheric boiling point of pure propane. Prismatic tanks enabled the ship’s cargo carrying capacity to be maximised, thus making fully refrigerated ships highly suitable for carrying large volumes of cargo such as LPG, ammonia and vinyl chloride over long distances.
• Today, fully refrigerated ships range in capacity from 20,000 to 100,000 m³.
• The main types of cargo containment system utilised on board modern fully refrigerated ships are independent tanks having rigid foam insulation. Older ships can have independent tanks with loosely filled perlite insulation. In the past, there have been a few fully refrigerated ships built with semi-membrane or integral tanks and internal insulation tanks, but these systems have only maintained minimal interest.

Liquefied Natural Gas (LNG) Carriers:

• At about the same time as the development of fully refrigerated LPG carriers was taking place, naval architects were facing their most demanding gas carrier challenge, this was the transport of LNG.
• Natural gas, another clean, non-toxic fuel, is now the third most important energy source in the world, after oil and coal, but is often produced far from the centres of consumption. Because a gas in its liquefied form occupies much less space, and because of the critical temperature of liquefied methane, the ocean transport of LNG only makes sense from a commercial viewpoint if it is carried in a liquefied state at atmospheric pressure; as such, it represents a greater engineering challenge than shipping LPG, mainly because it has to be carried at a much lower temperature; its boiling point being –162°C.
• LNG containment system technology has developed considerably since those early days: now about one-half of the LNG carriers in service are fitted with independent cargo tanks and one-half with membrane tanks.
• The majority of LNG carriers are between 125,000 and 135,000 m³ in capacity. In the modern fleet of LNG carriers, there is an interesting exception concerning ship size.

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Diagram about LNG Ships Membrane Tank Structure:

• Unlike independent tanks, membrane tanks are non-self-supporting structures. Their primary barrier consists of a thin layer of membrane (0.7 to 1.5 mm thick).
• The membrane is supported to the inner hull structure through an insulation that can range upto 10 mm thickness as per IMO IGC Code. Due to their non-self-supporting nature, the inner hull bears the loads imparted onto the tank.
• This way, the expansions and contractions due to thermal fluctuations are compensated by not allowing the stress to be taken up by the membrane itself. Membrane tanks are primarily used for LNG cargo.
• Often, there are two layers (primary and secondary) of insulation and membranes placed alternatively.
• The most common types of membrane tanks are the ones designed and developed by two French companies Technigaz and Gaz Transport. The Tehnigaz system makes use of a stainless steel system that is constructed with corrugated sheets in such a way that one sheet is free to expand or contract independent of the adjacent sheet. The Gaz Transport system uses Invar as the primary and secondary membranes.
• Invar has low coefficient of thermal expansion, which makes corrugations unnecessary.
• The insulation is usually made of materials like Reinforced Polyurethane.
• In GTT membrane tanks, the primary membrane is made of Corrugated SUS 304, and the secondary membrane is made of Glued Triplex. Below Figure  illustrates the anatomy of twin-membrane tanks.

Some of the advantages of membrane tanks are as follows:

• They are generally of smaller gross tonnage, that is the space occupied within the hull is lower for a given cargo volume.
• Due to the above reason, maximum space in the hold can be used for cargo containment.
• Since the height of tanks above the main deck is significantly lesser compared to the cases of Moss tanks, membrane tanks provide allow visibility from the navigational bridge. This also allows a lower wheelhouse.

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Preparation Procedure for loading LNG cargo:

Loading LNG cargo after dry docking:

LNG is a cryogenic substance and its main component is methane. It gasifies violently when directly introduced into a cargo tank at ambient temperature, rapidly increases the internal pressure of the cargo tank and makes the atmosphere into a flammable condition. In addition, the cargo tank is rapidly cooled, resulting tremendous thermal stress on cargo tank skins and cargo piping systems.

To avoid such damages, the preparatory work for cargo loading after dry docking must be done in the following sequence. During dry dock all the compartments of an LNG carrier are kept gas free.

After leaving the dry dock the vessel has to be prepared to load cargo, for that the following points to be considered with priority.

Drying of Cargo Tank:

During dry docking or inspection, cargo tanks which have been opened and contained humid air, must be dried to avoid the formation of ice when they are cooled down and the formation of corrosive agents if the humidity combines with sulfur and nitrogen oxides which might be present in excess in the inert gas.

The drying operation need not be performed independently by using dry air, instead during inerting operation by supplying dry inert gas, drying operation can be achieved. During such operation special attention is required to the delivery temperature of inert gas to prevent condensation of humid air inside the tank.

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Dry air, with a dew of -70ºC to -40ºC, can be produced by the onboard IGG system:

i)  It is essential that cargo tanks are thoroughly inspected for cleanliness, free of liquid, any loose objects and all fittings are properly secured. Once this inspection has been completed, the cargo tank should be securely closed and drying operation can be started

ii)  During drying operation, measure the atmosphere at different levels at regular intervals. When the dew point of the cargo tank drops below than the planned temperature, finish the drying operation.

Drying of Hold Spaces:

The drying operation of a hold space is carried out in order to prevent tank insulation damage due to condensation of moisture inside it prior to initial cool down operation and periodically during a voyage. Fresh air is dehumidified by the IGG and sent to a hold space as dry air with a dew point of -70ºC to -40ºC through its bottom section, humid air inside the hold space is released through the vent pipe provided in the upper portion of the tank. The hold space should be maintained at a higher pressure than the atmospheric pressure.

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Operation Procedures and Precautions:

1. Before delivering dry air into a hold space, completely dry up the bottom section of the hold space, particularly the bilge well.
2. When drying a hold space after completing the inerting operation of a cargo tank, purge relevant equipments and Inerting/aerating lines with dry air to prevent the ingress of inert gas into the hold space. This is because the hold space holding dry air sent into it is kept almost sealed till the next dry docking and, in addition, about 15% CO2 gas is present in the inert gas, which may corrode aluminium cargo tanks and destroy insulation materials.
3. During drying operation, measure the atmosphere at different levels at regular intervals. When the dew point of the hold space drops below than the planned temperature, finish the drying operation.

• Inerting of Cargo Tanks:

Before introducing the cargo into the tanks, the moisture content and oxygen content in the tanks shall be reduced simultaneously.

Cargo tanks filled with air shall be dried and inerted with inert gas supplied from the inert gas generator on board. Inert gas shall be led into the bottom of the cargo tank through the liquid filling line and displaced air shall be vented to the atmosphere through the vapour line and the vent mast. Drying and inerting shall be finished when the dew point and also the oxygen content in the cargo tank are less than the planned level.

The dew point and oxygen content shall be periodically measure by a portable instrument at the sampling lines in way of cargo tank dome.

• Inerting of Annular Space for Moss type vessels:

The space between the surface of a cargo tank and insulation is called annular space, insulation space or wedge space. Annular Space is inerted with nitrogen gas and continuously supplied from N2 generator through the N2 bleed line in service in order to ensure adequate path in the insulation space for the gas detection system.

A safety valve is installed in the N2 bleeding line of each hold in order to avoid over pressure of the insulation space.

• Inerting Inter Barrier Spaces (IBS) and Insulation Spaces (IS) for Membrane type vessels:

The space between the primary and the secondary barrier is called inter-barrier space (IBS). The space between the secondary barrier and the inner hull is called insulation space (IS). The pressure in these spaces shall be regulated at a pressure slightly above atmospheric pressure in order to prevent any air ingress.

In normal operation, IBS and IS shall be purged with nitrogen in relation with atmospheric pressure variations and cooling or warming of the spaces during loading or unloading, and IBS should be continuously purged with nitrogen if gas is detected by micro-leakage of the membrane.

• Gassing-up:

After lay-up or dry dock, the cargo tanks are filled with inert gas or nitrogen. If the purging has been done with inert gas, the cargo tanks have to be gassed up and cooled down when the vessel arrives at the loading terminal. This is because, inert gas contains about 14% carbon-dioxide, which will freeze at around -60ºC and produces a white powder which can block valves, filters and nozzles.

During gassing up, the inert gas in the cargo tanks is replaced with warm LNG vapor. This is done to remove carbon dioxide and to complete drying of the tanks.

• Supply of LNG for Gassing up:

LNG liquid is supplied from the terminal to the liquid manifold where it passes to the stripping/spray header via the appropriate ESDS liquid valve. It is then fed to the main vaporizer and the LNG vapour produced is passed at a temperature warmer than the dew point temperature existing within the cargo tanks through the vapor header and into each tank via the vapor suction fitted in the upper part of the tank.

This method of gassing up is called “Piston Flow Method”. In this the lighter specific gravity LNG vapor is injected from top and the heavier IG is displaced from bottom.

• Initial Cool Down:

Cool down is an operation to pre-cool cargo tanks and lines required before taking on cryogenic LNG. Cargo tank cool down is carried out by spraying LNG through the spray nozzles of each cargo tank, using LNG received from the shore terminal. The cool down operation from an ambient temperature (from a condition after gassing up) to a planned temperature, is called ‘initial cool down’ and is to be differentiated from an ordinary cool down operation carried out on ballast voyage.

Before LNG can be introduced into the cargo system of an LNG vessel, the system, and in particular the cargo tanks, have to be cooled down to a temperature close to that of the LNG which is to be loaded. The reasons for this are as follows:

• Vapor Generation:

If LNG is introduced directly into warm tanks, the LNG will almost immediately turn into vapour. LNG has a liquid to gas expansion ratio 1: 600. Therefore, to enable the liquid to be loaded into the tank at a reasonable loading rate, necessity of large compressors would be required to remove the vapour generated in the process.

By reducing the cargo tank temperature, the amount of heat that is available to transfer into and heat the LNG is minimized. Consequently the amount of vapour generated can be maintained within reasonable limits.

• Cargo Tank Material:

Most cargo tanks are constructed of stainless steel which is a material, that retains its flexibility and strength characteristics over the temperature range being considered (-180ºC – 50ºC). However problems could occur if the material is subjected to very local and rapid cooling such as when a small droplet of LNG comes into contact with a warm tank wall. Because of the transfer of the heat from the wall into the liquid, the temperature at the particular point will decrease rapidly causing large thermal stresses to arise between the point and the surrounding material. This could lead to stress cracking.

• Pipe Tower Construction:

The tower which supports the pipe-work within the tank is constructed of stainless steel bars. If subjected to rapid cooling thermal stress within the material can be excessive, leading to the material cracking.

All three reasons are of equal importance as each, if not carefully controlled, can have a significant impact on the tank structure and overall safety of the vessel.

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Loading Operation:

• LNG is loaded via the loading manifolds to the liquid header and then to each tank filling line. The boil-off and displaced vapour leave each tank via the vapour suction to the vapour header. The vapour is initially free-flowed to shore via vapour crossover manifold and, as tank pressure rises, one compressor is brought into operation to increase the gas flow to shore and limit the vapour main and cargo tank pressure.
• As the loading rate increases, it is important to monitor the tank pressures and to start one HD compressor. If the compressors are unable to cope with the volume of boil-off and displaced gas, it will be necessary to reduce the loading rate.

Fig: LNG bulk loading diagram

Bulk loading:

• When all lines and valves are fully cooled the vessel can commence ramping up the loading rate in the sequence agreed with the terminal. Deballasting should be commenced in accordance with the cargo plan. The cargo should be evenly distributed during the loading.
• Ensure the HD compressors are adjusted in line with loading rate to ensure that the tank vapour pressure remains at a level safely below the lifting pressure of the relief valves. Ensure Nitrogen system is performing correctly.
• Moss vessels will require the temperature gradient (with particular reference to the equator) to remain within certain limits, the tank temperatures are therefore to be closely monitored. Hourly temperatures are to be recorded in order that if required the vessel can verify that temperature has stayed within the manufacturers tolerances.
• If not already started membrane ships should start appropriate cofferdam heating. Communications with the terminal should be tested on a frequent basis. Remote gauging devices and valve position indicators should be verified against local readouts at regular intervals during the operation. Moorings should be diligently attended and vessel movement with respect to loading arms closely monitored, if required additional persons are to be called to assist with the moorings. If at any time the OOW is in doubt a senior officer or the Master should be called.

Topping off:

• As the vessel approaches completion of cargo operations the tanks should be staggered in line with the cargo plan, typically this would leave a gap of 10 to 15 minutes between completion of each tank. The terminal is to be notified well in advance and in line with the agreed procedure that the vessel is topping of and will need to reduce loading rate. Notification should be made at least 30 minutes before reducing rate.

Note: Membrane tanks normally fill to 98% whereas Moss vessels normally fill to 99.5%. On all vessels the independent alarms activate at preset filling levels, the upper alarm activates the ESD if previous alarms are ignored.

Deballasting:

• The deballasting operation is carried out simultaneously with the cargo loading operation. Before any de-ballasting commences, all ballast surfaces should be visually checked and confirmed as free from oil or other pollutants. This check must be carried out through inspection hatches / tank lids. This is particularly important for ballast tanks which are situated adjacent to fuel oil tanks. If fitted, gas detection / sampling systems may not indicate the presence of hydrocarbons particularly in small quantities.
• Deballasting is initially carried out by gravity discharge until the level in the ballast tanks approach the vessels water line when the ballast pumps are used.
• The ballast should be adjusted to keep a small stern trim to aid with the stripping of the ballast tanks. The flow rate of the ballast should be adjusted to keep the ship within 1 meter of the arrival draft or as specified by the terminal. Deballasting should normally be completed before the start of the topping off of the cargo tanks.

Filling Rate of Cargo Tanks:

• The IGC Code (International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk) came into force on July 1, 1986, in accordance with the International Convention on the Safety of Life at Sea, 1983 (the 1974 SOLAS Convention, as amended in 1983), and, following this, the Regulations Relating to the Carriage and Storage of Dangerous Goods by Ship was revised in Japan. The IGC Code contains a chapter for “Filling Limits for Cargo Tanks”.
• LNG carriers registered in Japan are NK-class ships and constructed on the basis of NK’s “Rules and Guidance for the Survey and Construction of Steel Ships – Part N”. These rules reflect the IGC Code, as it is, and, as a result, our LNG carriers, though built before the enforcement of the ’83 SOLAS Convention, meet requirements for new ships in the IGC Code.

Behaviour of LNG in the Cargo Tanks:

• When loaded in the cargo tanks, the pressure of the vapour phase is maintained substantially constant, slightly above atmospheric pressure.
• The external heat passing through the tank insulation generates convection currents within the bulk cargo, causing heated LNG to rise to the surface where it vaporizes.
• The heat necessary for vaporization comes from the LNG, and as long as the vapour is continuously removed by maintaining the pressure as substantially constant, the LNG remains at its boiling temperature.
• If the vapour pressure is reduced by removing more vapour that is generated, the LNG temperature will decrease. In order to make up the equilibrium pressure corresponding to its temperature, the vaporization of LNG is accelerated, resulting in an increase heat transfer from LNG to vapour.
• If the vapour pressure is increased by removing less vapour than is generated, the LNG temperature will increase. In order to reduce the pressure to a level corresponding to the equilibrium with its temperature, the vaporization of LNG is slowed down and the heat transfer from LNG to vapour is reduced.
• LNG is a mixture of several components with different physical properties, particularly the vaporization rates; the more volatile fraction of the cargo vaporizes at a greater rate that the less volatile fraction. The vapour generated by the boiling of the cargo contains a higher concentration of the more volatile fraction than the LNG.

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Preparation for loading LNG cargo – Inerting of Cargo Tanks:

• Inert gas (e.g. nitrogen) or mixture of gases, containing insufficient oxygen to support combustion. The introduction of inert gas into a space to reduce and maintain the oxygen content at a level at which combustion cannot be supported- with an Oxygen content of less than 1% and a dewpoint of less than -45 deg C is typically introduced into the bottom of cargo tank through the filling pipe.
• The Inert Gas is displaced from the top of each tank through the dome and vapour header and is discharged from the vent mast. During this process all the cargo piping and equipment forming part of the cargo system is to be purged with Inert Gas.
• Warning – Inert Gas will not sustain life. Great care is to be taken to ensure the safety of all personnel involved with any part of the operation including those working with the Inert Gas plant.

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Below is our guideline prior loading LNG cargo on board:

Inerting of Cargo Tanks:

• Vapours from the last cargo in the system are displaced by inert gas from the ship’s inert gas generator, or by pure nitrogen from shore. If the ship’s inert gas is used, the cargo piping system from the tank should be opened to the vent before the inert gas supply is connected as an additional precaution against the possible backflow of flammable vapour to the generator.
• Regulations regarding venting of cargo vapour in port should be observed. Such regulations may require that vented cargo vapours should be led to a flare or vent stack ashore. Inerting is continued until the required dew point or concentration of cargo vapour or oxygen level has been reached.
• Before introducing the cargo into the tanks, the moisture content and oxygen content in the tanks shall be reduced simultaneously.
• Cargo tanks filled with air shall be dried and inerted with inert gas supplied from the inert gas generator on board. Inert gas shall be led into the bottom of the cargo tank through the liquid filling line and displaced air shall be vented to the atmosphere through the vapour line and the vent mast. Drying and inerting shall be finished when the dew point and also the oxygen content in the cargo tank are less than the planned level.
• The dew point and oxygen content shall be periodically measure by a portable instrument at the sampling lines in way of cargo tank dome.

Inerting of Annular Space for Moss type vessels:

• The space between the surface of a cargo tank and insulation is called annular space, insulation space or wedge space. Annular Space is inerted with nitrogen gas and continuously supplied from N2 generator through the N2 bleed line in service in order to ensure adequate path in the insulation space for the gas detection system.
• A safety valve is installed in the N2 bleeding line of each hold in order to avoid over pressure of the insulation space.

Inerting Inter Barrier Spaces (IBS) and Insulation Spaces (IS) for Membrane type vessels:

• The space between the primary and the secondary barrier is called inter-barrier space (IBS). The space between the secondary barrier and the inner hull is called insulation space (IS). The pressure in these spaces shall be regulated at a pressure slightly above atmospheric pressure in order to prevent any air ingress.
• In normal operation, IBS and IS shall be purged with nitrogen in relation with atmospheric pressure variations and cooling or warming of the spaces during loading or unloading, and IBS should be continuously purged with nitrogen if gas is detected by micro-leakage of the membrane.

The Nitrogen provides a dry and inert medium for the following purposes:

1. To prevent formation of flammable mixture in the event of any LNG leak.
2. To permit easy detection of an LNG leak through a barrier.
3. To prevent corrosion.

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“BOIL OFF” on an LNG:

LNG Boil-off Vapour Handling System:

• Although it is technically quite feasible to re-liquefy LNG boil off vapours, the plant required is complex and expensive and to date has not been installed on board such ships. Instead, the boil off is used as fuel for the ship’s boiler during the sea passage.
• LNG, the predominant component of which is methane, is the only cargo used as fuel in this manner. Methane vapour is lighter than air at ambient temperature, whereas LPG vapours are always heavier than air. Therefore, in the event of leakage, methane tends to be dispersed upwards more easily.
• Where methane boil-off is used as fuel, it is very important to ensure that the correct procedures and safety precautions are followed.
• LNG ships use steam turbine driven axial flow compressors to handle boil off vapours produced during cool down, loading and during loaded and ballast passage. Normally, a low duty compressor handles the boil off whilst on passage, a high duty compressor handles vapours produced during cool-down and loading, returning these vapours to shore.
• Whilst on passage, the low-duty compressor collects the boil-off from a common header connected to each cargo tank, passing it through steam heater to the poop front. Here it enters a specially designed double duct trunking system heading to the boiler or diesel engine dual fuel systems. This trunking is continuously monitored for leakage and has automatic shutdown protection in the event of system malfunction or leakage. The compressors are provided with surge controls and protective devices.

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Single-Stage Direct Cycle:

• The single-stage direct cycle system is particularly suited to the semi-pressurised carrier.
• A simplified diagram of single-compression reliquefaction is shown in Figures 4.11(a) and (b).
• This cycle is suitable where suction pressures are relatively high, as in the carriage of semi-pressurised products. Boil-off vapours from the cargo tank are drawn off by the compressor — (a) in the diagrams. Compression increases the pressure and temperature of the vapour — to (b) in the diagrams. The high temperature allows it to be condensed against sea water in the condenser — at (c) in the diagrams.
• The condensed liquid is then flashed back to the tank via a float controlled expansion valve at (d) in the diagrams.
• The liquid/vapour mixture being returned the cargo tank may be either distributed by a spray rail at the top of the cargo tank or taken to the bottom of the tank to discourage re-vaporisation.
• The spray rail is normally used when the tank is empty and bottom discharge when the tank is full (see also 2.19 and Figure 2.16).

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Explanation on Heel maintained on LPG: Use of Coolants on LPG:

Heel on LPG: It is frequent practice in some refrigerated trades to retain a small quantity of cargo on board after discharge and the amount retained is known as the Heel. This product is used to maintain the tanks at reduced temperature during the ballast voyage but this procedure only applies when the same grade of cargo is to be loaded at the next loading terminal.

In general, the quantity retained on board as a Heel depends on:—

• Commercial agreements
• The type of gas carrier
• The duration of the ballast voyage
• The next loading terminal’s requirements, and
• The next cargo grade

In the case of a large LNG carrier, as much as 2,000 to 3,000 cubic metres of liquid may be retained in the tanks on departure from the discharge port; the actual volume, depending on the size and type of cargo containment, the length of the voyage and fuel policy. These ships are normally fitted with spray cool-down pumps in each cargo tank to provide liquid to spray lines fitted in the upper part of each tank. This system is used from time to time on the ballast voyage to minimise tank thermal gradients. The frequency of this operation will depend on ship size and type and the duration of the ballast voyage.

With LPG cargoes, the small amount of liquid remaining after discharge should be sufficient to provide the necessary cooling effect during the ballast voyage. This is carried out by intermittent use of the reliquefaction plant, returning the condensate to the tanks to ensure arrival at the loading port with tanks and product suitably cooled.

If the ship is proceeding to a loading terminal to load an incompatible product, none of the previous cargo should be retained on board but if small amounts exist they may be stored in the deck-mounted pressure vessels. This avoids contamination of the following cargo and allows the maximum quantity of the new cargo to be loaded.

Coolants on LPG: PRINCIPLES OF REFRIGERATION:

• The principles of heat transfer, evaporation and condensation are applied in refrigeration.
• Cold liquid refrigerant is vaporised in an evaporator which, being colder than its surroundings, draws in heat to provide the latent heat of vaporisation.
• The cool vapour is drawn off by a compressor which raises both the pressure and the temperature of the vapour and passes it to the condenser. Here, the vapour is condensed to a high-pressure liquid and the sensible heat from desuperheating, together with latent heat of condensation, is removed by means of the condenser coolant, which is warmed in the process.
• The high-pressure liquid then passes through an expansion valve to the low-pressure side of the refrigerator and, in doing so, flash evaporates to a two-phase mixture of cold liquid and vapour. This mixture then passes to the evaporator (cargo tank) to complete the cycle.

Information required prior to loading of a given Chemical Cargo in Bulk:

1. The correct chemical name of the cargo should be provided so that the appropriate data sheet in the Tanker Safety Guide (Chemicals) can be consulted.
2. Quantity in Weight.
3. Required quantity control – Contamination is measured in parts per million (ppm). Thus tanks & pipelines must be practically spotless. Degree of wall-wash required.
4. Specific Gravity – This is required in order that an estimation can be made of the probable volume that the weighed quantity will occupy.
5. Temperature – This is required for two purposes.
   • The loading temperature is used in conjunction with the specific gravity to obtain the probable volume of the particular parcel.
   • The temperature at which the cargo is to be carried will indicate if heating will be required on passage. Some chemicals will solidify or polymerize if a certain temperature is not maintained. Polymerization is a chemical reaction in which small molecules combine into larger or very large molecules, which contain thousand of the original molecules. Thus a free flowing liquid can become a viscous liquid or even solid.
6. Compatibility – Certain chemicals react with other chemicals and thus may not be stowed in adjacent compartments.
7. Tank coating compatibility – The tank coating must be suitable for the proposed cargo.
8. Corrosive Properties – This will also indicate the required tank coating and also possible damage to ship fittings.
9. Electrostatic generation – Some chemicals can accumulate static, the principles which apply to HC cargoes should be applied to chemical static accumulators.
10. Fire & Explosion Data – It has been previously noted that 50 % of the chemicals transported are derived from hydrocarbon oil and thus fire hazards are similar to those which pertain to petroleum products.
11. Toxicity – Chemicals which emit highly toxic vapours requires Closed Ventilation and Ullaging System.
12. The Health Hazard of the particular parcel.
13. Reactivity.
14. Action to be taken in the event of particular emergencies – Most of the above information and additional essential information can be found on the chemical data sheets in the safety guide.

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Publications which are referred to get info Prior to Loading Chemical Cargo:

1. On receipt of the name of the cargo, the certificate of fitness must be checked to verify if the said vessel is allowed to carry that particular cargo as enlisted in the COF.
2. Depending on whether the ship is constructed before / after 01-07-1986. The relevant IBC Code / BCH Code must be consulted. Chapter 17 of the IBC code – contains summary of minimum requirements & various information pertaining to the cargo can be obtained.
3. Additional information can be obtained from the chemical data sheet pertaining to that cargo – found in the ICS (International Chamber of Shipping) publication. Tanker Safety Guide – Chemical in Volume – I, II, III & IV.
4. Also added information can be obtained from USCG system. CHRIS – Chemical Hazard Response Information System, provided for essential decision making during emergencies involving the water transport of the hazardous chemicals.
5. The “Procedure & Arrangement” (P & A) manual which is ship specific, gives information such as tank arrangement, pumping & piping arrangements any special requirements to assists any loading can be obtained.
6. Annex 2 of the MARPOL 73/78 should be referred to obtain discharge criteria & procedure.
7. Paint compatibility guide – to check if the coating in the tank will withstand with particular cargo to load.

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Types of Chemical Tankers:

There are 3 basic types of Chemical Tankers, All 3 types are meant to carry chapter 17 cargoes of IBC code.

Type I ships

• It must be able to survive assumed damage anywhere in their length. Cargo tanks for the most dangerous products should be located outside the extent of the assumed damage and at least 760mm from the ship’s shell. 
• IMO type 1, 2 and 3. Other cargoes, which present a lesser hazard may be carried in tanks next to the hull – (incl diluted slops after tank washings)

See layout below–

• Some of the chemicals carried on type one ships are Chlorosulphonic acid, Dodecyl phenol, Phosphorous yellow/ white, Tricresyl Phosphate (>1% ortho-isomer), Trixylyl Phosphate.
• Maximum tank size is 1250 M3.
• Double side width B/5 or 11.5 mtrs whichever is less.
• DB depth B/15 or 6 mtrs at centre line, but not < 760mm.
• Auto ignition temperature of cargoes <65 deg C.
• Explosive range >50% by volume in air.
• Type 1 offers highest limit of containment.

Type II ships

• If more than 150m in length, must be able to survive assumed damage anywhere in their length; if less than 150m, the ship should survive assumed damage anywhere except when it involves either of the bulkheads bounding machinery spaces located aft. Tanks for Type II cargoes should be located at least 760mm from the ship’s shell and outside the extent of assumed grounding damage.
• Maximum tank size is 3000 M3.
• Capable of stripping tanks <100 litres.
• Auto ignition temp of cargoes <200 deg C.
• Explosive range >40% by volume in air.

Type III ships

• If more than 125m in length, should be capable of surviving assumed damage anywhere in their length except when it involves either of the bulkheads bounding the machinery space.  If less than 125m in length, they should be capable of surviving damage anywhere unless it involves machinery spaces. There is no special requirement for cargo tank location. No limit for size of tank.
• Capable of stripping tanks <300 litres with tolerance of 50 litres.
• Length > 125 m but < 225 m damage anywhere in length except including ER bulkheads.
• Length <125 m damage anywhere in length except machinery space

After 1 January 2007 vegetable oils are carried in chemical tankers complying with the revised IBC Code as a Ship Type-2 (double hull) with COF as Cat Y.

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Chemical Tankers: P & A Manual

• MARPOL Annex II requires that each chemical tanker be provided with a P&A Manual to achieve compliance with the regulations and to be able to demonstrate that compliance has been considered from the earliest design stage. The format of the P&A Manual and its contents must be as specified in MARPOL Annex II Appendix D, and be approved by the flag administration of the ship.

• The P & A Manual is concerned with the marine environmental aspects of cleaning of cargo tanks, and the discharge of cargo residues that may or may not be mixed with a washing medium. The results of the stripping test are recorded in it.

• Ships’ officers should familiarise themselves thoroughly with the P&A Manual, and adhere at all times to operational procedures with respect to cargo handling, tank cleaning, stop handling, residue discharge, ballasting and deballasting. The master is obliged to ensure that the ship does not discharge into the sea any cargo residues, or mixtures of residue with water, unless such discharges are made in full compliance with the operational procedures contained in the P&A Manual, and that the equipment required by the Manual for such discharge is used.
• The P & A Manual, together with the cargo record book and Certificate of Fitness, will be checked by the ship’s own flag administration and by port state control officers in order to confirm full compliance with the requirements of MARPOL Annex II.

• It is now recognised that almost any discharge from a ship into the surrounding environment needs to be carefully considered in advance. Not only are chemical cargo residues, oily water from machinery room bilges and overboard disposal of garbage strictly regulated, but funnel exhausts and ballast water have now been identified as requiring control.

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Cargo Related Documents required on board Chemical Tankers:

Transportation of chemicals by tankers is usually accompanied by considerable documentation. Documentation can be even greater when trading to and from less developed countries. The vessel’s management is presented with a great deal of documentation from parties to the cargo, authorities, etc. Furthermore vessel’s management must also issues papers serving to record evidence, claims etc.

Following are most needed documents:-

1. Deck Log Book
2. Sea Passage Report
3. Port Log
4. Notice of Readiness
5. Dead freight Statement
6. Protest of Difference Between Ship and Shore Figures
7. Pre arrival and Commencement – Cargo Operations Checklist
8. During Loading Ops Checklist
9. Completion of Cargo & Pre-departure Checklist
10. Prior to Use of Vapour Emission Control System Checklist
11. During Discharge Ops Checklist
12. Ullage Report
13. Pumping Record
14. Cargo Heating Report
15. Inert Gas Log
16. Tank Cleaning Record
17. ROB Report
18. Dry receipt
19. Vessel Experience Factor (Load)
20. Cargo Loading Plan
21. Cargo Discharge Plan
22. Chemical & Physical Properties
23. Pressure Log & O2 Log
24. Cargo Hose Record
25. Cargo Sampling Log
26. Tank Cleaning Plan
27. Tank Cleaning Schedule Checklist
28. Monitoring During Cleaning Operations
29. Wall Wash Test Results

Documents provided by the Shipper:-

1. Cargo quality certificate (analysis report)
2. Cargo quantity certificate.
3. Certificate of origin.
4. Cleanliness report.
5. Heating instructions.
6. Inhibitor certificate.
7. Cargo manifest.
8. Vessel’s experience factor.
9. Tank History.
10. Sample receipt.
11. Custom Clearance Reports / Papers

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Diagram, a ‘closed circuit’ loading operation, using a vapour return line on Chemical Tankers under the provision of IBC Code:

Closed loading guideline for various noxious liquid chemicals in bulk:

• Special precautions are necessary onboard a chemical tanker during closed loading of various grade liquid chemicals.
• Closed loading/discharge means loading or discharging with securely closed ullage, sounding and sighting ports. Additionally the venting must be controlled. Vessels equipped with a system such as Skarpenord (pressure gauges in the tanks) or radar ullage systems shall at all times carry out closed loading/unloading procedures for all cargoes. Closed loading should be used at all times unless not possible due to the design of the vessel or trade practices (e.g. vegetable oil trade loading over the top is normal.)
• For gauging e.g. ullaging and sounding closed devices must be used. The level alarm systems must be operated during the entire closed cargo operation. Closed cargo operations must be stopped as soon as any essential system for safe loading or discharging becomes inoperative. Sampling to be carried out with closed sampler whenever possible. When more than one grade of cargoes is loaded, use of same sampler for different grades will contaminate the cargo sample unless the sampler has been thoroughly cleaned.

Gauging, sounding and sampling:

• A closed gauging device penetrates the cargo tank, but is part of a closed system and prevents the cargo or its vapour being released. Examples are the float-type systems, radar systems, electronic probe, magnetic probe and protected sight-glass.
• For sampling and sounding, the Dovianus or Hermetic portable gauging and sampling systems may be used. It is important that sufficient of these devices are carried onboard and maintained in a fully operational and certified calibrated condition. The vessel must fully comply with ISGOTT “Measuring and Sampling Non-Inerted Tanks” and ISGOTT “Measuring and Sampling Inerted Tanks” as applicable.
• Vapour locks, where fitted, are to be calibrated and certified by a recognised cargo inspection company which will also approve the datum level corrections including list and trim corrections for tank volumes. The approval certificate is to be readily available during cargo surveys.

Cargo tank venting:

• Controlled venting must be established if closed cargo operations are required. A controlled tank venting system is a system with pressure and vacuum-relief valves (P/V-valve) fitted on each tank in order to limit the pressure or vacuum in the tank. The P/V valve should operate in such a manner that neither pressure nor vacuum is created in the cargo tank during cargo operations that exceed the tank design parameters.
• Secondary venting system must also be operational Information on maximum loading rates and venting capacities is to be readily available and displayed in the cargo control room.

Vapour emission control system onboard chemical tankers:

• Where required, VEC is to be used and operated in accordance with IBC Code, local regulations, and instructions contained in the vessel’s VEC System Operation Manual and in conjunction with the requirements and provisions of the shore installation.
• Masters and Officers must be aware that significant operational and safety implications are present, as the shore and the ship are effectively joined together as one unit.

The Primary Hazards include:

• The ship loses effective control of the tank atmosphere pressure, and is directly influenced by any changes which may occur within the terminals system.
• Associated pressure sensing devices on the vessel are well maintained.
• It is also essential that individual cargo tank P.V. valves are properly maintained and operate correctly.
• Check that the VECS alarms are correctly set and tested. (Secondary PV alarms are set 5-10% above PV valves setting as per Oil Major Requirements for normal operations).
• Whenever any of these alarms activates during cargo operations, the cargo operations shall be immediately stopped and cause of alarm activation rectified before resuming cargo operations.
• Vessels fitted with a VEC system must have an independent overfill alarm providing audible and visual warning. These are to be tested at the tank to ensure their proper operation prior to commencing loading, unless the system is provided with an electronic self-testing capability. Fixed gauging systems must be maintained in a fully operational condition at all times.
• The ship’s system is to be provided with means to collect and drain condensed vapour, which may have accumulated in the pipelines. Drains must be installed at low points within the ship’s piping system. These drains must be checked clear before each use of the VEC system and on a regular basis when the system is not in use.
• Care must be taken to ensure that no possibility of misconnection of Vapour and Liquid hoses can occur. The ship’s vapour connection is to be clearly identified. The outboard 1.0 metre of piping is to be painted with yellow and red bands (0.1m red, 0.8m yellow, 0.1m red) and marked with the word “Vapour” (not less than 50mm high). The vessel’s presentation flange is to be fitted with a stud to prevent an incorrect connection.
• To prevent electrostatic build up within the vapour return pipe work, all pipe work is electrically bonded to the hull. The integrity of these connections is to be periodically checked.
• VECS manual requirements to complied with respect to loading rate, vapour density, pressure drop etc.
• The full procedures for the use of the VEC system are to be clearly agreed at the pre-transfer meeting between the Terminal Representative and the Chief Officer.

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Typical tank & Piping Arrangement of any one type of Chemical Tanker:

The pipes leading from the cargo tanks to the pumps are termed as bottom lines, from the pump-room up to deck are called risers. The lines on deck are termed as deck lines. The lines which lead from the deck to the tanks are called drop lines. Besides these, there are Crude Oil washing lines on deck (COW lines). The COW main line usually branches off from the main discharge line in the pump-room. It further branches out to the various tanks on deck. There is also a small diameter line (Marpol line) which is used to discharge the last part of the cargo from the ship.

In the cargo tanks, the pipes terminate in a bellmouth. A tank may have two bellmouths – one main and one smaller stripper bellmouth. Alternatively, one bellmouth may serve the purpose of main as well as stripping discharge.

The piping system has evolved over the years to cater to varying cargo requirements. In a product tanker which is designed to carry many grades, we see that there are many more pipes so that many grades can be catered to. In a crude oil tanker, the piping is straightforward and simple.

There are three basic types of pipeline systems:

1. Direct Line system
2. Ring main system
3. Free flow system.

Each system has their uses and is designed to fulfill a need in a particular type of vessel.

Direct Line system:-

It consists of lines running longitudinally in the centre tanks and branching out to bellmouths in the centre and wing tanks. The system is uncomplicated and found on some crude carriers.

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Procedure for Tank Cleaning a Cargo Tank in a Chemical Tanker:

No cleaning can take place unless the mandatory prewash as required by MARPOL is done.

The master must enforce precautions like “no smoking “and “AC on recirc “.  It is important for all on a chemical tanker to know the location of AC fresh air intake, and the Anemometer, to use the relative wind direction to advantage.  Bio accumulative vapours and carcinogenic fumes can enter the engine room via intake vents and cause health problems for the engine staff. It is not all right to say that just the deck crew are exposed to toxic vapors.

Water washing can be done even if the solubility of the chemical in water is low to as much as 0.3%.  such solubility always increases if the water temperature is higher. Hence always try to use wash water at high temperature ( about 25 deg higher than MP after removing cold ballast interface ) unless the cargo being cleaned does not allow it, like drying oils and mineral oils with high paraffin content.  Solidified matter when melted flows away with the stripped out water, it need not be soluble.

Mineral oils with high paraffin content and certain crude oils which require heating during transportation should always be cleaned with ambient wash to prevent evaporation of lighter fractions which would leave a waxy residue on tank bulkheads. 

Drying oils if prewashed with hot water will polymerize. Drying oils must be washed immediately after dischg with ambient water. If they are left to dry polymerization takes place due to reaction with oxygen, and heat increases the reaction speed. This means by removing the air form the tank using Nitrogen this process can be slowed down. By the way, Ambient means upto 35 deg C. Moderate means upto 60 deg. Hot means >60 deg C.

Certain cargoes like acetic acid, benzene, luboils, caustic soda, paraffin, molasses, phenol, DINP, fatty alcohols, HMD , Hitec, WPAC, butyl acrylate , creosote etc  can be hot pre-washed.

However, if you hot prewash Styrene monomer or Acrylic acid polymerization will happen. The hotter the water, the faster the polymerization. Due to condensation of vapour, inhibitor free liquid is formed, as the inhibitor is not volatile. Inhibitor if not removed will affect the PTT test.

When a cargo is fully soluble in water, using tank cleaning chemicals in a mindless manner does more harm than good.

Cleaning the tanks is just not enough. Most often it is the tank appendages which cause huge cargo claims or tank rejection. PV valves, vent lines, fixed pipelines , portable manifold hoses, superstrip lines, sampling and drain cocks, air/ nitrogen/ steam portable connection stubs, pump internals , cofferdams –all can hold contaminant matter. With certain cargoes like LSHW FO even the butterfly valve Teflon seals can trap sludge and discolor the next cargo.

Washing tanks with portable/ fixed Butterworth machines from designated areas may not clean everything. This is why it is important to enter the tank and have a visual check before wasting fresh water and expensive tank cleaning chemicals after high MP cargoes like palm oil fatty acids. As soon as you open the tank dome you get a general idea of shadow areas.

The chief officer must know if the tank corrugations are horizontal or vertical. Horizontal corrugations mean that fixed tank cleaning machines which cannot be given drops are ineffective. Vertical corrugations mean that the position of the Butterworth port is important.

If the previous cargo is strong smelling like Acrylates or Crude Turpentine or MTBE a smell killer can be used. Gaskets emit smell, hence they must be flushed with methanol.

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STAGES OF TANK CLEANING:-

1) Precleaning with sea water:- Pre cleaning is different from MARPOL mandatory Prewash. Tank cleaning machines may have shadow sectors, and this can be rectified for main wash. Nondrying oils and fats can be steamed before the hot water precleaning.

Pre-cleaning is the first cleaning step, without cleaning agents in order to remove the majority of product residue. The cleaning temperature and the temperature of the adjacent tanks are important parameters for successful cleaning. When water is not allowed, pre- cleaning is carried out with a suitable solvent. Pre-cleaning generally takes several hours and the sooner pre-cleaning is done after discharge, the easier it is to remove the product residue. Pre-cleaning is very important, because it is very difficult to obtain a satisfactory result following an initial mistake

2) Main wash with sea water: – Do not attempt to use tank cleaning chemicals unless the cargo clingage is removed.

3) Tank Cleaning Chemical wash: – If cargo is not water soluble or residues remain , the use of  tank cleaning chemicals is justified.  If the previous cargo is not water soluble using a 0.04% detergent wash will be good enough for WW standard ( this is not WALL WASH ).  Graco barrel pump injecting into the tankcleaning line , is the best as the bottom can be kept stripped and tank cleaning chemical is not reused.  Inject at the rate of 2 litres chemical per cubic meter of wash water.  Discharge from both sides of the manifold to Annex 2 UW overboard.

Recirc using spider / octopus will clean dirty areas, but they will also dirty clean areas.  Also if the solvent is volatile it will evaporate. Recirc is more effective after a Graco injection wash. Prevent static charge dangers. Annex 1 mineral oils which are not soluble in water requires a emulsifier/ degreaser wash. Handspray with undiluted chemicals is only for local application, sufficient contact time must be allowed.

Most cleaning agents are additives which are used in combination with water to improve the water solubility of the cargo to be cleaned. Only very few cargoes which cannot be cleaned with water-based systems require a non-water-based solvent as a cleaner (sometimes in combination with an emulsifier).

To neutralize the odor of some chemicals, the use of an odor remover may be recommended in combination with an emulsifier. For most cargoes a variety of cleaning agents are available. Cleaning agents must be IMO-approved. Cleaning concentrations, times and temperatures in the final cleaning steps are recommended in the cleaning guides to achieve a satisfactory result. Cleaning generally takes several hours.

4) Rinsing with sea water:- This is done with tank cleaning machines, the main purpose is to get rid of the residues loosened up by the tank cleaning chemicals.

5) Flushing with fresh water:- This is done with tank cleaning machine using a low throughput, to remove the salt before they dry up . If you anticipate a delay , as your ship does not have a dedicated FW pump or line prevent the salt from drying up by steaming, more so on zinc adsorbent porous surface.

6) Steaming tank and pipelines (to bring level of chlorides down and wash down appendages):- Steaming is the introduction of saturated steam into the tank to evaporate volatile residue (odour removal). The steam will condense on the tank surfaces. The temperature should normally be  as  high as  possible during steaming. This is enhanced if the adjacent tanks (including ballast tanks) are empty Steaming removes traces of volatile substances. The steam must hence be allowed to escape constantly via the PV valve.

If the chloride level of the wash water is too high, the use of steam for removal of chloride is often the only feasible option. Clearly the steam quality depends on the construction of the boiler.  If the steam is used to remove chloride, the wall temperatures should be cool (in contrast to the evaporation method described above) – this results in condensation  and  water  film  running  down  the  tank  walls  to  wash  the chlorides off.

In case of Wall wash , with low chloride specs, for optimal results the chloride content of the water must be less than 0.1 mg/ litre  (distilled water, deionized water, demineralized water by microfiltration).

7) Draining tank sump:- Strip out the tank and the pump stack. Sometimes it will be necessary to use a Wilden pump and sponges to save time.  Mopping reduces drying time if there are water pools on the tank bottom. Make sure no lint is left .

8) Drying tank with ventilation:- While venting please remember that warm moist air condenses on a cold surface.  Ventilation removes water, moisture and odor, which is usually done by forced air circulation.

The advantages are that:

1. It is easy to operate and less training of personnel is required.
2. As there are fewer valves, it takes less time to set up the valve system before commencing a cargo operation.
3. Contamination is unlikely, as it is easy to isolate each section.

The disadvantages are that:

1. The layout is not as versatile.
2. A very rigid system which makes it difficult to plan

Ring-main systems:-

• It is also called the circular system. This type of piping system provides for the handling of several different types of oil. A particular tank can be pumped out either by a direct suction line or through another line by use of a cross-over. The system is very versatile.

• The pipeline system illustrated above in Diagram 1 is better suited to the centre line bulkhead type of ship. Each tank or oil compartment has two suctions — one Direct suction and one Indirect suction. The direct suctions for the port tanks are all on the port cargo line, and feed the port cargo pump.
• The indirect suctions for the port cargo tanks feed the starboard cargo line and the starboard cargo pump. Master valves are provided on each line between the tanks, so as to isolate each tank from the other when necessary.
• This particular vessel is not fitted with a stripping line and pump. This type of pumping system providing for the handling of several different types of oil was a natural development from the earlier types which were only suitable for one grade of oil.
• To drain the oil from the main tanks it was necessary to list first one way, and then the other, so as to keep the strum covered and to help the flow of oil towards the suction.
• Diagram 2 shows a vessel fitted with a Circular Line or Ring Main but adjusted for the twin bulkhead type of vessel.
• This ship is also fitted with a stripping system. Inspection of the pipeline system shows that the pipeline travels around the ship in the wing tanks, crossing over from one side to the other.
• Each wing tank has a suction on the line which passes through it. The centre tanks have two suctions, one on either side leading to the port and starboard lines respectively.
• It will be noted that the master valves provide separation between the tanks as in the earlier system.
• When the level of the oil in any particular tank has fallen to a foot or less, the main pumps are switched to another full tank, and the stripping pump is brought into operation.

Free-flow system:-

• In this system, the oil flows freely into the aft most tanks when the interconnecting gate valves are opened.
• Main suction bellmouths in a full free flow tanker will only be provided in the aft tanks. However, each tank is generally provided with a small stripping line.
• This system has the distinct advantage of having lesser and less complicated piping system in the tanks and is suitable for large tankers which usually do not carry many grades of oil.
• Obviously, the flexibility of operations is comparatively less as compared to other piping systems. Some ships are also designed as part free flow i.e. free flow system only between certain tanks, which is a hybrid or cross between a full free flow system and a ring main system.

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Phosphoric Acid Discharge & Tank Cleaning:

• For all cargo operations including stripping & tank cleaning procedures always refer to ship’s P & A manual.
• Phosphoric Acid is normally carried in rubber lined or Stainless Steel Tanks.
• Phosphoric Acid is generally carried on a “Type 3” Chemical tanker.

IMO Ship Type 3:- is a Chemical Tanker intended to transport products with sufficiently severe environmental & safety hazards. These products require a moderate degree of containment to increase survival capability in a damaged condition. There is no filling restrictions for chemicals assigned to Ship Type 3.

Some of the properties of Phosphoric acid is listed below:-

Pollutant Category: Z

Sp. Gravity:- 1.685 @ 25^OC (Water =1 )

Vapour Pressure: 0.3 k Pa (@ 20^OC)

Vapour Density: 3.4 (Air = 1)

• Easily soluble in hot water. Soluble in cold water.
• Very hazardous in case of skin contact (irritant), of eye contact (irritant) of ingestion.
• Phosphoric Acid is non-flammable.
• Reacts with metals to liberate flammable hydrogen gas.
• Minor corrosive effect on bronze. Sever corrosive effect on brass, corrosive to ferrous metals & alloys.
• Polymerization will not occur.

Tank Cleaning:-

• Ensure the prewash after dischg is with fresh water. Then use sea water till the pH is 7. Immediately after that wash again with fresh water to remove all chlorides from tank. This is crucial to avoid elephant skin.
• Any sediment at the bottom of tank can only be removed with more pure phosphoric acid like crude oil wash. It will take a long time to kew machine the cement off the bottom. So keep some good clean pac in 200 drums for this manual effort.
• Have a look at the first empty tank. If the sediment is too much –it is usual to recirc at end of discharge of each tank.
• Acid/sea water mixture remaining in lines and stainless steel hoses will soon result in pittings.
• For Stainless Steel (SS) Tanks: – After the tanks are thoroughly/finally cleaned, passivate the tanks with Nitric Acid as the Phosphoric destroys the passive oxide coating on the stainless.
• Category Z:- Noxious Liquid Substances which, if discharged into the sea from tank cleaning or deballasting operations, are deemed to present a minor hazard to either marine resources or human health and therefore justify less stringent restrictions on the quality and quantity of the discharge into the marine environment.
• Every ship constructed on or after 1 July 1986 but before 1 June 2007 shall be provided with a pumping & piping arrangements to ensure that each tank is certified for the carriage of substances in Cat X or Y does not retain a quantity of residue in excess of 900 ltrs in the tank of its associated piping.
• Similarly each tank certified for the carriage of substances in Cat Z does not retain a residue of quantity in excess of 300 ltrs in the tanks and associated piping.

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Discharge Criteria for Tank wash residues into Sea:-

• Underwater discharge criteria are applicable to all ships built after 1 Jan 2007.
• If outside any S.A. (Special Area)
  • Discharge tank washing 12 NM from Nearest land.
  • Depth of water must be more than 25 mt.
  • Speed of ship must be more than 7 kmts.
• S.A. designated for Annex II cargoes is Antartic Region.
• Discharge of tank washings is not permitted in the Baltic Region.

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IBC Code: Integral tank:

Ans:- Integral tanks: Integral Tank means a cargo-containment envelope which forms part of the ship’s hull and which may be stressed in the same manner and by the same loads which stress the contiguous hull structure and which is normally essential to the structural completeness of the ship’s hull.

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IBC Code: Gravity tank. (July-18)

Ans:- Gravity tank: Gravity tank means a tank having a design pressure not greater than C). 7 bar gauge at the top of the tank. A gravity tank may be independent or integral. A gravity tank should be constructed and tested according to recognized standards, taking account of the temperature of carriage and relative density of the cargo.

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IBC Code: Pressure tank. (July-18)

Ans:- Pressure tank: Pressure tank means a tank having a design pressure greater than 0.7 bar gauge. A pressure tank should be an independent tank and should be of a configuration permitting the application of pressure-vessel design criteria according to recognized standards.

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Hazards involved with Tank Cleaning in Type 1 Chemical Tankers:

A Hazard is a physical situation with a potential for human injury, damage to property, damage to the environment, to capital investment or some combination of these. Hazards can be identified through a review of the Physical Properties and Product Characteristics of the product to be cleaned.

• Fire & Explosion Three elements are necessary to create a fire: Fuel, an Oxidiser (usually air) and a Source of Ignition (energy). In theory, ignition is not possible, if any one of the 3 is eliminated. Most cleaning operations will be carried out in tanks that are filled with air, thus the oxidiser is present in most cases, unless the tank is inerted. Fuel as far as tank cleaning is concerned could be the product itself, if this product has a low flash point, or a flammable cleaning solvent. Under certain circumstances even substances with a high flash point can be ignited and must thus be considered as a fuel (mist). During many tank cleaning operations the atmosphere in the tank must be considered as flammable because the product to be cleaned is flammable and inertisation is not possible. Under these circumstances the only way to guarantee that an explosion cannot occur during cleaning is to make certain that there is no source of ignition. A potential source of ignition during tank cleaning is Electrostatic discharge. Especially during water spraying electrostatic charges could be induced.
• Undesired reactions Polymerization (Depletion of inhibitor or excessively high temperature) Saponification (Creation of hard soap forming a layer on the tank requiring acid cleaning or even removal by Hydroblasting) Drying/Hardening (Formation of hard debris that is no longer soluble, requiring treatment with a Solvent) Reaction with water (Violent reaction of an Isocyanate after Pre-Cleaning with water)
• Corrosion – Corrosive substances destroy human tissue on contact (e.g. skin, eyes and mucous membranes in the mouth and respiratory tract) Metal or other material used in ship construction could be corroded at an excessive rate.
• Overexposure to toxic substances (Death of operator after wiping Phenol residues by tank entry without wearing a full chemical suit and SCBA [self-contained breathing apparatus])
• Asphyxiation – Oxygen deficiency (Entry into a tank with an inert gas atmosphere)
• Emissions
• To the air: As always when ventilating, special care must be taken to prevent the risk of explosion (flammable products) or with regard to toxic vapors. All normal safety precautions must be taken. (No smoking, accommodation ventilation on recirculation etc.) The wind strength and wind direction must also be a decisive parameter for the Master to allow ventilation. To avoid a buildup of explosive or toxic vapors on deck the amount of gas to be escaped from the tanks should be limited. Never open and ventilate several tanks at the same time.
• To the water: Emissions to the water should be reduced to the absolute minimum. All on-board facilities must be operated carefully according to the P&A Manual to reduce the residues during unloading. All regulations, especially MARPOL I and II, must be followed strictly.

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Certificate Of Fitness (COF) on Chemical Tankers:

• An International Certificate of Fitness for the Carriage of Dangerous Chemicals in Bulk shall be issued after an initial or renewal survey to a chemical tanker engaged in international voyages which comply with the relevant provisions of the Code.
• Classification society issues the certificate of fitness on behalf of the administration.
• An International Certificate of Fitness for the Carriage of Dangerous Chemicals in Bulk shall be issued for a period specified by the Administration which shall not exceed 5 years.
• All ships will get a new COF after 1.1.07, considering IBC code and Marpol rules are revised as on 1.1.07.  IMO has decided that Chemical carriers can carry COF or NLS, not both.
• Ships with COF can carry IBC code chapter 17 XYZ cargoes also —–  while NLS certificated ships an carry IBC code Chapter 18, Category Z and  OS cargoes only.
• For ships carrying IBC code chapter “Other Substances” OS, there is no need for COF, they require only NLS certificate.
• COF is a certificate issued by Flag administration confirming that the structure, equipment and materials used in the construction of the chemical tanker are in compliance to carry a given list of chemicals and it gives the conditions of carriage.
• Upon receipt of cargo loading instructions, all products are to be checked against the Certificate of Fitness. Any irregularities are to be reported immediately to the chemical operator. However, it must be understood that this by itself is not enough. The tank lining resistance tables must be consulted and only if both agree can the chemical be considered to be loaded.
• Since new chemicals are being manufactured and re-evaluated for safe sea carriage regularly it may be possible that some chemical is not included in the COF list. In such a case permission may be obtained from flag administration or their representative for this particular chemical and attached to the COF as an addendum.
• The issuance of an Addendum to CoF may be done immediately based on the Tripartite Agreement. The submission of data and evaluation by GESAMP and ESPH may come afterwards.

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Cargo tank coatings can be categorized into two main groups:

Inorganic coatings – zinc silicates and ethyl silicate types.

Generally, the life of this coating is proportional to the thickness of the coat. This coating is one-layer coating, comprising of inorganic silicates pigmented with high percentage of zinc powder.

Organic coatings – epoxy and modified epoxy systems.

This type of coating consists an organic resin system, which form strong chemical bonds between the resin molecules. Those types of coating have the ability to resist in more strong acids or alkalis than inorganic coatings. And they tend to absorb significant quantities of cargo and contamination problems can occurs.

Coating Systems and Types:

Numerous types of coating have been used for cargo tank service in sea trades. Some of these coating have stopped to being used. And more reliable and flexible coating has been developed. Typical coating system can be categorized as Zinc and Epoxy coating

Zinc Silicates:- Zinc silicates are formulation of zinc powder plus organic or inorganic binder, and designed to be porous films, which can create problem in the tank cleaning process especially when vessel carry non-volatile cargoes.

Main Characteristic:

• Not resistant to strong acid or bases, including sea water which has a slow weakening effect
• High resistance and tolerance to aromatic hydrocarbon solvent, alcohols and ketones
• Volatile cargoes are desorbed very fast, and retain non-volatile oil like cargoes.
• Residues can result in contamination of next or after next cargo

Epoxy:

Generally suitable for the carriage of alkalis, animals fats and vegetable oils but they have limited resistance to aromatics such as benzene and toluene, alcohols such as ethanol and methanol

Main Characteristic:

• Resistant to most strong acids and bases
• Do not retain oil like cargoes. Solvent cargoes are absorbed
• Water wash before thorough ventilation and desorption of residues could result in serious damage of the coating
• Residues can result in contamination of next or after next cargo
• Suitable for carriage of animal fats and vegetable oils provided the free fatty acid content of 5%.

Coatings are required for any cargo tank which constructed from mild steel. Most of BLT Chembulk Group modern chemical fleet is SUS cargo tank. SUS are good materials for chemical tanks, because of their ability to create a passive layer on their surface. This passive layer is mainly consisted by chromium oxide, which is very resistant to corrosive environment.

However, in some environments like strong hot acids, chloride solutions and generally solutions which contain halogens, the passive film can break down locally and new film formulation can be disrupted. Generally, SUS is considered to be the ideal material of construction because it’s non-corrosive and easy to clean.

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Hazards associated with Carriage of Chemicals:

• FLAMMABILITY:
  • Vapour given off by a flammable liquid will burn when ignited provided it is mixed with certain proportions of air, or more accurately with the oxygen in air. But if there is too little or too much vapour compared to the air, so that the vapour-and-air mixture is either too lean or too rich, it will not burn. The limiting proportions, expressed as a percentage by volume of flammable vapour in air, are known as the lower flammable limit (LFL) and the upper flammable limit (UFL), and the zone, in between is the flammable range (see Definitions for further details).
  • In addition, a flammable liquid must itself be at or above a temperature high enough for it to give off sufficient vapour for ignition to occur. This temperature is known as the flash point. Some cargoes evolve flammable vapour at ambient temperatures, others only at higher temperatures or when heated. Safe handling procedures depend upon the flammability characteristics of each product. Non-combustible cargoes are those which do not evolve flammable vapours
  • Volatile and Non Volatile Cargoes.
  • If a cargo is being handled at a temperature within 10C of its flashpoint, it should be considered volatile.
  • Therefore a cargo with a flashpoint of 80C should be considered volatile if handled at a temperature of 70C or above.

• HEALTH HAZARDS:
  • Toxic means the same as poisonous. Toxicity is the ability of a substance, when inhaled, ingested, or absorbed by the skin, to cause damage to living tissue, impairment of the central nervous system, severe illness or, in extreme cases, death. The amounts of exposure required to produce these results vary widely with the nature of the substance and the duration of exposure to it.
  • Acute poisoning occurs when a large dose is received by exposure to high concentrations of a short duration, i.e. a single brief exposure. Chronic poisoning occurs through exposure to low concentrations over a long period of time, i.e. repeated or prolonged exposures. Prevention of exposure is achieved through a combination of cargo containment, which prevents toxic fumes or liquid from contaminating the workplace, and the use of personal protective equipment (PPE)
  • Suffocation: suffocation is unconsciousness caused by lack of oxygen, Any vapour may cause suffocation, whether toxic or not, simply by excluding oxygen in air. Danger areas include cargo tanks, void spaces and cargo pumprooms. But the atmosphere of a compartment may also be oxygen-deficient through natural causes, such as decomposition or putrefaction of organic cargo
  • Anaesthesia: Certain vapours cause loss of consciousness due to their effect on the nervous system. In addition, anaesthetic vapours may or may not be toxic.
  • Additional health hazards: Additional health hazards may be presented by non-cargo materials used on board during cargo handling. One hazard is that of frostbite from liquid nitrogen stored on board for use as atmosphere control in cargo tanks. Full advice on dealing with frostbite is contained in the MFAG. Another hazard is that of burns from accidental contact with equipment used while handling heated cargoes.

• REACTIVITY
  • Self Reaction:
    • The most common form of self-reaction is polymerisation. Polymerisation generally results in the conversion of gases or liquids into viscous liquids or solids. It may be a slow, natural process which only degrades the product without posing any safety hazards to the ship or the crew, or it may be a rapid, exothermic reaction evolving large amounts of heat and gases. Heat produced by the process can accelerate it. Such a reaction is called a run-off polymerisation that poses a serious danger to both the ship and its personnel. Products that are susceptible to polymerisation are normally transported with added inhibitors to prevent the onset of the reaction.
    • An inhibited cargo certificate should be provided to the ship before a cargo is carried. The action to be taken in case of a polymerisation situation occurring while the cargo is on board should be covered by the ship’s emergency contingency plan.
  • Reaction with water: Certain cargoes react with water in a way that could pose a danger to both the ship and its personnel. Toxic gases may be evolved. The most noticeable examples are the isocyanates; such cargoes are carried under dry and inert condition. Other cargoes react with water in a slow way that poses no safety hazard, but the reaction may produce small amounts of chemicals that can damage equipment or tank materials, or can cause oxygen depletion.
  • Reaction with air: Certain chemical cargoes, mostly ethers, may react with oxygen in air or in the chemical to form unstable oxygen compounds (peroxides) which, if allowed to build up, could cause an explosion. Such cargoes can be either inhibited by an anti-oxidant or carried under inert conditions.
  • Reaction with other cargoes: Some cargoes react dangerously with one another. Such cargoes should be stowed away from each other (not in adjacent tanks) and prevented from mixing by using separate loading, discharging and venting systems. When planning the cargo stowage, the master must use a recognised compatibility guide to ensure that cargoes stowed adjacent to each other are compatible.
  • Reaction with other materials: The materials used in construction of the cargo systems must be compatible with the cargo to be carried, and care must be taken to ensure that no incompatible materials are used or introduced during maintenance (e.g. by the material used for replacing gaskets). Some materials may trigger a self-reaction within the product. In other cases, reaction with certain alloys will be non-hazardous to ship or crew, but can impair the commercial quality of the cargo or render it unusable.

• CORROSIVENESS:
  • Acids, anhydrides and alkalis are among the most commonly carried corrosive substances. They can rapidly destroy human tissue and cause irreparable damage. They can also corrode normal ship construction materials, and create a safety hazard for a ship. Acids in particular react with most metals, evolving hydrogen gas which is highly flammable. The IMO Codes address this, and care should be taken to ensure that unsuitable materials are not included in the cargo system. Personnel likely to be exposed to these products should wear suitable personal protective equipment.

• PUTREFACTION:
  • Most animal and vegetable oils undergo decomposition over time, a natural process known as putrefaction (going off), that generates obnoxious and toxic vapours and depletes the oxygen in the tank. Tanks that have contained such products must be carefully ventilated and the atmosphere tested prior to tank entry.
  • It must not be assumed that all vapours produced by cargoes liable to putrefaction will in fact be due to putrefaction; some may not be obvious, either through smell or appearance of the cargo. Carbon monoxide (CO), for instance, is colourless and odourless and can be produced when a vegetable or animal oil is overheated.
  • Vapour given off by a flammable liquid will burn when ignited provided it is mixed with certain proportions of air, or more accurately with the oxygen in air. But if there is too little or too much vapour compared to the air, so that the vapour-and-air mixture is either too lean or too rich, it will not burn. The limiting proportions, expressed as a percentage by volume of flammable vapour in air, are known as the lower flammable limit (LFL) and the upper flammable limit (UFL), and the zone, in between is the flammable range (see Definitions for further details).
  • In addition, a flammable liquid must itself be at or above a temperature high enough for it to give off sufficient vapour for ignition to occur. This temperature is known as the flash point. Some cargoes evolve flammable vapour at ambient temperatures, others only at higher temperatures or when heated. Safe handling procedures depend upon the flammability characteristics of each product. Non-combustible cargoes are those which do not evolve flammable vapours
  • Volatile and Non Volatile Cargoes.
  • If a cargo is being handled at a temperature within 10C of its flashpoint, it should be considered volatile.
  • Therefore a cargo with a flashpoint of 80C should be considered volatile if handled at a temperature of 70C or above.

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Contents of Procedure and Arrangements (P & A) Manual as required under Annex II of Marpol 73/78:

MARPOL Annex II requires that each ship which is certified for the carriage of Noxious Liquid Substances in bulk shall be provided with a Procedures and Arrangements Manual. Scope of this plan is to provide the arrangements and equipment required to enable compliance with MARPOL Annex II. Plan is developed in line with IMO Legislation. Approval by the Administration or a Recognised Organisation (RO) on behalf of the Administration is mandatory.

Indicative Contents:

• Main Features of Marpol 73/78, Annex II
• Description of The Ship’s Equipment And Arrangements
• Cargo Unloading Procedures And Tank Stripping
• Procedures Relating To The Cleaning of Cargo Tanks, The Discharge of Residues, Ballasting And Deballasting
• Flow Diagrams & Drawings
• Heating requirement of cargo
• Control of heating system
• Method of temperature measurement
• Stripping requirement for the ship
• Cleaning & disposal procedure
• Prewash procedure
• Prewash for solidifying substances
• Minimum quantity of water to be used
• Required duration of prewash
• Ventilation procedure

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Table showing Control of Discharge of Category X, Y & Z NLS as per Marpol Annex II:

Marpol Annex II – Discharge Criteria:-

Category	BCH Ships Constructed before 31/7/1986	Existing IBC Constructed from 31/7/1986 but before 1/1/2007	New Buildings Constructed from 1/1/2007	Ships Other than Chemical Tankers Constructed before 1/1/2007
X	Pre-Wash Strip to 350 Litres 12 mile 25m water depth 7 knots, en-route	Pre-Wash Strip to 150 Litres 12 mile 25m water depth 7 knots en-route	Pre-Wash Strip to 75 Litres 12 mile 25m water depth 7 knots, en-route	Carriage Prohibited
Y	Pre-Wash for Solidifying for high viscosity substances Strip to 350 Litres 12 mile 25m water depth 7 knots, en-route	Pre-Wash for solidifying for high viscosity substances Strip to 150 Litres 12 mile 25m water depth 7 knots, en-route	Pre-Wash for solidifying for high viscosity substances Strip to 75 Litres 12 mile 25m water depth 7 knots, en-route	Carriage Prohibited
Z	Strip to 950 Litres 12 mile 25m water depth 7 knots, en-route	Strip to 350 Litres 12 mile 25m water depth 7 knots, en-route	Strip to 75 Litres 12 mile 25m water depth 7 knots, en-route	Strip to Maximum Extent 12 mile 25m water depth 7 knots, en-route
OS	No carriage Requirements	No Carriage Requirements	No Carriage Requirements
Underwater Discharge Required	Only X and Y cargoes	Only X and Y cargoes	X, Y and Z cargoes

WRT to above table, discharge of residues of category X for ship constructed before 31 July 1986.

• Stripping to 350 Ltrs
• Pre-Washed
• The resulting residues to be discharged to the reception facility.
• The concentration of substance in effluent must be at or less than 0.1% by weight.
• Ship proceeding enroute with speed of atleast 7 kts.
• More than 12 NM from nearest land.
• Depth of water not less than 25 mtrs.
• Discharge is made below the water line.
• P & A Manual shall be referred.

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Contents of SMPEP manual and who approves it:

SMPEP (Shipboard Marine Pollution Emergency Plan):

Background

MARPOL Annex II Regulation 17 requires every chemical tanker of 150 GT and above to carry a SMPEP. Scope of this plan is to provide guidance on the actions to be taken if a spill of oil or noxious liquid substance has occurred or is likely to occur. The plan is in line with IMO MEPC. 54(32), MEPC. 86(44) and the SMPEP guidelines in Resolution MEPC. 85(44). Plan Approval by the Administration or a Recognised Organisation (RO) on behalf of the Administration is mandatory.

Indicative Contents

• Reporting Requirements
• Steps To Control Discharge
• National And Local Co-Ordination
• Additional Information
• List Of Coastal State Contacts
• List Of Ship Interest Contacts
• Summary of Flow Chart And Checklists
• IΜΟ Resolution A.851(20)
• Vessel Specific Information

Info/Plans Required

• Ship Specific Information (Questionnaire to be submitted)
• General Arrangement Plan
• Capacity Plan
• Midship Section
• Lines Plan
• Tank Tables
• Benefits
• Master will have guidance with respect to the steps to be taken when a pollution incident has occurred or is likely to occur.
• All latest legislation and contact information will be included
• Easy reporting procedure for initial and follow up report (with examples)
• Quick and simple response with actions guide and responsible personnel
• Media handling guidance

We will ensure

• Full compliance with national and international regulations and common marine practice
• Real life documentation addressed to senior officers and crew onboard
• Full integration of any client specific requirements
• Full support provided after development in line with our Document Support Policy

Sketch and Explanation on Inert Gas of an Oil Tanker:

Inert gas system is the most important integrated system for oil tankers for safe operation of the ship.

Inert gas is the gas which contains insufficient oxygen (normally less then 8 %) to suppress combustion of flammable hydrocarbon gases.

Inert gas system spreads the inert gas over the oil cargo hydrocarbon mixture which increases the lower explosion limit LEL (lower concentration at which the vapors can be ignited), simultaneously decreasing the Higher explosion limit HEL (Higher concentration at which vapor explodes). When the concentration reaches around 10 %, an atmosphere is created inside tank in which hydrocarbon vapors cannot burn. The concentration of inert gas is kept around 5% as a safety limit.

Components and description of IG system:

The following components are used in a typical inert gas system in oil tankers:

1. Exhaust gases source: inert gas source is taken from exhaust uptakes of boiler or main engine as contains flue gases in it.
2. Inert gas isolating valve: It serve as the supply valve from uptake to the rest of the system isolating both the systems when not in use.
3. Scrubbing tower: Flue gas enters the scrub tower from bottom and passes through a series of water spray and baffle plates to cool, clean and moist the gases. The SO2 level decreases up to 90% and gas becomes clear of soot.
4. Demister: Normally made of polypropylene, it is used to absorb moisture and water from the treated flue gas.
5. Gas Blower: Normally two types of fan blowers are used, a steam driven turbine blower for I.G operation and an electrically driven blower for topping up purpose.
6. I.G pressure regulating valve: The pressure within the tanks varies with the property of oil and atmospheric condition. To control this variation and to avoid overheating of blower fan, a pressure regulator valve is attached after blower discharge which re-circulates the excess gas back to scrubbing tower.
7. Deck seal: Purpose of the deck seal is to stop the gases to return back which are coming from the blower to cargo tanks. Normally wet type deck seals are used. A demister is fitted to absorb the moisture carried away by the gases.
8. Mechanical non return valve: It is an additional non return mechanical device inline with deck seal.
9. Deck isolating valve: The engine room system can be isolated fully with the deck system with the help of this valve.
10. Pressure Vacuum (PV) breaker: The PV breaker helps in controlling the over or under pressurization of cargo tanks. The PV breaker vent is fitted with flame trap to avoid fire to ignite when loading or discharging operation is going on when in port.
11.  Cargo tank isolating valves: A vessel has numbers of cargo holds and each hold is provided with an isolating valve. The valve controls the flow of inert gas to hold and is operated only by a responsible officer in the vessel.
12.  Mast riser: Mast riser is used to maintain a positive pressure of inert gas at the time of loading of cargo and during the loading time it is kept open to avoid pressurization of cargo tank.

Safety and alarm system: The Inert gas plant is provided with various safety features to safeguard the tank and its own machinery.

Following are various alarms (with Shutdown) incorporated in the Inert Gas plant on board ship:

• High Level in scrubber leads to alarm and shutdown of blower and scrubber tower.
• Low pressure sea water supply (approx. 0.7 bar) to scrubber tower leads to alarm and shutdown of blower.
• Low pressure sea water supply (approx. 1.5 bar) to deck seal leads to alarm and shutdown of blower.
• High inert gas temperature (approx. 70 deg C) leads to alarm and shutdown of blower.
• Low pressure in line after blower (approx. 250mm wg) leads to alarm and shutdown of blower.
• Oxygen content high (8%) leads to alarm and shutdown of gas delivery to deck.
• Low level in deck seal leads to alarm and shutdown of gas delivery to deck.
• Power failure leads to alarm and shutdown of blower and scrubber tower.
• Emergency stop leads to alarm and shutdown of blower and scrubber tower.

Following are various alarms incorporated in the Inert Gas plant:

• Scrubber low level
• Deck seal High level
• Low O2 Content (1%)
• High O2 Content (5%)
• Low lube oil pressure alarm

Working of Inert Gas Plant:

The basis of inert gas production in the IG plant is the flue gas generated from the ship’s boiler. The high temperature gas mixture from the boiler uptake is treated in an inert gas plant which cleans, cools and supplies the inert gas to the individual tanks via PV valves and breakers to ensure safety of tank structure and atmosphere.

The system can be divided in to two basic groups:

a) A production plant to produce inert gas and deliver it under pressure, by means of blower(s), to the cargo tanks.

b) A distribution system to control the passage of inert gas into the appropriate cargo tanks at the required time.

Brief working procedure:

1. Boiler uptake gases are drawn to the scrubber unit via flue gas isolating valve(s) to the scrubber unit.
2. In the scrubber unit the gas is cooled, cleaned and dried before being supplied in to the tanks.
3. Motor driven inert gas blowers supplies the treated gas from scrubber tower to the tanks through. They are mounted on rubber vibration absorbers and isolated from the piping by rubber expansion bellows.
4. Regulation of gas quantity delivered to deck is taken care of by the gas control valves and the deck pressure is managed by pressure controller. If the deck pressure is lower than the set point the output signal will be raised to open the valve more, and vice versa if the deck pressure is lower than the set-point. These valves will then work in cooperation to keep both the deck pressure / blower pressure at their respective set point without starving or overfeeding the circuit.
5. Before entering the deck line, the gas passes through the deck water seal which also acts as non-return valve automatically preventing the back-flow of explosive gases from the cargo tanks.
6. After the deck seal the inert gas relief is mounted to balance built-up deck water seal pressure when the system is shut down. In case of a failure of both the deck seal and the non-return valve, the relief valve will vent the gases flowing from the cargo tank into the atmosphere.
7. The oxygen analyser which is fitted after the blower separates the “production” and “distribution” components of the plant and analyzes the oxygen content of the gas and if it is more than 8%, it alarms and shut downs the plant.

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Features of flammability diagram with respect of following: Purging:

• When it is required to gas free a tank after washing, it should first be purged with inert gas to reduce the hydrocarbon content to 2% or less by volume so that during the subsequent gas freeing no portion of the tank atmosphere is brought within the flammable range.
• The tank may then be gas freed.
• The hydrocarbon content must be measured with an appropriate meter designed to measure the percentage of hydrocarbon gas in an oxygen deficient atmosphere.
• The usual flammable gas indicator is not suitable for this purpose.
• If the dilution method of purging is used, it should be carried out with the inert gas system set for maximum capacity to give maximum turbulence within the tank.
• If the displacement method is used, the gas inlet velocity should be lower to prevent undue turbulence.

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Features of flammability diagram with respect of following: Inerting

• Before the inert gas system is put into service the tests required by the operations manual or manufacturer’s instructions should be carried out. The fixed oxygen analyser and recorder should be tested and proved in good order. Portable oxygen and hydrocarbon meters should also be prepared and tested.
• When inerting empty tanks which are gas free, for example following a dry-docking or tank entry, inert gas should be introduced through the distribution system while venting the air in the tank to the atmosphere.
• This operation should continue until the oxygen content throughout the tank is not more than 8% by volume.
• The oxygen level will not thereafter increase if a positive pressure is maintained by using the inert gas system to introduce additional inert gas when necessary.
• If the tank is not gas free, the precautions against static electricity given in Section 10.6.7 of ISGOTT should be taken.
• When all tanks have been inerted they should be kept common with the inert gas main and the system pressurised with a minimum positive pressure of at least 100mm water gauge.

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Features of flammability diagram with respect of following: Gas Freeing:

• In a gas freeing operation air is delivered into the tank, where it mixes with the existing tank atmosphere and also tends to mix together any layers that may be present.
• The resultant mixture is expelled to the outside atmosphere. Because the process is one of continuous dilution with the air, the highest hydrocarbon concentration is vented at the beginning of gas freeing and decreases thereafter.
• For example, on a non-inerted ship, gas freeing of a motor gasoline tank that has been battened down can give initial concentrations as high as 40% by volume, but in most circumstances the concentration in the vented gas is much lower, even at the start of the operations.
• On inerted ships, where purging to remove hydrocarbon vapour before gas freeing is a requirement, even the initial concentration will be low, 2% by volume or less.

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Functions and maintenances of cargo related equipments on oil tankers using sketches and diagrams: Pressure Vacuum Valve (P/V Valve):

Working:-

• The main components of this system are fixed washing machines which are fed from a 200 mm main line on upper deck.
• The washing fluid is supplied by any of the cargo oil pumps via a riser line from the pump discharge cross-over line.
• In the pump room, there is a steam heated washing water heater which is connected to the riser line in a by-pass arrangement with the water side isolated by means of stop valves and spectacle flanges.
• These flanges may be in open position only when required for hot water washing. The rated heating capacity is 280 m³h of sea water from 20^OC to 80^OC.
• During all washing, the pressure in the main line should be maintained at minimum 8.5 bar at the aft end in order to ensure satisfactory operating conditions also for the forward-most washing machines.
• A pressure less than 7.5 would considerably reduce the effectiveness of the washing operations. For pressure monitoring, there is fixed pressure gauge at the aft end of the main line as well as a boss for a portable pressure gauge at the forward end of the main line.

In order to ensure the correct functioning of PV valves the following should always be complied with:

• PV valves should be serviced and calibrated according to classification society requirements;
• Prior to loading and discharging, PV valves should be checked to ensure they function as designed;
• During cargo operations the correct functioning of PV valves should be monitored; and
• Pressure sensors fitted as the secondary system as a back up to the primary vent system should be checked to ensure that they function as designed and, where provided, that the alarms are correctly set.

Setting PV alarms:- High pressure alarms and low pressure alarms must be set to:

• Activate additional safety or other alarm systems;
• Support maintenance of correct positive inert gas pressure in tanks;
• Prevent air intake to tanks; and
• Comply with regulations.

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Working of a Pressure Vacuum Breaker (PV Breaker):

Pressure Vacuum Breaker or usually known as PV Breaker is a safety measure used in the IG line on deck.

The major functions of a PV Breaker are:-

• Abnormal rise of Pressure in Cargo tanks when loaded specified rate of gas outlets.
• Abnormal rise of Pressure in Cargo tanks when cargo is unloaded beyond specified rate of the inert gas blower.
• Abnormal rise or drop of pressure in cargo tanks when the breather valve does not operate properly for the fluctuation of the pressure in cargo tanks due to variation in atmospheric and sea water temperatures.

Operation:-

• When Pressure Rises: – When the pressure in the cargo oil tanks rise, the seal liquid rises in the inner pipe. At this time , if the pressure beyond the specific capacity of the breaker, the seal liquid will push out of the pipe to let the pressure inside the be out.

• When Pressure drops: – When the pressure in the cargo oil tanks fall, the seal liquid rises in the outer pipe. If the pressure beyond the specific capacity of the breaker, the seal liquid is drown into the cargo oil tanks, and atmospheric air will be inhaled in the tank.

How to ensure that P/V Breaker is protecting the cargo tanks effectively:

• Every inert gas system is required to be fitted with one or more pressure/vacuum breakers or other approved devices. These are designed to protect the cargo tanks against excessive pressure or vacuum and must therefore be kept in perfect working order by regular maintenance in accordance with the manufacturer’s instructions.
• When these are liquid filled it is important to ensure that the correct fluid is used and the correct level maintained for the density of the liquid used. The level can normally only be checked when there is no pressure in the inert gas deck main. Evaporation, condensation and possible ingress or sea water must be taken into consideration when checking the liquid condition, density and level.
• In heavy weather, the pressure surge caused by the motion of the liquid in the cargo tanks may cause the liquid in the pressure/vacuum breaker to be blown out. When cold weather conditions are expected, liquid filled breakers must be checked to ensure that the liquid is suitable for low temperature use, and if necessary anti-freeze is to be added.
• The P/V breaker(s) are to be clearly marked with their high pressure and vacuum opening pressures and also with the type and volumetric concentration of antifreeze (if water filled type), and minimum operating temperature.

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Pressure Vacuum Valve or PV Valve:

• Moderate pressures of 0.24 bar acting on large surfaces in liquid cargo tanks are sufficient to cause damage and rupture.
• The pressure on each unit of area multiplied by the total area gives a large loading on the underside of the top of a tank or other surface, which may then buckle or the metal plate may be torn.
• Similarly, pressure drop within a tank can cause damage due to greater atmospheric pressure on the outside.

• Pressure vacuum valve or pv valve in the ventilation system will prevent either over or under pressure. They are set usually so that tank pressure of about 0.14 bar will lift the main valve (The smaller valve will lift along with it) and release excess pressure. The vapour passes to atmosphere through a gauze flame trap. Drop in tank pressure compared with that of the outside atmosphere will make the small valve open downwards to equalize internal pressure with that outside.
• Pressure vacuum valve or pv valve can relieve moderate changes in tank pressure due to variations in temperature and vapour quantity. A drop towards vacuum conditions as the result of the condensation of steam will also be handled by the valve. Rapid pressure rise due to an explosion would not be relieved.
• The fast rate at which a tank is filled while loading produces a very rapid expulsion of the previous contents (vapour and inert gas). The pressure vacuum valve is not designed as a filling vent and neither should the tank hatch be left open. The latter method of venting can cause an accumulation of flammable vapours at deck level. Tanks should be vented while filling, through mast head vents or through special high velocity vents.

Use & Limitations of Draeger tubes on Oil Tankers:

Multigas Detector or Draeger Multiple detector:

• This is used to detect the presence of a variety of toxic gases inside the compartment.
• They work on the principle of chemical absorption of the gas to be detected by a re-agent which gets discoloured.
• A sample of the atmosphere is drawn into a tube containing crystals of the reagent.
• The tube is graduated and the level of discolouration indicates the concentration of the vapour in the sample.
• The amount of air drawn through the tube must be exactly the same each time, to ensure this the bellows must be fully compressed and allowed to expand to the full limit of the limiter chain.
• The tubes have a shelf life of two years.
• Both ends of the tube are broken before use and one end is fitted into the pump head.
• Different tubes are used for detection of different gases.

Use & Limitations of Explosimeter on Oil Tankers:

It is used for the detection and measurement of combustible gases and vapour. It depends for its operation on the heat developed by the actual combustion of the flammable portion of the sample. The sample is drawn over a heated filament which forms one arm of a balanced Wheatstone’s bridge circuit.

        The current for the circuit is provided by six standard dry cells. Combustible gas in the sample is burnt on the filament. Thus its temperature is raised and its resistance increases in proportion to the amount of combustible gas burnt i.e. in proportion to the amount of combustible gas in the sample. The circuit is now unbalanced which causes a deflection of the meter. The scale is graduated in percentage of the lower explosive limit. The scale is graduated in percentage of the lower explosive limit. The initial balance of the circuit is achieved in fresh air with the meter at zero by adjustment of a rheostat R, in the figure.

The Limitations of the Explosimeter are:-

• As the explosimeter only indicates the presence of flammable gases and vapours it may be dangerous to enter the compartment as no indication of toxicity is given or sufficiency of oxygen.
• A compartment which is initially safe may be rendered unsafe by future operations e.g. stirring or handling bottom sludge in a crude oil tank. Hence, frequent tests are required while the work in progress.
• If a compartment having a high boiling point liquid is heated by welding or other processes the vapour concentration will increase and such an atmosphere which originally showed a low concentration vapour may now be rendered explosive.
• When testing at a high temperature some of the vapour may condense in the sampling tube of the instrument, so only a small concentrate of vapour will be indicated by the instrument.
• As the instrument depends on combustion of the flammable portion of the sample it cannot detect in a steam or inert atmosphere due to the absence of O₂. In the case of inerted tanks of vessels carrying crude or refined petroleum products an instrument called a tankscope has been specially designed to detect and measure the concentration of hydro carbon vapour in the absence of oxygen.

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Use & Limitations of Tank Scope on Oil Tankers:

Diagram same as Explosimeter

• This is used to detect the presence of a variety of toxic gases inside the compartment.
• They work on the principle of chemical absorption of the gas to be detected by a re-agent which gets discoloured.
• A sample of the atmosphere is drawn into a tube containing crystals of the reagent.
• The tube is graduated and the level of discolouration indicates the concentration of the vapour in the sample.
• The amount of air drawn through the tube must be exactly the same each time to ensure this the bellows must be fully compressed and allowed to expand to the full limit of the limiter chain.
• The tubes have a shelf life of two years. Both ends of the tube are broken before use and one end is fitted into the pump head.
• Different tubes are used for detection of different gases.

Use & Limitations of Oxygen Analyzer on Oil Tankers:

The instrument is used to check the O₂ content of the atmosphere within a tank or other confined space. Samples of the atmosphere are drawn by means of a rubber aspirator bulb and passed over a sensor.

The sensor is the most important part of the instrument and can be of various types:

• Paragmagnetic Sensor:-
• The magnetic properties of oxygen is used to deflect a light, metal body suspended in a magnetic field. When the gas is drawn through the cell, the suspended body experiences a force proportional to the magnetism of the gas. An equal and opposing force is produced by an electric current passing through a coil would round the suspended body. This equalizing current is proportional to the magnetic force of the gas which depends on its O₂ content.
• Electrolytic Sensor:-
• In this type of oxygen is passed into an electrolytic cell causing a current to flow between two electrodes separated in a liquid electrolyte. The current flow between the electrodes is directly proportional to the O₂ concentration in the sample. In this type, certain gases may affect the sensor or poison the electrolyte giving rise to false readings.
• Chemical absorption liquid:- in this type a known volume of the sample gas is brought into contact with a measured volume of a liquid which absorbs O₂ causing a change in its volume. The change in volume is a measure of the O₂ content of the sample.
• Limitations:-
• Can only measure O₂ content.
• Regular calculation prior every use.

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Properties of Oxygen Analyzer:

These analyzers come in various makes and models and we will be studying about one such analyzer namely the continuous reading type analyzer.

The main property of oxygen which helps in its detection and measurement of its percentage in the given sample of air is that of Para-magnetism. Basically this means that oxygen gets attracted towards a magnetic field. The set up for measuring oxygen content using this property can be understood from the image shown below.

Hazards of Petroleum with Reference to:-

Toxicity:

ISGOTT 1.2 – Toxicity:- Toxicity is the degree to which a substance or mixture of substances can harm humans or animals.

Toxic substances can affect humans in four main ways: by being swallowed (ingestion); through skin contact; through the lungs (inhalation) and through the eyes.

Ingestion:

Petroleum has low oral toxicity, but when swallowed it causes acute discomfort and nausea. There is then a possibility that liquid petroleum may be drawn into the lungs during vomiting and this can have serious consequences, especially with higher volatility products, such as gasolines and kerosenes.

Skin Contact:

Many petroleum products, especially the more volatile ones, cause skin irritation and remove essential oils from the skin, leading to dermatitis. They are also irritating to the eyes. Certain heavier oils can cause serious skin disorders on repeated and prolonged contact.

Direct contact with petroleum should always be avoided by wearing the appropriate protective equipment, especially impermeable gloves and goggles.

Petroleum Gases:

Comparatively small quantities of petroleum gas, when inhaled, can cause symptoms of diminished responsibility and dizziness similar to drunkenness, with headache and irritation of the eyes. The inhalation of a sufficient quantity can be fatal.

These symptoms can occur at concentrations well below the Lower Flammable Limit.

However, petroleum gases vary in their physiological effects and human tolerance to these effects also varies widely. It should not be assumed that because conditions can be tolerated the gas concentration is within safe limits.

The smell of petroleum gas mixtures is very variable and in some cases the gases may dull the sense of smell. The impairment of smell is especially likely, and particularly serious, if the mixture contains hydrogen sulphide.

The absence of smell should never be taken to indicate the absence of gas.

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Primary & Secondary means of Venting on Oil Tankers:

Primary means of Venting : Vessels utilising a common gas / vapour system as the “primary means of venting” which is isolated from a cargo tank by a valve, or other means, which is shut due to the normal operation of the vessel (such as in the case of a vessel carrying parcel cargo with non-compatible vapours) are not in compliance with the requirements of SOLAS Reg. II-2/4.5.3 unless they have a second independent means of venting which cannot be isolated from the cargo tank.

Secondary means of Venting: (Reg. II-2/4.5.3.2.2) Where the arrangements are combined with other cargo tanks, either stop valves or other acceptable means shall be provided to isolate each cargo tank. Where stop valves are fitted, they shall be provided with locking arrangements which shall be under the control of the responsible ship’s officer. There shall be a clear visual indication of the operational status of the valves or other acceptable means. Where tanks have been isolated, it shall be ensured that relevant isolating valves are opened before cargo loading or ballasting or discharging of those tanks is commenced. Any isolation must continue to permit the flow caused by thermal variations in a cargo tank in accordance with regulation 11.6.1.1.

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Differentiate between PV valve & PV Breaker:

PV VALVE	P/V BREAKER
Connected On Tank Top Of individual Cargo Tank	Connected To Main Inert Gas Line
1 or 2 per tank	1 on main IG line
Mechanical Type	Gravity Type ( Liquid)
PRESSURE :+1400 mmaq	PRESSURE :+2100 mmaq
VACCUM : -350mmaq	VACCUM : -700 mmaq
Primary Means Of Protection	Secondary Means Of Pretection
Requires Regular Maintenance	Less Maintenance
Fixed Set-Point	SET POINT CANBE INCREASED OR DECREASED
Automatic Self-Closing Checklifts	Automatic Self-Closing Checklifts NOT AVAILABLE
Check Lift AVAILABLE TO CHECK THE OPERATIONAL CONDITION	WATER LEVEL GAUGE AVAILABLE TO CHECK THE PRESSURE& VACCUM SETTING
System Failure May Occur	100% Reliable
No extra precaution in cold climate	Antifreeze  required in cold climate

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Precautions to be taken on an Oil Tanker during Loading, Discharging and Tank Cleaning against Static Electricity Hazard:

• The following are essential only when loading static accumulator oils (conductivity < 50 pS/m): Restrict initial loading rates, when splashing and surface turbulence occur, to flow rates less than 1 meter/second (volume flow rate conversions available).
• Adequate inlet coverage’s are: side or horizontal entrance- 0.6 meter; downward pointing inlet- twice the inlet diameter.
• ISGOTT %B7 Loading rate conversions appear both in ISGOTT and Texaco. %B7 Restrict initial unloading rates to shore installations also, as long as inlets in the shore tank are not covered with liquid. The inlet fill pipe should discharge near the bottom of the tank. NFPA 77 %B7 Keep water and other impurities out of the incoming cargo stream as much as possible.
• Extra care with loading and unloading rates when presence of impurities (e.g., water, sulfur, metals) is suspected is essential. ISGOTT, NFPA 77 %B7 Avoid pumping entrained gases with cargo.
• NFPA 77 %B7 Degassing (to <20% of LFL at tank bottom) or inerting a ship’s tank eliminates loading rate restrictions due to static electricity. Texaco %B7 Reduced pumping speeds are used for discharge of slops and other “mixed-phase flow” (some ballast) to shore tanks.
• Prevention of charge accumulation – recommended by ISGOTT /NFPA 77.

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The following safety precautions have been developed to prevent the accumulation of static charge:

• Antistatic additives:- These additives raise the conductivity of a static accumulator; one specification calls for a minimum of 100 pS/m.
• ISGOTT Treatment is required for these fuels in Canada:- The Canadian General Standards Board specifies minimum conductivity of 50 pS/m for static accumulating fuels, especially aviation fuels .
• API 2003 recommends that these additives be introduced at the beginning of the “distribution train”, and notes that their positive effect may be reduced by repeated shipments or passage through clay filters. Safety precautions for the handling of static accumulating oils have historically been waived for those treated with antistatic additives.
• These precautions have, however, recently been extended to residual oils and oils treated with anti-static additive to raise conductivity above 50 pS/m (May 1991 amendment to ISGOTT).
• Space is too small to give full details. You can get ample literature / case histories of such accidents from published journals and published symposium. Basically, this problem can be a great extent mitigarted if:
• A firm earthing connection exists between tank top to bottom on all sides (four quandrantss), measurement of earthing measuremens once in 6 months to meet both Indian Electricity regulations, as well Indian Petrolem and over all for API construction code requirements, firm earthing at all jump-over points(especially piping joints in and out of tanks).
• Allowing enough settling time betwenn tank loading and allow time for tank discharge (i.e, withdrawal of naphtha), good lightning arrestor at the top of the tanks and the continuity of the same will help you to avoid major catastrophy in naphtha storage tanks – inspie of unpreventable static electric discharge.
• As a precaution, do not load the tank too fast or take fuel discharge during severe lightning time.

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Dirty Ballast:

• This intermitted discharge is composed of the seawater taken into, and discharged from empty fuel tanks to maintain the stability of the vessel. The seawater is brought into these tanks for the purpose of improving the stability of a vessel during rough sea conditions.
• Prior to taking on the seawater as ballast, fuel in the tank to be ballasted is transferred to another fuel tank or holding tank to prevent contaminating the fuel with seawater.
• Some residual fuel remains in the tank and mixes with the seawater to form dirty ballast.
• Dirty ballast systems are configured differently from Compensated ballast and Clean ballast systems.
  • Compensated ballast systems continuously replace fuel with seawater in a system of tanks as the fuel is consumed.
  • Clean ballast systems have tanks that carry only ballast water and are never in contact with fuel.
• In a dirty ballast system, water is added to a fuel tank after most of the fuel is removed.
• Thirty Coast Guard vessels generate dirty ballast as a discharge incidental to normal vessel operations. These Coast Guard vessels do so because their size and design do not allow for a sufficient volume of clean ballast tanks.
• The larger of these vessels discharge the dirty ballast at distances beyond 12 n.m. from shore, while the smaller vessels discharge the dirty ballast between 3 and 12 n.m. from shore. Coast Guard vessels monitor the dirty ballast discharge with an oil content monitor. If the dirty ballast exceeds 15 parts per million (ppm) oil, it is treated in an oil-water separator prior to discharge.

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Cloud Point:

In the petroleum industry, cloud point refers to the temperature below which wax in diesel or bio-wax in bio-diesels form a cloudy appearance. The presence of solidified waxes thickens the oil and clogs fuel filters and injectors in engines. The wax also accumulates on cold surfaces (e.g. pipeline or heat exchanger fouling) and forms an emulsion with water. Therefore, cloud point indicates the tendency of the oil to plug filters or small orifices at cold operating temperatures.

In crude or heavy oils, cloud point is synonymous with wax appearance temperature (WAT) and wax precipitation temperature (WPT).

The cloud point of a nonionic surfactant or glycol solution is the temperature where the mixture starts to phase separate and two phases appear, thus becoming cloudy. This behavior is characteristic of non-ionic surfactants containing polyoxyethylene chains, which exhibit reverse solubility versus temperature behavior in water and therefore “cloud out” at some point as the temperature is raised. Glycols demonstrating this behavior are known as “cloud-point glycols” and are used as shale inhibitors (see Talk). The cloud point is affected by salinity, being generally lower in more saline fluids.

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Spiked Crude Oil:

• “Spiked crude oil” (also called “enriched” or “tailored” crude) is crude oil, which has had hydrocarbons, added in gas or liquid form.
• The spiked crude may contain rather large amounts of added hydrocarbons and therefore emit heavy gasses under certain conditions (during loading, crude oil washing, discharging).

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Sour Crude:

• Sour crude oil is crude oil containing a high amount of the impurity sulfur. It is common to find crude oil containing some impurities. When the total sulfur level in the oil is more than 0.5% the oil is called “sour”.
• The impurities need to be removed before this lower-quality crude can be refined into petrol, thereby increasing the cost of processing. This results in a higher-priced gasoline than that made from sweet crude oil.
• Current environmental regulations in the United States strictly limit the sulfur content in refined fuels such as diesel and gasoline.
• The majority of the sulfur in crude oil occurs bonded to carbon atoms, with a small amount occurring as elemental sulfur in solution and as hydrogen sulfide gas. Sour oil can be toxic and corrosive, especially when the oil contains higher levels of hydrogen sulfide, which is a breathing hazard. At low concentrations the gas gives the oil the smell of rotting eggs. For safety reasons, sour crude oil needs to be stabilized by having hydrogen sulfide gas (H₂S) removed from it before being transported by oil tankers.

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Pour Point:

• The pour point is the lowest temperature at which a marine fuel oil can be handled without excessive amounts of wax crystals forming out of solution.
• At a lower temperature the fuel will gel, thereby preventing flow.

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Precautions you will observe while loading Crude Oil having very high Concentration of Hydrogen Sulphide:

Bunker fuels containing high H₂S concentrations may be supplied without advice being passed to the tanker beforehand. Tanker’s personnel should always be alert to the possible presence of H₂S in bunker fuel and be prepared to take suitable precautions if it is present.

• Before loading bunkers, the tanker should communicate with the supplier to ascertain whether the fuel to be loaded is likely to have any H₂S content.
• The design of bunker tank vents and their location makes managing the exposure to personnel more difficult, as closed loading and venting cannot usually be implemented.
• If bunkering with fuel containing H₂S above the TLV-TWA cannot be avoided, procedures should be in place to monitor and control the access of personnel to exposure areas.
• Ventilation to lower the concentration of vapour in the ullage space and in specific areas where vapours may accumulate should be carried out as soon as practicable.
• Even after the tank has been ventilated to reduce the concentration to an acceptable level, subsequent transfer, heating and agitation of the fuel within a tank may cause the concentration to reappear.
• Periodic monitoring of the concentration of H₂S should be continued until the bunker tank is refilled with a fuel oil not containing H₂S.

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Sketch Wet Type “Deck Seal” & the required water level is maintained:

• The seal is kept full using a continuously running seal water pump which may be backed up with a crossover from a secondary system as required.
• Should the pressure on the downstream side exceed the upstream side the water is pushed up the inlet pipe.
• The height of this pipe ensures that the head pressure generated is greater than either the pressure release valve or any water seals.

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Sketch High Velocity (HV) vent valve fitted in Cargo Oil Tanks:

• Tank vapours can be released and sent clear of the decks during loading through large, high velocity vent.
• The type shown above has a moving orifice, held down by a counter weight to seal around the bottom of a fixed cone.
• Pressure build up in the tank, as filling proceeds, causes the moving orifice to lift.
• The small gap between orifice lip and fixed cone gives high velocity to the emitted vapour.
• It is directed upwards with an estimated velocity of 30 meters per second.
• Air drawn in by the ejector effect dilutes the plume.

• The conical flame screen fixed to the moving orifice to give protection against flame travel will, like the moving parts, require periodic cleaning to remove gummy deposit.
• The cover is closed (as shown) when the vessel is on passage. A simpler design of a high velocity vent, having two weighted flaps which are pushed open by the pressure build up to achieve a similar nozzle effect.

Precautions to be taken on an Oil Tanker during loading/ discharging against Static Electric Hazard:

• Restrict initial loading rates, when splashing and surface turbulence occur, to flow rates less than 1 meter/second (volume flow rate conversions available). Adequate inlet coverage’s are: side or horizontal entrance- 0.6 meter; downward pointing inlet- twice the inlet diameter. ISGOTT
• Loading rate conversions appear both in ISGOTT and Texaco.
• Restrict initial unloading rates to shore installations also, as long as inlets in the shore tank are not covered with liquid. The inlet fill pipe should discharge near the bottom of the tank. NFPA 77
• Keep water and other impurities out of the incoming cargo stream as much as possible. Extra care with loading and unloading rates when presence of impurities (e.g., water, sulfur, metals) is suspected is essential. ISGOTT, NFPA 77
• Avoid pumping entrained gases with cargo. NFPA 77
• Degassing (to <20% of LFL at tank bottom) or inerting a ship’s tank eliminates loading raterestrictions due to static electricity.
• Reduced pumping speeds are used for discharge of slops and other “mixed-phase flow” (some ballast) to shore tanks.

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International Safety Guide for Tankers and Terminals – ISGOTT

• This Guide makes recommendations for tanker and terminal personnel on the safe carriage and handling of crude oil and petroleum products on tankers and at terminals.
• It was first published in 1978 by combining the contents of the ‘Tanker Safety Guide (Petroleum)’ published by the International Chamber of Shipping (ICS) and the ‘International Oil Tanker and Terminal Safety Guide’ published on behalf of the Oil Companies International Marine Forum (OCIMF).
• This latest edition takes account of recent changes in recommended operating procedures, particularly those prompted by the introduction of the International Safety Management (ISM) Code, which became mandatory for tankers on 1st July 1998.
• One of the purposes of the Guide is therefore to provide information that will assist companies in the development of a Safety Management System to meet the requirements of the ISM Code.
• This guide does not provide a definitive description of how tanker and terminal operations are conducted. It does provide guidance and examples of how certain aspects of tanker and terminal operations may be managed.
• Effective management of risk demands processes and controls that can quickly adapt to change. Therefore the guidance given is, in many cases, intentionally non prescriptive and alternative procedures may be adopted by some operators in the management of their operations.
• These alternative procedures may exceed the recommendations contained in this guide.
• Where an operator has adopted alternative procedures, they should follow a risk based management process that must incorporate systems for identifying and assessing the risks and for demonstrating how they are managed. For shipboard operations, this course of action must satisfy the requirements of the ISM Code.
• It should be borne in mind that, in all cases, the advice in the guide is subject to any local or national terminal regulations that may be applicable, and those concerned should ensure that they are aware of any such requirements.
• It is recommended that a copy of the guide be kept — and used — on board every tanker and in every terminal to provide advice on operational procedures and the shared responsibility for port operations.

For contents please refer to ISGOTT (Latest Edition)

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Slop Tanks:

Marpol Annex I- Regulations for the Prevention of Pollution by Oil

Chapter 4 – Requirements for the cargo area of oil tankers. Part A – Construction.

Regulation 29 – Slop tanks

1. Subject to the provisions of paragraph 4 of regulation 3 of this Annex, oil tankers of 150 gross tonnage and above shall be provided with slop tank arrangements in accordance with the requirements of paragraphs 2.1 to 2.3 of this regulation. In oil tankers delivered on or before 31 December 1979, as defined in regulation 1.28.1, any cargo tank may be as a slop tank.

2.1 Adequate means shall be provided for cleaning the cargo tanks and transferring the dirty ballast residue and tank washings from the cargo tanks into a slop tank approved by the Administration.

2.2 In this system arrangements shall be provided to transfer the oily waste into a slop tank or combination of slop tanks in such a way that any effluent discharged into the sea will be such as to comply with the provisions of regulation 34 of this Annex.

2.3 The arrangements of the slop tank or combination of slop tanks shall have a capacity necessary to retain the slop generated by tank washings, oil residues and dirty ballast residues. The total capacity of the slop tank or tanks shall not be less than 3 per cent of the oil-carrying capacity of the ship, except that the Administration may accept:

1} 2% for such oil tankers where the tank washing arrangements are such that once the slop tank or tanks are charged with washing water, this water is sufficient for tank washing and, where applicable, for providing the driving fluid for eductors, without the introduction of additional water into the system;

2} 2% where segregated ballast tanks or dedicated clean ballast tanks are provided in accordance with regulation 18 of this Annex, or where a cargo tank cleaning system using crude oil washing is fitted in accordance with regulation 33 of this Annex. This capacity may be further reduced to 1.5% for such oil tankers where the tank washing arrangements are such that once the slop tank or tanks are charged with washing water, this water is sufficient for tank washing and, where applicable, for providing the driving fluid for eductors, without the introduction of additional water into the system; and

3} 1% for combination carriers where oil cargo is only carried in tanks with smooth walls. This capacity may be further reduced to 0.8% where the tank washing arrangements are such that once the slop tank or tanks are charged with washing water, this water is sufficient for tank washing and, where applicable, for providing the driving fluid for eductors, without the introduction of additional water into the system.

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Tank Cleaning, Purging and Gas free Operation for tankers:

Responsibility: –

• The Chief Officer is in charge of and shall supervise as the person in charge of the Tank Cleaning, Hydrocarbon Gas (HC) Purging, Gas Freeing & Re-Inerting operations.
• He shall ensure that all activities carried out during such operations are in compliance with the latest edition ICS/OCIMF International Safety Guide for Oil Tankers and Terminals (ISGOTT).

Gas-Freeing for Cargo Tank entry:-

• Cargo Tank entry shall not be permitted unless the Oxygen Content is 21% and the hydrocarbon vapor content is less than 1% of the Lower Flammable Level (LFL).
• Follow company’s “Procedure for Entry into Enclosed Spaces” with related permits.
• If the previous cargo contains Hydrogen Sulfide (H2S) or other toxic contaminants which could evolve toxic gases (eg benzene, toluene, Mercaptans, etc), the tank should be checked for such gases. Refer to “Guidelines for Toxic Gases Hazards”.
• Carrying out “Hot Work” inside Tanks within the ‘Dangerous Area’ need special caution as per “Procedures for Hot Work” and carry out preparation accordingly.

Gas-Freeing or Purging for the Reception of Cargo:-

• If the intention of Gas-Freeing or Purging operations is to prevent the next cargo to be loaded from contamination due to the previous cargo oil hydrocarbon gas, use the gas content indicated by the Charterer as standard, but go on with the operations mentioned in (2) of Article 1 until the LFL decreases down to 40% or under.

Safety Precautions:-

• For the operations to be followed, (Tank cleaning, HC Gas Purging, Gas Freeing and Re-Inerting), the Chief Officer shall carry out the following precautions. Detailed guidance on preparations and safety precautions are also described within relevant sections of ISGOTT.
• Have persons engaged in the operations observe the necessary precautions as described in this section and the “Precautions during Gas-freeing Operations”. Complete the necessary sections of “Tank Cleaning, Purging and Gas Freeing Checklist” to confirm safety strictly at the appropriate time.
• Tank Preparation And Atmosphere Control During Operations.

Non Flammable Atmosphere:-

• On Tankers using the inert gas systems, the Chief Officer shall carry out the operations mentioned in Article 1 and should maintain the cargo tanks in a “Non Flammable” condition at all times.
• Refer to the “Flammability composition diagram- Hydrocarbon Gas/Inert/Air Gas Mixtures” from the ISGOTT. i.e. at no time should the atmosphere in the tank be allowed to enter the flammable range, as mentioned therein.
• Pyrophoric hazards on chemical reaction with Hydrogen Sulfide Gas Pyrophoric Iron Sulphide, forms when Hydrogen Sulfide Gas (normally present in most crude) reacts with rusted surfaces in the absence of oxygen (Inert conditions) inside cargo tanks.
• These substances, can heat to incandescence on contact with air. This risk is minimized, by following the correct purging procedure.
• Such procedures serve as a general guidance for the preparation procedures required and may differ as per ship type.

Atmosphere Control during Tank Cleaning Operations:-

• Tank atmospheres can be any of the following, However, ships fitted with an inert gas system, shall carry out the operations under the Inerted Condition, unless otherwise as instructed: It should be met with atmosphere containing less than 8% oxygen, and tank pressure of minimum 200 mmAq. Refer details to “ISGOTT”

Inerted Tanks:-

• An atmosphere made incapable of burning by the introduction of inert gas and the resultant reduction of the overall oxygen content. For the purposes of this procedure, the oxygen content of the tank atmosphere should not exceed 8% by volume.
• This is a condition where the tank atmosphere is known to be at it’s the lowest risk of explosion by virtue of its atmosphere being maintained at all times Non-Flammable through the introduction of inert gas and the resultant reduction of the overall oxygen content in any part of any cargo tank to a level not exceeding 8% by Volume, while being under positive pressure at all times.

Purging with Inert Gas (IG) :-

• For reduction in hydrocarbon (HC) content in tank atmosphere for Cargo  Vapor contamination reasons:
  • After tank cleaning operations the cargo tanks may be purged with inert gas to reduce the concentration of the hydrocarbon gas inside the tank atmosphere.
• Follow the procedures as laid out in the operation and equipment manual.
• Purge pipes, with proper flame screens shall be fitted, where provided.
• Carry out the operations of replacing the tank atmosphere by introducing IG of which oxygen content is 5% by Volume or less into the tanks.
• Go on with purging by IG until the hydrocarbon content reduces to the required / desired level.

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Segregated Ballast:

Annex I- Regulations for the Prevention of Pollution by Oil

Chapter 4 – Requirements for the cargo area of oil tankers

Part A – Construction

Regulation 18 – Segregated ballast tanks

Oil tankers of 20,000 tonnes deadweight and above delivered after 1 June 1982

• Every crude oil tanker of 20,000 tonnes deadweight and above and every product carrier of 30,000 tonnes deadweight and above delivered after 1 June 1982, as defined in regulation 1.28.4, shall be provided with segregated ballast tanks and shall comply with paragraphs 2, 3 and 4, or 5 as appropriate, of this regulation.
  • The capacity of the segregated ballast tanks shall be so determined that the ship may operate safely on ballast voyages without recourse to the use of cargo tanks for water ballast except as provided for in paragraph 3 or 4 of this regulation. In all cases, however, the capacity of segregated ballast tanks shall be at least such that, in any ballast condition at any part of the voyage, including the conditions consisting of lightweight plus segregated ballast only, the ship’s draughts and trim can meet the following requirements:
    • the moulded draught amidships (dm) in metres (without taking into account any ship’s deformation) shall not be less than:

                dm = 2.0 + 0.02L

• the draughts at the forward and after perpendiculars shall correspond to those determined by the draught amidships (dm) as specified in paragraph 2.1 of this regulation, in association with the trim by the stern of not greater than 0.015L; and
  • in any case the draught at the after perpendicular shall not be less than that which is necessary to obtain full immersion of the propeller(s).

• In no case shall ballast water be carried in cargo tanks, except:

1. the opinion of the master, it is necessary to carry additional ballast water in cargo tanks for the safety of the ship; and
2. in exceptional cases where the particular character of the operation of an oil tanker renders it necessary to carry ballast water in excess of the quantity required under paragraph 2 of this regulation, provided that such operation of the oil tanker falls under the category of exceptional cases as established by the Organization.

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Reid Vapour Pressure:

Reid Vapour Pressure (RVP) is measured by ASTM D-323 testing method. The sample is placed in a chamber at a constant temperature of 100^oF. RVP is slightly lower than the True Vapour Pressure (TVP) at 100^oF.

• The volatility characteristics of petroleum fuels are very important especially for gasolines. Motor and aviation gasolines are manufactured as liquids but they are consumed in the vapor phase.
• Consequently, gasoline volatility must be high enough to assure acceptable engine start-up, warm-up, acceleration and throttle response under normal driving (or flying) conditions.
• On the other hand, the maximum volatility of a gasoline must be restricted to avoid vapor lock, vaporization losses, air pollution, and unsafe storage and handling.
• The volatility considerations for other transportation fuels like kerosene and diesel are, to some extent, similar to those for gasoline.
• The Reid vapor pressure (RVP) is frequently used as an indication of volatility of liquid hydrocarbons.
• It is not equivalent to the true vapor pressure. In general, RVP is lower than the true vapor pressure due to some small sample vaporization and the presence of water vapor and air in the confined space.
• The apparatus and procedures for determining the RVP are standardized and specified in ASTM method D-323 and IP-402 [1]. The Reid vapor pressure test is widely used as a criterion for blending gasoline and other petroleum products.
• Once RVP of a fuel is known the methods provided in the API-TDB [2] can be used to estimate true vapor pressure of a fuel or a crude oil at any desired temperature.
• True vapor pressure is an important thermodynamic property related to volatility and phase equilibrium calculations.

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Hazards of Petroleum with Reference to:- Gas Density

ISGOTT – 2.3 :- DENSITY OF HYDROCARBON GASES:

The densities of the gas mixtures evolved from the normal petroleum liquids, when undiluted with air, are all greater than the density of air. Layering effects are therefore encountered in cargo handling operations and can give rise to hazardous situations.

The following table gives gas densities relative to air for the three pure hydrocarbon gases, propane, butane and pentane, which represent roughly the gas mixtures that are produced respectively by crude oils, by motor or aviation gasolines and by natural gasolines. These figures are not significantly changed if inert gas is substituted for air.

It will be seen that the density of the undiluted gas from a product such as motor gasoline is likely to be about twice that of air, and that from a typical crude oil about 1.5 times. These high densities, and the layering effects that result from them, are only significant while the gas remains concentrated. As it is diluted with air, the density of the gas/air mixture from all three types of cargo approaches that of air and, at the lower flammable limit, is indistinguishable from it.