Chemical Tankers

1 Chemical tankers The hazardous nature of many cargoes (not all cargos; such as wine or vegetable oils) are carried in chemical tankers. There exist various rules and regulations relating to tanker construction in order to safeguard both the ships and the environment. The International Maritime Organisation (IMO) has produced a “Code for the Construction and Equipment of Ships carrying Dangerous Chemicals in Bulk” This code provides a basis for all such vessel designs and an “IMO Certificate of Fitness” must be obtained from the flag state administration to indicate compliance. 2 • This code was included into the International Convention for the Safety of Life at Sea, 1974 (SOLAS 74), for chemical tankers built on or after 1st of July 1986. • Annex II of the Marpol 73/78 Convention and Protocol is also now in force and applies to hazardous liquid substances carried in chemical tankers. • The IMO Code defines three types of ships. • Types-I, II and III, which correspond to different classes of hazardous chemicals and the suitable location of the vessel's tanks. IMO Ship Types 3 Type I Type III Type II Ship type I is designed to provide maximum preventive measures with respect to the escape of it's cargo under the assumed conditions. Ship type II requires significant preventive measures. Ship type III is for products of a sufficient hazard to require a moderate degree of containment. The criteria considered in producing the tank configurations were: damage considerations following collision, stranding, minor ship side damages and also survival assumptions. 4 Ship Structure • The number of tanks, their volume and layout, will further be determined by the required frame spacing in the cargo space. • The tanks must be designed to withstand the dynamic forces within partially filled tanks and also the high specific gravity of some cargoes. • The bottom structure may be either single or double bottom along the length of the vessel, or a double bottom beneath the centre tanks and a single bottom beneath the wing tanks. • The choice will be influenced by the fact that IMO types I and II must have a double bottom and double bottom ballast tank capacity is needed for trimming and heeling. 5 6 • Bulkheads may be either horizontally or vertically corrugated or plane with stiffeners on the outside of the tank to facilitate tank cleaning. • The hull structure may be single or double skin. • IMO Type I and Type II cargoes reguire a double hull. The insulating effect of the double hull also helps to the heating or cooling of cargos within the cargo tanks. • Deck structure can be conventional with stiffeners in the tank space or may use a double skin or cofferdam in order to provide plane surfaces within the tanks. • Single decks with stiffeners outside the tank space have been used. 7 Cargo Tank Types • Cargo tanks may be “integral” or independent”. • An integral tank is built into the hull structure and contributes to the strength of the hull. • An independent tank is completely separated from the hull structure and does not contribute to the strength of the structure or the loads imposed upon it by the sea. • Where an independent tank is designed to withstand an internal pressure in excess of 0.7 bar gauge, it is a pressure tank. • Tanks which do not exceed this pressure are considered gravity tanks and can be either independent or integral. • Small cargo parcels can be carried by fitting deck tanks, which are usually cylindrical gravity tanks, independent of the hull. 8 Cargo handling • Due to the large numbers of tanks and the need to avoid contamination between different chemicals, usually deepwell pumps are used in each tank. • The pump may also be used as the filling line for the tank. • Tank ventilation may use open or closed vent piping depending upon the type of cargo. • The height of the main vent may be as low as 4 metres, for less toxic and as high as B/3 for highly toxic cargoes. • Pressure/vacuum valves are used to guard against overpressure in the tanks. 9 • Heating coils may be provided in some or all of the tanks. Steam, heating oil or , warm water can be used as the heating medium. • Tank cleaning systems are also required and must include a slop tank system to comply with Marpol 73/78. Tank coatings • Protection of the structural components is important and various types of coating are used in the cargo tanks. • Tanks may be constructed of mild steel, clad steel or stainless steel. • The choice of paint or other coating will depend upon the various cargoes to be carried and compatibility is essential. • The main types of coatings used are epoxy, phenolics, zinc silicate and polyurethane. • Rubber coatings are used in tanks carrying liquids such as hydrochloric acid. 10 Liquefied Gas Carriers • The bulk transport of liquefied gases requires the use of specialised vessels. • Natural gas and petroleum gas each require different transport arrangements and therefore the vessel types are particular to their cargo. • Specialist ships are now used to carry the various types of gas in a variety of tank systems, combined with arrangements for pressurizing or refrigerating the gas. 11 Liquefied Gas Carriers • Natural Gas is found and released as a result of oil-drilling operations. It is a mixture of such gases as methane, ethane, propane, butane and pentane. • The heavier gases, propane and butane, are separated by liquefaction and are termed ‘Petroleum Gases'. • The properties and therefore the behaviour of these two basic groups vary considerably, thus requiring different means of containment and storage during transportation. 12 • Natural Gas is, by proportion, 75-95 per cent methane and has a boiling point of -162°C at atmospheric pressure. • Methane has a critical temperature of -82°C. Above the critical temperature a gas cannot be liquefied by the application of pressure. A pressure of 47 bar is necessary to liquefy methane at 82°C. • Thus, natural gas cannot be liquefied by pressure at normal temperatures. • Liquid Natural Gas tankers are therefore designed to carry the gas in its liquid form at atmospheric pressure and a low service temperature in the region of -164° C 13 • The following design problems are encountered for LNG ships : •reducing the loss of gas, and, •avoiding the leakage of gas into the occupied regions of the ship. • protecting the steel hull structure from the low temperatures, • Petroleum gas consists of propane, propylene and butane or mixtures of these gases. All of these gases have critical temperatures above normal ambient temperatures. • Thus they can be transported either as a liquid at low temperature and pressure or at normal temperature and under pressure. • The design problems for this type of ship are similarly; •protecting the steel hull from low temperatures, reducing gas loss and • avoiding gas leakage, •added consideration of pressurising the tanks. 14 The IMO International Gas Carrier Code • In 1975 IMO adopted the Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk which provides international standards for ships which transport liquefied gases in bulk. • It became mandatory in 1986 and is generally referred to as the IMO International Gas Carrier Code. • The code covers; – damage limitations to cargo tanks and ship survival in the event of collision or grounding, – ship arrangements for safety, – cargo containment and handling, – materials of construction, – environmental controls, – fire protection, – use of cargo as fuel, etc. 15 Liquefied Natural Gas tankers • The tank types of LNG carrier are; – self-supporting and either prismatic, cylindrical or spherical in shape – or a membrane construction which is supported by insulation Materials used include aluminium, 90 % nickel steel or membranes composed of stainless steel or nickel iron. • Self-supporting or free standing tank is strong enough due to its construction to accept any loads imposed by the cargo it carries. • A membrane tank requires the insulation between the tank and the hull to support and carry the cargo load. • Single or double metallic membranes can be used, with insulation separating the two membrane skins. • The semi-membrane or semi-integrated design is similar to the membrane, except that the tank has no support at its corners. • A double-hull type of construction is used with each of the above designs, the space between being used for water ballast. 16 Tank arrangements for LNG Tankers Prismatic tank(self Supporting) Spherical Tank(self Supporting) 17 Tank arrangements for LNG TAnkers Membrane tank Cylindrical Tank (self Supporting) 18 Semi- membrane Tank Tank arrangements for LNG Tankers Double membrane Tank 19 • Comparison of ta nk types • Membrane and prismatic tanks use the underdeck cubic capacity most effectively. • Cylindrical and spherical tanks involve constructional problems by penetrating the upper deck but provide greater safety in the event of collision or grounding. • Membrane tanks are cheaper to build but the insulation, which must be load bearing, is more expensive. • The hull and machinery costs are about equal for each type. All the different types are in service, with the firmly established designs being prismatic, spherical and membrane types. • Boil-off • Liquefied natural gas is continually boiling in tanks when transported by sea. There is therefore a need to release this gas to avoid a pressure build-up in the tank. • It may be vented directly to atmosphere or burnt in boilers or in specially adapted dual fuel engines. • Burning the boil-off gas in a flare mounted on a boom remote from the ship is another possible solution. • Re-liquefaction is not economical because of the large power and huge cost of the machinery necessary. 20 Liquefied Petroleum Gas Tankers • Three basic types of LPG tankers are currently used- • the fully pressurized tank, • the semi-pressurized partially refrigerated tank, and • the fully refrigerated atmospheric pressure tank. • The fully pressurized tank operates at about 17.5-18.0 bar and requires heavy, expensive tanks of carbon steel which are usually cylindrical in shape. This high pressure is equivalent to the vapour pressure of the cargo at the highest possible ambient temperature, usually taken as 45°C. The tank domes penetrate the upper deck and have fitted all the necessary connections for loading, discharging, sampling, etc. • Semi-pressurized tanks operate at about 8 bar and a temperature of about -7°C must be maintained in the tanks. • Insulation is therefore required around the tank and, since some cargo will boil off, a re-liquefaction plant is needed. Horizontal cylindrical tank configurations are again used. Low temperature steels for temperatures down to around -45°C must be used for the tanks. 21 The capacity of fully pressurized ships is usually less than 2000 m3 of propane, butane or anhydrous ammonia carried in two to six uninsulated horizontal cylindrical pressure vessels arranged below or partly below deck. 22 The capacity of semi-pressurized ships ranges up to about 5000 m3. The cargoes carried being similar to fullypressurized ships. 23 • Fully refrigerated atmospheric pressure tank systems have service temperatures about -50°C and maximum working pressures of 0.28 bar. • The tanks are insulated, self-supporting and prismatic in shape. The tank material must be ductile at low temperatures and is usually a fine-grain heat-treated steel. • A secondary barrier capable of retaining the cargo in the event of main tank fracture is required by classification society rules. Three tank types are used with fully refrigerated LPG ships: 24 •The capacity of fully-refrigerated ships ranges from 10 000 m3 to 100 000 m3 •Tanks are almost exclusively of prismatic in shape, independent type with tops sloped to reduce free surface and bottom corners sloped to suit the bilge structure. 25 26 •A central trunk runs along the top for the length of the cargo tank. Wing ballast tanks are fitted, their inner surface acting as the secondary barrier •Three tank types are used with fully refrigerated LPG ships: Type 1 27 A large dome is situated aft at the top of the tank and wing ballast tanks are fitted The inner surface of the wing tanks acts as the secondary barrier. •Three tank types are used with fully refrigerated LPG ships: Type 2 28 A large dome is situated aft at the top of the tank but no wing ballast tanks are fitted. Hopper tanks are used for ballast when necessary. The hull itself acts as the secondary barrier and must be of low temperature carbon steel in way of the cargo tanks. •Three tank types are used with fully refrigerated LPG ships: Type 3 29 Comparison of tank types • The reduction in weight of tank material in a semipressurised tank design is offset by the need for refrigerating plant and insulation around the tank. • The use of low pressure tanks does, however, permit better utilisation of the underdeck cubic capacity of the vessel. • The fully pressurised tank has no need of insulation nor a secondary barrier. Construction aspects of LNG and LPG carriers • The various regulatory bodies have rules for the construction and classification of ships carrying liquid gases in bulk. These rules follow closely the IMO code for this type of vessel. • A complete or partial secondary barrier is required in all but pressure vessels operating at ambient temperatures down to -100C . This secondary barrier is a liquidresisting outer skin which will temporarily contain any leakage of the liquid cargo from the primary barrier or tank. The secondary barrier should also prevent the structure temperature from dropping and should not fail under the same circumstances as the primary barrier. 30 • Bulkheads or cofferdam arrangements are necessary between cargo tanks, depending upon the temperature of the cargo carried. • Cargo-pumping pipework systems must have no interconnection with other systems. Where a cargo tank has no secondary barrier a suitable drainage system must be provided which does not enter the machinery space. • Where secondary barriers are used drainage must be provided to deal with any leakage, again from outside the machinery space. Special ship survival arrangements are required which limit the width of tanks in relation to the ship's breadth. Doublebottom tank heights are also stipulated. Arrangements of tank design or internal bulkheads where possible must be used to "restrict cargo movement and the subsequent dynamic loading of structure. Membrane tanks, for instance, cannot have internal bulkheads and are tapered off in section towards the top. 31 General Arrangement of Gas Carriers • Gas carriers have a similar overall arrangement to tankers in that their machinery and accommodation are aft and the cargo containment is spread over the rest of the ship to forward where the forecastle is fitted. • Specific gravity of LPG cargoes can vary from 0.58 to 0.97 whilst LNG ships are often designed for a cargo specific gravity of 0.5 so that a characteristic of LNG ships in particular and most LPG ships is their low draft and high freeboards. • Water ballast cannot be carried in the cargo tanks. Therefore adequate provision is made for ballast within the double hull spaces, double bottom, bilge tank, and upper wing tank spaces. • The double hull feature of LNG carriers and many LPG ships is a required safety feature and the tanks of LPG ships which do not have this feature are required to be a minimum distance inboard of the shell. 32 Fore end and aft end structure is similar to that for other ships. The cargo section is transversely or longitudinally framed depending primarily on size in the same manner as other cargo ships. The inner hull receives special consideration where it is required to support the containment system. All gas ships have spaces around the tanks which are monitored for gas leaks and in many ships these spaces are alsa inerted. An inert gas system is fitted aboard the ship. Liquid gas cargoes are carried under positive pressure at all times so that no air can enter the tanks and create a flammable mixture. Displacement sizes for gas carriers range up to 60 000 tonnes with speeds 12-16 knots. 33 34 35 • SEMI-PRESSURIZED (OR SEMI-REFRIGERATED) TANKS • The capacity of semi-pressurized ships ranges up to about 5000 m3. The cargoes carried being similar to fully-pressurized ships. • The independent tanks are also designed as pressure vessels and are generally constructed of ordinary grades of steel suitable for a temperature of - 5o C and are designed for a maximum pressure of about 8 kg/cm2. • The outer surface of the tank is insulated and refrigeration or reliquefication plant cools the cargo and maintains the working pressure. • Cargo tanks are of ten horizontal cylinders mounted on two saddle supportsand many designs (see Figure 23.1) incorporate bio-lobe tanks to better utilize the underdeck space and improve paylaod. 36 FULLY - REFRIGERATED TANKS • The capacity of fully-refrigerated ships ranges from 10 000 m3 to 100 000 m3 • The smaller ships in the range being multi-product carriers whilst the larger vessels tend to be single product carriers on a permanent route. • Tanks are almost exclusively of prismatic in shape, independent type with tops sloped to reduce free surface and bottom corners sloped to su it the bilge structure. • In most cases they are subdivided along the centreline by a liquidtight bulkhead which extends to the underside of the dome projecting through the deck which is used for access and piping connections, etc. • The tanks sit on insulated bearing blocks