TYPES OF SHIPS

_{TYPES
OF SHIPS}

SPECIALISED REQUIREMENTS OF SHIPS –

GAS, CHEMICAL, OIL, BULK, PASSENGER, RO-RO AND OTHERS GAS CARRIERS

Gas carriers can be grouped into six different catagories according to the Cargo carried and the carriage condition i.e.

a) FULLY PRESSURIZED VESSELS : Cargo pressure is maintained at maximum 17bar & at ambient temperature, is normally used for LPG and short voyages for ammonia. Type 'C tanks are used which are normally cylindrical or spherical pressure vessels fabricated in carbon steel with a typical design pressure of 17.5bar, corres ponding to the vapour pressure of propane at 45°C. No thermal insulation or reliquefication is necessary and cargo can be discharged using special pumps or compressors. Because of their design pressure, the tanks are extremely heavy.
As a result, fully pressurised ships tend to be small with maximum cargo capacity of about 4,000 m³. Ballast is carried in double bottoms and in top wing tanks.Because these ships utilise Type 'C containment system, no secondary barrier is required and the hold space may be ventilated with air or inert gas.

b) SEMI - REFRIGERATED VESSELS : These ships are similar to fully pressurized ships in that they incorporate Type 'C tanks - in this case pressure vessel designed typically for a maximum working pressure of 5-7 bar. The ships range in size upto 7,500 m³ and are primarily used for carrying LPG. Compared to fully pressurized ships, a reduction in tank thickness is possible due to the reduced pressure, but at the cost of the addition of a refrigeration plant and tank insulation. Tanks are constructed of steels capable of withstanding temperature as low as -10°C, they can be cylindrical, conical, spherical or bi-lope in shape.

c) SEMI - PRESURISED/FULLY REFRIGERATED VESSELS :

Constructed in the size range of 1,500 to 30,000 m³, this type of gas carriers have evolved as the optimum means of transporting the wide variety of gases, from LPG and VCM to propylene and butadine, found in the busy costal gas trades around the Medfterramean and Northern Europa This type of ships are most popular today, amongst "operators of 'smaller size' gas carriers. This vessels also use Type 'C pressure vessel tanks and therefore do not require a secondary barrier. The tanks are made either from low temperature steels to provide for carriage temperatures of - 48°C, which is suitable for most LPG and chemical gas cargoes or from special alloyed steel or aluminium to allow the carriage of ethyleve at -104°C.

These ships, cargo - handling system is designed to be able to load from, or discharge to, both pressurised and refrigerated storage facilities. Tanks are insulated in prismatic insulation.

d) FULLY REFRIGERATED LPG VESSELS : Fully Refrigerated (FR) ships,carry their cargo at approximately atmospheric pressure and are generally designed to transport large quantities of LPG and ammonia. Four different cargo containment systems are used in FR ships : Independent tanks with double hull, Independent tanks with single side shell but double bottom and hopper tanks, Integral tanks and semi - membrane tanks, both these latter having a double hull. The most widely used arrangement is the independent tank with single side shell with the tank itself a Type 'A' prismatic free - standing unit capable of withstanding a maximum pressure of 0.7 bar. The tanks are constructed to low - temperature steels to permit cariage

temperature as low as -48°C. FR ships range in size from 10,000 to 1,00,000 m³.

A typical FR- LPG carrier will have upto six cargo tanks, each tank fitted with transverse wash plates, and a centre line longitudinal bulkhead to improve stability. The tanks are usually supported on wooden chocks and are keyed to the hull to allow expansion and contraction as well as prevent tank movement under static and dynamic loads.

The tanks are also provided with anti - floatation chocks. Because of the low temperature carriage condition, thermal insulation and reliquefaction plant must be fitted. The FR gas carrier is limited with respect to operational flexibility. However cargo heaters and booster pumps are often used to allow discharge into pressurised storage facility.

e) ETHYLENE VESSELS : This gas is normally carried fully refrigerated at atmospheric presure at -104°C. Ethylene carriers have capacities ranging from 1000 to 30,000 m³. If type 'C pressure vessel tanks are used, no secondary barrier is required, Type B tanks require a partial secondary barrier; Type A tank requires a full secondary barrier and because of cargo temperature at - 104°C, the hull cannot be used as a secondary barrier. Thermal insulation and a high capacity reliquefication plant is required on this type of vessel.

Many Ethylene Carriers can also carry LPG cargoes thus increasing their versatility. Ballast is carried in double bottom and wing ballast tanks and a complete double hull is required for all cargoes carried below - 55°C whether the tanks be of Type A, B or C.

f) LNG VESSELS : LNG carriers are specialised vessels built to carry LNG at atmospheric presure boiling point of - 1 63°C and 1,30,000 m³ and are normally dedicated to a specific project through out trjeir contract life for 20-25 years.
These ships are of three types :- (i) Gaz Transport membrane, (ii) Technigaz membrane and (iii) Kvaemer Moss spherical independent type B.

All LNG carriers have double hulls throughout their cargo length which provides adequate space for ballast, the membranes have a full secondary barrier, the spheres a drip - pan type protection. Another characteristic common to afl is that they burn the cargo boil - off as fuel (permitted with methane cargo, being lighter than
air at ambient temperature but not propane or butane which are heavier than air gases).

Hold spaces around the cargo tanks are continuously inerted except in the case of spherical Type B containment where hold spaces may be filled with dry air provided that there are adequate means for inerting such spaces in the event of cargo leakage being detected. Continuous gas monitoring of all hold spaces is required.

TYPES OF CARGO CONTAINMENT SYSTEM

The IMO Code identifies five different types of Cargo containment system :-

a) Independent tanks.

b) Membrane tanks.

c) Semi - membrane tanks.
d) Integral tanks

e) Internal insulation tanks.

Single stage direct reliquefaction

Two stage reliquefaction with intercooler

Figure 5 Caz Transport membrane containment system as utilised on larger-sized LNG carriers

Figure 6(3) Technigaz membrane containment system as utilised on larger-.sized LNG carriers

Figure 6(b) Detail of the Technigaz membrane's barrier and insulation construction

a) INDEPENDENT TANKS -

These type of tanks are completely self - supporting and do not form part of the ship's hull and do not contribute to the hull strength. Depending on the design pressure, there are mainly three types of independent tanks, Type A, B and C.

1) Type 'A' tanks are self - supporting prismatic tanks constructed primarily of plane surfaces with conventional internal stiffening. This means cargoes must be carried in a fully refrigerated conditions at or near atmospheric pressure (normally below 0.25 bar). In order to ensure safety in the event of cargo tank leakage, a secondary containment system is required to protect the ship's hull from low temperatures. The secondary containment system is known as "secondary barrier" and is a feature of all ships with Type 'A' tanks capable of carrying cargoes below - 10°C. For a typical fully refrigerated LPG Carier [which will not carry Cargoes below -55°C) the secondary barrier must be a complete barrier capable of containing the whole tank column at a defined angle of heel and may form part of the ship's hull, it is this approach which is generally adopted, i.e. appropriate parts of the ship's hull are of special steel capable of with standing low temperature. Alternative would be to build secondary barrier around each cargo tank. Any secondary barrier must be able to contain tank leakage for a period of around 1 5 days. The space between primary tank and sescondary barrier is known as the hold space. Where flammable cargoes are being carried, these spaces must be filled with inert gas to prevent a flammable atmosphere from being created in the event of primary tank leakage. Thermal insulation may be applied either to the outside of the primary tank or to the inner hull:

(ii) Type 'B' tanks are either constructed of plane surface or of pressure vessel type, the stress analysis must include fatigue life and crack propagation analysis. Type 'B' tanks are normally spherical tanks.and requires only a partial secondary barrier and usually cousists of a drip - tray and splash barrier. The hold space is normally filled with dry inert gas but may be ventilated with air provided that inertrip of the space can be achieved in the event of the vapour detection system for cargo leakage. A protective steel dome covers the primary barrier above deck level, and insulation is applied to outside of the primary barrier surface. Type 'B' spherical tanks is almost exclusively used for LNG ships where the cost reduction is achieved by the reduced secondary barrier requirement. Where a prismatic shape is used, the maximum design vapour press is limited to 0.7 bar.

(iii)Type 'C tanks are normally sphesical or cylindrical presure vessel with design vapour prressure higher than 2 bar. In case of fully pressurised ship, i.e. where the cargo is carried at ambient temperature, the tanks may be designed for a maximum pressure of 1 7 bar or more. For a semi - prressure/full refrigerated ship, the cargo tanks and associated equipments are designed for a working pressure of 5-7 bar and a 50% vacuum. The tanks steels are typically capable of withstanding carriage temperature of - 48°C for LPG and - 104°C for ethylene and LPG. Normally, no secondary barrier is required and the hold space can be filled with either inert gas or air.

b) MEMBRANE TANK I-

In this type, cargo containment is in very thin primary membrane barrier, which are supported through the insulation by the hull of the ship. They are not self -supporting and the inner hull forms the load bearing structure. Membrane tank

system must always be provided with a complete secondary barrier. The membrane is designed in such a way that thermal or other expansion and contraction is compensated for without undue stressing of the membrane itself.

There as two types of membrane system -

(i) Gaz Transport Membrane system - It comprises of 0.5 mm thick Invar primary barrier attached to the inner (cold) surface of 200 mm thick perlite - filled plywood boxes used as the primary insulation, these are attached as the inner layer of an identical 0.5 mm thick Invar secondary barrier and 200 mm thick perlite – filled plywood boxes for secondary insulation. Invar is chosen for the membrane because
of its very low co-efficient of thermal expansion, thus making expansion joints or corrugation, in the barriers unnecessary. New designs of the Gas Transport System utilise Invar membranes of 0.7 mm thickness and strengthened plywood boxes to hold the perlite insulation. The perlite is siliconized to make it impervious to water/moisture.

(ii) Technigaz membrane System - It consists of a primary barrier of 1.2 mm thick stainless steel with raised corrugations or waffles, to allow for expansion and contraction. The insulation that supports the primary membrane consists of laminated balsa wood panels between two plywood layers, the inner (cold) plywood layer forms the secondary barrier. In the latest design the balsa wood is replaced by cellular foam with an integral fibre glass cloth/aluminium laminate secondary barrier.

c) SEMI - MEMBRANE TANKS -

The primary barrier is much thicker than that of membrane system, having flat sides and large radiused corners. The tank is self - supporting when empty but non-self supporting in the loaded condition in that the liquid (hydrostatic) and - vapour pressures acting on the primary barrier are transmitted through the insulation to the inner hull as in the case with the membrane system.

d) INTEGRAL TANKS -

The IMO code state that Integral tanks form a structural part of the ship's hull and are influenced in the same manner and by the same load which "stress the hull structure. They further states that integral tanks are not normally allowed, if the cargo temperature is below - 10°C.

CONSTRUCTIONAL MATERIALS OF GAS CARRIERS

Choice of material depends on the cargo being carried. Most important is the low temperature toughness as most metals and alloys (except aluminium) become brittle below a certain temperature. Fine grain treatment of structural carbon steels can be used to achieve low temperature characteristies and the IMO codes specify low temperature limits for varying grades of such steel down to -30°C for grade E. For carrying fully refrigerated LPG cargoes, must have tanks capable of withstanding temperature down to -55°C. To achieve this service temperature alloy steels such as fully killed fine - grain Carbon manganese steel, sometimes alloyed with 0.5 per cent nickel (Ni) are used.

Where a ship has been designed specifically to carry fully refrigerated ethylene (with a boiling point at atmosphere pressure of -140°C) or LNG (atmospheric boiling point -163°C), nickel alloyed steels, stainless steel or aluminium must be

There as two types of membrane system -

(i) Gaz Transport Membrane system - It comprises of 0.5 mm thick Invar primary barrier attached to the inner (cold) surface of 200 mm thick perlite - filled plywood boxes used as the primary insulation, these are attached as the inner layer of an identical 0.5 mm thick Invar secondary barrier and 200 mm thick perlite – filled plywood boxes for secondary insulation. Invar is chosen for the membrane because of its very low co-efficient of thermal expansion, thus making expansion joints or corrugation, in the barriers unnecessary. New designs of the Gas Transport System utilise Invar membranes of 0.7 mm thickness and strengthened plywood boxes to hold the perlite insulation. The perlite is siliconized to make it impervious to water/moisture.

c) SEMI - MEMBRANE TANKS -

d) INTEGRAL TANKS -

used as the material of tank construction.

TANK INSULATION OF GAS CARRIERS

Purpose -

a) To minimise heat flow into cargo tanks and thus reduce boil - off,

b) To protect the general ship structure around the cargo tanks from the effects of low temperature.

Porperties -

i) Low thermal conductivity

ii) Non - flammable or self - extinguishing,

iii) Ability to bear loads.

iv) Ability to withstand mechanical damage.

v) Light weight.

vi) Material should not be affected by cargo liquid or vapour.

The insulatfon material's vapour - sealing properties to prevent ingress of water or moisture is important otherwise condensation and freezing of moisture can cause damage to insulation. Humidity must be kept low in hold spaces.

The following are the materials normally used for insulation in gas carrier construction-

a) Polyurethene - Pre-formed, sprayed or foamed - Thermal conductivity 0.02 - 0.03 w/mk (watt per meter per ° k)

b) Mineral wool - Slab or roll - form - Thermal conductivity 0.03 w/mk.

c) Balsa - Load bearing insulant used in LNG containment designs. Thermal conductivity 0.05-w/mk.

d) Perlite- Thermal conductivity 0.04 w/mk.

e) Polystyreve -Thermal Conductivity 0.036 w/mk
GENERAL GAS CARRIER LAY - OUT

Gas carriers have many features which are not found on any other types of tanker. It is not permitted for a cargo pumproom to be placed below the upper deck, nor may cargo pipework be run beneath deck level, therefore deepwell or submersible pumps must be used for cargo discharge. Cargo pipeworks to tanks beneath deck level must be taken through a cargo tank dome which penetrates the deck.

Where a gas tanker is fitted with a reliquefication plant, this plant is housed in a compressor house on deck. Close to this compressor house is an electric motor room which contains the motors for driving the compressors of the reliquefication plant and booster pumps when fitted. The electric motor room and compressor room must be separated by a gastight bulkhead.

The IMO codes detail the requirements for mechanical ventilation of these rooms.

which is fundamental to the safety of the gas tanker. Another safety feature concerns sealing of the driving shaft penetrating the gas-tight bulkhead between the compressor and motor room. The cargo tanks can not be used for ballast purposes and therefore separate ballast tanks are required.

The cargo containment and handling systems must be completely separated from accommodation spaces, machinery spaces etc with cofferdam separation or other means of gastight segregastion between the cargo area and the engine room, fuel tanks and chain lockers, the IMO code also gives specific re commendation for positioning of doors leading from accommodation spaces into cargo areas. In addition, air intakes for, accommodation and engine spaces must be sited at a minimum distance from ventilation outlets associated with gas dangerous areas. All air intakes into accommodation and service spaces should be fitted with closing devices.

Gas tankers are fitted with fixed water spray, system for fire protection purposes. This covers cargo tank domes, cargo tank areas above deck, manifold areas, the front of the accommodation area, boundaries of control room facing the cargo area etc. Minimum water flow rates of 10 litre/m² per minute for horizontal surfaces and 4 litre/m² per min for vertical surfaces should be achieved.

In addition to fixed water spray system, all gas tankers must be fitted with a fixed dry powder installation capable of fighting local cargo area fires. At least two hand hose lines must be provided to cover the deck area. The dry powder installation is activated by nitrogen which is stored in pressure vessels adjacent to the powder containers. Fixed CO₂ flooding of compressor and pumproom are to be provided.

CATAGORIES AND TANK LOCATION

. The IMO code divides.gas carriers into four catagories, ship types I G, II G/fl PG and IE G, which reflect the hazard rating of the cargoes to be carried. For example, e.g. Type I G ships, where the cargo tanks are looted at the greatest distance from the side shelf (and may also be restricted in capacity), must be used for cargoes representing the greatest hazard, e.g. chlorine. Ship Types II G/II P G and III G can carry cargoes which represents progressively decreasing environmental hazard and therefore progressively less stringent constructional requirements in the event of collision or grounding.

A fully refrigerated ship at atmosphere pressure with Type 'A' tanks, designed for LPG must comply with the requirements for tank location and survival capability of a catagory II G ship whereas a semi - refregerated ship with Type 'C tank carrying LPG can comply with the requirement either of a II G,or a II PG ship. For the latter case the Type 'C pressure vessel must be designed for a design pressure of at least 7 bar and a design temperature of not lower than - 55°C.

The II PG catagory takes into account the fact that the pressure vessel design provides incrased. survival capability when the ship is damaged by collision or grounding.

CERTIFICATION AND SURVEYS

As per IMO Codes, gas carriers are required to under go five different types of surveys and have then "Certifictae of Fitness" issued or endorsed on satisfactory completion of

a) An initial survey before the ship is put into service or before issuing' a certificate of Fitness for the first time. This survey includes a complete survey of the structure, equipment, fittings, arrangements and materials of the ship. This survey is carried out to ensure that the ship complies with the appropriate IMO Code.

b) Mandatory Annual Survey following the initial issue of the Certificate of Fitness, this survey includes a general survey of items as in (a).

c) Intermediate surveys not exceeding 30 months to ensure that basic safety equipment, pumping and piping systems etc comply with the appropriate code and in good working order.

d) Periodical Surveys at intervals not exceeding five years to ensure that structure, equipment, fittings, arrangements and material continue to comply with the provisions of the appropriate Code.

e) Additional survey, either general or partial according to the circumstances, following a surveyable defect or following important repair or renewals.

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