Liquefied Gas Carrier

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Cargo conditioning, reliquefaction and boil-off control requirement for a liquefied gas carrier

The term “cargo conditioning” refers to the care and attention given to the cargo on passage to ensure that:

(1) There are no undue losses in cargo quantity;

(2) Cargo tank pressures are kept within design limits; and

(3) Cargo temperature is maintained or adjusted as required.

These aims are achieved either by reliquefaction or, on most LNG ships, by using boil-off as propulsion fuel. Cargo conditioning may not be necessary on ships with pressure vessel tanks.

If reliquefaction plant is fitted the responsible personnel should have a thorough understanding of its operational principles. When running, the plant should be monitored so that anything which might adversely affect its safety or efficiency is quickly recognized and corrective action taken. Plant is normally fitted with shutdown devices to sense high liquid level, temperature or pressure.
LNG carrier underway

Fig : Gas carrier on sea passage

Reliquefaction and Boil-off Control

General guidance on safe procedures for reliquefaction and boil-off control is given below. The detailed instructions for any ship depend upon the system fitted, and manufacturers’ operating instructions should be closely followed.

There are several different types of reliquefaction system.

The most common involves compressing the cargo vapour and condensing it in a seawater-cooled condenser. Alternatively the condenser may be cooled by a refrigerant from a secondary refrigerating unit (cascade-type refrigeration).

Another type of reliquefaction is achieved by circulation of the refrigerant through coils inside the tank or through a separate heat exchanger outside the tank (indirect cooling). Cargo-incompatible refrigerants should not be used, nor refrigerants which are known to have a high ozone depleting potential.

The vapour of certain cargoes (e.g. ethylene oxide, propylene oxide) cannot be compressed. Such cargoes can only be refrigerated by indirect cooling and cargo compressors usually have to be isolated or blanked off.

LPG is normally reliquefied by direct compression and condensation in one or two stages, with condensation against water, in what is called a direct reliquefaction system. Colder cargoes such as ethylene, although still requiring direct compression, require a cascade system with the cargo condensing against a secondary refrigerant, which is condensed using water as the coolant.

A reliquefaction plant is not normally fitted to LNG carriers. Instead the boil-off is used as fuel for main propulsion machinery. During ballast passages the tanks are kept cold using cargo deliberately retained on board: this cargo is known as a “heel”. Boil-off from the heel is also used for propulsion during the ballast voyage. The retention of a heel requires consideration of sloshing loads: care has to be taken to ensure that the retained liquid is properly distributed.

A heel is often also retained on board fully refrigerated or semi-refrigerated LPG carriers to enable the tanks to be kept cold on the ballast voyage. As LPG boil-off is heavier than air, regulations do not permit it to be used as propulsion fuel and it is therefore reliquefied and returned to the tanks. Return should be by the spray line, if fitted, for best cooling efficiency.

The specific operating instructions for the system fitted should be observed in addition to the following precautions:

(1) The purpose of the reliquefaction system is to prevent loss of cargo and ensure that the cargo liquid is either kept at the loading temperature or is at the temperature required for discharge on arrival. In the latter case it may be necessary either to cool or to warm up the bulk liquid on passage.

If the system is used only to keep cargo tank pressure just below the relief valve set point, the cargo will warm up to a new temperature and it may be too hot for discharge at the terminal. If it is necessary to cool down the liquid on passage, the loading temperature and system capacity should be assessed to ensure that the necessary operations can be completed during the voyage.

(2) If two or more cargoes are carried simultaneously, they should be segregated throughout all cargo operations. Particular care is required with incompatible cargoes.

(3) Gas detection equipment in spaces containing reliquefaction plant, instrumentation and controls should always be activated. Upper and lower sample points (if fitted) should be selected according to the relative vapour density of the cargo.

(4) Ventilation equipment for the reliquefaction plant space should be started well in advance of activating the plant.

(5) Filters on the suction side of compressors should be checked and carefully cleaned if necessary. If they are blocked the efficiency of the plant may be reduced drastically.

(6) The lubricants used for all machines should be compatible with the cargo and suitable for the temperatures and pressures experienced both in operation and when stopped. Oil levels should be checked and crankcase heaters started if necessary before the plant is activated.

(7) All plant, instrumentation, control and shutdown equipment should be tested on a regular basis.

(8) The precautions on ice or hydrate formation, reactivity and cargo contamination should be observed.

(9) All pipelines and valves should be double-checked to ensure that they are correctly set before starting the plant.

(10) To prevent overheating, the cooling water supply to condensers should be established and the refrigerant system (where fitted) started before cargo compressors are run.

(11) Cargo compressors should never be operated with discharge valves shut.

(12) Sub-atmospheric pressures should normally be avoided in any part of system to prevent the ingress of air. Flammable vapour / air mixtures should never be passed through cargo compressors.

(13) Refrigerant or cargo vapour compressors should be started and suction valves opened very slowly to prevent damage from liquid carry-over.

(14) If the capacity of cargo or refrigerant compressors is controlled manually, plant should be started on the minimum setting and the capacity increased gradually as necessary.

(15) Operation of the reliquefaction plant will be affected by any incondensable gases in the vapour drawn from the cargo tanks. These incondensables may originate from the cargo itself (e.g. ethane, methane) or may be inert gas remaining from previous purging. Incondensables will cause abnormally high condenser pressure and will reduce condensation of the cargo vapour. To re-establish full condensation the incondensables have to be vented regularly.

Problems with incondensables mainly arise during the early stages of reliquefaction. Reliquefaction plant liquid levels should be checked regularly during operation to prevent overfilling of receivers or condensers, which may be caused by sticking control valves or expansion valves. It is desirable to keep comprehensive records so that any unexpected changes can be quickly noticed and remedial action taken.

(16) Care should be taken to prevent liquid cargo from entering compressors, particularly if liquid separation equipment is not fitted. In heavy weather this could be a significant problem which may require shutdown of compressors. Under certain conditions liquid entrainment may also occur during spray cooling of the tanks. Liquid entrainment in the vapour may cause severe mechanical damage to compressors.

(17) If condensate is returned to more than one tank simultaneously, or if vapour is taken from several tanks and is returned to a single tank, the liquid levels should be checked regularly and remedial action taken to avoid possible overfilling.

Use of Cargo as fuel

Boil-off from LNG cargo may be burnt as fuel in the main propulsion system. Two factors influence the sanctioning of this practice:

(1) LNG vapour, being mainly methane, is lighter than air at ambient temperatures. It is therefore safe to be used because if it were to leak into the machinery space it would escape through exhaust vents and not accumulate within the machinery space. Consequently LNG is the only cargo vapour allowed to be used as fuel.

(2) It is possible to burn LNG vapour in boilers, diesel engines or gas turbines. In each case cargo vapour is introduced into a space from which it is normally excluded, and the design of the cargo vapour-to-fuel system is therefore subject to strict requirements. It is vital to ensure that the integrity of the system is not impaired in any way.

LNG boil-off may be either vented or burnt (or both) to keep tank pressures at the required level. The decision whether to vent or burn the boil-off depends on many factors, some economic, some the result of regulations. Regulations may, for instance, either prohibit venting or the use of cargo as fuel in certain places. Such regulations should always be observed.

Note: Attention should also be paid to Chapter 16 of the IGC Code, Regulation II-2/15.1 of the SOLAS Convention, IMO recommendations concerning the use of low flashpoint cargoes as fuel e.g. IMO Resolution A565(14), and to classification requirements.

On the high sea, cargo vapour may provide the main fuel, though oil pilot burners are also required. In the case of steam plants, cargo vapour may also be burnt when propulsion machinery is not in operation provided that means for steam-dumping are installed.

The following precautions should be observed:

(1) Personnel should fully understand the system, its limitations, maintenance requirements and the danger of cargo leakage. The system should be kept clean and efficient and machinery performance logged so that changes can be identified.

(2) Ventilation fans for the machinery space and the fuel supply line trunking should be operated before and during gas burning operations. Attention should be paid to the ventilation of any areas near untrunked gas piping.

(3) Gas detection equipment for the system should be working throughout burning operations.

(4) Supply lines should be purged with inert gas immediately before and after burning operations.

(5) All operating instructions for the system should be observed. Safety equipment (such as interlocks) should not be overridden.

(6) If the gas flame goes out, the reason should be established before it is relit. If both oil and gas flames are lost all combustion spaces should be ventilated of flammable vapour before the flame is relit, otherwise an explosion could occur. Attention should be paid to flame failure sensors; low sensitivity will result in failure to shut down and high sensitivity will cause unnecessary shutdowns.

(7) Cargo tank pressures should be monitored during all burning operations: if boil-off is removed too fast, the pressure could be reduced below atmospheric and air drawn into the tank, creating a flammable mixture. Cargo tank pressures should be maintained above atmospheric at all times.

(8) Care should be taken to prevent liquid cargo from entering compressors, especially if liquid separators are not fitted. Rapid changes in supply pressures should be avoided, otherwise the flame will not be stable.

(9) The gas supply lines should be checked regularly for leaks. If a leak does occur, the fuel supply should be isolated immediately and not reconnected until the leak has been repaired.

(10) No modification whatsoever should be made to the system without the permission of a responsible authority.

(11) All joints in the supply line should be pressure tested after maintenance before the system is re-commissioned.

(12) Water should be drained from carbon steel fuel lines to prevent corrosion.

(13) Flame screens may be fitted in the supply line or within each burner: they have very small holes which are easily blocked, and should be cleaned regularly.

(14) The gas heaters should be regularly checked to ensure that no leakage occurs between the gas and steam systems. Steam condensate has to be returned to the feed water system via a ventilated drain tank: the water level in these tanks should be maintained and vents checked periodically for blockage which could cause gas to enter the feed system.

(15) Gas booster compressors should be carefully maintained and attention paid to the condition of shaft seals.

(16) All incidents, however trivial they may seem, should be recorded and brought to the attention of the responsible officer.

Related Information:

  1. Boil-off & vaporized gas (BVG) management system for LNG carriers

  2. Cargo containment systems for gas carriers

  3. Safety guideline for changing previous cargo

  4. Gas analyzing equipment

  5. Custody Transfer Measurement (CTM) System

Other info pages

Gas cargo containment systems - primary barrier (the cargo tank),secondary barrier, thermal insulation and more

Boil-off & vaporized gas (BVG) management system for LNG carriers

Ships preparations prior to a loading or receiving terminal

Cargo conditioning, reliquefaction and boil-off control requirement for a liquefied gas carrier

Cargo Containment Systems in Liquefied Gas Carriers

cargo emergency shutdown requirement

damage stability guideline for liquefied gas carriers

Various Cargo handling equipments onboard

Cargo hoses connection guideline

Documents accompanying a liquid gas cargo

How LNG transferred from shore to ships cargo tanks ?

Cargo operation guideline onboard a liquefied gas carrier

Cargo piping Systems in Liquefied Gas Carriers

cargo planning requirement

cargo and pumproom safety precautions

cargo stripping guideline

Emergency response for cargo system leaks

Emergency response for cargo tank rupture

Risk of overfilling of cargo tank during loading onboard a liquefied gas carrier

Preparation for cargo transfer

cargo transfer between vessels- safety guideline

Safety guideline for changing previous cargo

Gas analyzing equipment

Custody Transfer Measurement (CTM) System

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