DieselShip

Carriage of LNG Cargos

This deals mainly with the carriage of LNG cargoes although some reference is given to other types,

Cycle of Operations

Typically, the sequence starting from drydock and containing one loaded voyage and then returning to drydock would be;

• Inerting
• Drying
• Cooldown
• Loading
• Loaded passage
• Discharge
• Warm up
• Inerting
• Gas freeing

This pattern differs from normal trading in that once cooldown has been achieved it is normal to maintain tank temperatures as low as practical by keeping a 'heel' or small quantity of liquid in the tank. This quantity is normally initially purchased by the ship owner or charterer.

Inerting

This involves the reduction of oxygen in the atmosphere within the tank sufficiently to prevent combustion.

There are two main methods of inerting;

• Displacement of atmosphere with nitrogen
• Displacement of atmosphere with inert gas supled from inert gas generator

Nitrogen
Requires supply from a shore facility of tender vessel. Liquid nitrogen is passed first through a vaporizer then through a heater.
The gas is passed through the liquid header to the bottom of the tanks. The upward flow exits through the vapour line to atmosphere.

The vaporizer takes the form of a steam heated matrix heater. a gas detector may be fitted to the condensate drains from this heater to check for leaks. Alternately manual measurements may be taken on a routine basis.

The nitrogen is maintained at a temperature 10 to 15'C below the temperature of the atmosphere in the tank. This gives good stratification and reduces mixing allowing for a much more efficient inerting process

The tank pressure is adjusted by control of the liquid header isolating valves to the tank.

Inert gas
May be from boiler uptake, from onboard IG generator or from tender vessel. The former is the least favorable due to the poor quality of the gas.

Drying

The gas used for inerting contains moisture. Large drying plants may be used to dry this gas, alternately, warmed methane may be used.

The methane is pumped from shore facility to the vessels liquid header then via the bypass line to the spray line and on to the vaporizer.
The gas is heated and sent to the tanks via the vapour header.

The gas in the tanks is drawn out via the liquid loading lie by the compressors and return to shore via the vapour return or to the for'd mast riser. The latter is frowned upon due to atmospheric and safety concerns.

The limit for the temperature of the gas is governed by the tank insulation.

The dew point of the exit gas is checked and drying stopped when desired dew point and 100% CH₄ is achieved.

Cooldown

The cooldown process is required to ensure that excessive boil off, thus over pressurisation, is prevented during cargo loading. This procedure is performed before each loading operation.

Cooldown is achieved by supplying liquid CH₄ to the spray headers. The spray enters the tank via distribution grids and may be directed to various levels as required. Boil off vapour is passes to the high duty compressors back to shore via the vapour line.

Boil off occurs during loading of LPG cargos, whether this is passed to shore via the vapour line or must be liquified on board is very much dependent on the shore facility. Generally shore facilities much prefer it to be done on board ship therefore the liquifaction plant must be maintained in top condition.

Heat must be extracted from both the primary barrier and the insulation. Cooldown rates of 30 - 40'c per hour correspond to an overall rate of 8-10'C including the insulation and secondary barrier.

When a liquid level is detected in the bottom of the tank then the cooldown process is considered to be complete. Primary insulation and secondary barrier temperatures should be measured at -80 to -100'C

Tank pressure is maintained by use of the compressors and by varying the flow of liquid to the spray headers.

Cooling the tanks will lead to a drop in pressure in the inter barrier space. Nitrogen should be introduced as required. Test for methane in the barrier gas should be carried out during this procedure to ensure no leakage.

Loading

Before commencing loading the flow from shore used for cooldown is initially reduced.

The liquid loading line to each tank is opened, the spray shut down. As the liquid vaporisers in the header it is drawn out via the vapor line and return to shore.

Depending of the lie of the liquid in the header the tanks are isolated in sequence to ensure each loading line is cooled.

Loading takes place using normal safe working practices. That is,slow start building up gradually to full rate. Tank pressures are monitored, the capacity of the compressors to return vapour to shore governs the maximum load rate.

Ballasting takes place with loading. Great care must be taken with some designs of LNG carriers due to the 'L' shape of the ballast tank.The vertical section (sometime called the 'pipe') empties vary rapidly and has a large effect on the metacentric height.

Emergency Shut Down Systems-A formal of shut down is agreed with shore side facility sometimes taking the form of a remote stop placed on the vessel. Due to the potential damage caused by shutting a valve against a flow of liquid the shut down this takes the form of programmed sequence of events with timed parameters.

On completion of loading, the high duty compressors are stopped and the vapour line is disconnected. Once the tank pressure starts to rise it will be necessary to commence gas burning in the boilers, starting the low duty compressor and bringing into the use the steam dump system as required until the vessel sails.

Loaded passage

During the loaded passage a quantity of the Cargo will boil off removing heat that has leached past the insulation. This gas is used in the propulsion plant as fuel. Although progress has been made with fitting gas burning gear to large diesel plants , the vast majority of LNG tankers are steam powered.

The boil off gas is fed to the propulsion plant by a low duty compressor via a heater. The gas pipe entering the engineroom is surrounded by a gas tight enclosure whose atmosphere is monitored for hydrocarbons. The spaces around the burner platform are positively ventilated and again monitored for hydrocarbons.

The gas flame in the boiler is extremely clean reducing sootblowing from a daily to a fortnightly requirement. However, the flame is clear blue requiring either a special type of flame detector or for a small quantity of fuel to be burnt which can be seen by normal flame detectors.

The gas burner normally takes the form of a simple open ended pipe.

Cold spot inspection It is possible to check the integrity of the insulation during the loaded passage by entering the cargo and cofferdam spaces and look for cold spots were ice has formed. The ice is a very good thermal insulator and this causes the metal underneath to cool to levels which may allow damage to occur.
For severe cold spots then warming of the metal is required. This can take the form of wetting the surface or steam heating of the ballast.

Interbarrier spaces The inert gas in the interbarrier space is tested for hydrocarbons and moisture. The pressure in the space is slightly below the head pressure in the tank in membrane systems.

A test for membrane tanks during drydocking is to apply a small vacuum to the interbarrier space and then monitoring it over a period thereby checking for leaks.

On completion nitrogen is reintroduced slowly. I know one case were this gas was allowed to build up to an extent that the membrane 'blew' off the wall at huge cost. The C/E, a recipient of an MBE became known as the Membrane Busting Engineer

Inducing Boil off -When the LNG cargo is first loaded generally it is supercooled. On sailing very little boil off is experienced for a period which may extend to a few days. It is in the operators, though obviously not the charters, interest to bring on the amount of boil off as this will drastically reduce fuel oil consumption. The author has sailed on liner trade vessels which are capable of operating from discharge to next discharge only burning fuel oil when gas burning is ceased during cargo operations.

There are practices, although not good ones, available to the Marine Engineer to induce the boil off of the gasses. Two of which are;
On first sailing the ships is run at flat out creating hull vibrations. This shakes the cargo and frees gas bubbles trapped within.
Reducing the LP compressor suction pressure below the minimum set by the charterer. This can have the effect of creating a stable supercooled layer on the cargo. Therefore it may be necessary to give the cargo a little 'shake'.

There are other methods but these two are the most common and simplest to carry out, the author can vouch for there efficacy having seen them in action many times. It should be noted that inducing boil off is severely frowned upon by charter parties.

Entering Port

Before entering ports it will be necessary to resume fuel oil burning. In addition the propulsive load will be reduced. This may cause tank pressures to begin to rise due to insufficient boil off gas being burnt. Steam vessels are fitted with large steam dump facilities which allow steam to be dumped to a condenser increasing boiler load.

It should be noted that the majority of countries and/or port authorities will not allow venting with port limits for obvious reasons.

Discharge

The Liquid header is cooled down before arrival whilst still gas burning. To do this a spray pump is started filling the spray line then the liquid header via the cross-over. Boil off and excess liquid flow back to the tank via the branch pipe.

Cargo Pumps-Before starting the cargo pumps and electrical cables are megger tested.

The pumps are started sequentially against shut discharge valve. After starting the valves are slowly opened to prevent surging.