ECUACIONES INTEGRALES
4.7. Resolvente de operadores integrales
4.2.8a Ballast Voyage With Normal Boil-Off Gas Burning Issue: 1
Key
Degassing Line Into Main Cargo Pump Cable Penetration LNG Vapour ME001VR 450 451 FCV 460 453 421 FCV 441 404 494 493 492 491 FCV 452 FCV TCV TCV 454 431 411
Illustration 4.2.8a Ballast Voyage With Normal Boil-Off Gas Burning
1 2 1 2 To Engine Room To Insulation Spaces
Inert Gas from Engine Room LD Compressor No. 1 (Inboard) Dual Purpose Heaters Main Vaporiser Demister Forcing Vaporiser HD Compressor No. 1 (Inboard) HD Compressor No. 2 (Outboard) LD Compressor No. 2 (Outboard) Vent Gas Heater 1 2 1 2 Jettison
Tank 1
Tank 4
Tank 2
Tank 3
Liquid Dome Vapour Dome Liquid Dome Vapour Dome Liquid Dome Vapour Dome Liquid Dome Vapour Dome4.2.8 Ballast Voyage
A characteristic of the cargo tanks of the Gas Transport membrane type is that as long as some quantity of LNG remains at the bottom of the tanks, the temperature at the top will remain below -50°C.
However, if the ballast voyage is too long, the lighter fractions of the liquid will evaporate. Eventually most of the methane disappears and the liquid remaining in the tanks at the end of the voyage is almost all LPG with a high temperature and a very high specific gravity which precludes pumping.
Due to the properties of the materials and to the design of the membrane cargo containment, cooling down prior to loading is, theoretically, not required for the tanks. However, to reduce the vapour generation and to prevent any thermal shock on the heavy structures, eg the pump tower, loading takes place when the tanks are in a “cold state”.
Cold Maintenance During Ballast Voyage
Different methods are used to maintain the cargo tanks cold during ballast voyages.
1 For short voyages a sufficient amount of LNG is retained in each tank at the end of discharge. The level must never be above 10% of the length of the tank and the quantities can be calculated by considering a boil-off of approximately 0.18% per day and the need to arrive at the loading port with a minimum layer of 10cm of liquid spread over the whole surface of the tank bottom (with the ship even keel). These actual quantities will have to be confirmed after a few voyages.
With this method of cold maintenance, the tank bottom temperature should be below -150°C and the top below -50°C which allows loading without further cooling down.
2 During longer ballast voyages, the lighter parts of the liquid layer remaining in the tank, will evaporate, thus making the liquid almost LPG and at temperatures of higher than –100°C. The upper parts of the tanks will reach almost positive temperatures and under these conditions it will be necessary to cool down the tanks before loading.
Three methods of cooling down are possible, and the one selected will depend on the operating conditions of the ship.
2.1 Cool down the tanks with LNG supplied from shore 2.2 Cool down the tanks just before arrival at the loading
terminal. At the previous cargo discharge, a LNG heel is retained in one of the tanks, provided that the heel does not exceed 10% of the tank length. On top of the quantity to be sprayed, the amount of the LNG heel to be retained will be calculated by assuming a boil off equivalent of 50% of the boil off under laden conditions.
2.3 Maintain the cargo tanks at cold during the ballast voyage by periodically spraying the LNG so that the average temperature inside the tanks does not exceed -120°C/-130°C. As before, a LNG heel is kept in one of the tanks, provided that the level does not exceed 10% of the tank length. On top of the quantity to be sprayed, the amount of the LNG heel to be retained will be calculated by assuming a boil off equivalent of 50% of the boil off under laden conditions.
Cooling down is carried out by spraying LNG inside the tanks for whichever method is used. Each tank is provided
with two spray rings, one capable of a large flow rate
(equivalent to 55m3/h for all the tanks) and another of
small flow rate (equivalent to 35m3/h for all the tanks). Note: It is obvious that this system will generate more
boil-off than the first proposed system. The quantity of LNG to be retained on board will have to be calculated with enough margin to avoid the situation at mid-voyage where the residual is too heavy for the pump to operate.
Conservation of bunkers is important, consequently the co-operation of all members of the management team is essential to ensure as much boil-off gas as possible is used to supply boiler fuel demand, thus keeping fuel oil consumption to a minimum.
The LD. gas compressor is used for gas burning on the ballast voyage in the same way as on loaded, with control of compressor from vapour header pressures. (see section 4.2.8a gas burning operation)
If a long delay at the loading port is experienced, the remaining heel will slowly boil off and the gas available for burning will reduce, care must be taken to stop gas burning as the tank system pressures continue to drop as the temperature rises. The degree of natural warm-up will depend on the time factor, voyage and weather conditions.
After refit the first ballast voyage will have to be made using fuel oil only.
Due to the different calorific values of fuel oil and gas, engine power will require controlling to prevent overloading the boilers.
4.2 Ballast Voyage - Page 1