The Line Cool-Down at Sea operation is carried out while the vessel is en route to the discharge port. The goal is to keep the liquid lines at the required low temperature and ensure they are in a ready-to-discharge condition upon arrival. To maintain the liquid lines in a cold state, the vessel uses the spray pumps inside the cargo tanks to guide the LNG from the spray line into the liquid line, thereby cooling it down. This process ensures the lines remain cold and are ready for the custody transfer system upon arrival.
| Function | Lowering cargo line temperature before LNG transfer |
| Performance criteria | Cargo line temperature -100°C |
| Auxiliaries involved | Stripping/spray pump |
| Operation duration | 2 hours |
| Check points | Cargo line temperature sensors and visual frosting and icing |
The purpose of cooling down the LNG lines is to avoid thermal shock on the piping and associated valves which would occur if gradual cooling was not employed.
Therefore, before unloading LNG from the cargo tanks, the cargo liquid lines, crossovers, and cargo filling lines must be cooled down to cryogenic temperatures and this can be undertaken with LNG supplied from the vessel’s cargo tanks.
The cargo tanks will normally be at approximately -160ºC during a loaded voyage with fully loaded cargo tanks and with the supply of fuel gas for the Main Engines (ME) and the Generator Engines (GE).
A stripping/spray pump is used to supply an initial slow flow of LNG to the spray line and crossover into the cargo lines and to the cargo tank filling lines. The operation would normally be undertaken prior to arrival at the port.
The exercise begins by starting the required generators for use. The system is correctly lined up by opening the necessary valves. The spray pump is started, and parameters are verified to ensure the system is functioning properly.
