How does cargo discharging carried out onboard of gas carriers?
07 Aug '17, 03:25
Before commencement of discharging any cargo, the captain must get a written permission from the owner to discharge.
Certain information is needed before arriving at the discharge port, such as the discharge temperature given in charter party, backpressure on the terminal, and size of the terminal lines and flanges. The captain can receive this information either from the agents or directly from the terminal.
When all the information is received a discharging plan prepared. The discharging plan must contain which cargo tanks are to be discharged, and the sequence and rate of the discharging. If the cargo has to be heated and we need to use a booster pump, and agreement must be made with the terminal as to the minimum/maximum cargo temperature and maximum backpressure. Also, the correct reducer to be prepared to fit the terminal flanges.
The cargo pumps we planned to use have to be checked according to the manufacturer's operation manual. Check closely the oil and mechanical seals.
The vessels discharge plan must be discussed with the terminal before commencing discharging. When the discharge plan is agreed to, fill out the ship/shore safety checklist. The discharge plan and all checklists must be signed and followed by the vessel and the terminal. All deviations from the discharge plan or checklist must be noted in the deck logbook and cleared with the terminal.
There are different methods for discharging a gas carrier, and they depend on the cargo equipment onboard and the facilities at the terminal. There are two main methods used for discharging a gas carrier, either by pumps or by pressure. If shore backpressure is very high a booster pump must be used in addition to the ordinary cargo pumps or cargo tank pressure.
When cargo pumps used to discharge the cargo firstly check that the pump is free by turning the pump shaft by hand. When the terminal and the vessel are ready to commence discharging, start the process by using only one pump. The delivery valve on the pump is only slightly open as well as the one back to the cargo tank. When the line is chilled down close the valve down to the cargo tank and open the pressure valve a little bit more. Check that there is no leakage on the vessel's lines and valves and as well as the terminal lines. When both the vessel and terminal lines are chilled down increase the discharge rate according to the discharge plan and on the terminal advice. Do not increase the discharge rate more than what the terminal has asked for, in the worst case the terminal will stop discharging and the vessel is charged for the delay.
While discharging, it is always the terminal that sets the rate and required the cargo temperature. If the terminal asks the vessel to reduce the rate then vessel must comply. To reduce the rate either throttle the pressure valve on a pump or stop a pump. If the pressure valve throttled, the pressure generated in that cargo tank. To avoid increased cargo tank pressure it is best to stop the pump. If the vessel refuses to reduce the rate the terminal can stop the discharging and then hold the vessel responsible for the delay or damage caused by the high discharge rate.
If the shore tank pressure is too high the terminal must flare the excess pressure, or they can arrange a vapour return to the vessel. Reasons for the high shore tank pressure may be either a discharge rate that is high, too high temperature of the cargo or the cargo compressors on shore may have a too small capacity.
If the vessel must use the cargo cooling plant while discharging, permission to do so must be stated in the charter party or agreed to by the receiver so that the vessel will be compensated for the extra fuel consumption. Always while discharging record in the deck logbook if there have been any deviations from the discharge plan. Do the utmost to avoid any claims from the terminal.
Discharging by cargo tank over pressure
When discharging the cargo tank by pressure, the tank that receives our cargo must have a lower pressure than tank being discharged. This way of discharging is the simplest, but when the liquid level decreases, the cargo tank pressure also decreases. That means the loss of pressure to be compensated. That is done either by hot gas from the vessel or shore, or by nitrogen from shore. This way of discharging can be done by a semi and fully pressurized gas carriers.
Discharging with centrifugal pumps
The most common pumps on gas carriers are centrifugal pumps, either of the deep well or submerged type. Most gas carriers have one pump in each cargo tank and they are also normally equipped with one or more booster pumps on deck. When discharging fully refrigerated and semi-pressurized gas carriers normally only pumps in the cargo tanks used. If required discharge against high backpressure then the use of the booster pump either onboard or at the terminal needed. While running the cargo pumps check that the pump is running the correct direction and the liquid level is reduced in the cargo tank. Record in the discharge log pump pressure and ampere and discharge volume every hour. On new cargo pumps there is an anti rotating device to protect the pump from running the wrong way. Most of the pumps also have a non-return valve on the pressure side to avoid leaks if the pump has or are stopped. Check the oil level and the mechanical seal when the pump is running.
The booster pumps must be used if the backpressure is higher than maximum pressure for the pumps in the cargo tank. Booster pumps can be set up either in a series or parallel, depending on backpressure and the rate agreed upon. Never increase the pump pressure higher than the pressure limit on the terminal discharging hose/arm.
When transferring cargo from one vessel to another follow the company quality manual and the ICS Ship to Ship Transfer Guide. While transferring cargo either ship to ship or ship to shore maintain a good communication between the two parties. All changes from the discharge plan or checklists must be reported in the deck logbook.
Discharging through the cargo heater Sometimes required to discharge the cargo with a higher temperature than we have in our cargo tanks. In those cases, a cargo heater used while discharging. When using the cargo heater first check that the heating medium is circulating. Seawater or oil is used as a heating medium, however, some terminals do not accept water as a heating medium. When checked that the heating medium is circulating start pumping the cargo through the heater. Normally there is a by-pass besides the heater, but watch the temperature of the liquid manifold. Never pump cargo to shore with a lower temperature than the minimum temperature given in the checklist. In the worst case, the shoreline may be damaged. If water used water as a heating medium keep the seawater temperature from dropping below 5°C as it goes out of the heater. If the water is freezing, the cargo heater may be damaged and start leaking.
The cargo temperature on the vessel's liquid manifold depends on the amount of cargo that is pumped through the cargo heater. The manifold temperature is adjusted by bypassing the cargo heater. Less cargo through the heater will result in lower temperatures and higher flow 'discharge rate'. If the water temperature is 5°C or less do not use the cargo heater unless there are facilities to heat the water before we pump it to the cargo heater. Some terminals do not allow the use of cargo heaters that utilize water as a heating medium; they require oil as a medium. If that is the case heat the cargo at sea during the passage.
07 Aug '17, 04:27