The Discharging with Heel operation is carried out onboard an LNG carrier when the vessel arrives at the discharge port. The goal of this operation is to discharge the required amount of LNG using the cargo pumps and systems, while leaving a small amount of LNG (the “heel”) in the tanks for safety and operational purposes. This exercise is the first part of the discharging operation, where the user starts the discharging process and ensures a staggered discharge between different tanks.
The purpose of unloading with vapour return is to be able to empty the cargo tanks of LNG and replace this with cold LNG vapour returned from the terminal. The rate of discharge of LNG is balanced with the returning LNG vapour, keeping the vessel’s cargo tank within the pressure limits.
All cargo tanks are simultaneously unloaded using all the cargo pumps into the common liquid line and to the manifolds. It is usual for most terminals to have only three liquid lines and one vapour line connected.
The operation of the cargo pumps can be conducted from the IAS where the operator can select either manual control or auto control.
In manual control, the operator sets the valves to set values before starting each cargo pump. In auto control, the operator sets the valves to auto mode and each cargo pump will start in an auto sequence, when activated by the operator. All the interlocks must be clear to enable the cargo pump to start.
The final stop levels in all tanks should be pre-calculated to determine the amount of heel to be retained. In any event, the cargo pumps can be used to take out most of the LNG without needing to use the stripping/spray pumps.
To avoid unnecessary warming of the pump tower upper section, nozzles are fitted in the cargo pump discharge lines which will spray LNG to continue cooling the upper structure whenever the cargo pumps are running.
Precautions during discharging
The fixed gas detection system must be in operation during discharge;
- During a normal discharge, only the main cargo pumps should be used. For starting it is preferable to use the sequence provided by IAS as described in Cargo Operation Manual;
- Before starting the main cargo pumps it is necessary to fill the discharge column with LNG to avoid a pressure surge in the lines;
When starting the cargo pumps, the discharge valve should be opened fractionally to reduce start load & pressure surge; if necessary, re-circulate liquid carefully to cool deck lines;
- Most LNG cargo systems require that the discharge valves are opened to a minimum percentage prior to starting, to avoid “hammer” and so that liquid is re-circulated to the tank initially. It may also be necessary to fill the liquid header using a spray pump to reduce start-up “hammer”. The procedures given in the Cargo Operating Manual must be observed;
- The pump discharge valves should be adjusted to obtain optimum performance as indicated by electrical load, discharge pressure and pump graph;
- Tank pressures must be monitored and recorded during discharge. As the liquid is pumped out, tank pressures will tend to fall. If there is insufficient boil-off to maintain positive pressures, then vapour must be added to prevent a vacuum – use vapour from shore. Should the vapour return from shore be insufficient to maintain tank pressures, other means of supplying vapour to the tanks, either by using the tank sprayers or the LNG vaporiser, have to be used. If the above fail, discharging rate should be reduced or even stopped. In such cases the gas heater should be prepared and lined up for use well in advance in order to avoid venting cold LNG vapour;
- Void space/inter-barrier space pressures may also change during discharge. These should also be monitored, recorded and adjusted as required;
- Levels in all tanks should be monitored and recorded even if they are not being discharged. Any reading that does not change as expected or changes unexpectedly should be investigated;
- During the discharge period, the ship must be kept on an even keel. If it is required to empty a cargo tank, the ship is trimmed according to terminal maximum draught by the stern to assist in stripping the tank. However, the trim should not exceed 2.80 meters by the stern. For any deviation of the above, head office should be prior informed.
- When a tank is near empty, one pump should be stopped to avoid excessive turbulence at the tank bottom, which creates disturbance at the suction of both pumps;
- Pump cavitation should be avoided. The discharge valve should be throttled in when the tank is near empty and before stopping the pump to prevent cavitation, tripping on low current and to maximize the out-turn using spray/ stripping pumps;
- Cargo pumps (or any other pumps) should never be allowed to run dry as cargo liquid provides lubrication & cooling;
- On completion of discharge, all liquid lines and cargo arms must be drained & purged inboard with the nitrogen provided by the facilities;
- The shore line must not be disconnected until it is proven that it is clear of liquid and is also depressurised;
- All manifold & other isolating valves must be closed before the shore transfer arms are disconnected;
- The cargo arms must not be disconnected until gas readings obtained from a portable gas instrument are less than 2% Vol CH4 at the manifold vent and drain valves.
- The vapour arm should be purged with nitrogen from the shore terminal. After confirming that the gas content is less than 2% Vol CH4 at the manifold drain valve, the vapour arm can be disconnected.
- The pressure that vapour line can be disconnected should be determined taking into account the prevailing conditions at the terminal and the vessel.
Exercise Overview:
- Initial Setup:
- The user must configure the settings on the ESD system and prepare the generators for operation to start the cargo pumps.
- Configuring the correct settings for the ESD system.
- Preparing the generators.
- Starting the custody transfer for official tank aging.
- Once the correct lineup is complete, the pumps are started one by one to achieve the required discharge rate as specified in the exercise.
- Assessment Process:
- The assessment is carried out in milestones. The user must:
- Start each pump sequentially.
- Increase the flow to the required rate, as outlined in the exercise.
- Ensure the tanks are staggered in such a way that only one tank contains the remaining liquid heel at the end of the process.
- Completion Criteria:
- The exercise is considered complete when the required staggering between the tanks is achieved, and the user has maintained the required flow rate throughout the operation.
- The user’s performance is evaluated based on their ability to manage the discharging operation, including how they respond to various remarks, alarms, and cautions during the course of the exercise.
