5.1 THE DISCHARGE PLAN

The chief officer is responsible for preparing the cargo discharge plan, according to the owner's guidelines and the instructions of charterers and Receivers. (See section 2.8)
The principal considerations in the preparation of the discharge plan are:
• Preservation of cargo quality.
• Maximum cargo outturn (minimum ROB).
• Limitation of hull stress and trim to acceptable levels.
• Minimum discharge time.
• Safety of crew.
• Operation of cargo pumps within acceptable limits.
• Maintain freeboard within operating envelope of shore loading arms.
• Maintain adequate under-keel clearance in berth.
• Achieve vessel performance meeting or exceeding the requirements of the charter.
• Take all precautions to prevent any pollution incident.
• Ensure that the vessel remains safely moored throughout the discharge operation.

Case evaluation
The master and chief officer may not always be able to provide a discharge plan which perfectly satisfies the demands of the receiver(s). In that case, they must use the plan which protects both the safety of the ship and the quality of the cargo before any commercial considerations.

5.1.1 Charterer's instructions vs. receiver's instructions
In most cases, any discharge orders received by the ship will be very general in nature, indicating only the discharge port and quantities to be discharged. Occasionally a discharge sequence may be specified. If there is any discrepancy between the charterer's orders or the cargo discharge quantities and the discharge sequence requested by the receiving terminal, then a message requesting clarification should immediately be sent to the charterer. It is the responsibility of charterer to work out any differences between their intentions and those of the receiving terminal. The master should only advise charterer and then await his instructions. The chief officer's discharge plan must be in accord with the general instructions received from the charterer.

5.1.2 Trim, list and bulkhead stresses
The cargo tank discharge and ballast tank filling sequence must be scheduled in a way which minimises the stresses on the hull and keeps the trim and list within acceptable limits. The first tanks discharged should be wing tanks forward of amidship. Removal of that weight will reduce bending stresses and begin to trim the ship by the stern. Discharging of wing tanks first reduces bulkhead stresses and improves outturn of heated cargoes.
The chief officer's discharge orders should clearly state that no other cargo is to be discharged until the tanks designated to be started first have been well begun. The discharge plan should at an early time achieve a 6 to 7 meter trim aft to ensure good tank draining. Berth depth restriction may delay achieving optimum trim until later in the discharge than would be possible otherwise. Figure 5.1.2 shows a vessel trim programme designed to achieve good stripping trim of 7 meters in 9 hours without exceeding 15 meters draft aft. The 'midship draft' line is determined by the ship's overall pumping rate. The 'after draft for stripping' line is parallel and a one-half of the desired trim (7/2 = 3.5 meters), above the 'midship draft' line. The 'discharge plan aft draft' line (coloured red) is determined by calculation of the discharge plan effects on the ship, hour by hour.

1 Above shows draft and trim during a planned discharge. Draft restriction is 50 feet and time restriction is 30 hours. 2 The constraint of 50 feet draft at berth is shown at top of graph. 3 The constraint of maximum berth time is shown at right of graph. 4 Midship draft is plotted against time. 5 For minimum draining trim, a line is drawn parallel at half trim to indicate minimum after draft for effective draining. 6 By effective discharge sequencing, the after draft intersects the effective draining after draft at point 'A', which is 8 hours into the discharge. This gives a time of 16 hours for effective-stripping of cargo before completion of cargo discharge. 7 The objective of this cargo sequence is to bring point 'A' to the left and keep draft within the confines of available depth of water and total time within time constraint.

5.1.3 Discharge plan details
The discharge plan should specify a number of important details and limitations to be observed in controlling the discharge, including:
• Safety procedures to ensure that the discharge plan proceeds without incident or crew injury.
• The critical path for the discharge. The sequence of operations which should be started as soon as possible and expedited throughout to minimise total discharge time.
• A step-by-step discharge programme, with the setting of all valves
clearly indicated for each step. Valves to be open must be identified by number and all other valves must be closed. The offloading valve arrangement should be in the form of a checklist or other format providing for sign off by the cargo watch officer who starts each operation.
• The operating criteria for the pumps including operating speed, tank levels where main pump is to be stopped and maximum discharge pressures.
• Indications of how the main cargo pumps are to be regulated so that they are used to move the greatest amount of cargo, leaving minimum work for the stripping pumps.
• The sequence for stripping tanks and the stripping accumulation tank(s) to be used. Clear instruction is necessary regarding the level to which the stripping accumulation tank must be drained before stripping begins. Tanks used for stripping accumulation should be fitted with hi-level alarms.
• The ballast programme, with a clear indication of the progress of cargo discharge to be achieved before ballasting can begin and the correlation of the ballast sequence to the discharge progress. Procedures for sampling the cargo during discharge Procedures for changing grades of cargo on the same cargo system.
• When the chief officer is to be called.
• Requirements for inspection of pumprooms, sea valves, discharge manifolds and moorings.
• Procedures for emergency cargo pump stop and raising the alarm in case of fire or cargo spill.
• Procedures to avoid limitations due to known leaking pipelines, faulty valves, or inoperative pumps.

In preparing the discharge plan, the chief officer refers to the records of the vessel's previous performance at the port and with the-cargoes to be discharged, as well as the pump's 'flow rate/head at the ship's manifold' curves (if available). With that information, he estimates the pumping times required to discharge the current cargo. The chief officer considers any mechanical limitations while preparing the planned discharge programme.
The final cargo discharge plan should include a time bar graph of the cargo discharge, COW programme (if applicable) and the ballast programme. This diagram should be drawn on a single sheet so that he interrelationship between the various activities can be clearly seen and appreciated by all involved in the discharge operation.

A simple discharge sequence bar chart.

5.1.4 Heated cargo programme
The programme for discharging heated cargo should include consideration of:
• Discharging wing tanks first, because of the more rapid heat loss of the wing tanks.
• Early reduction of steam to the heating coils of cargo with a high vapour pressure, to reduce the chance is suction loss due to gassing of the pumps.
• Discharge of the more viscous grade of heated cargo first, so that the ship's lines can be cleaned by the less viscous cargo following.