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
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
Take all precautions to prevent any pollution incident.
Ensure that the vessel remains safely moored throughout the discharge
A 33-tank product tanker, carrying 8 grades of products, arrived in Lagos
as the first port of a multi-port discharge programme. Upon docking, the
chief officer was besieged by 7 or 8 receivers, each demanding their products
first. As a result of much shouting and pushing, the only recourse was
to remove all shore personnel from the ship's office and request that
just one representative should attend on everyone's behalf to discuss
the discharge plan, which would finally be decided by the chief officer
according to the many trim and stress considerations of the discharge
programme. It was subsequently discovered that the nearby International
Airport had all but run out of JET A1 inventory!
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
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
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
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
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
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
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.