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The cargo discharge operation is the culmination of the tankers work. After diligently preparing the tanks for cargo, carefully loading the cargo and exercising proper care and custody during the loaded passage, the crew needs only to get it safely ashore to complete their mission. However, the discharge operation offers more opportunities than the loading operation to have a cargo incident, including a spill! Only the diligent application of all necessary precautions will get the cargo into the shore tanks without accident.

Case study
A crude oil carrier berthed at a Thames refinery on a clear day in December. The arrival, berthing and inspections were without incident. Shortly after 10.00 hours, the ship began discharging cargo. About ten minutes after cargo discharge began, the cargo officer noted a sudden increase in discharge pressure. He reduced the pumping rate to bring the manifold pressure below maximum limit and called the chief officer. Almost immediately, the deck watch called to advise that one of the cargo tanks was overflowing. The cargo officer stopped the pump and closed tank valves. The deck watch was directed to close the manifold valve. About 400 litres escaped onto the deck and 40 litres overboard. All oil was recovered by vacuum lorry.
An immediate investigation discovered that ten minutes after the ship had begun discharging, the shore terminal had started an intra-terminal transfer. The terminal valve alignment was incorrectly set and the high-pressure shore pump began delivering oil into the dock receiving line and onto the ship.

Case analysis
While the shore terminal is clearly at fault in this case, the incident raises doubt about the efficiency of the vessel's main discharge line nonreturn valves. It is a good example of how nothing can be taken for granted during any cargo transfer. At the beginning of the discharge operation, the tanker typically has very little ullage space in the tanks which are first to be pumped. If there is an error in the alignment on the ship, an incorrect alignment of the shore booster pump, a defective check valve on the dock, of a pumping error on the shore, it can take only a few seconds for the ship's tank to be filled to overflowing by the unexpected entry of cargo.
These incidents are best avoided by:
Using written cargo system lineup orders and double-checking the setting of all valves.
Being extra vigilant during the first minutes of the discharge operation to ensure that the oil is moving out of the ship, not in.
Whenever possible, start the cargo discharge by pumping a slacktank, so that additional ullage space is available to accept and detect an incorrect cargo flow.

5.19.1 Starting communications
The ship-shore communications before start of discharge should reflect the example for the loading operation in section 3.16. The requirement is for short, clear, precise statements of the intentions and readiness of each party in the cargo transfer, conducted in a way that indicates without doubt when the ship can start its cargo pumps.

5.19.2 Starting cargo pumps
The typical cargo pump installation on a traditional tanker includes from two to four steam turbine driven centrifugal cargo pumps. The turbines are located and started in the engine room and they drive the cargo pumps (located in the pumproom), by shafts passing through gas-tight seals in the intervening bulkhead.
When the shore has declared that they are ready to receive cargo with all valves open, the pumpman will be directed to open the pumproom bulkhead and/or pump suction valves and flood the pump with cargo. The pump casing vent line should be used to verify that the pump is filled with cargo before it is started. When the pumpman advises the chief officer that the pump is ready to start, the cargo officer, or first officer, calls the engine room for the pumps to be started. Normally, the watch engineer starts the cargo pump turbine in the engine room. He runs it at slow speed for a few minutes until satisfied with its operation, then assigns control of the turbine/pump speed to the cargo control room control panel.
After pump rotation starts, the cargo officer opens or causes his watch personnel to open the correct manifold valve. The pumpman continues to monitor the performance of the pump, while the cargo watch officer must monitor the ullages of the tanks being pumped. When it is apparent that cargo is being discharged (tank ullage increases), the ship advises the shore terminal operator. When the shore terminal operator confirms that cargo is being properly received, then the pumpman or cargo watch officer can begin increasing the pump speed and discharge pressure up to the maximum permitted. This should be done in steps, permitting the pumpman and deck watch personnel to make several checks of the machinery, piping and shore connection as the pressure increases. If more pumps are to be used on the same shore line, the first pump(s) should be brought to only moderate speed until all pumps are on line, after which they can be brought up to speed in parallel.
Immediately after the start of the discharge, all inactive tanks must be checked to verify that their ullage is constant and particularly that it is not decreasing (tank filling up).
During this first hour of discharging, frequent inspections of the pumproom, other non-cargo spaces, the water surface in the berth and the deck piping should be made to permit early detection of any malfunctions or leaks.
On ships equipped with electrically driven deepwell pumps, it is important to ensure that the drive motor is started in the low speed range and the pump allowed to stabilise before increasing speeds to the maximum permitted. Ships equipped with deepwell pumps utilise their deck loading lines as discharging lines. Because of this arrangement, it is important to keep the tank discharge valves on all tanks closed until immediately before that tank is ready for discharging.

5.19.3 Starting against back-pressure
Some discharging terminals have an unavoidable level of back pressure which the ship must overcome before cargo will begin moving ashore. At these berths, unless the chief officer has absolute confidence in the shore and /or ship check valves, the manifolds valve should not be opened until the ship's pumps are producing a discharge pressure equal to the shore back-pressure. An accurate determination of the shore back-pressure must be made before the pump is started. The cargo pump is then started and increased in speed until the manifold pressure equals the shore pressure. The manifold valve is then opened and the pump speed increased, all the while carefully watching the ullage of the first cargo tank being discharged.
During the start to the discharge, the pumpman or cargo watch officer must be ready to immediately shut off the cargo pumps and manifold valve if a leak or misdirection of flow is detected.
If the shore terminal offers booster pumps to assist with discharging against high back-pressure, then the cargo discharge should be started at moderate speed, observing the pump discharge pressure carefully. If the pressure decreases suddenly after a few minutes, it is an indication that the shore booster pumps have been started correctly. If the discharge pressure suddenly increases the cargo watch officer must immediately call the shore terminal asking them to stop the booster pumps. The sudden pressure increase is an indication that the valve alignment of the booster pump(s) may have been reversed, causing the booster pump to deliver cargo toward the ship instead of to the shore tanks.
When booster pumps are in operation, indicate this fact in the logbook as an explanation of low ship discharging pressure (to defend against any later claim by charterers that ship was unable to maintain charter pumping pressure while discharging).

5.19.4 Shore line displacements
Shore line displacements are conducted for two main reasons:
1 To displace the product already in the shore pipeline, in which case this is normally a shore stop. Portland, Maine is a terminal at which pipeline displacements are routinely required.
2 To verify that there are no voids in the shore pipeline contents for cargo loss control purposes. These displacements are normally a ship stop.

The chief officer should take special interest in the line displacement operation, particularly where the reason is for loss control purposes and additionally:
a Confirm the accuracy of the independent or terminal surveyor's
calculations for the line displacement.
b Monitor the remaining tanks on the ship in case product has
entered or escaped to/from any other tank(s).
c Obtain a copy of the calculations of the quantity received ashore.

Despite numerous theories regarding temperature corrections for shore line displacements, the line displacement volumes should be documented uncorrected for temperature.
The shore terminal may request that the ship begin the discharge by displacing the shore pipeline, giving the amount of displacement required. The chief officer should request to know the grade of petroleum being displaced and whether the ship will be requested to stop the discharge at the end of the displacement.
To assist the shore terminal in accurately completing the line displacement, the ship should conduct it by discharging from a single tank which does not change the trim or list significantly and should calculate a stop gauge for the tank used (remembering to allow for the fact that the ship's deck lines are empty when starting cargo).
The chief officer should advise the shore terminal operator when they are nearing the displacement quantity requested and again tell them when it is reached, but should not stop the displacement unless instructed to do so by the shore terminal. The cargo pump should be run at the agreed speed throughout the line displacement, without speed changes. Enter the times and ship tank ullages for starting and completing the line displacement in the logbook.

5.19.5 Stripping tank water bottoms
If the vessel has significant water bottoms in the cargo tanks, the master should suggest to the terminal, in writing, that the water bottoms and slops be discharged first to a single shore tank to minimise the possibility of spreading the 'wet bottoms' throughout the shore tank system. The shore may request that this be done in any event, particularly if the cargo is home heating fuel. If this operation is to be conducted, the ship should be trimmed and listed as necessary for stripping and the water stripped from the tanks, discharging to the main cargo line. When all water bottoms have been stripped, the main cargo pump can be started.
If the main cargo pump is used to remove wet bottoms with the ship on an even keel, it may take up to one meter of cargo pumped from each tank to discharge 5 centimetres of free water bottoms. Several tanks with similar ullage should be opened first to reduce the drawing off rate from each tank. Forwardmost tanks should be dealt with first to improve the trim as the decanting progresses. De-bottoming by discharging one tank at a time with a MCP is generally not a successful operation.

5.19.6 Maximum discharge pressures and rates
The maximum permitted discharge pressure will be set according to the limits of the shore terminal or the ship, whichever is lower. The ship's maximum pressure limit will be as read on the gauges at the discharge side of the pumps. Pressure should always be built up gradually, with frequent visual checks of pumproom and deck piping and the manifold connections.
Initial pumping rates for static accumulator oils will be limited to those producing linear pipeline velocities of less than one meter per second (see section 7.4.5 of the International safety guide for oil tankers and terminals).
The shore terminal may request reductions in pumping rates for topping shore tanks or other reason. These requests should always be accommodated immediately and the shore terminal operator advised when the rate has been reduced. Enter the request and time of reduction in the log book. Also enter the time full pumping rate is requested and resumed.

5.19.7 Discharging to floating-roof tanks
Crude oils and gasolines will normally be stored in shore tanks with internal floating roofs ('floaters'). When these shore tanks are empty, the roof is supported one or two meters above the tank floor by numerous 'legs'. High velocity flow of cargo into empty 'floaters' can cause extensive damage to the floating roof, especially if constructed of aluminum. Normally the shore will request that discharge to empty 'floaters' proceed at a reduced rate until the roof is well afloat. This request should be strictly complied with and the time of reduced pumping rate entered in the log. Do not increase the rate until advised to do so by the shore terminal operator.

5.19.8 High vapour-pressure cargo
High vapour pressure cargo can be difficult to discharge efficiently and failure to follow some normal procedures can result in prologued cargo discharge and large ROB.
All cargo handling equipment must be in good working condition and well maintained. The pipeline and valve system must be leak free. The following procedures are recommended:
If the vessel is fitted with vacuum pumps, primavac systems, eductors, stripping pump(s) and separate stripping lines then this equipment should be used.
If the cargo pumps are fitted with a priming line to the stripping pump, this should be used to draw off the gas generated in the main pumps, thus improving suction. The vacuum pump should be used in the usual way.
When the tank has been discharged to a level below that of the pump intake (ie. when the pump has a negative suction head), the main cargo pumps, vacuum pumps and primavac systems should be closed down. The final stripping should be carried out by the eductor and/or stripping pump. The RPM of the pump driving the eductor must be reduced to avoid a top low vacuum which will cause gassing at a higher liquid level in the cargo tank. For the same reason, the stripping pump should be carefully watched and slowed down when necessary.
Put one pump only on each cargo system; do not make the suction lines common.
If necessary, the cargo pumps should be run at reduced RPM. This will be a deviation from the normally preferred procedure of shutting down a pump and keeping the others at full speed.
Retain one tank at a level sufficient to provide drive for the eductors and priming liquid for the pumps. There is an advantage in using a forward tank for priming. With the vessel trimmed by the stern, a positive head to the pump is achieved at all times. This can only be done if hull stresses and stripping requirements permit.
If eductors are fitted, use the smallest tank possible (e.g., a slop tank with its own stripping line) to provide the eductor drive fluid. This tank should be discharged last and is likely to be extremely difficult to drain. When suction is lost the cargo remaining will be substantially less due to the small size of the final tank.
As a means of recovering as much cargo as possible, the tank can be pressurised with inert gas. However, do not close the P/V valve, as this can overpressure and rupture the tank. Place the P/V valve in the automatic mode. The tank pressure must be carefully monitored by use of a gauge or manometer. The pressure must not be allowed to exceed 2 psi. In a vessel without IGS, low pressure air may be used, but not more than 2 psi. Reduce tank pressure to normal operating level as soon as discharge is complete. This procedure should not be used as a substitute for maintaining pumping equipment and cargo lines in good condition.
Maintain the vessel with the maximum practicable stern trim. Maximum trim is normally restricted by the amount of trim which the boilers an other machinery can accept without malfunction or damage.
Do not allow the oil level in one tank to get out of step with that in another if more than one pump is discharging into the same riser or hose. Failure to monitor tank levels can result in unbalanced pumps and loss of suction. This is due to the pump drawing from the lower level tank being unable to discharge against the discharge head from the pump connected to the higher tank.
Reduce the RPM of the pumps at approximately half tank level. Thereafter, reduce the flow as the ullage increases. Throttle in (partially close), the pump discharge valve to maintain an adequate back-pressure (80 to 100 psi at the pump). Eventually the flow rate may be reduced by as much as 90% of the rated capacity of the pumps.
Do not throttle in the suction valve at any time until the final stages of discharge.
If suction is lost, the pump must be primed from a tank with a positive head and the RPM further reduced. In most cases, the pump must be stopped before it can be properly primed.

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