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Cargo discharging operation in oil tankers

Oil cargo discharge operation involves various safety factors to be taken into consideration. Following are the most common elements and check items to be followed. The procedures explained here are only indicative, not exhaustive in nature and one must always be guided by the practices of good seamanship.

Supply of I.G to cargo tanks being discharged:

Confirm that the oxygen level in the IG main supply is less that 5% & supplied to tanks. The date, time, voyage number and description of operation should be entered on the IGS fixed pressure and oxygen density recorder.

Line up of the IGS:

Prior to starting of discharge, IGS must be set appropriately to maintain a Positive Gas pressure in all tanks at all times.

Cargo tanks IG inlet lines to the designated discharging tanks shall be re-checked and confirmed in desired position.

The control of the key to the locking arrangements for cargo tank IG inlet valves shall be with the Chief Officer.

For tanks which are required to be isolated by vapor (as per the Chartererís instructions), the individual I.G pressure shall be monitored Every 4 hrs.

Preventing for Cargo Contamination including Vapor Contamination Standard Oil Tankers except Product Oil Carriers are provided Single Main Inert Gas and Common Vent Lines which is connected with all cargo oil tanks.

In such vessels, IG Inlet (Cut-out) valves should be operated, if the charterer requests to prevent Vapor contamination, which may be restricted to monitor the main line pressure, so as to require to fit portable pressure gauges for cargo oil tanks which are isolated.
cargo transfer system for oil tanker
Fig: cargo transfer system for oil tanker

Also in a Product Oil Carrier, below precautions should be considered to prevent Cargo contamination.

a) Vapor contamination at the exhaust end in a Slop tank of AUS Vacuum Pump.
b) Liquid contamination with leakage of valves of Manifold Vapor Equal Line.
c) Liquid contamination with leakage of Manifold Drain Line.

Safety Confirmations and Clearance:

Once the Chief Officer is satisfied that all preparations have been made in accordance with the cargo oil discharge plan and the shore facility representative has confirmed that the facility is ready to receive cargo, he may order the designated manifold valve to be opened, the IG out put to the discharging cargo tanks and the discharge operations to commence in accordance with the discharge plan.

Commence discharging at reduced speed. Follow shore instructions & Increase the discharge rate once it has been confirmed that there are no oil leaks and shore receiving at their end, until the agreed cargo oil manifold pressure has been reached.

Deck Watch and Personnel Arrangement

The deck watch shall check for oil leaks in the cargo area throughout the cargo oil discharge operation.
At the beginning of the operations, confirm that no oil leaks from piping joints and that no oil in flowing into tanks other than the tank being used.
Keep continuous monitoring of the Oil Level from the discharging tanks, until settling down of cargo pumps and flow rate.
It has been occurred in the past that, due to excessive shore back pressure & height level of shore tanks, causing filling of shipís tanks and subsequent overflow, due to poor monitoring.
After reaching the desired full rate with all the required cargo pumps and confirmation reports have been received from all stations at deck / pump room watch, (including the cargo piping, pumps and sea surface around the vessel) the Chief Officer may dismiss the off duty crew and revert to the routine Watch Schedule
During discharging operations, watch manifold pressure all the time, and monitor portions where oil is likely to leak.

Starting of Discharge Operation pumps and adjusting Internal Pressure of Tanks

Starting of Cargo Pumps:

Open manifold valves, and start the first cargo pump at slow speed.

After confirming normal operation of each part, open delivery valves to start pumping oil. Then start other pumps.

Only after receiving reports of all safety checks confirmed from all stations of deck / pump room watch, then follow the instruction of the terminal side to increase the manifold pressure to the prescribed (agreed) pressure.

Close watch of the manifold pressure shall be maintained, until completion of setting down of shore side / shipís cargo pumps.

When starting pumps flows, pressures and flow of cargoes are to be maintained at minimum. Only on confirmation from the receiving terminal and completion of results from safety check at each designated (manned location), should flow / pressure be increased to max agreed.
However, at some discharge ports, this requirement of initial flow rate may be revised where line clearance / circulation techniques are employed.

Standard Practice for Operating Centrifugal Cargo Pumps:

Centrifugal pumps are to be run at their full nominal RPM during bulk discharge. Reduction of RPM leads to a rapid decrease of pumping rate and pumping efficiency.

When the tank ullage is nearing low liquid levels, it is preferable to partially close the discharge valve, rather than reduce pump revolutions, in order to reduce the flow rate (to prevent cavitations ie pump sucking gas at suction side due to vortex formation / gasification).
It is worth knowing that a 40% open butterfly valve will allow nearly the same flow rate as a 100% open butterfly valve. RPM may then be reduced as necessary

The use of the Auto Unloading System (AUS), where fitted shall be correctly and effectively utilized to reduce the overall discharge time.

Cavitations will occur when the pump tries to discharge more cargo than is able to enter the suction i.e. with high viscosity cargo or where the cargo is highly volatile (high RVP cargoes). In such cases, the cargo tank IG pressure could play an important contribution to increase of the cargo pumpís NPSH (Net Positive Suction Head).

There is a danger of drawing gas or air instead of liquid into the pump, the pump would then be operating in a partial vacuum instead of liquid.

Evidence of cavitations is increased vibration and reduced output which will cause damage to the pump. Always watch the suction pressure gauge of the cargo pump.

During the pump operation an officer must be on station at all times in the cargo control room to observe the tachometer and discharge pressure and be prepared to partially close ("throttle") the pump discharge valve if pressure falls off, or to shut down the pump if it loses suction

Precaution while operating Multi numbers of Cargo Oil Pumps:

It is important to run all pumps at the similar pumping condition (Speed & Delivery. Pressure) If one pump runs much slower than others or much lower delivery pressure, it may stop pumping and heat up, possibly to a dangerous level. If there is high back pressure it is doubtful whether it is worth running all pumps. Due to high shore back pressure, a pump running at reduced RPM may not be discharging at all; therefore, energy is converted to the heat and could be dangerous.

Shutting Down of Cargo Pumps:

The pump discharge valve should be closed at the same time as the pump RPM's are reduced. When the pump has stopped, the suction valve should be closed.
When pumps are kept on rolling RPM with the discharge valve shut, a careful watch should be maintained on the parameters and temperatures.
A lower RPM will generate lesser adverse effects. For prolonged periods, consider shutting down.

Recording during Discharging in Tanker Cargo Log Book:

Following items shall be recorded in Tanker Cargo Log Book hourly.
Discharged Quantity (Rate) to compare it with that of the terminal side,
The pumping performance record / pumping log shall be maintained correctly, the times corresponding to those times written in the port Log Book.
Pump Pressure and RPM,
Manifold Pressure / Temperature,
Draft & Trim
Monitor of levels in tanks not being discharged
The Stress and Stability of the vessel
Tank pressure

a) Charterers are not only concerned with a vesselís sea performance but also their pumping ability and as a result the duration of port stays, therefore, an informative and complete record of the discharge operation is required in order to be able to refute any claims of poor pumping performance.
b) An informative and complete record of RPM, rates, pump discharge pressures and suction pressures as well as pressures shown on the gauges at the manifold / ships rail is required in order to be able to refute any possible claims of poor pumping performance.

Chief Officerís Standing Order:

Cargo plan & instructions from Chief officer should be followed.
Chief officer shall give his written instructions to duty officer in his rest time.
Stress monitoring and print-outs of intermediate conditions shall be recorded during regular cargo operation. Loading computer shall be updated hourly for conditions on board.
Where possible, comparison of real & calculated draft & trim shall be carried out to give proactive warning of any unplanned or unobserved deviation from plan.

Crude Oil Washing

Carry out crude oil washing as per ďProcedures for Crude Oil OperationsĒ when the tank to be washed reaches the planned condition.

Ballasting and De-ballasting

Fill tanks with ballast water according to the ballasting plan.
Consider the precautions decided for heated cargo discharge.
Notify the terminal of the matter before the start of ballasting.
As a company standard, to avoid the possibility of ballast overflow when alongside, BALLASTING LEVEL OF ANY TANKS (WHILE ALONGSIDE) SHALL NOT EXCEED 90% OF THE TANKíS CAPACITY. Such levels shall be marked near the ballast gauges and shall be conspicuously displayed in the control room.

Trim and Draft

Checking water depth as the operation progresses, make a stern trim so that ample trim which is described in the COW manual may be obtained at the time when tank stripping starts.
Ensure the draft maintained, after allowing for tidal variation, is well within the limits of the height limitation of manifold / loading arms. The vessel shall always be maintained well within the operating limits (envelope) of the shore arms.

Stripping cargo

In the last stage of discharging, drop the number and speed of main cargo pumps and gradually reduce opening of delivery valves to change over to stripping. Stripping is conducted by the Auto Unloading System (AUS), Eductors or Stripping pumps.
For further confirmation of effective stripping with remote suction pressure gauge, closed system of manual sounding (dipping) of cargo tanks shall be done by the shipís crew to ascertain the sludge / oil condition of tank bottom.

Survey upon completion of discharge from tanks

(Dry certificate / ROB certificate):
The cargo tanks are to be gauged in the presence of the attending surveyor / berth master to confirm free from pumpable (liquid) cargo.
The vessel is to prepare dry certificate (or ROB certificate, if pumpable liquid), upon completion and the same shall be acknowledged from the attending official.
If available, copy of the surveyors document to be retained onboard.
Any remaining Un-pumpable cargo (by vesselís fixed pumping systems), if found, shall be documented with suitable remarks on the certificate.
Closed method of dipping such cargo tanks shall be followed.

Completion of Discharge

Close the manifold gate valves after confirming the completion of transferring oil to the terminal.
Upon completion of a dry survey by the terminal (receiving) side, drain hoses and arms at the manifold, and disconnect them.
Drain hoses and arms at the manifold. All manifold drain valves are to be operated under the knowledge of the Chief Officer; the duty deck officer must be stationed at the manifold and ensure that the correct valves are opened before confirming to the Chief Officer in the CCR that the valves are opened.
After draining of all oil in pipe lines, close tank valves and vent valves. Ensure connection is depressurized and isolated from the internal cargo tank IG pressure
All cargo in deck cargo lines should be dropped by gravity into a designated tank or tanks. Lines should not be dropped back to the pump room.
In parallel with draining work, measure the temperature and ullage in each tank to work out the loaded quantity.
On completion of gauging and sampling all ullage ports, vapour locks and any other tank openings should be confirmed closed.
Care should be taken to ensure that cargo lines do not become over pressurized due to high ambient temperatures
The IGS recorder shall be switched on to record and monitor the cargo tanks pressure. It shall be suitably marked for details of Voyage Number, date and time of turning on and corresponding present pressure. This record shall be in continuous operation until the final discharge port.

Stopping Inert gas system

Adjust inert gas pressure in tanks, and stop the system when the pressure reaches the prescribed value (normally 1,000 mmAq).

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