5.31 STRIPPING PIPELINES

The final part of the discharge operation, but one of the most important, is stripping the remaining cargo from the pumproom piping and cargo piping. Draining the top, or deck, cargo lines is critical on a non-SBT tanker which will use its cargo pumps for ballasting. Failure to effectively strip the top lines will create a head of oil in the lower pumproom system what may cause a pollution incident when the sea valve is opened.
All of the upper cargo lines will require draining. Apart from the last cargo line used, all lines should be dropped back to the accumulation tank prior to line stripping. If this is not done, the line stripping operation will take considerably longer than necessary.

Pump and line drainage arrangement and 'small-diameter' drainage discharge line.

When cargo discharged has been completed on a cargo system, the pipeline system should also be discharged ashore. The ideal arrangement for this operation is a stripping system which includes a separate, small diameter discharge (MARPOL), line to the midship manifold, with connections to each cargo manifold outboard of the manifold valve. If that arrangement is available, the following procedure can be used:
• Close the manifold valve.
• Open the stripping pump suction connection to the main cargo line in the pumproom.
• Open the deck drop valve on the main line.
• Open the main cargo pump bypass valve(s), (the pumproom loading/recirculating valve(s)), in the pumproom.
• Open the small diameter, (MARPOL), stripping discharge valve outboard of the manifold valve.
• Verify that the dock manifold valve is open.
• Start the stripping pump and run it at slow speed.

Method of draining and stripping cargo lines at completion of discharge. 1 Stop discharge (MCP). 2 Close manifold valve (A). 3 Open main cargo pump bypass valve(s) (C). 4 Open suction valve to accumulation/slop tank (D). 5 Open cargo line vent valve at manifold (B). 6 Open stripping suction to accumulation tank 'E'. 7 Strip accumulation tank ashore via small-bore MARPOL line and valve)(G). 8 When tank is empty close valves (D) and (E); open valve (F). 9 Strip line residues ashore. 10 Stop stripping pump, close all valves. 11 Note: If cargo is hi-pour or freezing weather is expected, the MARPOL line must immediately be drained back to the slop/ accumulation tank.

All of the lower pumproom lines containing cargo should be lined up to the stripping pump, the manifold valve closed and the MARPOL line set to discharge ashore outboard of the main manifold valve. As soon as the stripping pump is registering a vacuum, the cargo line vents should be carefully opened at the manifold crossovers. Have a bucket or other containment under the vents to catch any spillage. The crew member assigned to open the vents should remain at the vent valves during the whole line draining operation.
It is not unusual to find that the MARPOL line has become plugged during the MCP discharge due to one of the MARPOL line manifold valves leaking. This usually happens when discharge pressures have been high. The line should be checked several times during discharge and drained back if necessary. The leaking manifold valves should be repaired or replaced during the ballast passage.
To ensure that the forward sections of the main cargo line are emptied, the following procedure can be used:
• After the stripping pump has completed enough strokes to have emptied the main cargo line deck and deck drop sections, close the manifold vent valve.
• Allow the stripping pump to develop a partial vacuum on the main cargo line.
• Open the forward-most tank suction valve on the main cargo line.
• Continue operating the stripping pump at slow speed until suction is lost.
• Stop the pump and close all valves.

Upon completion of line stripping, the only cargo remaining in the vessel should be in the MARPOL line. It is important to fully drain the MARPOL line after the cargo inspection, particularly whe.re waxy or high-pour crudes were discharged.
If the ship does not have the independent stripping discharge connections to each manifold, it will not be possible to effectively strip the main lines individually. In the case of a straight cargo, the lines can be stripped after all cargo systems have completed discharging and the stripping discharge directed to one main line, so that it is the only one containing cargo when the discharge is finished.
Precise communication is required between the ship and the shore during cargo line stripping. The ship should not say that 'discharge has been completed, we are now stripping lines'. On more than one occasion, shore valve have been closed after receiving such a statement, making it very difficult for the ship's stripping pumps!
While the pipelines are being stripped, the cargo tanks should be checked again to ensure that no cargo has drained back into any tank.
Stripping lines effectively may take as long as one hour, but it may increase outturn quantities by as much as 600 barrels on a 100,000 DWT tanker.