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Large quantities of water are often found in crude oil cargoes. Some of this water is a result of poorly drained ballast piping on CBT ships. It is important that the CBT bottom lines be thoroughly drained after CBT discharge is completed. Since the lines cannot be vented by leaving the ballast tank suction valves open the following procedure is recommended:

  • Complete CBT discharge and close all valves.

  • Align stripping pump suction valves to take suction on the CBT main suction line.

  • Start stripping pump slowly and observe the suction gauge.

  • When stripping pump has developed a vacuum on the CBT line, open a tank suction valve on the forewardmost cargo tank connected to the CBT line.

  • Continue running the stripping pump at slow speed until the stripping discharge pressure drops or it is otherwise noted that the pump has lost suction.

  • Close all valves.
  • The pipeline strappings should never be discharged overboard even if the stripping pump, associated piping and forwardmost cargo tank have been thoroughly flushed before the vessel arrived in port. There are too many opportunities for previously undetected oil residues to be discharged with the final ballast withdrawn from the ballast tanks or pipeline. The most prudent procedure is to strip the pipelines ashore, or to the vessel's slop tank.

    Stripping of pipelines is important because the salt water residues from the pipelines, if left in the cargo tanks, can:

  • Increase tank bottom corrosion.

  • Seriously affect the quality of certain grades of cargo.

  • Cause damage to the receivers shore plant equipment.

  • Distort cargo measurement calculations.

  • Waste freight earning capacity.

  • Require special precautions with certain grades of cargo, such as static accumulators and highly heated cargoes.
  • Fresh water flushing of some pipelines or tanks may be required after ballast discharge to eliminate the possibility of salt contamination of the cargo.

    Cargoes where this is advisable include:
    Crude oils
    Tia Juana
    Black oils
    Virgin gas oil
    Cut feedstock
    Aromatic concentrate
    Aromatic tar
    RFD extract

    3.4.1 Contaminated ballast
    If ballast contamination has been discovered, the ballast cannot be discharged overboard and must be either retained on board or pumped to the shore facility. Most shore terminals have some facilities for receiving slop or ballast residues, but this capability depends on the capacity of the shore holding and processing facilities and problems such as dealing with residues containing lead (TEL).
    Even if clean ballast appears to have no contamination, it should never be discharged overboard entirely. The last half meter or meter of ballast innage should be stripped ashore or to the slop tank.

    3.4.2 Slop tank gauging
    When all ballast operations have been completed, the slop tank should be carefully gauged and an interface reading taken to determine quantities of oil and water contained. If the slop tank is to be used for load on top (LOT), operations, as much water as possible should be stripped ashore from the bottom of the slop tank, leaving little or no emulsion under the oil residues. After the tank has been stripped of water (pumped to shore), it must be gauged again and a 'Certificate of slops' prepared by the master. The slop tank may be gauged by the independent petroleum inspector prior to loading.
    If the slop tank will not be loaded into, then its contents must be disposed on in accordance with the provisions of MARPOL 73/78.

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