5.37 SLOP TANKS

When discharging a full cargo of one grade, a slop tank will normally be used to receive the tank strippings. If the vessel has arrived with the slop tank(s) filled with cargo, a portion of the slop tank contents must be discharged with the main cargo pump before it can receive tank strippings.
Sufficient cargo must be discharged to provide space for the anticipated stripping recovery, plus an allowance for safety. On many occasions, the stripping operation will recover more cargo than anticipated. This could cause a slop tank overflow if only the minimum space has been provided to receive strippings.
If the slop tank is fitted with an adjustable high-level alarm, it should be set to sound at the level which provides sufficient time to stop the stripping pumps before an overflow. The chief officer should decide on the level to set the alarm and indicate the level in the cargo orders or night orders. Enter the setting of the adjustable hi-level alarm in the log book. Slop tank fixed hi-level alarms should be checked for correct operation before beginning transfer of cargo to the slop tank.
The accuracy of the remote ullage indicator for the slop tank must be verified against a hand ullage measurement before, or soon after, stripping begins. Enter the completion of this check in the logbook.
The slop tank is normally the last tank to be discharged ashore. Enough cargo should be retained in the slop tank during stripping so that the main cargo pump can be used to discharge it after all other cargo tanks have been stripped. When the main cargo pump has discharged the tank to the lowest practicable level, the remainder is stripped ashore using the MARPOL small-bore stripping discharge line.

5.37.1 Closed cycle COW operations
In some cases, COW operations will be completed using oil from the slop tank and recirculated back to the slop tank. This 'closed cycle' washing technique is usually effective except when washing waxy (parafinic) crudes. Experience has frequently shown that when tanks are stripped or educted back to the slop tank during COW operations, that the crude in the slop tank will become progressively more waxy to the point that continuing to use it for COW operations will serve no useful purpose.
For COWing wax-rich crudes in cold climates, it is recommended that not more than four tanks be COWed by closed cycle washing unless the contents of the drive tank are then discharged ashore and it is refilled with new oil.