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
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.