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2.12 SLOP TANK OPERATIONS
Case study
In March 1990, an 80,000 tonne tanker, while anchored in Trieste was
investigated as a possible source of a heavy oil spill which had polluted
both the Italian and Yugoslav shorelines. While an extensive clean up
effort was being mobilised, the ship's side was found soiled with oil
which was identical to that spilled. The investigation showed that on
the previous night the ship had transferred oil between slop tanks.
However, after the transfer, 40 metric tonnes was found to be missing.
There were three valves in the overboard discharge line, all found to
be in good condition. The cause of the spill was inferred to be improper
valve line-up and insufficient checking before the transfer. Clean up
costs, fines and expenses of this incident totalled more than USD 800,000.
Case analysis
Slop tank transfers must be conducted according to a written transfer
plan, with the same degree of planning and attention as a cargo operation.
The plan should include enumeration of the valves to be used, double
check of the valve settings and overside monitoring of the water surface
during the transfer.
Under the provisions of MARPOL 73/78, tank washings and dirty ballast
residues cannot be discharged directly to the sea. It is now mandatory
that effective slop tank procedures be employed by all tankers. Slop
tanks cannot be properly or effectively used unless all equipment on
the vessel's IOPP certificate is maintained in a good and efficient
state.
Large quantities of oil-contaminated water are produced by the tank
washing operation. The slop tank receives the used tank washing water
from the stripping pump discharge line. As the tank washing program
progresses, the tank will fill with a mixture of oil and water. If the
tank is not carefully and regularly checked, it may overflow. The first
material ejected from the tank will be oil, causing a hazardous condition
and possible pollution incident. Slop tank(s) must be carefully monitored
throughout the tank washing operation.
2.12.1 Slop tank capacity
Before tank washing operations begin, the volume of space available
in the slop tank must be determined and a rough estimate made of the
washing time necessary to fill that volume. If the slop tank has a capacity
of 2400 tonnes available and four washing machines are to be used, each
supplying 30 tonnes of water per hour, then the slop tank will be filled
after less than twenty hours of tank washing. The slop tank will fill
at a faster rate than the rate of washing water use, because it also
receives the oil recovered from the cargo tanks along with the washing
water. The slop tank capacity is usually 3% of the cargo carrying capacity
of the ship and tankers over 70,000 DWT must have two slop tanks.
During tank washing, the slop tank must be regularly monitored to verify
the amount of space remaining to receive tank strippings. In automated
tankers, the slop tank level alarm should be set to alert the cargo
control room operator at 90% capacity.
2.12.2 Closed-cycle and open-cycle washing
Most tank washing operations begin with open cycle washing. As the slop
tank fills with tank strippings, it will begin to separate with the
water layer on the bottom and the oil residue on top. After a time,
the bottom of the tank contains water with very little oil and this
water can then be recirculated as tank washing water. If this is done
the washing operation becomes a closed-cycle one.
Closed cycle tank washing has the potential for producing static electricity.
Closed cycle washing should not be used unless the tanks are properly
inerted to less then 8% oxygen. Tank washing water should never be allowed
to free-fall into the slop tank. The slop tank must be filled to cover
the slop inlet pipe before washing begins.
The slop tank level must be regularly checked during closed-cycle operations,
since the quantity of slops will increase slowly as the washing operation
recovers oil residues from the cargo tanks.
2.12.3 Dirty ballast residues
After tanks have been washed for clean ballast and the clean ballast
has been taken in, dirty ballast can be discharged. The last meter or
two meters of dirty ballast in each tank is retained and transferred
to the slop tank. The chief officer must ensure the there is sufficient
capacity in the slop tank before stripping the last of the dirty ballast.
2.12.4 Decanting the slop tank
During tank washing, the method used to decant the contents of the slop
tank depends on the number of slop tanks on the vessel. If the ship
has two slop tanks, a continuous decanting process can be used. One
slop tank is used to receive the tank washing strippings. It acts as
a first stage separator, permitting the bulk of the oil to accumulate
on the surface of the tank. When the first slop tank has reached a sufficient
ullage, the decanting line to the second slop tank is opened. The comparatively
clean water from the bottom of the first slop tank then flows by gravity
to the second slop tank. As tank washing continues, the decanting process
delivers more water to the second slop tank until it contains enough
volume to begin discharging. If the tanker is outside prohibited zones
and can discharge from the second slop tank within permitted discharge
limits, then the second slop tank can be discharged through the oil
discharge monitor to the sea. Depending on the ship's draft and the
level of the overboard discharge, it may be possible to decant the tank
by gravitation. This method is preferred because it makes it impossible
to inadvertently discharge the whole of the slop tank contents overboard.
The level of liquid in the slop tank will be automatically maintained
at or above that of the overboard discharge or the sea level, whichever
is higher.
If pumping is required to remove the decanted water from the second
slop tank, then tank washing may have to be stopped while one of the
stripping pumps is used to decant the tank.
When the tanker has only one slop tank, or is using a single cargo tank
for a slop tank, then the opportunity to perform continuous slop tank
decanting depends on the arrangement of the delivery and suction lines
in the tank. If the tank washing water is introduced into the slop tank
at mid-height and at a point well removed from the slop tank suction
bellmouth, then it may be possible to perform continuous decanting.
If the used wash water is introduced to the slop tank through one of
the regular suction/fill lines and bellmouth, then continuous decanting
is not possible and batch decanting will be required.
Batch decanting of the slop tank involves continuous tank washing until
the slop tank is filled with wash water. Washing is then stopped and
the tank is allowed to settle until the water at the bottom is within
acceptable discharge limits. This will normally take twelve to fourteen
hours. The depth of the oil/water interface is measured using an approved
oil/water interface detector. (Full details of the use of the oil/water
interface detector are contained in the OCIMF Clean seas guide for
oil tankers.)
The slop tank is then decanted through the ODM, by gravity or by pumping.
Slow decanting continues until the water layer has been reduced to the
minimum, or until the permitted discharge limit are reached, whichever
occurs first. Decanting is stopped and tank washing resumes. The washing
followed by batch processing routine continues until all tank washing
liquids and the tops of the dirty ballast tank contents have been processed.
Throughout the slop tank filling and decanting process, the level of
the sloop tank(s) must be closely monitored to ensure that a safe ullage
is maintained.
Slop tank settling and decanting will be less effective if the vessel
is rolling or pitching. If this is the case, it may be necessary to
accumulate wash water in another tank, or to suspend tank washing or
slop tank decanting operations until weather conditions have improved.
2.12.5 Pumproom bilges
Pumproom bilges may be transferred to the slop tank at any time during
the ballast voyage so long as they do not contain any detergents,
cleaners or emulsifiers used to clean the bilges. Pumproom bilges
should be maintained in a dry condition and frequently inspected to
have the earliest indication of any cargo leaks from pipelines, valves,
or from other defects. If the pumproom is fitted with a gas or mist
detection system (recommended), it is important to keep the bilges oil
free. The gas detection system will then give timely warning of any
new leakage to the pumproom. The best way to keep pumproom bilges continuously
dry is by means of a small, permanently installed pump (air or hydraulic),
with a separate suction line and separate discharge line to the slop
tank filling line. Bilges can then be maintained dry regardless of the
ongoing use of the stripping pump(s).
Case history
In 1992, an OBO experienced a pumproom fire while discharging a cargo
of crude oil on the US east coast. The pumproom ventilation was immediately
stopped, all pumproom openings were closed and it was flooded with CO2.
The pumproom was kept sealed for 24 hours until internal temperatures
had diminished and the risk of re-ignition eliminated. The cause of
the fire was found to be an overheated bearing on the ballast pump.
The hot bearing broke off and fell into the bilges, igniting the oily
residues there.
Case analysis
Causes of this incident were possibly infrequent inspections of the
pumproom while running pumps and certainly the oily condition of the
pumproom bilges.
2.12.6 Engine room bilges
Engine room bilge water accumulations should normally be treated and
discharged through the engine room treatment system. If it is necessary
(due to defect in the engine room treatment system) to transfer engine
room bilge water to the slop tank, then the chief engineer must first
obtain the master or chief officer's permission for the transfer. Engine
room bilges should not be transferred if degreasers, or cleansers have
been used to clean the bilges.
Oil resides from the engine room may include heavier petroleum products
than the cargo tank and pumproom resides. Settling and decanting of
the slop tank(s) may be adversely affected by these heavier oils. Engine
room bilges should be transferred only after all other tank washing
and ballasting operations arf complete and the slop tank is ready for
final heating and decanting.
2.12.7 Slop tank heating
Tank washing after a cargo of high pour point or high viscosity oil
may encounter difficulty maintaining the recovered oil in a liquid state.
The slop tank must be continuously heated to permit gravity separation
and decanting and to keep the recovered oil liquid. It will usually
be necessary to maintain maximum heat on the slop tank(s) while tank
washing and decanting to keep the water and oil sufficiently hot. However
as decanting proceeds the heat must be reduced to avoid overheating
the oil. Recovered fuel oil should be heated to not more then 60 °C
and recovered crude oil (except some heavy crude oils), to not more
than 43 °C after removal of free water. High viscosity or high pour
point slops must be continuously heated until disposed of. If they are
allowed to cool, then heavy wax or sedimentation may occur, which cannot
be restored to a liquid at any temperature available by on-board heating.
2.12.8 Load-on-top operations
By a process of heating and careful decanting, the water content of
the slop tank can be slowly reduced until only the top oil layer, the
oil/water interface layer and a shallow bottom water layer are left.
If the characteristics of the slop oil and the nature of the next cargo
permit, the slops may be retained on board and the next cargo loaded
on top of them (see section 2.19). This is normally
done with crude oil cargoes when the same or a similar crude will be
carried on the next voyage. It may also be done with some products provided
the cargo shipper is advised and approves of the procedure.
If there is any doubt about retaining the slops, the master must communicate
this question to the owners and cargo owner or charterer and request
their advice. When requesting advice, the master must advise the quantity
of slops and the nature of their contents (including tetra-ethyl lead
or detergents).
Charterer may wish to LOT, in which case full freight will be normally
paid. If charterer does not wish to LOT, the slops must be segregated
and freight will not be paid on the space/deadweight used. Most charter
parties provide for the charterer to make the decision regarding the
disposition of slops.
Charterers may require the slops to be discharged. The time required
to do this normally constitutes laytime used.
2.12.9 Slops as bunkers
If slop residues are to be pumped ashore for final separation, with
no compensation for the recovered oil, or perhaps for a treatment charge,
owners may be tempted to consider using the slops as fuel. The principal
problems associated with using slops as fuel are safety risks and machinery
damage.
The safety risk is using slops as fuel is due to the flash point of
the slops. If the flash point is above 60 °C, then there is no significant
hazard. Crude oil slops, or petroleum product slops may have a flash
point well below 60 °C and can only be used for bunkers under special
circumstances. The following precautions must be observed to legally
use fuel with a flash point below 60 °C:
Heated fuel tanks must be vented to a safe position outside the machinery
spaces, with weather deck vent outlets fitted with flame screens.
Heated fuel tank drains must empty into spaces ventilated to prevent any
accumulation of oil vapours at temperatures close to the flash point of
the fuel
There must be no sources of ignition near the fuel tank vents or drain
outlets.
Fuel samples should be drawn and tested for flash point before each fuelling.
The consequences of failing to observe these precautions can be costly
or catastrophic. From 1978 to 1983 at least three tanker explosions
were attributed to use of bunkers with low flash points, including one
explosion which killed 76 persons. If port authorities find the atmosphere
in bunker tanks to be in the explosive range, then the vessel may be
ordered to vacate berth, clean out bunker tanks, obtain gas free certificates
and take on new fuel before being permitted to load.
Oil transferred from slop tanks may contain as much as 35% suspended
salt water. Use of such fuel in diesel engines can have the following
effects:
Choking of the exhaust belt and turbine inlet grid.
Rise in exhaust temperature.
Surging of the turbochargers.
Extensive wear of piston rings, liners caused by deposits of sodium which
build up behind the piston rings. This normally results in the top rings
seizing and leaves the lower rings to take the full load. This produces
heavy wear and subsequent blow-by due to micro-seizure, resulting eventually
in cylinder liner failure.
Sticking of fuel oil injection pumps and exhaust valves.
Aggravation of high temperature corrosion if any vanadium is present (see
section 5.42.5).
The ultimate result is an unseaworthy vessel, with all the exposure
to owners interests that such a condition implies.
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