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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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