Crude Oil Wash

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System explained basis my present vessel

Cargo oil tanks are crude oil washed to comply with both legislation (contingency  ballast  requirements) and  charterer’s  requirements  in  order to achieve  maximum  out-turn.  This would  usually  be No.4 COT port and starboard (nominated heavy weather ballast tanks) and one quarter of the remainder. However, no tank requires to be washed more than once in four months, with the exception of ballast requirements.

A programme for the regular crude washing of cargo tanks is to be maintained. Crude oil washing permits the removal of oil fractions adhering to or deposited on the tank surfaces. These deposits, which would normally remain on board after discharge, are then discharged with the cargo. As a consequence, the need to water wash to remove residues is virtually eliminated. Water rinsing will be necessary if the tank is to be used for clean ballast.

A typical crude oil washing program is as follows:

1st voyage

No.1 and 4 COTs and one slop tank

2nd voyage

No.2 and 4 COTs and one slop tank

3rd voyage

No.3 and 4 COTs

4th voyage

No.4 and 5 COTs

Leakage of crude oil from the COW system is a potential fire and pollution hazard. Before use, the system should be pressure tested to working pressure and any leaks made good. Reference should be made to the vessel’s approved Crude Oil Washing Manual.

CAUTION :The cargo stripping pump is a positive displacement pump and therefore must never be used to pressure test the COW line.

During COW operations the system must be kept under continuous observation and the tanks fully inerted. Crude oil washing must be stopped immediately if there are any signs of leakage or a malfunction is detected, or there is a failure of the IG system.

Before commencing COW it is necessary to debottom all COTs, including the slop tanks; this will remove any water that may have settled during transit. Debottoming will considerably reduce the risk of any static charges that may be created during washing.

Where any tank has been used for load on top and it is intended to use them for COW, they should be discharged in their entirety and then recharged.

If the slop tanks are to be used for COW, it is, subject to grade segregation, usually advisable to empty the slop tanks and recharge them with fresh dry crude prior to commencement. The levels to which the slop tanks are recharged are arbitrary, but sufficient ullage, (approximately 11m if using one tank and 8m if using both tanks) is required in the clean slop tank to allow for the cargo pump to maintain suction and the balance line to remain covered.

In the slop tanks the balance line outlet is at approximately 10m above the tank floor level in the starboard clean slop tank, and the inlet 1.5m above the tank bottom in the port dirty slop tank.

This method of COW allows for greater ullage and easier monitoring of the crude oil returns, but it is quite feasible to utilise a single slop tank for the operation, reducing the level occasionally to maintain a safe ullage.

During COW operations one of the major factors in ensuring the tank top and bulkheads are cleaned of residues, sludge etc., is the level of solvency in the COW liquid.  As the period of COWing continues this level of solvency will diminish if only one source is being used. As a number of tanks required to be COW’d as per the charterers instructions (full bottom wash of all tanks and at least two COT top washes) may well be above the minimum MARPOL requirement, it may well be necessary to either discharge the slop tanks and then recharge them after a number of tanks have been completed, or conduct half of the COW with one slop tank, then the other half with the second slop tank. During the transit period to the discharge port it is advisable when hand dipping  the tanks  to gauge  the degree  of sediment,  sludge  etc at the tank bottom. The degree of sediment present can influence the amount of time it will take to conduct a COW of the individual tanks and the level of ROB at the finish of discharge. Therefore when formulating the discharge plan, it may be necessary to take this information into account when setting the stagger for the tanks.

COTs are crude oil washed during discharge by pumping dry crude, at a back pressure of about 0.8MPa, bled from the discharge of one of the cargo oil pumps via the tank cleaning line to the tank cleaning machines.

The eductor is driven by the same cargo oil pump that is being used to drive the COW machines. It is also used to drain the oil fractions from the cargo tank bottom to a slop tank. Good draining is essential during COW operations. The stripping suctions are in wells and a slight port list during draining would be beneficial.


Cold washing (water rinsing) of COTs is required for the following purposes:

•    Prior to the ballasting  of COTs  which  have previously  been crude oil washed, where the ballast is to be treated as clean ballast.

•    Prior to refit, or the inspection of COTs that have previously been crude oil washed.

Procedure for the Operation of the Tank Cleaning System

When preparing the system for tank cleaning, it is important to ensure that all valves are in the closed position prior to setting the lines. A line wash must be carried out before the operation can commence.

a)     Ensure the COT(s) to be washed is inerted and that the oxygen content is less than 8%.

b)     Drain all crude oil from the tank cleaning main to the slop tank by opening a slop tank cleaning machine and one of the cleaning machines at the forward end of the tank cleaning main. Ensure these valves are closed prior to commencement of tank cleaning.

c)     As it is necessary to charge both slop tanks with clean sea water, the spectacle flange between valves 070VCP and 071VCP must be turned into the OPEN position. When the spectacle flange has been turned, keeping both the main sea valves shut, line up the stripping pump from the sea chest to the port slop tank.

d)     Open the slop tank balance line.

e)     Start the stripping  pump  in order  to create  a vacuum  at the inboard side of the sea suction.

f)      Open the inboard sea valve, check for a vacuum, then open the outboard sea valve, sea water will now be drawn through the sea water chest and discharged to the port slop tank.

g)     On the COP to be used for tank washing, a basic line wash is necessary. Line up the COP to take suction from the sea and discharge to the port slop tank via the eductor bypass line.

h)     Start the COP on tank cleaning duty.

i)       When the COP has suction, stop the stripping pump and shut it down.

j)      Charge the slop tanks to a level that is higher than the balance line outlet in the starboard  slop tank, the height of which is located at approximately 50% of the tank volume.

k)     Change over the COP from the sea suction to the starboard slop tank suction, discharging to the port slop tank through the eductor. The eductor is to be used to drain the tank being washed.

l)     Open the stripping suctions on the cargo oil tank to be washed. m)   Open the required tank cleaning machines.

Complete at least one bottom wash, one full cycle and one more bottom wash. The patterns given are for a general wash, the actual duration required will be found with experience. The stripping suction valves in the COTs are in stripping wells, therefore the vessel should be given a slight port list during draining.

n)     Monitor the slop tank ullages and total quantities carefully.

The above method of water wash is entitled ‘Closed Cycle’, and is considered to be the most manageable and controlled method of tank washing.

Closed Cycle Washing

Assuming No.4 COTs are being washed for inspection using No.1 COP as the tank cleaning pump and that the sea chest spectacle flange between valves

070VCP and 071VCP has been turned into the open position.

a)     Commence with all valves closed.

OpenStripping pump suction valve to sea chest064VCP 059VCP 060VCP 073VCP 002VCP 034VCP 037VCP
OpenStripping pump discharge valves to port slop tank065VCP 068VCP 024VCP
OpenOpen the slop tanks balance line129VCT 130VCT

b)     Start the stripping pump. When a vacuum shows on the suction side of the pump.

OpenIntermediate sea valve070VCP

c)     When a vacuum shows at the sea chest.

OpenMain sea valve071VCP

d)     Monitor and verify there is a positive flow to the port slop tank, then set up No. 1 COP to prime the slop tanks.

OpenNo.1 COP discharge line to both slop tanks015VCP 016VCP 023VCP 024VCP 021VCP 031VCP
OpenNo.1 COP suction valve003VCP

e)     Start the COPT, when at minimum speed open the pneumatic discharge valve.

OpenNo.1 COP discharge valveM01VCP

f)      When the rpm is steady and flow is established into the slop tanks, stop the stripping  pump and close the stripping  pump suction valves to the sea chest and discharge to port slop.

CloseStripping pump suction valves to sea chest002VCP 073VCP 060VCP 059VCP 064VCP
CloseStripping pump discharge valves to port slop tank065VCP 068VCP

g)     Fill the slop tanks until there is sufficient water in each to cover the levelling line outlet in the starboard slop tank.

h)     Change the cargo oil pump on tank cleaning duty to draw from the starboard slop tank cleaning suction and discharge back to the port slop tank via the eductor.

OpenStarboard  slop  tank,  tank  cleaning  suction valve032VCP
OpenEductor inlet valve and outlet valves020VCP 022VCP
OpenEductor suction valve063VCP
CloseEductor bypass valve021VCP
OpenEductor discharge valve to port slop tank023VCP 024VCP
CloseInboard and ship side sea valves070VCP 071VCP

i)       The vessel is now ready to carry out tank cleaning, using cold wash water from the slop tanks or, if hot water is required, the tank  cleaning  heater  and slop tank  heating  coils  will be required.

j)     Open  the  tank  cleaning  line  isolating  valve  019VCP  and 201VTC.

k)     Open  the  eductor  suction  into  No.1  line  and  the  stripping suction valves for No.4 COT.

OpenEductor suction valves to No.1 line060VCP 073VCP
OpenNo.4 COT stripping suction valves116VCT 118VCT
OpenNo.4 line bulkhead valve001VCP

l)     Open the individual tank cleaning machine isolating valves on No.4 COT.

OpenTank cleaning machines on No.4 COT013VTC 030VTC 031VTC 032VTC

No.4 cargo oil tanks are now being water washed on a closed cycle. Maintain the required  pressure  for the tank cleaning  machines  and eductor  drive by regulating the tank cleaning pump discharge valve or the speed of the pump.

As an alternative, any of the cargo pumps could be used to supply the drive fluid to eductor and tank cleaning machines.

Hot Water Wash

A tank cleaning heater is fitted in the top of the pump room capable of heating sea water from 20°C to 80°C with a throughput of 160m3/h. In addition heating coils are fitted in both slop tanks. Eight sets of coils are fitted to the port slop tank and are capable of heating the sea water contents from 15°C to 66°C over a period of 24 hours with a sea water temperature of 5°C and air temperature of 2°C. The starboard slop tank has two coils fitted

The closed cycle method of cleaning as described above, would be required for hot washing.

Slop Tank Heating Coils

Both slop tanks are fitted with heating coils in multi-tiers with specification as follows:

DescriptionPort Slop TankStbd Slop Tank
Maximum steam consumption (kg/h)10835650
Heating coil surface area (m3)11923
Heating coil ratio0.05050.0098
Heating coil length (m)513.692.1
Length  per  downcomer  and  exhaust combined (m)4847
Volume 50% (m3)1185.01185
Volume 98% (m3)23242324

The slop tank heating coils steam supply is fed from the engine room 0.6MPa system.

Each tank is fitted with a steam and a condensate header. There are drain valves on both headers which are used to test the quality of the condensate returns.

The condensate from the slop tank heating coils is led back to the feed filter tank through the atmospheric condenser and an inspection tank in the engine room.

The normal method of testing the coils is simply to crack steam on to the system and test the quality of the condensate returns.

WARNING :Water hammer in steam lines can be a problem resulting  in possible damage  to  the  pipe system  and  even  steam  line  failure  resulting  in scalding of personnel. It is essential that all steam lines are drained of condensate and that steam is supplied to cold lines gradually with line drain valves open. This allows the steam line to warm through and for the condensate to drain.

Procedure for the Operation of the Slop Tank Heating System

All valves and drains are closed.

a)     Open  the main condensate  return  valve  to the engine  room, directing the slop tanks heating condensate to the atmospheric condenser.

b)     Open  the  deck  steam  master  valve  to the  slop  tank  heating coils.

c)     Open the steam header drain valve, then crack open the header steam isolating valve. When the drains run clear close the drain valve.

d)     Open the condensate drain valve on the condensate header fully on each coil.

e)     Open the steam inlets to each coil and warm through each coil slowly, until the steam header isolating valve is fully open.

f)     Check the drain cocks on each coil for any contamination.

g)     When all the drains have run clear, open the condensate header return isolating valve.

h)     Open the condensate outlet valves from each coil then close the drain valves.

If traces of oil emerge at the condensate drains valves, inform the chief officer and shut off the steam supply to that coil.

i)     Monitor the observation tank for contamination.

Shutting Down the System

a)     Shut off all individual tank steam and condensate valves.

b)     Open drain valves to prevent a vacuum forming which could draw in oil through any pipe defects.

c)   Close drain valves when the coils have reached ambient temperature, this in order to prevent any ingress of sea water during heavy weather.

d)     Close the main supply and return valves.


If contamination should occur at the observation tank proceed as follows:

a)     Check the condensate drains on each slop tank and locate the defective coil.

b)     Isolate the defective heating coil and insert blanks in the steam inlet and condensate outlet lines.

c)     Proceed to heat the tank using the other coils.

Testing the Coils

The modern  materials  and the continuous  welded  construction  used in the heating coils tend to offer reliable service. Routine testing by checking the condensate outlet when putting the system into use will normally suffice. However, pin holes can develop at welds and loose pipe brackets can cause fretting.

If contamination occurs, test the defective coils at the next possible opportunity. This is done by supplying steam to the coil with the outlet valve closed, making a tank entry and locating the leakage.

A permanent repair will, in most cases, require welding. This would be carried out during refit. A near permanent repair can be carried out by cutting the coil in way of the defect and inserting a Yorkshire coupling.

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