4 General guidelines for discharge of dirty ballast and the decanting of slops at sea (section 7 of Manual)

  4.1.1 Discharge of dirty ballast at sea must always he performed under strict control of pumping and in compliance with Regulation 9 of Annex I of MARPOL 73/78. Procedures to ensure that the discharge is restricted to permitted limits should be observed as follows:

• .1 Before discharging the dirty ballast overboard, flush main cargo lines to be used for discharging the dirty ballast into the slop tank.

• .2 Before flushing, prime the system, establish suction, stop the pump and close all valves and allow the oil to separate out of the pipe walls.

• .3 Resume pumping after half an hour at moderate rate with output throttled on the discharge side of the pump.

• .4 Commence to discharge dirty ballast.

• .5 Reduce discharge rates from individual tanks on approaching a water depth of about 20 per cent of the tank depth.

• .6 Thereafter reduce pumping rates to avoid drawing surface oil into the suction by vortex or weir effects. Observe carefully the trend of the oil content monitor reading.

• .7 Stop discharge of individual tanks when a level has been reached which, for the particular ship, is known not to give rise to any entrainment of oil. When all dirty ballast tanks have been discharged to this level, all discharge overboard must cease.

• .8 The officer in charge must verify that the slop tank can take the volume of dirty ballast remaining. If ullage is insufficient, the slop tank may be partially discharged (see paragraph 4.2 of this section) to provide the necessary capacity taking care to ensure that an adequate depth of water remains beneath the oil residue layer.

• .9 Transfer the remaining dirty ballast into the slop tank, using the stripping system.

• .10 Transfer to the slop tank the contents of the pump-room bilges and any other bilges connected to the cargo stripping system.

• .11 Flush the stripping system, which will then be dirty, into the slop tank.

  4.2.1 During pipe flushing oil residues will collect in the slop tanks together with water. These residues have to be processed using conventional retention on board techniques. After proper decanting of the water content of the slop tank the residues may remain in the slop tank or may be transferred to a cargo tank and new cargo loaded on top of them or they may be pumped ashore if preferred by the owner/charterer. If cargo is loaded in the slop tanks it is important that there is sufficient ullage to receive the water flushings when the dedicated clean ballast system is cleaned.

  4.2.2 Decanting of the contents of the slop tank is a critical step in the retention of oil on board. Hence the timing of the various steps in the operation is important. Even a short delay in stopping a pump or closing a valve can allow oil to escape into the sea. The time required for oil and water to separate in the slop tank depends upon the motion of the ship as well as on the type of previous cargo. Under favourable conditions a few hours may be enough, but in most circumstances 36 hours or more should be allowed. Discharge from the slop tank must cease well before the interface is reached to avoid discharge of any oil-in-water emulsion overboard.

  4.2.3 Before starting to decant the contents of the slop tank, an accurate interface and ullage reading, using an oil/water interface detector, must be taken to determine the depth of the oil layer.

  4.2.4 Although every effort should be made to remove as much water as possible from the slop tank, the prime objective is to prevent oily water reaching the sea. Extreme care is therefore necessary, and a close check must be kept on the overboard discharge.

  4.2.5 Agitation of the contents of the slop tank must be kept to a minimum to avoid drawing oil into the suction by vortex or weir effects, particularly as the oil/water interface approaches the top of the structural members in the tank bottom. Pumping rates must be strictly controlled. The following detailed procedures should be followed:

• .1 Cargo lines to be used for decanting slop tanks must be flushed as outlined in paragraphs 4.1.1.1 to 4.1.1.3 of these guidelines.

• .2 Pump down the slop tank using one main cargo pump at slow speed until a water depth of about 20 per cent of the tank depth is reached.

• .3 Stop the cargo pump, then take an oil/water interface and ullage reading and re-calculate the remaining water depth.

• .4 Resume pumping of the slop tank, this time using the stripping system, until a predetermined water depth is reached which, for the particular size and construction of the slop tank, is known not to give rise to discharge of oil. Pumping, which may initially be at a moderate rate, should be slowed as this predetermined water depth is approached.

• .5 Observe carefully the trend of the oil content monitor reading.

• .6 If oil should appear before the predetermined water depth is reached, stop pumping.

• .7 Should this occur, further settling of the slop tank contents should be allowed for as long as possible before repeating the steps given above.

• .8 Any decanting beyond this limit must be carried out with extreme care and by strictly observing the oil content monitor reading. When the trend of the monitor reading indicates that the interface is being entrained, the discharge must be stopped immediately.

  4.3.1 After these operations have been completed the lines and pumps used will contain traces of oil. The lines and pumps which will be used to discharge ballast must therefore be thoroughly flushed into the slop tank, or while the ship is still more than 50 nautical miles from the nearest land and outside a special area thoroughly flushed to sea, ensuring that the permitted instantaneous rate of discharge of oil and the permitted total quantity of oil discharged are not exceeded.

  4.3.2 As a final preparation, pumps that will be used for the discharge of the arrival ballast should take suction from each tank containing arrival ballast for a short period of time.