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Before 1978, one of the tanker procedures which has most frequently resulted in operational pollution of harbours was taking ballast following the discharge of cargo. In 1978, the International Maritime Organization adopted the MARPOL 1978 Protocol. This comprehensive anti-pollution agreement includes requirements for certain ships to be provided with ballast tanks, pumps and pipelines which are completely 'segregated' from the ship's cargo systems. Ships whose ballast systems meet the applicable requirements are certified as Segregated Ballast Tank (SBT), tankers.
The SBT tanks are required to be protectively located on all 'new' crude carriers over 20,000 DWT and all 'new' product carriers over 30,000 DWT. The protective location is intended to reduce accidental pollution due to collision or stranding. The SBT tanks can contain sufficient ballast to achieve an amidship draft in meters of 2 + 0.02 x LBP, with the propeller immersed. This is enough for all but hurricane or typhoon conditions if course and speed are properly adjusted in heavy weather.
Additional ballast can be taken in the ship's cargo tanks if the master thinks it necessary. This is done by means of a special pumproom cross-over connection (removable spool piece or blind), between the ballast and cargo systems. Heavy weather ballast carried in cargo tanks which have been crude oil washed but not water washed must be handled as dirty ballast.

2.1.1 SBT pollution
SBT is not a fail-safe means of preventing pollution. Petroleum cargo can enter the SBT tanks through bulkhead leaks. SBT tanks can also be contaminated by cargo leaking into ballast piping which passes through cargo tanks, or by cargo leaking out of cargo piping which passes through ballast tanks. To maintain the maximum pollution safeguards while using SBT, routine operational checks and piping integrity tests must be carefully followed.

Bulkhead leaks or ballast piping leaks can be sources of SBT contamination producing pollution and explosion hazards.

Case study
In June 1987 and March 1989, two different vessels commenced taking on ballast water into segregated tanks by gravity (without use of the ballast pumps), soon after commencement of discharge of cargoes of crude oil. Unknown to the vessel's crews, the ballast lines had fractured within the cargo tanks through which they passed. Instead of ballast water flowing into the ship, the greater head pressure of oil in the cargo tanks caused about 25 barrels and 35 barrels, respectively, of oil to escape via the segregated ballast sea valves into the harbour.

Case analysis
If the vessel had maintained a program of ballast line integrity checks on each ballast voyage, the defects should have been discovered and the pollution incident avoided. If there is any doubt about the integrity of ballast lines on a ship, then ballast should only be loaded by pumping. In this case, t he ballast tanks would have later been found contaminated by cargo, but that would have been an easier problem to fix than a pollution incident.

2.1.2 SBT operations in the discharge port
Before taking on ballast at the discharge port, it is advisable to vapour test the atmospheres of the ballast tanks. Before taking on SBT, open the tank filling valve a few minutes prior to opening any other valves. Then inspect the ballast tank for any oil ingress as stated. Presence of hydrocarbon vapours in a ballast tank will indicate either a bulkhead or pipeline leak. Whatever the cause, the ballast piping to the affected tank will have to be flushed, and the tank washed before it can be used for clean ballast. The tank may be ballasted without washing, but the ballast will have to be handled as 'dirty ballast' when it is discharged. The required entry must be made in the oil record book.

2.1.3 SBT contamination procedures
Once any contaminated ballast tanks have been suitably cleaned and ballasted, the vessel may depart for sea.
After the vessel is at sea, the cargo tank(s) adjacent to the contaminated ballast tank can be washed, ventilated and entered to examine the adjacent ballast tank bulkhead for leaks. (Keep in mind that some cracks will leak with pressure on one side, but will be forced closed by pressure on the other side - 'one-way leaks'!) If the leak is located, the ballast tank can be emptied and the leak cold-patched with composition adhesive. The location of the leak must be reported to owners/managers for permanent repair at the earliest opportunity. After the ballast is discharged at the loading port, the ballast tank can be ventilated and entered and the other side of the leak temporarily repaired, if necessary, before cargo is loaded.
If no bulkhead leak can be found, and particularly if the contamination appears in more than one ballast tank, a pipeline leak should be suspected. Test for a pipeline leak by leaving the ballast suction/fill valve to an affected ballast tank open during the ballast voyage and look for an accumulation of water in the empty cargo tanks. The water will indicate the location of the leaking pipe section. When the leak is located the cargo tank must be washed and ventilated and the affected section of pipe clamped to restore ballast system integrity until a permanent repair can be made. The installation of the pipe clamp, the type of clamp used, exact location of the leak, and the names of personnel who made the temporary repair must all be recorded in the preventive maintenance program record. The leaking pipe section must be scheduled for a permanent repair at the earliest opportunity, not later than the next shipyard period.

2.1.4 SBT precautions
Normally SBT tanks are filled while the cargo is being discharged, and emptied while cargo is being loaded. SBT provides an economic saving by permitting the vessel to be able to sail immediately after completing discharge of cargo. Without SBT, when the cargo has been discharged, the ship would be in a 'hogged' condition, with the amidship empty and the heavier bow and stern structures tending to arch the keel of the ship. To prevent these stresses, ballast will normally be taken in the amidship segregated ballast tanks first. The forepeak and afterpeak ballast tanks are filled last.
SBT hatches must be kept closed when handling cargo or ballast to avoid drawing hydrocarbon vapours into the tanks. (SBT tanks are not normally connected to the IGS system.)
Do not overflow the ballast tanks in port: it may wash oily residues from the deck overboard, causing pollution.
Vessels which ballast in rivers or shallow harbours tend to accumulate sediment in their ballast tanks. Tanks with sediment accumulation should be washed and if necessary entered and descaled on the loaded passage. All confined space entry procedures must be followed while cleaning ballast tanks.

2.1.5 SBT corrosion
Many SBT tanks are fitted with sacrificial anodes to protect the uncoated steel structure. The chief officer must remember that the anodes only protect those parts of the structure which are completely immersed in ballast water. When ballast tanks are filled they should be pressed-up to the deckhead after departure, and maintained 100% full throughout the voyage. Any slack (ullage) in ballast tanks will promote accelerated corrosion of the under-deck structure.

Reduce ballast tank corrosion by pressing up ballast tanks to the deck while at sea.

2.1.6 Ballast tank icing
Vessel's entering Canadian, Norwegian, or other high-latitude waters during the winter season should replace any fresh water ballast with salt water to reduce the freezing point of the ballast. In extreme cold weather, when even salt water ballast may freeze, tanks should be left slack. Before discharging, fill the tanks sufficiently to break up any ice against the under-deck structure. This will prevent the ice from forming on or clinging to the under-deck area in large chunks which may damage ullaging equipment or ladders when falling to the tank bottom.

2.1.7 Written ballast plan
The chief officer must prepare a ballasting plan for each loading and discharge port. The plan may be part of the general loading or discharge plan, in which case it should be set out in a separate section. The plan is prepared by considering the berth draft and freeboard restrictions and stresses on the hull resulting from the off-loading plan. The plan should be issued to the watch officers before arrival at the port so that it can be reviewed by them and discussed as necessary.

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