DEBALLASTING RECOMMENDATIONS As per Capt.J.Isbester, BULK CARRIER PRACTICE, Ch.7 Ballast Management, Nautical Institute, 1993 Opening of airpipes cowls. Before deballasting can commence, the ballast tank airpipes must be open.

​DEBALLASTING RECOMMENDATIONS

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​DEBALLASTING RECOMMENDATIONS

DEBALLASTING RECOMMENDATIONS

As per Capt.J.Isbester, BULK CARRIER PRACTICE, Ch.7 Ballast Management,

Nautical Institute, 1993

  1. Opening of airpipes cowls.

Before deballasting can commence, the ballast tank airpipes must be open. If this forgotten the ballast pumps will begin to labour as they try to remove water from a sealed tank, or the structure of the tank may be damaged as a vacuum is created within it.

  1. Keeping ship upright.

If the ship develops a list when loading and deballasting the effect may be caused by bad distribution of cargo, or it may be a fault with the deballasting, causing one of a pair of tanks to discharge more slowly than the other.

  1. Optimum trim and list.

Achieving a good discharge of ballast requires a combination of competence, thoroughness and good organization, and also a clear understanding of the physical positions of the ballast suction and the sounding pipe within the tank.

It is normal for the ballast suctions in DBT to be situated in the inboard after corner of the tank. To achieve the maximum discharge of ballast from No.2 DBT stbd the vessel should be trimmed by the stern and listed to port, thus tipping any ballast which remains in the tank towards the position where the suction is located. The sounding pipe is situated close to the ballast suction, so that an accurate measure of the depth of water at the suction can be quickly obtained.

The factors which govern the deballasting and stripping on most bulkers are the following:

Good stripping of the DBT takes time and cannot be done effectively at the end of

loading,

when the ship is trimmed even keel or nearly.

The final stripping of the DBT is best done with the ship trimmed well by the stern, and listed 2-3 degrees. The ship can be listed to port, say, using a TST. The loading foreman must be informed when this is done and asked to continue to pour the cargo on the ship’s centerline to avoid an uneven spread of cargo across the ship. Once the ship is listed to port, all the stbd DBTs can be stripped dry.

By the time that the final tank is stripped dry, more water will probably have trickled through the tank floors and side girders in the first tank..

The discharge of ballast is more difficult, final ballast stripping cannot be pumped out and accurate soundings cannot be obtained.

  1. Preventing Ballast pumps from tripping.

To maximize the pump discharge the load (the pump amperes) should be monitored regularly (say, every half hours) and maintained at the makers’ recommended value by adjusting the pump outlet valve.

When the depth of water is reduced to 15-20 cm the flow of water to the suction will start to be interrupted, as the ballast water cannot flow fast enough across the base of the tank, through the drain holes in the floors and intercostals. In consequence the pump will begin to pump a mixture of air and water.

Modern pumps are fitted with degassing devices, which enable to keep pumping when air is mixed with the water.

When air starts to pass through the ballast pump the vacuum is switched on automatically and removes it. When the ballast pump is again pumping water the vacuum pump is switched off automatically. In older vessels, where the ballast pumps are not fitted with degassing devices, the pumps will race and then trip (i.e., cut out, or switch off) when they gas up.

Another cause for the ballast pump to trip is if the ballast suction in the tank becomes blocked with mud – the sediment, which has settled out of the ballast water in the tank.

Sediment in tanks can build up to substantial levels during a period of months or years. Where sediment is suspected of being the cause of a stoppage of discharging the quickest way to clear sediment from around the suction is to flood water backing the tank from another tank with a good head of water, or from the sea. The rush of water into the tank will wash the sediment away from the suction, but this of course, is only a temporary solution. When sediment is causing problems in a tank, traces of mud will probably be seen on the sound rod.

  1. Leaking Ballast line.

So long as the hole in the ballast line is below the level of water in tank, the system will perform normally, but once the water level has dropped below the hole, the system will start to take in air and the pump is liable to gas up and trip.

Such a leak should be suspected if difficulty is regularly experienced in pumping out the last 0.5 – 1.0 metres of ballast in tank, and if the sounding in an after tank falls slowly whilst a forward tank is being discharged, or rises slowly whilst the forward tank is being filled.

The leak can quickly be found by inspecting the ballast line within the after tank whilst the forward tank is full and the tank valve is opened. Under these conditions water will be squirting out through the leak. Repairs to a leaking expansion joint can be effected by tightening or renewing the bolts, or by wrapping the joint temporarily in plastic sheeting.

On most occasions the pump trips because the water cannot flow to the suction fast enough, or because mud is blocking the suction.

  1. Deballasting problems.

Aboard a minibulker which had no stripping pumps and where all deballasting was done with the main ballast pumps, the pumps used to lose suction when the sounding was still 40 cm – the height of the bottom of the lightening holes. The engineers believed that the drainage holes in the floors and side girders were blocked with mud.

The true problem was next: when the pump was pumping at full speed it emptied the small bay where the suction was located in two or three seconds, much faster than the water could flow into the bay through the drainage holes, which were not blocked. The only way that the tank could be pumped dry was by reducing the pumping rate.

  1. Precaution when deballasting.

When ballast is discharged from topside tanks by way of dump valves (otherwise known as drop valves) the process looks after itself. This is convenient for ship’s personnel, but is thought to have resulted in numerous occasions in personnel forgetting to close the dump valves after all ballast has been discharged. It is recommended that the closing of all dump valves immediately on completion of deballasting be checked and logged.

  1. Importance of goods records and thorough, methodical approach.

Records of sounding obtained and of stages in the deballasting should be carefully recorded, so that they can easily be checked by OOW.

When the deballasting of a tank has been started the airpipe should be checked to verify that air is being sucked in, thus confirming that water is being pumped out. Tanks, which have been deballasted and recorded as empty should be rechecked at a later time, preferably when there is a good stern trim.

  1. Ballast residues.

Typical ballast residues for well run ships in the laden condition, as measured by accurate surveys are approximately:

Mini-bulkers – 20 tonnes,

Handy-sized bulkers – 50t,

Panamax-size bulkers – 100t,

Cape-sized bulkers – 200t.

MAINTENANCE OF BALLAST COMPARTMENTS

  1. Removal of mud by shovel and bucket
  2. Hosing of topside and forepeak tanks
  3. Hosing of double-bottom tanks.
  4. High-pressure hosing of double bottom tanks
  5. Use of sediment remover
  6. Removal of scale from ballast tanks
  7. Patching of leaks in ballast tanks
  8. Maintenance of coatings
  9. Inspection of ballast tanks

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