The magnitude of the bodily sinkage and change of trim is difficult to quantify, however, it will be a function of the underkeel clearance, speed of the water, the cross section of the hull in relation to the cross section of the river (blockage factor) and the shape of the hull (full form vessels are affected more).
Generally full form vessels will trim by the head, and fine line vessels will trim by the stern. If a vessel is passing at the time that the draughts are being read, this may exacerbate the error due to the increased blockage of the river and consequently the vessel may trim and sink more than normal.
FURTHER FACTORS
Other areas that can lead to errors in the draught survey calculation include the following:
The hydrostatic data and tank sounding tables may not be accurate due to changes to the ship’s structure.
On most vessels the ballast tanks suctions are located at the aft end of the tanks, as the normal convention is for a vessel to be trimmed by the stern. However, when trimmed by the head, although a ballast pump may have lost suction indicating that the tank is empty, there may be un-pumpable ballast pooled in the forward part of the tank. In addition, if the sounding pipe is
towards the aft end of the tank and the vessel is trimmed by the head, then the sounding may also show the tank to be empty, when that is not the case.
Leaking tank valves – when a tank has been sounded and found to be empty, subsequent ballasting / de- ballasting operations could lead to water leaking into the tank, similarly, when ballast tanks are overflowed during discharge operations to show that they are full, water could leak out of the tank prior to the draught survey taking place.
Whilst on passage, depending on the nature of the cargo, water could migrate from the cargo to the cargo hold bilges and be subsequently pumped overboard. If water is found to be accumulating in the bilges while at sea, accurate records of the quantity of bilge water pumped overboard should be kept to help counter any subsequent cargo shortage claim.
Some vessels are susceptible to unsymmetrical hull deflection, this particularly affects smaller vessels due to their larger engine room in relation to their size which pushes the location of maximum sag forward of amidships, leading to inaccurate draught readings.
On older vessels in particular, problems can be encountered due to set in shell plating between the upper wing and double bottom ballast tanks. This can lead to actual displacements less than shown in the hydrostatic data for a given load
42 | The Report • September 2018 • Issue 85
draught, therefore there can be less cargo onboard than expected.
The position of the hatch covers can affect the deflection of the hull, particularly on smaller vessels when the open hatch covers may be stowed at one or both ends of a single hold. For draught surveys on all vessels it is recommended that the hatches remain closed to ensure consistency.
Whilst alongside any bunkering, taking of potable water, and discharge of sludge or oily water should be considered when completing the final draught survey. Where there is any notable difference in the constant between loading and discharge, then this is an indicator that there is an error somewhere in the draught survey, and this should be thoroughly investigated. Throughout a cargo voyage, the constant should be, as the name suggests, fairly constant.
In the event of a cargo discrepancy arising as a result of a draught survey, deck officers are advised to consider the possibility that one or more of the foregoing factors may have contributed to the difference. In particular any substantial change in the constant should be viewed with suspicion and investigated.
Reprinted by kind permission of West of England P&I Club, Tower Bridge Court, 226 Tower Bridge Road, London SE1 2UP. Telephone: +44 (0) 20 7716 6000 Web:
www.westpandi.com
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