Key errors leading to inaccuracies in draught surveys

The West of England P&I Club has said that it regularly receives claims arising from alleged shortages of solid bulk cargoes as a result of discrepancies between draught survey results. Draught surveys by their very nature are not an exact science and the accuracy of the calculated cargo figure may vary typically by between 0.5% and 1.0%. However, there are occasions when considerable differences in the cargo figures derived from the draught surveys have been obtained.

The commonly forgotten weights are:
– Bilge water present in the cargo holds, machinery spaces, duct keel, void spaces and the chain lockers.
– Swimming pool water.
– Anchor and anchor cable on the seabed, either when at anchor or alongside and an anchor has been deployed as part of the mooring arrangement.
– Silt and mud can accumulate in the double bottom tanks of vessels regularly ballasting in rivers or estuaries. – The extra weight can give rise to an apparent increase in the cargo loaded.

Squat is the bodily sinkage and trimming of a vessel making way with limited underkeel clearance. However, a vessel alongside a river berth may also be susceptible to squat, leading to inaccurate draught readings. Squat will occur when there is limited space for the water to flow between the vessel and the river bed.

In order for the river water to pass between the flat bottom and the river bed, there will be an increase in velocity of the water flowing past the hull, and consequently a decrease in the water pressure. The decrease in pressure leads to the ship bodily sinking and also a change of trim. 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.

Other areas that can lead to errors in the draught survey calculation include:

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.

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.

Read the Inaccuracies in Draught Surveys document in full: West-of-England-Club-Inaccuracies-in-Draught-Surveys-2018_05

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