Question

Define the purpose of cofferdams.

State where cofferdams are most likely to be found on :

(i) Dry cargo ships;

(ii) Oil tankers.

(c)

(i) State what information is available about danger of entering void spaces.

(ii) Identify, with reasons, the precaution to be observed before and during entry to cofferdams.

ANS

a) The purpose of cofferdams are to prevent leakage of one tank or double bottom to another ie, there would be a cofferdam between fuel tanks and fresh water tanks. They also allow for inspection of tanks, cofferdams should also be sounded regularly to check for leaks.

b)

i) On dry cargo vessels, cofferdams would be found between fuel tanks and cargo, ballast tanks and cargo, also found around the main engine LO drain tank.

ii) On oil tankers, cofferdams would be found between the oil cargo tanks and the accommodation or engine room, in some cases this can be the pump room, also there will be a cofferdam around the main engine LO drain tank.

c) Precautions taken before entry into an enclosed space are as follows; An entry permit must be obtained from a responsible officer, you may be the issuing officer. The space is to be well ventilated and oxygen levels tested by an 02 meter. Safety gear, such as self contained breathing apparatus sets must be at hand, rescue lines, communications. Also while anyone is in the enclosed space, someone must be on standby at the entrance who must be in communication with the people in the space and also must be able to raise the alarm if they are to get into difficulties.

Official guidance on entry into enclosed spaces can be found in "M" notices and also in the "Code of Safe working practices".

CAVITATION

The thrust of a propeller varies approximately as the square of the revolutions. Thus as the speed of rotation is increased there is a considerable increase in thrust. The distribution of pressure due to thrust over the blade section is approximately as shown in Fig

The net pressure at any point on the back of the blade is the algebraic sum of the atmospheric pressure, water pressure and negative pressure or suction caused by the thrust. When this suction is high at any point, the nett pressure may fall below the vapour pressure of the water at water temperature, causing a cavity or bubble to form on the blade. This cavity is filled with water vapour and with air which disassociates from the sea water. As the blade turns, the bubble moves across the blade to a point where the nett pressure is higher, causing the cavity to collapse. The forming and collapsing of these cavities is known as cavitation.

When the cavity collapses, the water pounds the blade material, and since the breakdown occurs at the same position each time, causes severe erosion of the blades and may produce holes in the blade material several inches deep. Cavitation also causes reduction in thrust and efficiency, vibration and noise. It may be reduced or avoided by reducing the revolutions and by increasing the blade area for constant thrust, thus reducing the negative pressure. Since cavitation is affected by pressure and temperature, it is more likely to occur in propellers operating near the surface than in those deeply submerged, and will occur more readily in the tropics than in cold regions.