Answer Construction Question 7
Question 7: A. Define the purpose of cofferdams,
B. 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.
Answer: (A) A
cofferdam is a void or empty space between two bulkheads or floors
which prevents leakage from one to another.Normally they are about half
to one frame space width with provision of entry for inspection and also
fitted with an air pipe to relieve any excess pressure. Between DB
tanks the cofferdam height will be up to the tank top and in deep tanks
up to the tank depth
(B) In tankers the pump room placed between the machinery space and the cargo tanks is considered as a cofferdam. In addition some tankers have a cofferdam just aft of the collision bulkhed separating the forepeak tank and the forward oil cargo tank. This space is sometimes used to place the emergency fire pump.
In dry cargo ships the cofferdams are under the main machinery providing separation between the lub oil storage, diesel oil tanks and feed tanks.
(C) (i) Any void space is considered as an enclosed space and may contain inert or noxious vapours depending on the source of leakage into the space. If it is adjoining an oil cargo tank it may contain hydrocarbons. If it is an empty ballast tank or a cofferdam in which water has leaked it will contain carbon dioxide.
(ii) Safe procedure for entry:- When ever a void space or cofferdam is opened for inspection, the following procedure must be followed for safe entry.:
Risk assesment for the entry must be carried out.
The space must be vented using a portable blower for at least 8 to 12 hours .
The space must be tested for oxygen content, using an oxygen meter. If the space is big it may be necessary for the engineer to enter the tank donning an SCBA for checking the oxygen content.
Entry must only be allowed when the safe level of oxygen content in the space is at least 21 percent.
(B) In tankers the pump room placed between the machinery space and the cargo tanks is considered as a cofferdam. In addition some tankers have a cofferdam just aft of the collision bulkhed separating the forepeak tank and the forward oil cargo tank. This space is sometimes used to place the emergency fire pump.
In dry cargo ships the cofferdams are under the main machinery providing separation between the lub oil storage, diesel oil tanks and feed tanks.
(C) (i) Any void space is considered as an enclosed space and may contain inert or noxious vapours depending on the source of leakage into the space. If it is adjoining an oil cargo tank it may contain hydrocarbons. If it is an empty ballast tank or a cofferdam in which water has leaked it will contain carbon dioxide.
(ii) Safe procedure for entry:- When ever a void space or cofferdam is opened for inspection, the following procedure must be followed for safe entry.:
Risk assesment for the entry must be carried out.
The space must be vented using a portable blower for at least 8 to 12 hours .
The space must be tested for oxygen content, using an oxygen meter. If the space is big it may be necessary for the engineer to enter the tank donning an SCBA for checking the oxygen content.
Entry must only be allowed when the safe level of oxygen content in the space is at least 21 percent.
A suitable check list is to be documented and approved by the C/E.
A person on lookout must stand at the man hole opening when person/s have entered the space for inspection.
A person on lookout must stand at the man hole opening when person/s have entered the space for inspection.
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