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The sea chest is one of the most important openings in a ship. It lets sea water into the cooling and fire-fighting systems. It is also one of the weak points of the Hull. Sea chest blanking and plugging in Nagoya Port allows these openings to be closed temporarily, so work can be done without dry-docking.
A sea chest is a box fitted into the Hull, usually near the bottom or the side, with a grating on the outside and a valve on the inside. Water enters through the grating, passes through the chest and flows to the ship's pumps. A ship may have several, and they are often placed high and low on the Hull to give a choice of suction.
Reasons include repair or replacement of the valve, a leaking flange or pipe, a damaged grating, a crack in the chest or the need to clean or inspect it from the inside. Blanking closes the opening on the outside, and the chest can then be drained and entered or worked on. Plugging is a similar action used for smaller openings.
A blanking plate is made to match the opening or the grating frame. It is lowered by the Diver and fixed by bolts or clamps over a gasket. Some blanks are designed to be held by the pressure of the water and fastened with studs. The plate must be strong enough for the water pressure at the depth of the opening, and the seal must be tested. We design and check each blank before it is used.
Small openings, such as drain holes or damaged fittings, can be plugged with metal or wooden plugs, rubber plugs or expanding plugs. The plug is fixed and sealed, and the area is checked for leaks. These are quick and often used in emergencies.
The ship's engineers isolate and drain the system, close the valve and lock it. They confirm that no pump will draw from the chest. The Diver does not start until this is confirmed. After the work is done, the blank is removed, the grating is checked and the system is returned to service. A test is made to confirm that everything is working.
A blank is only as good as its seal. We use a second barrier, usually the ship's valve, as a back-up. If the grating or opening is badly corroded or cracked, a blank may not hold, and we report that clearly. In such cases, a dry dock may be needed.
You receive video, photographs and a note of the blank used, the time it was fitted and removed, and the test result.
Q1. What is a sea chest?
A box fitted into the Hull with a grating on the outside and a valve on the inside, which brings sea water to the ship’s pumps.
Q2. Why would a sea chest be blanked?
To repair or replace the valve, flange or pipe, to repair a damaged grating or to inspect or clean the chest from inside, without dry-docking.
Q3. How is the blank held in place?
By bolts or clamps over a gasket, and designed for the water pressure at the depth of the opening. It is tested before use.
Q4. What back-up barrier is used?
The ship’s own valve, closed and locked, acts as a second barrier while work proceeds.
