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Some repairs need a weld that is as good as one made in a shipyard. When the damage is below the waterline and the ship cannot dock, a cofferdam can give the welder dry working conditions. Cofferdam installation and dry welding in Nagoya brings workshop quality to a hull repair afloat.
A cofferdam is a steel box that is fitted against the Hull and sealed with a gasket. Once the box is in place, the water inside is pumped out, leaving the damaged plate exposed to the air. Welders can then work from inside the box in normal conditions, using standard welding methods. When the work is done, the box is flooded and removed.
Cofferdams are used for repairs that need good weld quality, such as cracks, insert plates, doublers and the renewal of damaged areas. They are especially useful on the Hull side near the waterline, and in some cases on the bottom with a specially designed chamber. The class society often prefers a dry weld for permanent repairs.
The cofferdam is designed to fit the curve of the Hull and to resist water pressure. Its size depends on the repair, the depth and the ship's shape. The Diver takes measurements, and the box is made or adapted on the surface. A sealing gasket runs around the edge. At the berth, the work is planned around the ship's loading condition and the tide.
The box is lowered by crane or lifted by a workboat. Divers guide it into place and fix it with clamps, bolts or magnets, depending on the design. A pump removes the water, and the seal is checked. If the box leaks, the Diver adjusts the seal. When it is dry and safe, the welders enter.
With the plate dry, the surface is cleaned, defects are removed and the weld is made to an approved procedure. Welders work in good light and normal conditions. The weld can be inspected with standard methods, including visual checks, dye penetrant or magnetic particle testing, and ultrasonic testing where required. The surveyor may attend and witness the work.
A cofferdam is a confined space, so ventilation, gas testing and entry procedures are needed. Structural safety is also important, since a large box carries heavy water pressure. We check strength, seal and supports before pumping out. Tanks behind the plate must be gas-free if hot work is planned.
Preparation takes time, usually a few days. Installation and pumping out take hours. The welding depends on the repair size. We give an estimate after the inspection.
Q1. What is a cofferdam in ship repair?
A steel box fitted against the Hull and sealed with a gasket, which is then pumped dry so welders can work on the damaged plate in normal conditions.
Q2. Why use dry welding instead of wet welding?
Dry welding gives workshop-quality welds that can be inspected by standard methods, and class societies often prefer it for permanent repairs.
Q3. What inspection methods are used after dry welding?
Visual checks, dye penetrant or magnetic particle testing, and ultrasonic testing where required.
Q4. What safety checks apply to a cofferdam?
Ventilation, gas testing, entry procedures, structural strength and seal checks before pumping out, and gas-free tanks behind the plate if hot work is planned.
