UMC came up with an ingenious cofferdam design consisting of a bolted hexagonal steel ring, integrated ballast tanks and a flexible membrane that reaches from the ring to the underside of the spudcan. A series of secondary inflatable seals reduced the possibility of leaks. When assembled, the complete cofferdam system measured 16m across by 2m deep. The benefits of the design over an all steel construction meant reduced weight, costs and level of clearance required for installation.

Manufactured and trial assembled in the UK, the entire structure was disassembled, transported to the port of Esbjerg in Denmark and reassembled prior to arrival of the rig. Once the rig had berthed, the complete cofferdam assembly was then brought to the rig, adjusted via the built in buoyancy tanks and fastened underneath. When in position over the spudcan and up against the hull, air was fed into the ballast tanks. The resulting upthrust forced the cofferdam against the hull and compressed the main seal, closing the entire system to the environment outside.

A series of pumps were installed into the leg well from the upper deck of the rig to extract the water. When fully drained, surface workers could safely access the top of the spudcan and carry out the desired NDT and welding in a dry and safe environment.

Since the entire operation on the three spudcans was successfully completed in six weeks, the designers are confident that the same principles can be used for similar repairs, providing a cost effective and practical alternative to drydocking.