Since 1995, DML Composites (Plymouth, U.K.) has been one of the pioneers in the use of customized wet layup repairs for offshore pipework and other platform component repairs. A project completed by DML in 2000 involved the in-service repair of a steel caisson on a platform in the North Sea. A caisson is a large-diameter pipe or tubular that spans the interval between the topside and the water surface, through the splash zone, and is generally used to lift water to the platform or protect more sensitive piping or equipment inside it.

    “In this case the caisson support was 56 ft/17m above sea level, so the lever arm was quite long, requiring a laminate thickness of about 1.2 inches/30 mm at the top of the caisson,” says DML’s Paul Hill. Because bending stiffness was the critical parameter, due to wave load in the splash zone, the repair was accomplished with a combination of unidirectional carbon fiber tape and stitched multiaxial carbon fabric, wet out with epoxy resin. Laminate thickness was decreased in the lower part of the caisson, where bending loads were reduced. To complete the repair, technicians were required to rappel down from the platform deck on ropes to apply the wet out material to the steel.

    In addition to pipework, DML — in an alliance with Furmanite Worldwide Inc. (Kendal, Cumbria, U.K.), an oil industry service firm specializing in leak sealing and fire protection — has completed repairs on a flare boom, on flare lines with through the wall defects and on steel pressure vessels, such as closed-drain sump tanks.