Composite tube repair

Rust buster

Who

Engineers at the Devonport Royal Dockyard in Plymouth have developed a composite repair technology as an alternative to the replacement of corroded sea tubes on the Vanguard class Trident submarines.

Technology

The method has been used during the current three-year overhaul and refuel programme on HMS Vigilant at Devonport. The submarines’ 11 sea tubes, typically measuring up to a metre long and with a 200mm outside diameter, allow seawater to pass from the sea through ballast tanks into the pressure hull via a hull valve. The tubes are prone to corrosion, particularly in the heat-affected zone of flange welds, where variations in metallic property exist.

Typically, the sea tubes have been removed and replaced during submarine refits but this is time-consuming and complex and requires the manufacture of flanges from high-quality forgings and bending each tube to tight tolerances. The removal and replacement process typically involves lifting the sea tubes in and out by crane at least four times to make sure they fit correctly, a time-consuming and risky process.

The team at Devonport has developed an in-house composite repair technology, which enables rehabilitation of the corroded sea tubes to extend their design life, rather than replacing them. The sea tubes are lifted out of the submarine, cleaned and weld-repaired to ensure they are watertight. They are then glass-coated on the inside (using a glass flake vinyl ester technology) to provide long-term corrosion and abrasion resistance. Externally, the carbon fibre and epoxy resin composite technology is applied in a layering process to restore both hoop and axial strength and the sea tubes are then reinstalled. The process is one continuous series of operations and avoids the need for repeated removal and refitting of the tubes.

A number of criteria are taken into account in calculating the carbon thickness required, including the degree of wall thinning caused by corrosion, the size and scope of the damaged area, design pressures, and the tube geometry. Typically, a repair of up to 25mm thickness will be applied to restore full strength to the sea tubes for a 15-year life.

Application

This is the first application of composites to restore submarine sea tubes. The approach has delivered considerable time and cost benefits over traditional cut-and-replace methods, and the composites themselves do not corrode. The time taken to refurbish the 11 tubes has been cut by around 75% from around eight months to two. The composite repair method also offers health and safety benefits, with minimised manual handling in difficult spaces, and a significantly reduced welding requirement.