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An alternative to replacement of corroded sea tubes on the Vanguard class Trident submarines has been developed and applied on HMS Vigilant, using composite repair technology.
The new method, which offers significant time and cost savings, has been identified during the current three year Long Overhaul Period and Refuel (LOP(R)) on HMS Vigilant, being undertaken by Babcock at Devonport Royal Dockyard, and implemented by Babcock’s Composite Technology Group.
The submarines’ 11 sea tubes, typically measuring up to a metre long with a 200mm outside diameter, allow sea water services to pass from the sea through ballast tanks into the pressure hull, via a hull valve. Being surrounded by seawater they are prone to corrosion, particularly in the heat affected zone (HAZ) of flange welds, where variations in metallic property exist.
Typically the sea tubes have been removed and replaced during submarine refits, but manufacture of new sea tubes is time-consuming and complex, requiring manufacture of flanges from high quality forgings (which are then welded on site to fit the submarine profile), and bending of each tube (which is bespoke to its location on the submarine) to tight tolerances. Further, 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 – taking time, and generating a number of health and safety challenges. Moreover, the replacement sea tubes are equally prone to corrosion.
Working together to look at alternative methods, Babcock and the MoD design authority identified the new approach using Babcock’s in-house composite repair technology, which enables rehabilitation of the corroded sea tubes to extend their design life, rather than replacing them.
The benefits are significant in terms of reduced cost, time savings, minimised health and safety issues, and superior long-term corrosion performance. The time taken to refurbish the 11 tubes has been cut by around 75% (from around eight months, to two). Further, the composite repair method offers health and safety benefits, with minimised manual handling in difficult spaces, and a significantly reduced welding requirement (traditionally around 100 hours per tube).
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 applied to the internal surface of the sea tubes) to provide long term corrosion and abrasion resistance. Externally, Babcock’s 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 re-installed. The process is one continuous series of operations and avoids the need for repeated lifting in and out that traditional removal and replacement methods can involve.
The thickness and strength of the composite repair is determined by the design calculations, and the repair design produced by Babcock’s Composite Technology Group was subject to Finite Element Analysis (FEA), carried out by Babcock company Frazer-Nash Consultancy, and validated and approved by the MoD design authority. A number of criteria are taken into account in calculating the carbon thickness required, including the degree of wall thinning caused by the corrosion, the size and scope of the damaged area, design pressures, and the tube geometry, among other factors. Typically a repair of up to 25mm thickness will be applied to restore full strength to the sea tubes for a 15 year life, meeting the requirement to restore the sea tubes to their original design specification, with the added advantage of being non-corroding and therefore maintenance-free for the submarine’s next commission.
This is the first application of composites to restore submarine sea tubes, although the repair method is well proven in other applications (including pipes and other components of offshore oil rigs), and can be validated by standard strength calculations verified by FEA. The primary consideration is to ensure that the original sea tube design criteria are met following the repair.
Babcock Composites Technology Group general manager Graham Clark said: “The composite repair solution we’ve provided to restore the sea tubes offers some considerable time and cost benefits over the traditional cut and replace methods. Moreover, the composites themselves do not corrode, so the repaired sea tubes are now corrosion-free.”
Babcock submarine operations director, Devonport, Gavin Leckie, remarked: “This is an example of the partnering approach being taken on the HMS Vigilant LOP(R), with Babcock working in close collaboration with the MoD, ship’s staff and Rolls Royce Marine. This is driving some new approaches and innovative engineering to better previous performance, save time and cost, improve safety or quality standards, or resolve issues, for example. The new sea tube repair technique has a valuable role to play in helping to maintain schedules and budgets in this LOP(R), and can now be applied in subsequent Vanguard class refits.”
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