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A permanent repair to a closed drain vessel on EnCana Scott recently demonstrated the capabilities of FD Alliance’s advanced composites technology, in a repair designed for a 20 year lifespan that represented significant savings.
The need arose due to internal corrosion in the 3.5 metre diameter vessel, which had caused wall thinning over a 40 by 70mm area and a 15mm diameter through-wall defect. EnCana called in FD Alliance to design a permanent repair using its advanced composites capabilities not only to provide pressure containment (assessed against the vessel’s design pressure of 3.5 bar and operating temperature of 116°C), but also to restore structural strength, and allow for continued growth of the hole without compromising the integrity of the vessel.
Importantly, unlike pipe repairs – for which composites are now more widely accepted and available – vessel repair such as this is far more technically demanding, and is something no other composites repair provider has the specialist expertise to deliver. FD Alliance technical manager Dr Paul Hill explains: “Standard solutions simply can’t be applied in such cases. The repair has to be designed to support both hoop and axial stresses, and being a patch rather than a wrapping process, it involves more challenging engineering in the design. High quality and high performance are all-important.
“Operating pressures and temperatures, installation temperature, substrate type, defect size and shape, repair materials, surface preparation, geometry, extent of sufficient good substrate to overlap onto, post-cure requirements and so on all have to be considered here, as with all composites repairs – but in the case of vessel repair such as this there are additional factors. The laminating, for instance, is a key factor in a successful vessel repair, and requires a far higher, more specialist level of skill than for pipework, beyond the capabilities of most providers. Our NVQ in composites technology ensures that our technicians have the specialist knowledge and expertise required – and their wide experience in using composites for structures from boats to trains and other structural components gives them a strong basis to work from.”
Traditionally where a vessel is concerned the primary option would be shutdown and replacement or welding. FD Alliance’s repair was designed using multi-directional carbon fibre and epoxy resin applied in-situ, and require no prefabrication or hotwork.
The epoxy resin-impregnated carbon fibre plies were built up to the specified thickness using a layering process, following thorough surface preparation using abrasive blasting (critical to achieving the bond required for assured long-term performance) and the application of an e-glass tie-coat. Eleven carbon plies were applied, with a taper and overlap onto good steel in accordance with the repair design calculations, followed by a sacrificial peel-ply layer which was removed once the specified post-cure requirements were complete.
The design provides full pressure integrity even given a through-wall defect of up to 78mm diameter, fully anticipating continued corrosion of the carbon steel substrate during the life of the repair, and ensuring integrity would be maintained throughout.
Commenting on the work, EnCana integrity group leader Henrik Andersen said: “FD Alliance’s ability to design and install this closed drain vessel repair using composites technology brought valuable savings, in that it negated the requirement for both hot-work and vessel entry. The immediate saving by employing composites technology was in the reduction of production down time, compared to welding or vessel entry; in the longer term the saving is that the repair is designed for 20 years.”
FD Alliance business manager Paul Smith adds: “Given operators’ needs to extend the life and gain maximum return from maturing assets in the North Sea, our extensive composites capabilities covering complex geometries and structures as well as pipe repairs, have a valuable role to play.”
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