In-depth | WELDING
Water-backed welding offers potential solution to FPSO maintenance issues
A UK-based company believes that the challenges inherent to using an alternative to conventional below the water line maintenance on FPSOs can be overcome
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s an FPSO ages, an increasing number of defects can start to develop in the hull structure and
where these are found in areas below the waterline they can be especially challenging to address. As drydocking may not always be a financially, or operationally, viable option for fixing defects found below the waterline, alternative methods of addressing such issues are actively being sought within the industry. A specialist FPSO repair contractor
based in Scotland, Marine Technical Limits (MTL), has recently undertaken extensive research and in-house welding trials to better understand welding afloat with a water backing – and its associated challenges – as a solution to this problem. Danielle Milne, mechanical engineer at MTL, says, “Te development and application of new techniques and technology is crucial as the industry ventures into further flung locations and deeper water to keep up with the world’s demand for energy.” FPSO units have played a key role in
extreme offshore areas for many years. However, harsh offshore environments, vessel motions and production demands can put FPSO structures under stress. Ineffective inspection and maintenance regimes not only negatively impact the safety of the asset but can introduce onerous and unnecessary costs. MTL has identified that water-backed
welding could be a viable solution to the on-site maintenance problems faced by FPSO operators. Milne notes: “As it is not a commonly used or well-established welding process, there is limited classification society code guidance on whether it could be a viable option. But at MTL we empower our team to tackle these challenges, develop groundbreaking concepts, appraise them through rigorous trials and testing, and ultimately deliver them to the market.”
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MTL has carried out extensive trials on water-backed welding techniques
MTL’s research into the concept
confirmed that water-backed welding creates more demanding conditions compared to conventional welding processes. At the same temperature, the water acts as a greater heat sink than air, which significantly increases the cooling rate of the weld. Tis rapid cooling results in a harder, more brittle weld that is not considered compliant with welding rules and standards. Terefore, it is not possible to achieve a suitable weld without altering the welding process or introducing pre-heat, a process which is against some classification
society rules, during water-backed weld procedure qualification. A thorough examination of the welding
process and the specific alterations required was needed to render this technique feasible. Trials were undertaken using a bespoke welding test tank connected to an industrial water chiller unit, allowing the simulation of the conditions and flow rates found offshore, such as water temperatures below 0°C. Throughout these trials, welding variables such as the welding process, position, technique, consumable and steel composition, as
Shiprepair & Maintenance 4th Quarter 2018
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