Published on 22 January 2013
The need to attach studs to submerged platforms has led to interest in friction stud welding - a process capable of operating under water. Thompson Welding & Inspection Limited developed a prototype underwater welding machine, and TWI undertook trials to demonstrate welding 19mm diameter steel studs to carbon manganese steel plates. One application for which the machine was subsequently used was to retrofit sacrificial anodes, where a tapered stud was friction welded to the base substrate through an electrical take-off strap.
In TWI's laboratories, the first phase of the underwater friction welding study was carried out in a one cubic metre water tank over which a vertical friction stud welding machine was mounted. The laboratory equipment was used to weld simple studs to plates before leading to selection of workpiece design, to satisfy electrical connection requirements.
Here, the concept of friction welding a tapered stud through a strap representing the electrical take-off lead from the anode was investigated. Welds were made in air, in water at atmospheric pressure and in water at a pressure of 21 bar for a range of stud diameters and stud/strap end preparations. Various materials such as polystyrene, polyurethane foam and monocast nylon were used to provide a protective shroud for many welds made in water.
In Phase 2 a similar series of trials was undertaken using the special underwater machine. Practical limitations dictated that only the welding head was totally immersed and pressurised in the hyperbaric chamber. The primary objectives of this particular phase were to establish the operational characteristics of the equipment and to demonstrate that weld properties comparable to those obtained in Phase 1 could be achieved.
A natural progression to site trials, at Loch Linnhe in Scotland, formed the programme for Phase 3. After total immersion of the whole welding facility, an appraisal was made of its operating function at various water depths, down to the maximum available of 150 metres. A colour television camera with recording facility was positioned to monitor the weld performance at depth. Selected welds were assessed to ensure no undesirable changes were introduced as a result of complete immersion of the machine.
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