Floating assets such as FPSOs, FSUs, FPUs, semi-subs and ships are required to stay longer and longer at sea, without returning to port for dry-docking and hull maintenance. As a result, original build defects in welding, and fatigue cracks in the steel are becoming more common problems in ageing vessels. Whittaker Engineering from Aberdeenshire in Scotland were asked to carry out repairs to a 42-year-old semi-submersible floating production unit which had developed a number of significant fatigue cracks on the inside of the pontoons and bracers below the waterline. These cracks were not leaking yet but were very close to that point.

An Analogy

Picture yourself in an open field with in an air temperature of 5°C. It would feel cold but would be easily survivable in outdoor clothing. Now picture yourself, in the same clothing, in the North Sea with a water temperature of 5°C. You would be unlikely to survive longer than 15 minutes. Your body temperature, and the temperature of the surrounding environment, is the same in both cases. However, the rate of heat loss in water is far higher than in air. The same applies to welding of water-backed steel hulls, so that establishing a ‘normal’ preheat temperature and using conventional welding techniques on the dry side of the plate is not guaranteed to avoid excessive cooling rates and hardened weld deposits.

Onshore Experiments

A 20,000-gallon test tank was filled with water and maintained at 5°C to simulate conditions in the North Sea. The test tank included a 20mm thick insert plate, equal to the hull thickness, containing a realistic defect which was ground to give a remaining wall thickness of 6mm. One of the most important parameters in the welding of structural steels is the time taken for the weld deposit to cool from 800°C to 500°C. Test welds were conducted and thermocouples, connected to a data logger, were plunged into weld metal as it began to solidify, measuring the weld-cooling rate. It was found that conventional electric preheat mats were not sufficient to control the weld-cooling rate. In fact, depending on atmospheric conditions and the remaining wall thickness, electric mats were barely sufficient to prevent condensation at the weld site. A high-power induction heating unit was eventually sourced in Europe, and this proved sufficient to provide 100°C weld preheat, to control the weld cooling rate, and to prevent condensation. In fact, the unit could locally boil the water on the reverse side of the plate!

Welding Procedure Qualification

Armed with the knowledge gained from this experimental work, the company was ready to embark on a qualification of welding procedures for formal approval by the Classification Society. The test tank used for the in-house experimental work was also used for the procedure qualification work. The welding procedure relied on the use of the high-power induction heating plant, and on the use of a specially developed welding technique designed to maximise heat input. The completed welds showed extremely good strength, ductility, toughness and hardness, with no evidence of untampered martensite in the heat affected zone. In fact, the weld properties were found to exceed those usually achieved when welding under normal conditions, easily satisfying Classification Society requirements.

Offshore Welding

The cracks found below the water line were far more numerous than had been originally anticipated. Whittaker Engineering is, five years later, still working to repair them whilst the vessel remains fully operational. Had the vessel been returned for dry-dock repair, the lost operating revenue would have been enormous and not economically viable. Although the onshore trials were very successful, working offshore presented additional challenges. The worksite and induction coils were positioned 75m from the induction heating power supply, necessitating the design and manufacture of bespoke water-cooled extensions cables. The welding and plating work was carried out in a very confined area, requiring a great deal of skill whilst working in uncomfortable and cramped conditions. The same technique was also applied to another FPSO when extensive pitting was discovered in the bottom of one of the cargo tanks, with only 5mm of hull thickness remaining. This timely intervention prevented the need for a dry-docking. Crucially, over the last three years, the hull repairs have not resulted in any production shutdowns.

Global Potential

According to Ken Whittaker, Managing Director of Whittaker Engineering, “this technique, allowing weld repairs to submerged hull structures without resorting to dry-docking, has global potential. The method has already been applied in the North Sea and it is anticipated that other locations, with warmer and calmer waters, will pose fewer challenges. The quality of the repair welds is equal to those of welds carried out under dry-dock conditions, and the welding procedures have been approved by Classification Societies. “Dry-docking is extremely expensive, and this heating and welding technique has great potential for extending the useful lifetime of vessels and structures, the repair of which would not otherwise be economically feasible.”