Sea Technology

JAN 2017

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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Page 43 of 76 January 2017 / st 43 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 min. The constants in both cases are your body temperature and the temperature of the surrounding environment. How- ever, the rate of heat loss in water is far higher than in air. The same applies to welding of water-backed steel hulls. If normal preheat temperatures and conventional welding techniques are used on the dry side of the plate, this leads to excessive cooling rates and hardened weld deposits. To ad- dress this, we have developed a welding technique that can be performed at sea on submerged hulls, thus eliminating the need for dry docking. Onshore Experiments A 20,000-gallon test tank was filled with water, and the water temperature was maintained at 5° C to simulate conditions in the North Sea. The test tank included an in- sert plate, equal to the rig hull thickness, containing a realistic defect that was ground to give a remaining wall thickness of 6 mm. 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 solid- ify, measuring the weld cooling rate. It was found that conven- tional electric preheat mats were not sufficient to control the weld cooling rate. Depend- ing on atmospheric conditions and remaining wall thickness, electric mats were barely suf- ficient to prevent condensation at the weld site. A high-power induction heating unit was eventually F loating assets such as FPSOs, FSUs, FPUs, semisubs and ships are being required to stay for increasingly longer periods at sea without returning to port for hull maintenance and dry docking. As a result, original build defects in weld- ing and fatigue cracks in the steel are becoming more com- mon in aging vessels. Whittaker Engineering, based in Aberdeenshire, Scot- land, was contacted by oil and gas operator EnQuest to carry out a number of repairs to a 37-year-old semisubmers- ible floating production unit that had developed a number of significant fatigue cracks on the inside of the pontoons and bracers below the waterline. These cracks were not yet leaking but were very close to doing so. An Analogy Picture yourself in an open field with an air temperature of 5° C. It would feel cold, but would be easily survivable in outdoor clothing. Water-Backed Welding Procedure In-Situ Repair Work Prevents Dry Docking of Vessels By Ken Whittaker Whittaker Engineering's water-backed weld- ing technique allows weld repairs to sub- merged hull structures to be completed with- out resorting to dry docking.

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