Heidrun Is a Tension Leg Platform (TLP) structure with sixteen 270m long tendons, fabricated from flush-ground girth-welded tubulars. The tendons are subjected to cyclic loading in service; fatigue was therefore considered with respect to reliability and in-service inspection. As a part of a comprehensive qualification test programme, fatigue tests were conducted on typical production tendon girth welds in order to demonstrate their fatigue performance. This paper presents the fatigue endurance test data, both from flush-ground production girth welds and from flush-ground girth welds containing deliberately-introduced embedded defects. The fatigue crack growth rate behaviour of the parent materials (pipe and forging) in a seawater environment Is also presented.
In 1995, Conoco Norway Inc. (CNI) completed the development of the Heidrun field in the Haltenbanken area off the coast of Norway. The Heidrun Tension Leg Platform (TLP) is situated in 345m deep water, and is attached to the seabed using sixteen 270m long tendons, each fabricated from 12m lengths of steel pipe, with an outside diameter (OD) of 1118mm and a wall thickness of 38mm. The pipes are joined together by two-sided flush-ground girth welds, and the final connection to the seabed is made through a series of forged connections. All the welds are subjected to a comprehensive NDT procedure to ensure that they are free from any significant flaw. An operational objective for the Heidrun tendon system is that it should be engineered to limit the need for extensive inspection during service. In order to meet this objective, particularly with respect to tendon girth welds, the fatigue performance of the tendon system was assessed and considered a principal factor. The European fatigue design classification for girth welds made from both sides is Class E (UK Department of Energy, 1990), but enhancement of this category to Class C is claimed to be possible by flush-grinding the weld cap.
