The Wave Induced Motions of a Tension Leg Platform in Deep Water

Abstract This paper deals with the behaviour of a four column semi-submersible type Tension Leg Platform in 450 m water depth under the influence of regular and irregularlong-crested waves. Results of computations based on three-dimensional potential theory are compared with results of model tests carried out in the Seakeeping Basin of the Netherlands Ship Model Basin. This comparison not only concerns the wave frequency oscillatory motions of the platform and the forces in the mooring tethers but also the mean and low frequency motions induced by second order wave drift forces. Some non-linearities appear to be present in the platform motions in the vertical plane and the forces in the mooring tethers, but in general the applicability of advanced computational methods for predictingin the design stage the motion behaviour and the mooring forces of a semi-submersible TLP in deep water is confirmed. Introduction Offshore exploitation of gas and oil sources has mostly taken place from fixed steel platforms. How deep the steel structures can go depends on the offshore environment. The steel platform built for Shell's Cognac field in the US Gulf of Mexico stands in a water depth of approximately 300 m. Platforms of this size would be prohibitively expensive for the more hostile conditions of the North Sea. As shown by Kypke [1] andCapanoglu [2] the costs of fixed platforms increase exponentially with water depth and severity of environmental forces. It was stated that in general a water depth of 150-220 m may be a practical limit for fixed structures in a harsh environment. As the offshore industry moves into deeper water with severe environmental conditions, the tension leg platform will be one of the economically viable engineering solutions to meet the demand. A tension leg platform is a positively buoyant floating platform, which is secured to the seabed by tensioned moorings which limit the platform motions and provide a restoring force to keep the. platform on. station. Examples of the tension leg platforms are: the Conoco TLP scheduled to operate at the North Sea Hutton Field [3],the Aker tethered production platform [4] and the design of Tecnomare for a water depth of 600 m [5]. In designing a tension leg platform a good known edge of the motion behaviour and mooring line forces is important. In this paper, a numerical method to predict these quantities is briefly described. The results of the calculations are presented for a tension leg platform operating in a water depth of 450 m. Also model tests were conducted both in regular and irregular seas. The results of the model tests were used to judge the applicability of the computation method.