Statistical ULS Evaluation of Tension Leg Slackline for Lightweight TLP Platforms in Unidirectional Long Crest Wave Using Linear and Second-Order Potential Flow Solvers

Abstract Lightweight tension leg platforms whose ratio of mass to buoyancy is lower than 0.80 on average have been used commonly as exchange power stations supporting offshore energy production plants. The outstanding difference between mass and buoyancy leads to a massive pretension load endured by the tendons. The massive pretension loads cause notable acceleration at fairleads when the structure is exposed to the range of high-crest wave frequency. The resultant large amplitude motion reduces the reserve buoyancy and correspondingly the pretension loads in tendons which may lead to slackline occurrence throughout tendons. Hence, the ultimate limit state is dictated by the regulations and standards along with the basic design, which imposes load factors for evaluating slackline. Accordingly, the environmental loads such as the 100-year wave record must be taken into account while obtaining 3-hour elapsed tension loads along tendons for statistical analysis and determining the extreme tension loads. Therefore, an opportunity is taken in the present study to compare the numerical results of linear and second-order potential flow solutions for slackline evaluation while the GICON-TLP as a lightweight modularized structure interacts with long crest unidirectional waves. The second-order potential model uses the full quadratic transfer function to consider the second-order wave components derived by the interference of frequency pairs. Moreover, the steady drift force is involved in the pretension load estimation where the linear stiffness of tendons is assumed.