Fatigue Analysis of a Tension Leg Platform: Fatigue Life Improvement

Abstract

A fatigue life analysis of a Tension Leg Platform (TLP) was performed; concretely it was verified how the fatigue life (F.L) of a TLP is affected by means of structural modifications in critical fatigue areas. The main aim is quantify the effect of such modifications to guarantee a good fatigue design without over-dimension certain structural elements. For the study, a standard design of TLP is used to obtain realistic results, exposed to waters of the North Sea. A Stochastic fatigue analysis using linear wave theory was considered. A Potential Flow Solver combined with Morison Formulation coupled with a Finite Element solver, is used to determine the pressure distribution generated by the waves on the structure and mooring line loads. The loads on tendon elements are neglected in this study. After determined, loads are transferred to the Global model and the fatigue study is performed to identify the critical fatigue areas. For the structural assessment, a quasi-static analysis is used since dynamic effects are not to be so relevant in this particular case of study. Once critical areas are identified, a local analysis is developed considering the hydrodynamic loads from the global model and the boundary conditions are applied in terms of displacements. From the local model, the hotspots are identified and the fatigue life is determined. Since known the loads and boundary conditions to be applied in the local model, structural modifications of the identified critical areas are proposed and the effect of such modifications in the F.L is quantified. In addition, the effect of surface treatments in the F.L is also verified. The treatments considered for the study are Grinding, TIG dressing and Hammer peening. It is expected that when increasing the thicknesses of certain structural elements, the F.L will tend to increase proportionally to such increments. Nevertheless, the aim is to quantify in an accurate way the cyclic life improvement to provide a guideline to guarantee a good fatigue design without over-dimensioning, valid for TLP’s to be installed in the North Sea