TLP Hull Fatigue Analysis Due to Sum-Frequency Wave Effects

The paper outlines the use of Quadratic Force Transfer Functions to compute hull response for a Tension Leg Platform, TLP, caused by the fast varying Sum-Frequency wave effects. The direct linear wave effects are also included in the procedure. Previously, this method has been applied for the analysis of tether fatigue by the first author on several occasions. This paper describes how the tether fatigue procedure is extended to cover important structural details at any point on a TLP structure. Since Sum-Frequency effects have been shown to have a pronounced effect on tether fatigue one was concerned that the fatigue life of certain critical structural elements on the TLP hull could also be affected by this form of wave loading. Steel decks on a large concrete TLP was identified as an area of concern. The computational procedure, developed for tether fatigue analysis, was extended to convert TLP hull rotational accelerations, caused by Sum-Frequency excitations, to deck stress transfer functions. These deck transfer functions were then used in a spectral fatigue analysis along the same lines as for the tether fatigue analysis. A highly useful concept of designing against fatigue damage was arrived at during the course of the work. Given the required lifetime, material properties and the type of structural detail to be analyzed, the computer code was enhanced to iteratively compute the maximum allowable Stress Concentration Factor. Knowing the required SCF of given areas, the designer would then apply the necessary care to detailing in order to satisfy this requirement. As expected, Sum-Frequency effects were found to be of minor concern to fatigue damage for the analyzed structure.