Effect of High-Frequency Response on TLP Tendon Fatigue

High-frequency vibrations of Tension Leg Platforms (TLPs), commonly known as ringing and springing, have challenged TLP designers since the first full-scale TLP was installed in the North Sea in 1984. Although current design codes recognize the significance of the ringing and springing response for tendon design, no widely accepted modeling approach for their calculation has yet emerged. This paper presents a nonlinear time-domain model of a TLP that exhibits the ringing and springing response of the vessel. The analysis model uses large displacement theory for the vessel and tendons and a semi-empirical wave model based on a modified linear wave theory. Predictions of vessel motions and tendon loads made with the analysis model were compared to model tests and were found in good agreement with the measurements. The analysis model was also was used to investigate the fatigue damage in the tendons caused by the vessel’s high-frequency response. Tendon stress time histories were computed for nine different unidirectional sea-states. These sea-states represent a condensed wave scatter diagram for the Gulf of Mexico (GoM). The tendon fatigue was calculated from the stress histories by rainflow counting. Fatigue contributions from different frequency ranges were identified by Fourier analysis. The analysis showed that high-frequency response was present in all sea-states even though ringing occurred only in sea-states with significant wave heights above 10 ft. Tendon fatigue damage contribution from high-frequency loads were found to be significant in every sea-state. For all sea-states combined 73% of the up-wave tendon fatigue damage was due to high-frequency response. For the down-wave and the cross-wave tendons, the high-frequency contributions were 57% and 34%, respectively. This paper demonstrates the importance of considering high-frequency response for the fatigue design of TLP tendons. Another finding of the study is that the analysis model using a modified linear wave theory can describe the ringing and springing behavior of a TLP provided other significant nonlinearities of the system are considered.