Abstract

As a typical complaint offshore structure, the tension leg platform (TLP) has been observed that it is very sensitive to the state of its foundations where the seabed is seismically active. In this paper, a program considering several nonlinearities induced by finite displacements are used for computing TLP responses and tether tensions under random waves. The seismic loads are simulated using IBC spectrum and added explicitly to the position of tether anchorages. Parametric analyses, including wave heading angles, PGAs, ground motion directions, and seismic components in different directions, are carried out in detail to find the major factors. Results indicated earthquakes mainly affect the translational motions of the platform and will lead to sharp change in the heave motion due to resonance in the vertical direction. Earthquakes results a sharp change in tether tensions and play a major role in the coupling effects of earthquakes and waves. The vertical ground motion is the major component of seismic ground motions and determines the changes in tether tensions as it is parallel to the direction of heave motion. The upward ground motion results a sharp decrease in tether tensions, which may lead to an overall buckling of tethers. The downward ground motion results a sharp increase in tether tensions, which is much below the ultimate tensions but still need attentions.

Introduction

Tension leg platform (TLP) is a typical form of compliant platforms, which is often used in deep water and ultra-deep water offshore oil and gas exploitation. In addition to the common surface wave loads, it has been observed that the underwater seabed is often seismically active (Wang K, 2019). A TLP has been observed that it is very sensitive to the state of its foundations where the seabed is seismically active. Under the seismic loads, the foundations of tethers for TLP may move randomly, which may greatly change the dynamic characteristics of such compliant platforms and lead to certain risks of operation failure or even offshore accident. Therefore, dynamic analysis of offshore structures under seismic excitation is necessary and essential to design the structures that have good resistance against strong ground motion.

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