Seismic assessment of tension-leg platforms

Tension-leg-platforms (TLPs) are becoming increasingly popular for offshore drilling at deep-water sites. Seismic response of TLPs is of significant importance in the design or assessment of these platforms since seismic excitations, especially the vertical component, can directly affect the total tensile force of tendons. This paper focuses on the impact of seismic ground motions on the TLPs, especially on tendons. The first part of this paper discusses the development of an advanced methodology for seismic assessment of TLP structure considering the following effects; pile-soil interaction, soil radiation damping, soil degradation due to cyclic loading, current effects, hydrodynamic damping, and marine growth. The second part of the paper presents a case study to demonstrate application of the developed methodology. Two types of analyses are carried out; response spectrum analysis for extreme level earthquake (ELE) and dynamic nonlinear analysis for abnormal level earthquake (ALE). The response spectrum analysis is carried out using SACS and the time history simulations are performed using nonlinear dynamic structural analysis program, CAPFOS. Multiple interactions among soil, fluid and platform is numerically quantified in this study, providing insights into the response of TLPs under complicated dynamic loading conditions.