Influence of the setdown of a tension leg platform on the extreme airgap response

One distinguishing characteristic of a tension leg platform (TLP) is the setdown of the hull when the platform moves in its compliant modes (surge, sway and yaw). The nonlinear setdown has profound implications in various aspects of TLP design, but this paper focuses on its impact on the available airgap. Although there is general consensus that setdown should be included in the airgap assessment, to date there is no systematic procedure of analysis, and related literature is scarce. This paper aims to develop a simple method for incorporating setdown in the extreme response prediction of the airgap. The proposed method requires only the covariances of the platform motions, and these are available from a frequency domain analysis. From a case study, the crossing rates calculated by the proposed method is found to be in good agreement with Monte Carlo simulation, with only a slight disparity at high threshold levels. This work has also afforded physical insight; for example, it is discovered that the wave-frequency (WF) motions are more critical to the extreme airgap compared to the low-frequency (LF) motions, because the surface elevation is correlated with the WF motions, but not with the LF motions.