Nonlinear Response of Tension Leg Platform Subjected to Wave, Current and Wind Forces

Abstract

Tension leg platform (TLP) superstructure and submerged areas are being subjected to wind and wave forces during their lifetime, yet the magnitude of each respective energy density affects the TLP response. The present work aims to study the influence of possible load combinations on the response of the TLP. In order to achieve this objective, the Joint North Sea Wave Project (JONSWAP) spectrum and Simiu and Leigh wind spectrum were used for the computation of wave and wind spectral densities. Airy wave theory and Morison equation were adopted for the calculation of wave kinematics and forces on the platform. The variable submergence, drag force in the Morison equation, tension fluctuation from large displacement were the sources of nonlinearities considered. Subsequently, mathematical formulation for the dynamic analysis of intact and a tether-missing TLP was developed in FORTRAN software. The nonlinear equation of motion was solved using Newmark-Beta scheme coded in the software. From our findings, the sea state with high significant wave height and wave period suppressed the effect of current and wind loads on the TLP motion response as compared to the sea state with low sea state parameters. The effect of wind drag force increases the surge and heave responses, but the overall influence of the current drag depends on the magnitude of the wave energy spectrum. There was no abrupt behaviour when a tether was missing although the loss of tether’s stiffness slightly affects the TLP motion and tension variation.