Reliability analysis of a tension leg platform in a tension leg platform in a sea depth of 5000 ft has been undertaken. Prior to the reliability analysis, deterministic parametric analysis of the maximum tendon group force using Chakrabarti¹s non-linear stretching approximation in evaluating the wave kinematics up to the free water surface was conducted. The reliability implications of calculating the wave kinematics up to the free water surface rather than to the mean water level have been explored.
Reliability parametric studies of the metocean variables (using failure of a tendon under axial and hydrostatic load as the limit state) have also been undertaken. The statistics of the maximum tendon group forces have been studied using both regular waves and irregular waves and the resultant cumulative distribution functions from FORM have been approximated by normal, lognormal, and Rayleigh distributions using least squares method to establish the appropriate linear relations. The implications of reduction in the tendon¹s axial stiffness due either to foundation effects or longer tendons (sea depths greater than 5000 ft) have been investigated.
Higher harmonic forces due to second order forces lead to excessive forces in the tendons if multiples of the wave frequency coincide with either the heave or pitch natural frequency. The Rayleigh distribution is the best approximation for the cumulative distribution function (CDF) of the maximum group tendon force estimated by FORM for regular wave analysis; whereas the lognormal distribution gives the best approximation for FORM CDF for irregular wave analysis.