In this study a variation of the environmental contour technique is used to examine the sensitivity of basic reliability estimates for an idealized Tension Leg Platform (TLP) tendon subject to random sea excitation generated using the Ochi-Hubble and Torsethaugen spectral models. It is recognized that the response of offshore platforms is significantly influenced by the spectral shape as well as the storm severity. Often operational and storm seas are multi-peaked and this study examines the response of deepwater tendons whose natural period of vibration is in the range of peak spectrum energy. Initially, the primary peak is varied along the environmental contour, reflecting a specified return period, while the secondary peak is held constant at a natural period of vibration of the tendon. Reliability estimates are then computed for a deterministic displacement limit state using Monte Carlo simulation. The limit state is chosen with typical tendon spacing as a guideline. Then the most probable location of the secondary peak given the first is determined. Simulation is performed along the contour and a second reliability estimate made for this case. In the second part of the study the sensitivity of the reliability index as a function of the second peak is examined. Comparisons of these reliability estimates to estimates made utilizing typical single peaked JONSWAP spectrum generated seas are presented and discussed.
As the next generation of compliant platforms are being designed there is a need for more accurately characterized random seas. Compliant platforms are dynamically sensitive and their response can be significantly influenced by the shape of the wave elevation spectrum. The use of a spectrum with a single peak can be viewed as an over simplification of many offshore environments. Recent statistical work has shown that "true" ocean spectra have as many as four peaks (Borgman 1993).
