Fatigue Design Criteria for TLP Tendons

A framework for the development of fatigue design criteria for components (joints) of tendons of tension leg platforms (TLP) is presented. Two approaches are considered. First, a target reliability for the tendon (as an element) is specified from which a component target reliability, ${\beta }_0\text{,}$ is derived. The second approach requires that a maximum probability of replacing any tendon during the service life be specified; ${\beta }_0$ is derived from elementary system analysis. The safety check format proposed requires that linear fatigue damage, D, computed by conventional methods using nominal values, be less than a target damage level, ${\Delta }_0\text{,}$ and derived from ${\beta }_0\text{.}$ A simplified approach to the computation of the probability of progressive tendon failure is presented as a mechanism to ensure that ${\beta }_0$ is chosen to be sufficiently large to avoid loss of platform. No specific recommendation is made herein regarding target reliabilities. However, the suggestion is made that the choice of ${\Delta }_0$ not be made without consideration of ultimate strength requirements based on system reliability analysis.