Fatigue assessment of offshore jacket structures: A case study

Abstract

Offshore fixed jacket structures are vulnerable to fatigue damage from hazardous environment in North Sea. In this thesis, an offshore jacket structure was checked for the ULS 100- year return period of wave and checked for FLS using a long-term stress distribution. Sesam software was used to perform analysis for both the limit states. Ultimate limit state analysis shows satisfactory results, but the deterministic fatigue limit state results are not satisfactory for the members and joints in splash zone of the offshore jacket structure. Therefore, the thesis work is extended to see the effect on fatigue life by changing some of the fatigue parameters. Fatigue parameters considered for checking the effect are hydrodynamic property, joint types on structure and SN curves for tubular joint. It was observed that, no significant improvement in fatigue lives even after changing the fatigue parameters. Therefore, Stress concentration factor (SCF), which is the most sensitive parameter in estimation of fatigue life of tubular joint is studied further in this thesis. The SCF parameter is applied on each joint to determine the hot-spot stresses on the intersection region between the chord and brace on a tubular joint. Efthymiou (parametric) equation validity is first checked manually for one of the critical joint on offshore jacket based on the fatigue analysis results. Then the factor is calculated manually and compared to with the SCF calculated from Framework analysis. Discussion and conclusions are made at end of this thesis.