System reliability in design and maintenance of fixed offshore structures

摘要

Offshore structures are usually redundant, and several components must fail before structural collapse occurs. This work deals with system reliability for fatigue and overload of typical steel truss and frame structure such as jackets and jack-ups. The application of system reliability in design and maintenance is addressed. A formulation for a failure sequence of fatigue and overload is presented. The basic failure event for fatigue, is defined in terms of time to section failure. For overload, realistic failure criteria are applied. After an overload failure, the member replacement technique is used to account for the post collapse behavior. The branch and bound algorithm is used to identify important failure sequences, and system failure occur if any of these collapse sequences occur. The first order reliability methods are efficiently used to calculate the failure probabilities. In this work the importance of uncertainties in load and ultimate strength for a jacket and a jack-up structure is discussed. A joint environmental model for wave and current loading is applied. Ultimate capacity is calculated by nonlinear pushover analysis, and the uncertainties identified by Monte Carlo simulation. Fatigue may be an important failure mode for offshore structure, and in-service inspections are frequently performed. Underwater inspections are, however, very expensive and it is therefore important that the inspections are performed in such a way that they significantly increase our knowledge about the safety of the structure. In this work an inspection importance factor is defined. This importance factor can be used to identify the most important members in the system from an inspection point of view. Inspection for unexpected damages are also discussed. 122 refs., 72 figs., 50 tabs.