Fatigue and overload reliability of offshore structural systems, considering the effect of inspection and repair

Systems reliability methods are very important for offshore structures, since these structures usually are redundant, implying that several components must fail before structural collapse occurs. The problem is, however, that the traditional way of describing systems failure in a reliability sense, i.e. by a set of limit state functions, is very time-inefficient. A system reliability formulation, in which the whole system is described by a single continuous limit state function, is thus proposed. Correlations are properly accounted for by this formulation. Three structural systems are investigated, i.e. a TLP tether system, a catenary mooring system for a stationary located storage tanker, and a pipeline for transport of oil and gas. For the tether system example, failure is assumed to be caused by both fatigue crack growth and extreme stresses. Two possible failure modes with respect to overloading are considered, i.e. failure of all tethers simultaneously due to a single extreme load, and failure of one tether per extreme load. A framework for reliability updating is also provided, both with respect to inspection results and known tether failures. Effect of fabrication errors are investigated as well. The catenary mooring system example is analysed with respect to failure due to extreme line tension and fatigue. Effects due to both low-frequency vessel motions generated by waves and wind, and wave-frequency vessel motion due to waves are considered. In the pipeline example the failure mode is assumed to be yielding due to extreme external pressure. Correlation properties between pipeline components and the effect of pressure reduction with increased distance to pumpstations are studied.