System failure probability of offshore jack-up platforms in the combination of fatigue and fracture

As it is known, fatigue is the process of damage accumulation in material due to the stress fluctuation. The failure occurs when accumulated damage exceed critical level. In practice, uncertainties are incorporated in fatigue estimation especially in the loads and in the capacity of structures. Reasonably these uncertainties can be incorporated within reliability framework or probability context. It is possible to estimate the fatigue reliability based on crack propagation in fracture mechanics and also using reliability context. However the first failure of structural element under fatigue degradation may not lead to the collapse of system, an approach to combine in the system failures is needed. aper presents a new approach to estimate structural system failure probability of jack-up platforms in combination of fatigue and fracture. The probability of failure for each component is initially estimated using typical fatigue and fracture failure scenario. But for the second element failure in combination of fatigue and fracture, a new approach is presented. This method is based on the Monte Carlo Simulation of crack size in accordance with the crack growth approach in fracture mechanics. Important sequences leading to the structural collapse are identified within the branch and bound technique. Finally the system failure probability through combination of important paths leading to the structural collapse is obtained. Advantage of this method in addition to its simplicity in application is possibility of using the FORM technique to estimate each failure path individually and finally combine them together to determine system reliability. It is shown the system failure under combination of fatigue and fractures has higher probability of occurrence than the purely first or in combination with the second fatigue failures only.