System Reliability of Jack-Up Platforms Under Fatigue and Fracture Failures

Jack-up is a rig for oil drilling and exploration in shallow water. The ability of movement from one location to another place, in addition to the possibility of lifting up the hull during operation, increase demand for the use of this platform in deeper water and harsher environments. Fatigue is the process of damage accumulation in material due to stress fluctuation caused by variation of loads in service time. The fatigue failure occurs when accumulated damage has exceeded a critical level. In practice, some uncertainties are included in loads and characteristics of capacity demand. It will be reasonable to take these uncertainties into account within reliability framework. Based on the crack propagation approach and fracture mechanics, a reliability of structural element under fatigue degradation can be estimated. However, the first failure of structural element under fatigue degradation may not cause system failure. This paper presents a new approach to estimate structural system reliability of jack-up platforms in combination of fatigue and fracture failure modes. The probability of failure of each component is firstly estimated using fatigue limit state and the subsequent failure probability is calculated extending fracture modes. In fracture mode, the crack size is required. In this paper a new approach based on Monte Carlo Simulation is presented to estimate the statistical crack size in accordance to the fatigue limit state. Important sequences of failure are then identified utilizing branch and bound technique and finally the system reliability through combination of important failure paths leading to system failure has been calculated. The advantage of this method is that the FORM or SORM technique can be applied to compute each failure path individually and finally determine system reliability based on combination of significant failure paths identified in branch and bound search technique. It is shown the system failure in this sequence has higher probability than the entirely first and second failure under fatigue failure modes.