A probabilistic methodology is developed to quantify the risks associated with structural failure of a steel production riser and in deepwater oil and gas production. First, background on the steel production riser is presented, detailing its functions, main components, operational phases and design considerations. Next, the study riser, developed by Stress Engineering, is described and its design basis is presented. A fault tree of failure scenarios is then developed and analyzed. Fatigue is identified as the primary failure mechanism affecting the risk for a production riser between its base at the mud-line and its top at the sea level. Bursting, collapsing and yielding are not likely for the riser. Probabilistic fatigue analysis is carried out to better understand and quantify the probability of a fatigue failure. This analysis includes development of probabilistic model and quantifying the uncertainties in the input to the model.
It was concluded that the probability of fatigue failure in a 20 yr design life is on the order of 1 in 1000 or smaller. This probability is consistent with the levels that are accepted in the offshore industry. The risk is very sensitive to the parameters of the weld, including the size and frequency of cracks in the weld and the properties of the intact weld material. The size and frequency of the crack in the weld depend on the quality of initial weld and the level of quality control in inspecting the weld for defects to the repaired. The critical location in the production riser with respect to the fatigue is at its base.
