The Reliability Analysis And Maintenance Decision-Making For Existing Submarine Pipelines

Submarine pipelines are the lifeline engineering of the oil field on the sea. Their security is the right important thing that connects the existing of the whole oil field. At present, people have achieved in design, construction, survey, maintenance of the submarine pipeline. But, because of the bad conditions around them, their states are changing. As a result, the submarine pipeline cannot work normally according to the designed states when the free spanning is becoming longer increasingly, that is to say whether they can complete the preconcerted function in the spare time should be considered. In the paper, the reliability theory is used to confirm the ultimate length of the free spanning which is the dependence of the maintenance decision-making of the submarine pipeline.The aimed reliability index is used as the criterion of the reliability assessment. Thus the safety factor is firstly confirmed by fuzzy optimization method, and then the different aimed reliability indexes are established according to their own limit states in the different conditions.One of the two limit states is failure of the strength of the submarine pipeline in seismic load, so the dynamic reliability analysis of them is researched according to the intensity invalidation pattern. As a result, an ultimate length of the free spanning is found.The other limit states are the fatigue of the submarine pipeline due to wave lift forces after the free spanning conies into being. Therefore, the fatigue reliability assessment is necessary for them, and the other ultimate length of the free spanning is confirmed.The results indicate that the ultimate length of the free spanning confirmed by fatigue limit states is far less than the one by limit states of failure of the strength of the submarine pipeline via the contrast of the reliability assessments in the two limit states. Thus it can be seen that the fatigue due to vibration of the suspended pipeline in the lift forces should be regarded as the main controlled factor. Besides, the existing submarine pipelines should be maintained in case their lengths of the free spanning reach the one settled by fatigue limit states.