Fatigue reliability test and life prediction of submarine pipeline

Key words:  X60 steel  submarine pipeline  welded joint  fatigue life  P S N curve

Abstract:

Considering the fatigue life of submarine pipeline free span under the wave force, a high cycle multi level fatigue test was carried out with the specimens from the submarine pipeline free span (X60) used in oilfield. Then the fatigue lives and their probability statistical distribution of X60 steel were obtained in different stress levels. The distribution of the specimen 's fatigue life was obtained with parameter hypothesis test method, and the linear relationship between the logarithmic fatigue life and the equivalent cyclic stress was established under different survival rates. The P S N curve of X60 steel as specimens was modified by introducing into the composite correction factor and the unsymmetrical cycle coefficient. Then the fatigue life prediction formula for the practical submarine pipeline free span was developed. The results show that the fatigue lives of specimens obey lognormal distribution, and the relationship between the logarithmic fatigue life and the equivalent cyclic stress is linear, and the prediction results agree well with the test results.