Comparison of Safety Factors Used for Determination of Remaining Life of Girth Welded Pipelines Using Fracture Mechanics and Conventional Endurance Assessment Techniques

This paper compares the effect of using commonly applied safety factors for the estimation of the remaining life of welded components. Two different design philosophies are compared, conventional design (based on the S-N approach) and a fracture mechanics (Paris Law) approach. Both methods use a safety factor based on two standard deviations of the mean experimental data. However, there is no theoretical reason as to why the two methods should give similar degrees of conservatism when used to assess the remaining life of structures or pipelines. In this study a fracture mechanics approach is used to predict a hypothetical initial flaw size such that the mean life of the welded component (in this case a girth welded pipe) is compatible with that predicted from mean S-N data and the effect of growing the fatigue flaw at various rates (based on selected numbers of standard deviations of the Paris Law curve) is used to illustrate the effect of the safety margins used in both methods. From a comparison of the predicted life with the S-N design curve it is possible to compare the conservatism of the two techniques and highlight inconsistencies between the two approaches. The paper then proceeds to describe how the use of the S-N approach and the fracture mechanics approach is incorporated into the procedure for assessing the criticality of welded joints under operational conditions as given in DNV OS F101 Appendix A.