X65 Steel Welded Pipeline Fatigue Assessment Based On BS7910

Submarine pipeline' safety is very important during its installation and subsequentservice because any fracture occurrence may cause serious economic damage or fatalaccidents. So much importance has been attached to the assessment of the submarinepipeline when defects have been confirmed to be existence. In order to meet therequirements of fracture assessments in pipeline project, we programmed the softwareof ECA (Engineering Crack Assessment) used for welded pipeline.BS7910 standard (Guide on methods for assessing the acceptability of flaws inmetallic strucres) has been widely used as a main criterion of marine projects atpresent. In this article, we have introduced the BS7910 standard and the method offatigue assessment based on BS7910. Experiments on X65 steel have been performedin order to research its crack propagation property during this period. The experimentresults reveal that the crack growth velocity of base material is faster than those ofweld bead crack and heat-affected zone crack However there are less differencesbetween weld bead crack and heat-affected zone crack Then we draw a conclusionthat the crack propagation resistance of weld bead and heat-affected zone is betterthan that of base metal. We performed a fatigue assessment of X65 steel pipelineaccording to the experiment results and the assessment results has been used to checkthe software's accuracy.More than one method has been used in the software of ECA (Engineering CrackAssessment) used for welded pipeline. These methods include Paris integration,cycle-by-cycle method, finite crack growth increment method, blocks of stress cyclesmethod, crack accumulation damage method. All the methods are suitable for bothconstant loading and spectrum loading. We have researched into the accuracy of themand summarized the accuracy affecting factors. It has been found that each methodhas both merits and shortcomings respectively. However we can conclude that thecycle-by-cycle method is the most accuracy among all these methods. Because thismethod not only presents a relatively accurate result but also reflects the changeinstantly when the crack is propagating. In practical assessment, we stronglyrecommend that the cycle-by-cycle method be used when more accurate result isneeded. The finite crack growth increment method can also be used when the totalnumber of stress or increment is small.