Failure Assessment Diagram (FAD) analysis of fatigue test results for X65 welded joints

Abstract

Abstract. Offshore pipelines transfer oil and gas from seabed to production facility on the surface. The long pipelines are formed by welding of pipe segments, where these welded joints are a source of stress concentration and defects from which fatigue cracks can grow. This work aims to study the behaviour of deep fatigue cracks. In this current work, finite-element based on a parametric study of four-point bending is used to assess the stress intensity factors (SIFs) of deep surface cracks in X65 specimens, while considering local limit load the remaining load bearing ligament. These deep cracks take on a non-regular shape and have widths that exceed that of the specimen. They will be compared to empirical expressions from derived standards such as British Standards BS7910, which may be more conservative. The existing large flaw is also assessed via the failure assessment diagram (FAD). The effects of limit load solutions and reference stresses used to determine the FAD diagram will be discussed.

Application of Failure Assessment Diagram (FAD) for Steel Welded Connection Based on BS7910 and DNV-RP-108

“…A Finite-element based on a parametric study of four-point bending is used to assess the stress intensity factors (SIFs) of deep surface cracks in X65 specimens. The test specimen (cut from a pipe) was modeled as a plate using the extended finite element method (XFEM) to study the deep flaws in the specimens [13]. The four-point bending fatigue test was conducted at various loads (load ratio 0.1) until the coupons could no longer support fatigue cyclic loading.…”

“…They concluded that the current assessment of the tested X65 specimens with large cracks shows local throughthickness yielding is likely to occur before global ligament yielding. This work demonstrates a more accurate approach to assess large flaws by considering the SIFs and limit loads by finite element analysis [13].…”