A strain-controlled fracture assessment for submarine thin-walled pipes with V-groove welds and circumferential embedded cracks

摘要

In the offshore industry, oil and gas are commonly transported with thin-walled clad pipes. Reel-lay is a widely used method to install clad pipes. During the installation and service process, engineering critical assessment (ECA) is usually required to guarantee safety. Most of the ECA methods pertain to surface cracks. The DNV-RP-F108 standard indicates that an embedded crack can be transformed into a surface crack in evaluation, and the result is still conservative. However, the evaluation can be over-conservative at times, meanwhile, the effect of the weld material as well as groove shape is not considered. In this work, a more direct assessment of welded clad pipes with a circumferential embedded crack is proposed based on a modified reference strain approach. A strain-controlled boundary condition is applied, and various pipe and weld geometrical shapes are considered in the analysis matrix. Two plugins based on Python script are developed to circularly create a lot of numerical models. Hundreds of regression analyses based on the linear relationship between normalized J integral and nominal strain are implemented. An analytical solution of the peak J integral of the embedded crack is proposed, and the solution is proved to be close to FEA.