Dynamic reconstruction and weld fatigue evaluation of top-tensioned riser based on strain monitoring

Top-tensioned riser (TTR) is essential for offshore oil and gas transportation, which is welded by several standard steel pipes. The intricacies of monitoring, and the particularity of welded joints, render the riser's girth welds present potential service hazards. Here, we propose a riser monitoring and fatigue evaluation method (RMFE), which uses the riser's reconstruction results with strains, to assess girth weld damage. By implementing the orthogonal strain sensors layout and applying a least-squares function, we not only achieve high-precision real-time riser shape reconstruction with high mean stress in global status, but also achieve the structural stress reconstruction of the girth weld at the element level. The research results show the riser's weld fatigue life by RMFE system, based on the “strain reconstruction”, aligns with the results from ABAQUS and FEM theory, with most errors within 3% and the maximum error not exceeding 16%. Furthermore, smaller mesh sizes in the RMFE system can reduce calculation errors but increase computation time. Importantly, the structural stress method, incorporated in the RMFE system, can thoroughly consider mean stress correction for weld fatigue, and eliminate subjective error in selecting the master S-N curve, which has been extended to three-dimensional space to better meet engineering needs.