Numerical investigation on crack identification using natural frequencies and mode shapes of a drilling riser during deployment and retrieval

Abstract

Deployment and retrieval of drilling risers are among the most highly stressed marine operations processes, and allowable imperfections or missed-inspection cracks are likely to expand into limited defects, making crack identification essential during the deployment and retrieval of risers. This study's main objective is to investigate the performances of natural frequencies and modal shapes on crack identification in a drilling riser during installation and retrieval. First, a computation code is programmed to extract the natural frequencies and modal shapes of the intact and cracked risers, and its validity is verified by two numerical methods, i.e., precise integration method and differential transformation method in the literature. Second, the crack-identification ability of natural frequencies versus crack depth and riser-suspension length is studied in detail. An approach using the normalized fourth derivative of modal shapes is developed for crack identification compared with the traditional modal-curvature and modal-curvature difference approaches. Finally, the cracks near the middle span and both ends of the risers are identified, respectively, during the deployment. From the research on crack identification during installation, several conclusions and suggestions are drawn to provide some valuable references for the safety of drilling and production operations.