Nonlinear Static And Dynamic Analyses Of Deepwater Flexible Risers

Both conventional flexible risers, which are made of concentric extruded polymer and reinforcing helical metallic layers, and steel catenary risers (SCRs) fall into the category of slender structures that are commonly used in ocean engineering, as they behave with similar dynamic response characteristics, accommodating floating platform motion and hydrodynamic loading by changing their shape significantly. In other words, geometric nonlinearity is dominant in their structural analysis. Bearing this in mind, this thesis uses flexible risers to represent both conventional flexible risers and SCRs hereafter, and seeks to tackle the relatively difficult geometric nonlinearity problem.Among the three main modeling methods that are capable of tackling large displacement, namely lumped mass, finite difference, and finite element method (FEM), a special FEM absolute coordinate based slender rod model is adopted to analyze flexible risers. The model is formulated directly in a global coordinate system by employing global position displacements and their derivatives as nodal variables, and thus obviates all transformations involving trigonometric functions, which is typically required in traditional finite element method. The governing equation derived with internal flow in consideration is discretized using FEM in space domain. The static problem is then solved iteratively by the classic Newton-Raphson method, while the dynamic response is integrated in time domain by Adams-Moulton method and Newmark--βmethod, respectively. With respect to the riser-seabed interaction, both the seabed support and friction effect are considered, with the former modeled by a nonlinear quadratic spring, allowing for a consistent derivation of the tangent stiffness matrix.Based on the above theory, the author developed a computer program to conduct static and dynamic analyses for flexible risers. The program has been validated against two theoretical examples. By analyzing several flexible risers-flexible jumper, steel catenary riser, lazy wave riser, and steep wave riser, it is demonstrated that this program is capable of dealing with structural analysis of typical flexible risers. And it serves as source code for further study about flexible risers'structural response behavior under various loadings in marine environment.