ANALYSIS OF FLEXIBLE RISER SYSTEMS SUBJECT TO 3-D SEASTATE LOADING

The application of flexible risers and pipelines in the development of innovative floating production systems for deeper waters with more severe weather environments has created a need for a comprehensive 3-D analysis capability. This paper describes a complete 3-D nonlinear finite element solution scheme which includes arbitrarily large displacements and rotations about all axes and the computation of axial, bending and torque generalised forces. Major features of the solution scheme include a mixed variational formulation for the element and a variable step size time integration scheme. This hybrid element overcomes the numerical ill-conditioning due to the near axial inextensibility of the flexible pipeline, and the variable step allows economical simulation times for the computation of statistical and extreme values of important parameters. Sample computations for a catenary, steep-wave and multiple steep-wave flexible riser configurations attached to floating vessels are presented and demonstrate the utility and accuracy of the approach.