Dynamic Analysis of a Flexible Hanging Riser in the Time and Frequency Domain

Abstract

This paper outlines the dynamic analysis of flexible risers in the time and frequency domain using lumped mass discretization, where tension and bending are modeled with extensional and rotational springs respectively. For the time domain analysis, integration is carried out using the Wilson-theta implicit scheme, which allows the use of relatively large time steps without compromising stability. This increases computational efficiency and automatically filters the high frequency axial responses. The time domain code is validated with the commercial software Orcaflex, which employs an explicit scheme, and results are found to match for the same number of elements. The relative merits of implicit and explicit integration schemes are discussed. For the frequency domain analysis, the added mass, damping, axial/bending stiffness matrices are formulated in global coordinates. The nonlinear drag force is linearized iteratively for both regular and random waves. The range of accuracy for the linearized frequency domain simulations is assessed by methodical comparisons with the nonlinear time domain results for varying loading amplitudes. One problem encountered during the early development of an analytical tool is the lack of published results for validation, especially where access to commercial packages and test facilities is unavailable or limited. Hence, the simulation results presented herein are for a flexible hanging riser with simple boundary conditions and load cases to facilitate benchmarking.