A Comparison of Time Domain and Frequency Domain Analysis of a Flexible Marine RiserUndergoing Large Deformations by Using a Lumped Mass Approach

摘要

A comparison of time domain and frequency domain analysis of aflexible marine riser undergoing large deformations by using a lumpedmass approach is described in this paper. The riser is modeled as a setof lumped masses connected by axial springs with a structural axialdamping and the external forces acting on it are due to the surfacewaves, the hydrodynamic loads represented by a modified form of theMorison's equation, and to the top end motion of the pipe due to thevessel's motion. The effect of the shear force along the riser is alsoconsidered.This work describes the non linear dynamic behavior of the riser in thetime domain, obtained by using an implementation of the Runge-Kutta113 algorithm provided in Matlab. Moreover the natural frequenciesand the natural mode shapes of the riser are obtained by using a tangentstiffness matrix due to the sum of the axial stiffness and the bendingstiffness.The main aim of the paper is to compare results from the time domainanalysis with those yielded by a linearised frequency domain analysis.This delineates the range of validity of the frequency domain approach,which is computationally efficient but inherently approximate.