A comparison of time domain and frequency domain analysis of a flexible marine riser undergoing large deformations by using a lumped mass approach is described in this paper. The riser is modeled as a set of lumped masses connected by axial springs with a structural axial damping and the external forces acting on it are due to the surface waves, the hydrodynamic loads represented by a modified form of the Morison's equation, and to the top end motion of the pipe due to the vessel's motion. The effect of the shear force along the riser is also considered. This work describes the non linear dynamic behavior of the riser in the time domain, obtained by using an implementation of the Runge-Kutta 113 algorithm provided in Matlab. Moreover the natural frequencies and the natural mode shapes of the riser are obtained by using a tangent stiffness matrix due to the sum of the axial stiffness and the bending stiffness. The main aim of the paper is to compare results from the time domain analysis 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.

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

MARANO, Giuseppe Carlo;MOSSA, Michele
2006

Abstract

A comparison of time domain and frequency domain analysis of a flexible marine riser undergoing large deformations by using a lumped mass approach is described in this paper. The riser is modeled as a set of lumped masses connected by axial springs with a structural axial damping and the external forces acting on it are due to the surface waves, the hydrodynamic loads represented by a modified form of the Morison's equation, and to the top end motion of the pipe due to the vessel's motion. The effect of the shear force along the riser is also considered. This work describes the non linear dynamic behavior of the riser in the time domain, obtained by using an implementation of the Runge-Kutta 113 algorithm provided in Matlab. Moreover the natural frequencies and the natural mode shapes of the riser are obtained by using a tangent stiffness matrix due to the sum of the axial stiffness and the bending stiffness. The main aim of the paper is to compare results from the time domain analysis 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.
The proceedings of the Sixteenth International Offshore and Polar Engineering Conference : ISOPE 2006. San Francisco, USA, May 28-June 2, 2006
9781880653661
International Society of Offshore and Polar Engineers
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11589/13100
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