Vol. 4, No. 7-8, 2009

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ISSN: 1559-3959
Determination of offshore spar stochastic structural response accounting for nonlinear stiffness and radiation damping effects

Rupak Ghosh and Pol D. Spanos

Vol. 4 (2009), No. 7-8, 1327–1340
Abstract
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A study of the dynamic behavior of a combined dynamic system comprising a spar structure, a mooring line system, and top tensioned risers (TTR) by buoyancy can is presented. Not only the nonlinear restoring force of the mooring lines, the Coulomb friction at the compliant guides and the spar keel, and the hydrodynamic damping forces are considered, but also the effect of the frequency-dependent radiation damping is readily incorporated in this formulation. The dynamic model is subjected to input force and moment time histories that are compatible with a spectral representation (Jonswap spectrum) of a 100-year hurricane in the Gulf of Mexico. The response of the system is first determined by direct numerical integration of the equations of motion. In this regard, particular caution is exercised to treat properly the frequency-dependent terms which involve convolution transforms in the time domain. Next, a novel approach for determining the system responses is proposed. It is based on the technique of statistical linearization which can accommodate readily and efficiently the frequency-dependent elements of the dynamic system. This is achieved by appropriate modification of the system transfer function and by proper accounting for the system nonlinearities. The time domain analysis results are used to demonstrate the reliability of the statistical linearization solution. Further, the effect of the radiation damping, and the effect of the hydrodynamic forces are investigated.

Keywords
spar offshore structure, sea wave spectrum, nonlinear dynamic analysis, radiation damping, statistical linearization
Milestones
Received: 29 September 2008
Revised: 28 May 2009
Accepted: 28 May 2009
Published: 26 December 2009
Authors
Rupak Ghosh
Pol D. Spanos
Rice University
Departments of Civil and Mechanical Engineering
6100 Main Street
Mail Stop 321
Houston, TX 77005-1892
United States