2000 Fiscal Year Final Research Report Summary
Perturbation analysis on non-linear behaviors of the offshore structure
| Project/Area Number |
09450376
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| Research Category |
Grant-in-Aid for Scientific Research (B).
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
海洋工学
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| Research Institution | University of Tokyo |
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Principal Investigator |
SANO Ikou Institute of Industrial Science, University of Tokyo, Research Associate, 生産技術研究所, 助手 (90238220)
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| Co-Investigator(Kenkyū-buntansha) |
KINOSHITA Takeshi Institute of Industrial Science, University of Tokyo, Professor, 生産技術研究所, 教授 (70107366)
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| Project Period (FY) |
1997 – 2000
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| Keywords | wave drift damping / slow drift motion / ringing / springing / wave drift force |
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| Research Abstract |
Firstly hydrodynamic forces including a wave drift damping for a slow drift motion of a platform are investigated both experimentally and theoretically.
It is well known that the 2^<nd> order hydrodynamic forces acting on the marine structures in both current and waves vary with the wave number. We measured the 2^<nd> order wave forces and moment acting on a platform model in both slow current and regular wave trains of various incident angles. The experimental results were compared with the results of semi-analytical solutions based a perturbation theory and eigenfunction expansion.
The model is restricted to the linear responses to wave excitations. An assumption of a quasi-steady state for each position of rotation is made in data processing for the yaw mode of slow motion. To calculate wave drift damping, the slow oscillations are approximated by the corresponding steady motions. Some new treatments to deal with the integral over the free surface are presented to improve the convergence and accuracy.
It was confirmed that the yaw moment is significantly induced by oblique waves and current, and that those phenomena are well explained by the theory.
Secondly the diffraction problem for a truncated circular cylinder is formulated up to the third order referring to the wave slope. The problem is solved by means of eignfunction expansion. The Green function suitable for the pressure distribution problem is used to give a special solution which satisfies the inhomogeneous free-surface condition in higher-order problems. The special solution is expressed by an integral over the free surface and the singularity contained in the integrand is discussed. A new method to improve the convergence of the solution is proposed, which has the advantage of simplicity compared with the previous approach. A convergence test also shows satisfactory results. The third-order wave forces are evaluated and compared with the previous results.
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