Second-Order Sum-Frequency Wave Exciting Forces on Column-Footing Type Floating Bodies

The paper presents an experimental investigation of the first order and second order wave exciting forces acting on a body of simple geometry subjected to long crested irregular waves. The body is axis-symmetric about the vertical axis, like a vertical cylinder with a rounded bottom, and it is restrained from moving. Second order spectral analysis is applied to obtain the linear spectra, coherence spectra and cross bi-spectra of both the incident wave elevation and of the horizontal and vertical wave exciting forces. Then the linear and quadratic transfer functions (QTF) of the exciting forces are obtained. The QTF obtained from the analysis of irregular wave measurements are compared with results from experiments in bi-chromatic waves and with numerical predictions from a second order potential flow code.

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Second-order sum-frequency wave loads on large-volume structures

Applied Ocean Research ◽

10.1016/s0141-1187(05)80052-5 ◽

1991 ◽

Vol 13 (6) ◽

pp. 287-296 ◽

Cited By ~ 10

Author(s):

M.H. Kim

Keyword(s):

Large Volume ◽

Second Order ◽

Wave Loads ◽

Frequency Wave ◽

Sum Frequency

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Wave forces on three-dimensional floating bodies with small forward speed

Journal of Fluid Mechanics ◽

10.1017/s002211209100006x ◽

1991 ◽

Vol 227 ◽

pp. 135-160 ◽

Cited By ~ 56

Author(s):

Jan Nossen ◽

John Grue ◽

Enok Palm

Keyword(s):

Velocity Potential ◽

The Body ◽

Wave Forces ◽

Forward Speed ◽

Floating Bodies ◽

Wave Drift ◽

Exciting Forces ◽

Wave Exciting Forces ◽

Wave Drift Damping ◽

The Mean

A boundary-integral method is developed for computing first-order and mean second-order wave forces on floating bodies with small forward speed in three dimensions. The method is based on applying Green's theorem and linearizing the Green function and velocity potential in the forward speed. The velocity potential on the wetted body surface is then given as the solution of two sets of integral equations with unknowns only on the body. The equations contain no water-line integral, and the free-surface integral decays rapidly. The Timman-Newman symmetry relations for the added mass and damping coefficients are extended to the case when the double-body flow around the body is included in the free-surface condition. The linear wave exciting forces are found both by pressure integration and by a generalized far-field form of the Haskind relations. The mean drift force is found by far-field analysis. All the derivations are made for an arbitrary wave heading. A boundary-element program utilizing the new method has been developed. Numerical results and convergence tests are presented for several body geometries. It is found that the wave exciting forces and the mean drift forces are most influenced by a small forward speed. Values of the wave drift damping coefficient are computed. It is found that interference phenomena may lead to negative wave drift damping for bodies of complicated shape.

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Turbine Floater-Tether Coupled Dynamic Analysis Including Second-Order Sum-Frequency Wave Loads for a TLP-Type FOWT (Floating Offshore Wind Turbine)

Volume 8: Ocean Renewable Energy ◽

10.1115/omae2013-11261 ◽

2013 ◽

Cited By ~ 3

Author(s):

Y. H. Bae ◽

M. H. Kim

Keyword(s):

Dynamic Analysis ◽

High Frequency ◽

Second Order ◽

Offshore Wind Turbine ◽

Frequency Wave ◽

Wave Condition ◽

Sum Frequency ◽

Coupled Dynamic Analysis ◽

Resonance Frequencies ◽

Fully Coupled

In the present study, a numerical simulation tool has been applied for the time-domain turbine-floater-tether fully-coupled dynamic analysis of a FOWT. The fully coupled dynamic analysis includes aero-blade-tower dynamics and control, mooring dynamics, and platform motions. In particular, the effects of second-order sum-frequency wave excitations on the coupled dynamic analysis are investigated. The fully coupled simulations with full blade operation are compared with those with parked condition (without blade rotation). For this purpose, a mono-column TLP with 5MW turbine in 200m water depth is selected as an example. The time histories and spectra of the FOWT motions and accelerations as well as tether top-tensions are presented for the given random collinear wind-wave condition. The shift of original floater natural frequencies due to the inclusion of tower flexural modes is demonstrated. The increase of aero damping in the case of rotating blades is also explained. The second-order sum-frequency wave loading introduces high-frequency excitations near pitch-roll resonance frequencies or lowest tower flexural modes. Its effects are more clearly seen in the blade-parked condition than the blade-fully-operational condition. The increased high-frequency responses may significantly increase tower-top accelerations and accumulated fatigue.

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