Frequency-domain analysis of non-linear wave effects on offshore platform responses

Estimate of the pitch motion of an oil offloading Catenary Anchor Leg Mooring (CALM) buoy is presented. Linearization of the quadratic drag/damping term is investigated by the frequency-domain analysis. The radiation problem is solved to estimate the added mass and radiation damping coefficients, and the diffraction problem is solved for the linear wave exciting loading. The equation of motion is solved by considering the linearized nonlinear drag/damping. The pitch motion response is evaluated at each wave frequency by iterative and various linearization methods of the nonlinear drag term. Comparisons between the linear and nonlinear damping effects are presented. Time-domain simulations of the buoy pitch motion were also compared with results from the frequency-domain analysis. Various linearization methods resulted in good estimate of the peak pitch response. However, only the stochastic linearization method shows a good agreement for the period range of the incident wave where typical pitch response estimate has not been correctly estimated.

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Influence of Viscosity and Non-Linearities in Predicting Motions of a Wind Energy Offshore Platform in Regular Waves

Volume 1: Offshore Technology ◽

10.1115/omae2018-78127 ◽

2018 ◽

Author(s):

José del Águila Ferrandis ◽

Ricardo Zamora Rodríguez ◽

Chryssostomos Chryssostomidis ◽

Luca Bonfiglio

Keyword(s):

Frequency Domain ◽

Stokes Equations ◽

Offshore Platform ◽

Operating Conditions ◽

Boundary Element Methods ◽

Linear Wave ◽

Floating Bodies ◽

Computational Costs ◽

Non Linear ◽

Flow Theories

Motion prediction of floating bodies in waves represents one of the most challenging problems in naval hydrodynamics. The solution of the seakeeping problem involves the study of complex non-linear wave-body interactions that require large computational costs. For this reason, over the years many seakeeping models have been formulated in order to predict ship motions using simplified flow theories, usually based on potential flow theories solved within the linear assumption. Neglecting viscous effects in the computation of radiation forces might largely underestimate the energy dissipated by the system while moving at the free surface. This problem is particularly relevant for unconventional floating bodies at resonance. In these operating conditions the linear assumption is no longer valid and conventional Boundary Element Methods, solved in the frequency domain, might predict unrealistic large responses if not corrected with empirical damping coefficients. The application considered in this study is an offshore platform to be operated in a wind farm requiring operability even in extreme meteorological conditions. In this paper, we compare heave and pitch Response Amplitude Operators, predicted for an offshore platform using three different seakeeping models of increasing complexity; namely a frequency-domain BEM, a high-order BEM solved in time-domain and a non-linear fully viscous model based on the solution of the Unsteady Reynolds Averaged Navier-Stokes equations (URANS). Results are critically compared in terms of accuracy, applicability and computational costs.

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Numerical Study of Non-Linear Dynamic Behavior of an Asymmetric Rotor for Wave Energy Converter in Regular Waves

Processes ◽

10.3390/pr9081477 ◽

2021 ◽

Vol 9 (8) ◽

pp. 1477

Author(s):

Yoon-Jin Ha ◽

Ji-Yong Park ◽

Seung-Ho Shin

Keyword(s):

Frequency Domain ◽

Wave Height ◽

Cfd Simulation ◽

Domain Analysis ◽

Frequency Domain Analysis ◽

Linear Motion ◽

Regular Waves ◽

Cfd Simulations ◽

Non Linear ◽

Wave Heights

This study conducted a numerical investigation on the non-linear motion problems between a Salter duck-type rotor and large waves using CFD simulations. Regular waves of five different wave heights were generated. First, the linear motion of the rotor from the CFD simulation was verified by comparing it with the existing experimental and frequency domain analysis results. Then, a series of CFD simulations were performed to investigate the non-linear motions of the rotor. In the case of a lower wave height, the CFD simulation results were in good agreement with the experimental and frequency domain analysis results. However, as the wave height increased, the resonance periods were different in each other. In addition, the magnitudes of normalized pitch motions by the wave heights decreased as the wave heights increased. To investigate the aforementioned phenomena, the pitch motion equation was examined using separate CFD simulations. The results showed that changing the restoring moments induced changes in the maximum pitch motions and magnitudes of the normalized pitch motions. In the case of a higher wave height, non-linear phenomena and the changing restoring moments induced non-linear motion.

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