scholarly journals Hydrodynamic Responses of a 6 MW Spar-Type Floating Offshore Wind Turbine in Regular Waves and Uniform Current

Fluids ◽  
2020 ◽  
Vol 5 (4) ◽  
pp. 187
Author(s):  
Zhiping Zheng ◽  
Jikang Chen ◽  
Hui Liang ◽  
Yongsheng Zhao ◽  
Yanlin Shao
Keyword(s):  
Perturbation Method ◽  
Time Domain ◽  
Strong Influence ◽  
Offshore Wind ◽  
Regular Waves ◽  
Wave Drift ◽  
Uniform Current ◽  
The Time Domain ◽  
Wave Drift Forces ◽  
Drift Forces

In order to improve the understanding of hydrodynamic performances of spar-type Floating Offshore Wind Turbines (FOWTs), in particular the effect of wave-current-structure interaction, a moored 6MW spar-type FOWT in regular waves and uniform current is considered. The wind loads are not considered at this stage. We apply the potential-flow theory and perturbation method to solve the weakly-nonlinear problem up to the second order. Unlike the conventional formulations in the inertial frame of reference, which involve higher derivatives on the body surface, the present method based on the perturbation method in the non-inertial body-fixed coordinate system can potentially avoid theoretical inconsistency at sharp edges and associated numerical difficulties. A cubic Boundary Element Method (BEM) is employed to solve the resulting boundary-value problems (BVPs) in the time domain. The convective terms in the free-surface conditions are dealt with using a newly developed conditionally stable explicit scheme, which is an approximation of the implicit Crank–Nicolson scheme. The numerical model is firstly verified against three reference cases, where benchmark results are available, showing excellent agreement. Numerical results are also compared with a recent model test, with a fairly good agreement though differences are witnessed. Drag loads based on Morison’s equation and relative velocities are also applied to quantify the influence of the viscous loads. To account for nonlinear restoring forces from the mooring system, a catenary line model is implemented and coupled with the time-domain hydrodynamic solver. For the considered spar-type FOWT in regular-wave and current conditions, the current has non-negligible effects on the motions at low frequencies, and a strong influence on the mean wave-drift forces. The second-order sum-frequency responses are found to be negligibly small compared with their corresponding linear components. The viscous drag loads do not show a strong influence on the motions responses, while their contribution to the wave-drift forces being notable, which increases with increasing wave steepness.

Time-Domain Hydrodynamic Model for Mooring Analysis of a Spread Moored Fpso With Calibration of Wave Drift Forces

2021 ◽  
Author(s):  
Min Zhang ◽  
Junrong Wang ◽  
Junfeng Du ◽  
Nuno Miguel Magalhaes Duque Da Fonseca ◽  
Galin Tahchiev ◽  
...  
Keyword(s):  
Time Domain ◽  
Hydrodynamic Model ◽  
Wave Drift ◽  
Wave Drift Forces ◽  
Drift Forces

A Hybrid Method for Predicting Ship Maneuverability in Regular Waves

2020 ◽  
Vol 143 (2) ◽  
Author(s):  
Tianlong Mei ◽  
Yi Liu ◽  
Manasés Tello Ruiz ◽  
Evert Lataire ◽  
Marc Vantorre ◽  
...  
Keyword(s):  
Hybrid Method ◽  
Panel Method ◽  
Regular Waves ◽  
Ship Maneuvering ◽  
Wave Drift ◽  
Hydrodynamic Derivatives ◽  
Ship Maneuverability ◽  
Wave Drift Forces ◽  
Maneuvering Motion ◽  
Drift Forces

Abstract Traditionally, ship maneuvering is analyzed under calm water condition. In a more realistic scenario, such as a ship sailing in waves, the importance of taking the wave effects into account should be stressed. In this context, this paper proposes a hybrid method for predicting ship maneuverability in regular waves by combining a potential flow theory based panel method and a Reynolds-averaged Navier–Stokes (RANS)-based computational fluid dynamics method. The mean wave drift forces are evaluated by applying a three-dimensional time-domain higher-order Rankine panel method, which takes the effects of ship's forward speed and lateral speed into consideration. The hull-related hydrodynamic derivatives in the equations of ship maneuvering motion are determined by using a RANS solver based on the double-body model. Then, the two-time scale method is applied to predict ship maneuvering in regular waves by integrating the seakeeping model in a three degrees-of-freedom MMG model for ship maneuvering motion. The numerical results of a laterally drifting S175 container ship, including the wave-induced motions, wave drift forces, and turning trajectories in regular waves, are presented and compared with the available experimental data in literature. The results show that the proposed hybrid method can be used for qualitatively predicting ship maneuvering behavior in regular waves.

Numerical Evaluation of Ship Maneuvering Performance in Waves

2018 ◽  
Author(s):  
Min-Guk Seo ◽  
Bo Woo Nam ◽  
Yeon-gyu Kim
Keyword(s):  
Irregular Waves ◽  
Time Signal ◽  
Regular Waves ◽  
Ship Maneuvering ◽  
Wave Drift ◽  
Mmg Model ◽  
Free Running ◽  
Drift Force ◽  
Wave Drift Forces ◽  
Drift Forces

This paper considers a numerical computation of ship maneuvering performance in waves. For this purpose, modular-type maneuvering model (MMG model) is adopted and wave drift forces and moments are included in maneuvering equation of motion. Wave drift forces ware calculated using a seakeeping program based on higher-order Rankine panel method. When calculating the wave drift force acting on a ship, the forward speed, wave heading, wave period and drift angle of the ship are considered as key parameters. It means that ship’s lateral speed is also included to calculate wave drift force. Numerical simulations are carried out in regular waves using S175 containership and computation results are validated by comparing them with results of free-running model test. Using the developed program, numerical simulation in irregular waves are, also, conducted and discussion is made on the sensitivities of time signal of wave elevation on turning performance.

Simulation of Low Frequency Motions in Severe Seastates Accounting for Wave-Current Interaction Effects

2017 ◽  
Author(s):  
Babak Ommani ◽  
Nuno Fonseca ◽  
Carl Trygve Stansberg
Keyword(s):  
Test Data ◽  
Time Domain ◽  
Low Frequency ◽  
Interaction Effects ◽  
Force Coefficients ◽  
Wave Drift ◽  
Current Interaction ◽  
Drift Force ◽  
Wave Drift Forces ◽  
Drift Forces

Today’s industry practice assumes wave drift forces on floating structures can be computed from zero current wave drift force coefficients for the stationary floater, while simplified correction models introduce current effects and slow drift velocity effects. The paper presents an alternative approach which overcomes some of the limitations of today’s procedures. The method, to be applied together with a time domain solution of the low frequency motions, is based on pre-calculation of mean wave drift force coefficients for a range of current velocities. During the low frequency motions simulation, the wave drift forces induced by the irregular waves are computed from the mean drift coefficients corresponding to instantaneous relative velocity resulting from the current and the low frequency velocities. A simple interpolation model, based on a quasi-steady assumption, is applied to obtain the drift forces in time-domain. Since calculation of the wave drift forces on Semi-submersibles in severe sea states with fully consistent methods is out of reach, a semi-empirical model is applied to correct the potential flow wave drift force coefficients. This model takes into account viscous effects, that are important in high seastates, and wave-current interaction effects. The paper compares the wave drift forces and the related low frequency motions computed by the proposed method, with results applying “standard” methods and with model test data. The test data was obtained in the scope of the EXWAVE JIP, with model tests designed to investigate wave drift forces in severe seastates and assess the wave-current interaction effects.

Time-Domain Hydrodynamic Model for Mooring Analysis of a Spread Moored FPSO With Calibration of Wave Drift Forces

2020 ◽  
Author(s):  
Min Zhang ◽  
Junrong Wang ◽  
Junfeng Du ◽  
Nuno Fonseca ◽  
Galin Tahchiev ◽  
...  
Keyword(s):  
Numerical Model ◽  
Test Data ◽  
Time Domain ◽  
Transfer Functions ◽  
Low Frequency ◽  
Linear Wave ◽  
Mooring System ◽  
Wave Drift ◽  
Wave Drift Forces ◽  
Drift Forces

Abstract The paper presents calibration and validation of a time domain numerical model for mooring analysis of a spread moored FPSO in moderate seastates with and without current. The equations of motion are solved in the time domain with a fully coupled method, accounting for linear wave frequency (WF) radiation and diffraction, second order wave drift forces and nonlinear low frequency (LF) damping. The mooring system dynamics is solved by a FEM. Uncalibrated numerical models are based on input from the mooring system, vessel mass, radiation/diffraction analysis, decay tests and current coefficients. WF responses are very well predicted by standard radiation/diffraction linear analysis, therefore the focus is on the LF responses. LF motions are underpredicted by the uncalibrated numerical model. Calibration is performed by comparing simulations with model test data and adjusting hydrodynamic coefficients known to be affected by uncertainty. These include wave drift force coefficients and LF damping. Correction of the drift coefficients is based on empirical quadratic transfer functions (QTFs) identified from the test data by a nonlinear data analysis technique known as “cross-bi-spectral analysis”. The LF damping coefficients are then adjusted by matching low frequency surge and sway spectra from the model tests and from the simulations.

scholarly journals Calibration of a Time-Domain Numerical Hydrodynamic Model for Mooring Analysis of a Semi-Submersible

2018 ◽  
Author(s):  
Nuno Fonseca ◽  
Carl Trygve Stansberg
Keyword(s):  
Test Data ◽  
Time Domain ◽  
Transfer Functions ◽  
Numerical Models ◽  
Low Frequency ◽  
Added Mass ◽  
Frequency Wave ◽  
Wave Drift ◽  
Wave Drift Forces ◽  
Drift Forces

The paper presents calibration of a time domain numerical model for the motions of the Exwave Semi in high seastates with current. The time domain equations of motion combine linear radiation, linear diffraction and second order wave drift forces, based on MULDIF diffraction code, with nonlinear forces from quadratic damping and from the mooring system. Calibration is performed by comparing simulations with model test data and adjusting hydrodynamic coefficients known to be affected by uncertainty. These include wave drift force coefficients, damping and added mass coefficients. Correction of the drift coefficients is based on empirical quadratic transfer functions (QTFs) identified from the test data by a nonlinear data analysis technique known as “cross-bi-spectral analysis”. Initial “uncalibrated” numerical models are based on input from the mooring, vessel mass, MULDIF hydrodynamic analysis, decay tests and current coefficients. They need adjustments for surge and sway. Empirical drift coefficients, natural periods and damping coefficients are then adjusted by matching low frequency surge and sway spectra. Wave-frequency coefficients need no adjustment. Low frequency wave drift forces, damping and added mass need increase in high sea states, in particular with current. Final motion simulations show 30%–40% underestimation in initial simulations, while final calibrated simulations are close to the measured records.

On Approximations of the Wave Drift Forces Acting on Semi-Submersible Platforms With Heave Plates

2016 ◽  
Author(s):  
Bernard Molin ◽  
Jean-Baptiste Lacaze
Keyword(s):  
Wave Field ◽  
Scattered Wave ◽  
Diffraction Radiation ◽  
Morison Equation ◽  
Wave Drift ◽  
Horizontal Wave ◽  
Heave Plate ◽  
Drift Force ◽  
Wave Drift Forces ◽  
Drift Forces

The horizontal wave drift force acting on a vertical floating column, without then with a heave plate, is considered. Computations are performed with a diffraction-radiation code and through the Morison and Rainey equations. Focus is on wave frequencies around the heave resonance where the drift force may be significant, even though the scattered wave-field being weak. It is found that the Morison equation overpredicts the drift force while Rainey equations perform rather well.

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