Heave Plates Research Articles

Influence of heave plates on the dynamics of a floating offshore wind turbine in waves

This paper presents an analysis of the effects of heave plates with large skirts on the dynamics of a semi-submersible FOWT in waves. The analysis was based on a preliminary hull configuration envisaged for Brazilian waters. A numerical model based on linear frequency-domain hydrodynamics evaluated with panel methods was used for predicting the effects of the heave plates on the natural periods of motions. The predictions were compared to the results of model tests conducted in a wave basin involving ten different configurations of plate diameters and skirt heights. The experimental results confirmed the validity of the numerical predictions and provided quantitative results regarding the variations in viscous damping associated with the dimensions of the heave plates. The data will be used for future calibration of a parametric model for hull geometry optimization.

A review of the researches on the added mass and damping coefficients for the heave plates of the offshore platforms at translational and rotational oscillations

A review of the researches on the added mass and damping coefficients for the heave plates of the offshore platforms at translational and rotational oscillations

Effects of heave plate on dynamic response of floating wind turbine Spar platform under the coupling effect of wind and wave

Stability of the platform is the most fundamental guarantee for floating offshore wind turbines. In order to explore hydrodynamics of floating wind turbine Spar platform under the coupling effect of wind and wave, whole model of floating wind turbine is established based on the OC3-Hywind Spar Buoy platform and the NREL 5 MW wind turbine. Taking the research experiences of traditional marine engineering, this work proposed the structural design method of installing heave plate on main body of Spar. Based on the Bladed Element Momentum theory, this paper adopts FORTRAN language to accomplish the secondary development of hydrodynamic software AQWA to establish the coupled aerodynamic - hydrodynamic - mooring line system model. The amplitude-frequency curves of the added mass, radiation damping, wave exciting force and response amplitude operators (RAO) were obtained based on radiation/diffraction theory to investigate the hydrodynamic performance of floating wind turbine Spar platform in frequency domain. In the time-domain analysis, the three kinds of working conditions include wave-only, non-coupling effect of wind-wave and coupling effect of wind-wave were conducted comparatively to verify the importance of wind load and wind - wave interaction. In addition, the actual motions of original Spar and Spar with heave plates were analyzed to explore the effects of the heave plate and its installation position on the nonlinear dynamic response characteristics of Spar under the coupling effect of wind and wave. The research results indicated that the installation position of the heave plate has little influence on the surge response. In view of the actual heave motion, the heave plate can play a positive effect no matter where the heave plate is installed. When the heave plate is located on the upper part of the platform, the pitch response and the intensity of the fluctuation of the platform will be greatly reduced. Finally, time domain mooring analysis for Spar platform with and without heave plate is performed to illustrate the feasibility of the present approach for mooring modeling.

Experimental Studies of Interaction Forces Affect the Position of Vertical Plates on Oscillating Heave Plates with Cylindrical Bodies in Regular Waves

This paper discusses an experimental study of a wave energy converter (WEC) without using reaction from the seabed. The WEC uses buoys and heave plates, which can react to their self-reacting. The interaction force between heave plates and buoys can absorb energy from ocean waves better. The heave plate model affects the output of energy produced. It is presented in this study with variations in the position of upright plates. The research aims to measure the influence of the place of the addition of vertical plates into heave plates on the WEC on the hydrodynamic performance (coefficient of mass increase, drag coefficient, and KC value) and the interaction of the force it produces with the buoy on regular waves. The conclusion is the vertical plate position makes the coefficient of mass added Ca increase with an increasing amount of KC, and an almost linear relationship was observed between them. As the frequency increases, the value of C increases slightly, but it is not clear. Thus, the oscillating frequency has little effect on the mass coefficient of added heave plates with vertical plates. Thus, the change in the vertical plate position has only a powerful effect on KC < 0.75. ©2020. CBIORE-IJRED. All rights reserved

Connections between resonance and nonlinearity in swimming performance of a flexible heaving plate

We investigate the role of resonance in finite-amplitude swimming of a flexible flat plate in a viscous fluid. The role of resonance in performance remains unclear for two reasons: i) a lack of definition of resonance for the fully-coupled fluid-structure interaction system in a viscous flow, and ii) the presence of nonlinear effects, which makes it difficult to disentangle resonant and non-resonant mechanisms in finite-amplitude swimming. We address point i) and provide an unambiguous definition for system resonance by computing global linear stability modes of the fully-coupled fluid-structure interaction system that account for the viscous fluid, the plate, and the coupling between them. We then resolve point ii) by considering high-fidelity nonlinear simulations of systematically increased amplitude. By comparing the results for different amplitudes with one another and with the linear stability modes, we separate linear and/or resonant effects from nonlinear and/or non-resonant effects. Resonant behavior is observed over a wide range of plate stiffnesses, with peaks in trailing-edge motion and thrust occurring near the resonant frequency defined by the global linear analysis. The peaks broaden and weaken with increasing heave amplitude, consistent with an increased damping effect from the fluid. At the same time, non-resonant mechanisms are present at large heave amplitudes. The input power exhibits qualitatively different dynamics at large heave amplitudes compared to smaller heave amplitudes, where resonance dominates. Moreover, leading-edge separation is present for stiff plates at large heave amplitudes, which can drastically alter the performance characteristics from what one would expect through linear predictions.

A Simple Approach for Vortex Core Visualization

Abstract We propose a method to visualize vortex cores based on manipulation of the pressure field produced by isolated vortices in incompressible flow. Under ideal conditions, the function D=2|∇p|/∇2p yields an approximate distance to vortex centerlines. As opposed to local methods to identify coherent structures, isosurfaces of D produce a field of vortex tubes equidistant to the vortex core center which, ideally, are independent of vortex intensity or size. In contrast to other line-vortex identification methods, which typically rely on algorithms to detect vortex core lines and frequently need complex implementations, the proposed method can be computed from the local Eulerian velocity and pressure fields as easily as vortex identification methods such as the Q and λ2 criteria. D=2|∇p|/∇2p results in the exact distance to the core center for a Rankine vortex and is in general valid for the region of a vortex where there is pure rotation, yielding an approximation to the distance farther from the core in other simple one-dimensional vortex models. The methodology performs well in all tests we attempted, though limitations are presented and discussed. The method is demonstrated for a canonical Burgers vortex, a Bodewadt vortex, homogeneous isotropic turbulent flow, the wake of a propeller, a heaving plate, and a turning containership. The proposed method helps to better visualize vortical flow fields by displaying vortex cores, complementing methods like Q and λ2 which display vortical volumes.

Using inerter-based control device to mitigate heave and pitch motions of semi-submersible platform in the shallow sea

Semi-submersible platforms (SSPs) are widely applied for energy mining in the ocean. During its service life, SSP may be subjected to excessive heave and pitch motions induced by sea waves, which in turn may result in fatigue problems of structural components or even catastrophic capsizing of the platform. It is therefore important to mitigate the heave and pitch motions of SSP by all means. In the present study, a novel inerter-based control system, rotational inertia damper (RID), is proposed to simultaneously mitigate the heave and pitch motions of SSP in the shallow sea. The responses of SSP equipped with RID systems subjected to six typical wave conditions are calculated in the frequency and time domains respectively based on the developed analytical models and Simulink models. For comparison, the models and responses of SSP without and with traditional control devices, i.e. fixed heave plate (FHP) and tuned heave plate (THP), are also developed and calculated. The results reveal that the RID system can achieve almost the same or even better control effectiveness compared to the conventional systems with a much smaller additional mass to the SSP especially when the SSP is under harsher waves.

Hydrodynamic analysis of a novel modular floating structure system with central tension-leg platforms

ABSTRACT The present work proposed a novel modular structural system with central tension-leg platforms and outermost wave energy converters (marked as MTLPW). The hydrodynamic interaction effect and the mechanical coupling effect between the WECs and the TLP have been taken into consideration, and the effects of key design parameters of the WEC on the hydrodynamic characteristics of the MTLPW system have been clarified. The results indicate that the outermost WEC can significantly reduce the surge response of the central TLP and produce considerable wave energy. In addition, the effects of different connection types, pretension levels and additional heave plates on the hydrodynamic performance of the MTLPW system have been further investigated, and the results indicate that both the outermost WEC module and the additional heave plate can be promising and available for improving the hydrodynamic performance of the initial MTLPW system.

Hydrodynamic Characteristics of Square Heaving Plates with Opening Under Forced Oscillation

The existence of the heaving plates can improve the heaving motion performance of an offshore structure significantly by providing both extra added mass and damping. In the current research, numerical investigation is carried out on the hydrodynamic characteristics of both isolated square heaving plate and double square heaving plates with opening by an immersed boundary-lattice Boltzmann method. The effects on hydrodynamic performance of plates due to Keulegan-Carpenter (KC) number, frequency number, opening ratio, opening distribution and spacing of plates are examined. It is found that the heaving plates with optimized opening ratio can provide additional damping compared with the plates without opening. Better hydrodynamic characteristics of double plates can be obtained with the increase of plate spacing.

Hydrodynamic characteristics of the modified V-shaped Semi-floating offshore wind turbine with heave plates

In recent years, there is a great need to develop offshore wind energy on a global scale due to the greenhouse effect and energy crisis. Great efforts have been devoted to developing a reliable floating offshore wind energy technology to take advantage of the large amount of wind energy resources that exist in deep water. In this paper, a novel concept of a floating offshore wind turbine (FOWT), namely, the modified V-shaped semi-floating with heave plate, is proposed, and its hydrodynamic characteristics are studied. A numerical model based on ANSYS/AQWA is used to investigate the dynamic motion, response characteristics and mooring performance of the new concept. Moreover, the response amplitude operators (RAOs) of the different response quantities are also elaborated. A comparative study of the dynamic response of the different response quantities of the modified V-shape and original V-shaped semi-floating is carried out for operational environmental conditions. It is found that the modified V-shape semi-floating shows relatively better performance in platform motion and mooring line response.

Mooring System Design for the 10MW Triple Spar Floating Wind Turbine at a 180 m Sea Depth Location

This works presents a methodology for the design of a mooring system for the Triple Spar floating wind turbine that supports the INNWIND 10MW wind turbine. The mooring system keeps the platform at the desired location and avoids the drift caused by wind, currents and nonlinear hydrodynamics. A semi-taut mooring system configuration, combining steel chain and polyester is chosen to reduce the cost. The basic configuration is defined using static equations and provides a smooth relationship between the restoring force and the displacement, to prevent snap loads. A dynamic analysis for the environmental conditions of the Gulf of Maine is performed to verify the performance of the design. The tensions on the lines are well below the maximum breaking limit for the different materials. Nevertheless, the chain weight can not be decreased as it provides the required mooring system stiffness. It has been verified that the polyester segment does not contact the seabed that could potentially damage it. The dynamic analysis also shows that the anchors do not experience vertical forces and the lines do not impact the platform heave plates. A complete load analysis must be performed to fully validate the proposed design.

Numerical study of distributed hydrodynamic forces on a circular heave plate by large-eddy simulations with volume of fluid method

ABSTRACT Distributed hydrodynamic forces on circular heave plates are investigated by large-eddy simulation (LES) with volume of fluid (VOF) method. First, the predicted added mass and drag coefficients for a whole heave plate is validated by published experimental data. The distributions of hydrodynamic loads on the circular heave plates are then investigated. It is found that maximum dynamic pressure occurs at the plate centre and decreases monotonically towards the outer regions. Finally, formulas of distributed added mass and drag coefficients in the radial direction are proposed based on present numerical simulations, and effects of aspect ratio and diameter ratio on the distributed added mass and drag coefficients are investigated.

Numerical research of an effective measure for stabilising floating wind turbines in shallow water

In order to stabilise floating wind turbines, an innovative motion stabilisation measure is proposed and verified here through conducting a series of numerical researches with the aid of SESAM. In the research, the numerical model of a spar-supported 5 MW floating turbine was developed first to investigate its motion stability in different depths water and under different wave conditions. Then, a new concept of motion stabiliser is proposed, which consists of a number of heave plates that are connected to floating turbine foundation via structural arms. The influences of both the number of heave plates and their arm length on motion reduction are then investigated in order to explore an optimal design of the proposed stabiliser. Considering the dynamic motions of a floating turbine is mainly affected by sea waves, the motion stabilising capability of the proposed stabiliser is investigated over a wide range of wave period 4–36 s. It has been found that after using the proposed motion stabiliser, both the pitch and heave motions of the floating turbine are successfully limited within the most range of wave period, especially when the wave period exceeds 12 s.

Hydrodynamic performance of multiple co-axial heave plates with different diameters

ABSTRACTHeave plates are structural components of a floating structure whose primary purpose is to increase damping and added mass in heave or pitch direction. If multiple plates are stacked along a common axis, then the relative diameters and spacing in between are important factors affecting the performance. In the present study, hydrodynamic coefficients of single, double and triple disks are experimentally investigated in different arrangements as Uniform, Concave and Convex edge shapes. It is observed that each disk in a multiple arrangement with lower diameter ratio have better hydrodynamic performance compared to the cases with higher diameter ratio when the spacing is small compared to the mean diameter. Some hydrodynamic aspects affecting floating structure design are also discussed in this paper.

Experimental Research for Stabilizing Offshore Floating Wind Turbines

Floating turbines are attracting increasing interest today. However, the power generation efficiency of a floating turbine is highly dependent on its motion stability in sea water. This issue is more marked, particularly when the floating turbines operate in relatively shallow water. In order to address this issue, a new concept motion stabilizer is studied in this paper. It is a completely passive device consisting of a number of heave plates. The plates are connected to the foundation of the floating wind turbine via structural arms. Since the heave plates are completely, rather than partially, exposed to water, all surfaces of them can be fully utilized to create the damping forces required to stabilize the floating wind turbine. Moreover, their stabilizing effect can be further amplified due to the application of the structural arms. This is because torques will be generated by the damping forces via the structural arms, and then applied to stabilizing the floating turbine. To verify the proposed concept motion stabilizer, its practical effectiveness on motion reduction is investigated in this paper. Both numerical and experimental testing results have shown that after using the proposed concept stabilizer, the motion stability of the floating turbine has been successfully improved over a wide range of wave periods even in relatively shallow water. Moreover, the comparison has shown that the stabilizer is more effective in stabilizing the floating wind turbine than single heave plate does. This suggests that the proposed concept stabilizer may provide a potentially viable solution for stabilizing floating wind turbines.

Scale Effects on Heave Plates for Semi-Submersible Floating Offshore Wind Turbines: Case Study With a Solid Plain Plate

Abstract In the case of SPAR or semi-submersible platforms for floating wind turbines, it is beneficial in some cases to use heave plates that reduce their heave motion amplitude and/or tune their heave natural period. As part of the Hiprwind project, it was decided to study scale effects on the hydrodynamics of this element. To this aim, models of one leg of the platform, equipped with a heave plate without any reinforcements, were built. This model is a simplified representation of the actual one, which incorporates a vertical flap on the heave plate edge. The scales were 1:20, 1:27.6, and 1:45.45, with the former leading to added mass values of the order of 300 kg, becoming one of the largest models for which experiments with heave oscillations have been carried out. Decay tests starting from various amplitudes and forced oscillations tests for a range of frequencies and amplitudes were performed. It is shown in the paper that the influence of the scale factor on the hydrodynamic coefficients is weaker than the effect that the motion amplitude (characterized with the Keulegan–Carpenter (KC) number produces in them. This result is relevant because the selection of a representative KC is an important and somewhat arbitrary aspect to be set in the linear potential simulation codes in order to add viscous damping. What has been shown herein is that a right selection of KC has a larger impact on the models than the uncertainties due to eventual scale effects in the heave-plates dynamics.

The surge-heave-pitch coupling motions of the Φ-type vertical axis wind turbine supported by the truss Spar floating foundation

The motions of the floating vertical axis wind turbine (VAWT) with consideration of the coupling between the aerodynamics and the hydrodynamics are calculated in this paper. The surge-heave-pitch coupling nonlinear equations of the floating VAWT are established. The aerodynamic loads are obtained by applying the double-multiple-stream tube theory with consideration of the dynamic stall and the floating foundation motion. The motion performances of a 5 MW Φ - Darrieus type floating VAWT are studied with its foundation of the Spar type with heave plates. It is shown that the amplitude of the heave motion of the floating VAWT is small when the heave plates are equipped. The effects of the wind and wave loads on the floating VAWT motions are assessed. The results show that the mean values of the surge and the pitch of the floating VAWT are mainly related to the wind loads; the standard deviations of the surge, the heave, and the pitch are mainly related to the wave loads. In the regular wave cases, the frequencies of the surge, the heave and the pitch are dominated by the wave frequencies, and the components of the 2P responses (caused by the aerodynamic loads) of the pitch are small. The 2P responses of the pitch are more significant in the irregular wave cases as compared with those in the regular wave cases.

Effects of heave plates on the global performance of a multi-unit floating offshore wind turbine

This paper aims to investigate the heave-plate effects on the behavior of a multi-unit floating offshore wind turbine (MUFOWT). The global performance of the large-scale, square-shape, semi-submersible floating platform equipped with 4 wind turbines at each corner, and 24 wave energy converters on frames along near-waterline is numerically simulated in time domain by using the in-house turbine-floater-mooring fully-coupled dynamic analysis program, MUFOWT. Cases with and without heave plates are systematically compared to identify the role of heave plates. For verification, an independent 1/50-scaled experiment was conducted by authors in the Korea Research Institute of Ships and Ocean Engineering's (KRISO) 3D wave basin. The numerical results with heave plates are extensively compared against the physical model-test results. Finally, the global performance results under survival-storm conditions, in which the WECs and blades of wind turbines are fixed, with and without heave plates are systematically investigated. The findings indicate that the heave plates are effective in reducing heave and pitch motions, and shift their natural frequencies with a minimal increase of mass.

Iterative Frequency-Domain Response of Floating Offshore Wind Turbines with Parametric Drag

Methods for coupled aero-hydro-servo-elastic time-domain simulations of Floating Offshore Wind Turbines (FOWTs) have been successfully developed. One of the present challenges is a realistic approximation of the viscous drag of the wetted members of the floating platform. This paper presents a method for an iterative response calculation with a reduced-order frequency-domain model. It has heave plate drag coefficients, which are parameterized functions of literature data. The reduced-order model does not represent more than the most relevant effects on the FOWT system dynamics. It includes first-order and second-order wave forces, coupled with the wind turbine structural dynamics, aerodynamics and control system dynamics. So far, the viscous drag coefficients are usually defined as constants, independent of the load cases. With the computationally efficient frequency-domain model, it is possible to iterate the drag, such that it fits to the obtained amplitudes of oscillation of the different members. The results show that the drag coefficients vary significantly across operational load conditions. The viscous drag coefficients converge quickly and the method is applicable for concept-level design studies of FOWTs with load case-dependent drag.

Mitigation of heave response of semi-submersible platform (SSP) using tuned heave plate inerter (THPI)

The undesirable motions resulting from wave loading can lead to the long-term fatigue damage or even catastrophic sinking of offshore semi-submersible platforms (SSP). It is therefore by all means necessary to suppress the excessive vibrations of SSP. Many methods have been proposed to mitigate the heave motion of offshore platforms, such as using a fixed heave plate (FHP) to increase the draft and damping of the system, or adopting a tuned heave plate (THP) to form a tuned mass damper (TMD) system. In this paper, a novel inerter-based control system, namely a tuned heave plate inerter (THPI), is proposed for control of SSP heave vibrations. In this system, an inerter device, which can transform the linear motion into the high-speed rotational motion and thus significantly amplifies the physical mass of the system, is added to the THP to further improve the performance of conventional THP. Analytical studies are performed to investigate the effectiveness of the proposed method. The mean square heave motions of SSP without control device and with FHP, THP and THPI are stochastically formulated, and the optimal design parameters for THP and THPI are derived. Parametric studies are conducted to investigate the influences of the size and original depth of heave plate on the optimal performances of FHP, THP and THPI. Finally, a novel waterwheel inerter is developed to realize the suggested device. The analytical results show that THPI is more effective to mitigate the heave motion of SSP compared to the conventional methods, and the novel waterwheel inerter is capable of generating a large apparent mass by using a small waterwheel.