Floater Motions Research Articles

An effective bidirectional solving method for the motion and mooring tension of moored floaters

With more and more moored floaters having been applied in ocean engineering globally, the security of moored floaters is becoming a key concern. Although there are extensive studies on the motion and mooring tension of moored floaters, few of them addressed the direct relation between the motion and mooring tension of floaters. This study establishes a direct relation between floater motion and mooring tension through the fairlead position and proposes a new effective bidirectional solving method on the floater motion and mooring tension in time domain. Noted that this method is independent to specific environmental factors. To validate the proposed method, analytical solutions are firstly obtained and then compared with the 2-D experimental measurements and 3-D numerical simulations. The comparison shows that the predictions by the proposed method match well with the measurements and simulations. Therefore, given the time history on either motion or mooring tension of floaters, the other can be predicted by the proposed method efficiently. For operational environments offshore, this method can be used to predict the motion or mooring tension, which is also saving in cost and time.

A semi-analytical solution for energy harvesting via the elastic motion of the circular floater of aquaculture cages attached with piezoelectric

This work's unique research objective is to assess the possibility of energy harvesting by attaching the piezoelectric patches on the high-density polyethylene floater of aquaculture fish cages. Using Euler-Bernoulli's curved beam theory, the vertical elastic motion of the floater in regular linear sea waves was modeled semi-analytically based on slender body theory and matched asymptotic expansions. Regular linear waves refer to the single frequency waves with small amplitudes compared to the wavelength, for which the linear airy wave theory can approximate the wave characteristics. When the floater elastic motions are determined, the resultant strain is implemented to the governing equation of piezoelectric patches to calculate the generated power. The effect of some variables such as wavenumber, floater bending stiffness, relative floater dimension, and the number of patches in the circumference of the floater on the generated power was investigated. The results indicate that by installing the patches in the proper place, the floater can produce the required electrical power supplying some low-powered electric sensors for monitoring sea environmental conditions or other applications. Also, as a suggestion, installing piezoelectric patches throughout the perimeter of a semi-submerged circular elastic floater is a way of energy harvesting from sea waves.

A dynamical study on sea waves interaction with an array of circular elastic floaters of aquaculture fish cages

In this paper, linear oblique wave incident to circular elastic floaters of aquaculture fish cages was studied semi-analytically. This work's unique research objectives include considering the array of elastic floaters where each floater consists of two tori with minor differences in their main radii. Hydrodynamic interaction between the inner and outer of each floater body and hydro-elastic interaction between floaters in the array are the particular features of the problem. Using slender body theory and matched asymptotic expansions, each 3D problem was decomposed into two separate, more straightforward problems named near-field and far-field problems. This approach helps derive fundamental solutions for symmetric and antisymmetric motions. Also, using linear wave theory, the diffracted and radiated waves due to the presence of floaters were expressed in Fourier modes of symmetric and antisymmetric motions. Moreover, Graf's and Gegenbauer's addition theorems for Bessel functions are used to shift between different coordinate systems. This approach is helpful to bridge on calculating related integrals numerically. The developed model verification by BEM-based software demonstrates that the model solves the mathematical formulas with enough accuracy. The effect of various parameters such as floater dimensions, wavenumber, the distances between floaters, and the bending stiffness of floater on the floater's motions was studied. The results demonstrate that as the wavenumber increases, the floater's hydro-elastic interaction becomes essential. Furthermore, the upstream floaters do encounter larger wave loads than a single floater in waves.

Cost-based mooring designs and a parametric study of bridles for a 15 MW spar-type floating offshore wind turbine

This paper highlights a cost-based design method for feasible and cost-effective mooring configurations. A parametric study of mooring designs considering bridle impact is performed for a 15 MW spar-type floating offshore wind turbine (FOWT). This site-specific FOWT is based on the preliminary model from EU funded H2020 Project COREWIND. Without running numerical simulations, the cost-based design method generates 1443 mooring configurations with normalized costs ranging from 1.00 to 1.667. Six configurations with the lowest normalized costs are selected for OpenFAST simulations. The ultimate test results verify the structural strength of mooring configurations. Bridle designs cause significant impact on the ultimate mooring tensions and floater motions in surge, sway and yaw, which is clearly influenced by the mass ratio of a bridle to a mooring line. When the mass ratio decreases from 13% to 5%, peak mooring tensions increase by up to 17%. The influence of bridle designs on mooring tension fatigue is less prominent and the effect of mass ratio on mooring fatigue varies with wind speed. Near the rated wind speed, deviations in mooring fatigue loads are within 5% for all six configurations. This cost-based method promotes feasible and economical mooring designs. The parametric study of bridle designs provides solid support for optimal mooring designs of spar-type FOWTs.

An engineering modification to the blade element momentum method for floating wind turbines

The design of next-generation offshore wind rotors for floating applications carries large uncertainties that make it harder to bring costs down. A significant share of this uncertainty is associated with the use of Blade Element Momentum (BEM) methods to run aeroelastic design and loadcase calculations, because the assumptions underlying the momentum theory are violated by the floater motion. This work presents an engineering method for BEM that allows to model generic floater motions and improves the aerodynamic modelling of floating wind turbines. The new model overcomes the challenge of estimating the apparent wind caused by the platform motion in a simple way, which proves to be very effective for rigid aerodynamic simulations. To verify its impact, BEM results with and without the proposed correction are compared to high-fidelity free vortex wake simulations, imposing the same harmonic motion first to the pitch and then to the yaw degree of freedom of the platform. While little differences are found for the yaw motion, the new floating model clearly improves predictions for the platform pitch case in terms of both aerodynamic loads and induction, finding a very good match with the high-fidelity results. An analysis of the verification data provides new valuable insights on the effect of these motions on aerodynamic loads and performance of a floating wind turbine.

Impact of Turbulent Time Scales on Wake Recovery and Operation

Enhancing wake recovery behind wind turbines has the potential to significantly improve the power production and efficiency of large wind farms. Rather than investigating turbine control strategies, floater motion or global turbulent quantities such as turbulence intensity, this work aims to study wake stability and recovery through a focus on the turbulent scales of the inflow. Using Large Eddy Simulations of a single turbine, sinusoidal streamwise forcing is applied to the inflow with a constant amplitude and mean flow velocity, but differing time scales between 80s and 140s. For all applied time scales the turbine wake is characterised by the rolling-up of the near wake into the periodic shedding of vortex rings, and an excitation of the applied forcing frequency resulting in velocity fluctuations in the wake several times larger than that at the inflow. For shorter time scales (80s - 90s) a more aggressive and earlier wake roll-up led to a shorter near wake region, faster overall recovery and significantly improved the expected power output from 6R downstream onwards. An inflow time period of 80s gave rise to more than a 50% increase in power output of a fictive downstream turbine placed at 14R downstream, compared to an inflow time scale of 140s.

Wind field reconstruction using nacelle based lidar measurements for floating wind turbines

In this work we investigate the influence of floater motions on nacelle based lidar wind speed measurements. The analysis is focused on wind field characteristics which are most relevant for performance analysis, namely rotor effective wind speed, shear and turbulence intensity. A numerical approach, coupling the publicly available in-house lidar simulation framework ViConDAR (Virtual Constrained turbulence and liDAR measurements) with an aeroelastic floating offshore wind turbine simulation is employed. Synthetic turbulent wind fields are generated with the open source turbulence generator TurbSim. The dynamics of the floating offshore wind turbine are simulated in the aeroelastic simulation code OpenFAST. Turbine dynamics and the corresponding synthetic wind field are then passed to the lidar simulation module of ViConDAR, which has been adapted for the consideration of turbine dynamics in all six degrees of freedom to simulate the lidar measurements under influence of motion. The simulation framework is demonstrated in a case study, simulating a lidar system on the nacelle of the IEA 15 MW turbine in combination with the WindCrete floater concept. Two different realistic lidar patterns are investigated under different metocean conditions. Different motion cases are examined individually and combined to evaluate the influence of rotational and translational degrees of freedom. Results show an increase of mean absolute error between lidar estimated and full wind field rotor effective wind speed of 0 to 25% depending on the environmental conditions. Observed overestimation of mean rotor effective wind speed was found to be in the region of up to 1%.

Slack coupled modeling method and dynamic analysis on floating vertical axis wind turbine with helical blades

The working performance of floating vertical axis wind turbine (FVAWT) with helical blades under complex ocean environment is greatly influenced by its dynamic characteristics. For better grasping the dynamic characteristics of helical blade FVAWT, a slack coupled modeling method is proposed in this paper. During the modeling process, the wind turbine system is divided into two configurations, which are connected through the transmission of large overall motion of the blade. Then the reliability of the proposed model is verified by validating the motions of floating foundation and deformations of blade, respectively. Afterwards, the dynamic characteristics of a 5 MW helical blade FVAWT are analyzed. Results show that the amplitude and the equilibrium position of floater motions are influenced by wave loads and wind loads, respectively. And the blade deformation is dominated by aerodynamic frequency components. In addition, the relative angle between orientation of blade section and direction of the wind are sensitive to the helical twist angle, which will lead to the phase difference of aerodynamic loads and the blade deformations between different sections. This study could provide a reasonable mathematical model on design and investigation of the helical blade FVAWT.

機構解析による新型式FOWT 浮体の波浪中動揺及び弾性体応答の評価に関する研究

Floating offshore wind turbines (FOWTs) have been attracting a lot of attention as a main energy source for a carbon-neutral society. Although it is said that the costs of commercialization of FOWTs are still high, many studies have been conducted to rationalize costs. The main cost is construction of the floating structure, and now many novel FOWTs concepts have been proposed to reduce it. The unique shape and structure of one of those FOWTs, which consist of a steel tower and concrete lower hulls with wires holding them rigid, make it difficult to evaluate their safety using existing numerical analysis methods. In this study, we investigated a multi body dynamics analysis method for the novel floater. The analysis method was employed to reproduce the behavior in regular waves, considering the elastic response of the floating body. A model test was also carried out to verify the accuracy of the analysis method. In the model test, a special large-size scale model was used, which was manufactured in the same way as the actual model. Comparison of both results, i.e., the analysis and the model test, showed good agreement in floater motions, mooring tensions, and wire tensions. On the other hand, differences were also observed between them for some wave period conditions. It is possible that the asymmetry of the floating structure and its columns that penetrate the water surface at an angle caused the differences in the estimation of the fluid force acting on the columns. Through the analysis, the effectiveness of the FOWTs concept on reduction its tower base load was also confirmed.

On the aerodynamic loading effect of a model Spar-type floating wind turbine: An experimental study

Aerodynamic loading is one of the most dominating environmental excitations of Floating Wind Turbines (FWTs) and plays an important role in the FWT dynamics. In this study, we developed a model Spar-type FWT and then constructed a dedicated experiment apparatus to reveal the aerodynamic loading effects. As for the floater motion, the wind loading serves as an external exciting force, as well as potential damping source and equivalent added mass item. To take all these roles into account, we proposed a concept of aerodynamic loading effect. The presence of aerodynamic loading effect is validated by free decay tests and white noise wave tests. Results show that the aerodynamic loading effect alters the natural frequencies and damping ratios of the FWT system. We suggest the FWT designers refer to the altered natural frequencies when designing the floater and the FWT controllers. We experimentally observed that the increased aerodynamic loading seems to suppress the pitch resonance vibration while amplifies the resonance vibration at surge frequency. Besides, the nacelle motions, blade loads, and the tower dynamics, are all significantly impacted by the aerodynamic loading effect. The presented results are potentially helpful for optimizing FWTs and developing advanced FWT controllers.

Assessing focused wave applicability on a coupled aero-hydro-mooring FOWT system using CFD approach

The present study examines the wave type and wave steepness impacts onto the floating offshore wind turbine (FOWT) hydro/aerodynamics. With the aid of using a high-fidelity aero-hydro-mooring CFD solver, the current modelling focuses on the analysis of NREL 5 MW semi-submersible FOWT dynamic motion response, tension load of the mooring line, wind turbine thrust, power output and tower base bending moment. Totally, three types of waves, i.e., focused wave, irregular wave and reconstructed focused wave are adopted to examine the wave type impacts on FOWT performance. Our results find that, given a same wave spectrum, a significant difference is observed for the floater motion response prediction between a focused wave and an irregular wave. However, with the use of the reconstructed focused wave, the results show very similar FOWT hydrodynamic characteristics as those obtained with the irregular wave, indicating that the reconstructed focused wave can be an alternative of the irregular wave for extreme wave studies. Moreover, nonlinear effect is well captured for various wave steepness tested, which is revealed by the occurrence of wave diffraction and large wave run-ups near the side columns. The examination on FOWT aerodynamics found that both wave type and wave steepness have little impact on turbine aerodynamic performance which is evident by a close agreement of thrust and power prediction under different incident waves.

Investigation of the floating IEA Wind 15 MW RWT using vortex methods Part I: Flow regimes and wake recovery

AbstractFloating wind turbines have the potential to enable global exploitation of offshore wind energy, but there is a need to further understand the complex aerodynamic phenomena they can encounter due to floater induced rotor motion. Aerodynamic models traditionally used in the wind energy sector, like the Blade Element Momentum (BEM) theory, may not be capable of capturing the dynamic phenomena that occur when the rotor moves in and out of its own wake. In the present paper, we therefore compare an industry standard BEM‐based code to a state of the art vortex solver, to investigate the phenomena in detail and further clarify the capabilities and limitations of both methods. An initial benchmark of the two codes using the IEA Wind 15 MW RWT mounted on the WindCrete spar‐buoy floater is carried out. Three different scenarios are taken into account: a bottom‐fixed, a floating case, and a floating case subject to regular waves. Growing discrepancies between the codes have been observed with the increasing complexity of the simulations. Moreover, large differences between the wake generated by a bottom fixed and a floating turbine have been observed, with the latter one experiencing a faster recovery. To further explore the floating turbines behaviors that can affect the rotor performance and wake, a systematic investigation of the mean tilt angle influence in the wake development has been carried out. Further, to account for the oscillatory motion of a floating turbine, a parametric study where the floater motion is prescribed in both pitch and surge degrees of freedom (DoF) is designed. The study covers a large variety of scenarios; a wide range of relevant frequencies and amplitudes are taken into account in under and above‐rated wind conditions. A total of more than 28 unique cases have been defined and simulated with both fidelity models. The results include the downstream evolution of the wake recovery and intensity of the turbulent disturbances induced by the rotor. The generic nature of the study allows to characterize the flow and performance effects and enables subsequent generalization to floater designs of given natural frequency and motion amplitude. It has been found that the BEM and LL predictions of the maximum loading in the blade root and tower bottom compare quite well, except for the case of large oscillation frequency in above rated conditions, where the BEM method under‐predicts the loads. Moreover, the use of a vortex solver makes it possible to look in depth into the wake characteristics, in which large differences are observed between the bottom‐fixed and the floating case in non‐turbulent inflow conditions. It has been found that the frequency and amplitude of the turbine oscillations can have a strong impact on the recovery of the wake. Moreover, we have found a link between short time intervals of large FA blade tip displacements and a faster break down of the wake. Finally it has been shown that a floating wind turbine with large and fast oscillations can transition between the different rotor states, including the non‐conventional propeller and vortex ring states, which are identified and characterized.

Application of SADA method on full-scale measurement data for dynamic responses prediction of Hywind floating wind turbines

Floating offshore wind turbines (FOWTs) are promising solutions for offshore renewable energy harvesting, with the successful installation and operation of the world's first commercial floating wind farm, Hywind Scotland, in 2017. However, both academia and industry are still constantly facing challenges in the aspects of cost reduction, monitoring, safety and sustainability improvement for the design and maintenance of FOWTs. The purpose of this paper is to demonstrate an engineering application of a novel Artificial Intelligence knowledge-based method, named SADA, on the full-scale measurement data of an Hywind FOWT. The SADA method was applied to perform numerical optimization and dynamic responses prediction of the FOWTs, based on the full-scale data from one Hywind FOWT in Scotland. The methodology of SADA and the key technology of the application are introduced firstly. Then, the selection of Key Discipline Parameters (KDPs) is introduced, followed with the training of AI-based numerical models with full-scale measurement data, including Floater motions, wind, wave and current data. After that, the numerical model imbedded in SADA is trained to be intelligent for the objective Hywind FOWT under different sea states. The intelligent SADA model is used to do comparisons and predictions. The comparison results show that using SADA method, the AI-trained numerical model can predict the motions of Hywind supporting Floater in higher accuracy. In addition, other physical quantities that cannot be obtained directly in full-scale measurement easily but are of great concern by industry, can also be obtained from a more believable perspective. This AI-based SADA method brings an innovative vision for FOWTs' full-scale measurement technology in the future.

Assessment of mooring configurations for the IEA 15MW floating offshore wind turbine

To achieve cost-effective deployment of floating offshore wind farms, it is necessary to reduce mooring costs of Floating Offshore Wind Turbines (FOWTs). Beyond the cost, in terms of environmental impact, the seabed disrupted area due to mooring lines should be mitigated with care. The objective of this paper is to shed light on design parameters for cost- effective and low-footprint mooring configurations for FOWTs using coupled dynamic analyses. A design space is explored for mooring configurations with different pretension ratios, laid down length ratios and clump weight sizes. Ultimate and fatigue load cases are simulated in OpenFast to compute the floater motions, mooring line tensions and fatigue damage. With constant pretension ratio of 0.15 and adding clump weights of 40t, mooring line length, mooring footprint and peak tension can be reduced by 14%, 15% and 9% respectively, while maximum surge and fatigue damage increase by 25% and 12% respectively. This paper will serve as a basis for further work on mooring design in the EU H2020 funded project COREWIND and provide a practical reference for the mooring system design for FOWTs.

Comparison of concrete and steel semi-submersible floaters for 10MW wind turbines

Prestressed concrete floaters are gaining growing interest for floating wind turbines, while different material properties can affect the characteristics of floater motions and the range of suitable floater geometries. In this study, a basic geometry of a semi-submersible floater for 10MW is designed, based on which sensitivity study of the material property on floater design is conducted. It was found that minimum floater pitch angles achieved by selecting the optimum floater geometries were similar for the three types of material, while larger values of geometrical variables were required for concrete floaters than for steel floaters for a floater pitch limitation. If larger wall thickness was to be required for concrete structures due to, for example, corrosion protection and crack limitation, generally larger side column width and lower hull length were required for concrete floaters. As the sensitivity of pitch angle to the wall thickness was more affected by wind excitation than wave excitation for normal concrete, it is indicated that the design space for the length of hull can be widened by combining the design with the wind turbine controller methods in the treatment of the maximum thrust force. Lightweight concrete had advantages over normal concrete in the lower center of gravity. The raw material cost of lightweight concrete was higher than normal concrete but significantly lower than steel floaters.

Bag and floater motions of a fabric membrane cage

We investigate bag and floater motions of a scaled fabric membrane cage model. Model tests with an assumed model scale of 1:27 were performed at SINTEF Ocean’s towing tank, where the cage model consisted of an enclosing fabric (or a bag), an elastic floater, a mooring system, and an internal water mass. The model was moored in a towing tank and subjected to regular waves with different periods and steepnesses. The first harmonic component of the floater and bag motions time series were analyzed, and image analysis was performed for the latter. Several interesting responses of the fabric membrane cage are discussed: cage set-down, wavelike bag motion, and floater motions. The magnitude of the cage set-down is dependent on the incident wave period and steepness; the magnitude increases in response to a longer and steeper wave. The bag also exhibits a wavelike motion that propagates from the floater to the bottom part of the cage and has apparent implications for the dynamics of the bag bottom and the snap loads on the fabric material. Simplified numerical simulations were performed using WAMIT, which assumed a rigid cage model. A floater motions comparison between model tests and WAMIT results shows the importance of taking the bag and floater flexible modes into account when performing hydrodynamic analysis of a fabric membrane cage.

Experimental field study of floater motion effects on a main bearing in a full-scale spar floating wind turbine

In this paper we present a full-scale experimental field study of the effects of floater motion on a main bearing in a 6 MW turbine on a spar-type floating substructure. Floating wind turbines are necessary to access the full offshore wind power potential, but the characteristics of their operation leave a gap with respect to the rapidly developing empirical knowledge on operation of bottom-fixed turbines. Larger wind turbines are one of the most important contributions to reducing cost of energy, but challenge established drivetrain layouts, component size envelopes and analysis methods. We have used fibre optic strain sensor arrays to measure circumferential strain in the stationary ring in a main bearing. Strain data have been analysed in the time domain and the frequency domain and compared with data on environmental loads, floating turbine motion and turbine operation. The results show that the contribution to fluctuating strain from in-plane bending strain is two orders of magnitude larger than that from membrane strain. The fluctuating in-plane bending strain is the result of cyclic differences between blade bending moments, both in and out of the rotor plane, and is driven by wind loads and turbine rotation. The fluctuating membrane strain appears to be the result of both axial load from thrust, because of the bearing and roller geometry, and radial loads on the rotating bearing ring from total out-of-plane bending moments in the three blades. The membrane strain shows a contribution from slow-varying wind forces and floating turbine pitch motion. However, as the total fluctuating strain is dominated by the intrinsic effects of blade bending moments in these turbines, the relative effect of floater motion is very small. Mostly relevant for the intrinsic membrane strain, sum and difference frequencies appear in the measured responses as the result of nonlinear system behaviour. This is an important result with respect to turbine modelling and simulation, where global structural analyses and local drivetrain analyses are frequently decoupled.

Experimental study on the motion response of an integrated floating desalination plant and offshore wind turbine on a non-ship platform

An on-grid floating desalination plant (FDP), powered by conventional fossil fuel, was recently proposed to support freshwater demands in some remote coastal cities that have electricity grid networks. The aim of this study is to investigate feasibility of integrating a wind turbine into the same FDP platform to be suitable for off-grid services with sustainable clean energy resources. The new proposed concept is a fully self-contained mobile system powered by wind. Using a 1:100 scale model of the proposed concept, an experimental study was performed to investigate the floater's motion behavior in Egypt. According to historical sea statistical data for Red Sea in Egypt and taking combined dynamic responses of turbine and floating platform into consideration, frequency and time history dynamic analyses have been done. Furthermore, the possibility of using five different wind turbines in the same FDP platform was studied for upgrading purposes. Results show that the proposed FDP concept has capability to support all tested turbines and to operate safely in Egyptian environmental conditions. Based on the FDP concept with and without turbine comparisons, there are minor motion changes in heave responses, while pitch and surge responses show major changes in time history analyses due to turbine operation.

Effects of upstream rotor tilt on a downstream floating wind turbine

This work quantifies the relationship between the design tilt of an upstream rotor and the structural response of a spar floating offshore wind turbine located in its wake. Three wind speed scenarios are considered: below, at, and above rated operation. The inflow is generated with the Mann model and the wake and loads are simulated with FAST.Farm. As the upstream rotor tilt goes from 0° to 6°, we find that the mean wake is displaced upward by more than 0.2 rotor diameters (D) by a downstream distance of 7 D. The vertical velocities increase by up to 35 cm/s in the center of the wake. As a result, the downstream rotor is partially waked and experiences a rotated velocity vector. With a higher upstream rotor tilt, the velocities and moments on the downstream turbine increase their mean axial value and their lateral and vertical standard deviation. These changes affect the blade and tower loading and the floater motion primarily in the out-of-plane direction: the damage-equivalent loads for the tower pitch moment and blade-root moment increase by up to 10% because of higher variability at the first mode for the tower and at one blade passing frequency for the blade root. Lesser effects are observed for the roll moments and for floater sway and heave. When the joint effect of rotor tilt and platform motion is considered, the load response on the downstream system is amplified primarily for tower pitch and blade out-of-plane moment.

A Numerical Solution for Modelling Mooring Dynamics, Including Bending and Shearing Effects, Using a Geometrically Exact Beam Model

During the operation of moored, floating devices in the renewable energy sector, the tight coupling between the mooring system and floater motion results in snap load conditions. Before snap events occur, the mooring line is typically slack. Here, the mechanism of energy propagation changes from axial to bending dominant, and the correct modelling of the rotational deformation of the lines becomes important. In this paper, a new numerical solution for modelling the mooring dynamics that includes bending and shearing effects is proposed for this purpose. The approach is based on a geometrically exact beam model and quaternion representations for the rotational deformations. Further, the model is coupled to a two-phase numerical wave tank to simulate the motion of a moored, floating offshore wind platform in waves. A good agreement between the proposed numerical model and reference solutions was found. The influence of the bending stiffness on the motion of the structure was studied subsequently. We found that increased stiffness increased the amplitudes of the heave and surge motion, whereas the motion frequencies were less altered.