Platform Yaw Research Articles

About control of bi-wind turbine single-point-moored floating offshore wind turbines

In the last years, several novel floating offshore wind energy concepts have appeared in the market, whose goal is the reduction of floating offshore wind energy costs. Among them, bi-wind turbine floating offshore wind turbines, which use a single floating platform to place two wind turbines, are one of the most promising ones. However, they face some challenges in their full deployment. For example, depending on the wind direction, one of the rotors could be found downstream of the other one and hence be affected by its wake. To avoid this, the turbines of this type of systems do not use a conventional active yaw mechanism, but a single-point-mooring configuration, to allow the platform rotation around the vertical axis. This way, the alignment of the rotors with the wind can be achieved, avoiding wake issues. However, as stated in the literature, at least for single-rotor wind turbines, this alignment is not straightforward and some yaw drift could appear in the platform. Therefore, the main goal of this paper is to confirm that this platform yaw drift could also arise in bi-wind turbine single-point-moored floating wind energy systems, and analyse its impact in terms of performance, while shedding some light on the performance improvements when applying new advanced control strategies.

Numerical analysis of aero-hydrodynamic wake flows of a floating offshore wind turbine subjected to atmospheric turbulence inflows

The influence of atmospheric turbulence inflow on coupled dynamic behaviors and wakes of floating offshore wind turbine (FOWT) is pronounced, especially as blade size increases. In this study, we explore the effects of atmospheric inflow on aero-hydrodynamics and wake characteristics of a spar-type FOWT. A well-validated in-house computational fluid dynamic (CFD) solver, FOWT-UALM-SJTU, is utilized to conduct the numerical simulations. To generate the realistic atmospheric inflow condition, the large eddy simulation (LES) with long-duration is employed. Two other cases involving uniform inflow and shear inflow are conducted to provide some comparable results. Compared to uniform inflow and shear inflow, the power and thrust of FOWT under atmospheric inflow display greater instability, with their power spectrums exhibiting higher intensity in high frequency region. The variation of yaw moment in atmospheric scenario is significantly drastic, leading to a remarkable response in platform yaw motion. Notably, the dominant frequency in this scenario is the blade passage frequency, as opposed to the incident wave frequency observed in the uniform and shear scenarios. The atmospheric inflow promotes the manifestation of wake breakdown and wake meandering, resulting in the faster wake recovery of FOWT. Besides, the meandering in the far wake is more significant compared to uniform and shear scenarios. The absence of vortex rings in atmospheric scenario is observed, along with the presence of more complex and smaller vortices in far wake. Our findings highlight the remarkable impacts of atmospheric inflow on dynamic responses and wake evolution of FOWT, and it is recommended that the atmospheric inflow be taken into account when assessing wake interactions between multiple FOWTs.

The impact of yaw motion on the wake interaction of adjacent floating tidal stream turbines under free surface condition

Several works put emphasis on the maturation of floating tidal stream turbine systems, deemed viable for sites at deep water depths. The objective of this research is to determine, through a CFD model, the yaw effect on the wake and functioning of a floating twin-turbine system under wave-induced motion. The simulation certainty is corroborated against experiments on a 1:7 piled twin-turbine system (Froude similarity). Collectively, the variation and deficiency of the power and thrust coefficients are amplified with the yaw frequency and amplitude. Whereas the individual wakes collide, depending on the yaw feature and turbine cross-current spacing, resembling a smoke-plume-type pattern. The larger cross-current spacing shortens the wakes and reduces near-wake deficit at mid-row due to the greater energy exchange from the ambient flow. Regardless of the yaw feature, the transversal wake deficits turn from double (majority symmetrical) at near wake into triple distributions thenceforth, before flattening at far wake. Importantly, the wake unsteadiness and recovery rate are more obvious with period, as opposed to amplitude effect. As yaw operation is imminent, contributed by flow skewness and platform yaw motion, it will be constructive to implement natural frequency analysis to prevent vortex-induced vibrations and mechanical weariness in the floating system.

Benefits of individual pitch control on offshore wind turbine submerged in upstream wake

The present study investigates the benefits of an individual pitch control (IPC) strategy on a DTU 10MW with a floating platform subjected to an upstream wind turbine wake. The wind turbine wakes are generated via large eddy simulation, with which the dynamic response of the downstream wind turbines are calculated using OpenFAST with IPC engaged. Three wind speeds and six arrangements with varying longitudinal and lateral offsets of wind turbine pairs are investigated. Findings show that the large spatial variance of the wind velocity in the rotor-swept area leads to a higher fatigue load of out-of-plane blade-root moment (MOoP). The IPC is effective in suppressing the 1P peak of the power spectral density of MOoP, as well as the yaw fluctuation of the platform motion, which saw a maximum 20% reduction in the damage equivalent load of MOoP and up to 74% reduction in the platform yaw fluctuation in the staggered wind turbine layout.

Dynamic Response of 15 MW Floating Wind Turbine With Non-Redundant and Redundant Mooring Systems Under Extreme and Accidental Conditions

AbstractThis work focuses on examining the dynamic behavior of large floating offshore wind turbine (FOWT) exposed to extreme (wind and wave with 50-year return period) and accidental (mooring line breakage) loadings. The FOWT considered is 15 MW reference turbine supported on semi-submersible platform stationed using catenary moorings. As the mooring configuration greatly affects the response of FOWT, two different mooring configurations namely non-redundant (three-line) and redundant (six-line) systems were studied and compared. The coupled multi-body dynamic system was solved using Openfast. When simulating the mooring line failure, both the operating and extreme loading conditions were considered. Failure of one mooring was considered at a time. The response of the coupled system due to breakage of the mooring indicates high displacements in surge and sway directions in comparison to the intact system especially for the non-redundant mooring system. Furthermore, change in platform yaw angle and increased tension in the other intact mooring lines were observed. The findings from this study will be helpful in accidental limit state design and preventing failure of similar large FOWT systems. In addition, insights into using non-redundant and redundant mooring configurations for such large structures with respect to extreme and accidental loadings were also discussed.

Platform yaw drift in upwind floating wind turbines with single-point-mooring system and its mitigation by individual pitch control

Abstract. This work demonstrates the feasibility of an individual pitch control strategy based on nacelle yaw misalignment measurements to mitigate the platform yaw drift in upwind floating offshore wind turbines, which is caused by the vertical moment produced by the rotor. This moment acts on the platform yaw degree of freedom, being of great importance in systems that have low yaw stiffness. Among them, single-point-mooring platforms are one of the most important ones. During recent years, several floating wind turbine concepts with single-point-mooring systems have been proposed, which can theoretically dispense with the yaw mechanism due to their ability to rotate and align with environmental conditions (weather-vaning). However, in this paper it is proven that the vertical moment overcomes the orienting ability, causing the yaw drift. With the intention of reducing the induced yaw response of a single-point-mooring floating wind turbine, an individual pitch control strategy based on nacelle yaw misalignment is applied, which introduces a counteracting moment. The control strategy is validated by numerical simulations using the 5 MW National Renewable Energy Laboratory (NREL) wind turbine mounted on a single-point-mooring version of the DeepCwind OC4 floating platform to demonstrate that it can mitigate the yaw drift and therefore maintain the alignment of the wind turbine rotor with the wind.

Aero-hydro-servo-elastic coupled modeling and dynamics analysis of a four-rotor floating offshore wind turbine

This paper developed a simulation framework, including multi-body dynamics, aerodynamics, hydrodynamics, controller dynamics, and mooring system, for the fully coupled dynamic analysis of a four-rotor floating offshore wind turbine (FOWT). The coupled framework is constructed based on SIMPACK, MATLAB/Simulink, user defined force element, and third-party modules to be suitable for a four-rotor FOWT. Then, the coupled framework is verified by a code-to-code method. The simulations under normal operating conditions indicate that the framework is able to investigate the dynamic responses of a four-rotor FOWT. Further capability of the framework is investigated through a case study of a parked rotor situation. The results show that the effect of an upper parked rotor is generally more significant than that of a lower parked rotor. The parked rotor reduces the tower base fore-aft loads and mooring line fairlead tensions, but increases the fluctuation of the tower base side-to-side loads. Additionally, it can be concluded that the yaw control system design is necessary because the shutdown of one of four rotors significantly affects the platform yaw responses. Furthermore, the developed coupled framework can also be extended to model bottom-fixed offshore and onshore wind turbines with different numbers of rotors.

Platform Oscillation Reduction of a Floating Offshore Wind Turbine

This paper focuses on the design of an individual blade pitch controller to reduce the platform oscillation of a floating offshore wind turbine with repositioning capacity. Such reduction is helpful for captured power regulation in wind farm control via turbine repositioning. In this study, a baseline 5MW wind turbine on a semi-submersible platform with long mooring lines is used for demonstration purposes. The model for the controller design is obtained as a state equation using the linearization functionality of the wind turbine simulator OpenFAST, activating only platform pitch and platform yaw as degrees of freedom. Based on the model, a linear quadratic regulator (LQR) is designed for platform vibration suppression. During each revolution, the controller prioritizes producing restoring moments to address platform pitch oscillation as the blades approach the vertical centerline of the rotor plane. Conversely, as the blades rotate close to the horizontal centerline of the rotor plane, the controller shifts its focus towards generating restoring moments to counteract platform yaw fluctuations. It is demonstrated in OpenFAST that the designed LQR-based individual blade pitch controller effectively reduces platform surge, roll, pitch, and yaw oscillation, resulting in improved power regulation during turbine repositioning.

Comparative dynamic analysis of two-rotor wind turbine on spar-type, semi-submersible, and tension-leg floating platforms

Multi-rotor floating offshore wind turbines have been recently proposed as an innovative technology to further reduce the cost of offshore wind energy. Even though examples of commercial prototypes are present, the literature lacks studies on the dynamic performance of such systems. This work presents a comparative analysis of a two-rotor wind turbine concept mounted on spar-type, semi-submersible, and tension-leg platforms. Their short-term performance is assessed by considering six different load cases considering directionally congruent turbulent wind profiles and irregular sea states. The analysis is carried out through an in-house fully-coupled code developed in Modelica. AeroDyn v15 within FAST v8 by NREL is coupled to the Modelica code to achieve blade-element momentum capabilities. Results indicate that platform yaw motion is an important dynamic mode of the systems, particularly for the spar configuration. Stiffer station-keeping lines and longer fairlead distance to the platform centerline reduce significantly yaw motion, as in the case of the semi-submersible and tension-leg configurations. Large tower base bending moment standard deviations and the associated concentration of energy at the platform heave and pitch motion frequencies indicate an increased risk for fatigue damage for the TLP configuration, especially at above-rated wind speeds. Moreover, large tendon loads can pose concerns in terms of fatigue and limit state performance. Large mean platform pitch angle and yaw standard deviation contribute to the reduction of electric power output quality. Extreme storm conditions greatly increase the response standard deviation, especially for the semi-submersible configuration.

Linear Quadratic Regulator Optimal Control of Two-Rotor Wind Turbine Mounted on Spar-Type Floating Platform

Abstract Interest is steadily growing for multi-rotor wind turbine concepts. This type of wind turbine offers a practical solution for scaling issues of large wind turbine components and for the reduction of costs associated with manufacturing, logistics, and maintenance. However, the literature lacks thorough knowledge of the dynamic performance of multi-rotor wind turbine concepts installed on floating platforms. Previous research studied the dynamic response of a two-rotor wind turbine concept mounted on a spar-type floating platform (2WT). Platform yaw motion is a significant dynamic factor directly caused by differential turbulence intensity experienced by the two hubs coupled with the distribution of thrust loads on the tower structure. Blade-pitch control analysis also showed how the 2WT yaw response is extremely sensitive to the control strategy employed. In this work, a linear quadratic regulator (LQR) is used to design an optimal controller for the 2WT prototype. Three LQR gain schedules corresponding to three operation regions are considered. An in-house tool for the dynamic analysis of multi-rotor floating wind turbines is used for linear state-space extraction and dynamic analysis. The control performance in different load conditions is assessed against the baseline OC3 proportional-integral (PI) control strategy and a PI-P control strategy in a previous article presented by the authors.

Floating-Platform High-Frequency Hybrid Sky-Surface Wave Radar: Simulations and Experiments

A theoretical sea echo modeling of the floating-platform high-frequency hybrid sky-surface wave radar (HSSWR) is presented. The ionosphere is considered to be a no-tilt reflecting plane moving in the vertical direction. Subsequently, the first-order sea clutter cross section with consideration of the platform sway and yaw motion is derived. Simulations are conducted to explore the influences of the ionospheric movement and the platform oscillation motion on the sea clutter power spectrum. Simulation results indicate that the platform sway motion may induce additional peaks that distribute symmetrically concerning the Bragg peaks, while the platform yaw motion and the ionospheric movement may broaden even split the sea clutter. Experiments are conducted with the newly-developed HSSWR system, of which the receiving arrays are simultaneously deployed on a floating platform and the shoreside. Experimental measurements concerning the frequency shift, Doppler width, and the arrival angle of the E-layer/F-layer ionosphere reflected direct wave are compared and analyzed. The study demonstrates the feasibility of the proposed modeling and provides valuable guidelines for the sea clutter characteristics analysis and the data quality assessments of the floating-platform HSSWR.

Dynamic analysis of two-rotor wind turbine on spar-type floating platform

The dynamic response of a two-rotor wind turbine mounted on a spar-type floating platform is studied. The response is compared against the baseline OC3 single-rotor design. Structural design shows how the two-rotor design may lead to a mass saving of about 26% with respect to an equivalent single-rotor configuration. Simulations predict significant platform yaw response of the two-rotor floating wind turbine — about 6 deg standard deviation at the rated operating wind speed. It is shown how the platform yaw response is directly caused by the turbulence intensity at the hub coupled with the transversal distribution of thrust loads on the structure. A coupled control strategy for the rotor-collective blade pitch controller is proposed, in which a simple proportional control mitigating platform yaw motion is superimposed to the baseline OC3 PI controller. Numerical simulations show how platform yaw response is reduced by about 60%, at the cost of mean power loss at below-rated wind speeds of about 100 kW and maximum increase of the rotor-collective blade-pitch angles standard deviation of about 2 deg. Parametric analysis of mooring lines design shows how an equivalent mass density of the line of at least 190 kg/m is needed to avoid vertical loads at the anchors.

Ocean Surface Current Inversion With Anchored Floating High-Frequency Radar: Yaw Compensation

Deploying a high-frequency (HF) radar on one anchored floating platform for the maneuverability appears to be a good alternative. In this case, because the yaw rotation during one coherent integration time may be rather large, an effective method to get precise direction-of-arrival (DOA) estimations is important for reliable ocean surface current mapping. In this article, we propose an adaptive data channel conversion method to compensate for this kind of rotation. In this method, radar data can be converted from rotating antenna channels into pseudo-fixed beam channels by following the minimum-mean-square-error rule between the wanted pseudo-fixed beam and a reference beam. Simulations with a uniform circular array and one kind of yaw rotation, the range of which is 90 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> , are conducted. After compensation, the DOA estimation of one simulated object becomes unbiased and performs nearly as good as that on a fixed platform. Besides, the high root-mean-square errors of radial ocean currents, caused by the yaw rotation, also decrease. Furthermore, we introduce a field experiment of one anchored floating HF radar, during which the DOA estimations of two onshore radars' direct waves improved drastically. The field ocean currents, deduced from this floating HF radar, also indicate the feasibility of the proposed method. This method offers a new idea to compensate for a floating platform's yaw rotation and can help introduce the traditional onshore HF radar to a much more flexible offshore scenario.

Photon Pressure Force on Space Debris TOPEX/Poseidon Measured by Satellite Laser Ranging

AbstractThe (TOPography EXperiment) TOPEX/Poseidon (T/P) altimetry mission operated for 13 years before the satellite was decommissioned in January 2006, becoming a large space debris object at an altitude of 1,340 km. Since the end of the mission, the interaction of T/P with the space environment has driven the satellite's spin dynamics. Satellite laser ranging (SLR) measurements collected from June 2014 to October 2016 allow for the satellite spin axis orientation to be determined with an accuracy of 1.7°. The spin axis coincides with the platform yaw axis (formerly pointing in the nadir direction) about which the body rotates in a counterclockwise direction. The combined photometric and SLR data collected over the 11 year time span indicates that T/P has continuously gained rotational energy at an average rate of 2.87 J/d and spins with a period of 10.73 s as of 19 October 2016. The satellite attitude model shows a variation of the cross‐sectional area in the Sun direction between 8.2 m2 and 34 m2. The direct solar radiation pressure is the main factor responsible for the spin‐up of the body, and the exerted photon force varies from 65 μN to 228 μN around the mean value of 138.6 μN. Including realistic surface force modeling in orbit propagation algorithms will improve the prediction accuracy, giving better conjunction warnings for scenarios like the recent close approach reported by the ILRS Space Debris Study Group—an approximate 400 m flyby between T/P and Jason‐2 on 20 June 2017.

Quantitative Evaluation of Stereo Visual Odometry for Autonomous Vessel Localisation in Inland Waterway Sensing Applications.

Autonomous survey vessels can increase the efficiency and availability of wide-area river environment surveying as a tool for environment protection and conservation. A key challenge is the accurate localisation of the vessel, where bank-side vegetation or urban settlement preclude the conventional use of line-of-sight global navigation satellite systems (GNSS). In this paper, we evaluate unaided visual odometry, via an on-board stereo camera rig attached to the survey vessel, as a novel, low-cost localisation strategy. Feature-based and appearance-based visual odometry algorithms are implemented on a six degrees of freedom platform operating under guided motion, but stochastic variation in yaw, pitch and roll. Evaluation is based on a 663 m-long trajectory (>15,000 image frames) and statistical error analysis against ground truth position from a target tracking tachymeter integrating electronic distance and angular measurements. The position error of the feature-based technique (mean of ±0.067 m) is three times smaller than that of the appearance-based algorithm. From multi-variable statistical regression, we are able to attribute this error to the depth of tracked features from the camera in the scene and variations in platform yaw. Our findings inform effective strategies to enhance stereo visual localisation for the specific application of river monitoring.

Evaluation of a new body-sideslip-based driving simulator motion cueing algorithm

This paper describes a new motion cueing algorithm for motion-based driving simulators. The algorithm uses the simulated vehicle’s body sideslip angle as the demand for the motion platform’s yaw degree of freedom. The current state of the art for motion cueing algorithms involves some form of filter or controller that limits the bandwidth of the vehicle motion before using this as the motion platform demand; the algorithm is tuned such that the platform does not exceed its limits. However, this means that information about the vehicle state that is contained within the motion is removed indiscriminately. Since the body sideslip angle will fit within the platform yaw limit under normal conditions, it does not need to be filtered beforehand, and thus no information must be removed. The implementation of the body-sideslip-based algorithm is described, as is a set of tests using human participants wherein the body sideslip algorithm was compared against the three most popular existing algorithms (namely the classical, adaptive and linear quadratic regulator algorithms) for normal road driving. The results of these tests indicate that the body sideslip algorithm performs as well as, or marginally better than, the other algorithms; future work will test the algorithm under limit handling conditions, to see whether the approach of preserving vehicle state information improves the simulator driver’s perception.

Rotation and direction judgment from visual images head-slaved in two and three degrees-of-freedom.

The contribution to spatial awareness of adding a roll degree-of-freedom (DOF) to telepresence camera platform yaw and pitch was examined in an experiment where subjects judged direction and rotation of stationary target markers in a remote scene. Subjects viewed the scene via head-slaved camera images in a head-mounted display. Elimination of the roll DOF affected rotation judgment, but only at extreme yaw and pitch combinations, and did not affect azimuth and elevation judgement. Systematic azimuth overshoot occurred regardless of roll condition. Observed rotation misjudgments are explained by kinematic models for eye-head direction of gaze.

Measurement of Ocean Wave Directional Spectra Using Doppler Side-Scan Sonar Arrays

A technique is presented for extraction of ocean wave directional spectra using Doppler side-scan sonars. Two 103-kHz steerable side-scan beams from a freely drifting subsurface platform are used to estimate horizontal water surface velocity due to waves. The side-scan targets are clouds or layers of microscopic air bubbles, confined to within a few meters of the ocean surface. Upward-looking sonars are used to estimate the surface height power spectrum. The side-scans are steered to compensate for any platform yaw, and corrections to the measured wave velocity due to horizontal and vertical platform motion must be applied. Measurements of wave velocity with the side-scans in this geometry are found to have acceptable spectral signal-to-noise properties at horizontal ranges up to 150 m in the frequency range 0.05–0.35 Hz, with degraded performance at greater range. Time series of horizontal wave velocity at 16 range points along each of two orthogonal beams are used in a maximum-likelihood array processing technique to estimate the directional spreading functions versus frequency. Numerical and experimental tests suggest that the maximum-likelihood method can reconstruct adequately unimodal and bimodal wave fields. These methods are demonstrated with ocean data from the northeastern Pacific.

Vector Representation of Finite Rotations

line in-plane swing angle and the platform pitch angle are calculated to be cxeq = 0eq = 0.0372 rad. Figures 2 and 3 show variations of the real part of the least-damped oscillatory mode for both subsystems, with the Q and R matrices. Only by trial and error can one arrive at suitable values for Q and R that result in the desired closed-loop system response. The crosses show the optimal design point, arrived at after a series of parametric studies. Figures 4 and 5 show the transient response of the differential length, the platform pitch, and the tether line inplane swing angle, as well as platform yaw, roll, and tether line out-of-plane swing angle for initial conditions of 105 m in tether length, length rate 5.5 x 10~ m/s and 0.05 rad in all the variational angles and angular rates of 5.5 x 10~ rad/s. It is seen that it takes much more time for the out-of-plane subsystem to reach equilibrium than for the in-plane subsystem.