Effects Of Platform Motion Research Articles

Mechanical analysis for deepwater drilling riser system with structural parameters uncertainty

Uncertain structural parameters increase the mechanical failure risk of the drilling riser system. The effect of uncertain structural parameters on the mechanical characteristics of the drilling riser system is unclear. Therefore, the mechanical characteristics of the drilling riser system with structural parameter uncertainty are analyzed. A mechanical model with uncertain structural parameters is established based on the basic analysis model and the local average theory. The basic analysis model takes into account the combined effects of platform motion, tensioner, riser string, and marine environment. The local average theory is suitable for situations where there is a lack of uncertainty information. A solution approach is proposed using the second-order central perturbation method and the Newmark-β method. The solution method reduces the load by simplifying degrees of freedom and random variables. The correctness of the solution method is verified using the Monte Carlo method. The analysis results indicate that uncertain structural parameters significantly affect the static response, while the dynamic response is less affected. The failure risk due to static axial stress and static shear stress is heightened by uncertain structural parameters. The failure risk arising from static shear stress at the bottom of the riser system experiences a significant increase.

On the characteristics of the wake of a wind turbine undergoing large motions caused by a floating structure: an insight based on experiments and multi-fidelity simulations from the OC6 project Phase III

Abstract. This study reports the results of the second round of analyses of the Offshore Code Comparison, Collaboration, Continued, with Correlation and unCertainty (OC6) project Phase III. While the first round investigated rotor aerodynamic loading, here, focus is given to the wake behavior of a floating wind turbine under large motion. Wind tunnel experimental data from the UNsteady Aerodynamics for FLOating Wind (UNAFLOW) project are compared with the results of simulations provided by participants with methods and codes of different levels of fidelity. The effect of platform motion on both the near and the far wake is investigated. More specifically, the behavior of tip vortices in the near wake is evaluated through multiple metrics, such as streamwise position, core radius, convection velocity, and circulation. Additionally, the onset of velocity oscillations in the far wake is analyzed because this can have a negative effect on stability and loading of downstream rotors. Results in the near wake for unsteady cases confirm that simulations and experiments tend to diverge from the expected linearized quasi-steady behavior when the rotor reduced frequency increases over 0.5. Additionally, differences across the simulations become significant, suggesting that further efforts are required to tune the currently available methodologies in order to correctly evaluate the aerodynamic response of a floating wind turbine in unsteady conditions. Regarding the far wake, it is seen that, in some conditions, numerical methods overpredict the impact of platform motion on the velocity fluctuations. Moreover, results suggest that the effect of platform motion on the far wake, differently from original expectations about a faster wake recovery in a floating wind turbine, seems to be limited or even oriented to the generation of a wake less prone to dissipation.

Numerical study of the effect of central platform motion on the wave energy converter array

Placing multiple wave energy converters (WECs) on a central floating platform has become a popular approach of ocean energy conversion in recent years. The strong interaction between the WECs and the platform brings a number of technical challenges for the design of such devices. The present study investigates the effect of the platform motion on the WEC array of a typical ocean energy harvesting device, consisting of a central support platform and eight point-absorber WECs arranged in a circular array. The numerical studies are performed in four stages based on the potential flow solver ANSYS-AQWA. The power absorption of each WEC in isolation and of eight WECs operating simultaneously is evaluated and compared in the first and second stages respectively, revealing the interaction among the WECs. The effect of a fixed and a floating platform on the power absorption of the WEC array is investigated in the next two stages. The systematic studies demonstrates that the heave motion of the platform improves the power absorption of the WEC array for most wave frequencies tested, due to the increase in the wave-surface elevations around the WECs and the phase differences of the heave motion between the WECs and the platform. The opposite is true for the pitch motion of the platform. Furthermore, the WEC dimension, the platform dimension, and the distance between the WECs and the platform should be optimized based on the actual operating sea conditions to increase the wave-surface elevations and the phase differences. Otherwise, the motion of the platform should be restricted as much as possible.

Comparative analysis of bent and basic winglets on performance improvement of horizontal axis wind turbines

The winglet has been widely deployed in the optimization of the blade design as it reduces the tip loss of the blade and increases the swept area of the rotor. However, previous studies have not adequately investigated the effect of platform motion on winglet performance in wind turbines. The objective of this study is to propose a novel bent winglet structure for wind turbines to improve performance under both stationary and surge conditions. To achieve this, the NREL Phase VI horizontal axis wind turbine (HAWT) is treated as a baseline. The numerical method employed is validated by comparing the simulated power and pressure coefficients of the HAWT with experimental data from the literature. The performance of the conventional winglet with the proposed novel winglet is compared in detail, taking into account the cant, twist, expansion direction, length, and winglet number of the conventional winglet. The results show that the novel bent winglet exhibits superior performance, with a 14.5% improvement in performance compared to the conventional winglet under surge motion. This study provides a feasible scenario for the optimization of onshore and offshore wind turbine designs.

Shipborne HFSWR Target Detection in Clutter Regions Based on Multi-Frame TFI Correlation

High-frequency surface wave radar (HFSWR) is an important marine monitoring technology, and this new regime of radar plays an important role in large-scale, continuous early-warning monitoring at sea. In particular, shipborne HFSWR has wider applications in detecting interesting sea areas, with the advantages of flexible deployment and extended detection capability. Due to the large amount of sea clutter accompanying the echo signals of shipborne HFSWR and the spread of sea clutter due to platform motion, the detection of targets in clutter regions is extremely difficult. In this paper, a multi-frame time-frequency (TF) analysis–based target-detection method is proposed. First, the sea clutter spreading area in the HFSWR echo signal is modeled, and the effects of platform motion and currents on the sea clutter spread are analyzed to determine the sea clutter coverage area; this paper focuses on frequency modeling. Then the TF image (TFI) of each range cell is obtained by TF analysis of the cells within a certain range of the echo signal, and the range cells of possible target points are determined by binary classification of the TFI through a convolutional neural network. Finally, the location of the final target point is obtained by correlation of multi-frame TFIs. Shipborne HFSWR field experiments show that the proposed detection method performs well in detecting targets concealed by sea clutter.

Modeling of near wake characteristics in floating offshore wind turbines using an actuator line method

Fast and effective numerical models describing the effect of platform motion on the performance of floating offshore wind turbines (FOWTs) are fundamental to assess energy harvesting potential in large offshore wind farms. The purpose of this paper is to implement a CFD-based Computationally-Efficient approach based on an actuator line model (ALM) for FOWTs aerodynamics. Such a tool aims at complementing reasonable accuracy and affordable computational effort while being able to investigate the effects of the platform motions on the wake evolution. The actuator line model for FOWTs is developed by implementing a dedicated C++ library in the OpenFOAM toolbox. In addition, a tip treatment is applied to involve the tip effects induced by the pressure equalization from the suction and pressure sides. Results show that employing ALM decreases computational cost and preprocessing time for producing appropriate computational grids, as just about 400k and 600k grids are necessary for solving two representative test cases of fixed-bottom turbines (NREL Phase VI and NREL 5-MW) with reasonable accuracy. The inclusion of platform motion is then introduced, and the results showed that ALM is capable of capturing vortices trajectory, potential blade-vortex interactions, and vortex pairing and vortex ring state phenomenon in FOWTs.

A Numerical Investigation of the Geometric Characteristics of Floating Wind Turbine Wakes under Axial and Yawed Rotor Conditions

The physics of wind turbine wakes on fixed foundations have been extensively studied through numerical modelling and measurements over the recent decades. While research initiatives in the field of floating wind turbines have increased significantly, various aspects characterising floating turbine wakes are still not well understood. This paper applies a free-wake vortex model to derive the geometric characteristics of the wake of a surging rotor under axial and yawed rotor conditions. The study examines the influence of platform motion on the wake boundaries and centreline up to four rotor diameters downstream. It is shown that floating turbine wakes experience a higher degree of perturbation as compared to those of fixed turbines. This is highly dependent on the sea state and suggests the need to develop new analytical wake models for the wind industry that are able to account the effect of platform motion on wake behaviour.

Modeling of Wake Characteristics in Floating Offshore Wind Turbines Using an Actuator Line Method

Fast and effective numerical models describing the effect of platform motion on the performance of floating offshore wind turbines (FOWTs) are fundamental to assess energy harvesting potential in large offshore wind farms. The purpose of this paper is to implement a CFD-based Computationally-Efficient approach based on an actuator line model (ALM) for FOWTs aerodynamics. Such a tool aims at complementing reasonable accuracy and affordable computational effort while being able to investigate the effects of the platform motions on the wake evolution. The actuator line model for FOWTs is developed by implementing a dedicated C++ library in the OpenFOAM toolbox. In addition, a tip treatment is applied to involve the tip effects induced by the pressure equalization from the suction and pressure sides. Results show that employing ALM decreases computational cost and preprocessing time for producing appropriate computational grids, as just about 400k and 600k grids are necessary for solving two representative test cases of fixed-bottom turbines (NREL Phase VI and NREL 5-MW) with reasonable accuracy. The inclusion of platform motion is then introduced, and the results showed that ALM is capable of capturing vortices trajectory, potential blade-vortex interactions, and vortex pairing and vortex ring state phenomenon in FOWTs.

Ocean Surface Cross Section for Bistatic HF Radar Incorporating a Six DOF Oscillation Motion Model

To investigate the characteristics of sea clutter, based on ocean surface electromagnetic scattering theory, the first- and second-order ocean surface scattering cross sections for bistatic high-frequency (HF) radar incorporating a multi-frequency six degree-of-freedom (DOF) oscillation motion model are mathematically derived. The derived radar cross sections (RCSs) can be reduced to the floating platform based monostatic case or onshore bistatic case for corresponding geometry setting. Simulation results show that the six DOF oscillation motion will result in more additional peaks in the radar Doppler spectra and the amplitudes and frequencies of these motion-induced peaks are decided by the amplitudes and frequencies of the oscillation motion. The effect of the platform motion on the first-order radar spectrum is greater than that of the second-order, and the motion-induced peaks in the first-order spectrum may overlap with the second-order spectrum. Furthermore, yaw is the dominant factor affecting the radar spectra, especially the second-order. Moreover, the effect of platform motion on radar spectra and the amplitudes of the second-order spectrum decreases as the bistatic angle increases. In addition, it should be noted that the amplitudes of the Bragg peaks may be lower than those of the motion-induced peaks due to the low frequency (LF) oscillation motion of the floating platform, which is an important finding for the applications of the floating platform based bistatic HF radar in moving target detection and ocean surface dynamics parameter estimation.

Effect of Platform Motion on Aerodynamic Performance and Aeroelastic Behavior of Floating Offshore Wind Turbine Blades

In the present study, a numerical framework for predicting the aerodynamic performance and the aeroelastic behavior of floating offshore wind turbine rotor blades involving platform motion was developed. For this purpose, the aerodynamic and structural analyses were conducted simultaneously in a tightly coupled manner by exchanging the information about the aerodynamic loads and the elastic blade deformations at every time step. The elastic behavior of the turbine rotor blades was described by adopting a structural model based on the Euler-Bernoulli beam. The aerodynamic loads by the rotor blades were evaluated by adopting a blade element momentum theory. The numerical simulations were conducted when the platform of the wind turbine independently moves in each of the six degrees-of-freedom directions consisting of heave, sway, surge, roll, pitch, and yaw. It was observed that flexible blades exhibit complicated vibratory behaviors when they are excited by the aerodynamic, inertia, and gravitational forces simultaneously. It was found that the load variation caused by the platform surge or pitch motion has a significant influence on the flapwise and torsional deformations of the rotor blades. The torsional deformation mainly occurs in the nose-down direction, and results in a reduction of the aerodynamic loads. It was also found that the flapwise root bending moment is mainly influenced by the platform surge and pitch motions. On the other hand, the edgewise bending moment is mostly dictated by the gravitational force, but is not affected much by the platform motion.

Effects of platform motions on aerodynamic performance and unsteady wake evolution of a floating offshore wind turbine

The capacity of offshore wind power has increased recently because of the emerging environmental and social problems in onshore wind turbines. A floating offshore wind turbine (FOWT) system experiences the additional six-degree-of-freedom (6DoF) motions caused by both wind and wave loads. These motions are associated with the distortion of the wake structure and the oscillation of aerodynamic performance. This study focused on the unsteady wake characteristics of FOWTs. A nonlinear vortex lattice method (NVLM) was coupled with a vortex particle method (VPM) and used for simulation of the NREL 5-MW wind turbine undergoing periodic motions. Translational (heave, sway, and surge) and rotational (yaw, pitch, and roll) motions were imposed on the wind turbine, and the displacements of the floating platform were defined as a sinusoidal function. Significant variations in the thrust force and power output were observed for the streamwise motions. In addition, the platform motions affected the wake evolution strongly, thus resulting in periodic deformation of the wake structure and the rapid breakdown of helical wake vortices for all motions. A discussion of the current study could facilitate in understanding the wake-induced phenomena and the unsteady wake behavior of FOWTs.

Effects of second-order wave forces and aerodynamic forces on dynamic responses of a TLP-type floating offshore wind turbine considering the set-down motion

A coupled 6 degree of freedom (DOF) aero-hydro-restoring model is developed to study the dynamics of a tension leg platform (TLP)-type offshore wind turbine. This model includes second-order wave loads, pitch control strategies, and effects of platform motion on aerodynamic performance. The second-order wave force is calculated using the full-field quadratic transfer function. The coupled effects of the horizontal motions (such as surge, sway, and yaw motions) and the set-down motion are taken into consideration through the nonlinear restoring matrix. Different load scenarios are chosen to simulate the platform's dynamic response and the rotor's aerodynamic performance in the time domain. The analysis shows that second-order wave forces will induce slow-drift and springing resonances. Due to the large motion in surge, a significant set-down motion which is an important part of heave motion is induced by the second-order difference-frequency force. When different wind types are chosen, the aerodynamic damping effect is found in the slow-drift motion in surge and the springing motion in pitch. On the other hand, low-frequency set-down motion is excited and the heave response is amplified by the wind. Compared to the steady wind, the effect of aerodynamic damping is reduced under the turbulent wind. Slow-drift frequency, wave frequency, and springing frequency are all observed in the aerodynamic results.

First‐order bistatic high‐frequency radar ocean surface cross‐section for an antenna on a floating platform

The first-order bistatic high-frequency radar cross-section of ocean surface is derived for the case of a fixed receiver and a floating, but tethered, transmitter. A general expression for the bistatically received first-order electric field is obtained from earlier work based on fixed antennas. A small displacement caused by the platform motion is added in the source term to modify the stationary antenna model. Based on the assumption that the ocean surface can be described as a Fourier series with coefficients being random variables, the first-order bistatic radar cross-section is derived. The effect of the platform motion is found to produce a sum of Bessel functions in the final cross-section result, varying in order from zero to infinity. Under appropriately specified conditions, the bistatic model with antenna motion is verified to reduce to the monostatic model with antenna motion and the bistatic stationary model, respectively. Assuming a simple model in which the platform motion is caused by the dominant ocean wave, simulations are made to show the effect of platform motion on the radar cross-section under a variety of sea states and operating frequencies.

CFD simulation of a floating offshore wind turbine system using a variable-speed generator-torque controller

Prediction and control of rotor rotational velocity is critical for accurate aerodynamic loading and generator power predictions. A variable-speed generator-torque controller is combined with the two-phase CFD solver CFDShip-Iowa V4.5. The developed code is utilized in simulations of the 5 MW floating offshore wind turbine (FOWT) conceptualized by the National Renewable Energy Laboratory (NREL) for the Offshore Code Comparison Collaboration (OC3). Fixed platform simulations are first performed to determine baseline rotor velocity and developed torque. A prescribed platform motion simulation is completed to identify effects of platform motion on rotor torque. The OC3’s load case 5.1, with regular wave and steady wind excitation, is performed and results are compared to NREL’s OC3 results. The developed code is shown to functionally control generator speed and torque but requires controller calibration for maximum power extraction. Generator speed variance is observed to be a function of unsteady stream-wise platform motions. The increased mooring forces of the present model are shown to keep the turbine in a more favorable variable-speed control region. Lower overall platform velocity magnitudes and less rotor torque are predicted corresponding to lower rotor rotational velocities and a reduction in generated power. Potential improvements and modifications to the present method are considered.

The Second-Order Bistatic High-Frequency Radar Cross Section of Ocean Surface for an Antenna on a Floating Platform

. The 2nd-order bistatic high-frequency radar cross section of ocean surface is derived for the case of a fixed receiver and a floating transmitter. The 2nd-order radar cross section contains both the hydrodynamic contribution and the electromagnetic contribution. The 2nd-order hydrodynamic contribution is obtained based on the 1st-order bistatic model with antenna motion. The derivation of the 2nd-order electromagnetic contribution begins with a general expression for the bistatically received 2nd-order electric field for the case of a floating antenna. A new bistatic electromagnetic coupling coefficient, which, unlike some earlier versions, produces no nonphysical singularities in the radar cross section, is derived. The new bistatic radar cross section model is verified by comparing it with that of a monostatic swaying antenna case and that of the bistatic case without antenna motion. The effect of platform motion on simulated Doppler spectra is considered for a variety of sea states and operating frequencies. The motion-induced 2nd-order peaks appearing in the spectrum are seen to have significantly less energy than those in the 1st-order case.

Investigation of the effects of platform motion on the aerodynamics of a floating offshore wind turbine

Along with the flourishing of the wind energy industry, floating offshore wind turbines have aroused much interest among the academia as well as enterprises. In this paper, the effects of the supporting platform motion on the aerodynamics of a floating wind turbine are studied using the open source CFD framework OpenFOAM. The platform motion responses, including surge, heave and pitch, are superimposed onto the rotation of the wind turbine. Thrust and torque on the wind turbine are compared and analysed for the cases of different platform motion patterns together with the flow field. It is shown that the movement of the supporting platform can have large influences on a floating offshore wind turbine and thus needs to be considered carefully during the design process.

Bistatic High Frequency Radar Ocean Surface Cross Section for an FMCW Source with an Antenna on a Floating Platform

The first- and second-order bistatic high frequency radar cross sections of the ocean surface with an antenna on a floating platform are derived for a frequency-modulated continuous wave (FMCW) source. Based on previous work, the derivation begins with the general bistatic electric field in the frequency domain for the case of a floating antenna. Demodulation and range transformation are used to obtain the range information, distinguishing the process from that used for a pulsed radar. After Fourier-transforming the autocorrelation and comparing the result with the radar range equation, the radar cross sections are derived. The new first- and second-order antenna-motion-incorporated bistatic radar cross section models for an FMCW source are simulated and compared with those for a pulsed source. Results show that, for the same radar operating parameters, the first-order radar cross section for the FMCW waveform is a little lower than that for a pulsed source. The second-order radar cross section for the FMCW waveform reduces to that for the pulsed waveform when the scattering patch limit approaches infinity. The effect of platform motion on the radar cross sections for an FMCW waveform is investigated for a variety of sea states and operating frequencies and, in general, is found to be similar to that for a pulsed waveform.

Moving Target Detection in Distributed MIMO Radar on Moving Platforms

This paper examines moving target detection in distributed multi-input multi-output radar with sensors placed on moving platforms. Unlike previous works which were focused on stationary platforms, we consider explicitly the effects of platform motion, which exacerbate the location-induced clutter non-homogeneity inherent in such systems and thus make the problem significantly more challenging. Two new detectors are proposed. The first is a sparsity based detector which, by exploiting a sparse representation of the clutter in the Doppler domain, adaptively estimates from the test signal the clutter subspace, which is in general distinct for different transmit/receive pairs and, moreover, may spread over the entire Doppler bandwidth. The second is a fully adaptive parametric detector which employs a parametric autoregressive clutter model and offers joint model order selection, clutter estimation/mitigation, and target detection in an integrated and fully adaptive process. Both detectors are developed within the generalized likelihood ratio test (GLRT) framework, obviating the need for training signals that are indispensable for conventional detectors but are difficult to obtain in practice due to clutter non-homogeneity. Numerical results indicate that the proposed training-free detectors offer improved detection performance over covariance matrix based detectors when the latter have a moderate amount of training signals.

Transfer of Training on the Vertical Motion Simulator

This paper describes a quasi-transfer-of-training study in the NASA Ames Vertical Motion Simulator. Sixty-one general aviation pilots trained on four challenging commercial transport tasks under one of four different motion conditions: no motion, small hexapod, large hexapod, and Vertical Motion Simulator motion. Then, every pilot repeated the tasks in a check with Vertical Motion Simulator motion to determine if training with different motion conditions had an effect on task performance. New objective motion criteria guided the selection of the motion parameters for the small and large hexapod conditions. Considering results that were statistically significant, or marginally significant, the motion condition used in training affected 1) longitudinal and lateral touchdown position, 2) the number of secondary stall warnings in a stall recovery, 3) pilot ratings of motion utility and maximum load factor obtained in an overbanked upset recovery, and 4) pilot ratings of motion utility and pedal input reaction time in an engine-out-on-takeoff task. Because the training motion conditions revealed statistical differences on objective measures in all the tasks performed in the Vertical Motion Simulator motion check, with some in the direction not predicted, trainers should be cautious not to oversimplify the effects of platform motion. Evidence suggests that the new objective motion criteria may offer valid standardization benefits because increases in the training motion fidelity, as predicted by the two conditions covered by the criteria, resulted in expected trends in pilot ratings and objective performance measures.

A derivation and discussion of a mutual information lower bound for dynamic acoustic response functions under a Gauss Markov law

A derivation and discussion of a mutual information lower bound for broadband acoustic channels under a Gauss-Markov assumption is provided. This lower bound is associated with dynamic broadband acoustic response functions that obey a Gauss Markov law. Considered here is the difference of the mutual information between source and receive signals given the acoustic response and the mutual information between acoustic response and receive signal given the source signal. The derivation illuminates the features of this mutual information and permits identifying two key information loss mechanisms. The first is an added self noise term proportional to signal power that is only associated with the innovation variance of the Gauss Markov response. The second term also increases with the innovation variance but is associated with the uncertainty in the state of the channel operator at the initial time of signaling. This later variance is identified as a forward-backward channel estimation error. Platform motion effects and their impact on these two loss mechanisms are discussed and the structure of an effective signal to noise ratio (eSNR) ceiling at high SNR are identified.