Surge Motion Research Articles

Analysis of floating platform-mooring system-pile-soil interactions under wave loadings: A case study of the triangle-shaped semi-submersible platform

The floating platform-mooring system-pile-soil interactions under wave loadings represent a complex yet understudied aspect of floating offshore wind turbines (FOWTs). This paper presents the development and application of an Incompressible Material Point Method-Finite Element Method-Material Point Method (IMPM-FEM-MPM) model to simulate these interactions for a triangle-shaped semi-submersible platform (Tri-SSP), which serves as the foundation of China's first deep-sea FOWT, named ‘Fuyao.’ The proposed model is validated by studying the hydrodynamic responses of the OC4-DeepCwind platform. Under regular waves, increasing wave heights lead to heightened motion amplitudes, with the Tri-SSP exhibiting distinct phase differences between surge, pitch, and heave motions. Anchor Group 1 experiences significant peak tensions, ranging from 2.26 MN to 4.47 MN, which influence soil displacement, shear forces and bending moments along the pile shaft, emphasizing the impact of wave loading on pile-soil interactions. Under 1-h irregular waves, tension in Anchor Group 1 reaches extreme levels, with tensions up to 8 MN observed, underscoring the critical role of mooring systems in mitigating wave-induced forces. The calculated maximum stress within the pile indicates the structural integrity of the DH36 steel pile under these conditions. Comparisons of hydrodynamic responses, both with and without pile-soil interactions, show minimal discrepancies in motion amplitudes, indicating a minor influence on the dynamics of the platform and mooring system. These findings contribute to the understanding of complex interactions in deep-sea FOWTs, informing design and operational strategies for enhanced stability and performance.

Experimental study on the hydrodynamic performance of a multi-DOF WEC-type floating breakwater

A hybrid system consisting of wave energy converters and breakwaters can be applied for both wave energy conversion and shoreline protection. Besides, the cost of wave energy conversion is believed to be reduced by such a combination. In this paper, a two-dimensional asymmetric wave energy converter-type breakwater with a triangle-baffle bottom that can convert wave energy through its heave motion is proposed and investigated experimentally in regular waves. A supporting system is designed and established to allow the model to move in a single degree of freedom or prescribed combination of multiple-degree-of-freedoms. The effects of the geometric asymmetry, the bottom slope, the width-to-draft ratio, and the multiple-degree-of-freedom motion on the wave energy conversion and attenuation performances are analyzed. The results show that the triangle-baffle wave energy converter-type breakwater has much higher wave energy conversion efficiency than its rectangular counterpart with the same displacement. Increasing the bottom slope and allowing pitch motion can improve efficiency, allowing surge motion will reduce efficiency, and the effect of increasing the width is uncertain due to two conflicting effects. The asymmetry, bottom slope, and width only have a limited influence on the wave attenuation, whereas surge and pitch motions reduce the wave attenuation in short waves, analogizing a wavemaker.

A rapid motion forecast strategy for ships in waves using seakeeping and maneuvering modules

This paper develops a new navigation assisted system for ships with time-varying wave disturbances. A new framework is presented using seakeeping and maneuvering modules to achieve the fast forecast of ship motions. S175 containership and KVLCC2 tanker are adopted to verify the seakeeping module. Based on the validated module, the heave, roll and pitch motions of a cruise ship in different conditions are calculated by using three-dimensional Rankine panel method. The horizontal surge, sway and yaw motions of ships can be rapidly evaluated by using MMG model considering the wave drift forces. Finally, the motion amplitudes of 6°-of-freedom can be forecasted rapidly by using seakeeping and maneuvering modules. The study provides the insight into the motions of ships in waves. It is of great engineering application value to evaluate the total ship motion amplitudes and to improve the safety of navigation of ships in waves.

Surge motion-induced dynamic inflow effects in floating offshore wind turbines: A State Prediction Model

The impact of surge motion on aerodynamic unsteadiness in modern floating offshore wind turbines (FOWT) is a well-recognized phenomenon. When coupled with advanced controllers integrated into these turbines, it can lead to fluctuations in power output. This paper introduces a state-space model that incorporates dynamic inflow effects resulting from surge motion and a PID pitch controller. Additionally, an observation equation is formulated to predict the system power output. Through time series validations, the proposed state-space model demonstrates its ability to accurately capture power responses influenced by surge motions. The developed model serves as a valuable tool for the comprehensive analysis of FOWT, allowing for the exploration of the intricate interplay between unsteady aerodynamics, advanced control mechanisms, and power output.

Multi-fidelity actuator-line modelling of FOWT wakes

Large Eddy Simulations (LES) are considered the proper tool for predicting the physics of a wind turbine wake, thereby establishing a solid foundation for investigating the interaction among floating turbines within wind farms. In this work the Actuator Line Model, implemented in the OpenFoam environment, is combined with both U-RANS and LES simulations to underline the differences in accuracy when reproducing the near and far wake of a single turbine. Both a fixed-bottom and a surge motion case are tested to highlight the wake phenomena strictly generated by the platform motion. The use of LES simulation becomes fundamental by virtue of its ability to accurately simulate turbulence and mixing with free-stream flow, hence, this research aims at advancing the knowledge of wake dynamics from multiple perspectives while ensuring reliability thanks to the use of the experimental wake data from the UNAFLOW test campaign on the scaled laboratory model of the DTU 10 MW. In the near wake, limited flow unsteadiness and similar mean velocity are predicted by the two models. In the far wake, instead, the LES approach estimates a strong rise of flow unsteadiness which, in case of surge motion, affects the mean velocity value making evident the difference between LES and RANS estimation.

Data-driven forecasting of FOWT dynamics and load time series using lidar inflow measurements

This study focuses on forecasting the fairlead tension and floater dynamics time series of floating offshore wind turbines (FOWTs) using a data-driven approach that incorporates onboard sensor measurements and lidar inflow data. Sensors on FOWTs can provide data on turbine dynamics, such as rotational and translational movements, and load metrics like mooring line loads. However, these sensors are limited to current state measurements and do not provide future signal projections. In this research, we investigate a data-driven forecasting methodology using a simulated dataset. This dataset encompasses FOWT responses to diverse environmental conditions and the associated lidar measurements. Utilizing a Long Short-Term Memory (LSTM) sequence-to-sequence model, this study forecasts the time series of fairlead tension, surge, and pitch for forecasting horizons of 20, 40, and 60 seconds, considering lidar ranges from 100m to 500m. The performance of these forecasting models is benchmarked against a simple persistence model. The results indicate that incorporating lidar inflow measurements significantly improves the forecasts of fairlead tensions and platform motions. The enhancement for pitch motion forecasts is observed across all forecasting horizons. For fairlead tension and surge motion, the enhancement is observed for the longer horizons of 40s and 60s. These findings underscore the value of lidar data in accurate forecasting and emphasize the need to account for the interplay between lidar range, wind speed, and forecasting horizon to achieve optimal forecast accuracy.

Insights into the dynamic induction in FOWT surge motion using an actuator-line model

This research investigates the impact of surge motion on Floating Offshore Wind Turbines (FOWTs) through dynamic inflow analyses using an in-house Actuator Line Model (ALM). The study explores surge reduced frequencies in the range from 1 to 10 to analyse the returning wake phenomenon occurring at blade passage frequency. The findings highlight the dampening effect of the rotor thrust and torque by 25% due to induction field inertia. Noteworthy observations include the filter-like effect on the span-wise induced velocity trends: with the frequency increment, the induction field is ’synchronised’ with the apparent wind and forces. The results of the analyses evidence the suitability of the induction parsed in the absolute reference frame to capture the induction field features, amplitude and phase shift, at increasing surge frequencies.

Dynamic response of a wind turbine wake subjected to surge and heave step motions under different inflow conditions

The development of floating offshore wind turbines poses new challenges since the floating platform introduces complex dynamics in the wind turbine wake. These wake dynamics are intricately tied to the advection velocity, referring to the velocity of the downstream propagation of the air flow, and used in the context of wind farm modelling. The present article investigates the far-wake dynamic response of a wind turbine model subjected to heave (up-down translation) and surge (fore-aft translation) step motions under two distinct inflow conditions. Wind tunnel experiments were conducted with hot-wires in a realistic turbulent inflow and a low shear and no ground effect inflow, achieved by varying the hub height of the wind turbine model in the atmospheric boundary layer developed in the test section. The results show that the dynamic response of the wake under the low shear and no ground effect inflow conditions aligns with a second-order system with the presence of undershoots and overshoots. In contrast, under realistic conditions, it appears like a first-order system with undershoots and overshoots less evident in most cases. Despite these variations the determined advection velocity remains roughly the same and consistent with the literature for both heave and surge step motions, regardless of the inflow conditions.

Numerical simulation of stabilisation of floating wind with submerged hydrofoil

This research focuses on the optimal design and method of attaching a submerged hydrofoil to an offshore platform to enhance stabilisation. The flapping hydrofoil, exhibiting a hybrid motion combining heave and pitch, is engineered to convert incoming wave energy. It generates a distinctive wake that effectively counteracts incoming waves, thereby reducing wave impact. In this study, a NACA0030-type hydrofoil was strategically positioned between two columns of the platform model. Comprehensive analyses were conducted to evaluate the free-floating platform’s response to regular waves, with a focus on the attached hydrofoil. The results indicate that the hydrofoil significantly reduces the surge motion and drifting speed of the platform, affirming its effectiveness in enhancing stabilisation.

Wake characteristics and vortex structure evolution of floating offshore wind turbine under surge motion

Offshore wind energy is booming and floating offshore wind has been proved to be a promising clean energy. Wake effect is one of the biggest challenges in large-scale floating wind farm which results in power loss and increased fatigue load. In this paper, actuator line model (ALM) coupled with unsteady Reynolds-averaged Navier-Stokes (URANS) is adopted to investigate the wake characteristics and evolution of floating wind turbine under motion. Besides, modal analysis is adopted to extract typical features of turbine wake. Velocity fluctuation is the prominent characteristic of floating turbine wake. The increase of surge frequency results in the decrease of velocity fluctuating region and large surge amplitude leads to bigger velocity deficit fluctuations. In addition, the surge motion brings a faster recovery rate than fixed wind turbine. The induced velocity field generated by vortex ring structure is the key to understanding complex evolution of turbine wake and the vortex ring structure is analyzed in this paper. With the deep understanding of FOWT wake, an innovative vortex ring structure model is proposed in this paper. This work is the basic work of developing the engineering wake model of floating wind turbine, to cope with the wake effect challenge in the large-scale floating wind farm.

Study on aerodynamic and structural performance of floating offshore wind turbine with fusion winglets

ABSTRACT The surge motion of the platform affects the aerodynamic and structural performance of the floating offshore wind turbine (FOWT) with tip-fusion winglets. Hence, this paper aims to study the aerodynamic and structural performance of the wind turbine with and without tip-fusion winglets under the surge motion by ANSYS software. Comparison of the differences in the pressure distribution, flow fields, modal analysis, and stress of the wind turbines under surge motions. The results show that the tip-fusion winglets have an obvious influence on the pressure distribution and flow fields, and the influence mainly concentrates on the tip of the blade. In addition, the frequency of the wind turbine with fusion winglets is less than without winglets from the first mode to the third mode. The main vibration type of wind turbines with and without winglets is flapping. Besides, the maximum stress of the wind turbine with fusion winglets is increased by 19.91%, and there is a stress concentration phenomenon at the connection between the blade tip and the winglet, which is easy to cause fatigue damage. The location of the maximum stress appears in the transition section between the Cylinder airfoil and the DU airfoil.

Going beyond BEM with BEM: an insight into dynamic inflow effects on floating wind turbines

Abstract. Blade element momentum (BEM) theory is the backbone of many industry-standard wind turbine aerodynamic models. To be applied to a broader set of engineering problems, BEM models have been extended since their inception and now include several empirical corrections. These models have benefitted from decades of development and refinement and have been extensively used and validated, proving their adequacy in predicting aerodynamic forces of horizontal-axis wind turbine rotors in most scenarios. However, the analysis of floating offshore wind turbines (FOWTs) introduces new sets of challenges, especially if new-generation large and flexible machines are considered. In fact, due to the combined action of wind and waves and their interaction with the turbine structure and control system, these machines are subject to unsteady motion and thus unsteady inflow on the wind turbine's blades, which could put BEM models to the test. Consensus has not been reached on the accuracy limits of BEM in these conditions. This study contributes to the ongoing research on the topic by systematically comparing four different aerodynamic models, ranging from BEM to computational fluid dynamics, in an attempt to shed light on the unsteady aerodynamic phenomena that are at stake in FOWTs and whether BEM is able to model them appropriately. Simulations are performed on the UNAFLOW 1:75 scale rotor during imposed harmonic surge and pitch motion. Experimental results are available for these conditions and are used for baseline validation. The rotor is analyzed in both rated operating conditions and low wind speeds, where unsteady aerodynamic effects are expected to be more pronounced. Results show that BEM, despite its simplicity, can adequately model the aerodynamics of FOWTs in most conditions if augmented with a dynamic inflow model.

Effect of combined surge and pitch motion on the aerodynamic performance of floating offshore wind turbine

Floating offshore wind turbines (FOWTs) can encounter variations in the phase difference and amplitude of combined surge-pitch platform motion. The phase difference between surge and pitch movement can result in the superposition or cancellation of wind turbine motion, potentially leading to significant changes in its aerodynamic characteristics. Herein, a wind turbine model was developed using the computational fluid dynamics (CFD) method to investigate the aerodynamic performance of the wind turbine. The findings revealed significant variations in the aerodynamics characteristics of the wind turbine when subjected to surge and pitch motions. When the rotor moves forward with a large relative velocity, dynamic stall can be triggered and induce the violent fluctuation of aerodynamic load. Moreover, vortex ring state (VRS) may occur when the wind turbine moves backward, leading to the recirculation of tip and root vortices and possibly causing negative thrust and torque. The trend of the drag force acting on the tower exhibits an almost complete opposition to that of the rotor thrust with the change of phase difference in the combined surge-pitch motion. It is also noted that the tower lift fluctuates asymmetrically, and as the platform motion amplitude increases, the tower lift gradually increases towards one side.

Multi-freedom effects on a raft-type wave energy convertor- hydrodynamic response and energy absorption

This research delves into the effects of multiple degrees of freedom on a raft-type wave energy converter (WEC), using both experimental and computational methods. The WEC features two interconnected floating bodies linked by a specialized parallelogram joint, allowing for distinct heave, pitch, and surge motions. The power take-off (PTO) system, integrating Coulomb damping, is analysed for its impact on energy absorption. Despite varying damping amplitudes, peak energy absorption is consistent across different wave periods, as shown in scaled model tests. Computational simulations further investigate the multi-freedom effects by imposing constraints on various degrees of freedom, thus elucidating their individual and combined influences on energy absorption. Preliminary results indicate that the addition of degrees of freedom can enhance the WEC's efficiency, especially through coordinated release of the bodies' relative pitch and surge motions. The study also finds that the energy absorption efficiency from pitch motion depends on the release of surge motion. Moreover, the energy distribution among each degree of freedom is significantly affected by the device's multi-freedom characteristics. This research provides valuable insights and practical guidelines for the development of advanced multi-freedom WEC technologies.

An experimental study on the influence of wind-wave-current coupling effect on the global performance of a 12 MW semi-submersible floating wind turbine

Investigating the coupling effect between different marine environmental conditions is essential to understand the dynamic response of wind turbines. This paper presents the study findings from the model tests conducted in a wave basin, exploring the coupling effect of wind, waves, and currents on the global performance of a 1:70 scale model 12 MW semi-submersible floating wind turbine (FWT). A multi-blade large-scale wind generation system with a rectifier network was improved and fabricated to provide a reliable wind field for the experiment. The stiffness-matched and geometry-matched tower models were designed and fabricated, achieving an accurate simulation of the tower's flexibility and windward areas and their impact on the global dynamic response. Following the calibration of marine environmental conditions and the characterization of the model FWT system, the study focused on the operational and extreme responses of the FWT concept. The aerodynamic damping effect of wind in pitch and surge motions is remarkable, especially in pitch motion, primarily manifesting by reducing the standard deviation of motions. Mooring load responses are predominantly induced by the hydrodynamic effect, with the wave-frequency response gradually becoming dominant as wave parameters increase. Tower-top load responses are primarily induced by the hydrodynamic effect of waves and the aerodynamic load effect of wind. Furthermore, the phenomenon where the action of current strengthens the aerodynamic damping effect of wind is observed. The coupling effect between wind and current amplifies surge motion while diminishing pitch motion, effectively reducing the tower-top and tower-base load responses.

Effects of pore-fluid pressure on the motion of debris flows

ABSTRACT Pore-fluid pressure mediates the shear strength between soil particles and significantly controls the movement of debris flows. This study analyzes the effects by proposing a numerical model to solve the pore-fluid diffusion equation coupled to changes in surge motion of debris flows. The model was established by combining a Lagrangian method solving the depth-averaged one-dimensional equations for the debris-flow motion and a Fourier-series solution for the pore-fluid diffusion equation. Owing to the source term related to the change in surge depth, the Fourier-series solution shows that the excess pore-fluid pressure near the bed dissipates faster and exhibits a non-monotonic profile. The results of numerical calculations reveal that the diffusivity of excess pore-fluid pressure and the liquefaction condition have significant impacts on the depth profiles and velocities of debris-flow surges.

Forced surge motion of a floating bridge pontoon to evaluate the hydrodynamic damping properties

Dedicated model tests were carried out to investigate the hydrodynamic force coefficients in surge for a pontoon that is designed to support a floating bridge. The experiments were carried out by oscillating the pontoon at various Keulegan–Carpenter (KC) numbers and oscillation periods in forced motion. A pontoon designed for the floating bridge in Bjørnarfjorden, Norway, is investigated herein. Several numerical simulations are performed to supplement the experiments. The estimated damping coefficients in surge are of significant importance at the first horizontal resonant mode of the floating bridge, which may be excited due to swell sea in the fjord. The first horizontal resonant mode of the floating bridge is around 15–16 s. The model tests were conducted with and without turbulence triggers. A finding from the model tests were that turbulence triggers did not prove to be necessary to eliminate model scale effects in oscillatory flow.The KC-dependent added mass and damping coefficient in oscillatory flow is investigated. The added mass and damping coefficients are clearly KC-dependent. The wave radiation damping in surge is small at the tested oscillation periods, and the damping in surge is dominated by the damping due to flow separation.Numerical simulations using a hybrid potential flow-CFD solver with linear free-surface conditions, PVC3D, are carried out with both linear and nonlinear body-boundary conditions. The simulations with nonlinear body-boundary conditions predict the hydrodynamic force coefficients with reasonable accuracy, whilst the simulations with linear body-boundary conditions fail to predict the hydrodynamic coefficients with the desired accuracy — both used to investigate the viscous damping which arises due to flow separation along the pontoon body.

Wind turbine rotors in surge motion: new insights into unsteady aerodynamics of floating offshore wind turbines (FOWTs) from experiments and simulations

Abstract. An accurate prediction of the unsteady loads acting on floating offshore wind turbines (FOWTs) under consideration of wave excitation is crucial for a resource-efficient turbine design. Despite a considerable number of simulation studies in this area, it is still not fully understood which unsteady aerodynamic phenomena have a notable influence on the loads acting on a wind turbine rotor in motion. In the present study, investigations are carried out to evaluate the most relevant unsteady aerodynamic phenomena for a wind turbine rotor in surge motion. As a result, inflow conditions are determined for which a significant influence of these phenomena on the rotor loads can be expected. The experimental and numerical investigations are conducted on a two-bladed wind turbine rotor subjected to a tower-top surge motion. A specialised wind tunnel test rig has been developed to measure the aerodynamic torque response of the rotor subjected to surge motions with moderate frequencies. The torque measurements are compared to two free-vortex-wake (FVW) methods, namely a panel method and a lifting-line method. Unsteady contributions that cannot be captured using quasi-steady modelling have not been detected in either the measurements or the simulations in the covered region of motions ranging from a rotor reduced frequency of 0.55 to 1.09 and with motion velocity amplitudes of up to 9 % of the wind speed. The surge motion frequencies were limited to a moderate range (5 to 10 Hz) due to vibrations occurring in the experiments. Therefore, a numerical study with an extended range of motion frequencies using the panel and the lifting-line method was performed. The results from both FVW methods reveal significant unsteady contributions of the surge motions to the torque and thrust response that have not been reported in the recent literature. Furthermore, the results show the presence of the returning wake effect, which is known from helicopter aerodynamics. Additional simulations of the UNAFLOW scale model and the IEA 15 MW rotor demonstrate that the occurrence of the returning wake effect is independent from the turbine but determined by the ratio of 3P and surge motion frequency. In the case of the IEA 15 MW rotor, a notable impact of the returning wake effect was found at surge motion frequencies in the range of typical wave periods. Finally, a comparison with OpenFAST simulations reveals notable differences in the modelling of the unsteady aerodynamic behaviour in comparison to the FVW methods.

Comparative study on dynamic responses of integrated installation process of a 5-MW and a 15-MW offshore wind turbine considering a pre-installed foundation

The integrated installation method for offshore wind turbines has the characteristics of high efficiency and low economic cost due to the small amount of construction work. However, the complexity of the entire system is exacerbated by environmental loads and mechanical couplings. Hence, it is urgent to investigate the dynamic response of OWTs during integrated installation procedures. This work compares the dynamic responses between NREL 5 MW and IEA 15 MW OWT installation systems. The hydrodynamics are calculated using the potential flow theory and the Morison equation, while the aerodynamics under wind turbine parked conditions are computed using the blade element momentum theory. Time domain simulations, considering various wind and wave conditions, are conducted via SIMA. This paper discusses the effects of wind and wave loads on the motion responses of catamaran installation vessel and spar, the relative motion at the mating point, and the sliding gripper forces, respectively. The results reveal that both the surge motions of the catamaran and the spar and the pitch motion of the spar exhibit sensitivity to wind loads. During installation, the motion response of the IEA 15 MW model is more dramatic than that of the NREL 5 MW system. Under wave-only conditions, the radial distances of the mating points for both NREL 5 MW and IEA 15 MW systems reach 1.65 m and 2.51 m, respectively. However, the influences of wind loads and wave incident angle are limited.

Floating wind turbine motion signature in the far-wake spectral content – a wind tunnel experiment

Abstract. The growing interest in floating offshore wind turbines (FOWTs) is rooted in the potential source of increased offshore energy production. As the technology is still in a pre-industrial state, several questions remain to be addressed where little field data are available. This study uses physical modelling at a reduced scale to investigate the signature of the floating motions into the wake spectral content of a simplified FOWT model. A wind turbine model based on the actuator disc concept is placed in an atmospheric boundary layer wind tunnel and subjected to a range of surge, heave and pitch motions. The signatures of idealised sinusoidal motion and realistic broadband motion on the model’s wake at distances of 4.6 D (D being the disc diameter) and 8 D are measured through the use of a rake of single hot wires. The spectral analysis shows that harmonic motion leaves clear signatures in the far wake's energy spectra, mainly in the top tip region, while broadband motion does not leave easily detectable signatures.