Tension Leg Platform Wind Turbine Research Articles

Towards gradient-based design optimization of fully-flexible tension-leg platform wind turbines

Novel technologies and design methods are needed to enable cost-competitive development of wind turbines with floating foundations. Tension-leg platforms have an established history in the oil and gas industry, though the complexity in coupled analysis of floating wind turbines generally has limited the exploration of novel concepts. The application of efficient, gradient-based optimization models has shown promise to overcome these difficulties and develop innovative designs. The core of this design optimization approach is an efficient coupled aero-hydro-servo-elastic dynamic model for a generic tension-leg platform wind turbine design, referred to as TLPOpt. The equation of motion is defined based on the generalized elastic mode shapes of the combined main column and tower. TLPOpt is implemented in the OpenMDAO framework for optimization, and analytical derivatives are defined throughout to increase efficiency. Stochastic dynamic analysis in the frequency-domain allows for efficient assessment of fatigue and extreme conditions. Verification steps show good agreement between the linearized analysis and higher-fidelity analysis tools. Early optimization studies suggest the optimization is able to improve upon a reference design, though more realistic constraint and objective functions are needed to draw further conclusions.

Numerical Assessment of a Tension-Leg Platform Wind Turbine in Intermediate Water Using the Smoothed Particle Hydrodynamics Method

The open-source code DualSPHysics, based on the Smoothed Particle Hydrodynamics method for solving fluid mechanics problems, defines a complete numerical environment for simulating the interaction of floating structures with ocean waves, and includes external libraries to simulate kinematic- and dynamic-type restrictions. In this work, a full validation of the SPH framework using experimental data available for an experimental test campaign on a 1:37-scale floating offshore wind turbine tension-leg platform (TLP) is presented. The first set of validation cases includes a surge decay test, to assess the quality of the fluid–solid interaction, and regular wave tests, which stimulate the mooring system to a large extent. During this phase, tendons (tension legs) that are simulated by MoorDyn+ are validated. Spectral comparison shows that the model is able to capture the surge and pitch dynamic amplification that occurs around the resonant fundamental mode of vibration. This work concludes with a numerical investigation that estimates the response of TLP under extreme events defined using multiple realizations of irregular sea states; the results suggest that the tendon loads are sensitive to the sea-state realization, providing maximum tendon peak forces in a range of ±10% about the mean. Furthermore, it is shown that the load pattern that forms from considering the relative position of the tendons to the incident wave direction leads to higher forces (≈20%).

Simulation of a Ship and Tension Leg Platform Wind Turbine Collision

Collisions between ships and tension leg platform wind turbines (TLPWTs) result in the TLPWT damage, including tower damage and tension leg breakage. This paper details a collision between a ship and TLPWT simulated using the LS-DYNA software package. During simulation, the behaviors of the water around the ship and of the TLPWT were processed using the constant added mass and the fluid–structure interaction methods, respectively. The results indicate that the energy conversion during collision is generally consistent with the law of conversion of energy, suggesting that the simulation’s calculations are acceptable. The TLPWT simulation results imply that two distinct collisions occurred during the broader collision period because of TLPWT pitch or roll, and the simulated damage mainly occurred in the contact areas. All tension legs were not slack during collision because of prestress, and the responses of the symmetrical tension legs were largely equivalent. The response of the TLPWT decreased over time due to water resistance. The results of this paper can be consulted to prevent and reduce losses caused by ship–TLPWT collisions.

Stochastic Analysis of Short-Term Structural Responses and Fatigue Damages of A Submerged Tension Leg Platform Wind Turbine in Wind and Waves

Stochastic Analysis of Short-Term Structural Responses and Fatigue Damages of A Submerged Tension Leg Platform Wind Turbine in Wind and Waves

Optimised design of screw anchors in tension in sand for renewable energy applications

The offshore deployment of floating offshore structures such as wind turbines or wave energy converters is expected to strongly increase during the next decade, to face the appetite for green energy sources. The growing size of these structures’ dimensions, inducing very large mooring forces, makes the anchoring solution adopted a critical issue for the commercial success of floating marine energy farms.The upscaling of the screw anchor technology from onshore to the offshore environment has been recently proposed as an efficient way of providing a large tension capacity while their installation generates far less noise and vibrations than impact pile driving. Most of recent studies on screw anchors have focused on separated geotechnical problems such as their uplift capacity or installation requirements. This paper incorporates within a single procedure geotechnical and structural constraints to calculate the optimal anchor geometry able to maximise the uplift capacity available.Performance envelopes for screw anchors have been derived in a parametric study, covering a broad range of soil conditions as well as in a case study, representative of offshore conditions. Results show that single screw anchors are more efficient (e.g. shorter and lighter) than driven piles to sustain tension loading. The results presented in this study support the applicability of screw anchors to be used as part of the mooring system for wave energy converters. However, tension requirements for tension-leg platform wind turbines would probably require the use of group of anchors.

Numerical Modelling of the MIT/NREL TLP Wind Turbine and Comparison with the Experimental Results

In this study, numerical analysis of a tension leg platform wind turbine is conducted and the responses with focus on surge motions and tendon tension are compared with available experimental test data. The main scope of the study is to establish the numerical model for which the damping coefficients for rigid-body motions are tuned based on the comparison of the sway free decay test results (natural periods and damping ratios) between the numerical and the experimental studies. The differences between the test model properties and the numerical model information have been discussed. Numerical model tuning with available test data resulted with relatively good accordance but also slight to moderate differences in the responses. These differences are credited for the uncertainties in the model testing and the solution methodology of the numerical model. Numerical study is under development with regular and irregular wave analyses and analyses including wind excitation.

Experimental and numerical study on the multi-body coupling dynamic response of a Novel Serbuoys-TLP wind turbine

The floating wind turbine (FWT) based on the tension leg platform (TLP) plays an important role in addressing the issue of energy pinch, but the central characteristic for TLP is that in vertical direction it is similar to a rigid structure with tension tendons to restrain motion, while in horizontal direction, it seems to be compliant with less constrain. The large amplitude motions in the horizontal plane caused by this compliance will seriously affect the performance of the platform under harsh environmental conditions. To address this problem, a new type of tension leg platform wind turbine connected with a series of buoys (Serbuoys-TLP) is proposed. A time-domain simulation program (DUTMST) for the TLP and Serbuoys-TLP based on MATLAB is provided, with consideration of the multi-body coupling effect between the body, tension leg and buoy. That is, each buoy can be analysed as a separate mass structure connected on the tension leg. The accuracy of DUTMST is discussed via comparison with the results of an experiment and AQWA. The dynamic responses of TLP and Serbuoys-TLP are presented in DUTMST. The results show that the Serbuoys-TLP can significantly reduce the horizontal response of the TLP. In addition, the horizontal response suppression effects of Serbuoys-TLP are investigated by varying the wave factor and parameters of buoys. Additionally, the parameters of buoys adapted to the East China Sea are given.

Loads and Response of a Tension Leg Platform Wind Turbine with Non-Rotating Blades: An Experimental Study

This paper describes model testing of a Tension Leg Platform Wind Turbine (TLPWT) with non-rotating blades to better understand its motion and tendon responses when subjected to combined wind and unidirectional regular wave conditions. The TLPWT structure is closely based on the National Renewable Energy Laboratory (NREL) 5 MW concept. Multiple free decay tests were performed to evaluate the natural periods of the model in the key degrees of freedom, whilst Response Amplitude Operators (RAOs) were derived to show the motion and tendon characteristics. The natural periods in surge and pitch motions evaluated from the decay tests had a relatively close agreement to the theoretical values. Overall, the tested TLPWT model exhibited typical motion responses to that of a generalised TLP with significant surge offsets along with stiff heave and pitch motions. The maximum magnitudes for the RAOs of surge motion and all tendons occurred at the longest wave period of 1.23 s (~13.0 s at full-scale) tested in this study. From the attained results, there was evidence that static wind loading on the turbine structure had some impact on the motions and tendon response, particularly in the heave direction, with an average increase of 13.1% in motion amplitude for the tested wind conditions. The wind had a negligible effect on the surge motion and slightly decreased the tendon tensions in all tendons. The results also showed the set-down magnitudes amounting to approximately 2–5% of the offset. Furthermore, the waves are the dominant factor contributing to the set-down of the TLPWT, with a minimal contribution from the static wind loading. The results of this study could be used for calibrating numerical tools such as CFD codes.

Dynamic analysis of a floating wind turbine in wet tows based on multi-body dynamics

In this study, a submerged tension leg platform wind turbine (STLPWT) which can be constructed near the quayside and then wet-towed to the installation site as a unit was proposed. This transportation method will consequently reduce the use of heavy offshore cranes. However, as a high-rise structure, the floating wind turbine may sustain large overturning moments induced by wind, wave, current, and towing force in the transport phase. In order to study the stability of the floating wind turbine and the towline force in wet tows, a numerical model of the towing system including a towboat, towline, and STLPWT was established based on multi-body dynamics. Then, the environmental load effects on the towing stability of the floating wind turbine were investigated. In addition, the comparison of the bollard pull and the height of towing points was performed. The results show that the STLPWT was stable under the rough sea towing condition: a significant wave height of 5 m and a wind speed of 17 m/s. An appropriate bollard pull should be chosen, in practice, as it involves the time and costs of the towing process. In addition, it may be a better choice to set the height of the towing points near the mean sea level to lower the pitch motions of the STLPWT as well as the towline force.

Comparison of wave load effects on a TLP wind turbine by using computational fluid dynamics and potential flow theory approaches

Tension Leg Platform (TLP) is one of the concepts which shows promising results during initial studies to carry floating wind turbines. One of the concerns regarding tension leg platform wind turbines (TLPWTs) is the high natural frequencies of the structure that may be excited by nonlinear waves loads. Since Computational Fluid Dynamics (CFD) models are capable of capturing nonlinear wave loads, they can lead to better insight about this concern. In the current study, a CFD model based on immersed boundary method, in combination with a two-body structural model of TLPWT is developed to study wave induced responses of TLPWT in deep water. The results are compared with the results of a potential flow theory-finite element software, SIMO-RIFLEX (SR). First, the CFD based model is described and the potential flow theory based model is briefly introduced. Then, a grid sensitivity study is performed and free decay tests are simulated to determine the natural frequencies of different motion modes of the TLPWT. The responses of the TLPWT to regular waves are studied, and the effects of wave height are investigated. For the studied wave heights which vary from small to medium amplitude (wave height over wavelength less than 0.071), the results predicted by the CFD based model are generally in good agreement with the potential flow theory based model. The only considerable difference is the TLPWT mean surge motion which is predicted higher by the CFD model, possibly because of considering the nonlinear effects of the waves loads and applying these loads at the TLPWT instantaneous position in the CFD model. This difference does not considerably affect the important TLPWT design driving parameters such as tendons forces and tower base moment, since it only affects the mean dynamic position of TLPWT. In the current study, the incoming wave frequency is set such that third-harmonic wave frequency coincides with the first tower bending mode frequency. However, for the studied wave conditions a significant excitation of tower natural frequency is not observed. The high stiffness of tendons which results in linear pitch motion of TLPWT hull (less than 0.02 degrees) and tower (less than 0.25 degrees) can explain the limited excitement of the tower first bending mode. The good agreement between CFD and potential flow theory based results for small and medium amplitude waves gives confidence to the proposed CFD based model to be further used for hydrodynamic analysis of floating wind turbines in extreme ocean conditions.

Small scale experimental study of the dynamic response of a tension leg platform wind turbine

A floating Tension Leg Platform (TLP) wind turbine was constructed at scale 1/200 and its dynamic response was analysed experimentally in co-directional wind and waves. The wind turbine was Froude scaled and a new rotor was designed to yield maximum power and Froude scaled thrust at the low model Reynolds number. Physical limitations due to the large scaling ratio further meant that some structural adjustments were necessary. Nacelle and floater accelerations were measured by means of two accelerometers. The TLP was moored with four different tendon configurations and exposed to different constant wind speeds and irregular sea states as well as a range of regular waves. It was found that an increase in wind speed reduces the wave-induced floater motion but causes slightly larger nacelle displacements. Further, the orientation of the spokes relative to the direction of wind and waves influences the pitch stiffness and thereby the nacelle displacements. Inclining the tendons towards the wind turbine reduces the nacelle displacements significantly and reduces the occurrence of slack tendons, but increases the inline tilt-motion of the rotor. Application of a very stiff mooring configuration increases the occurrence of slack tendons and the magnitude of the pitch accelerations. In a robust commercial design, however, slack tendons must be avoided. The experiments demonstrate the ability of the wind turbine model and the experimental setup to give insight to the dynamic characteristics of floating TLP wind turbines.

Ringing loads on tension leg platform wind turbines

The present work identifies realistic wave (and associated wind) conditions which could induce ringing responses in tension leg platform wind turbines (TLPWTs). The simulation results show the importance of ringing forces, the effects of turbine operation, and the sensitivity of the ringing response to platform stiffness and viscous damping. To model the ringing loads, the second order quadratic transfer function and a bandwidth-limited summation formulation for the third order wave forces were implemented. The chosen formulation avoids the spectrum cut-off dependency and the low-frequency components of a direct implementation of the irregular wave Faltinsen, Newman, Vinje (FNV) formula. Depending on the natural period and damping, the difference between a direct implementation and this formulation was 5–25%. Ringing-type responses were simulated for 50-year wind and wave conditions. Various hydrodynamic models were used to isolate physics in different approaches. For platforms with 14–18m diameters, ringing loads resulted in larger extreme loads and increased short-term fatigue damage in the tendons and tower. Ringing effects were particularly severe for platforms with a pitch/bending natural period of 3–4s. The viscous damping coefficient had a negligible influence on the ringing response, while aerodynamic damping could be important in damping the oscillations following the initial maximum.

Fully Coupled Three-Dimensional Dynamic Response of a Tension-Leg Platform Floating Wind Turbine in Waves and Wind

A dynamic model for a tension-leg platform (TLP) floating offshore wind turbine is proposed. The model includes three-dimensional wind and wave loads and the associated structural response. The total system is formulated using 17 degrees of freedom (DOF), 6 for the platform motions and 11 for the wind turbine. Three-dimensional hydrodynamic loads have been formulated using a frequency- and direction-dependent spectrum. While wave loads are computed from the wave kinematics using Morison's equation, the aerodynamic loads are modeled by means of unsteady blade-element-momentum (BEM) theory, including Glauert correction for high values of the axial induction factor, dynamic stall, dynamic wake, and dynamic yaw. The aerodynamic model takes into account the wind shear and turbulence effects. For a representative geographical location, platform responses are obtained for a set of wind and wave climatic conditions. The platform responses show an influence from the aerodynamic loads, most clearly through quasi-steady mean surge and pitch responses associated with the mean wind. Further, the aerodynamic loads show an influence from the platform motion through a fluctuating rotor load contribution, which is a consequence of the wave-induced rotor dynamics. Loads and coupled responses are predicted for a set of load cases with different wave headings. Further, an advanced aero-elastic code, Flex5, is extended for the TLP wind turbine configuration and the response comparison with the simpler model shows a generally good agreement, except for the yaw motion. This deviation is found to be a result of the missing lateral tower flexibility in the simpler model.

Design considerations for tension leg platform wind turbines

Tension leg platform wind turbines (TLPWTs) represent one potential method for accessing offshore wind resources in moderately deep water. Although numerous TLPWT designs have been studied and presented in the literature, there is little consensus regarding optimal design, and little information about the effect of various design variables on structural response. In this study, a wide range of parametric single-column TLPWT designs are analyzed in four different wind-wave conditions using the Simo, Riflex, and AeroDyn tools in a coupled analysis to evaluate platform motions and structural loads on the turbine components and tendons. The results indicate that there is a trade-off between performance in storm conditions, which improves with larger displacement, and cost, which increases approximately linearly with displacement. Motions perpendicular to the incoming wind and waves, especially in the parked configuration, may be critical for TLPWT designs with small displacement. Careful choice of natural period, diameter at the water line, ballast, pretension, and pontoon radius can be used to improve the TLPWT performance in different environmental conditions and water depths.