Article

V-shaped Semisubmersible Offshore Wind Turbine Subjected to Misaligned Wave and Wind

AIP Publishing
Journal of Renewable and Sustainable Energy
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Abstract

The dynamic behavior of the V-shaped semisubmersible offshore wind turbine subjected to misaligned wave and wind loads in operational conditions is presented in this paper. During the life time of an offshore wind turbine wave and wind can be misaligned which may affect the dynamic response and as a result the functionality of the floating wind turbine. Especially for asymmetric floating structures such as the V-shaped semisubmersible, the misalignment of the wave and wind may result in unexpected behavior. In the present study, integrated aero-hydro-servo-elastic analysis for coupled mooring-floater-turbine is carried out in order to investigate possible effects under misaligned wave and wind conditions. For misaligned wave and wind conditions, the wave-induced as well as the wave-wind-induced motions, tension of mooring lines and functionality of the turbine such as power production, rotational speed and controller actions like blade-pitch-angle are studied and presented. The results show that the V-shaped semisubmersible offshore wind turbine is not affected in an undesirable way by the misaligned wave and wind loads in operational conditions and can be considered as enough robust in such environmental conditions. Also, the functionality and power production of the current concept is not affected by the misalignment of the wave and wind. The wave-induced responses of the V-shaped floating wind turbine are relatively small compared to wave-wind-induced responses. The dynamic responses of the V-shaped semisubmersible offshore wind turbine in coupled wave-wind-induced analyses are mainly dominated by the wind loads effects.

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... Most of the floating WTs in the prototype stage adopt the semisubmersible concept, including the WindFloat concept [26,27], Dutch Tri-floater concept [28,29] and V-shaped semisubmersible concept [30,31]. In addition, several semisubmersible floating WTs have been validated by different numerical tools and model tests. ...
... Among those concepts, the VSWT does not have braces; hence, it is less prone to fatigue at the welded joints. The advantages of the VSWT are further discussed in the following papers [30,36,37]. ...
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... Kvittem et al. [230] investigated the dynamic response of WindFloat semisubmersible with NREL 5 MW wind turbine. Dynamic response of V-shaped semisubmersible in moderate and deep water and also for misaligned wind and wave conditions were explored [161,231,232]. Robertson et al. [139,233] compared the response of OC4 Deep-Cwind semisubmersible with NREL 5 MW wind turbine using the different numerical simulation tools considering the potential flow theory and Morison equation for hydrodynamics and blade element momentum theory for aerodynamics. ...
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... However, the probability of experiencing such direction-independent extreme loadings (i.e., single or combined) is very low and has often led to inconsistent estimation of environmental action effects associated with a particular recurrence period in other directions different than the typically considered direction (i.e., mean or spectral peak directions), which could result in inefficient and unreliable designs [10][11][12], especially in case of directional dependent structures such as non-axisymmetric support structures and mooring systems. This is due to the fact that these parameters are in reality correlated and direction-dependent so that due to the existence of usual multi-hour time lag between wind and wave fields during extreme events, a wind field with most likely less severe magnitude and different direction can be combined with a wave field during its extreme state [13][14][15]. ...
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Thesis (Ph. D.)--Technische Universiteit Delft, 2001. Includes vita. Includes bibliographical references (p. 247-264).
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A semi-empirical model is formulated to represent the unsteady lift, drag, and pitching moment characteristics of an airfoil undergoing dynamic stall. The model is presented in a form which is consistent with an indicial formulation for the unsteady aerodynamics under attached flow conditions. The onset of vortex shedding during dynamic stall is represented using a criterion for leading edge or shock induced separation based on the attainment of a critical leading edge pressure. The induced vortex lift is represented empirically along with the associated pitching moment which is obtained by allowing the center of pressure to move in a time dependent manner during dynamic stall. Significant nonlinearities in the airfoil behavior associated with trailing edge separation are represented using a Kirchhoff flow model in which the separation point is related to the airfoil behavior. These effects are represented in such a way as to allow progressive transition between the dynamic stall and the static stall characteristics. It is shown how the above features may be implemented as an algorithm suitable for inclusion within rotorcraft airloads or aeroelasticity analyses. Validation of the model is presented with force and moment data from two-dimensional unsteady tests on the NACA 0012, HH-02, and SC-1095 airfoils.
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In the middle of 2013, the offshore installed wind capacity was 6.5 GW in European countries while by 2020 the total installed capacity is expected to reach 43 GW mainly by application of offshore wind turbines; the growth of offshore wind industry necessitates introducing concepts with potential of being classed without being much affected by different water depths. Initially, recent patents of offshore wind turbines are presented in this article. Afterwards, stochastic dynamic mo- tion responses and generated power of a semisubmersible floating wind turbine for different water depths of 100 m and 200 m are examined for selected environmental conditions. The mooring line design is correspondingly modified to pro- vide proper station keeping as well as appropriate interactive influence on the dynamic responses of the semisubmersible wind turbine without affecting the generated power. Fully coupled aero-hydro-servo-elastic numerical models have been developed and integrated time-domain approach has been applied in order to investigate the dynamics of the semisub- mersible floating wind turbine. The motion responses, mooring lines effective tension as well as wind turbine perform- ance are compared for different water depths and environmental conditions. Hence, the present semisubmersible floating wind turbine can be classified which means an optimized design can easily be modified for new offshore site with differ- ent water depth without the need of major modifications.
Article
The dynamic responses of a spar, tension leg platform (TLP), and two semisubmersible floating wind turbines (FWTs) in selected misaligned wind and wave conditions are investigated using numerical simulation with an aero-hydro-servo-elastic computational tool. For a range of representative operational conditions, the platform motions and short-term fatigue damage in the tower base and tower top are examined. Although some misalignment conditions result in increased motions both parallel and perpendicular to the wave direction, aligned wind and waves cause the largest short-term tower base fatigue damage for the studied platforms and conditions. Several factors which lead to larger fatigue damage for certain platforms in particular conditions are identified, such as tower resonance due to the 3p blade passing frequency in low wind speeds; surge and pitch motions, particularly in the wave frequency range; and the variations in first-order hydrodynamic loads due to wave direction. A semisubmersible platform with large displacement suffers the least damage at the base of the tower.
Article
For current offshore wind farms, monopiles are by far the most popular support structure type. However, for deeper water and/or larger turbines, the fatigue loading is becoming critical and the monopile dimensions are exceeding the current economical feasibility. Especially in cases of misaligned wind and waves, the side-to-side fatigue loading at the support structure can become a design driver. Since the industry aims to use standardized rotor-nacelle- assembly (RNA) designs, the goal of this paper is to show the effectiveness of using the RNA as adjustable controller device for site-specific offshore support structure configurations by using active control devices.
Article
Korea is a peninsula which is surrounded by the Yellow Sea (shallow sea), the southern sea and the East Sea (deep sea). These circumstances always make us consider that a platform could have good motion performances in both shallow and deep seas. In this paper, the typical spar type platform of the Offshore Code Comparison Collaboration Hywind Floating Offshore Wind Turbine (FOWT) has been modified and a new concept FOWT platform is suggested for both seas. Its motion performances are evaluated by both 1:80 scale model tests and full scale numerical simulations.
Article
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.
Conference Paper
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.
Article
A coupled dynamic analysis of a floating wind turbine system has been performed to investigate effect of wave direction relative to wind on the system. Hydrodynamic loads are calculated by linear frequency domain approach and aerodynamic effect is taken into account by increasing hydrodynamic damping and restoring matrices with aerodynamic damping and gyroscopic stiffness. A modal analysis of the system was made to explain the calculated motions. It brings out the natural frequencies, natural modes and modal damping coefficients of the system. Excitation of natural modes, by waves explains the motion observed in the response amplitude operators, and the effect of wave direction relative to wind. This modal analysis helps to better understand the behaviour of floating wind turbine systems.
Conference Paper
Wind power has the primary potential among renewable energies. Because Japan consists of little flat land and little shallow coast, floating wind turbine must be developed to make wind farms in Japan. Therefore, Japanese national demonstration project of Floating Offshore Wind Turbine (FOWT) was started in 2010FY by Ministry of the Environment and a SPAR-type FOWT is under construction at present. The floater is planned to be hybrid, consists of upper part by steal and lower part by pre-stressed concrete. Four fins are attached around the floater to suppress yaw motion. The floater is moored by three catenary chains. In order to confirm the safety of the FOWT in storm condition, experiments of a scale of 1/34.5 model were carried out at Ocean Engineering Basin of National Maritime Research Institute (NMRI), Japan. The draft of SPAR, the height of hub above sea level and the diameter of rotor of the model are 1.07m, 0.68m and 0.64m, respectively. In all experiments, blades are fixed to the hub under feathering condition and the hub is irrotational and fixed to the tower because this wind turbine is assumed to be under the storm condition, but wind blows transversely to the nacelle to give the maximum wind force. Water depth of the basin is smaller than the planned sea area on a reduced scale of model, therefore, springs and wires were used instead of chains in order to correspond to characteristics of horizontal mooring tension. Environmental forces are wind, wave and current in 50-year return period. Tensions of the 3 moorings and the motion of the model are measured in condition of wind and/or wave and/or current. Three kinds of direction of wind are adopted. One is the same direction as the wave and current, another is perpendicular to the wave and current, and the other is against to the wave and current. Besides the intact conditions a mooring-line-cut experiment in a storm condition was also conducted. Moreover, the effect of vortex induced motion (VIM), which occurs in current, was discussed. The results of the model experiment are reported to show the sufficient safety of this FOWT.
Conference Paper
Coupled time domain analyses of a semi-submersible wind turbine are performed with the intention to study motions affecting fatigue damage at the base of the tower. The software applied is SIMO/RIFLEX with the extension TDHmill, which gives the wind thrust force and gyro moment on the wind turbine as point loads in the tower top. Short term environmental conditions are chosen from a joint wind and wave distribution for a site in the Northern North Sea. Variance spectra, mean value, standard deviation, kurtosis, skewness and Vanmarcke’s bandwidth parameter are calculated for stresses at the base of the tower. Damage is calculated for each short term condition by two methods; rainflow counting and narrow band approximation. The accuracy of narrow band approximation estimates for fatigue are examined for the structure in question. Time domain simulations are carried out for different sea states and fatigue damage is calculated for each case. Simulations show that turbulent wind dominates the response at low wind speeds and the response spectral density function tends to be very wide-banded. For wave dominated response, spectra have lower bandwidth, and narrow banded approximation for fatigue damage gives estimates 20–50% above rainflow counted damage.
Article
This paper presents a stochastic dynamic response analysis of a tension leg spar‐type wind turbine subjected to wind and wave actions. The dynamic motions, structural responses, power production and tension leg responses are analyzed. The model is implemented using the HAWC2 code. Several issues such as negative damping, rotor configuration (upwind or downwind rotor) and tower shadow effects are discussed to study the power performance and structural integrity of the system. The operational and survival load cases considering the stochastic wave and wind loading are analyzed to investigate the functionality of the tension leg spar‐type wind turbine. Amelioration of the negative damping applied for this concept reduces the structural dynamic responses, which are important for fatigue life. It is found that the responses induced by wave and wind actions at the wave frequencies are not affected much by the aerodynamic excitation or damping forces. Because of the nonlinear effects of the tension leg, all of the motion responses are strongly coupled. The global responses of upwind and downwind versions of the turbine are found to be close because the tower shadow has a limited effect on the global responses. However, the structural dynamic responses of the blades are more affected by the tower shadow. In this study, the extrapolation methods are applied to efficiently estimate the maximum responses. The maximum response is found to occur in the survival cases as a result of the wave actions and the increased aerodynamic drag forces on the tower. The results show that the maximum responses corresponding to the up‐crossing rate of 0.0001 (corresponding to the maximum response within a 3 hour period) can be expressed by the mean plus 3 to 5 standard deviations. Copyright © 2012 John Wiley & Sons, Ltd.
Article
This report describes a three-bladed, upwind, variable-speed, variable blade-pitch-to-feather-controlled multimegawatt wind turbine model developed by NREL to support concept studies aimed at assessing offshore wind technology.
Article
This manuscript summarizes the feasibility study conducted for the WindFloat technology. The WindFloat is a three-legged floating foundation for multimegawatt offshore wind turbines. It is designed to accommodate a wind turbine, 5 MW or larger, on one of the columns of the hull with minimal modifications to the nacelle and rotor. Potential redesign of the tower and of the turbine control software can be expected. Technologies for floating foundations for offshore wind turbines are evolving. It is agreed by most experts that the offshore wind industry will see a significant increase in activity in the near future. Fixed offshore turbines are limited in water depth to ∼30–50 m. Market transition to deeper waters is inevitable, provided that suitable technologies can be developed. Despite the increase in complexity, a floating foundation offers the following distinct advantages: Flexibility in site location; access to superior wind resources further offshore; ability to locate in coastal regions with limited shallow continental shelf; ability to locate further offshore to eliminate visual impacts; an integrated hull, without a need to redesign the transition piece between the tower and the submerged structure for every project; simplified offshore installation procedures. Anchors are significantly cheaper to install than fixed foundations and large diameter towers. This paper focuses first on the design basis for wind turbine floating foundations and explores the requirements that must be addressed by design teams in this new field. It shows that the design of the hull for a large wind turbine must draw on the synergies with oil and gas offshore platform technology, while accounting for the different design requirements and functionality of the wind turbine. This paper describes next the hydrodynamic analysis of the hull, as well as ongoing work consisting of coupling hull hydrodynamics with wind turbine aerodynamic forces. Three main approaches are presented: The numerical hydrodynamic model of the platform and its mooring system; wave tank testing of a scale model of the platform with simplified aerodynamic simulation of the wind turbine; FAST, an aeroservoelastic software package for wind turbine analysis with the ability to be coupled to the hydrodynamic model. Finally, this paper focuses on the structural engineering that was performed as part of the feasibility study conducted for qualification of the technology. Specifically, the preliminary scantling is described and the strength and fatigue analysis methodologies are explained, focusing on the following aspects: The coupling between the wind turbine and the hull and the interface between the hydrodynamic loading and the structural response.
Article
Phase IV of the IEA Annex XXIII Offshore Code Comparison Collaboration (OC3) involves the modeling of an offshore floating wind turbine. This report documents the specifications of the floating system, which are needed by the OC3 participants for building aero-hydro-servo-elastic models.
Article
Kenneth B. Amer has served as a helicopter consultant for the RAND Corporation since 1986. Mr. Amer was graduated from New York University in 1944 with a BS in Aeronautical Engineering, and he received an MS from the Massachusetts Institute of Technology in 1947. Following graduation from MIT, he spent six years in helicopter flight research with the NACA, NASA's forerunner. He began his career with Hughe, Helicopters (now McDonnell Douglas Helicopter Co.) in 1953 as a Research Project Engineer. In 1965, Mr. Amer was appointed Manager, Hughes Technology Department. Some of the technical development projects for which he was responsible included the OH-6A (U.S. Army Light Observation Helicopter, now called the Model 500), the XV -9A Hot Cycle Research Helicopter, and the AH-64 Apache Attack Helicopter. Mr. Amer's career at Hughes/McDonnell Douglas Helicopter Co. culminated in his selection as McDonnell Douglas Engineering and Research Fellow in 1985. Throughout his professional life, Mr. Amer has been active in the American Helicopter Society. He presented his first paper at the AHS Annual National Forum in 1950. He has authored more than 30 technical papers since then. In 1971, he served as AHS Annual National Forum Technical Chairman, and was AHS Technical Director in 1972. Mr. Amer's many honors include the 1976 AHS Alexander Klemin Award, and an AHS Honorary Fellowship in 1985.
Article
This work presents a comprehensive dynamic–response analysis of three offshore floating wind turbine concepts. Models were composed of one 5  MW turbine supported on land and three 5  MW turbines located offshore on a tension leg platform, a spar buoy and a barge. A loads and stability analysis adhering to the procedures of international design standards was performed for each model using the fully coupled time domain aero-hydro-servo-elastic simulation tool FAST with AeroDyn and HydroDyn. The concepts are compared based on the calculated ultimate loads, fatigue loads and instabilities. The loads in the barge-supported turbine are the highest found for the three floating concepts. The differences in the loads between the tension leg platform–supported turbine and spar buoy–supported turbine are not significant, except for the loads in the tower, which are greater in the spar system. Instabilities in all systems also must be resolved. The results of this analysis will help resolve the fundamental design trade-offs between the floating-system concepts. Copyright © 2011 John Wiley & Sons, Ltd.
Description of the relation of wind, wave and current characteristics at the offshore wind farm Egmond aan Zee (OWEZ) location in 2006
  • S Barth
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Barth, S. and Eecen, P. J., "Description of the relation of wind, wave and current characteristics at the offshore wind farm Egmond aan Zee (OWEZ) location in 2006," Technical Report No. ECN-E-07-104, ECN, 2006.
for Fukushima Floating Offshore Wind Farm Demonstration Project
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User's Manual, 2012. MARINTEK, Simo User's Manual
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MARINTEK, Riflex User's Manual, 2012. MARINTEK, Simo User's Manual, 2015.
Global analysis of floating wind turbines: Code development, model sensitivity and benchmark study
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Ormberg, H. and Bachynski, E. E., "Global analysis of floating wind turbines: Code development, model sensitivity and benchmark study," in Proceedings of the 22nd International Offshore and Polar Engineering Conference, Rhodes, Greece, 17-22 June, 2012.