Bottom-fixed Offshore Wind Turbines Research Articles

Characterizing ramp events in floating offshore wind power through a fully coupled electrical-mechanical mathematical model

Floating Offshore Wind Turbines (FOWTs) exhibit a noteworthy nonlinear low-frequency dynamic response during rated and higher wind-wave scenarios, leading to a substantial exacerbation of its power characteristics, power stability, and wind energy forecast reliability. This study develops a fully coupled FOWT model to investigate the impact of wind and wave loads on wind power ramp events (WPREs), which integrates mechanical and electrical factors, including a spar buoy FOWT with generator, converter, and aero-hydro-servo-elastic (AHSE) dynamics. Results indicate that the floating structure enables periodic and significant WPREs of FOWTs, as wave load and platform natural motion causing low and ultra-low frequency response. In contrast to bottom-fixed offshore wind turbines (OWT), FOWTs exhibit a supplementary decline in rated power output ranging from 7.9 % to 40.5 % during WPREs. Moreover, ramp peak mainly depends on the aerodynamic loads, but become sensitive to wave loads characterized by wave heights over 2.52 m. FOWT power performance is highly unstable within the rated wind speed range, under WPREs within ultra short time period, resulting in failure to meet grid standards, emphasizing the external need for targeted power compensation and power signal processing. Overall, this study highlights the importance of pitch motion and wave load impact for WPRE study of FOWTs.

Study on the power performance of wave energy converters mounted around an offshore wind turbine jacket platform

With the deepening of renewable energy research, the economic-friendly multi-energy coupling development method is gradually favored. This paper proposes a jacket-mounted hybrid wind-wave energy converter, which includes a bottom-fixed offshore wind turbine (OWT) and a microarray composed of three point-absorbing wave energy converters (WECs). Numerical simulations were conducted to study the power performance of the wave energy converter array, and a model test with a scale of 1:8 was carried out to calibrate and validate the numerical model. The array converts wave energy through a hydraulic system, and a dynamic-link library was created to simulate the hydraulic power take-off (PTO) system that can take into account the real-time pressure changes of the accumulator. The time domain simulation results show that the maximum capture width ratio (CWR) of the WEC array exceeds 0.5 when the wave period is nearly to the natural period, and the optimal CWR of a single WEC exceeds 0.8. When the wave period is less than 2 times the natural period, the energy capture behavior of WEC is sensitive to the change in wave direction, and this sensitivity is weakened when the device forms an array. Specifically, a single WEC prefers waves to be incident from the back side, while the WEC array prefers waves to be incident obliquely. This work will provide design guidance for the engineering application of stationary hybrid wind-wave energy concepts.

Influence of wind and seismic ground motion directionality on the dynamic response of four-legged jacket-supported Offshore Wind Turbines

This paper aims at investigating the influence of the direction of wind and seismic ground motion on the structural response of four-legged jacket support substructures for bottom-fixed Offshore Wind Turbines located in areas with non-negligible seismic risk. To do that, a parametric analysis that considers thirteen seismic shaking directions, seven wind directions and four earthquake records is carried out. The results are presented in terms of accelerations, axial forces, bending moments and shear forces in the jacket substructure. The NREL 5 MW wind turbine supported on the jacket designed for the phase I of the OC4 project is considered. The response of the system is simulated using an OpenFAST model that takes into account multi-support seismic input, soil–structure interaction and kinematic interaction. The results show that load combinations with aligned wind and ground motion directions are never the worst-case scenario. On the contrary, the largest accelerations are found when the shaking direction acts along the side-to-side direction, and the most significant internal forces are usually found when the ground motion is aligned with one of the diagonals of the base of the jacket and not aligned with the wind direction. The paper also discusses the specific ranges of the angle of misalignment between wind and seismic shaking directions within which the maximum internal forces are expected to be found.

Efficient floating offshore wind realization: A comparative legal analysis of France, Norway and the United Kingdom

Floating offshore wind (FOW) has the potential to play a key role in many states' decarbonization policies going forward. The reason is that the majority of the world's wind energy potential is located offshore at depths where deployment of conventional bottom-fixed offshore wind turbines is not economically profitable. To realize FOW's potential as a key decarbonization technology policy makers and legislators must design legal frameworks that properly address the three main barriers to efficient deployment: (a) spatial competition, (b) need for state financing and (c) allocation of grid connection responsibilities. This article analyses and compares Norway, France and the United Kingdom's regulation of the three main barriers to efficient FOW deployment with respect of their ability to ensure efficient realisation. The article concludes with best practices and recommendation to policy makers and legislators on how to regulate the three main barriers to accomplish efficient FOW deployment.

Numerical Simulation of a Floating Offshore Wind Turbine Incorporating an Electromagnetic Inerter-Based Device for Vibration Suppression and Wave Energy Conversion

Offshore wind turbines (OWTs) are considered vital to the promotion of the development of renewable energy. Especially, floating OWTs can be deployed over a larger area than bottom-fixed OWTs. The floating OWTs, however, are vulnerable to vibration induced by disturbances and require a backup power supply in the case of power outage. On the one hand, various kinds of inerter-based devices have been proposed especially for vibration suppression of civil structures subjected to earthquake loadings. Recently, combined with electromagnetic devices, the inerter technologies have also been applied in the field of vibration energy harvesting such as point absorber wave energy converters. Thus, this paper proposes a novel floating OWT consisting of two bodies combined with inerter-based power take-off (PTO) devices which accomplishes vibration suppression and wave energy conversion at the same time. To investigate the vibration suppression and energy conversion capabilities of the proposed floating OWT with a variety of inerter-based PTO devices for ocean waves, numerical simulation studies employing WEC-Sim are conducted, and the performance of each system is compared for regular and irregular waves. Results show that the proposed floating OWT with the appropriately designed inerter-based PTO devices for the incident wave period has great potential for both vibration suppression and wave energy conversion in a specific frequency range.

Mind the gap – an experimental study on the need for grouting suction buckets in sand under vertical cyclic loading

Suction bucket jackets are increasingly being used to support bottom-fixed offshore wind turbines. It is generally assumed that contact between the underside of the bucket lid and the seabed is mandatory to safeguard against additional settlement. Lid contact is usually ensured by underwater grout injection, albeit this has a significant impact on the duration and costs of installation. Although grouting is routine, there are limited available data regarding the benefits of ensuring contact between the lid and the soil. This paper presents data from centrifuge tests of completely and partially installed buckets in dense sand subjected to vertical cyclic loading to investigate the role of lid contact. Lid contact was shown to have a discernible effect on displacement accumulation when buckets were loaded beyond the drained skirt capacity in compression. Under these conditions, settlement of a completely installed bucket was limited by lid contact with the soil plug, while partially installed buckets experienced continuous settlement, as expected. The physical evidence indicates complex load transfer mechanisms that are influenced significantly by the drainage conditions. Hence, an understanding of the soil drainage characteristics at a particular site would provide value when evaluating the requirement for under-lid grouting.

An optimisation methodology for suspended inter-array power cable configurations between two floating offshore wind turbines

Floating offshore wind turbines (FOWTs) convert wind energy to electrical energy in deeper water areas where installing bottom-fixed offshore wind turbines is not feasible. However, technological obstacles have to be overcome to make them cost-effective, with inter-array power cable designs being one of the potential areas to be investigated. This study presents a novel configuration of suspended inter-array power cables using subsea buoys between two 5MW OC3-Hywind FOWTs. In this configuration, the inter-array cable is kept afloat between the seabed and the sea surface by attaching several buoys distributed along the cable. The effects of marine growth at a water depth of 320 m are considered. First, steady-state analyses are performed in the Start of Life (SOL) and End of Life (EOL) states to select the configurations which satisfy the minimum breaking load (MBL) and minimum bend radius (MBR) acceptance criteria. The selected configurations are then further investigated by considering a fitness optimisation criterion. Subsequently, full dynamic analyses are performed on the best configurations identified through the fitness optimisation criterion, considering the operating and feathered modes of the FOWTs. Copper and aluminium cables with two differently sized buoys are evaluated. The results indicate that the suspended inter-array power cable concept is feasible using the proposed buoy setup and optimisation methodology. Smaller buoys yield better designs for both cable types than larger buoys. The aluminium cable configuration has a lower effective tension but also a lower safety margin to its MBL limit than the copper cable configurations. The copper cable configurations result in smaller horizontal excursions. The configurations with closely spaced buoys result in large bend radii.

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

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

Failure analysis of spar buoy floating offshore wind turbine systems

Floating offshore wind energy is a new form of marine renewable energy which is attracting a great deal of attention worldwide. However, the concepts of floating offshore wind turbines (FOWTs) are still in early stages of development and their failure properties are not yet fully understood. Compared to bottom-fixed wind turbines, FOWTs are subject to more extreme environmental conditions and significant mechanical stresses which may cause a higher degradation rate and shorter mean-time-to-failure for components/structures. To fill the research gap, this paper aims to conduct qualitative and quantitative failure studies on an OC3 spar-type FOWT platform with 3 catenary mooring lines. The failure analyses are performed based on two well-established reliability engineering methodologies, namely, fault tree analysis (FTA) and failure mode and effects analysis (FMEA). The most critical FOWT components are prioritized according to their failure likelihood as well as the risk-priority-number. Our results show a good agreement between the two methods with regard to failure criticality rankings. However, some differences between the results are also observed that are attributed to the difference between FTA and FMEA methodologies as the former incorporates the causes of various failure modes into analysis, whereas the latter is mainly adopted for a single random failure analysis. The results obtained from the FMEA study for the FOWT system will also be compared with those reported for bottom-fixed offshore wind turbines and some interesting conclusions are derived.

Effects of Simulation Length and Flexible Foundation on Long-Term Response Extrapolation of a Bottom-Fixed Offshore Wind Turbine

Abstract This paper deals with statistical and modeling uncertainty on the estimation of long-term extrapolated extreme responses in a monopile offshore wind turbine. The statistical uncertainty is addressed by studying the effect of simulation length. Modeling uncertainty is explored by evaluating the effects of considering a rigid and flexible foundation. The soil's flexibility is taken into account by considering the improved apparent fixity method. To identify the most relevant environmental conditions, the modified environmental contour method is used. The analysis focuses on the fore-aft shear force (FASF) and the fore-aft bending moment (FABM) at the mudline. The results show that using a simulation length of 10 min does not provide sufficient accuracy. It was found that for the FASF, simulation lengths of at least 30 min are required to achieve an accuracy of about +/−5%. For the FABM, it was found that both the extrapolations made with 20 min and 30 min simulations achieved similar levels of accuracy of about 20%. Meanwhile, the results obtained from 10 min simulations reached deviations of about 40%. Finally, from the comparison made between a rigid and flexible foundation, it was found that the extrapolated responses exhibit maximum deviations up to around 5% and 10% for the FASF and the FABM, respectively. Also, for the FABM, it was observed that the consideration of a flexible foundation causes the critical wind speed to shift from 16.5 m/s (rigid) to 18 m/s (flexible).

Prospects and Economics of Offshore Wind Turbine Systems

In recent years, floating offshore wind turbines have attracted more attention as a new renewable energy resource while bottom-fixed offshore wind turbines reach their limit of water depth. Various projects have been proposed with the rapid increase in installed floating wind power capacity, but the economic aspect remains as a biggest issue. To figure out sensible approaches for saving costs, a comparison analysis of the levelized cost of electricity (LCOE) between floating and bottom-fixed offshore wind turbines was carried out. The LCOE was reviewed from a social perspective and a cost breakdown and a literature review analysis were used to itemize the costs into its various components in each level of power plant and system integration. The results show that the highest proportion in capital expenditure of a floating offshore wind turbine results in the substructure part, which is the main difference from a bottom-fixed wind turbine. A floating offshore wind turbine was found to have several advantages over a bottom-fixed wind turbine. Although a similarity in operation and maintenance cost structure is revealed, a floating wind turbine still has the benefit of being able to be maintained at a seaport. After emphasizing the cost-reduction advantages of a floating wind turbine, its LCOE outlook is provided to give a brief overview in the following years. Finally, some estimated cost drivers, such as economics of scale, wind turbine rating, a floater with mooring system, and grid connection cost, are outlined as proposals for floating wind LCOE reduction.

Experimental Evaluation of Dynamic Rock Scour Protection in Morphodynamic Environments for Offshore Wind Jackets

Bottom-fixed offshore wind turbines are generally built on continental-shelf sections that are morphodynamically active due to their shallow depths and severe wave and current conditions. Such sites are commonly protected against scour to prevent the loss of structural stability. Scour protection can be designed using static or dynamic solutions. Designing dynamic protection requires experimental validation, especially for singular or unconventional structures. This article presents an experimental method for the laboratory analysis of scour protection for jacket foundations placed at morphodynamically active sites. The test campaign was conducted within the project East Anglia ONE (UK) as part of the asset owner studies and aimed to evaluate operation and maintenance (O&M) aspects, independent of the contractor’s original design assessments. The physical experiments explored morphodynamic changes on the sea bottom and their importance to scour protection, as well as the importance of the history of the wave loads to the deformation of the rock scour protection. This was explored by repeating different cumulative tests, including a succession of randomly ordered sea states (Return Period (RP) 1-10-20-50 years). The experimental results show that the deformation of the rock sour protection was the greatest when the most energetic sea states occurred at the beginning of the experimental test campaign. The maximum deformation was at 5D50 when the first test was also the most energetic, while it was at 3D50 when not included as the first test, yielding a 40% reduction in the scour protection deformation.

Assessing the potential areas for developing offshore wind energy in Japanese territorial waters considering national zoning and possible social conflicts

This study identified areas suitable for offshore wind energy development in Japan based on a low probability of conflicts with stakeholders, such as fishery groups, shipping agents, and residents (minor conflict areas). Using geographic information systems, this study uses a two-stage approach. The first stage reviews Japanese zoning rules and excludes non-conforming areas with rules. The second stage examines case studies of stakeholder conflicts to identify minor conflict areas using three parameters, i.e., distance from the shore, shipping density, and existence of fishery rights, by considering local concerns regarding the seascape and conflicts resulting from shipping routes and fishery rights. Although previous studies have assessed massive offshore wind energy potential areas (> 140,000 km2) with various approaches, the areas that conform to the zoning rules (53,665 km2) and minor conflicts areas (7,213 km2 or 2% of Japanese territorial waters) are significantly limited. Furthermore, this study revealed that concerns regarding the seascape are a key issue inhibiting the expansion of bottom-fixed offshore wind turbines in Japan. Thus, the approach, which considers both zoning rules and stakeholder conflicts, can reduce the risk of offshore wind energy potential overestimation. For offshore wind energy capacity targets, relevant authorities should carefully examine both the zoning rules and stakeholders.

Numerical and analytical analysis of a monopile-supported offshore wind turbine under ship impacts

Offshore wind turbines in the vicinity of ship traffic are exposed to increased risks of ship collisions. To better understand the impact mechanism, this paper evaluates the dynamic responses of a monopile-supported wind turbine under ship impacts, using both numerical and analytical methods. The nonlinear finite element method is applied during the numerical simulations, and the wind load effects, soil conditions, and rigid and deformable ship bows are considered. The analytical approach, originally developed based on the energy method, is extended here to address the damping effects of monopile-supported wind turbines. In the case study, the impacts are studied between a 4600-ton vessel and a 5-MW offshore wind turbine. The effects are presented of the aerodynamic damping, ship impact velocity, mean wind speed, wind direction, and ship bow stiffness on the collision responses. A comparison between the numerical and analytical results shows a generally good agreement for the maximum contact force. Under an impact velocity of 1 m/s and 3 m/s, the discrepancy between the two methods is 5% and 7%, respectively. The developed engineering approaches can be used to address accidental collision problems between ships and bottom-fixed offshore wind turbines.

Applying multiple tuned mass dampers to control structural loads of bottom-fixed offshore wind turbines with inclusion of soil-structure interaction

Using structural control methods to suppress structural loads of offshore wind turbines is an economical and feasible solution. Note that the dynamic responses of offshore wind turbines are generally analyzed under the parked condition, and the blades are usually modeled with a lumped mass. Actually, the geometrical properties and rotational speed of blades can significantly affect the wind turbine performances. Therefore, these simplifications may result in inaccurate evaluation for the wind turbine dynamic responses, and then further influence the evaluation of the load mitigation effect of a control system. Moreover, the vibration behaviors and structural responses of the wind turbine can be influenced by soil-structure interaction. In this work, the structural responses of a bottom-fixed offshore wind turbine that suffers combined wind and wave excitations are explored. The blade model is explicitly established and soil-structure interaction is also taken into account. The dynamic responses of the wind turbine as well as the vibration mitigation effects of a single tuned mass damper (STMD) and a multiple TMDs (MTMD) system are examined. Furthermore, the sensitivity analysis for the TMD tuning frequency and the robustness of the MTMD system are investigated under the assumption that some TMDs in MTMD do not work properly.

Wind-wave directional effects on fatigue of bottom-fixed offshore wind turbine

The current trend for offshore wind energy is that larger turbines are placed on monopile foundations at increasing water depth. This requires larger foundations, increasing the importance of hydrodynamic loading. It is well established that wave loads perpendicular to the wind direction are important for the fatigue damage in monopile foundations. However, this is normally only taken into account considering wind-wave misalignment. In this paper, the effect of assuming short-crested waves in design calculations is considered. The lifetime fatigue damage may increase significantly for hydrodynamically sensitive support structures when modelling the waves as short-crested rather than long-crested. For the turbines in this paper, the fatigue damage increased with up to 80 %. At the same time, the changes in fatigue damage were small for support structures that are less hydrodynamically sensitive. The work performed in this paper shows that the typical design assumption of long-crested waves may be both conservative and non-conservative. This fact is important to be aware of when designing support structures for offshore wind turbines.

Motion Control of Pentapod Offshore Wind Turbines under Earthquakes by Tuned Mass Damper

The dynamic characteristics of a bottom-fixed offshore wind turbine (OWT) under earthquakes are analyzed by developing an integrated analysis model of the OWT. Further, the influence of the interactions between the rotor and support system on the structural responses of the OWT subjected to an earthquake is discussed. Moreover, a passive control method using a tuned mass damper (TMD) is applied to the OWT to control the responses under earthquakes. The effects of the mass ratio, location and tuned frequency of the TMD on controlling structural responses of the OWT under different recorded seismic waves are studied.

Site selection for floating offshore wind turbines in Japanese coasts by quantitative risk assessment

Floating offshore wind turbines (FOWTs) are now at the stage of demonstration experiments in Japan. They have a specific risk of “drift”, which is negligible for bottom-fixed offshore wind turbines. Taking account of the risk of drift, it is necessary to consider the spatial relations between a FOWT and surrounding sea-lanes, coastal facilities, and ecosystem, in addition to the profitability and stakes in fishing industry, when selecting an appropriate site for installing the FOWT. In this paper, the authors propose the method of site selection for installing a FOWT by quantitative risk assessment, taking account of the spatial relations between the FOWT and surrounding sea-lanes, coastal facilities and ecosystem. Based on the calculation results in the case study of the quantitative risk assessment, the following procedure is recommended: first, extract the area where the risks related to the sea-lane are small enough, and second, select the position within the extracted area with the smallest risk of collision with coastal facilities or ecosystem in case of drift of the FOWT by quantitative evaluation of the risk distribution under the environmental conditions in the area. The proposed procedure is commonly applicable in most areas of Japanese coast.

Impact assessment of a wind turbine blade root during an offshore mating process

Single-blade installation is a popular method for installing blades on bottom-fixed offshore wind turbines. A jack-up crane vessel is often employed, and individual blades with their roots equipped with mechanical joints and bolted connections are lifted to the tower-top height and mated with a pre-assembled hub. The final mating phase is challenging and faces significant risks of impact. Due to relative motions between the blade and the hub, substantial impact forces may arise and lead to severe structural damages at root connections, thereby causing delays in the installation task. The present paper considers a realistic scenario of the mating process and investigates the consequences of such impact loads. Here, a single-blade model with tugger lines and a monopile model were established using a multi-body formulation, and relative velocities under collinear wave and wind conditions were obtained. A three-dimensional finite element model was developed for the blade root with T-bolt connections, and an impact investigation was performed for the case in which a guiding connection impacts the hub. The results show severe bending and plastic deformation of the guide pin bolt together with failure of the adjoining composite laminate at the root connection. Based on the type of damage obtained for the different environmental conditions considered, this paper also discusses its consequence on the installation tasks and suggests onboard decision making in case of an impact incident. The results of this study provide new insights regarding the mating phase and can be utilised to establish response-based operational limits.

Model Uncertainties for Soil-Structure Interaction in Offshore Wind Turbine Monopile Foundations

Monopiles are the most common type of foundation used for bottom-fixed offshore wind turbines. This investigation concerns the influence of uncertainty related to soil–structure interaction models used to represent monopile–soil systems. The system response is studied for a severe sea state. Three wave-load cases are considered: (i) irregular waves assuming linearity; (ii) highly nonlinear waves that are merged into the irregular wave train; (iii) slamming loads that are included for the nonlinear waves. The extreme response and Fourier amplitude spectra for external moments and mudline bending moments are compared for these load cases where a simpler static pile-cap stiffness and a lumped-parameter model (LPM) are both considered. The fundamental frequency response of the system is well represented by the static pile-cap stiffness model; however, the influence of higher modes (i.e., the second and third modes with frequencies of about 1 Hz and 2 Hz, respectively) is significantly overestimated with the static model compared to the LPM. In the analyzed case, the differences in the higher modes are especially pronounced when slamming loads are not present.