Offshore Wind Turbine Substructures Research Articles

Reliability analysis of fatigue damage extrapolations of wind turbines using offshore strain measurements

While substructures of offshore wind turbines become older and begin to reach their design lifetimes, the relevance of measurement based lifetime extension increases. To make well-founded decisions on possible lifetime extensions, damage extrapolations based on measurements are needed. However, although for all substructures, fatigue damage calculations were conducted during the design process, there is no consensus on how to extrapolate 10-minute damages to lifetime damages. Furthermore, extrapolating damages is an uncertain process and its actual reliability is unknown. Therefore, the current work uses data of offshore strain measurements to assess different approaches of extrapolating damages, and to investigate the reliability of damage extrapolations. For the present data, the most reliable lifetime estimations are possible, if the damage data is split up into wind speed bins. For each wind speed bin, the occurrence probability should be based on data rather than on design documents. Moreover, using mean damages in each bin is the best practice. Furthermore, our results suggest that strain measurements of about 9 to 10 months lead to a relatively representative and unbiased data set. Therefore, if there are no significant changes of the turbine or the environmental conditions over the lifetime, damage extrapolations based on such a time period are sufficiently accurate.

A Comparative Study between Three-Legged and Tripod Sub-structures in Design of Offshore Wind Turbines in the Transition Water Depth

This research aimed at investigating tripod and three-legged offshore wind turbine substructures. A comparison between the two substructures based on their weight as well as the installation method of piles, i.e. pre-piling and post-piling, was carried out. The in-place (Ultimate Limit State), Dynamic, natural frequency check and fatigue (Fatigue Limit State) analyses were conducted considering aerodynamic and hydrodynamic loads imposed on substructures in 50m water depth. An optimisation process was carried out in order to reduce the mass of substructures. The results revealed that the three-legged substructure is more cost effective with 25% lesser structure mass. However, the construction of the three-legged structure usually takes more time due to increased number of members and subsequently welding joints. The results, furthermore, showed that the pre-piling method reduces the time and cost of offshore installation, and reduces the weight of piles by 50%.

Impact of turbulence induced loads and wave kinematic models on fatigue reliability estimates of offshore wind turbine monopiles

The cost of offshore wind turbine substructures has a significant impact on competitiveness of the wind energy market and is affected by conservative safety margins adopted in the design phase. This implies that an accurate design load prediction, especially of those resulting in fatigue damage accumulation, may help achieve more cost-effective solutions. In this article, the impact of turbulence and wave loads on fatigue reliability of pile foundations is investigated for a 5-MW offshore wind turbine. Loads obtained by varying turbulence percentiles are compared with those obtained from the full joint probability distribution of wind speed and turbulence through Monte Carlo (MC) simulations, and from the equivalent turbulence level currently adopted by IEC standards. The analyses demonstrate that a lower equivalent turbulence percentile leads to a more realistic and less conservative estimation of fatigue loads. Subsequently, the research focuses on studying the effects of uncertain marine environments on the fatigue load distribution, showing that the latter is insensitive to the random variability of the hydrodynamic coefficients. With respect to the wave kinematic model, a comparison between nonlinear and linear waves clearly suggests that hydrodynamic forces depend significantly on the kinematic model adopted and the operational conditions of the turbine. Furthermore, a term is derived to correct the error introduced by Wheeler stretching at finite water depths. The respective model uncertainties that originate from the nonlinear irregular wave model and Wheeler correction are quantified and employed in a reliability analysis. In a case study, the results are finally compared in terms of estimated probability of failure, with the aim to quantify the influence of environmental models on monopile reliability.

A model for quick load analysis for monopile-type offshore wind turbine substructures

Abstract. A model for quick load analysis, QuLA, of an offshore wind turbine substructure is presented. The aerodynamic rotor loads and damping are pre-computed for a land-based configuration. The dynamic structural response is represented by the first global fore-aft mode only and is computed in the frequency domain with phases using the equation of motion. The model is compared to the state-of-the-art aeroelastic code Flex5. Both lifetime fatigue and extreme loads are considered in the comparison. In general there is good agreement between the two models. Some deviations for the sectional forces are explained in terms of the model simplifications. The differences in the sectional moments are found to be within 10 % for the fatigue load case and 10 % for the extreme load condition.

Prediction of the shape of inline wave force and free surface elevation using First Order Reliability Method (FORM)

In design of substructures for offshore wind turbines, the extreme wave loads which are of interest in Ultimate Limit States are often estimated by choosing extreme events from linear random sea states and replacing them by either stream function wave theory or the NewWave theory of a certain design wave height. As these wave theories super from limitations such as symmetry around the crest, other methods to estimate the wave loads are needed. In the present paper, the First Order Reliability Method, FORM, is used systematically to estimate the most likely extreme wave shapes. Two parameters of maximum crest height and maximum inline force are used to define the extreme events. FORM is applied to first and second-order irregular waves in both 2D and 3D. The application is validated against the NewWave model and also the NewForce model, which is introduced as the force equivalent of NewWave theory, that is, the most likely time history of inline force around a force peak of given value. The results of FORM and NewForce are linearly identical and show only minor deviations at second order. The FORM results are then compared to wave averaged measurements of the same criteria for crest height and peak force value. Relatively good agreement between the FORM results of free surface elevation including the second order effects, and the wave averaged measurements is observed. However, the inline force time series reproduced using the numerical method are not as consistent with the measurements as the free surface elevation time series. The discrepancies between the FORM results and the measurements is found to be a result of more nonlinearity in the selected events than second order and negligence of the drag forces above still water level in the present analysis. This paper is one step toward more precise prediction of extreme wave shape and loads. Ultimately such waves can be used in the design process of offshore structures. The approach can be generalized to fully nonlinear models.

Numerical study of irregular breaking wave forces on a monopile for offshore wind turbines

The substructures of offshore wind turbines are subjected to different types of hydrodynamic loads from sea states. The wave forces exerted by irregular breaking waves are one of the serious concerns because of the uncertainties involved in defining the breaking wave and the resulting force calculations. In the present study, irregular breaking wave forces on a vertical pile structure are investigated using an open-source Computational Fluid Dynamics (CFD) model REEF3D. The Level Set Method (LSM) is used for modelling the free surface. The Bretschneider spectrum is used for the irregular wave generation. This is validated in the numerical wave tank by comparing the numerical wave spectrum with the experimental wave spectrum. The wave free surface is calculated at three wave gauge locations and compared with experiments. It is observed that the peak of spectral wave density is higher for the wave gauge located besides the cylinder due to shoaling, wave run up and reflections from the cylinder and the peak of spectral wave density is lower for the wave gauge located behind the cylinder due to wave breaking. Further, simulations are performed to study the wave forces on a monopile due to the depth-limited breaking waves. A good match is observed with the experimental and numerical results. Numerical wave energy spectra at different locations along the tank are compared to study the changes in the wave surface elevations due to the interaction of irregular breaking waves with a monopile. The statistical parameters for free surface elevation and wave forces are further investigated. The free surface features around the monopile during its interaction with waves are also studied.

17.05: Advanced framework for suction bucket jacket design

ABSTRACTThe suction bucket is a well know foundation concept from the oil and gas industry and the concept is now adopted in the foundation of jacket substructures for offshore wind turbines. The suction buckets transfer the loads from the jacket to the soil and it is essential to include the jacket‐bucket‐soil interaction.Ramboll has developed a framework for the design of suction buckets, where both in‐place and installation scenarios are included.In this paper, we will discuss the important features included in the framework and the steps required delivering an optimized and fabrication friendly bucket design. A major challenge is to determine the foundation loads and corresponding soil stiffness for the jacket design and installation support conditions for the bucket design.It is essential to assess the short and long‐term global and local interaction between jacket/bucket as well as the bucket/soil interaction during installation to ensure that the strong suction pressure can be safely applied. The interfaces between the structural and geotechnical designs are further challenging for multiple foundations in a wind farm with varying water depths and soil conditions. The importance of focus on the safe design of this thin walled structure and the importance of managing the interfaces in such projects are highlighted in this paper.The framework for verification of suction bucket foundations involves the interfaces between geotechnical design, efficient load calculation, structural design and production. A detailed design involves numerous calculations on the buckling capacity of the bucket skirt and lid subjected to ultimate loads as well as design for fatigue.

DeRisk — Accurate Prediction of ULS Wave Loads. Outlook and First Results

Loads from extreme waves can be dimensioning for the substructures of offshore wind turbines. The DeRisk project (2015-2019) aims at an improved load evaluation procedure for extreme waves through application of advanced wave models, laboratory tests of load effects, development of hydrodynamic load models, aero-elastic response calculations and statistical analysis. This first paper from the project outlines the content and philosophy behind DeRisk. Next, the first results from laboratory tests with irregular waves are presented, including results for 2D and 3D focused wave groups. The results of focused wave group tests and a 6-hour (full scale duration) test are reproduced numerically by re-application of the wave paddle signal in a fully nonlinear potential flow wave model. A good match for the free surface elevation and associated exceedance probability curve is obtained. Finally, the utilization of DeRisk's results in practical design is discussed.

Irregular Wave Forces on a Large Vertical Circular Cylinder

Substructures of offshore wind turbines are exposed to irregular sea states that are generally defined using wave spectra. The waves undergo transformations due to non-linear wave-wave interactions and due to interaction with the structures they are incident on. In the present study, simulations of regular and irregular wave interaction with a vertical cylinder are carried out using the open- source Computational Fluid Dynamics (CFD) model REEF3D. The model solves the Reynolds Averaged Navier-Stokes (RANS) equations over the entire domain and provides detailed information regarding the wave hydrodynamics including fluid pressure, velocities and the free surface. The non-linear wave-wave, wave-structure interactions and the turbulence in the flow are accounted for in the solution of the RANS equations. In this way, detailed flow features around the cylinders can be visualised and analysed. The numerical model is verified with the analytical equations for the loads on a cylinder under regular waves. Further, simulations are carried out for irregular waves generated using the JONSWAP wave spectrum and the wave force spectrum is calculated. The wave spectrum for the different wave gauges around the cylinder are compared. The free surface features around the cylinder are visualised and correlated to the wave forces on the cylinder. It is observed that the regular waves with higher steepness show a clear diffraction pattern around the cylinder. For the irregular waves, the diffraction pattern is less developed and irregular.

A model for Quick Load Analysis for monopile-type offshore wind turbine substructures

A model for Quick Load Analysis, QuLA, of an offshore wind turbine substructure is presented. The aerodynamic rotor loads and damping are precomputed for a load-based configuration. The dynamic structural response is represented by the first global fore-aft mode only and is computed in the frequency domain using the equation of motion. The model is compared against the state of the art aeroelastic code, Flex5, and both life time fatigue and extreme loads are considered in the comparison. In general there is good similarity between the two models. Some derivation for the sectional forces are explained in terms of the model simplifications. The difference in the sectional moments are found to be within 14% for the fatigue load case and 10% for the extreme load condition.

Fresh and hardened properties of steel fiber-reinforced grouts containing ground granulated blast-furnace slag

Fresh and hardened properties of steel fiber-reinforced grout (SFRG) containing ground granulated blast-furnace slag (GGBS) were investigated for applications to prestressed concrete substructures of offshore wind turbines. The addition of short smooth steel fibers (6mm in length, 0.2mm in diameter) generally decreased the flow, bleeding, and time of setting of SFRG, whereas the addition of GGBS for the replacement of 40% of the cement by weight ratio clearly increased its flow and the time of setting. The effect of adding GGBS on bleeding was different according to the type of cement. Despite these outcomes, it was observed that the addition of GGBS notably enhanced the chloride penetration resistance of SFRGs, while the addition of steel fibers significantly increased their flexural strength.

Monopod Bucket Foundations Under Cyclic Lateral Loading

The monopod bucket foundation can be a cost-reducing sub-structure for offshore wind turbines. To avoid problems during the turbine operation, the long-term effect of cyclic loading must be considered in the design of the foundation. In this paper a 1g testing rig is adopted to extend the knowledge on bucket foundations under lateral cyclic loading. The test setup is described in detail and a comprehensive experimental campaign is presented. The foundation is subjected to cyclic overturning moment, cyclic horizontal loading and constant vertical loading, acting on the same plane for thousands of cycles. Three buckets with different embedment ratios are tested. The data interpretation is focused on the long-term permanent rotation of the foundation and, particularly, on understanding how the controlling variables influence the potential for rotation accumulation. New and more general parameters of an empirical model predicting the long-term plastic rotation are proposed on the base of the experimental results.

Design and Analysis of Jacket Substructures for Offshore Wind Turbines

This study focused on investigating various existing types of offshore jacket substructures along with a proposed twisted-tripod jacket type (modified jacket (MJ)-structures). The architectures of the three-leg structure, as well as the patented twisted jacket structure motivated the design of the proposed MJ-structures. The dimensions of the structures were designed iteratively using static stress analysis to ensure that all structures had a similar level of load-carrying capability. The numerical global buckling analyses were performed for all structures after the validation by the scaled-down experiments. The local buckling strength of all compressive members was analyzed using the NORSOK standard. The results showed that the proposed MJ-structures possess excellent structural behavior and few structural nodes and components competitive with the patented twisted jacket structures, while still maintaining the advantages of low material usage similar to the three-leg jacket structures. This study provides alternatives for the initial selection and design of offshore wind turbine substructures for green energy applications.

Analytical gradient-based optimization of offshore wind turbine substructures under fatigue and extreme loads

Design optimization of offshore wind turbine support structures is an expensive task due to the highly-constrained, non-convex and non-linear nature of the design problem. A good depth of detail in the problem formulation can give useful insights in the practical design process, but may also compromise the efficiency. This paper presents an analytical gradient-based method to solve the problem in an effective and efficient way. The design sensitivities of the objective and constraint functions are evaluated analytically, while the optimization procedure is performed in the time domain, subjected to sizing, eigenfrequency, extreme load and fatigue load constraints. A case study on the OC4 and UpWind jacket substructures show that the method was reliable and consistent in delivering superior efficiency and accuracy in the optimization study, as compared with the conventional finite difference approach. The global optimum was probably achieved in the design optimization process, where the large number of design constraints implemented can possibly be the blessing in disguise, as they seem to enable the optimizer to find the global optimum. Both the buckling and fatigue load constraints had significant influence over the design of tubular members and joints, while each component is oriented to maximize the utilization against the prescribed limit state functions.

Pitch motion mitigation of spar-type floating substructure for offshore wind turbine using multilayer tuned liquid damper

The possibility of controlling the pitch motion of a spar-type floating substructure for offshore wind turbines by using a tuned liquid damper (TLD) was numerically investigated. First, the motion of a spar-type substructure scale model excited by surface waves was predicted, and the results were compared to the measured data. There was reasonably good agreement between the two results, which confirmed the validity of the present numerical methods. Then, the effects of a TLD on the motion of the floating substructure were quantitatively assessed by a comparison between the predicted results with and without a TLD. The pitch motion of the scale model could be reduced by using a TLD. Based on this result, multilayer TLDs were proposed to effectively reduce the pitch motion of the floating substructure at the fixed target frequency. This expectation was confirmed when the predicted results were compared with that of the single-layer TLD. Finally, a theoretical model was derived to compute the rate of reduction in the pitch motion of floating substructures depending on the number of TLD layers. This model allowed the minimum number of layers for TLDs to reduce the pitch motion of the substructure by the required amount to be determined.

Experimental investigation on the development of wear in grouted connections for offshore wind turbine generators

Relative displacements between grout and steel have been observed in grouted connections used for offshore wind turbine substructures, which appear to be linked to the unexpected settlements that have occurred in some offshore wind farms. A literature review has highlighted a lack of understanding of the implications that this relative movement has on the grout wear. Experimentation has therefore been undertaken to determine the influence of various factors on the wear development, including compressive stress, displacement amplitude, surface roughness and the presence of water, looking at conditions typically experienced by offshore grouted connections. These experiments have indicated that wear of the steel and grout surfaces occur, even at low magnitude compressive stresses. The presence of water has the most significant impact on wear rate, being up to 18 times higher than for the equivalent dry condition. The presence of water can also significantly reduce the coefficient of friction to values lower than typically recommended for evaluation of grouted connections. These findings demonstrate that wear of the grouted connection is likely to occur over the life of this type of offshore structures and should therefore be considered when evaluating their integrity and assessing their behaviour.

하이브리드 해상풍력발전 지지구조물의 콘크리트 베이스-슬리브 연결부에 대한 실험 연구

In this paper, concrete base to sleeve connections of hybrid substructures for offshore wind turbines were suggested and investigated experimentally. Punching shear strength tests with well-instrumented three connections under different reinforcement ratios and loading conditions were conducted to investigate the punching shear strength and the behavior of the concrete base to a sleeve connection. The test results showed that the punching strength and stiffness of the connections are affected mainly by the reinforcement ratios. The loading conditions with an axial load and proportional moment cannot affect the stiffness but affect the strength of the connections because of the axial load-moment interaction. The punching shear failure and critical section of the each test specimen are also discussed.

Numerical analysis of a hybrid substructure for offshore wind turbines

For the reliable design of substructure supporting offshore wind turbines it is very important to reduce the effects of wave forces. Since the substructure is strongly influenced by the effects of wave forces as the size of substructure increases. In the present study, the hybrid substructure with multi-cylinder is newly suggested to reduce the effects of wave forces. Using diffraction theory the scattering waves in a fluid region are expressed by an Eigenfunction expansion method with three dimensional potential theory to calculate the wave force acting on the hybrid substructure. The wave force and wave run-up acting on the hybrid substructure is presented to examine the water wave interaction according to the variation of cylindrical size and the distance among cylinders. It is found that the suggested hybrid substructure with multi-cylinder is very useful to reduce the effects of wave forces acting on the substructure for offshore wind turbines.

Feasibility study of offshore wind turbine substructures for southwest offshore wind farm project in Korea

Korea has huge potential for offshore wind energy and the first Korean offshore wind farm has been initiated off the southwest coast. With increasing water depth, different substructures of the offshore wind turbine, such as the jacket and multipile, are the increasing focus of attention because they appear to be cost-effective. However, these substructures are still in the early stages of development in the offshore wind industry. The aim of the present study was to design a suitable substructure, such as a jacket or multipile, to support a 5 MW wind turbine in 33 m deep water for the Korean Southwest Offshore Wind Farm. This study also aimed to compare the dynamic responses of different substructures including the monopile, jacket and multipile and evaluate their feasibility. We therefore performed an eigenanalysis and a coupled aero-hydro-servo-elastic simulation under deterministic and stochastic conditions in the environmental conditions in Korea. The results showed that the designed jacket and multipile substructures, together with the modified monopile, were well located at soft–stiff intervals, where most modern utility-scale wind turbine support structures are designed. The dynamic responses of the different substructures showed that of the three substructures, the performance of the jacket was very good. In addition, considering the simple configuration of the multipile, which results in lower manufacturing cost, this substructure can provide another possible solution for Korean’s first offshore wind farm. This study provides knowledge that can be applied for the deployment of large-scale offshore wind turbines in intermediate water depths in Korea.

Hydrodynamic response of alternative floating substructures for spar-type offshore wind turbines

Hydrodynamic analyses of classic and truss spar platforms for floating offshore wind turbines (FOWTs) were performed in the frequency domain, by considering coupling effects of the structure and its mooring system. Based on the Morison equation and Diffraction theory, different wave loads over various frequency ranges and underlying hydrodynamic equations were calculated. Then, Response Amplitude Operators (RAOs) of 6 DOF motions were obtained through the coupled hydrodynamic frequency domain analysis of classic and truss spar-type FOWTs. Truss spar platform had better heave motion performance and less weight than classic spar, while the hydrostatic stability did not show much difference between the two spar platforms.