Offshore Wind Turbine Support Structures Research Articles

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Considering the effect of dynamic response amplification, a reliability analysis of an offshore wind turbine support structure under an earthquake is presented. A reliability analysis based on the dynamic response requires a large amount of time when using not only a level 3 approach but also level 2 such as a first order reliability method (FORM). Moreover, if a limit state is defined by using the maximum stress at a structural joint where stress concentration occurs, a three-dimensional element should be used in the finite element analysis. This makes the computational load much heavier. To deal with this kind of problem, two techniques are suggested in this paper. One is the application of a quasi-static structural analysis that takes the dynamic amplification effect into account. The other is the use of a stress concentration factor to estimate the maximum local stress. The proposed reliability analysis is performed using a level 2 FORM and verified using a level 3 simulation approach.

A method for identification of an effective Winkler foundation for large-diameter offshore wind turbine support structures based on in-situ measured small-strain soil response and 3D modelling

A procedure is presented for the derivation of an effective small-strain soil stiffness governing the soil–structure interaction of large-diameter monopiles. As a first step, geophysical measurements are used to estimate the depth-dependent shear modulus G of the soil stratum. The second step is to use this modulus and an estimated Poisson’s ratio and density in a 3D model, which captures the deformation of both the monopile and the soil. As a final step, a new method is proposed to use the computed 3D response for identification of a depth dependent stiffness of an effective Winkler foundation. This stiffness can be used in a 1D model, which is more fit for design purposes. The presented procedure is deemed more appropriate than the often used “p–y curve” method, which was once calibrated for slender flexible piles and for which the input is based on the large-strain cone penetration test. The three steps are demonstrated for a particular design location. It is also shown that the displacements of the 3D model are smaller and the resulting fundamental natural frequency is higher than calculated with the p–y method.

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.

Evaluation of fatigue damage model predictions for fixed offshore wind turbine support structures

This work deals with the evaluation of the spectral fatigue damage prediction of a tripod offshore wind turbine support structure subjected to combined stochastic wave and wind – induced loads. The stochastic loadings are defined using the sea states based on a scatter diagram related to the North Atlantic. Further, the power spectral density of the hot spot stress is estimated accordingly. The prediction of fatigue damage is evaluated in several spectral fatigue damage models including the Rayleigh, Wirsching–Light, Tunna, α0.75, Tovo and Benasciutti, Zhao–Baker, Rice and Dirlik models. Critical hot spot locations, which experience the most fatigue damage, are analysed based on the finite element method and the S–N fatigue damage approach. The time-domain solution based on the rainflow cycle counting method is assumed to be the “real” data and the model that best fits the fatigue damage of the wind turbine support structure is identified with the Akaike’s Information Criterion.

Reliability analysis of laterally loaded piles for an offshore wind turbine support structure using response surface methodology

With an increasing demand of a renewable energy, new offshore wind turbine farms are being planned in some parts of the world. Foundation installation asks a significant cost of the total budget of offshore wind turbine (OWT) projects. Hence, a cost reduction from foundation parts is a key element when a cost-efficient designing of OWT budget. Mono-piles have been largely used, accounting about 78% of existing OWT foundations, because they are considered as a most economical alternative with a relatively shallow-water, less than 30 m of seawater depth. OWT design standards such as IEC, GL, DNV, API, and Eurocode are being developed in a form of reliability based limit state design method. In this paper, reliability analysis using the response surface method (RSM) and numerical simulation technique for an OWT mono-pile foundation were performed to investigate the sensitivities of mono-pile design parameters, and to find practical implications of RSM reliability analysis.

Dynamic reliability analysis of offshore wind turbine support structure under earthquake

Seismic reliability analysis of a jacket-type support structure for an offshore wind turbine was performed. When defining the limit state function by using the dynamic response of the support structure, a number of dynamic calculations must be performed in a First-Order Reliability Method (FORM). That means analysis costs become too high. In this paper, a new reliability analysis approach using a static response is used. The dynamic effect of the response is considered by introducing a new parameter called the Peak Response Factor (PRF). The probability distribution of PRF can be estimated by using the peak value in the dynamic response. The probability distribution of the PRF was obtained by analyzing dynamic responses during a set of ground motions. A numerical example is presented to compare the proposed approach with the conventional static response-based approach.

Statistical Estimation of Extreme Loads for the Design of Offshore Wind Turbines during Non-Operational Conditions

The International Electrotechnical Commission (IEC) design standard (IEC 61400-3) for offshore wind turbines includes a design load case which considers loads on the turbine during extreme conditions, when the wind turbine is not operating and the blades are feathered. The recommendation of this design standard is to simulate 6 one-hour periods, each with wind and wave fields modeled as random processes, and to calculate design loads as the mean of the maximum from the six simulations. Previous studies have investigated this recommendation for fixed bottom offshore wind turbines and raised concerns about the stability of the estimate of the design load calculated using these guidelines. The research presented in this paper calculates statistics of extreme design loads as a function of the number of one-hour simulations considered for three offshore wind turbine support structures: a fixed-bottom turbine supported by a monopile and two floating turbines, a spar buoy and a semi-submersible. The study considers one extreme design load, the moment at the tower base, one set of 50-year metocean conditions representive of the Northeast U.S. Atlantic coast, and five combinations of wind and wave models, including linear and nonlinear representations of irregular waves. The data generated from this study are used to assess the stability of the estimate of the extreme load for various numbers of one-hour simulations for each turbine type. The study shows that, for the considered metocean conditions and models, the monopile exhibits the least stability in the estimate of the extreme load and therefore requires more one-hour simulations to have comparable stability as the two floating support structures. Explanations for this result are provided along with a probabilistic formulation for estimating variability of design loads more efficiently, using fewer one-hour simulations.

Fatigue damage assessment of fixed offshore wind turbine tripod support structures

The objective of this work is to carry out a fatigue damage assessment of a fixed offshore wind turbine support structure due to combined wave and wind – induced loading. Considering the operational mode of the wind turbine 3 loading conditions are identified and for each loading condition 4 critical regions located in different zones of the supporting structure are studied. The stress transfer function is obtained by carrying out dynamic finite element analysis in the frequency domain. A wave scatter diagram of the North Atlantic is used in order to account for the environmental effects. The stress transfer function is formulated such that a sufficient number of frequencies can be used in the Inverse Fast Fourier Transform with random phase angles in order to obtain the stress time histories. An up-to-date rainflow cycle counting method is adopted to count the total number of cycles related to stress range bins. The S–N approach is employed to estimate fatigue damage.

Dynamic Models for Load Calculation Procedures of Offshore Wind Turbine Support Structures: Overview, Assessment, and Outlook

One of the main mechanisms for driving down the cost of offshore wind energy is to install ever larger wind turbines in larger wind farms. At the same time, these turbines are placed further offshore in deeper waters. As a result, traditional monopile foundations are not always feasible and multimembered foundations, such as jackets and tripods are required. Typically, thousands of load cases need to be simulated for the design and certification of offshore wind turbines (OWTs). As models of such foundations are significantly larger than their monopile counterparts, model reduction is often applied to limit the computational costs. Additionally, the foundation design is generally done by a specialized company, which bases its design on the results of the load simulations. Hence, an accurate estimation of the stresses in load simulation is essential to predict the integrity and the lifetime of different designs. The effect on the load accuracy of both the model reduction as well as the postprocessing method used by foundation designers (FDs) are investigated in this paper. A case study is performed on a jacket-based wind turbine model to verify and quantify the findings. First, it is observed that the effect of the reduced foundation model on the wind turbine loads is negligible. However, both the reduction method and the postprocessing method applied by the FD have a large influence on the fatigue loading in the jacket. It is shown that the popular Guyan reduction results in significant errors on the fatigue damage and that a static postprocessing analysis leads to serious underestimations of the fatigue loads. Finally, an outlook is given into future developments in the field of load calculations for OWT foundation design.

Simplified fatigue load assessment in offshore wind turbine structural analysis

AbstractThe estimation of fatigue lifetime for an offshore wind turbine support structure requires a large number of time‐domain simulations. It is an important question whether it is possible to reduce the number of load cases while retaining a high level of accuracy of the results. We present a novel method for simplified fatigue load assessments based on statistical regression models that estimate fatigue damage during power production. The main idea is to predict the total fatigue damage only and not also the individual damage values for each load case. We demonstrate the method for a jacket‐type support structure. Reducing the number of simulated load cases from 21 to 3, the total fatigue damage estimate exhibited a maximum error of about 6% compared with the complete assessment. As a consequence, a significant amount of simulation time can be saved, in the order of a factor of seven. This quick fatigue assessment is especially interesting in the application of structural optimization, with a large number of iterations. Copyright © 2015 John Wiley & Sons, Ltd.

Optimization of Offshore Wind Turbine Support Structures Using an Analytical Gradient-based Method

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. This paper presents an analytical gradient-based method to solve this problem in an efficient and effective way. The design sensitivities of the objective and constraint functions are evaluated analytically while the optimization of the structure is performed, subject to sizing, eigenfrequency, extreme load and fatigue load constraints. A case study was carried out for the OC4 jacket substructure to evaluate the method. Results show that an optimal jacket design with 52 percent structural mass reduction was attained in 27 iterations, while satisfying all design constraints under the simplified load cases used. Besides, it is shown that the analytical sensitivity analysis was more accurate and efficient than the often used finite difference approximations. It could avoid numerical artifacts that typically occur in the analysis of extreme load constraint sensitivities.

Integrated Multidisciplinary Constrained Optimization of Offshore Support Structures

In the current offshore wind turbine support structure design method, the tower and foundation, which form the support structure are designed separately by the turbine and foundation designer. This method yields a suboptimal design and it results in a heavy, overdesigned and expensive support structure. This paper presents an integrated multidisciplinary approach to design the tower and foundation simultaneously. Aerodynamics, hydrodynamics, structure and soil mechanics are the modeled disciplines to capture the full dynamic behavior of the foundation and tower under different environmental conditions. The objective function to be minimized is the mass of the support structure. The model includes various design constraints: local and global buckling, modal frequencies, and fatigue damage along different stations of the structure. To show the usefulness of the method, an existing SWT-3.6-107 offshore wind turbine where its tower and foundation are designed separately is used as a case study. The result of the integrated multidisciplinary design optimization shows 12.1% reduction in the mass of the support structure, while satisfying all the design constraints.

Reliability analysis of offshore wind turbine support structures under extreme ocean environmental loads

Reliability analysis of jacket type offshore wind turbine (OWT) support structure under extreme ocean environmental loads was performed. Limit state function (LSF) of OWT support structure is defined by using structural dynamic response at mud-line. Then, the dynamic response is expressed as the static response multiplied by peak response factor (PRF). Probabilistic distribution of PRF is found from response time history under design significant wave load. Band limited beta distribution is used for internal friction angle of ground soil. Wind load is obtained in the form of thrust force from commercial code called Bladed and then, applied to tower hub as random load. In numerical example, response surface method (RSM) is used to express LSF of jacket type support structure for 5 MW OWT. Reliability index is found using first order reliability method (FORM).

Fatigue design of offshore steel mono-pile wind substructures

Offshore wind turbine support structures experience tens of millions of load cycles throughout their design lives, such that these structures are prone to high-cycle fatigue damage. This paper focuses on steel mono-pile substructures, by far the most common type of offshore wind installation, and examines the origin of current fatigue design guidance and what needs to be done to develop guidelines in order to support designers and operators to better optimise offshore wind support structures. The paper discusses some of the incrementally developed techniques for fatigue design from the oil and gas sector and questions whether or not these are entirely appropriate for the rapidly developing offshore wind industry.

Simplified critical mudline bending moment spectra of offshore wind turbine support structures

Offshore wind turbines are subjected to multiple dynamic loads arising from the wind, waves, rotational frequency (1P) and blade passing frequency (3P) loads. In the literature, these loads are often represented using a frequency plot where the power spectral densities (PSDs) of wave height and wind turbulence are plotted against the corresponding frequency range. The PSD magnitudes are usually normalized to unity, probably because they have different units, and thus, the magnitudes are not directly comparable. In this paper, a generalized attempt has been made to evaluate the relative magnitudes of these four loadings by transforming them into bending moment spectra using site-specific and turbine-specific data. A formulation is proposed to construct bending moment spectra at the mudline, i.e. at the location where the highest fatigue damage is expected. Equally, this formulation can also be tailored to find the bending moment at any other critical cross section, e.g. the transition piece level. Finally, an example case study is considered to demonstrate the application of the proposed methodology. The constructed spectra serve as a basis for frequency-domain fatigue estimation methods available in the literature

Condensation of long-term wave climates for the fatigue design of hydrodynamically sensitive offshore wind turbine support structures

Cost-efficient and reliable fatigue designs of offshore wind turbine support structures require an adequate representation of the site-specific wind–wave joint distribution. Establishment of this wind–wave joint distribution for design load calculation purposes requires typically a correlation of the marginal wind and wave distribution. This is achieved by condensation of the site-specific wave climate in terms of wave period or wave height lumping, subsequently used as input for a correlation with the corresponding wind climate. The quality of this resulting wind–wave correlation is especially important for hydrodynamically sensitive structures since the applied met-ocean parameters have a non-linear influence on calculated fatigue design loads. The present article introduces a new wave lumping method for condensation of the wave climate. The novelty is predominantly based on refined equivalence criterions for fatigue loads aiming at preservation of the fatigue damage distribution over either the wave height or wave period distribution. This new method is assessed in comparison with different other traditional wave lumping methods on the basis of the site-specific wave climate for the offshore wind farm project Gemini which has kindly been made available by the developer Typhoon Offshore. It is shown that the new method allows for a significantly better preservation of the hydrodynamic fatigue in comparison to the traditional methods.

Incremental wind-wave analysis of the structural capacity of offshore wind turbine support structures under extreme loading

Offshore wind turbine (OWT) support structures are subjected to non-proportional environmental wind and wave load patterns with respect to increases in wave height and with respect to wind and wave combined loading. Traditional approaches to estimating the ultimate capacity of offshore support structures are not ideally suited to analysis of OWTs. In this paper, the concept of incremental wind-wave (IWWA) analysis of the structural capacity of OWT support structures is proposed. The approach uses static pushover analysis of OWT support structures subject to wind and wave combined load patterns corresponding to increasing mean return period (MRP). The IWWA framework can be applied as a one-parameter approach (IWWA1) in which the MRP for the wind and wave conditions is assumed to be the same or a two-parameter approach (IWWA2) in which the MRPs associated with wind and wave conditions are related to a joint probability density function characterizing the wind and wave conditions at the site. Example calculations for monopile and jacket supported OWTs at Atlantic marine sites are performed under both one parameter and two parameters IWWA framework. The analyses illustrate that: the results of an IWWA analysis are site specific; and structural response can be dominated by either wind or wave conditions depending on structural characteristics and site conditions. Finally, reliability analyses for both examples excluding uncertainties in structural resistance are estimated based on their IWWA results and probabilistic models for site environmental conditions.

Reliability-based design optimization of monopile transition piece for offshore wind turbine system

This paper presents a reliability-based design optimization (RBDO) method for a monopile transition piece in an offshore wind turbine system. Two design approaches are investigated for the cost-effective and reliable design of the monopile transition piece: deterministic optimization (DO) and RBDO. First, dynamic response analysis of a reference offshore wind turbine is conducted to estimate design loads considering site conditions off the southwest coast of Korea. Second, DO for minimizing the mass of a conical monopile connection is carried out, including an assessment of the reliability of the DO design. Next, RBDO is performed to achieve a design with the desired reliability while concurrently minimizing the mass of the monopile transition piece. The present study shows that the structural design of the monopile connection is mostly dictated by the fatigue limit state and that DO does not guarantee structural reliability even though the design satisfies all limit state function conditions. The proposed RBDO process is shown to speed up the design cycle and enhance the reliability of the grouted connection for offshore wind turbine support structures.

Fatigue loading on a 5MW offshore wind turbine due to the combined action of waves and current

In the design of an offshore wind turbine the natural frequencies of the structure are of importance. In the design of fixed offshore wind turbine support structures it cannot be avoided that the first eigenmode of the structure lies in the frequency band of wave excitation. This study indicates that wave-current interaction should be taken into account for support structure design load calculations.Wave-current interaction changes the shape of the wave spectrum and the energy content in the wave frequency range of 0.2 – 0.35Hz. This is in the range of natural frequencies fixed offshore wind turbine structures are designed for. The waves are affected by the current in two ways. First there is a frequency shift, Doppler effect, for the fixed observer when the wave travels on a current. Second the shape of the wave is modified in case the wave travels from an area without current into an area with current. Due to wave-current interaction the wave height and wave length change. For waves on an opposing current the wave energy content increases, while for wave on a following current the wave energy content slightly reduces.Simulations of normal production cases between cut-in and cut-out wind speed are performed for a 5MW wind turbine in 20m water depth including waves with 1) a following current, 2) an opposing current and 3) no current present. In case of waves having an opposing current, the 1Hz equivalent fore-aft tower bending moment at the seabed is about 10% higher compared to load cases with waves only.

Influence of second-order random wave kinematics on the design loads of offshore wind turbine support structures

The impact of wave model nonlinearities on the design loads of wind turbine monopile foundations is delineated based on a second-order nonlinear random wave model that satisfies the boundary conditions at the free surface and by including the effects of convective acceleration in the inertial loads. The second-order nonlinear water kinematics is developed based on a Gram Charlier series expansion using the first four stochastic moments of the wave process. The wave surface velocities and accelerations are expressed using a Taylor series expansion about the mean sea level, which satisfies to the second-order, the unsteady Bernoulli equation and normal flow condition at the free surface. The operating design loads on the monopile are computed using fully coupled and uncoupled methods. The computation of the mud level loads based on the inclusion of nonlinear wave surface kinematics is compared with those obtained when using linear waves and Wheeler stretching. The effect of the spatial derivatives of the wave velocity on the wave surface kinematics is quantified and shown to determine the wave spectral cut-off frequency limit. The spatial derivatives of wave velocity also participate in the expression for the wave convective acceleration, whose effect is demonstrated on the inertial loads on the foundation in the presence of ocean currents. The effect of nonlinear water kinematics on the monopile design load reveals the large frequency bandwidth of wave structure interaction, but the phase differences between the hydrodynamic loads with the rotor loads tend to lower the probability of joint simultaneous extreme peaks in hydrodynamics and rotor loads.