Search

A numerical investigation of floating-type substructures for wind turbine generators was conducted by using time-domain simulation. A Tri-Star floater for 8–10 MW generators, which was developed by Samsung Heavy Industries (SHI), was chosen as the floating substructure. To make the anchor system, catenary mooring lines, considering redundancy, were installed on the floater. The main sources of external force on the wind turbine generator are wind, waves, and currents. To consider severe environmental conditions, Design Load Cases (DLCs) 1.6 and 6.1 of the IEC guidelines (IEC 61400-3-1) were chosen. From the measured environmental data for the installation site, the main parameters for the simulation conditions were obtained. The tilt angle and horizontal movement of the floater and the mooring tension for the different mooring systems were checked. The response of the floater during the failure of the mooring was also studied, and the critical failure of the mooring was confirmed. During the failure of the mooring, the redundancy system worked well, in which the movement of the floater was constrained within the criteria for all scenarios. [ABSTRACT FROM AUTHOR]

Wind turbine towers and their flange connections are highly fatigue loaded structures. Previous investigations have shown that initial parallel gaps between L‐flange connections, which are resulting from their respective flatness of the contact surfaces, can increase fatigue loads of the connecting fasteners significantly. Flange flatness and/or gap sizes in between flanges must comply with limitations given in relevant standards and therefore must be measured after the manufacturing process and/or during installation. More than 1900 flatness measurements of several offshore wind projects were collected and used to determine initial gaps between flanges analytically. A statistical evaluation allows to determine the distribution of expected gap sizes. The increase of gap height with increasing gap length can be described with linear correlations. Furthermore, the distributions of gap heights are approximated with log‐normal distributions. Parameters for generic log‐normal distributions are derived considering all performed mating simulations. Following the described methodology, gaps can be determined from any large number of flatness measurements. It is proposed to consider the results for flange design. [ABSTRACT FROM AUTHOR]

This paper investigates the structural dynamics of 10 MW offshore wind turbines (OWTs) supported by different substructures (Monopile, Tripod and Jacket) under wind, wave, current and earthquake loadings. The support structures are modeled using ANSYS, a finite element models (FEM) software package, by considering the nonlinear soil structure interaction (SSI) effects and earthquake loadings. A spring-displacement method has been employed to model the earthquake excitations on the support structures. FAST and AQWA tools have been used to analyze wind and wave loads which are fed into ANSYS via a dynamic link library (DLL) as external loads for combination with the earthquake loads to predict the support structures' responses. Earthquakes of different magnitudes have been simulated in the study. Under the earthquake with a magnitude of 7 measured on a Richter scale, the tower top displacements of the tripod, jacket and monopile are 1.42 m, 1.75 m and 1.80 m, respectively. The relative position of the displacement trajectory of the tripod OWT is nearest to the centroid of the yaw bearing. The maximum stresses of the jacket and tripod are respectively 5.7 times and 2.3 times that of the monopile. The average stress of the jacket and tripod are respectively 0.74 times and 0.56 times that of the monopile. This phenomenon shows that the tripod and jacket have a good stability, but with a high risk of local failure under earthquakes. The responses of the piles are mainly dominated by the seismic loads, rather than the wind and current loads. It was observed that the piles of the jacket and tripod OWT, which have small diameters and thin wall thickness, are more sensitive to earthquakes. Thus, the jacket and tripod OWTs may rely on the large stiffness offered by substructures to improve their stability. These phenomena can provide powerful insight into the seismic design of different substructures. [ABSTRACT FROM AUTHOR]

This paper extends the conventional failure mode and effect analysis methodology by introducing weights of its indices that are severity, occurrence, and detection as a basis to analyze the failures of the support structure of a generic floating offshore wind turbine. Critical failure causes, failure modes, as well as systems of the support structure of the floating offshore wind turbine are ascertained. Moreover, based on the analysis, recommendations on corrections and preventive actions are suggested aiming at ensuring the safe and economic operations of the support structure. The validation of the proposed technique is finalized by a comparison study between the results of conventional failure mode and effect analysis methodology and that of the proposed method. The comparison indicates that the proposed technique is more in line with practice and flexible for use and has the merit in removing the limitations of conventional failure mode and effect analysis methodology that different failure causes generate the same risk priority number. • The conventional FMEA is extended to analyze the failures of the support structure of a generic floating wind turbine. • Critical failure causes, failure modes, and systems of the support structure of the floating wind turbine are ascertained. • Recommendations on corrections are suggested aiming at ensuring the safe and economic operation of the support structure. • A comparison is made between the results of conventional FMEA methodology and that of the proposed method. • The proposed technique is more flexible for use and has the merit in partly removing the limitations of conventional FMEA. [ABSTRACT FROM AUTHOR]

This paper investigates efficient design of offshore wind turbine (OWT) support structures under ultimate loads and proposes three schemes to overcome excessive computer time due to many required external loads. The first is the assumption of a rigid support structure to find blade wind forces, so that these forces are only dependent on wind profiles, which limits different cases in the structural analyses. Since the blade information is often confidential in turbine companies, this two-stage analysis allows the hub force to be the input data for the support structure design. The second is using a few control loads to perform the steel design between the second and the second-last design cycles. The third is using parallel computational procedures, since all loading cases can be independently executed in different CPU cores and computers. The test cases, with 5044 loading cases, indicate that the proposed method is fully parallel and can complete the design procedures using a few personal computers within several days. Test cases include IEC 61400-3, tropical cyclone, and seismic loads; although there are many loads to be considered, steel design is governed by a limited number of load cases, which are discussed in this paper. [ABSTRACT FROM AUTHOR]

The existence of uncertainties in material properties, environmental loads and soil properties as well as the presence of nonlinearities introduced by the control systems have a remarkable influence on the dynamic response of offshore wind turbine (OWT) support structures. The reliability computations of these structures need to consider implicit expensive-to-evaluate limit state functions, implying large computational costs. This paper addresses these limitations by proposing a computationally-efficient reliability framework for OWT support structures, based on the use of a kriging model to approximate the response of the system, capturing both the dynamic behaviour of the structure and inherent uncertainties. The surrogate model is built with sample points from stochastic fully coupled simulations in the time-domain. A thorough sensitivity study is performed on the influence of number of sample points, the seeds used to obtain each point, the range of the variables and the inherent variability in turbulent wind and stochastic waves. The framework is used to evaluate the reliability of the NREL 5 MW turbine model, mounted on a monopile with a flexible foundation for the severest Design Load Cases (DLCs) from the IEC 61400-3. The results agreed with the general literature showing that the structure is far from failure. [ABSTRACT FROM AUTHOR]

The article presents the results of the research conducted within the framework of the project entitled WIND-TU-PLA (ERA-NET, MARTEC II), the general aim of which was to design and analyse supporting structures for wind turbines intended for operation on the South Baltic area. The research part described in the article aimed at developing a preliminary design for a jack-up platform which can operate on water areas with depth of 40 m. The main task was to determine optimal dimensions of platform legs and the radius of their spacing. Two jack-up platform concepts differing by spacing radius and hull dimensions were designed with the intention to be used as a supporting structure for a 6-MW offshore wind turbine. For each concept, the parametric analysis was performed to determine optimal dimensions of platform legs (diameter Dleg and plating thickness tleg). Relevant calculations were performed to assess the movements of the platform with parameters given in Table 1 in conditions simulating the action of the most violent storm in recent 50 years. The obtained results, having the form of amplitudes of selected physical quantities, are shown in comprehensive charts in Fig. 6 and 7. Based on the critical stress values (corresponding to the yield stress), the area was defined in which the impact strength conditions are satisfied (Fig. 14). Then, the fatigue strength analysis was performed for two selected critical leg nodes (Fig. 12). Its results were used for defining the acceptable area with respect to structure's fatigue (Fig. 14). Geometric parameters were determined which meet the adopted criteria, Table 6. The decisive criterion turned out to be the fatigue strength criterion, while the yield point criterion appeared to be an inactive constraint. [ABSTRACT FROM AUTHOR]

This paper investigates the structural monitoring concept and data analysis strategy using an example of an operating offshore wind turbine in the North Sea. As part of the data evaluation strategy; a design verification is shown for the bending moments calculated from monitoring data, operational modal analysis is performed to identify the first three dominant natural frequencies. Then an FE Model is prepared and validated with the identified natural frequencies so that, the model fairly represents the as built healthy physical structure. Finally, variation of the natural frequencies are traced for increasing sea bed scouring (change in boundary condition). This study is done to make up for the absence of such monitored events and as part of the introduced data evaluation strategy to initiate thresholds for damage sensitive global parameters. [ABSTRACT FROM AUTHOR]

By accounting for almost 25% of the capital cost of an OWT (offshore wind turbine), optimisation of support structures provides an efficient way to reduce the currently high cost of offshore wind energy. In this paper, a structural optimisation model for OWT support structures has been developed based on a coupled parametric FEA (Finite Element Analysis) and GA (Genetic Algorithm), minimising the mass of the support structure under multi-criteria constraints. Contrary to existing optimisation models for OWT support structures, the proposed model is an integrated structural optimisation model, which optimises the components of the support structure (i.e. tower, transition piece, grout and monopile) simultaneously. The outer diameters and section thicknesses along the support structure are chosen as design variables. A set of constraints based on multi-criteria design assessment is applied according to standard requirements, which includes vibration, stress, deformation, buckling, fatigue and design variable constraints. The model has been applied to the NREL (National Renewable Energy Laboratory) 5 MW OWT on an OC3 (Offshore Code Comparison Collaboration) monopile. The results of the application of the integrated optimisation methodology show a 19.8% reduction in the global mass of the support structure while satisfying all the design constraints. It is demonstrated that the proposed structural optimisation model is capable of effectively and accurately determining the optimal design of OWT support structures, which significantly improves their design efficiency. [ABSTRACT FROM AUTHOR]

The structure of an offshore wind turbine accumulates damage during its service life, while its surrounding soil is continually scoured. However, the existing damage detection technique based on structural mode ignores scouring, which will cause damage misjudgment in engineering application. Therefore, the effect of scour depth on structural damage detection need to be studied urgently. In this paper, the pile and its surrounding soil are constructed into a vibration system, and the change of soil constraint state caused by scouring is reflected in structural mode. Based on that, a new structural damage detection method considering the pile-soil interaction is developed, which is suitable for engineering reality. Then, from a comparison between the calculated result and preset damage in the numerical simulations and model experiments, the proposed method is validated. The results show that the interference factors caused by scouring increase with the increase of scour depth, which causes the deterioration of damage identification effects gradually, and even masks the preset damage in the end. Finally, an optimized FE model based on the actual scour depth is presented, which reduces the number of interference factors and improves the effect of damage detection significantly. • A structural damage detection method considering pile-soil interaction and seabed scouring is proposed, which breaks through the limitations of the fix-end constraint. • This method can identify the damage of the structure under the constraint state that is closer to the actual scoured seabed, which is more meaningful for engineering. • The interference caused by scour increases with the increase of scour depth, which causes the deterioration of damage identification effects, and eventually masks the preset damage. • An optimized FE model based on the actual scour depth is presented, which improves the effect of damage detection significantly. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

This paper presents a description of an effective method for determining loads due to waves and current acting on the supporting structures of the offshore wind turbines. This method is dedicated to the structures consisting of the cylindrical or conical elements as well as (truncates) pyramids of polygon with a large number of sides (8 or more). The presented computational method is based on the Morison equation, which was originally developed only for cylindrically shaped structures. The new algorithm shown here uses the coefficients of inertia and drag forces that were calculated for non-cylindrical shapes. The analysed structure consists of segments which are truncated pyramids on the basis of a hex decagon. The inertia coefficients, CM, and drag coefficients, CD, were determined using RANSE-CFD calculations. The CFD simulations were performed for a specific range of variation of the period, and for a certain range of amplitudes of the velocity. In addition, the analysis of influence of the surface roughness on the inertia and drag coefficients was performed. In the next step, the computations of sea wave, current and wind load on supporting structure for the fifty-year storm were carried out. The simulations were performed in the time domain and as a result the function of forces distribution along the construction elements was obtained. The most unfavourable distribution of forces will be used, to analyse the strength of the structure, as the design load. [ABSTRACT FROM AUTHOR]

In comparison with monopile support structures commonly used for offshore wind turbines (OWTs) in shallow water, fixed bottom support structures for deeper water such as jackets tend to lack axisymmetry and have different capacities when loaded in different directions. Design of such a structure may therefore benefit from consideration of the directional characteristics of loading. This paper focuses on a rational and efficient approach to assess the structural safety of jacket type OWTs under directionally dependent extreme environmental loads. The Incremental Wind–Wave Analysis (IWWA) framework is reviewed and used to assess capacity of jacket-type OWTs under directional environmental wind–wave conditions. The approach uses static pushover analysis of OWT jackets subject to combined wind and wave load patterns corresponding to increasing mean return periods (MRPs). The wind and wave conditions are calculated independently and assumed to occur simultaneously. The loading direction and jacket orientation are both included in the analysis. To illustrate the approach, metocean conditions at different sites along the U.S. Atlantic coast are obtained from the historical database of the National Oceanic and Atmospheric Administration (NOAA) Data Buoy Center. Two models are introduced for estimating the occurrence probability of wave direction, one of which directly uses the frequency of wave directions and the other uses a Gaussian kernel to represent the range of wave directions represented by given directional spectra. For each combination of wind–wave direction and structural orientation, an IWWA analysis gives the capacity in terms of the MRP conditions leading to formation of a fully developed plastic mechanism in the jacket. Those capacities, convolved with MRP models for loading intensity, yield direction-dependent structural reliabilities, and when those reliabilities are convolved with the probability density function for load direction, a total failure probability is obtained that accounts for wave directionality. Example analyses are conducted for an OWT supported by a four-leg jacket adapted from a design published as part of the European Union UpWind Project. Effects of extreme load directionality, structural orientation, structural geometry and site specification on the ultimate capacity of the jacket are carefully discussed. [ABSTRACT FROM AUTHOR]

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. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Meeting climate and air quality targets, while preserving the focus on the reliability and cost-effectiveness of energy, became a central issue for offshore wind turbine engineers. Floating offshore wind turbines, which allow harnessing the large untapped wind resources in deep waters, are highly complex and coupled systems. Subsystem-level optimisations result in suboptimal designs, implying that an integrated design approach is important. Literature saw a few attempts on multidisciplinary design analysis and optimisation of floating wind turbines, with varying results, proving the need for an efficient, and sufficiently accurate, integrated approach. This paper reviews the state-of-the-art approaches to multidisciplinary design analysis and optimisation of floating support structures. The choice of the optimisation framework architecture, support platform design variables, constraints and objective functions are investigated. The techno-economic analysis models are closely examined, focusing on the approaches to achieving the optimum accuracy–efficiency balance. It is shown that the representation of the fully coupled system within the optimisation framework requires the introduction of a more complex multidisciplinary analysis workflow. Methods to increase the efficiency of such frameworks are indicated. Non-conventional support structure configurations can be conceived through the application of more advanced parametrisation schemes, which is feasible together with design space size reduction techniques. The set of design criteria should be extended by operation and maintenance cost, and power production metrics. The main technical limitations of the frameworks adopted so far include the inability to accurately analyse a diverse range of support structure topologies in multiple design load cases within a common framework. The cost approximation models should be extended by the chosen aspects of pre-operational phases, to better explore the benefits of the floating platforms. • Multidisciplinary Design Optimisation leads to better support structure designs. • Frameworks are limited in the range of support structures and design load cases. • Multi-level and multi-fidelity methods for optimisation of coupled systems are useful. • Along the capital cost, O&M costs should be considered in optimisation studies. [ABSTRACT FROM AUTHOR]

This study shows the first results of a long‐term monitoring campaign on an offshore wind turbine in the Belgian North Sea. It focuses on the continuous monitoring of the resonant frequencies and damping values of the most dominant modes of the support structure. These parameters allow to better understand the dynamics of offshore wind turbines and are crucial in the fatigue assessment during the design phase. They can also help to minimise operation and maintenance (O&M) costs and to assess the lifetime of the offshore wind turbines structures during their operation. To do an accurate continuous monitoring of these parameters, a state‐of‐the‐art operational modal analysis technique has been automated, so that no human‐interaction is required and the system can track small changes in the dynamic behaviour of the offshore wind turbine. The study will analyse the resonant frequencies and damping values of the most dominant modes shapes while the wind turbine is in parked conditions. [ABSTRACT FROM AUTHOR]

It is believed that offshore wind farms may satisfy an increasing portion of the energy demand in the next years. This paper presents a comparative study of the fatigue performances of tripod and jacket steel support structures for offshore wind turbines in waters of intermediate depth (20-50 m). A reference site at a water depth of 45 m in the North Atlantic Ocean is considered. The tripod and jacket support structures are conceived according to typical current design. The fatigue behavior is assessed in the time domain under combined stochastic wind and wave loading and the results are compared in terms of a lifetime damage equivalent load. [ABSTRACT FROM AUTHOR]

The objective of the present study is to provide a review of the structural integrity of fixed support structures for offshore wind turbines, focusing on the physics behind the failure mechanism to assess the structural integrity. It covers the historical background of the fatigue and fracture phenomena, physics-based modelling techniques, methods to estimate the extent of the crack and its criticality, and finally, different models to predict the remaining life of the support structures in-service. The methodologies used for the fatigue design and the damage-tolerant maintenance planning of the offshore wind turbine support structures have benefited immensely from the experience and know-how gained from the onshore wind and offshore oil & gas industries. Thus, the present paper also reviews those studies which have been influential for both structural integrity and structural reliability of offshore wind turbine support structures, which is essential for the remaining life estimation and life-cycle management decisions. Furthermore, a risk-based framework for a structural integrity assessment is discussed considering the uncertainties involved in the numerical and analytical modelling, loads, structural response, and material properties, leading to building quantitative models to predict the impact of different actions on structural conditions and economic liabilities from a strategic perspective. • An extensive review of the structural integrity of offshore wind turbines is provided. • Studies performed to understand the critical failure mechanisms and developed practices to maintain structural safety are reviewed. • Methodologies for the fatigue design and damage-tolerant maintenance planning of the offshore wind structures are analysed. • A risk-based framework for a structural integrity assessment is discussed considering the uncertainties involved. [ABSTRACT FROM AUTHOR]

To obtain reliable estimations of the dynamic responses of high-rising marine structures such as offshore wind turbines with obvious nonzero initial conditions, traditional frequency-domain methods cannot be employed because they provide only steady-state results. A novel frequency-domain transient response estimation method for offshore wind turbines is presented in this paper. This method builds upon a recent, significant theoretical development, which found that expressions of external loads in the frequency domain can be obtained by discretizing their eigenvalues and corresponding complex coefficients rather than directly by discrete Fourier transform (DFT) analysis, which makes it possible to deal with nonzero conditions in the frequency domain. One engineering advantage of this approach is its computational efficiency, as the motion equations of the system can be solved in the frequency domain. In order to demonstrate this approach, a case of a monopile-supported wind turbine with nonzero initial conditions was investigated. The numerical results indicate that the approach matches well with the time-domain method, except for a small, earlier portion of the estimated responses. A second case study of a sophisticated, jacket support wind turbine, involving practical issues such as complex external loads and computation efficiency, is also discussed, and comparisons of the results with the time-domain method and traditional frequency-domain method using the commercial software ANSYS are included here. [ABSTRACT FROM AUTHOR]