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Maintenance costs in offshore wind farms are one of the main reasons why it is still not competitive with its onshore counterpart. To lower the price of energy, the authors want to improve the modeling of offshore wind farms in order to help improving the applied maintenance strategies. In this paper, we investigate the repair of wind turbine components as an alternating renewal process. The time it takes to repair (renew) a component in a turbine, directly influences the downtime of this turbine, or even larger parts of the wind farm. This downtime leads to production losses, which in turn raise the cost of energy. Previous investigations by two of the present authors showed that a variation in repair times has a significant influence of the production losses. In Maintenance costs in offshore wind farms are one of the main reasons why it is still not competitive with its onshore counterpart. To lower the price of energy, the authors want to improve the modeling of offshore wind farms in order to help improving the applied maintenance strategies. In this paper, we investigate the repair of wind turbine components as an alternating renewal process. The time it takes to repair (renew) a component in a turbine, directly influences the downtime of this turbine, or even larger parts of the wind farm. This downtime leads to production losses, which in turn raise the cost of energy. Previous investigations by two of the present authors showed that a variation in repair times has a significant influence of the production losses. In this paper, an alternate renewal process is investigated as tool to calculate the influence of the repair time duration on the production losses. We show that if we assume an exponential distribution of repair times, the distribution of the downtimes can be calculated analytically. The results are demonstrated with a case study, based on available failure rates and repair times. For other distributions, the calculation has to be done numerically, after fitting the parameters of the distribution to the available data. Further work with other distributions and actual repair time distributions is planned. [ABSTRACT FROM AUTHOR]

Key performance indicators (KPI) are tools for measuring the progress of a business towards its goals. Although wind energy is now a mature technology, there is a lack of well-defined best practices to asses the performance of a wind farm (WF) during the operation and maintenance (O&M) phase; processes and tools of asset management, such as KPIs, are not yet well-established. This paper presents a review of the major existing indicators used in the O&M of wind farms (WFs), as such information is not available in the literature so far. The different stakeholders involved in the O&M phase are identified and analysed together with their interests, grouped into five categories. A suggestion is made for the properties that KPIs should exhibit. For each category, major indicators that are currently in use are reviewed, discussed and verified against the properties defined. Finally, we propose a list of suitable KPIs that will allow stakeholders to have a better knowledge of an operating asset and make informed decisions. It is concluded that more detailed studies of specific KPIs and the issues of their implementation are probably needed. [ABSTRACT FROM AUTHOR]

Lifetime extension becomes increasingly crucial for industry, since the first offshore wind farms face the end of their design lifetime. The remaining useful lifetime of offshore foundations is driven by fatigue design and loading; both are affected by uncertainty. This paper presents a conditional probability model to link results from inspections with numerical simulations of fatigue cracks. Crack sizes are simulated with a fracture mechanics model applying Paris’ Law. The probability of detection of an existing crack depends on inspection technique and crack size. Results show that uncertainty about remaining useful lifetime significantly reduces after considering inspection outcomes. This decreases risks for the decision-making on lifetime extension of offshore wind turbines. [ABSTRACT FROM AUTHOR]

Fatigue reassessment within the decision process of lifetime extension might be uneconomical when individually performed for each turbine of a large offshore wind farm. This paper analyses the possibilities to extrapolate results from fatigue reassessment of wind turbines at selected design positions to other wind turbines of the wind farm. Five monopile-based turbines placed in a generic offshore wind farm were assessed with integrated aero-hydro-elastic simulations. A fatigue assessment was performed for each out of the five selected turbines using site specific environmental data commonly available during the design process. The results were compared to a fatigue reassessment where environmental data were modified in order to account for changes in environmental conditions during the service lifetime of the wind turbines. Results indicate that an extrapolation is feasible for selected parameters when changes in environmental conditions are small. This is an important step towards an effective and efficient assessment methodology for lifetime extension of offshore wind turbines. [ABSTRACT FROM AUTHOR]

In harsh sea conditions, it is possible for offshore wind turbines (OWTs) to be exposed to slamming loads due to breaking waves, especially plunging breaking waves. These slamming loads lead to significant structural responses and can affect the ultimate limit state (ULS) design and the fatigue limit state (FLS) design of OWTs. However, detailed consideration of slamming loads is not a common practice in the design of primary structures in offshore wind industry. Studies on integrated dynamic analysis of OWTs with consideration of slamming loads are very limited. When applying slamming loads on OWTs, several aspects should be considered, such as the detection of breaking waves, the calculation of slamming loads, and the approaches to integrate the slamming loads in fully coupled analysis, etc. This paper provides an extensive review of key issues concerning these aspects, which can benefit the application of slamming loads on OWTs. [ABSTRACT FROM AUTHOR]

Wind turbine jacket structures are complex structures, whose joints design is generally driven by fatigue. These joints, along with their complex welds, are of special interest in terms of cost reduction. Therefore, a thorough analysis and understanding of the background behind the assessment proposed in guidelines is motivated. The paper presents a study of the influence of meshing for the assessment of tubular K-joints following the hot-spot approach using numerical methods. The accuracy of the results is discussed for several mesh layouts. The influence of the mesh density, element shape and element type are investigated. Furthermore, a parametric study is performed in order to see the variation of the results for several diameters, thicknesses and brace inclination combinations. The hot-spot method is proved to be robust regarding mesh regularity. However, the efficiency of irregular mesh models is very low and a high number of elements is needed in order to find an asymptotic behaviour that tends to a constant solution for increasing mesh refinement. Conclusions can be drawn for which cases it is worth to invest time in semiautomatic meshing. A discussion is done regarding which element size and type is better regarding accuracy and computational time. [ABSTRACT FROM AUTHOR]

One increasingly popular type of foundations for offshore wind turbines in shallow water are truss structures. These are exposed to wave slamming forces from breaking wave events. These forces depend, among other parameters, on the slamming factor C s that typically takes values between π -2π. So far, several studies about slamming forces have been done for monopile structures, but not much work has been done regarding truss structures such as offshore jackets. This paper is based on the WaveSlam project in which an instrumented multi-membered truss model has been subjected to regular and irregular waves in the large wave flume at FZK Hannover in 2013. The structure was built at scale 1:8 and equipped with force transducers along the bracings and columns that measured the structural response from the breaking waves. The goal of this study was to characterize the breaking wave forces acting on the front bracings of the structure, in order to estimate the respective slamming factors. The tested structure has been modelled and validated with a finite element model in ANSYS and a wave run test is analyzed. The wave loads have been defined as uniform loads with a triangular force time history acting along the bracings. Through a fitting procedure, the initial responses from ANSYS and from the data were matched with a relative error of 3% for one single wave test. From this, a slamming factor of 4.78 is found in the highest part of the front bracings. The characterization of more breaking wave loads is recommended in order to get an estimate of the largest slamming factor, which is relevant for design purposes. [ABSTRACT FROM AUTHOR]

Time domain analyses of Offshore Wind Turbines involve a lot of computational effort. In this paper, we present a method for reducing this effort by using results from analyses with shorter simulation lengths to predict the results of the longer simulations lengths required by the standards. This involves using simple statistical treatment of the first 10 minutes of simulation data, and a subsequent linear regression, to predict the damage equivalent load of the full 60 minutes simulation. With some reservations about the general applicability of the method, the results are promising. Some suggestions for further investigations and developments of the presented methods are discussed. [ABSTRACT FROM AUTHOR]

Considerable variations in environmental site conditions can exist within large offshore wind farms leading to divergent fatigue loads on support structures. An efficient frequency-domain method to calculate wave-induced fatigue loads on offshore wind turbine monopile foundations was developed. A verification study with time-domain simulations for a 4MW offshore wind turbine showed result accuracies of more than 90% for equivalent bending moments at mudline and interface level. This accuracy and computation times in the order of seconds make the frequency-domain method ideal for preliminary design and support structure optimization. The model is applied in sensitivity analysis of fatigue loads using Monte-Carlo simulations. Local and global sensitivity studies and a probabilistic assessment give insight into the importance of site parameters like water depth, soil stiffness, wave height, and wave period on fatigue loads. Results show a significant influence of water depth and wave period. This provides a basis for design optimization, load interpolation and uncertainty analysis in large wind farms. [ABSTRACT FROM AUTHOR]

In this paper it is investigated if, by coupling the catenary mooring systems of a number of turbines, the number of lines and anchors needed and subsequently the costs could be reduced and what kind of dynamic phenomena such a mooring system will exhibit. A row, triangular and rectangular arrangement of floating wind turbines is analyzed in terms of their cost saving potential and their dynamic properties in the frequency domain. The row arrangement is further investigated in the time domain, with a one dimensional simplified model. The most important forces to evaluate the station keeping performance, except current, are applied. These consist of irregular wave forces, employing the Morison equation, estimated wave drift and wind forces. Diffraction effects have been estimated by employing the McCamy-Fuchs theory. Four operational and a 50-year extreme load case have been applied. The results are showing that significant cost reductions of up to 60% in mooring system and 8% in total system costs could be achieved. The dynamic analysis shows the general feasibility of integrated mooring systems. No resonance problems seem to exist. However displacements are increasing with the number of floaters, and cost savings diminish for larger numbers of turbines, as the required diameters, lengths and costs of mooring chains increase. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

One increasingly popular type of foundations for offshore wind turbines in shallow water are truss structures. These are exposed to wave slamming forces from breaking wave events. These forces depend, among other parameters, on the slamming factor C s that typically takes values between π -2π. So far, several studies about slamming forces have been done for monopile structures, but not much work has been done regarding truss structures such as offshore jackets. This paper is based on the WaveSlam project in which an instrumented multi-membered truss model has been subjected to regular and irregular waves in the large wave flume at FZK Hannover in 2013. The structure was built at scale 1:8 and equipped with force transducers along the bracings and columns that measured the structural response from the breaking waves. The goal of this study was to characterize the breaking wave forces acting on the front bracings of the structure, in order to estimate the respective slamming factors. The tested structure has been modelled and validated with a finite element model in ANSYS and a wave run test is analyzed. The wave loads have been defined as uniform loads with a triangular force time history acting along the bracings. Through a fitting procedure, the initial responses from ANSYS and from the data were matched with a relative error of 3% for one single wave test. From this, a slamming factor of 4.78 is found in the highest part of the front bracings. The characterization of more breaking wave loads is recommended in order to get an estimate of the largest slamming factor, which is relevant for design purposes. [ABSTRACT FROM AUTHOR]

Considerable variations in environmental site conditions can exist within large offshore wind farms leading to divergent fatigue loads on support structures. An efficient frequency-domain method to calculate wave-induced fatigue loads on offshore wind turbine monopile foundations was developed. A verification study with time-domain simulations for a 4MW offshore wind turbine showed result accuracies of more than 90% for equivalent bending moments at mudline and interface level. This accuracy and computation times in the order of seconds make the frequency-domain method ideal for preliminary design and support structure optimization. The model is applied in sensitivity analysis of fatigue loads using Monte-Carlo simulations. Local and global sensitivity studies and a probabilistic assessment give insight into the importance of site parameters like water depth, soil stiffness, wave height, and wave period on fatigue loads. Results show a significant influence of water depth and wave period. This provides a basis for design optimization, load interpolation and uncertainty analysis in large wind farms. [ABSTRACT FROM AUTHOR]

Time domain analyses of Offshore Wind Turbines involve a lot of computational effort. In this paper, we present a method for reducing this effort by using results from analyses with shorter simulation lengths to predict the results of the longer simulations lengths required by the standards. This involves using simple statistical treatment of the first 10 minutes of simulation data, and a subsequent linear regression, to predict the damage equivalent load of the full 60 minutes simulation. With some reservations about the general applicability of the method, the results are promising. Some suggestions for further investigations and developments of the presented methods are discussed. [ABSTRACT FROM AUTHOR]

In this paper it is investigated if, by coupling the catenary mooring systems of a number of turbines, the number of lines and anchors needed and subsequently the costs could be reduced and what kind of dynamic phenomena such a mooring system will exhibit. A row, triangular and rectangular arrangement of floating wind turbines is analyzed in terms of their cost saving potential and their dynamic properties in the frequency domain. The row arrangement is further investigated in the time domain, with a one dimensional simplified model. The most important forces to evaluate the station keeping performance, except current, are applied. These consist of irregular wave forces, employing the Morison equation, estimated wave drift and wind forces. Diffraction effects have been estimated by employing the McCamy-Fuchs theory. Four operational and a 50-year extreme load case have been applied. The results are showing that significant cost reductions of up to 60% in mooring system and 8% in total system costs could be achieved. The dynamic analysis shows the general feasibility of integrated mooring systems. No resonance problems seem to exist. However displacements are increasing with the number of floaters, and cost savings diminish for larger numbers of turbines, as the required diameters, lengths and costs of mooring chains increase. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: Jacket substructures for offshore wind turbines show strong potentials in water depths from 25 up to 70m. A review of state-of-practice and enhanced state-of-the-art modeling of offshore wind turbine jackets is conducted regarding detailed joint properties. The state-of-the-art approach takes advantage of an accurate description of the local joint behavior by use of superelements, enabling more accurate load simulations. Studies conducted in the past highlighted both strong benefits as well as shortcomings of this approach, whereas the drawbacks were mainly related to the size of superelements and the application of local wave loading. This work develops a smart sizing for detailed joint models taking into account the loading and location of the jacket joints. A concept of local wave loading is presented as well. Advice on recommendable parameters is given and enables an optimized superelement application for jacket substructures. As an example the potential for fatigue load reduction is shown using the NREL offshore 5-MW baseline wind turbine and the OC4 reference jacket. The predicted fatigue lifetime was increased by about 15%. [Copyright &y& Elsevier]

Abstract: A multivariate Markov chain model is presented for generating sea state time series based on observed time series. The sea state is represented by the wave height, wind speed, wave period, wind direction and wave direction. Two ways of capturing the seasonal variation in the sea state parameters resulted in two distinct models. Their quality was assessed by comparing their statistical properties to what was obtained from observed time series. In one of the models (Model 1) transition probabilities were estimated separately for each month, while in the other (Model 2) a monthly transformation of the data were performed. Two different sea state data sets were considered in the validation, and it was found that both models compared favorably to the empirical data. It was concluded that Model 1 worked best for the longest data set considered, but was challenged by the shorter time series, where Model 2 worked best. Model 2 uses the observed data more efficiently, but relies on stationarity after removing the monthly variability. This seems to be a reasonable approximation for the data considered. The effect of changing the wave height resolution in the modeled time series was also investigated. [Copyright &y& Elsevier]

Abstract: Performing a dynamic analysis of an offshore wind turbine with an aero-elastic solver relies on the inclusion of a model to represent the behavior of the piled foundations. The most commonly used solution is the p-y method, which was developed for offshore oil and gas applications and has been extended to the offshore wind energy industry. There are several shortcomings with this method, however, which can affect the accuracy of wind turbine simulations. These shortcomings are identified and explained in this work. More advanced pile foundations models which account for many nonlinear behaviors ignored by the traditional p-y method have been developed in the past for applications unrelated to offshore wind turbines. These models can nevertheless be applied to offshore wind turbine simulations and are discussed herein. A proposed method for incorporating these so called ‘dynamic p-y’ models into wind turbine simulations is laid out and discussed. [Copyright &y& Elsevier]

Abstract: An alternative support structure for offshore wind turbines is presented that removes the need for an expensive transition piece and allows for significant savings in steel weight. The concept is based on replacing both the traditional wind turbine tower and the usual support structure with a single lattice tower that extends from the sea bottom to the rotornacelle- assembly. An overview is given of studies performed and results obtained for this design so far, of its advantages and disadvantages, and of future plans for improved automatic optimization of the design. Although it is diffcult to show the economic viability of the full-height lattice tower concept, due to the general problem of reliably quantifying fabrication and installation costs, the major reduction in steel weight makes this an attractive concept that should be given further consideration, especially for future offshore projects in deep waters. [Copyright &y& Elsevier]

Abstract: Among several possible support structure types for offshore wind turbines, a full-height lattice tower is one design option. Advantages of this design are the smaller amount of steel used for the structure compared to other concepts, and the possibility to install the whole structure in one operation. Based on the complexity of dynamic loadings on the support structure by wind and wave as well as operational loads, an initial lattice tower design with constant member dimensions over the tower height shows a large optimization potential and can be optimized section by section. This paper presents basic considerations for an iterative optimization approach and identifies sensitivities for the optimization process of a full-height lattice tower. It was found that an analysis with constant member dimensions over the tower height gives an indication about the required dimensions for an optimized design. [Copyright &y& Elsevier]

Abstract: This paper describes a new tool for the optimization of catenary mooring systems for floating wind turbines with a semi-submersible support structure. Each source of environmental loading -wind, current and waves -is spread over a range of compass directions, based upon probability distributions determined by meteorological and oceanographic measurements. Frequency domain analysis is applied to determine the linear response of the platform. Mean forces due to wind and current are determined to find the mean position of the platform and thereby the linearized mooring stiffness matrix. Subsequently, the stochastic loads from wind sea, ocean swell and turbulent wind are applied in the form of spectral loading matrices. The resulting mooring system design is site-specific, accounting for the directionality of environmental loads. A large number of different scenarios can be evaluated quickly and approximately. Hence the tool is useful for preliminary design of floating wind turbines.As an example application, a symmetric semi-submersible design with three columns is analyzed. Results for two sites in the North Sea, with different water depths and distinct directionality of environmental loads, are compared and indicate that spectral wind loads should be considered when designing mooring systems for floating wind turbines. [Copyright &y& Elsevier]

Abstract: Up to one third of the total cost of energy from offshore wind generation is contributed by operation and maintenance (O&M). Compared to its onshore counterpart, this fraction is significantly higher. Costs are not only caused by spare-parts and repair actions, but also by production losses due to downtime. The accessibility of a turbine in case of a failure is one main aspect affecting downtime. Therefore, a tool has been developed and implemented in MATLAB to simulate the operating phase of a wind farm with special emphasis toward the modeling of failures and repair. As an example application, a site at the UK east coast was chosen, and a few distinct scenarios were considered. Results include how sensitive availability changes with respect to changes in maintenance fleet and maintenance scheduling strategy. A quantification of potential cost savings due to an increase in availability is also stated. [Copyright &y& Elsevier]

Abstract: This paper investigates two different approaches for individual pitch control for wind turbines. The first one is a gain scheduled decentralised control design and the second one is a robust H∞ loop shaping control design. Both controllers work well in the region above rated wind speed, exhibiting a response that is mostly independent of wind speed. The investigation is conducted based on the NREL 5MW benchmark wind turbine. Turbine modeling and control is conducted in FAST and Simulink. [Copyright &y& Elsevier]