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This paper presents the fatigue reliability prediction for welded planar tubular joints of offshore fixed jacket support structures. A coupled model is employed to calculate the dynamic response of the jacket support structure under wind and wave loads. Stress concentration factors (SCFs) are evaluated using Efthymiou parametric equations, and the hot-spot stresses at eight locations on the chord and brace of the tubular joints are derived by summing the stress components from axial, in-plane, and out-of-plane actions. The stress distribution is determined using the rainflow counting method, fatigue damage from an S–N curve, and Miner's rule. Uncertainties associated with the fatigue lifetime evaluation procedure are identified, quantified, and modeled. The focus is specifically given to modeling the uncertainty related to the SCFs, and a framework is proposed to evaluate the fatigue reliability of the joint by incorporating individual uncertainties for each degree of freedom of the SCFs (i.e., axial load crown, axial load saddle, in-plane bending moment, and out-of-plane bending moment). The results reveal that assuming the same uncertainty level (Coefficient of Variation of 0.2) as recommended by DNV-RP-C210 for each degree of freedom of the SCFs and including the uncertainty after obtaining the stress distribution from rainflow counting does not consistently yield conservative reliability index values, as it depends on the critical hot spot. It is essential to note that the simplified assumptions derived from the oil and gas sector may not directly translate to the offshore wind industry due to differences in loading conditions. A reduction of up to approximately 24% in the reliability index is observed for one critical hot spot, while an increase of up to 20% was observed for another. Overall, this paper provides valuable insights into the complex interplay of factors affecting the reliability of joints in offshore fixed jacket support structures, shedding light on the need for tailored approaches to address uncertainties in the specific context of the offshore wind industry. • Fatigue reliability of welded tubular joints in offshore wind turbine (OWT) jacket. • The stress distribution is determined using the rainflow counting method. • Fatigue damage is assessed using an S–N curve and Miner's rule. • Incorporates individual uncertainties for each degree of freedom (DOF) of SCFs. • SCFs uncertainty most impacts hot spot reliability index in OWT jacket joints. • The same uncertainty in each DOF of SCFs does not ensure conservative reliability. [ABSTRACT FROM AUTHOR]

A complete fatigue assessment for operational conditions for offshore wind turbines involves simulating thousands of environmental states. For applications such as optimization, where this assessment needs to be repeated many times, that presents a significant computational problem. Here, we propose a novel way of reducing the number of simulated environmental states (load cases) while maintaining an acceptable accuracy. From one full fatigue analysis of a base design, the OC3 monopile (with the NREL 5MW turbine), the distribution of fatigue damage per load case can be used to estimate the lifetime fatigue damage of a range of modified designs. Using importance sampling and a specially adapted two‐stage filtering procedure, we obtain pseudo‐optimal sets of load cases from which the fatigue damage is estimated. This is applied to seven different designs that have been modified to emulate iterations of an optimization loop. For several of these designs, sampling less than 1% of all load cases can give damage estimates with median errors of less than 2%. Even for the most severe cases, using 3% of the environmental states yields a maximum error of 10%. While further refinement is possible, the method is considered viable for applications within design optimization and preliminary design. [ABSTRACT FROM AUTHOR]

During the bidding or very early design phases of jacket structures for offshore wind turbines, there may be very limited information available on meteorological conditions, soil conditions, turbine specifications, etc. However, it is still important to quickly produce near‐optimal designs with production costs similar to that of the final support structure. Numerical optimization methods can be used to this purpose. This paper investigates three gradient‐based optimization methods, where preliminary designs are produced by mass optimization. The mass is reduced by changing tube diameter and thickness of the structural members, and the optimization considers both frequency and fatigue constraints. The three methods are based on (1) damage equivalent loads, (2) quasi‐static analysis, and (3) dynamic analysis. The optimizations are conducted using in‐house software JADOP (jacket design optimization), and the optimized designs are evaluated using state‐of‐the‐art integrated time‐domain simulation software FEDEM Windpower. The findings show that each analysis can be applied with success. However, if excitations of structural frequencies contribute significantly to the overall damage, special care must be taken with quasi‐static and static modeling. It is observed that wave loading does not contribute considerably to the fatigue damage. Additionally, the aerodynamic loading does not change significantly with changes of tube geometry within the optimization ranges. The optimized designs are partly driven by reducing stress concentration factors, which can be achieved by reducing the chord diameter to thickness ratio. Thus, the optimized designs resemble each other to a certain extent. [ABSTRACT FROM AUTHOR]

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]

The 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. [ABSTRACT FROM AUTHOR]

The ultimate capacity of support structures is checked with extreme loads. This is straightforward when the limit state equations depend on a single load component, and it has become common to report maxima for each load component. However, if more than one load component is influential, e.g., both axial force and bending moments, it is not straightforward how to define an extreme load. The combination of univariate maxima can be too conservative, and many different combinations of load components can result in the worst value of the limit state equations. The use of contemporaneous load vectors is typically non-conservative. Therefore, in practice, limit state checks are done for each possible load vector, from each time step of a simulation. This is not feasible when performing reliability assessments and structural optimization, where additional, time-consuming computations are involved for each load vector. We therefore propose to use Pareto-optimal loads, which are a small set of loads that together represent all possible worst case scenarios. Simulations with two reference wind turbines show that this approach can be very useful for jacket structures, whereas the design of monopiles is often governed by the bending moment only. Even in this case, the approach might be useful when approaching the structural limits during optimization. [ABSTRACT FROM AUTHOR]

Stochastic representations of turbulent wind and irregular waves are used in time domain simulations of offshore wind turbines. The variability due to finite sampling of this input loading is an important source of simulation error. For the OC4 reference jacket structure with a 5 MW wind turbine, an error of 12-34% for ultimate loads and 6-12% for fatigue loads can occur with a probability of 1%, for simulations with a total simulation length of 60 min and various load cases. In terms of fatigue life, in the worst case, the lifetime of a joint was thereby overestimated by 29%. The size of this error can be critical, i.e., ultimate or fatigue limits can be exceeded, with probability depending on the choice of number of random seeds and simulation length. The analysis is based on a large simulation study with about 30,000 time domain simulations. Probability density functions of response variables are estimated and analyzed in terms of confidence intervals; i.e., how probable it is to obtain results significantly different from the expected value when using a finite number of simulations. This simulation error can be reduced to the same extent, either using several short simulations with different stochastic representations of the wind field or one long simulation with corresponding total length of the wind field. When using several short-term simulations, it is important that ultimate and fatigue loads are calculated based on the complete, properly combined set of results, in order to prevent a systematic bias in the estimated loads. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

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: 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]

The cover image, by Jacob Oest et al., is based on the Research Article On gradient‐based optimization of jacket structures for offshore wind turbines, https://doi.org/10.1002/we.2206. [ABSTRACT FROM AUTHOR]

Abstract: Traditionally wind turbines are built in the upwind configuration, but the alternative of a downwind rotor has distinct advantages. A main issue with such a configuration is the tower shadow effect. The presence of the tower generates a complex wind field, consisting of an averaged velocity deficit, unsteady fluctuations from vortex shedding processes and turbulence. Since this tower shadow is commonly implemented using parametric steady wake models, the dynamic behaviour of the wake is not directly accounted for. The present paper introduces a general method for calibrating the parametric steady wake models and an effective turbulence intensity (accounting both for the velocity profile and the unsteady effects) from computational fluid dynamic simulations of the unsteady structural shadow. To demonstrate its potential the method is used in a blade fatigue comparison study. A 15 percent more flexible and lighter blade for the downwind mounted rotors showed a decrease in blade fatigue loads of three, four, and two percent compared to the conventional upwind mounted rotor on a monopile tower for the monopile tower and truss towers at 0 and 22.5 degrees, respectively. This example shows that the truss tower at 0 degree angle seems to result in the lowest blade fatigue loads. The proposed calibration method is completely general and can be used to obtain efficient, effective engineering models for the aerodynamic flow behind complex, multi-membered structures. [Copyright &y& Elsevier]

Lifetime extension needs low-cost assessments that can identify the remaining useful life of offshore wind monopiles. A novel concept for load monitoring was developed that only needs strain gauges installed at one level of the support structure. Damage equivalent loads were calculated from strain measurements and extrapolated along a monopile using a regression algorithm. In this paper, the assumptions behind the load extrapolation algorithm were verified with two consecutive months of measurement data from an offshore wind park. The verification was performed separately for two offshore wind turbines. Both turbines had strain gauges installed at a distance of approximately 15 m and 25 m. Results show that monthly damage equivalent loads can be predicted with errors smaller than 4% based on measurement data only. Prediction using linear regression resulted in similar results for the total fatigue damage as a nonlinear k-nearest neighbor approach, but individual 10-min damage equivalent loads showed larger differences than for the more robust k-nearest neighbor algorithm, especially for small loads. These results are very promising and should motivate further research. • Strain gauges installed at one level of the transition piece can be used to monitor loads of the monopile also. • Measured loads can be extrapolated along a monopiles using aero-hydro-elastic simulations and a regression algorithm. • Verification with data from two offshore wind turbines confirms good performance of the load extrapolation algorithm. • Results are similar for two offshore wind turbines located in the same wind farm. • Results are insensitive to the distance between measurement and extrapolation location. [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. 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. [ABSTRACT FROM AUTHOR]