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The secure operation of offshore wind turbines is at risk due to local scour. To mitigate this risk, effective countermeasures can be implemented to either halt or slow the progression of scour. In this paper, a thorough review of over one hundred and fifty references has been conducted to summarize the latest advancements in preventing scouring for offshore wind turbine monopile foundations. The scour countermeasures have been categorized into three groups based on the mechanisms that influence scouring: armouring protection, flow-altering protection, and comprehensive protection. Additionally, a comprehensive overview of more than twenty scour countermeasures has been provided, including commonly used measures such as riprap, experimental measures like collars that have demonstrated effectiveness, and measures with potential for future development such as biological protection. Furthermore, a comprehensive comparison and evaluation of these scour countermeasures has been conducted, taking into account factors such as effectiveness of protection, adaptability, cost, environmental impact, and additional effects. The strengths and weaknesses of these scour countermeasures have been thoroughly analyzed. The research presented in this paper serves as a valuable reference for selecting appropriate scour countermeasures in practical engineering applications. [ABSTRACT FROM AUTHOR]

Ko, D.-H. and Lee, U.-J., 2023. Reliability analysis of offshore wind structures considering the Korean met-ocean conditions. In: Lee, J.L.; Lee, H.; Min, B.I.; Chang, J.-I.; Cho, G.T.; Yoon, J.-S., and Lee, J. (eds.), Multidisciplinary Approaches to Coastal and Marine Management. Journal of Coastal Research, Special Issue No. 116, pp. 603-607. Charlotte (North Carolina), ISSN 0749-0208. IEC and DNV documents, which are developed for the design of offshore wind turbines, have been developed based on the experiences of the offshore wind industry. Moreover, IEC and DNV documents adopt the reliability design method, a partial safety factor based on the limit state design. However, since these documents were developed under European met-ocean conditions, it is necessary to develop a reliability design method suitable for the Korean met-ocean conditions to satisfy the safety level required by overseas documents. Thus, this paper conducted the reliability analysis of offshore wind structures considering the Korean met-ocean conditions. The Monte Carlo Simulation method was used as a reliability analysis technique. Moreover, design variables such as waves and winds of the Korean west sea were calculated by numerical simulation, and design parameters were estimated using extreme analysis. In addition, partial safety factors were suggested to satisfy the target reliability required by IEC. As a result, in the case of the Korean west sea, where typhoons are dominant, the reliability index was calculated lower than in Europe due to the higher variation of design variables such as wind. These results showed that a more careful approach should be taken to installing offshore wind turbines in areas affected by typhoons. Furthermore, it is judged that it is necessary to calibrate the partial safety factors considering the met-ocean conditions of the specific sites. [ABSTRACT FROM AUTHOR]

In this article, the seismic response of four different Offshore Wind Turbines (OWTs) from 5 to 15 MW, founded on monopiles embedded in homogeneous and non-homogeneous soil profiles with equivalent shear-wave velocities from 100 to 300 m/s is analysed. Two types of variable soil profiles with a semi-parabolic variation of the shear-wave velocity as depth increases are considered. The system seismic response under ten different accelerograms is computed through a finite element substructuring model in frequency domain. The foundation behaviour is obtained by a continuum model including kinematic and inertial interaction. Several models, in addition to that of rigid base condition, are considered to determine the influence of soil-structure interaction (SSI), as well as the contribution of each kinematic interaction (KI) factor. The seismic response of the OWTs is obtained in terms of maximum shear forces and bending moments at mudline level, as well as the acceleration amplification factor at hub level. It is found that the maximum response is produced when SSI is considered, including both the inertial and kinematic interaction. The differences arising due to the soil profile definition are shown to be mainly related to the rotational KI factor, which significantly affects the second vibration mode of the system. The largest responses are obtained for the homogeneous equivalent profile. • The seismic response of monopiled OWTs on different seabed profiles is studied. • The effects of SSI and of each KI factor on the seismic response are analysed. • Higher responses are obtained when a homogeneous profile and SSI are considered. • KI only has a significant effect if the rotational KI factor is taken into account. • The influence of the soil profile arises when the rotational KI factor is included. [ABSTRACT FROM AUTHOR]

For offshore wind development in Southern China, typhoon represents the extreme environmental loading condition to be considered in the design of the turbine structures. For the geotechnical design of the foundations, it is specified by the design code that the effect of cyclic loading on the soil strength and stiffness must be considered. This necessitates the complete time history of loads on the foundation experienced during a design typhoon. Idealised storm profiles with staged mean wind speeds and significant wave heights are commonly used as input to time-domain simulations to generate the foundation load history. However, no such design typhoon profile is provided in the Chinese design standard which creates considerable difficulty and uncertainty in the design process. In this paper, the methodology and the rationale that were applied to derive the design typhoon profiles for an offshore wind project in Guangdong province are presented. The wind and wave characteristics generated by typhoon are described in detail and the implications for the foundation design are discussed. To demonstrate application of the proposed typhoon profile, a case study for the ULS design of a suction bucket jacket is presented. • Design typhoon profiles are developed for an offshore wind farm in Southern China. • The typhoon wind and wave characteristics are described and implications for geotechnical foundation design are discussed. • The methodology and the rationale for developing the design typhoon profiles are presented. [ABSTRACT FROM AUTHOR]

The structural integrity of marine structures is significantly influenced in many practical applications by their fatigue behavior, especially when dealing with dynamically sensitive structural systems, e.g., offshore wind substructures. The propagation of cracks found in structural components can be estimated through fracture mechanics-based methods, in which the crack growth under cyclic loading is mainly driven by the stress intensity factor (SIF). To simplify the computation of the SIF, closed-form solutions are often suggested in industrial standards, yet their applicability is limited to specific geometries and loading conditions. Overcoming the aforementioned constraints, we propose here a general methodology for numerically simulating the SIF and growth of multiple cracks by computing the SIF of adjacent cracks in an integrated finite element analysis, in which interaction effects are implicitly considered. In our proposed method, the coalescence of bordering cracks is also adequately modeled and fatigue failure is defined based on fracture mechanics principles. With the objective of enabling an efficient SIF computation, we also provide the necessary modeling details for the appropriate implementation of the corresponding finite element analysis. The proposed methodology is then tested and validated in a finite thickness plate under cyclic loading setting, whereas in a more practical case study, we investigate the fatigue analysis of interacting surface cracks in an offshore wind structural connection. The results emphasize the importance of considering interaction effects and crack coalescence when simulating the fatigue evolution of structural details in practical applications, and within the study, specific insights are additionally provided for the fatigue analysis of offshore wind structural components. With respect to the investigated fatigue failure criteria, the results show that a through-thickness limit state might yield slightly conservative fatigue life estimates compared to the fatigue limit that results when failure is defined based on the material fracture toughness. • Fatigue crack growth and coalescence of interacting surface cracks. • Computation of stress intensity factors in an integrated finite element analysis. • Definition of fatigue failure criteria based on fracture mechanics principles. • Systematic investigation of surface crack interaction effects. • Fatigue analysis of interacting cracks in offshore wind structural connections. [ABSTRACT FROM AUTHOR]

With the continuously increasing installed capacity of offshore wind turbines, highly adaptable vertical axis wind turbines (VAWTs) are facing new opportunities. Large-scale offshore platforms need a megawatt installed capacity, at least 1 ∼ 2 MW, which requires the feasibility analysis of aerodynamic characteristics for large-scale VAWTs. Due to the unsteady aerodynamic phenomenon, i.e. the dynamic stall phenomenon, the power output of VAWTs is very sensitive to the variations of Reynolds number and reduced frequency, which are closely related to the scale of wind turbines. In order to explore the large-scale VAWTs for offshore platforms, three design methods are proposed: Increasing the Reynolds number; Decreasing reduced frequency; Forming an array. Their feasibility and economies are verified by a high-resolution numerical method, and the results show that increasing the Reynolds number could improve their power coefficients to 0.259 for a one-blade VAWT, while an excessive decrease of reduced frequency would lead to power losses. The form of installing one or more levels of VAWT arrays on a single platform is a better design scheme with averaged power coefficient of 0.326 and a blade weighing 2.7 tons per 10 MW, and for large offshore platforms, it could reach the lowest levelized cost of energy. • Different design schemes of offshore VAWTs are analyzed. • Turbine performance is investigated by a high-resolution numerical method. • The most suitable design scheme for large-scale VAWTs is proposed. • Feasibility of various scales and designs are studied based on cost analysis. [ABSTRACT FROM AUTHOR]

Pitch bearings are key components for the pitch system of wind turbines, and their reliability is closely associated with normal operation of wind turbines. In this paper, to solve the low efficiency problem of the GH Bladed and MATLAB software in design and analysis due to its operation in a single-user, stand-alone and single-key mode, GH bladed and MATLAB were combined based on Python for the integrated load spectrum calculation of pitch bearings through the full life cycle. Based on theories of contact strength analysis and fatigue life, finite element analysis and sub-modelling refinement techniques are established to investigate the relation between structural parameters and raceway stress of the bearings. Based on the Artificial Fish Swarm Algorithm (AFSA), the structural parameters are optimized to minimize the maximum contact stress on the bearing raceways and maximize the fatigue life of the bearings. The results show that the maximum contact load and the maximum contact stress on the pitch bearing raceways are significantly reduced and the bearing fatigue life is notably increased by 8.7%. The method proposed in the present study applies not only to pitch bearings, but also to other large four-point contact ball bearings. • B/S architecture and web service framework can help to improve the efficiency of calculation. • Refinement of sub-modelling is established to analysis the contact characteristics. • Methodology for structural optimization of pitch bearings are proposed based on AFSA. [ABSTRACT FROM AUTHOR]

• A procedure to establish fragility and vulnerability models of OWTs is developed. • Nonlinear behaviors induced by extreme loadings and servo dynamics are considered. • Influences of different operational conditions on fragility and vulnerability are studied. • Fragility and vulnerability models of benchmark OWTs with various substructures are developed. Wind technology is an effective solution for the carbon neutral over the world. Offshore wind turbines (OWTs) have therefore seen unprecedented development over the decades. Fragility and vulnerability of OWTs are vital in structural performance evaluation and risk assessment of infrastructural systems under natural hazards. Previous research mainly focused on the fragility of OWTs with monopile substructure in the normal operating or parked conditions. In the present study, a realistic situation with respect to the emergency shutdown mode is considered for the OWTs with monopile, tripod, and jacket substructures. A procedure to establish fragility and vulnerability models is developed. Numerical models using a combination of FE software are implemented and validated for dynamic analyses and pushover analyses. Then, the fragility and vulnerability models of the main structural subsystems including rotor, nacelle, tower, and substructure are separately established according to the developed procedure, so as to the fragility and vulnerability models of the whole wind turbine. Besides, impact of operational condition and characteristics of substructure on fragility and vulnerability is comprehensively compared and discussed. [ABSTRACT FROM AUTHOR]

• Introducing the extended transient current region by adopting short-time overcurrent capacity of grid-side converter. • Current relaxation region (CRR) is proposed for description of transient current boundary and look-up table design. • A novel CRR-based LVRT strategy is proposed to alleviate active current reduction while maintaining the reactive current ability. This paper proposes a current relation region (CRR)-based low-voltage ride-through (LVRT) strategy for offshore wind turbines (OWTs). The proposed strategy seeks to minimize the detrimental shortfall of OWT's active current on the premise of maintaining the reactive support ability by considering short-time overcurrent capacity of grid-side converter (GSC) during low-voltage fault. Firstly, the extended transient current region (ETCR) is introduced to improve the transient current ability of OWTs with comparison to conventional transient current region (CTCR) based on rated current capacity of GSC. Combined with the introduced ETCR, grid code constraint and wind speed constraint are investigated for detailed description of OWT's transient current boundary of CRR. Furthermore, we design the CRR-based LVRT strategy with look-up tables to facilitate dispatching the transient current instructions to OWTs' controllers. Finally, extensive case studies based on PSCAD are performed and the results show that the proposed strategy can improve active current and reactive current simultaneously over the conventional strategy. In the scenario of the rated wind speed (11.4 m/s) and critical voltage (0.2 p.u.), this achievement can be clearly seen by the value improvement in the active current and reactive current by 0.328 p.u. and 0.05 p.u., respectively. [ABSTRACT FROM AUTHOR]

A fully coupled hydro-servo-aero-elastic simulator for the analysis of floating offshore wind turbines (FOWTs) is presented. All physical aspects are addressed, and the corresponding equations are concurrently solved within the same computational framework, taking into account the wind and wave excitations, the aerodynamic response of the rotor, the hydrodynamic response of the floater, the structural dynamics of the turbine-floater-mooring lines assembly and finally the control system of the wind turbine. The components of the complex multi-physics system of a FOWT interact with each other in an implicitly coupled manner leading to a holistic type of modeling. Different modeling options, of varying fidelity and computational cost, are made available with respect to rotor aerodynamics, hydrodynamic loading of the floater and mooring system dynamics that allow for timely routine certification simulations, but also for computationally intense simulations of less conventional operating states. Structural dynamics is based on nonlinear multibody analysis that allows reproducing the large rigid body motions undergone by the FOWT, as well as large deflections and rotations of the highly flexible blades. The paper includes the description of the main physical models, of the interaction and solution strategy and representative results. Verification is carried out by comparing with other state-of-art tools that participated in the Offshore Code Comparison Collaboration Continuation (OC4) IEA Annex, while the advanced simulation capabilities are demonstrated in the case of half-wake interaction of floating wind turbines by employing the free-wake aerodynamic method.

The displacement of monopile supporting offshore wind turbines needs to be strictly controlled, and the influence of local scour can not be ignored. Using p–y curves to simulate the pile–soil interaction and the finite difference method to calculate iteratively, a numerical frame for analysis of lateral loaded pile was discussed and then verified. On the basis of the field data from Dafeng Offshore Wind Farm in Jiangsu Province, the local scour characteristics of large diameter monopile were concluded, and a new method of considering scour effect applicable to large diameter monopile was put forward. The results show that, for scour of large diameter monopiles, there was no obvious scour pit, but local erosion and deposition. Under the test conditions, the displacement errors between the proposed and traditional method were 46.4%. By the proposed method, the p–y curves of monopile considering the scour effect were obtained through ABAQUS, and the deformation of large diameter monopile under scour was analyzed by the proposed frame. The results show that, with the increase of scour depth, the horizontal displacement of the pile head increases nonlinearly, the depth of rotation point moves downward, and both of the changes are related to the load level. Under the test conditions, the horizontal displacement of the pile head after scour could reach 1.4~3.6 times of that before scour. Finally, for different pile parameters, the pile head displacement was compared, and further, the susceptibility to scour was quantified by a proposed concept of scour sensitivity. The analysis indicates that increasing pile length is a more reasonable way than pile diameter and wall thickness to limit the scour effect on the displacement of large diameter pile.

The use of bolted flange connections in the offshore wind industry has steeply risen in the last few years. This trend is because of failings observed in other modes of joints such as grouted joints, coupled with enormous economic losses associated with such failures. As many aspects of bolted flange connections for the offshore wind industry are yet to be understood in full, the current study undertakes a comprehensive review of the lessons learned about bolted connections from a range of industries such as nuclear, aerospace, and onshore wind for application in offshore wind industry. Subsequently, the collected information could be used to effectively address and investigate ways to improve bolted flange connections in the offshore wind industry. As monopiles constitute an overwhelming majority of foundation types used in the current offshore wind market, this work focusses on large diameter flanges in the primary load path of a wind turbine foundation, such as those typically found at the base of turbine towers, or at monopile to transition piece connections. Finally, a summary of issues associated with flanges as well as bolted connections is provided, and insights are recommended on the direction to be followed to address these concerns.

Due to the high cost of failures of wind turbines, redundancy designs are commonly applied in wind turbines for improving the reliability and availability of systems. For this reason, replacing failed components with other working components of the same type in redundant systems is becoming an attractive option of maintenance strategies towards more resilient systems. To quantitatively evaluate system’s reliability, this paper focuses on the reliability analysis of redundant systems of offshore wind turbines based on swapping existing components. The survival signature-based component swapping method is introduced to describe the new structure-function of the system upon swapping. Furthermore, the reliability model of redundant systems is established using the fault tree and survival signature. Following this, the influences of component swapping on component reliability importance measure (marginal reliability importance and joint reliability importance) without and with considerations of the imprecision of failure rates are explored. Finally, a 5MW offshore wind turbine is presented to show the applicability of the proposed approach for redundant systems, and the results show that the proposed approach can obtain realistic reliability assessment of redundant systems and considering component swapping can significantly improve system reliability.

In 2007, the EU established challenging goals for all Member States with the aim of obtaining 20% of their energy consumption from renewables, and offshore wind is expected to be among the renewable energy sources contributing highly towards achieving this target. Currently wind turbines are designed for a 25-year service life with the possibility of operational extension. Extending their efficient operation and increasing the overall electricity production will significantly increase the return on investment (ROI) and decrease the levelized cost of electricity (LCOE), considering that Capital Expenditure (CAPEX) will be distributed over a larger production output. The aim of this paper is to perform a detailed failure mode identification throughout the service life of offshore wind turbines and review the three most relevant end of life (EOL) scenarios: life extension, repowering and decommissioning. Life extension is considered the most desirable EOL scenario due to its profitability. It is believed that combining good inspection, operations and maintenance (O&M) strategies with the most up to date structural health monitoring and condition monitoring systems for detecting previously identified failure modes, will make life extension feasible. Nevertheless, for the cases where it is not feasible, other options such as repowering or decommissioning must be explored.

Different from the fixed-based wind turbines, the floating type wind turbines are regarded as under a free-free end operating condition. The tower structure of a floating offshore wind turbine is an integrated part connecting the nacelle and support platform. An analytic solution is presented in this technical brief for the free-vibration of the tower structure of a spar-type offshore wind turbine. The tower structure is modeled as a free-free beam based on Euler-Bernoulli beam-column theory. The platform and the nacelle are considered as two large mass components connected by torsion springs at two tower ends with different stiffness. The effects of system parameters on the natural frequencies are investigated under a range of variables, including the tower structure parameters, platform and nacelle parameters, and the connection types. Nonlinear relationships between those variables and the natural frequency of the tower structure are numerically found and some design issues are discussed for the spar-type floating wind turbines. [ABSTRACT FROM AUTHOR]

This paper focuses on load mitigation by implementing controllable trailing-edge slotted flaps on the blades of an offshore wind turbine (OWT). The benchmark NREL 5 MW horizontal axis OWT is subjected to coupled stochastic aerodynamic-hydrodynamic analysis for obtaining the responses. The OWT is supported on three different fixed-bottom structures situated in various water depths. Blade element momentum (BEM) theory and Morison's equation are used to compute the aerodynamic and hydrodynamic loads, respectively. Presently, the load reduction obtained by means of the slotted flaps is regulated using an external dynamic link library considering the proportional-integral-derivative (PID) controller. BEM theory is presently modified to account for unsteady effects of flaps along the blade span. The present analysis results show reduction up to 20% in blade and tower loads for the turbine with different support structures on implementing controllable trailing edge flaps (TEFs). This study can form the basis for evaluating the performance of large-scale fixed OWT rotors. [ABSTRACT FROM AUTHOR]

We present an approach for sizing optimization of jacket structures and apply it to investigate the conceptual design of jackets for offshore wind turbines. Conceptual design is an input to early structural and financial models, and we assume simplified analysis and load models. A four‐legged jacket for the DTU 10‐MW wind turbine in 50‐m water depth is modelled by Timoshenko beam finite elements, and the structural dimensions of the beam cross sections are considered as continuous design variables. A structural optimization problem is formulated to minimize the jacket mass, with constraints on fatigue and ultimate limit states. The optimal design problem is then used to investigate how the optimized mass depends on the number of bays and the jacket leg distance. The conceptual design investigation led to a new conceptual design with 14% lower mass compared with the original conceptual design. We conclude that structural optimization can provide useful insights in the conceptual design phase and lead to a better starting point for the further design and planning processes. [ABSTRACT FROM AUTHOR]

The local scour around a new pile-group foundation of offshore wind turbine subjected to a bi-directional current was physically modeled with a bi-directional flow flume. In a series of experiments, the flow velocity and topography of the seabed were measured based on a system composed of plane positioning equipment and an ADV. Experimental results indicate that the development of the scour hole was fast at the beginning, but then the scour rate decreased until reaching equilibrium. Erosion would occur around each pile of the foundation. In most cases, the scour pits were connected in pairs and the outside widths of the scour holes were larger than the inner widths. The maximum scour depth occurred at the side pile of the foundation for each test. In addition, a preliminary investigation shows that the larger the flow velocity, the larger the scour hole dimensions but the shorter equilibrium time. The field maximum scour depth around the foundation was obtained based on the physical experiments with the geometric length scales of 1:27.0, 1:42.5 and 1:68.0, and it agrees with the scour depth estimated by the HEC-18 equation. [ABSTRACT FROM AUTHOR]

Pile foundation design is conventionally conducted using a process of trial and error, where the dimensions of a pile are estimated and the performance is computed and compared with design criteria. The dimensions are varied and the process is repeated in order to converge to a safe and economical design. In this paper, this time-consuming and labor intensive process is replaced with an automated approach using the example case of an offshore monopile supporting a wind turbine. The optimum length and diameter of the monopile are determined with the aim of minimizing the pile weight while satisfying both serviceability and ultimate limit state criteria. The approach handles general soil and loading conditions and includes an ability to incorporate cyclic loading. [ABSTRACT FROM AUTHOR]

As offshore wind turbines supported on floating platforms extend to deep waters, the various effects involved in the dynamics, especially those resulting from the influence of moorings, become significant when predicting the overall integrated system response. The combined influence of waves and wind affect motions of the structure and induce tensile forces in mooring lines. The investigation of the system response under misaligned wind-wave conditions and the selection of appropriate mooring systems to minimize the turbine, tower, and mooring system loads is the subject of this study. We estimate the 50-year return response of a semisubmersible platform supporting a 13.2 MW wind turbine as well as mooring line forces when the system is exposed to four different wave headings with various environmental conditions (wind speeds and wave heights). Three different mooring system patterns are presented that include 3 or 6 mooring lines with different interline angles. Performance comparisons of the integrated systems may be used to define an optimal system for the selected large wind turbine. [ABSTRACT FROM AUTHOR]

Scour protection is an important component of fixed bottom foundations for offshore wind turbines. Depending on the hydrodynamic conditions, they might be indispensable to avoid the structural collapse of the foundation due to scour phenomena. The design of scour protections is typically deterministic, which often results in overestimated mean diameters of the armour layer. Moreover, the design methodologies currently applied do not provide a measure of safety associated with the proposed design. The present research proposes a novel methodology to assess the safety of the protection and to perform the probabilistic design of static and dynamic scour protections. A case study based on Horns Rev. 3 offshore wind farm is used to show how to select the mean stone diameter according to a pre-defined probability of failure of the protection. The results show that a dynamic scour protection could be safely designed with a reduction of the mean stone diameter up to 15 cm, when compared with the statically stable protection. [ABSTRACT FROM AUTHOR]

Summary: Considering the size and dimension of offshore wind turbine structures, structural optimization of such structures, notwithstanding being outstandingly fruitful, is a tedious task. Nonetheless, in this paper, a metaheuristic algorithm named as colliding bodies optimization is employed when investigating the optimal design of jacket supporting structures for offshore wind turbines. The OC4 reference jacket is considered as the case study, validating the outcomes of this research. To do so, MATLAB is utilized in modeling the structure. The structural optimization is then performed when both ultimate limit state and frequency constraints are being considered. During the optimization process, the weight of the structure is approximately halved, and its first and second frequencies are kept within the considered soft–stiff range (0.21–0.32 Hz). [ABSTRACT FROM AUTHOR]

Wind and wave loadings induce stochastic bending moments in grouted connections for monopile offshore wind turbines; thus, the fatigue behavior under bending moment is extremely important. This paper presents test results on five cylindrical monopile grouted connection specimens with shear keys, which have an outer diameter of 612 mm, and different lengths of 4365 mm, 4475 mm, and 4585 mm. They were designed and tested under constant range of fatigue cyclic bending moments. The load ranges were established based on the ultimate limit state bending capacity of each specimen. It was found out that, although some local degradation exists, the whole grouted connection in the first four specimen can still survive two million load cycles with no obvious degeneracy in bending capacity. Furthermore, no obvious cracks were found at the end section of the grouted connections. Meanwhile, in the longest specimen, due to its maximum load range, the welded structures cracked after about 1.28 million load cycles indicating the end of testing. In addition, no destruction was found in the grouted connection itself. Moreover, after comparing the specimens with different lengths, the authors determined that the recommended minimum design length for grouted connections with shear keys was conservative in the commonly used design code. [ABSTRACT FROM AUTHOR]

Structural optimization of offshore wind turbines is a tedious task due to the complexity of the problem. However, in this article, this problem is tackled using two meta-heuristic algorithms - Colliding Bodies Optimization (CBO) and its enhanced version (ECBO) - for a jacket supporting structure. The OC4 reference jacket is chosen as a case study to validate the methods utilized in this research. The jacket supporting structure is modeled in MATLAB and its optimal design is performed while both Ultimate Limit State (ULS) and frequency constraints are considered. In the present study, it is presumed that both wind and wave phenomena act in the same horizontal direction. As a result, all resultant forces and moments will act in-plane and the substructure can therefore be modeled in 2D space. Considerable weight reduction is obtained during the optimization process while fulfilling all constraints. [ABSTRACT FROM AUTHOR]

This paper reviews foundations for offshore wind energy convertors considering the significant growth of offshore wind energy since the early 2000s. The characteristics of various foundation types (i.e., gravity, pile, suction caisson, and float type) and the current status of field application are discussed. Moreover, the mechanical characteristics of soil are described in the sense that these characteristics including modulus, strength, damping, and modulus degradation of soil play critical roles for the design of offshore foundations. By using these mechanical properties of soil, theoretical studies to consider structure-soil interaction are classified (into equivalent spring models, distributed spring models, and continuous element models) and explained. Field and laboratory experiments on the response of structure embedded in soil to static and dynamic loads are discussed. Based on the review of previous studies, directions for future research and study on offshore wind turbine are suggested. [ABSTRACT FROM AUTHOR]

Offshore wind turbines suffer from excessive bi-directional vibrations due to wind-wave misalignment and vortex induced vibrations. However, most of existing research focus on unidirectional vibration attenuation which is inadequate for real applications. The present paper proposes a three dimensional pendulum tuned mass damper (3d-PTMD) to mitigate the tower and nacelle dynamic response in the fore-aft and side-side directions. An analytical model of the wind turbine coupled with the 3d-PTMD is established wherein the interaction between the blades, the tower and the 3d-PTMD is modeled. Aerodynamic loading is computed using the Blade Element Momentum method where the Prandtls tip loss factor and the Glauert correction are considered. JONSWAP spectrum is adopted to generate wave data. Wave loading is computed using Morisons equation in collaboration with the strip theory. Via a numerical search approach, the design formula of the 3d-PTMD is obtained and examined on a National Renewable Energy Lab (NREL) monopile 5 MW baseline wind turbine model under misaligned wind, wave and seismic loading. Dual linear tuned mass dampers (TMDs) deployed in the fore-aft and side-side directions are utilized for comparison. It is found that the 3d-PTMD with a mass ratio of 2 % can improve the mitigation of the root mean square and peak response by around 10 % when compared with the dual linear TMDs in controlling the bi-directional vibration of the offshore wind turbines under misaligned wind, wave and seismic loading. [ABSTRACT FROM AUTHOR]

An efficient technique called NVSM (Nonlinear Vertical Slices Model) for analyzing soil/structure interaction (SSI) problems is presented. This procedure approximates a full 3D problem into a series of 2D vertical interacting panels which can be solved using a standard 2D FE discretization, while allowing the slices interaction to be accounted for using the finite difference method. Duncan-Chang basic soil model is described first since it is simple to implement in a computer code, then, the extension of the VS Model to deal with nonlinear problems is detailed and supported by a flowchart giving the necessary steps of stress computations. The written Fortran computer code for predicting the response of large diameter monopiles supporting OWTs under combined loading is assessed against three case studies in which the authors used independently powerful commercial packages to study this kind of foundations embedded in offshore subsoils formed mostly of dense sand. An excellent agreement between the present predictions and those of the other numerical codes has been obtained. [ABSTRACT FROM AUTHOR]

The article proposes an optimal design approach to minimize the mass of load carrying structures with discrete design variables. The design variables are chosen from catalogues, and several variables are assigned to each part of the structure. This allows for more design freedom than only choosing parts from a catalogue. The problems are modelled as mixed 0-1 nonlinear problems with nonconvex continuous relaxations. An algorithm based on outer approximation is proposed to find optimized designs. The capabilities of the approach are demonstrated by optimal design of a space frame (jacket) structure for offshore wind turbines, with requirements on natural frequencies, strength, and fatigue lifetime. [ABSTRACT FROM AUTHOR]

Monopiles are used as foundations for offshore wind energy towers. These large diameter monopiles (4-6 m) which result in extremely stiff short monopiles do not follow the same elastic deformation patterns as those exhibited by small diameter (0.5-1.5 m) monopiles usually used for supporting structures in offshore oil and gas industry. Design recommendations (API and DNV among others) which have been developed on the basis of full-scale load tests on long, slender and flexible piles are not suitable for designing foundations for offshore wind turbine structures. Furthermore, as these facilities are very sensitive to rotations and dynamic changes in the soil-pile system, the precise prediction of monopile head displacement and rotation constitutes a design criterion of great importance. In this article, the semi-analytical finite element analysis which combines the conventional finite element method with Fourier series representation of displacements is used for the study of monopiles for offshore wind turbines under lateral loading in homogeneous soils. In order to develop functional forms of design equations for large diameter monopiles like the ones used for supporting offshore wind turbines, an extensive parametric study has been carried out. The analyzed parameters including pile slenderness ratio and soil stiffness allow to obtain simple and approximate closed-form solutions for static head stiffnesses of both rough and smooth short monopiles embedded in homogeneous soils. It has been found that lateral stiffness KL, rotational stiffness KR and cross coupling stiffness KLR for both rough and smooth interfaces depend only on the value of the monopile slenderness Lp/Dp rather than the relative soil/monopile rigidity Ep/Es usually found in the offshore platforms designing codes (DNV code for example). These analytical expressions have been incorporated into the expressions of the Offshore Wind Turbine (OWT) natural frequency of two wind farm sites. Excellent agreement has been found between the computed and the measured natural frequencies. [ABSTRACT FROM AUTHOR]

Environmental conditions created by winds blowing oblique to the direction of the waves are necessary to conduct some survivability tests of offshore wind turbines. However, some facilities lack the capability to generate quality waves at a wide range of angles. Thus, having a wind generation system that can be rotated makes generating winds that blow oblique to the waves possible during survivability tests. Rotating the wind generation system may disrupt the flow generated by the fans because of the effect of adjacent walls. Closed or semiclosed wind tunnels may eliminate the issue of wall effects, but these types of wind tunnels could be difficult to position within a wave basin. In this work, a prototype wind generation system that can be adapted for offshore wind turbine testing is investigated. The wind generation system presented in this work has a return that minimizes the effect that the walls could potentially have on the fans. This study characterizes the configuration of a wind generation system using measurements of the velocity field, detailing mean velocities, flow directionality, and turbulence intensities. Measurements were taken downstream to evaluate the expected area of turbine operation and the shear zone. The dataset has aided in the identification of conditions that could potentially prevent the production of the desired flows. Therefore, this work provides a useful dataset that could be used in the design of wind generation systems and in the evaluation of the benefits of recirculating wind generation systems for offshore wind turbine research. [ABSTRACT FROM AUTHOR]

In this paper, a joint distribution of all relevant environmental parameters used in design of offshore structures including directional components is presented, along with a novel procedure for dependency modelling between wind and wind sea. Probabilistic directional models are rarely used for response calculation and reliability assessments of stationary offshore structures. However, very few locations have the same environment from all compass directions in combination with a rotationally symmetric structure. The scope of this work is to present a general environmental joint distribution with directional descriptions for long term design of stationary offshore structures such as offshore wind turbines. Wind, wind sea and swell parameters will be investigated for a chosen location in the central North Sea. [ABSTRACT FROM AUTHOR]

The purpose of the present study is to review the state-of-the-art in research and development of offshore wind turbines in order to address the latest findings and trends in structural design and response analysis. This could enhance a development of sophisticated offshore wind turbines. To complete such a task, a detailed review of offshore wind renewable energy potential, wind, wave and sea current loading as well as structural analysis and design procedures and experimental work are presented. [ABSTRACT FROM AUTHOR]

Purpose: Generating electricity from wind turbines is currently a viable option to meet the electric power requirements in many countries. The structure of offshore wind turbines is constantly subjected to external dynamic loads due to wind, waves and rotor loads due to the rotation of blades. The frequency content of these dynamic loads is in the range of natural frequencies of the wind turbine. Therefore, determining the in situ dynamic characteristics of a wind turbine is very beneficial, as it can lead to further improvements in its design, performance and safety.Methods: In this paper, the dynamics of the structure of an offshore wind turbine is investigated numerically and experimentally. The finite element (FE) model of structural components is developed based on design specifications. The model takes the effect of rotor-nacelle assembly into account by considering its mass and moment of inertias relative to the top centre of the tower. In situ modal tests using impulse excitations were carried out on the actual wind turbine when the rotor blades were not rotating to identify the modal parameters.Results: This paper presents predicted natural frequencies from the FE model based on design conditions and those identified from the modal and ambient excitation tests.Conclusion: It was observed that the first two bending modes are close to the natural frequencies identified experimentally by the in situ modal tests. It was also observed that the in situ modal tests were not able to excite other higher natural frequencies of the structure. [ABSTRACT FROM AUTHOR]

The wide-shallow composite bucket foundation (WSCBF) is a new type of offshore wind power foundation that can be built on land and rapidly installed offshore, thereby effectively reducing the construction time and costs of offshore wind power foundation. In this study, the horizontal bearing capacity is calculated by finite element simulation and compared with test results to verify the validity of results. In this process, the vertical load and bending load are respectively calculated by the finite element simulation. Under the vertical load effect, the bucket foundation and the soil inside are regarded as a whole, and the corresponding buckling failure mode is obtained. The ultimate vertical bearing capacity is calculated using empirical and theoretical formulas; the theoretical formula is also revised by finite element results. Under bending load, the rotational center of the composite bucket foundation (in a region close to the bucket bottom) gradually moves from the left of the central axis (reverse to loading direction) to the nearby compartment boards along the loading direction. The H-M envelope line shows a linear relationship, and it is determined that the vertical and bending ultimate bearing capacities can be improved by an appropriate vertical load. [ABSTRACT FROM AUTHOR]

Large diameter caissons are being considered as plausible foundations for supporting offshore wind turbines (OWTs) where reductions in overall cost and environmentally friendly installation methods are expected. The design calculations required for optimization of dimensions/sizing of such caissons are critically dependent on the foundation stiffness as it is necessary for SLS (Serviceability Limit State), FLS (Fatigue Limit State), and natural frequency predictions. This paper derives closed form expressions for the 3 stiffness terms (Lateral stiffness K L , Rotational Stiffness K R and Cross-Coupling term K LR ) for suction caissons having aspect ratio between 0.5 and 2 (i.e. 0.5 < L/D < 2) which are based on extensive finite element analysis followed by non-linear regression. The derived stiffness terms are then validated and verified using studies available in literature. An example problem is taken to demonstrate the application of the methodology. [ABSTRACT FROM AUTHOR]

The fatigue damage sensitivity to wave directionality on large-diameter monopile foundations for use in the offshore wind turbine industry is investigated. A frequency-domain approach with the Dirlik method is used to estimate fatigue damage and to evaluate the effect of wave spreading and swell separation compared to a total sea representation. To evaluate the long-term impact, 30 years of hindcast data from the Dogger Bank area are used. Furthermore, a computationally efficient time-domain model of a 10-MW offshore wind turbine is used to compare with the frequency-domain results. Results show that benefits in terms of structural fatigue can be obtained with directional considerations. [ABSTRACT FROM AUTHOR]

Soil-structure interaction (SSI) is a vital aspect in the performance of bottom fixed offshore wind turbine (OWT) structures. This paper investigates the response of a jacket supporting a 5-MW OWT in intermediate water depths of 70 m, under the influence of hydrodynamic and aerodynamic loading. SSI is incorporated by means of a Winkler-spring formulation and sands of varying densities are considered. A spectrum of wind speeds are considered, increasing from the cut-in value of 3 m/s to an extreme case of 45 m/s. Response charts have been proposed in terms of rotations at various levels of the jacket, to serve as design-aids for similar structures. The effect of SSI becomes predominant for OWT in loose sands (with friction angle less than 30 ∘ ). Parametric resonant responses have been observed for the jacket in loose sands, in the region bounded by the rated and cut-out wind speeds. [ABSTRACT FROM AUTHOR]

Pile foundations for energy infrastructure, such as electric transmission towers, offshore platforms, or wind turbines, are usually rigid and close to the lateral response of rigid shafts. This paper presented an analysis method for the deflection of laterally loaded rigid pile in cohesive soil. The proposed method considers a broken-line variation of the ultimate horizontal soil resistance with depth. According to the elastic-perfectly plastic constitutive law and some calculation assumptions, there are four cases occurred with the increase of lateral load. For greater generality, the solutions have been given in dimensionless form. Comparing with numerical simulation and full-scale tests, the reasonability of the proposed method is demonstrated. The proposed method offers a simple alternative approach to the deflection of laterally loaded rigid pile in cohesive soil. [ABSTRACT FROM AUTHOR]

Suction caissons are skirted foundations frequently used in offshore environments supporting various structures. Among those lie the offshore wind turbines (OWT), which are currently one of the most common renewable energy systems. In this paper we study the tensile capacity of caisson foundations of OWT in dense sands, where the generation of excess pore pressure is analyzed. The tensile capacity of suction buckets under various loading rates is investigated using a series of quasi-static axisymmetric finite element analyses with coupled flow-deformation formulation. DeltaSand model is utilized for the soil, which is an elasto-plastic state-dependent model capturing the response of sands at varying relative densities. Numerical results are validated with available test data of scaled physical model experiments. Particular emphasis is placed on the interaction between suction-bucket and the soil. Parameters governing the soil-caisson interface playing a key role in the uplift response are identified. Lastly, a case study that can be observed in offshore foundations is simulated. Results show that the developed numerical model is applicable to capturing the uplift response of bucket in an ocean environment. The outcome of this study will contribute to closing the gap in the design of suction caissons of OWT in existing standards. • Tensile capacity of caisson foundations of OWT in dense sands is studied. • Tensile capacity is investigated withaxisymmetric finite element analyses using a coupled flow-deformation formulation. • Elemental behavior of dense sand is governed by a sophisticated material model, which is an elasto-plastic state-dependent model to capture the mechanical response of sand. • Results are validated with available scaled tests in terms of load-displacement and suction-displacement relationships. [ABSTRACT FROM AUTHOR]

Dynamic numerical analyses on an offshore wind turbine (OWT) supported by a suction bucket foundation were performed to investigate the effects of soil spatial variability on the ground liquefaction and tilts of OWT, and to provide suggestions on selecting a representative percentile of the random distributions of a clean sand equivalent SPT blow count, (N 1) 60 cs , for deterministic models. The material parameters of an elastoplastic constitutive model (Cyclic Mobility model) in the numerical model were derived from the (N 1) 60 cs (spatial variability). Monte Carlo simulations were used to describe the seismic responses of the laterally loaded suction bucket foundation in spatially variable soil. Simulation results for deterministic models (uniform properties of soil) were compared to results for random models (spatially variable properties of soil). Tilts of OWT and liquefaction potential index of ground are chosen as criteria, and those obtained from the random models and deterministic models were compared to identify the representative (N 1) 60 cs , based on the principle that the deterministic models produce reasonable consistency with the upper limit of the random model responses. The effects of site conditions, seismic conditions, and other factors were evaluated, and it is found that the coefficient of variation (COV), (N 1) 60 cs , acceleration, and frequency have significant influence. • The effects of soil spatial variability on ground liquefaction and tilts of OWT were investigated. • The study provides suggestions on selecting representative (N 1) 60 cs for deterministic models. • The material parameters of the Cyclic Mobility model were derived from the (N 1) 60 cs. • The effects of site conditions, seismic conditions, and other factors were evaluated. [ABSTRACT FROM AUTHOR]

The coupled effects of waves and a subsurface current on the hydrodynamic loads on offshore slender structures in coastal water regions have been examined in this paper, especially the coupling of the weak nonlinearity of surface waves and the depth variation of a current profile. Based on a narrowband assumption, this paper firstly derives a novel semi-analytical framework which extends potential flow theory and the Direct Integration Method (DIM) proposed by Li and Ellingsen (2019), permitting the interaction between weakly nonlinear surface waves and an arbitrary depth-dependent subsurface current. This framework is numerically implemented, validated, and used to lead to the current-modified Morison equations which are used to analyze the loading on a bottom-fixed, vertically installed cylinder. We demonstrate the clear differences between the features of surface elevation and hydrodynamics loading exerted by the coupled interaction of irregular waves and a subsurface current in both operational and extreme sea states. Especially, we find that the occurrence probability of extremely large events represented by the statistical features of surface waves can be insignificant whereas the coupled effects of waves and current on the hydrodynamic loads are considerable, compared with the decoupled cases. This suggests that the conventional approach, which uses the statistical features of surface elevation to represent the extremely large wave events, and therefore risks posing to the safety of offshore structures, may not be sufficient when both the waves and a subsurface current are present and play a role in the hydrodynamic performances of offshore structures. Both the depth variation of a current profile and the direction between the wave propagation and current velocity are found to play an important role in the loads on offshore slender structures. A following and an opposing uniform current lead to overestimated and underestimated loads, respectively, compared to either a decoupled model or a coupled model with a more realistic representation of the current profile. • The coupling between the nonlinearity of water waves and a current is examined. • Hydrodynamic loads on offshore slender structures are analyzed. • An approximate theory more general than potential flow theory is used. • Differences between the proxies to extreme wave events and wave kinematics are shown. • A following current leads to increased loads, compared with the decoupled models. • An opposing current leads to decreased loads compared with the decoupled models. [ABSTRACT FROM AUTHOR]

Digital twins (DTs) are thought to be promising tools for offshore wind turbines (OWTs) in real-time updating, optimized design, intelligent operation, and maintenance. When creating DT analysis tools, it is critical to update model parameters to create high-fidelity, high-accuracy virtual models. In this paper, a Bayesian updating framework based on Transitional Markov Chain Monte Carlo (TMCMC) is proposed to construct the desired virtual model of monopile OWTs to address this difficult problem. The likelihood function for Bayesian inference was created using measurements from physical entity monitoring as well as natural frequencies and damping ratios from the monopile OWT finite element (FE) model. Then, through a series of relatively simple intermediate likelihood functions, it gradually converges to the target function. During this time, the artificial neural network (ANN) surrogate model calculates the intermediate likelihood function of each sample, and a parallel computing strategy is used to enhance the computational efficiency. The results show that the posterior distribution of the uncertain parameters can be successfully estimated from the Markov chain using this method to update the finite element model. In addition, the proposed Bayesian updating method is resistant to measurement noise. • Using modal frequencies and damping ratios, a Transitional MCMC-based Bayesian model updating framework has been made. • For the first time, the TMCMC-based Bayesian method is used for the monopile offshore wind turbine structure updating. • The ANN surrogate model and a parallel computing strategy is used to enhance computational efficiency of Bayesian updating. • The effects of noisy data and FE model parameter uncertainties on the result of model updating are investigated. [ABSTRACT FROM AUTHOR]

This paper presents a study on the role of kinematic interaction effects on the seismic response of large monopile-supported offshore wind turbines. For this purpose, a finite element substructuring model is used to analyse the behaviour of three reference wind turbines with rated powers of 5, 10 and 15 MW founded on several soil profiles and subjected to different accelerograms. The foundation response is computed through a continuum model which includes soil–structure interaction. The effects of inertial and kinematic soil–structure interaction on the system seismic response is evaluated also through comparisons with results corresponding to the infinitely rigid base condition. It is found that neglecting kinematic interaction factors could lead to underestimate the seismic response of large monopile-supported offshore wind turbines. The rotational kinematic interaction factor is shown to increase the OWTs seismic response. • Kinematic interaction effects on the seismic response of large OWTs are studied. • The NREL 5 MW and the IEA Wind 10 MW and 15 MW reference OWT are studied. • Kinematic interaction is only relevant if the rotational KI factor is considered. • Neglecting kinematic interaction leads to underestimate the seismic response. • Kinematic interaction effects are found to be similar for all soil profiles and OWTs. [ABSTRACT FROM AUTHOR]

This paper presents a simplified analytical approach to rapidly simulate the elasto-plastic response of standalone tubular Offshore Wind Turbine (OWT) supports (e.g. monopiles and spar floating platforms) that are subjected to a ship collision. The wind turbine, idealized as a constant cross-section cantilever tube with a tip mass, is assumed to be struck by a rigid impactor. In a first step, a series of preliminary finite element simulations enable the identification of three successive phases in the deformation process: (i) local elastic denting, (ii) local plastic denting combined with global elastic bending, and (iii) plastic buckling at the base of the structure. In a second step, from these observations, existing analytical formulations extracted from the literature are integrated into a time-stepping algorithm. Finally, resulting force-penetration and energy balance curves are compared to finite element results. Simulations are performed considering mid and quarter-length impacts of a 6000-tonnes Offshore Supply Vessel approaching at different velocities. Seven different OWTs are tested. A good correlation between analytical predictions and numerical simulations is found in the vast majority of the investigated collision scenarios. Limitations are encountered for stockier supports, where the contribution of the global elastic bending mode is underestimated by the proposed analytical model. Overall, the method presents a promising alternative for risk assessment and structural optimization applications. • Numerical impact simulations of offshore wind turbine supports were performed. • The deformation process consists of local denting, global bending and buckling. • A rapid assessment simplified methodology was developed for pre-design applications. • The method was validated against numerical results and presents suitable accuracy. [ABSTRACT FROM AUTHOR]