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This article presents an innovative structural system for offshore wind turbines supported on monopiles. The novelty of this system lies in its enhancement of the lateral stiffness of the tubular tower through the incorporation of an outrigger system, widely employed in high-rise buildings. This novel system is modeled as a Timoshenko beam-column element under both static and dynamic loading scenarios, incorporating geometric nonlinearity and accounting for the effect of soil-structure interaction. A comprehensive numerical analysis was conducted to evaluate the system's behavior under various static and dynamic load cases, utilizing soil data from the Red Sea region of Egypt. A modal analysis is also incorporated to investigate the eigenmodes of the proposed system. This study offers valuable insights for designers and researchers, providing guidelines and diagrams for the implementation of the proposed system. [ABSTRACT FROM AUTHOR]

AbstractScour occurs inevitably around monopiles of offshore wind turbines (OWTs), negatively impacting the operation of the turbines. For the purpose of ensuring the long-term safety and stability of the foundations, solidified soil which provides higher scour resistance can be used to remediate scour considering its technical convenience. In this study, flume tests were conducted under unidirectional flow conditions to investigate the dynamic remediation of the local scour hole, which was applied to different scour development processes, using solidified soil. Various remediation schemes were simulated by controlling the design elevation of the solidified soil layers to investigate their performance under different conditions. Meanwhile, the phenomenon and characteristics of edge scour during remediation were also analyzed. Results show that different remediation elevations and initiation would lead to varying degrees of edge scour. Early remediation costs less solidified soil but results in severe edge scour, while late remediation needs more materials and experiences milder edge scour. In addition, an evaluation method assessing solidified soil remediation was established. This method was utilized to evaluate the comprehensive benefit and edge scour risk for different remediation schemes. [ABSTRACT FROM AUTHOR]

The evidence of soil nonuniformity has been reported in several site investigation reports and is well-confirmed for any soil in general. However, studies related to the spatial soil variability of offshore ground are somewhat limited, and current design practices for monopile foundations also overlook the uncertainty of the offshore ground. This paper presents the probabilistic evaluation of the serviceability limit state of monopile foundations considering the spatial variability of offshore ground. The nonuniformity of the ground is modeled utilizing the site-specific undrained shear strength (su) as a stochastic variable, using a spatially correlated Gaussian random field. A hardening soil constitutive model is utilized in 3D finite element analysis of offshore ground, with a focus on parameter calibration. The reliability of the serviceability limit state is assessed based on the allowable load on the pile head. The stochastic behavior of the maximum bending moment and maximum shear force along the depth of the monopile is also investigated. Besides, the paper also highlights the importance of incorporating ground nonuniformity during the design process of a monopile foundation. [ABSTRACT FROM AUTHOR]

In this paper, a numerical formulation was developed to generate the consistent mass matrix for Timoshenko tapered beam-column element for geometrically nonlinear transient analysis. The basis of the suggested solution is the exact shape functions and their derivatives, which describe the non-uniformity of the element properties. To illustrate the variations in section properties along the tapered element length, the section properties were presented as exponential functions with tapering indices. The model can simulate elements with various solid and hollow cross-sections. The suggested formulation is integrated into a Visual Basic program to perform the analysis, along with numerous examples to verify its precision and effectiveness. As a practical application of the proposed formulation, a structural system was proposed for offshore wind turbines with high elevations, and a geometrical nonlinear dynamic analysis was conducted. The results demonstrated the proposed model's ability to perform this type of complex calculation with minimal computational effort. [ABSTRACT FROM AUTHOR]

AbstractThe monopile-supported offshore wind turbines are located in a complex marine environment, bearing various long-term cyclic loads such as wind loads, waves, and currents. To explore the weakening of soil-structure interaction under long-term lateral load for the pile foundation, a series of model tests were conducted in the laboratory under normal gravity conditions, and the evolving trends for the weakening coefficients of varying soil depths and pile diameters were analyzed, leading to the proposal of a new expression for the weakening coefficient considering the governing factors in soil-structure interaction. The new formula for the weakening coefficient proposed in this paper is validated through the experimental results of this study and other references. Through a numerical example, affirming the viability of the soil stiffness attenuation model based on the finite element analysis method and revealing the weakening law of foundations for different cycling processes. This research contributes valuable insights for designing and analyzing offshore turbine foundations, particularly in the context of weakening phenomena. [ABSTRACT FROM AUTHOR]

The foundation of an offshore wind turbine (OWT) accounts for up to 20% of the total cost; therefore, investigations into reliable and efficient foundations are important for the offshore wind energy industry. This article describes an innovative winged caisson foundation for OWTs, which adapts the conventional caisson foundation to produce higher overturning capacities. The behaviour of this proposed novel foundation in the sand was investigated through experimental and numerical modelling using a series of 1 g tests (1/70th scale), centrifuge model tests (70 g ), and finite element (FE) simulations. Models with various wing sizes installed in different sand densities were tested in the laboratory by applying an overturning load. In this article, the moment–rotation performance of the foundation under monotonic loading is compared against that of conventional caissons to assess the potential benefits of adding wings to caisson foundations. Results show that there is a significant increase in overturning capacity provided by the novel foundation. FE models were first validated against experimental results, then, used to conduct a parametric study to better understand the behaviour of the winged caissons beyond the scaled model tests. In addition, the FE models were used to evaluate the drained response of the proposed foundation under combined loading. [ABSTRACT FROM AUTHOR]

This paper presents a literature review of analytical, numerical, and experimental methods for evaluating the structural response of Offshore Wind Turbines (OWT's) during ship collision events. The different energy transfer mechanisms involved throughout a collision against fixed or floating OWT substructures are examined, whilst documenting some of the most common procedures in the assessment of wind load effects, soil–structure interaction, mooring lines contribution and hydrodynamic coupling. A survey of internal mechanics and external dynamics approaches developed for ship-structures collision events is carried out with a discussion on their potential application in analyses involving different types of substructures. Moreover, different hydrodynamic coupling schemes found in the literature are studied, highlighting the importance of hydro-effects in the collision analysis. Common practices in the use of metallic and cementitious material constitutive models in ship-collision analysis are also presented, whereas the structural behaviour of reinforced concrete structures submitted to impact loads, which has been rarely studied outside high-velocity impact/blast applications, is discussed in the current context. Finally, some of the most important limitations of the analytical and numerical models used in ship-OWT collisions are identified, from which suggestions are provided for future research endeavors. [ABSTRACT FROM AUTHOR]

In this study, a series of centrifuge tests are performed to investigate the ultimate lateral capacity of a pile group for offshore wind turbines. The validated Finite element models (FEMs) are established. The rotation center and the maximum bending moment occur at 9 D and 3 D of the pile length. A sensitivity study is performed by considering the soil parameters through numerical tests. The internal friction angle is the most influential factor by introducing 32% change in lateral capacity. The key factors of pile number, pile diameter, loading height, and pile spacing are investigated in the following parametric study. Increasing the pile number and the pile diameter is effective for enhancing the lateral capacity. In contrast, a larger eccentricity has a negative effect on the foundation stability. The failure mode of the pile group foundation tends to change from strain softening to strain hardening when the loading height reaches a threshold magnitude. The p-multiplier is calculated to demonstrate the shadowing effect in a pile group foundation. An optimal design method is proposed by integrating the pile number and the pile spacing factors. This study provides some instructions for the application of pile group foundations in the offshore wind industry. [ABSTRACT FROM AUTHOR]

It is necessary to understand the response of monopile foundations for large offshore wind turbines (OWTs) with rated powers of 6 − 9 M W under cyclic loading. For this purpose, a monopile model test study was conducted in saturated medium dense sand under long-term unidirectional lateral sinusoidal cyclic loading for 12 , 000 cycles. The model pile was constructed with displacement transducers on the pile head and strain gauges along the pile length. The experimentally obtained bending moment, displacement, soil reaction evaluation along the pile length, pile head displacement accumulation, and lateral soil-pile stiffness were substantially different from the results of the American Petroleum Institute (API)-recommended method of determination and were significantly affected by the number of loading cycles. The experimentally based soil reaction (p) -pile displacement (y) curves obtained considering the maximum soil reactions for particular cycles were compared with the p - y curves recommended by the API. The maximum soil reaction (p) during the increase in pile displacement (y) obtained experimentally was significantly higher than the soil reaction obtained with the API method. The p–y curves obtained experimentally show similarities with those determined with the API method at only small pile displacements in the initial stage of the curves. [ABSTRACT FROM AUTHOR]

Exposed to the cyclic action of wind and waves, offshore wind structures are subject to fatigue deterioration processes throughout their operational life, therefore constituting a structural failure risk. In order to control the risk of adverse events, physics-based deterioration models, which often contain significant uncertainties, can be updated with information collected from inspections, thus enabling decision-makers to dictate more optimal and informed maintenance interventions. The identified decision rules are, however, influenced by the deterioration model and failure criterion specified in the formulation of the pre-posterior decision-making problem. In this paper, fatigue failure criteria are integrated with Bayesian networks and Markov decision processes. The proposed methodology is implemented in the numerical experiments, specified with various crack growth models and failure criteria, for the optimal management of an offshore wind structural detail under fatigue deterioration. Within the experiments, the crack propagation, structural reliability estimates, and the optimal policies derived through heuristics and partially observable Markov decision processes (POMDPs) are thoroughly analysed, demonstrating the capability of failure assessment diagram to model the structural redundancy in offshore wind substructures, as well as the adaptability of POMDP policies. [ABSTRACT FROM AUTHOR]

In this study, the Local Joint Flexibility (LJF) of tubular T/Y-joints with doubler plate under in-plane bending (IPB) load is investigated. In the first step, a finite element (FE) model was developed and verified using the available experimental results. In the next step, 252 FE models were generated to investigate the effect of the joint geometry on the LJF factor (fLJF). Results showed that the increase of the doubler plate size results in the considerable decrease of the fLJF. Also, the fLJF of a T/Y-joint reinforced with doubler plate can be down to 26% of the fLJF of the corresponding unreinforced joint. Despite the considerable effect of the doubler plate on the fLJF, there is not any study on the fLJF of the tubular joints with doubler plate. Hence, after a parametric study, FE results were used to propose a parametric formula for determining the fLJF. [ABSTRACT FROM AUTHOR]

Monopiles are popular solutions as the foundation to support offshore wind turbines (OWTs), which suffer from various amplitudes of continuous lateral cyclic wind and wave loading. The accumulation of tilt and change of the natural frequency of OWTs under cyclic loading have become crucial issues for OWT design. Hence the monopile-tower-soil system was established at the small-scale level based on similarity laws with a suit of constant-amplitude and multi-amplitude tests in dense sand. Two sets of gear-driving assembly were successfully set up to achieve the application of cyclic loading to the model OWT. The amplitude and symmetry of cyclic loading were specified as three indicative regions according to those of real wind farms. On the basis of test results, the two-way loading was found to be the most hazardous situation and the preloading condition led to a high accumulation of tilt. The change of natural frequency was controlled by the load magnitude and tended to increase during cyclic loads, while a sharp decrease was observed when sand subsidence was generated. The post-cyclic lateral capacity seemed to be a slight increase compared with the static capacity. The relevant analyses based on test results can provide practical recommendations for OWT design. [ABSTRACT FROM AUTHOR]

Mono-pile foundations have been widely used for offshore wind turbines principally due to their convenient construction and cost-effective nature. So far, little attention has been paid to large diameter "semi-rigid" piles that have distinct behaviours from flexible or ideally rigid piles. This paper presents a series of centrifuge model tests to study the deforming and bearing characteristics of a 5.9 dia. semi-rigid pile under lateral loadings in kaolin clay. For monotonic loading, a modified p–y curve analysis model considering rotational soil flow near the rotation centre of pile was proposed, highlighting the limitation of classic plane-strain based plasticity models to evaluate the ultimate lateral pile-soil resistance. For cyclic loading, a strong correlation between the degree of soil degradation and cyclic load amplitude was identified. Besides, a degradation factor model, accounting for various cyclic stress levels and soil depths, was proposed, which can be used to assess the accumulative displacement of semi-rigid piles under cyclic loadings in soft clay. [ABSTRACT FROM AUTHOR]

Bucket foundations can be regarded as a comparatively new foundation system for offshore structures. The installation of a bucket foundation using suction pressure can lead to seepage from the outside to the inside of the bucket, change the laws governing extrusion and decrease the friction coefficient between the wall and the sand. In this paper, three different bucket foundation wall thicknesses are examined when static and suction pressures are used to drive the foundation into the sand. The relationship between the pressure of the earth and the friction resistance of the inner and outer walls as a function of the penetration depth and the tip resistance was researched in the experiments. The mechanism behind the decrease in the penetration resistance and its magnitude based on the experimental results was revealed. Laws influencing the wall thickness to penetration resistance ratio for different penetration depth to diameter (h/D) ratios are proposed. [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]

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]

This paper analyzes the influence of scour on the overall response of monopile-supported offshore wind turbines (OWTs) in 20-m water depth. Scouring effects on OWTs have been often studied within the geotechnical domain, considering static loads at the mudline. The present work attempts to address the scour-induced problems in OWTs by making use of an integrated aerodynamic–hydrodynamic load approach in sandy soils. The OWT analysis is simulated for operational and shut-down (parked) condition. Under parked situations, the OWT blades are feathered, and power production is suspended, owing to structural safety concerns. The 50 Monte Carlo responses of stochastic sea-state condition (wind speed with turbulence, significant wave height, and peak spectral period) are generated. Irregular, long-crested waves are generated using the Joint North Sea Wave Project (JONSWAP) spectrum. Then from each simulation, the ensemble response is obtained. Sandy soils of varying densities are considered. Results indicate that OWTs founded on loose sands suffer significant stiffness (and hence natural frequency) reductions, shifting the structure into the resonance regime. Lateral responses also show an escalation with reduction in density of sandy soil. [ABSTRACT FROM AUTHOR]

Direct-driven ironless-stator machines have been reported to have low requirements on the strength of the supporting structures. This feature is attractive for offshore wind turbines, where lightweight generators are preferred. However, to produce sufficient torque, ironless generators are normally designed with large diameters, which can be a challenge to the machine’s structural reliability. The ironless multi-stage axial-flux permanent magnet generator (MS-AFPMG) has the advantages of ironless machines but a relatively small diameter. The objective of this article is to present the design optimization and performance investigation of the ironless MS-AFPMG. An existing design strategy, which employs two- and three-dimensional static finite element analyses and genetic algorithm for machine optimization, is improved with the aim of reducing the calculation load and calculation time. This improved design strategy is used to investigate the optimal ironless MS-AFPMG. Some intrinsic features of this kind of machine are revealed. [ABSTRACT FROM AUTHOR]

Wind/wave-induced vibrations of jacket-type offshore wind turbines (JOWTs) are suppressed by placing a passive vibration absorber, called tuned liquid column gas damper (TLCGD), on the turbine nacelle. Adopting an ensemble of 75 wind/wave combinations, three different JOWTs and a SimuLink-based nonlinear model in time domain, main parameters of the TLCGD are optimised to reach the minimum standard deviation of nacelle displacement. Obtained results indicate that contribution of the proposed TLCGD is more pronounced in the case of regular excitations such as those from sea waves and less turbulent winds. Depending on the wind/wave combination, TLCGD can result in reductions up to 45% and 51% in nacelle displacement standard deviation and maximum acceleration, respectively. As a result, TLCGD deemed to be well suited to protect fatigue critical JOWTs as well as acceleration-sensitive devices of the nacelle. [ABSTRACT FROM AUTHOR]

Selection of the optimum support structural configuration for offshore wind turbines is a decision that should take into account a variety of both technical and non-technical criteria in order to aggregate cumulatively the performance of several concepts such as jackets, tripods, monopiles, spars, semi-subs, tri-floaters and others with varying suitability and relevance to offshore wind applications. In the preliminary phases of a project, consideration of qualitative attributes based on experts’ opinions is a common practice due to the limited availability of quantitative data for attribute scores and relevant weights, which are mainly based on past experience. This practice can introduce a bias in the considered scores reducing the confidence of the qualifying solution. This paper documents the extension of the widely used TOPSIS method (Technique for Order of Preference by Similarity to Ideal Solution) to explicitly consider stochastic inputs (statistical distributions) following a systematic gathering of data through surveys and questionnaires. The methodology proposed has been efficiently implemented in a numerical tool and the case study that is presented on the decision analysis of an offshore wind turbine supports structure under given deployment conditions, illustrates its applicability and allows for a sensitivity analysis of the effect of the resolution of simulation, selection of statistical distributions and weighting of experts’ opinions based on their perceived level of expertise. [ABSTRACT FROM AUTHOR]

A probabilistic formulation is proposed to assess the performance of the support structure of offshore wind turbines based on their probability and expected time of exceeding specified drift thresholds. To this end, novel probabilistic models are developed to predict the mean and standard deviation of the drift ratio response of wind turbine support structures operating under day-to-day loads as a function of the wind turbine geometry and material properties, and loading conditions. The proposed models are assessed using a database of virtual experiments generated using detailed three-dimensional (3D) nonlinear finite element (FE) models of a set of representative wind turbine configurations. The developed models are then used in a random vibration formulation to estimate the probability and expected time of exceeding specified drift thresholds. As an example, the probability and expected time of exceeding specified drift thresholds are estimated for a typical offshore wind turbine at different wind speeds. A comparison is made between the results obtained based on the proposed models, those obtained using simulators commonly used in practice and detailed 3D nonlinear FE analyses. [ABSTRACT FROM PUBLISHER]

Wind turbines on offshore sites (OWECs) are subjected to combined loads from wind and waves. These dynamic loads, with a frequency content within the range of the natural frequencies of the structures, cause fatigue-effective stresses in the substructures of wind turbines. Therefore, the examination of natural frequencies is an important part within the design process of wind turbines. The quality of the numerical models for such calculations is of great importance, since the certification guidelines permit only small uncertainties in modal analysis results. The accuracy of the parameters of the numerical model can only be achieved through a comparison of simulation results with corresponding test results. Therefore, it is necessary to measure the dynamic behaviour of all components of the wind turbines simultaneously. This is true not only for the design verification, but also for monitoring the OWECs in operation. The potential of integrated systems for monitoring-based maintenance optimisation should thus be used. [ABSTRACT FROM AUTHOR]