Hysteretic damping model for laterally loaded piles

Fatigue assessment is a critical design aspect for many offshore structures. Soil-foundation interaction has a direct impact on the system dynamic response of these structures. While the stiffness of the soil-foundation interaction influences the system's natural frequency, the damping influences the amplification of the structural response to environmental excitations. This paper presents a simplified model for estimating the soil damping due to nonlinear soil response for pile foundations, which have wide applications in the offshore industry, such as for supporting jacket platforms, wind turbines and wellhead facilities. The proposed model is fundamentally linked to the damping response of the soil measured at element level therefore it offers design engineers an efficient and accurate way to estimate soil-pile interaction damping based on site-specific soil data. Approaches to include the suggested model for structural analysis are also proposed.     

海上风电场风机桩基局部冲刷计算

建设海上风电场时,风机所处的海洋环境十分复杂,在风、浪、流等气象水文要素的影响下,风机桩基周围会出现冲刷坑,进而影响风机基础的稳定。因此,风机桩基的局部冲刷深度是工程设计的重要参数。采用埕岛油田海洋平台桩基局部冲刷现场观测资料,对国内常用的2个计算桩基局部冲刷深度的经验公式(韩海骞公式和王汝凯公式)进行验证,运用波流合成速度代替纯潮流速度。结果表明该海域波浪对泥沙的作用不可忽略。韩海骞公式重新计算的结果与实测值吻合良好。     

海上风机大直径单桩双钩翻身技术

中国海上风电是绿色能源的重要一支,现已进入大规模发展阶段,其中大直径无过渡段单桩是国内海上风电场主流基础形式。为高效、安全的进行大直径单桩施工,本文通过研究一种单台起重机双钩空中翻身的技术,采用左右大张角双钩起重机、大吨位吊梁、翻桩夹具等手段,在仅需一台大型起重设备的情况,完成了大直径单桩的翻身起重作业,提高了施工效率,降低海上风电场建造成本和安全风险。     

Estimation of natural frequencies and damping using dynamic field data from an offshore wind turbine

The dynamic characteristics of offshore wind turbines are heavily affected by environmental loads from wave and wind action and nonlinear soil behaviour. In the design of the monopile structures, the fatigue load due to wind and wave loading is one of the most important problems to consider. Since the fatigue damage is sensitive to the foundation stiffness and damping, increasing the accuracy of analysis tools used in the design and optimization process can improve the reliability of the structure and reduce conservatism, thereby leading to a more cost-efficient design. In this context, analysis of field data is important for calibrating and verifying purposes. This paper presents analysis of measured accelerations and strains from a wind farm in the North Sea with monopile foundations. Field data during idling conditions, collected over long periods of operation, are analysed and the natural frequencies are determined, and damping is estimated. The measured natural frequencies are compared to calculated values using an aero-servo-hydro-elastic code, showing a good agreement in the frequency range below 2 Hz. Variation of the natural frequencies with intensity of loading may indicate effect of soil nonlinearity on the overall OWT response. Since the first natural bending modes have the largest potential to mobilize soil reactions, they are of primary interest in this context. The effect of load (wave, wind and dynamic bending moment) on the first natural frequency is investigated using different analysis techniques in the frequency domain and time domain. A clear correlation between load level and first natural frequency is demonstrated. A simple nonlinear SSI model of the tower/soil system is employed to numerically investigate the observed changes in the measured first natural frequency with the level of loading and increased overall damping. The simulated results reproduce the general trends in the observed reduction in the first natural frequency and increased damping ratio with the load level. However, the effect of the load level is less than that observed in the measurements, indicating contribution also from other factors than soil nonlinearity.     

The wet-towing resistance of the composite bucket foundation for offshore wind turbines

The composite bucket foundation (CBF) is a new and environmentally-friendly foundation for offshore wind turbines. This foundation can be prefabricated in batches onshore followed by integrated transport and installation at sea. The structure itself has a subdivision air cushion structure that enables the foundation to float stably on the water surface and realize long-distance towing of the foundation. The mechanism of this air-liquid-solid coupling towing process is complicated, and the influence of the bulkheads on the towing resistance is not clear. In this paper, the influence of the subdivision structure on the towing resistance of the CBF is compared with the tow test in hydrostatic water. The structural motion characteristics and the change of the cushion pressure are also analysed. Experiments are used to verify numerical calculation results. The flow field difference between the CBF with bulkheads, the CBF without bulkheads and the real floating structure was analysed. The dynamic pressure coefficient was used to analyze the force at surfaces of different CBF's. For the tow test and numerical calculation of multiple CBFs, the optimal multi-CBF tow distance and towage number are obtained through the calculation of energy consumption rate.     

Crack growth direction effects on corrosion-fatigue behaviour of offshore wind turbine steel weldments

In this study corrosion-fatigue tests have been conducted on fracture mechanics specimens extracted from an S355 G10+M structural steel welded plate. The tests have been performed on compact tension specimens with the crack tip located in the heat affected zone. The corrosion-fatigue test results from this study have been compared with the data available on the base metal as well as air tests on the same material. Moreover, the obtained results have been compared with the corrosion-fatigue data available in the literature on a wide range of steels and also the fatigue trends for welded joints in free-corrosion condition recommended in the BS7910 Standard. The effect of the specimen orientation, with respect to the weld region, is also examined in this study and it has been found that higher corrosion-fatigue crack growth rates are generally observed in the tests with 0° orientation. The results have also shown that the corrosive environment has significant effects on the fatigue crack growth acceleration at the beginning of the tests; however, as the crack propagates, the environmental damage effect on crack growth behaviour becomes less pronounced. The results presented in this study are discussed in terms of improvement in the structural integrity assessment of offshore wind turbine monopiles.     

Dynamic analysis of a dual-spar floating offshore wind farm with shared moorings in extreme environmental conditions

The concept of a shared mooring system was proposed to reduce mooring and anchoring costs. Shared moorings also add complexity to the floating offshore wind farm system and pose design challenges. To understand the system dynamics, this paper presents a dynamic analysis for a dual-spar floating offshore wind farm with a shared mooring system in extreme environmental conditions. First, a numerical model of the floating offshore wind farm was established in a commercial simulation tool. Then, time-domain simulations were performed for the parked wind farm under extreme wind and wave conditions. A sensitivity study was carried out to investigate the influence of loading directions and shared line mooring properties. To highlight the influence of the shared line, the results were compared to those of a single spar floating wind turbine, and larger platform motions and higher tension loads in single lines are observed for the wind farm with shared moorings. The loading direction affects the platform motions and mooring response of the floating offshore wind farm. Comparing the investigated loading directions to the 0-deg loading direction, the variation of mean mooring tension at the fairlead is up to 84% for single lines and 16% for the shared line. The influence of the shared line properties in the platform motions and the structural responses is limited. These findings improve understanding of the dynamic characteristics of floating offshore wind farms with a shared mooring system.     

Calibration of safety factors for offshore wind turbine support structures using fully coupled simulations

The offshore wind industry experienced a boost during the last decade in terms of size of wind farms and rated capacity of the wind turbines: towers are getting taller and blades are getting longer, constantly facing new and complex challenges. Because of the relative immaturity of the wind industry, and the fact that the offshore design standards stemmed from the oil and gas industry, it is generally acknowledged that the reliability levels achieved, although not very well understood, might result in partial safety factors not optimal for OWT. This paper addresses this situation by studying the reliability levels delivered by the current standards and assessing the validity of the safety factors through a reliability-based code calibration. The combination of the low probability of failure imposed on the design of OWTs and the computational cost of the aero-elastic time-domain simulations brings out the need to develop new approaches for reliability analyses. In this paper, the reliability analysis is performed using a Kriging surrogate model to approximate the load-effect from the aero-elastic simulations converting expensive-to-evaluate limit state functions to explicit functions. Subsequently, a calibration of the safety factors is carried out using the probabilistic models from literature. The approach is applied to an industry-reference turbine and support structure. The results showed very low probabilities of failure for the most severe design cases and confirm that the safety factors from the IEC are mostly adequate.     

Experimental study on seismic vibration control of an offshore wind turbine with TMD considering soil liquefaction effect

Wind energy is clean and sustainable. Taiwan is establishing offshore wind farms using wind turbines in the Taiwan Strait. However, these are located in an earthquake-prone area with sandy seabed conditions. To ensure their safety and reliability, the turbines’ support structure must be protected against wind, waves, and seismic loads. Tuned mass dampers (TMDs) are commonly employed to reduce structural vibrations. A TMD is more simply incorporated into turbine structures than are other energy dissipation devices. In this study, a 1:25-scale test model with a TMD was constructed and subjected to shaking table tests to experimentally simulate the dynamic behavior of a typical 5-MW wind turbine with a jacket-type support structure and pile foundation. The scaled-down wind turbine model has a nacelle without rotating blades; therefore, the aerodynamic and rotational effects due to the rotating blades were ignored in this study. A large laminar shear box filled with saturated sandy ground was used to simulate the typical seabed conditions of Taiwanese offshore wind farms. The TMD system was designed to be tuned the first-mode frequency of the test model. Two ground accelerations, selected by considering wind farm site condition and near-fault characteristics, were used for excitation in the test. The responses of the test model with and without the TMD system were compared, and the influence of soil liquefaction on the effectiveness of TMD vibration control was addressed.     

Static and dynamic loading behavior of a hybrid foundation for offshore wind turbines

Considering the deficiencies of the traditional monopile foundation for offshore wind turbines (OWTs) in severe marine environments, an innovative hybrid foundation is developed in the present study. The hybrid foundation consists of a traditional monopile and a wide–shallow bucket. A series of numerical analyses are conducted to investigate its behavior under the static and dynamic loading, considering various loading eccentricities. A traditional monopile with the same steel volume is used as a benchmark. Although the monopile outperforms the hybrid foundation in terms of the ultimate moment capacity under each loading eccentricity, the latter can achieve superior or the same performance with nearly half of the pile length in the design loading range. Moreover, the horizontal load and moment are mainly resisted by the bucket and the single pile in the hybrid foundation respectively. The failure mechanism of both the hybrid foundation and the monopile is excessive rotation. In the rotation angle of 0.05 rad, the rotation center is located at the depth of approximately 0.6–0.75 times and 0.65–0.75 times the pile length for the hybrid foundation and the monopile respectively. The increasing loading eccentricities can lead to increasing moment bearing capacity, increasing initial stiffness and upward movement of the rotation center of the two foundations, while decreasing load sharing ratio of the single pile in the hybrid foundation. Three scenarios are considered in investigating the dynamic loading behavior of the hybrid foundation. Dynamic response results reveal that addition of the bucket to the foundation can restrain the rotation and lateral displacement effectively. The superiority of the hybrid foundation is more obvious under the combined wave and current loading.     

Strain estimation for offshore wind turbines with jacket substructures using dual-band modal expansion

Structural fatigue is a design driver for offshore wind turbines (OWT). In particular, the substructures, like jackets, are strongly affected by fatigue. Monitoring the fatigue progression in the welds is vital for the maintenance and a potential lifetime extension. However, inspections of critical locations are costly due to the limited accessibility of the mostly submerged jacket. Considering the high number of potentially critical welds, it is regarded as economically unfeasible to equip all fatigue hot spots with sensors. Thus, an indirect method to monitor the fatigue progress of the structure and point out critical locations is desirable. For a consistent support of ongoing maintenance, it has to yield reliable results for varying operational and environmental conditions. This paper applies a virtual sensing approach to jacket substructures. From a small set of sensors on the tower, fatigue at every desired location of the jacket is estimated using dual-band modal expansion. Simulations using the OC4 jacket design are performed to show potentials and limitations of the method. Namely fatigue progress on leg welds of K-joints is predicted with high accuracy over a wide range of load cases. However, some difficulties in fatigue prediction of X-joints due to the occurrence of local modes and limitations in the extrapolation of wave loading have to be resolved in future work.     

Research on the calculation method of penetration resistance of bucket foundation for offshore wind turbines

The cost of foundations for offshore wind turbines constitutes approximately 35% of the total cost of an offshore wind farm. The bucket foundations show significant potential due to their superior transportation and installation efficiencies compared to pile foundations, leading to potential cost savings for the foundation of up to 30%. For a bucket foundation to be installed successfully, the penetration resistance must be predicted. However, there is currently a lack of clarity on how to select a suitable calculation method for penetration resistance based on known geological parameters to guide construction. In order to evaluate the current methods of calculation for bucket foundation penetration resistance, this study combines theoretical calculation methods with two sets of practical measurement data from the field. The calculation methods of penetration resistance for bucket foundation are first reviewed and categorized. The applicability range of each method and the parameters needed for calculation are given. Next, according to the measured data in the process of penetration of bucket foundation on site, the evolution of compartment pressure, tilt angle, resistance and required suction in the process of penetration are analyzed. Finally, the reviewed methods are compared to the results of two practical projects in order to analyze the differences between them and make recommendations for the calculation technique. The findings can be used as a guide for calculating the bucket foundation's penetration resistance in complex geological conditions.     

Effects of structural flexibility on the dynamic responses of low-height lifting mechanism for offshore wind turbine installation

Installation complexities are one of the major challenges in the floating offshore wind turbine (OWT) industry. The modern concept introduced by the SFI-MOVE project is an effort to overcome the complexities by utilizing a low-height lifting mechanism. It is common to idealize a crane in the lifting mechanism as a rigid body since the structural deflections are smaller than the responses introduced by the other system components. However, structural flexibility can play an essential role in demanding offshore operations with smaller acceptable tolerances. In this study, lifting cranes are modeled using the finite element method and simplified by implementing equivalent 3D beam elements. Dynamic analysis is performed for various environmental conditions, and the responses of the crane structure and the OWT are calculated for each load case. This research reveals that crane structure flexibility influences the relative motion between a floating spar buoy and an OWT during mating operations. Crane structural flexibility contributes significantly to the OWT rotations. In addition, the response deviation between using rigid and flexible cranes increases as the excitation force increases. Therefore, it is recommended to consider the crane structural flexibility in the calculation when strict installation tolerances are needed.     

基于能力曲线的某风电安装船动力定位系统失效分析

风能是一种优秀的绿色能源,随着风电开发延伸到海上,带有动力定位系统的风电安装船开始得到广泛应用。文章基于动力定位能力曲线对一艘风电安装船的动力定位系统进行失效分析,寻找出最危险的工况,为实际工程提供参考,以便在作业时尽量避开这些工况。     

Fatigue tests on grouted connection segment specimens in offshore wind turbine structures considering water ingression

Grouted connections (GCs) are widely used to connect superstructures and driven piles in offshore wind turbine structures. They resist fatigue loading in marine splash zones and even submerged environments. In this paper, six GC segment specimens were designed and tested under fatigue loading in both the air and water ingression conditions. The results in the air condition showed that for the specimens with lower loading ranges, the strain distributions and residual displacements stabilized after 20 thousand load cycles. These conditions persisted until the end of the test with two million cycles, with only a few tiny cracks appearing on the grout material surface. Meanwhile, wide cracks and grout material exfoliation were found in specimens with higher loading ranges. The residual displacement accumulated gradually, which eventually caused the termination of the test when it reached 5 mm after 1.3 million cycles. The results in the water ingression condition showed that the water had entered into the micro-cracks of the grout material, which severely degraded the fatigue behavior of the GC specimens. Even in lower loading ranges, specimens W-1 and W-2 only endured 0.264 million and 64 thousand load cycles before the displacement of the top clamp reached −10 mm. Compared with two specimens tested in the air, with a total displacement of less than −0.7 mm after two million load cycles, the severe deteriorating effect of water ingression on the fatigue behavior of GCs was apparent.     

Modified environmental contour method for predicting long-term extreme responses of bottom-fixed offshore wind turbines

Predicting extreme responses is very important in designing a bottom-fixed offshore wind turbines. The commonly used method that account for the variability of the response and the environmental conditions is the full long-term analysis (FLTA), which is accurate but time consuming. It is a direct integration of all the probability distribution of short-term extremes and the environmental conditions. Since the long-term extreme responses are usually governed by very few important environmental conditions, the long-term analysis can be greatly simplified if such conditions are identified. For offshore structures, one simplified method is the environmental contour method (ECM), which uses the short-term extreme probability distribution of important environmental conditions selected on the contour surface with the relevant return periods. However, because of the inherent difference of offshore wind turbines and ordinary offshore structures, especially their non-monotonic behavior of the responses under wind loads, ECM cannot be directly applied because the environmental condition it selects is not close to the actual most important one.The paper presents a modified environmental contour method (MECM) for bottom-fixed offshore wind turbine applications. It can identify the most important environmental condition that governs the long-term extreme. The method is tested on the NREL 5 MW wind turbine supported by a simplified jacket-type support structure. Compared to the results of FLTA, MECM yields accurate results and is shown to be an efficient and reliable method for the prediction of the extreme responses of bottom-fixed offshore wind turbines.     

Support condition monitoring of monopile-supported offshore wind turbines in layered soil based on model updating

Monopile-supported offshore wind turbines (OWTs) are dynamically sensitive structures whose fundamental frequencies may be close to those of environmental and turbine-related excitations. The changes in fundamental frequencies caused by pile-soil interaction (PSI) may result in unwanted resonance and serious O&M (Operation and Maintenance) issues, which have been identified as major challenges in the research field. Therefore, a novel model updating framework with an implicit objective function is proposed to monitor both the stiffness and damping variation of the OWT system based on the measured vibration characteristics, which is further verified by laboratory tests. In particular, layered soil was considered in the tests to simulate the practical soil conditions of Chinese seas. Different pile lengths were introduced to consider the long-term PSI effects for rigid piles and slender piles. The results showed that the variation in the fundamental frequency is significantly reduced in layered soil compared with the pure sand scenario. For the OWT systems in layered soil, the variation in foundation stiffness is negatively related to the burial depth under cyclic loading. The proposed model updating framework is proven reliable for support condition monitoring of OWT systems in complicated soil conditions.     

A procedure for predicting the permanent rotation of monopiles in sand supporting offshore wind turbines

Foundations for Offshore Wind Turbines (OWTs) are designed following the limit state philosophy. One of the considered states is the Serviceability Limit State (SLS), which verifies that the permanent rotation of the foundation generated from accumulated strains in the soil is below a project specific criterion. Despite design codes requiring an estimation of the permanent rotation, there is not clear guidance on how to implement this. This paper describes a methodology to estimate the monopile permanent rotation for SLS and discusses its advantages and limitations. The methodology combines an accumulation method with results from 3D Finite Element Analyses (FEA) and a soil model that accounts for strain accumulation as a function of the number of cycles, relative density and load characteristics. The performance of the proposed methodology is compared against experimental centrifuge tests and results from advanced 3D FEA, indicating that it can predict the permanent rotation with satisfactory accuracy, and with a considerable reduction in computational effort. This is important for the design of OWTs, where different load histories might be required to be checked – often under tight time constraints – to find which load history leads to the largest permanent rotation, and therefore is more critical to SLS design.     

Bottom effects on the tower base shear forces and bending moments of a shallow water offshore wind turbine

In this paper the tower base shear forces and bending moments of a shallow water offshore wind turbine have been rigorously calculated by using a nonlinear simulation method taking into account the bottom effects. It has been found that nonlinearly simulated realistic waves with bottom effects should be used as inputs in the stochastic time domain simulation in order to design an un-conservative support structure for the offshore wind turbine. In order to further improve the simulation efficiency, a transformed linear simulation method has been utilized in this paper for generating equivalent waves as those obtained from the nonlinear simulation method. The accuracy and efficiency of the transformed linear simulation method have been convincingly substantiated through the subsequent calculation examples in this article.