Wind Turbine Foundations Research Articles

Plate armor foundation (PAF) - Innovative for intertidal wind turbines

The plate armor foundation (PAF) is an innovative type of foundation for intertidal wind turbines. A PAF consists of a top plate that is made of reinforced concrete and a side wall which is made of Larsen sheet piles. The steel tie rods are used to connect the top plate and the side wall. The Larsen sheet piles are interconnected using the edge locks. This connection mode is generally considered to be hinged connect. The PAF has significant advantages of cost effectiveness and fast installation. Model tests were conducted to investigate the monotonic lateral bearing capacity and its influence factors of the PAF in saturated marine fine sand. 3D numerical modeling was performed to explore the cyclic behavior of the PAF. Results show that the PAF with sheet pile hinged connection can provide higher lateral bearing capacity and effectively limit lateral deflection compared with no connection and welded connection between sheet piles. It was found that when the L'/L (L' is the length of the sheet pile edge lock and L is the height of the side wall) equals 1/3, there is no significant change in the lateral ultimate bearing capacity compared with the L'/L value of 1, while the lateral ultimate bearing capacity decreases by 14.3% when L'/L equals 2/3. Additionally, under a certain length of the sheet pile edge lock (L'/L value of 1/3), the sheet pile edge lock position (at the upper, middle, and lower part of the side wall) has a slight effect on the lateral ultimate bearing capacity. In addition, the lateral bearing capacity increases with increasing the aspect ratio of PAF; however, decreases with the increase of the loading eccentricity. During lateral loading, rotating behavior between neighboring sheet piles occurs. The incremental rate in the rotation angle decreases considerably with increasing the lateral load. Furthermore, the failure mode of the PAF changes from a combination of horizontal translation and rotation to pure rotation during loading. Numerical simulation results show that the PAF's accumulated deflection and accumulated rotation increase with increasing the loading cycle and loading amplitude. The excess pore pressure increases rapidly in the early stage of the cyclic loading.

Study on Dynamic Response Characteristics of Circular Extended Foundation of Large Wind Turbine Generator

In order to study the dynamic response characteristics of circular extended foundation of wind turbine in mountainous areas, a 1:10 scaled model test was carried out on the circular extended foundation of 2MW wind turbine, and the deformation characteristics of wind turbine foundation under random wind load were analyzed by ABAQUS numerical calculation. The results show that: (1) The wind turbine foundation has different stress types on the windward side and the leeward side. The components of the windward side foundation are subjected to tensile stress, while the components of the wind turbine leeward side foundation are subjected to compressive stress. (2) The strain of the foundation bolt, the strain of the foundation ring, and the strain of the foundation plate are within the allowable range of material deformation, but the relative deformation of the windward side and the leeward side is quite different. (3) The numerical calculation results of wind turbine foundation under strong wind load are compared with the failure results of scale model experiment, which shows that the overall overturning failure of foundation is a dynamic response mode of wind turbine foundation. In the design and construction, it is necessary to strengthen the research on the windward side and the leeward side and strengthen the anti-overturning design of the wind turbine expansion foundation.

Accumulated displacement and unloading stiffness variations of modified suction caisson in sand under multidirectional lateral cyclic loading

The modified suction caisson (MSC) is an innovative foundation for offshore wind turbines. Model tests were conducted to study the bearing behavior of the regular suction caisson (RSC) and the MSC in sand under multidirectional lateral cyclic loads. Each test includes two stages: (a) the load was applied in the initial loading direction, and (b) the load was applied in the direction which rotate through a deflection angle from the initial loading direction. Results show that in the first stage, the translational motion direction of suction caisson in plane view coincides with the loading direction. In the second stage, the motion direction of the suction caisson doesn't coincide with the loading direction. This phenomenon becomes more obvious under low loading amplitude. When the deflection angle is less than 90°, total accumulated displacement increases with increasing the load cycles. The maximum normalized total displacements for the RSC (ζb = 0.833) and the MSC (ζb = 0.649, 0.833) were obtained when the deflection angles equal 45°, 45° and 30°. When the deflection angle value is higher than 90°, the total displacement decreases with increasing the deflection angle. Changing the loading direction has little effect on the unloading stiffness values for both the RSC and the MSC.

Static-dynamic Tests of Bearing Capacity of Wind Turbine Piles

The paper presents the static load tests (SLT) and dynamic load tests (DLT) of the piles and numerical analyses of piles in interaction with the soil of wind turbines. Pile tests were conducted using the equipment with counter-weights for SLT and an autonomous system for the control of DLT of the piles. Numerical DLT analyses were performed by the signal matching method. Tests proved that the design resistance of piles of SLT and mobilized static resistance of DLT are higher than the design forces obtained from the design of the wind turbine foundations. Also, the need to develop a plan for testing the bearing capacity of piles, concerning large and significant buildings was emphasized.

Coupled Eulerian-Lagrangian finite element analysis of penetration processes in silty clay for bucket foundation

ABSTRACT Bucket foundations, as an innovative type of offshore wind turbine foundation, have been widely adopted for their cost competitiveness and time effectiveness in construction and installation. For a successful installation, the penetration process prediction is crucial. This paper investigates the penetration process of both thin-walled and thick-walled bucket foundations in silty clay through small-scale experiments. Based on the experimental results, empirical formulas of penetration resistance are modified. Next, a Coupled Eulerian-Lagrangian (CEL) numerical method is validated and used to simulate the sinking process of engineering prototypes. The penetration resistance obtained by the calculated and numerical methods is in good agreement. Additionally, through the 3D CEL analysis, the disturbance of bulkheads and skirts on soil plugs is gained. The Eulerian volume fraction shows that the soil surface within the outer compartments is prone to rise to form a soil plug, which will bring out a sudden sharp increase in the penetration resistance due to the contact of lid and soil, and consequently affect the ultimate installation depth.

Hydrodynamic loading effects on the fatigue behavior of gravity-based offshore wind turbine foundations in Brazilian waters

Hydrodynamic loading effects on the fatigue behavior of gravity-based offshore wind turbine foundations in Brazilian waters

Development of an advanced constitutive model for spoil-structure interaction problems

Revitalisation options for mine spoil heaps is an important issue to consider when planning for post-mining land use. One option is the installation of wind turbines on the spoil heaps, which provide a sustainable energy source. This option poses challenges for foundation design due to the nature of the spoil material, which is usually highly heterogeneous due to the way spoils are deposited, and can contain high proportions of high-plasticity cohesive soils (clay). This paper presents results obtained using an advanced constitutive model that was developed to simulate cohesive mine spoil behaviour and presents model results for the prediction of foundation response under both monotonic and cyclic loading. The model is a modified version of the original isotropic bounding surface plasticity model that additionally involves a damaged-based plastic modulus in order to realistically represent soil strength degradation induced by cyclic action. The model is used for the simulation of shallow foundations for onshore wind turbines on a cohesive clay with a heterogeneous (linearly increasing) undrained shear strength (random spatial variability is not considered). Two different loading scenarios of a shallow strip foundation are considered in the paper: a pure moment-vertical loading (M-V), and a moment-horizontal-vertical loading (M-H-V). The effect of the adopted spoil-foundation interface type is also considered: (1) a fully-bonded interface and (2) a tensionless interface. Results demonstrate that, though the vertical bearing capacity is only slightly affected by the interface properties, the moment capacity is shown to be strongly influenced by the predefined contact conditions. For the footing system under M-H-V loading, a dominant footing uplift mechanism is obtained for light structures and the opposite holds for heavily loaded structures, where significant settlement accumulation occurs during cyclic loading. Based on the analysis, the moment bearing capacity of the footing system decreases as the slenderness ratio and/or the vertical safety factor increase, and the opposite mechanism is obtained as the strength heterogeneity degree increases.

Transient analysis of a novel full submersible floating offshore wind turbine with CFRP tendons

Floating offshore wind turbines (FOWTs) to exploit steady and abundant wind resources have attracted a great deal of attention in recent decades. The deep sea is rich in wind energy, but wind farms in these areas face some extreme environmental conditions, e.g., typhoons. A conceptual anti-typhoon full submersible platform (FSP) with carbon-fiber-reinforced polymer (CFRP) tendons, combining the advantages of existing various types of floating wind turbine foundations is proposed. The stability and tendon stresses of the proposed FSP were assessed through a combination of ANSYS-AQWA and FAST, and a comparison with other traditional platforms was made to verify the performance superiority of the proposed FSP. Additionally, the effects of mooring system type on the stability of the FSP under extreme wind speed conditions were investigated. Numerical results showed that the displacements of the proposed FSP in the surge and heave directions were significantly reduced, but in the pitch direction, they were not much different from those of the tension-leg platform. A hybrid mooring system can significantly reduce the surge displacements of the platform, which is essential for a high power output from a FOWT. In general, the proposed FSP is a promising and competitive offshore wind structure.

A simplified method for estimating the permanent accumulated rotation of an offshore wind turbine monopile throughout its design life

The permanent accumulated rotation is of great importance to the design of monopile foundations for offshore wind turbines because it is critical to the final dimensions of the monopile as well as its cost. Although design specifications require that the permanent accumulated rotation meet tolerances, they do not provide engineers with an appropriate method to calculate this value. This paper proposes a simplified method for estimating the permanent accumulated rotation of a monopile throughout its design life. To establish this method, a series of 1-g model tests were conducted in medium-dense sand to investigate the accumulative tendency of monopile rotation under typhoon and non-typhoon conditions. The results showed that the total accumulated rotation was mainly caused by typhoon events, further indicating that the static rotation generated by the maximum load magnitude among typhoon load sequences may maintain a certain proportional relationship with it. Then, the procedure for determining the two parameters in the method was illustrated by an NREL 5 MW wind turbine mounted on a monopile at a water depth of 28 m in the northern South China Sea. This method can provide a convenient tool for calculating the permanent rotation of a monopile foundation throughout its design life.

Unstated impacts of the green energy industry on the habitat of a coastal delphinid: Turbid‐turbulent wakes induced by offshore wind turbine foundations

Abstract Offshore wind farms (OWFs) are a green energy solution to reducing CO2 emissions, but often encroach on major habitats of coastal delphinids. Clustered OWF construction in shallow waters may have cumulative negative impacts on coastal delphinids and marine ecosystems. Although the impacts of piling noise have been discussed extensively, the potential impacts of oceanographic alterations induced by wind turbine foundations are rarely discussed. Two OWFs were constructed near the core habitat of a critically endangered subspecies of humpback dolphin (Sousa chinensis taiwanensis) in the eastern Taiwan Strait. Turbid‐turbulent wakes induced by wind turbine foundations were inspected using Landsat images. Sighting rates of the humpback dolphin that were obtained from boat‐based line‐transect surveys near the northern OWF (OWFn) since the late 2000s were examined. Indicators of surface turbidity were used to highlight turbid‐turbulent wakes extending in the direction of the main current. Sighting rates of the humpback dolphins near the OWFn have been declining since the 2000s, but decreased significantly after the OWFn was constructed and began operating. The long‐term decline in sighting rates implies an alarming decline in the population size of humpback dolphins. Decreased sighting rates after the OWFn installation indicate the reduced utilization of a once‐important habitat. Offshore wind farm construction may influence prey abundance and composition and cause acute stress from piling noise; operational noise may mask the chorus of prey fish and turbid‐turbulent wakes may disturb the benthic ecosystem. Site selection committees for future OWFs should avoid the habitats of coastal delphinids and areas subject to strong ocean currents. Ecological impact assessments for OWF projects should examine the impacts of turbulent wakes on benthic ecosystems and of acoustic disturbance on marine mammals. Long‐term environmental monitoring programmes are recommended, which can be funded by wind energy companies as a compensatory measure.

Effects of scour on the first natural frequency of monopile in different layered foundations

Scour is ubiquitous in offshore wind farms and threatens the stable state of the natural frequency of offshore wind turbines. The foundation of offshore wind turbines is a layered seabed, which mainly consists of clay, silt, and fine sand layers. However, little published literature can be found on the natural frequencies of monopiles in layered foundations with scour. The objective of this study was to reveal the influence of scour in layered foundations on the natural frequencies of monopoles and study its mechanism based on the dynamic characteristics of layered foundations. Therefore, based on an actual layered seabed, three typical layered foundations were established, and the first natural frequency of the monopile in terms of sensitivity to scour as well as the ability to resist continuous scour in the layered foundations were revealed. Moreover, the experimental results were analysed based on the stiffness and damping variations of the layered foundations under scour conditions. Importantly, a competitive relationship between the stiffness and damping of the layered foundations on the variation of the first natural frequency with scour was found and elaborated for the first time. The competitive relationship was influenced by the proportion of clay in the layered foundation.

An Investigation of the Effect of Utilizing Solidified Soil as Scour Protection for Offshore Wind Turbine Foundations via a Simplified Scour Resistance Test

Offshore wind power is rapidly developing as a source of clean energy. However, as local scour of the foundation of an offshore wind turbine can create serious safety risks to the normal operation of the turbine, it is necessary to protect the foundation from scour. In this paper, a new scour protection countermeasure using solidified soil has been investigated via an updated apparatus for a simplified scour resistance test (SSRT). Two types of tests were carried out: an unconfined compressive test to determine geotechnical parameters and an SSRT test to reflect the scour resistance of the soil samples. The results show that unconfined strength is approximately related to the critical flow velocity of the scour resistance as a power function. Soil samples having an unconfined compressive strength of 300 kPa can resist erosion under flow conditions above 3.14 m/s after solidification. In addition, the solidification state of the solidified soil has a great impact on the scour resistance of the soil sample, and the critical scour velocity of the final solidified soil is increased by 80–150% as compared to an initial solidified soil having the same final unconfined strength. These results suggest that attention should be paid to the state of the solidified soil during the construction process. The engineers should control the ratio of cement, water, and soil of the solidified soil according to the hydraulic parameters at the time of construction so that no great loss of solidified soil will occur during the construction process.

Energy flux analysis for quantification of vibratory pile driving efficiency

In this paper, the energy flux in a pile modeled as an elastic shell, is studied theoretically and experimentally. Based on this analysis, a new procedure is proposed to quantify the pile installation efficiency. This procedure is of importance for vibratory installation of the foundations of offshore wind turbines and it is believed to be the first procedure that relies directly on the energy propagating down the pile rather than the energy supplied by the vibratory shaker. The proposed approach is tested on piles installed by two distinct vibratory techniques, i.e. the axial vibratory driving and the recently developed Gentle Driving of Pile (GDP) method. The field data obtained during an experimental campaign were analyzed with the proposed energy flux approach. The cumulative energy flux in the pile normalized by the energy input of the shaker is found to be the best measure for quantification of the installation efficiency. Correspondingly, the main proposition of this paper is that the installation efficiency will be maximized provided that the normalized cumulative energy flux is at its maximum.

Prediction of Chloride Induced Corrosion Using Dynamic Bayesian Updating and Monitoring Data

Corrosion of steel rebars represents the main cause of deterioration of reinforced concrete (RC). Corrosion of steel rebars occurs with the break of the passivation protective layer on the surface of rebars due to carbonation or chloride contamination of concrete cover. Estimating the risk of corrosion initiation in time can provide reliable information to protect rebars and plan maintenance of RC structures. This paper presents a framework for a probabilistic modelling of corrosion initiation in RC structures induced by chloride contamination. The approach is based on a dynamic Bayesian network updated over time using corrosion monitoring data. The dependencies between variables are represented in a directed graph and joint probability density functions are computed using Fick’s law and Monte Carlo sampling. The approach is illustrated with the CorroVolta® sensor, an embedded corrosion sensor designed and manufactured at Socotec Monitoring. Results prove the feasibility of using the dynamic Bayesian approach for corrosion monitoring of RC structures and would support a reliable decision basis for reinforced-concrete bridges and wind-turbine foundations.

Transient analysis of the slamming wave load on an offshore wind turbine foundation generated by different types of breaking waves

In this study, a computational fluid dynamics model was employed to simulate the impact of a breaking wave on the foundation of an offshore wind turbine. The accuracy of the numerical simulation was validated in comparison with published experimental results for the Large Wave Channel (GWK) at Hannover, Germany. To understand the influence of the wave-breaking location on the monopile support structure, a series of shoaling waves with different spilling and plunging breaking wave types were considered in terms of the surf-similarity parameter (ξ0) and relative breaking distance (xb′). After processing the total wave load with an empirical mode decomposition method and a low-pass filter, the instantaneous nature of the slamming wave load generated by different breaking wave types was obtained. Subsequently, an exponential-type refined dynamic model was established to consider the characteristics of the filtered slamming wave load. ξ0 and xb′ were found to be dominant factors for the run-up phenomena, slamming characteristics, and wave-dependent parameters. The results demonstrated that these two coefficients can be applied to determine the nature of the run-up and slamming wave loads.

Soil-structure interaction of a laterally loaded hybrid foundation for offshore wind turbine

With the ramping up of the capacity of the offshore wind turbine (OWT), the water depth, and the weight of the superstructure, the need of foundation structure with higher performance but reasonable cost demands innovation from geotechnical and structural engineering. The novel hybrid foundation takes advantage of the mature monopile technology, and innovatively integrates a loading wheel-bucket component around the monopile in the upper layer of soil. The addition of the integrated hybrid component is proven to enhance the overall performance of the foundation by previous research. The soil-structure interaction (SSI) of the hybrid foundation is the key to optimize the design of the structure. In this paper, a series of finite element (FE) analysis of the hybrid foundation under monotonic load are performed. The results of the FE analysis are compared with that of the geotechnical centrifuge tests. The soil pressure distributions on the pile and the hybrid component are investigated. The variation of the soil pressure profile with the increase of rotation angle is also studied. The soil resistance on the skirt of the bucket component is illustrated in the analysis. The FE models also prevail the failure mechanism of the hybrid foundation.

A test method to study scour effects on the lateral response of rigid piles in sand under real scour conditions

Rigid piles are common foundations for offshore wind turbines. Scour around piles leads to the loss of surrounding soil, which seriously threatens the safe operation of offshore wind turbines. However, most of the existing experimental studies on the scour effects on marine foundations are carried out in the test box without a hydraulic environment, resulting in deviations between the test results and the actual ocean engineering. Therefore, a test method to study scour effects on the lateral response of rigid piles is proposed. The key to this test method lies in the design of four types of parameters: pile, soil, hydraulic (wave and current), and load. Considering the characteristics of scour tests and scour effects tests, the design method of the four types of parameters is proposed. Combined with an example, the test method is described in detail. Subsequently, the defects of this test method are discussed. The research shows that this test method is a good attempt to study the loss of lateral ultimate capacity and the increase of cyclic cumulative deformation of rigid piles under real scour conditions.

Bearing failure mechanism of rock-socketed monopile foundation for offshore wind turbine in weathered-granite seabed

Large-scale constructions of offshore wind power are carried out in southeast coast of China, where the seabed is covered in weathered granite. However, the bearing capacity and failure mechanism of monopile foundation in weathered-granite seabed are rarely researched. In this paper, the stress-strain characteristics and strength parameters of granite with different weathering degrees were obtained through laboratory tests, and the pre-strain test method was used to study mechanical parameter variation of moderately weathered granite in the crushing process. Experiment found that the strength parameters decrease mainly after the strain reaching 50% of the ultimate strain of moderately weathered granite. In addition, the bearing capacity and failure mode of rock-socketed monopile under unidirectional horizontal loading was investigated numerically. Analysis showed that the failure mode of rock-socketed monopile under unidirectional horizontal loading is progressive and presents two obvious failure zones. More critical, the existence of vertical loading effectively improves the horizontal bearing capacity of monopile foundation in the weathered rock seabed, and the failure envelope of the monopile foundation in H-V loading space is elliptic-shaped. This provides further understanding of the mechanical properties of submarine weathered granite, and the helpful reference for stability evaluation of rock-socketed monopile foundation in weathered granite seabed.

Monotonic and cyclic properties of silty sand and sandy silt for foundation design of offshore windfarms

This paper presents results of a series of monotonic and cyclic direct simple shear laboratory tests performed on reconstituted samples of two silty sands and one sandy silt. The laboratory program aimed to determine the cyclic shear strength and deformation characteristics of soils to be used in the offshore wind turbine foundation design. Various cyclic contour diagrams including cyclic shear strength and average and cyclic shear strains for different number of cycles were provided. The effect of strain rate for drained monotonic tests and overconsolidation ratio effect on both monotonic and cyclic properties were discussed. This study indicated that the fines content plays an important role in static and cyclic shear strength of silty soils, in addition to mineralogy and coefficient of uniformity.

Centrifuge modelling of monotonic and cyclic lateral responses of a hybrid monopile-bucket foundation for offshore wind turbines

Various hybrid systems combined with several foundation elements have been proposed to improve the lateral performance of monopiles. Among them, the hybrid monopile-bucket foundation which consists of a traditional monopile and a wide-shallow bucket has received more and more attention and has been used in offshore wind pilot projects for supporting OWTs. However, deep insights into its monotonic and cyclic responses are still lacking. This study aims to understand the monotonic load-bearing, the evolution of cumulative displacement, stiffness and bending moment of the hybrid foundation in soft clay. A series of centrifuge model tests were conducted to simulate a hybrid monopile-bucket foundation and a monopile subjected to lateral monotonic and multi-stage cyclic loading. The monopile with the identical pile diameter and embedded length is used as a benchmark. The experimental results show that by the addition of the bucket, the hybrid foundation shows a 30.1% increase in ultimate capacity. Compared to the monopile, the cumulative displacement, unloading stiffness and the growth of bending moment of the hybrid foundation during cyclic can be reduced by up to 25%, 30% and 35.3%, respectively, implying the effectiveness and superiority of the hybrid foundation. Apart from the centrifuge tests, supplementary three-dimensional finite element analyses have also been performed to reveal the bearing mechanism and the sharing ratios of external load and moment carried by the single pile and the bucket of the hybrid foundation. The test and numerical results presented in this study are expected to provide design references for further practical applications of the hybrid foundations.