Large Diameter Monopiles Research Articles

The Behaviour of a Large-Diameter Monopile under Two-Way Lateral Cyclic Load

The Behaviour of a Large-Diameter Monopile under Two-Way Lateral Cyclic Load

Study on time effect of bearing capacity of Offshore Large Diameter Monopiles

In order to improve the evaluation ability of bearing capacity of offshore large-diameter monopile, the initial high strain detection and repeated high strain detection with an interval of 10-84 days were carried out on 6 large-diameter monopiles with a diameter of 7.2m-7.4m in the offshore wind field dominated by cohesive soil layer. The results show that the time-dependent increase of tip resistance, pile side resistance and total resistance of large-diameter monopiles in the same offshore wind farm has great discreteness, and the axial force increment of pile has a consistent change trend. This paper puts forward the prediction interval of 95% guarantee rate of bearing capacity increment of offshore large-diameter single pile based on depth, which provides a basis for the design of large-diameter monopile.

Effect of nearby strip loading on lateral capacity of offshore large-diameter monopiles in clay slope

This study is devoted to investigating the effect of nearby strip loading on the lateral bearing capacity of offshore large-diameter monopiles at the crest of marine clay slope by the finite element limit analysis (FELA) method. After the successive validations by cases of rigid piles and strip footings at the clay slope crest, the safe domains for the laterally loaded large-diameter monopiles are defined by the failure envelopes relating the dimensionless lateral bearing capacity H u/(c u D 2) to dimensionless strip loading q/c u. Particularly, all failure envelopes are depicted by closed-form expressions and design charts can then be given for a quick and straightforward assessment of dimensionless lateral bearing capacity for a wide range of frequently-encountered cases. In addition, the interaction between the pile bearing capacity and nearby strip loading is characterized by introducing the relationship of normalized parameters H u/(c u D 2) and q/c u, then graphically explained by plastic multiplier contour plots. Several kinds of representative curves are given successively to further clarify the effects of dimensionless factors on the interaction.

Predicting accumulated deformation of offshore large-diameter monopile using hollow cylinder apparatus

Offshore wind turbine structures are commonly subjected to cyclic lateral loading, from waves and wind. The cyclic loading can cause an accumulated deformation of monopile. This study investigates cyclic accumulated deformation of offshore monopiles by using soil cyclic shear tests and numerical simulation. A series of soil axial-torsional bi-directional cyclic load tests are conducted to obtain the cyclic behavior of sand with different relative densities. The test results are analyzed in terms of soil secant modulus from accumulated shear strain, and the results are then used to improve a degradation stiffness method. This improved method was verified by comparing with a pile test. Finally, the cyclic accumulated deformation of large-diameter monopile was studied in loose over dense sand, medium-dense over dense sand and dense sand by using this improved method. The results show that the secant modulus decreases rapidly in the initial cycles of loading, and then the rate of decrease diminishes and remains below an asymptote. The depth of the max bending moment increases with increasing the cyclic number.

Numerical model for dynamic installation of large diameter monopiles

This paper presents the numerical code VibPile for simulation of the nonlinear dynamic response of large monopiles under harmonic loading and installation by vibration or impact driving. VibPile is based on a nonlinear FE model of the pile-soil interaction along the shaft and the tip by elasto-plastic springs representing the near field and elasto-dynamic elements (spring and dashpot) for the far field. While for the shaft friction and tip resistance the classical engineering solutions, such as those by standards or guidelines (e.g. API) and literature, are used in the numerical simulations, new computational models are developed for the dynamic response of the soil inside the pile and the far-field elasto-dynamic springs.What primarily differentiates the present model from the existing ones is the treatment of the soil inside the monopile together with the stiffness at the pile tip. Most existing models are based on the solutions for piles with solid sections or pipe piles in which the soil inside the pile follows the pile vibrations like a solid section. For large diameter monopiles commonly used for offshore wind turbines, both the soil response inside the pile and the stiffness of the pile tip are different. The developed model is verified against the vibro-pile test data collected at Altenwalde, Germany. The results include axial pile strains, accelerations, and shear stresses along the pile shaft and at the pile tip. Both the strains and accelerations as well as the rate of pile driving measured at Altenwalde are reproduced by VibPile with reasonable accuracy. Sensitivity analyses are presented to highlight the effects of the shear wave velocity and shear strength at the pile-soil interface on the pile driving rate. Moreover, the effect of the natural period of the inner soil on the pile driving is investigated.

Responses of model monopile to cyclic lateral loadings in clay

Large diameter monopiles are usually used as the foundation of offshore wind turbines. Monopile foundations are subjected to laterally cyclic marine environmental loadings which tends to accumulate displacements and rotation of the monopile. To preliminarily evaluate the cyclic responses of monopile in marine clay, 1g model tests have been conducted to combine with reported centrifuge tests to efficiently illustrate the behavior. This study investigates the laterally monotonic and cyclic responses of the model pile embedded in kaolin clay. As expected, obvious differences are observed between results of the 1g model tests and the centrifuge tests owing to the effect of stress levels, but important quantitative relations between results of these two types are revealed: two equations closely related to cyclic loading magnitude ratio ζb with same cyclic loading symmetry ratio ζc, could be qualitatively obtained to predict the pile responses in centrifuge tests by using the result of 1g model tests. The normalized unloading stiffness of model pile in 1g test is less than that of obtained from centrifuge tests with the same ζb values. This result not only highlights that it is necessary to account for the stress level of soil to understand cyclic lateral responses of monopile foundations, but also reveals that it is possible to qualitatively insight responses of monopile through 1g model tests.

An analysis of monopile with super large diameter

With the development of ocean engineering, monopiles with a large diameter foundation have gradually replaced those with small diameter and been applied to more and more ocean structures. The responses of monopiles with small diameter under wind, wave, current and seismic loads have been extensively studied. However, not too much work on large-diameter monopile (>9 m) has been carried out. In this study, finite element method (FEM) is adopted to study a monopile with a diameter of 9 m, and the mechanical responses are presented. Some mechanical responses are presented by ABAQUS.

Analysis of the Dynamic Characteristics of the Top Flange Pile Driving Process of a Novel Monopile Foundation without a Transition Section

As the most widely used foundation type, the pile-driving capacity of large-diameter monopile foundations and the hammer force of pile top flanges is increasing, as are the stress and strain of traditional flange structures under the resistance to the cyclic impact load of pile hammers. This paper conducts an empirical study on the Rudong 150 MW offshore wind farm project in Longyuan, Jiangsu Province. The numerical simulation and calculation method of large-diameter pile fatigue damage is mastered and proposed, and a simulation analysis and comparison is conducted, as is an analysis of the sensitivity and feedback of the key software parameters. Based on the results of the above analysis, a pile simulation analysis of the offshore wind power projects under construction and those that are about to be started is performed. The pile hammer stroke energy and pile top hammer stroke process curve are extracted. According to the time range curve of limit and hammer stroke forces, the flange limit stress and distribution of the pile roof are simulated. Combined with the process characteristics of large-diameter pile sinking and analysis in the case of high hammer energy and high acceleration, the effect of the impact on the sensor is discussed. The results show that the instantaneous loading analysis could load at the flange of the pile, the maximum instantaneous stress was above 500 MPa, and the final maximum stress was approximately 307 MPa. Different loading methods vary greatly in the calculation results of the structure, so choosing the appropriate loading simulation method is key to pile sinking analysis.

A Numerical Analysis on Lateral Resistance of Pile–Bucket Foundation for Offshore Wind Turbines

The large-diameter single pile has been widely used in marine wind turbine foundations. In order to improve its lateral bearing capacity, the suction bucket foundation (BF) can be used as a reinforcement method, which surrounds the periphery of the monopile foundation (PF), forming a new wind turbine foundation, i.e., pile–bucket foundation (P–BF). By using the general finite element software ABAQUS, in this paper, several numerical models are established to investigate the influence of pile diameter and bucket diameter on the lateral bearing capacity of large-diameter monopiles. The numerical results show that the internal force distribution along the pile shaft for the case of P–BF is similar to the case of PF. Compared to bucket height, bucket diameter has more of an effect on lateral capacity and the p-y curve in the P–BF. In the combined P–BF, the buckets can provide lateral bearing capacity, and the piles can provide anti-overturning moments, resulting in higher lateral bearing capacity. In this paper, the p-y curve in the API specification is modified based on the results from the finite element simulation. The modified p-y curve fits well with the results of the finite element calculation and can be used as a reference for the design of the P–BF in actual engineering.

Centrifuge model tests on large-diameter monopiles in dense sand subjected to two-way lateral cyclic loading in short-term

To evaluate the lateral resistance of rigid monopiles for wind turbines in dense sand under lateral cyclic loading, centrifuge model tests are performed, focusing on the base resistance and degradation of the soil resistance under two-way lateral cyclic loading in the short-term. The slenderness ratio (embedded pile length to diameter) is varied from 3.75 to 8 and the loading frequency is in the range of 0.002 to 0.4 Hz in the prototype scale. Under cyclic loading with a maximum horizontal displacement of 5% of the pile diameter, the build-up of excess pore water pressure is observed, but the maximum value of the average excess pore water pressure ratio is around 50% in the steady-state for dense sand whose relative density is 80%. A simple analytical model for the rigid piles, considering the base resistance, is derived and then used to quantify the significance of the resistance at the pile base and the degradation of the soil resistance under cyclic loading. When the slenderness ratio is less than 5, a significant contribution of the moment resistance at the base is confirmed. The estimation of the degradation of the horizontal subgrade reaction coefficient using the simple analytical model suggests that, through cyclic shear tests for the determination of the deformation properties of the soil in a laboratory, it is possible to estimate the degradation of the soil stiffness and the parameters for the reduced sway-rocking type of foundation model.

Seismic Analysis of 10 MW Offshore Wind Turbine with Large-Diameter Monopile in Consideration of Seabed Liquefaction

With the increasing construction of large-scale wind turbines in seismically active coastal areas, the survivability of these high-rated-power offshore wind turbines (OWTs) in marine and geological conditions becomes extremely important. Although research on the dynamic behaviors of OWTs under earthquakes has been conducted with consideration of the soil-structure interaction, the attention paid to the impact of earthquake-induced seabed liquefaction on OWTs supported by large-diameter monopiles remains limited. In view of this research gap, this study carries out dynamic analyses of a 10 MW OWT under combined wind, wave, and earthquake loadings. This study uses a pressure-dependent multisurface elastoplastic constitutive model to simulate the soil liquefaction phenomenon. The results indicate that the motion of the large-diameter monopile leads to more extensive soil liquefaction surrounding the monopile, specifically in the zone near the pile toe. Moreover, compared with earthquake loading alone, liquefaction becomes more severe under the coupled wind and earthquake loadings. Accordingly, the dynamic responses of the OWT are apparently amplified, which demonstrates the importance of considering the coupling loadings. Compared with wind loading, the effect of wave loading on the dynamic response and liquefaction potential is relatively insignificant.

Impact of cyclic strain accumulation on the tilting behaviour of monopiles in sand: An assessment of the Miner’s rule based on SANISAND-MS 3D FE modelling

Offshore monopiles accumulate permanent tilt under long-lasting cyclic environmental loads. Accurate prediction of monopile tilt is key to assessing their serviceability, and requires a fundamental understanding of loading history effects. While both experimental and numerical studies are shedding light on this matter, this work uses step-by-step implicit 3D FE modelling to investigate loading history effects in the response to cyclic lateral loading of monopiles in sand and to identify links between local soil behaviour and relevant features of global pile behaviour. For this purpose, the recently developed SANISAND-MS model is adopted to achieve a reliable simulation of sand’s cyclic ratcheting. In particular, the validity of an up-scaled Miner’s rule for monopile tilting under multi-amplitude cyclic loading is assessed based on the results of 3D FE parametric analyses, with emphasis on the role played by the engineering idealisation of random environmental loading. The validity of such a rule has been numerically investigated both in terms of local soil element response and global foundation behaviour — for the particular case of a large-diameter monopile. In respect, the effect of the loading history idealisation is presented, and it is concluded that Miner’s rule does not always rigorously apply to all the cases considered herein. The translation of irregular loading histories into a regular version with loading packages sorted in ascending amplitude order is shown to be a reasonable approach, at least when the possibility of cyclic pore pressure build-up is disregarded.

Evaluation of sand p–y curves by predicting both monopile lateral response and OWT natural frequency

Abstract Extending the use of the p–y curves included in the regulation codes API and DNV to design large-diameter monopiles supporting offshore wind turbines (OWTs) was unsuccessful as it resulted in an inaccurate estimation of the monopile behavior. This had prompted many investigators to propose formulations to enhance the performances of Winkler model. In this paper, two case studies are considered. A case consisting of an OWT at Horns Rev (Denmark) supported by a monopile in a sandy soil was studied first. Taking the FEA using ABAQUS as reference, results of WILDOWER 1.0 (a Winkler computer code) using the recently proposed p–y curves giving design parameters were plotted and evaluated. In order to see the ability of proposed p–y curves to predict the monopile head movements, and consequently the first natural frequency (1st NF), a second case study consisting of a monopile supporting an OWT at North Hoyle (UK) was selected. The monopile head stiffness in terms of lateral, rocking, and cross-coupling stiffness coefficients, necessary for the 1st NF, were computed using both ABAQUS and WILDPOWER 1.0. Comparisons with the measured 1st NF showed that with the exception of one p–y model, none of other proposed Winkler methods is able to predict accurately this parameter.

Numerical investigation of the monotonic drained lateral behaviour of large-diameter rigid piles in medium-dense uniform sand

This paper presents a numerical investigation of the monotonic lateral response of large-diameter monopiles in drained sand with configurations typical of those employed to support offshore wind turbines. Results from new centrifuge tests using instrumented monopiles in uniform dry sand deposits are first presented and used to illustrate the suitability of an advanced hypoplastic constitutive model to represent the sand in finite-element analyses of the experiments. These analyses are then extended to examine the influence of pile diameter and loading eccentricity on the lateral response of rigid monopiles. The results show no dependency of suitably normalised lateral load transfer curves on the pile diameter and loading eccentricity. It is also shown that, in a given uniform sand, the profile with depth of net soil pressure at ultimate lateral capacity is independent of the pile diameter because of the insensitivity of the depth to the rotation centre for a rigid pile. A normalisation method is subsequently proposed which unifies the load–deflection responses of rigid piles with different diameters at a given load eccentricity.

Effect of Reinforced Bucket on Bearing Capacity and Natural Frequency of Offshore Wind Turbines Using Pile–Bucket Foundation

Large‐diameter monopiles have been widely used for constructing offshore wind turbines. The bearing capacity of a monopile foundation is a significant research problem. In this study, a new type of foundation, known as the pile–bucket foundation, was investigated to improve the bearing capacity of monopiles. A finite element software was used for establishing several numerical models of monopile and pile–bucket foundations to analyze the reinforcement afforded by the bucket attached to the monopile foundation. Furthermore, considering that offshore wind turbines are prone to resonance under the excitation of wind and wave loads, the natural frequencies of the monopile and pile–bucket foundations were determined and compared using both analytical and numerical methods. The results show that compared to the monopile foundation, the pile–bucket foundation has a significantly higher bearing capacity, mainly for large bucket diameters. The natural frequencies of the pile–bucket foundations are slightly higher than those of the monopile foundations. The addition of the bucket can effectively improve the natural frequency without changing the diameter of the monopile and thus saving the foundation cost.

Lateral load response of large-diameter monopiles in sand

Due to the increasing demand for renewable clean energy, there has been rapid growth in the development of offshore wind energy resources in recent years. The majority of offshore wind turbines are founded on single, large-diameter, open-ended pipe piles known as monopiles. The design of these piles is often done using the p–y method, which models the soil simply as non-linear springs placed along the pile length and assumes the pile to behave as a one-dimensional beam. To develop a design method that accounts for three-dimensional pile–soil interaction and that is applicable to general conditions, a series of finite-element (FE) analyses covering a wide range of pile dimensions, wall thicknesses, slenderness ratios, load eccentricities, sand types and sand relative densities were performed using an advanced sand constitutive model. In this paper, a set of equations is proposed that can be used to estimate the critical pile length, Lcrit (the pile length beyond which the pile lateral capacity no longer increases), the lateral capacity, H, and the lateral load–rotation curve for monopiles. The proposed method produces estimates of the lateral capacity of monopiles that are in very close agreement with those from the FE analyses.

Influence of soil scour on lateral behavior of large-diameter offshore wind-turbine monopile and corresponding scour monitoring method

Soil scour is a major cause of foundation failures for offshore wind turbines (OWT), and has emerged as a significant concern for safety analysis and design. Most previous studies focus on investigating scour mechanisms but not their consequences. Therefore, this paper mainly investigated the scour effects on the lateral behavior of large-diameter offshore monopile and the corresponding scour monitoring method. Firstly, the effect of the soil scour on the lateral behavior of large-diameter monopile foundation of OWT was investigated numerically. It revealed that the horizontal load-displacement response, the bending moment and horizontal displacement of pile body, and the p-y curve were significantly affected by scour. However, the soil displacement field was not disturbed and remained “two-failure zone,” i.e., wedge failure zone near the ground surface and rotational failure zone near the pile toe. Furthermore, a monitoring method for scour depth of soil seabed around monopile foundation based on natural frequency was proposed and verified through model tests. It was found that test environment and conditions had little effect on the natural frequency of the measuring rod, which only depended on its free end length. This provides a theoretical basis for in-situ monitoring of soil scour for offshore wind turbine foundation.

Effect of the installation process on monopile lateral response

Most offshore wind turbines (OWTs) are supported by large-diameter monopiles, installed by impact driving. OWTs are dynamically sensitive structures, with a narrow design range for the eigenfrequency. Fatigue and serviceability limit states can be critical in design, so the foundation stiffness plays an important role. The pile installation process results in changes to the soil state, investigated here through large-deformation numerical analyses using the coupled Eulerian–Lagrangian approach with a hypoplastic constitutive relation to model the sand. The soil state is then mapped to a small-strain model to evaluate the effect of the installation process on the lateral response. Complementary physical modelling results from monopile installation and lateral load testing in a centrifuge provide confidence in the numerical model. Analyses for two different sands highlight commonalities and differences. Comparison with results featuring wished-in-place or jacked installation illustrates the effect of changes in the soil state on the lateral response. The results characterise the changes in soil state and allow quantification of their effects as impact driving tends to produce a stiffer lateral response, with the expected variations due to pile dimensions and sand relative density. Further research will provide insights into the role of pore fluid response during impact driving and investigate vibro-driven piles.

Experimental study on the effect of second order wavemaker theory on the response of a flexible large diameter monopile in irregular sea

Motivated by the need for larger offshore wind turbines, large diameter monopile foundations are being developed. To ensure safe design, there is a need for model testing and validation of hydrodynamic load models. Scaled model tests with a piston-type wavemaker commonly apply first order wavemaker theory for irregular waves. This approach results in the generation of second (and higher) order spurious (also known as parasitic) free waves in the tank. In this study, the effect of superharmonic spurious waves on the response of a monopile with eigenfrequency close to three times the wave peak frequency is examined experimentally. The bending moment response statistics are not found to be significantly affected by the wavemaker correction. Different wave breaking patterns are observed for individual events, but our results do not indicate any clear relation between breaking waves and the wave generation technique.

Performance of tripod foundations for offshore wind turbines: a numerical study

Around 80% of offshore wind turbines (OWTs) are founded on deep seabeds using large-diameter monopiles that reach a length up to 60 m. In a race to seek cost-effectiveness, tripod foundations made with suction caissons represent a promising solution. The deformation mechanism induced by cyclic lateral loading on the tripod bucket embedded in sand reveals recovery of dynamic stiffness with the number of cycles that influences the subsequent dynamic response of the structure–foundation complex. This ‘self-healing’ and the coupled increase of settlement rate are caused by uneven change in soil fabric and density among the buckets. However, the capability of the recently developed constitutive models to predict this effect is limited. To address this issue, a series of centrifuge tests are herein back-analysed with a three-dimensional finite-element model. The implicit approach has been implemented using an advanced rate-independent hypoplastic model coupled with the conventional intergranular strain theory. The model captures the magnitude of the peak and residual cumulative rotation. The results indicate that the push–pull mechanism turns into a horizontal translation, with active wedges formed behind the pulled bucket. After about 100 cycles, due to significant redistribution of mean stresses, wedges are formed around both pushed and pulled buckets.