Response Of Monopile Research Articles

An experimental study on the parameters affecting the cyclic lateral response of monopiles for offshore wind turbines in sand

The design of monopile foundations for offshore wind turbine structures is dominated by requirements resulting from serviceability and fatigue limit state. To fulfil these criteria, the load deflection-behaviour and therefore long-term accumulations of permanent deflections and rotations of the monopile foundation due to cyclic occurring wind and wave loads have to be predicted. In this paper a brief overview on current design code practice as well as other proposed methods for the prediction of accumulated deflections or rotations is given. Further, the results of a systematic model test study dealing with the response of monopiles to lateral cyclic loading in medium dense sand at different cyclic load ratios, load eccentricities and pile embedment lengths are described and evaluated. The observations of the model test study are supplemented by results of a second test series involving the visualisation of displacement fields around laterally loaded piles by means of particle image velocimetry. Based on the findings and the results of previous experimental investigations, recommendations regarding the prediction of displacement accumulations for large diameter monopiles in sand are given.

Cyclic performance of a monopile in spatially variable clay using an advanced constitutive model

The performance of monopiles in cohesive soils is of great interest for future offshore wind farm developments, particularly under the cyclic loads experienced in the ocean environment. Clay behaviour during undrained cyclic loading is complex and involves the accumulation of plastic strains, generation of excess pore-water pressures and degradation of initial stiffness. In this paper, the cyclic performance of a laterally-loaded monopile in spatially variable clay is investigated for the first time. A kinematic hardening constitutive model is used in a 3D finite element analysis to capture the hysteretic stress-strain behaviour of the clay. The monopile is installed in overconsolidated London Clay, which is present at several offshore wind farms in the Thames Estuary. The finite element model is coupled with random field representations of initial stiffness and clay structure. The statistical characterisation of the random fields was undertaken considering parameter ranges observed in laboratory tests. Under one-way cyclic loading, the monopile showed ratcheting behaviour, where pile rotation accumulates with increasing numbers of load cycles. The cyclic secant stiffness also increased due to the generation of negative excess pore-pressures in the clay. This behaviour occurred in both homogeneous and spatially variable clay. The monopile was also subjected to an extreme dynamic event and the soil response around the monopile showed increasing variability in stress-strain response and generation of excess pore-water pressure over time as plastic strain accumulated. However, the overall behaviour of the foundation was governed by a spatial average of the mobilised clay. The range in monopile response demonstrates how the natural spatial variability of clay can have a strong influence on monopile performance.

Monopile rotation under complex cyclic lateral loading in sand

Monopiles supporting offshore wind turbines experience combined moment and horizontal loading which is both cyclic and complex – continuously varying in amplitude, direction and frequency. The accumulation of rotation with cyclic loading (ratcheting) is a key concern for monopile designers and has been explored in previous experimental studies, where constant-amplitude cyclic tests have shown rotation to accumulate as a power-law with cycle number. This paper presents results from laboratory tests in dry sand, which explore the rotation response to constant and variable amplitude, unidirectional and multidirectional cyclic loading. The tests are designed to inform model development and provide insight into key issues relevant to monopile design. Unidirectional tests show behaviour consistent with previous studies and provide a basis for interpreting more complex tests; multidirectional tests provide new insight into the monopile response to multidirectional cyclic loading; and multi-amplitude storm tests highlight salient features of the response to realistic loading. Tests are conducted in both very loose and dense sand, where the behaviour is found to be qualitatively similar.

Seismic response of monopiles to vertical excitation in offshore engineering

This paper studies the dynamic response of a large diameter and thin-walled monopile, embedded in a partially saturated porous seabed with saturation degrees larger than 95%, under a seawater layer, subjected to vertically incident P wave excitations. The ground is a semi-infinite homogenous half-space with a horizontal surface. The solution of the coupled water-pile-soil vibration problem is constructed using a rigorous semi-analytical method. By considering the coupled boundary conditions at the pile-soil and water-soil interfaces, the earthquake induced response is reduced to integral equations and solved numerically. Results for the free field soil displacement and kinematic interaction factor are obtained, which are different from the results reported in the literature for onshore earthquake engineering applications as these do not have the effects of the water layer and the vibration induced dynamic water pressures. The existence of a water layer will always decrease the vertical response of the seabed, but may amplify the monopile's displacement near some resonant frequencies, compared to the case of no water. These results are useful in understanding the kinematic interaction of soil and monopile in offshore engineering applications.

Nonlinear lateral response of offshore large-diameter monopile in sand

This paper presents a nonlinear analysis method for laterally loaded large-diameter monopiles. The method accounts for the effects of pile diameter and soil deformation on the modulus of horizontal subgrade reaction by considering a modified modulus of horizontal subgrade reaction. The solution considers the force and moment equilibrium of soil-pile system to calculate the response of the monopile to lateral loads. The influence of the soil side friction reaction on the ultimate horizontal resistance is evaluated considering the degradation of modulus of horizontal subgrade reaction due to large pile displacement. The predictions of the developed method compared favorably with the results obtained from full-scale field tests, centrifuge model tests and laboratory model tests as well as existing analytical methods. The developed analytical method is then applied to study the rigid rotation behaviour of offshore monopiles. The results demonstrated that under lateral loading at the pile head, the depth for rigid rotation point lies within 0.62L–0.73L (L is pile length), and the depth of the rotation point increases as the pile diameter increases. It was also found that the load eccentricity has a significant effect on the depth of the rotation point.

Offshore wind turbine monopile foundations: Design perspectives

Perspectives on design of offshore wind turbine (OWT) monopile foundations are provided in this paper by logically adopting optimal analysis choices with a balance between accuracy and computational efficiency. A newly developed analysis framework is used for this purpose. The analysis framework consists of dynamic analysis with linear viscoelastic soil and static analysis with nonlinear elastic soil. The framework is used to show that the Timoshenko beam theory produces most accurate monopile response although the Euler-Bernoulli beam theory produces optimal results maintaining a balance between accuracy and computational efficiency. The rigid beam theory can be used only under very restricted soil and monopile conditions. The analysis further demonstrates that static analysis is sufficient and optimal in producing monopile-soil stiffness values that can be used for determining the natural frequency of vibration of the OWTs. Moreover, nonlinear elastic analysis is sufficiently accurate and quick to produce monopile responses that can be used in design and there is no need for elasto-plastic analysis. The aspect of cyclic degradation of soil stiffness over the design life of monopiles is not considered. Thus, an optimal pathway for monopile design is described using the newly developed analysis framework and demonstrated through a design example.

Earthquake response of monopiles and caissons for Offshore Wind Turbines founded in liquefiable soil

Monopile has been the most widespread foundation type for Offshore Wind Turbines (OWTs) in shallow waters. Caisson (skirted) foundations have also been evaluated in some projects as an economical alternative. While the main concern in design of offshore foundations has been the environmental loads, the recent growth in construction of OWTs in seismic regions with the possibility of soil liquefaction has necessitated evaluation of the impact of earthquake and liquefaction from strong shakings on these structures. Several studies have reported the consequences of soil liquefaction for buildings and onshore structures; However, the effects of liquefaction on offshore foundations have not been sufficiently studied. This paper investigates the use of advanced liquefaction modeling in assessment of the response of monopiles and caissons for offshore wind turbines. The software FLAC3D and the SANISAND constitutive model are used to conduct the nonlinear dynamic analyses for OWTs. Excess pore water pressure during earthquake shaking and earthquake-induced displacements are computed at various points in the soil medium around the considered monopile and caisson foundations. The analyses reveal that SANISAND model is capable of simulating the pore pressure generation in the free-field as observed in a recent centrifuge test. The numerical results also indicate that both monopile and caissons in liquefiable soil deposits experience considerable rotations under the combined action of wind loads and earthquake shaking when liquefaction occurs.

Centrifuge study on effect of installation method on lateral response of monopiles in sand

Monopiles used as foundations for offshore wind turbines can be installed using different methods including jacking, vibratory driving and impact driving. Significant research efforts have been dedicated to the characterisation of monopile−soil interaction under lateral loading, mainly using p–y curves. There has also been extensive research in quantifying the effect of different installation methods on the axial response using numerical modelling and physical modelling techniques. Little attention has been paid to the effect of the installation method on the subsequent lateral response of a monopile under the in-service condition. In this paper, a purpose-designed apparatus is described that allows in-flight installation using different installation methods followed directly by lateral loading without stopping the centrifuge and thus retaining the installation-induced stress state. Test results from three lateral loading tests are discussed, with the piles either jacked at 1g and Ng or impact driven at Ng into a dry medium dense sand, allowing the effect of the installation method on the initial stiffness and ultimate capacity to be examined. The successfully conducted tests illustrate the capabilities of the new apparatus for centrifuge testing of laterally loaded driven piles.

Empirical approach based on centrifuge testing for cyclic deformations of laterally loaded piles in sand

A systematic study into the response of monopiles to lateral cyclic loading in medium dense and dense sand was performed in beam and drum centrifuge tests. The centrifuge tests were carried out at different cyclic load and magnitude ratios, while the cyclic load sequence was also varied. The instrumentation on the piles provides fresh insights into the ongoing development of net stresses, bending moments and deflections as cycling progresses. Parallels between the test results and corresponding cyclic triaxial tests are drawn. The paper combines the results from this study with those from previous experimental investigations to provide empirical design recommendations for monopiles subjected to unidirectional cyclic loading.

Soil reaction curves for monopiles in clay

Large diameter steel pipe pile foundations, typically known as monopiles, are the predominant foundation solution for offshore wind turbines. This paper deals with soil reaction curves for monopile analysis under lateral loading in clay. Developed from extensive parametric finite element analyses, models are proposed that allow for construction of site-specific soil reaction curves based on the stress-strain responses measured in laboratory tests. Assisted by failure mechanisms revealed by finite element analyses, a conceptual framework is proposed to analyse the monopile response under lateral loading using the beam-column approach. The conceptual framework makes use of the p-y model for wedge failure developed in this study along the monopile above the rotation point, existing p-y model for the flow-around failure suggested in the literature below the rotation point and base shear s-u model developed in this study at the pile tip. Validation exercises against finite element simulations and field pile testing results demonstrate excellent capability of the framework to predict monopile responses. The soil reaction models proposed in this paper provide practising engineers with a simple yet powerful approach to use site-specific soil reaction curves in monopile design based on element soil behaviour measured in laboratory, without the need for advanced numerical analyses.

Influence of vertical shear stresses due to pile-soil interaction on lateral dynamic responses for offshore monopiles

It has recently been pointed out that the vertical shear stresses at pile-soil interface play an important role in static responses of offshore monopiles with large diameters, and traditional p - y curves cannot fully capture this influence. However, the influences of the vertical shear stresses on the dynamic response of monopiles are still not clear. This is the main focus of this study. The coupled horizontal and rocking vibration of a monopile embedded in a fully saturated poroelastic seabed is studied in this paper. The focus of the study is the difference between the responses with and without the vertical shear stresses on the pile. The monopile dynamic response is studied by the integral equation method, with both 3D elastodynamic theory and Euler-Bernoulli beam theory. Selected numerical results for highlighting the influence of the vertical shear stresses on the dynamic contact load distributions, pile displacements, and lateral dynamic impedance functions are examined for different length to diameter (l/d) ratios as well as poroelastic materials and frequencies of excitation. These results confirm that exclusion of the vertical shear stresses will lead to potentially very conservative design for monopiles with l/d < 6 in soft soil, and monopiles with l/d < 10 in medium to stiff soils. The computed dynamic responses of the monopile when the vertical shear stress is ignored can be up to 30% larger than the case when the vertical shear stress is considered.

A macro-element model for multidirectional cyclic lateral loading of monopiles in clay

This paper investigates numerically the impact of multidirectional cyclic loads on the response of a monopile in clay supporting an offshore wind turbine (OWT). The results indicate that multidirectional loading modifies the foundation stiffness and hysteretic damping, which affects the OWT eigenfrequencies and damping. Based on these results, a macro-element model to represent the monopile response in the time-domain integrated load simulations employed in OWT design is proposed and verified against finite element analyses (FEA). The verification indicates the macro-element can simulate the monopile response with almost the same accuracy as FEA, but with a considerable reduction in computational effort.

Numerical study of diameter effect on accumulated deformation of laterally loaded monopiles in sand

The design of offshore monopile is generally governed by the accumulated deformation under lateral cyclic loading, e.g. loads induced by winds and waves. Although methods have been proposed to predict the accumulated deformation of laterally loaded monopiles under cyclic loading, these models are mainly based on small diameter pile test results with number of load cycles far less than that experienced by an offshore wind turbine during its lifetime. Additionally, the effect of monopile dimension, e.g. diameter effect, on the cyclic response of monopile is not well examined. Based on the degradation stiffness model, this paper investigates diameter effect of monopiles on the accumulated deformation via a finite difference program, which shows that pile diameter has a remarkable influence on the development rate of the accumulated deformation of laterally loaded monopiles. A design model, which takes account of the effect of loading cycle numbers, load amplitudes and monopile diameter, is proposed based on a series of numerical simulation. The proposed design model is validated against measured results from field test and centrifuge test on cyclically loaded monopiles, which proves the validity of the proposed design model.

Deformation mechanisms for offshore monopile foundations accounting for cyclic mobility effects

There has been a huge surge in the construction of marine facilities (e.g., wind turbines) in Europe, despite the many unknowns regarding their long-term performance. This paper presents a new framework for design strategy based on performance measures for cyclic horizontally loaded monopile foundations located in saturated and dry dense sand, by considering pile deformations and pore pressure accumulation effects. A three-dimensional finite element model was developed to investigate the behavior of large-diameter piles. The model accounts for nonlinear dynamic interactions in offshore platforms under harsh combined moment and horizontal environmental loads, with emphasis on the cyclic mobility of the surrounding cohesionless subsoil and associated shear.The maximum moment applied in the cyclic analyses is varied from 18% to 47% of the ultimate resistance. The considered data reflect behavior at the expected load amplitudes and cycle numbers during the service life of operation.For low numbers of load cycles (<1000 cycles), there were no differences between the power law and logarithmic approaches in terms of describing the accumulated deformations; however, for high numbers of cycles (<10,000 cycles), the logarithmic law was less suited to describe the accumulation response. Magnitude of cyclic loads was found to cause a linear increase in the accumulated rotation. The results from short-term and long-term dynamic response of monopiles indicate that few load cycles with higher load levels are the main concerns in accumulation of pile rotation rather than thousands of load cycles with low amplitudes.

Static Response of Monopile to Lateral Load in Overconsolidated Dense Sand

AbstractLarge diameter, rigid monopile foundations have been extensively used in the fast-growing offshore wind energy industry over the last two decades. In view of the offshore environment, later...

Biaxial Loading of Offshore Monopiles: Numerical Modeling

AbstractIn this study, finite-element (FE) modeling was used to examine the influence of biaxial lateral loading of a monopile foundation. Two different soil models were adopted to investigate the influence of state-dependent and anisotropic behavior of soil on monopile response. A series of analyses was carried out in which both cyclic and static lateral loads were applied to the pile. Results show that biaxial loading significantly influenced the pile-head load-displacement response in addition to the development of significant accumulated displacements and coexistent soil loosening and densification. Moreover, predictions determined using a state-independent soil model were shown to be significantly nonconservative; these findings have important implications for environmental loading of offshore monopiles.

Numerical Simulations of Wave–Structure–Soil Interaction of Offshore Monopiles

AbstractOcean energy converters (OECs) are becoming a more popular source of electricity generated from the ocean. The main obstacle associated with the use of OECs is their high initial installation cost. In light of this, geotechnical engineers have been challenged with revisiting the foundation system, aiming toward more economical design. In this study, finite-element (FE) modeling was used to examine the influence of wave-induced lateral loads on a monopile, a common foundation system for OECs, where wave–structure–soil interaction was considered explicitly. Measured data were used to arrive at two different wave scenarios in conjunction with both regular and irregular wave-surface profiles; the wave-induced lateral forces on the monopile foundation were calculated for each case using diffraction theory and used as input loads in the FE model. A parametric study was then undertaken to examine the influence of a range of wave characteristics and pile/soil parameters on monopile response. The results o...

Field tests to investigate the cyclic response of monopiles in sand

Field tests to investigate the cyclic response of monopiles in sand

Field tests to investigate the cyclic response of monopiles in sand

Design models for offshore wind turbine monopiles tend to be based on those developed for flexible piles in the offshore oil and gas sectors. However, wind monopiles tend to be shorter and wider, resulting in a stiffer structure that rotates rather than bends when laterally loaded. New methods have been proposed to calculate the load–displacement response of piles to monotonic lateral loading. Those based on correlations with in situ tests such as the cone penetration test seem particularly promising. There is a dearth of experimental test data where monopiles have been subjected to extensive cyclic loading with which to extend such models to consider these effects. To address this, field tests were performed on two 340 mm diameter driven piles with an embedded length of 2·2 m, which were cyclically loaded with up to 5000 loading cycles. The piles had a slenderness ratio (embedded length over diameter) of 6·5, which is a typical value used in the offshore wind sector. The test results show that the accumulated pile head response primarily depends on the soil strength and stiffness, the previous loading history and loading characteristics. A cyclic loading design procedure consisting of cone penetration test based static design and Miner's rule based superposition is presented.

Lateral and Axial Capacity of Monopiles for Offshore Wind Turbines

Offshore wind has enormous worldwide potential to generate increasing amounts of clean, renewable energy. Monopile foundations are considered to be viable in supporting larger offshore wind turbines in shallow to medium depth waters. In this paper, the lateral and axial response of monopiles installed in undrained clays of varying shear strength and stiffness is investigated using three-dimensional finite element analysis. A combination of axial and lateral loads expected at an offshore wind farm located in a water depth of 30 m has been used in the analysis. Numerically derived monopile axial capacities will be compared to those calculated using an established method in the literature. In addition, the lateral monopile capacity will be determined at ultimate limit state and compared to that at the serviceability limit state. Through a parametric study, it will be shown that with the exception of extremely high axial loads that border on monopile axial capacities, variation in axial loads does not have a significant effect on the ultimate lateral capacity and lateral displacement of monopiles.