Monopiles In Sand Research Articles

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.

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.

Experimental and numerical investigation of the effect of vertical loading on the lateral behaviour of monopiles in sand

The influence of combined loading on the response of monopiles used to support offshore wind turbines (OWTs) is investigated in this paper. In current practice, resistance of monopiles to vertical and lateral loading is considered separately. As OWT size has increased, the slenderness ratio (pile length, L, normalised by diameter, D) has decreased and foundations are tending towards intermediate footings with geometries between those of piles and shallow foundations. Whilst load interaction effects are not significant for slender piles, they are critical for shallow footings. Previous research on pile load interaction has resulted in conflicting findings, potentially arising from variations in boundary conditions and pile slenderness. In this study, monotonic lateral load tests were conducted in a geotechnical centrifuge on vertically loaded monopiles in dense sand. Results indicate that for piles with L/D = 5, increasing vertical loading improved pile initial stiffness and lateral capacity. A similar trend was observed for piles with L/D = 3, when vertical loading was below ≈45% of the pile’s ultimate vertical capacity. For higher vertical loads considered, results tended towards the behaviour observed for shallow footings. Numerical analyses conducted show that changes in mean effective stress are potentially responsible for the observed behaviour.

Influence of Pile Diameter and Aspect Ratio on the Lateral Response of Monopiles in Sand with Different Relative Densities

The large-diameter monopiles are the most preferred foundation used in offshore wind farms. However, the influence of pile diameter and aspect ratio on the lateral bearing behavior of monopiles in sand with different relative densities has not been systematically studied. This study presents a series of well-calibrated finite-element (FE) analyses using an advanced state dependent constitutive model. The FE model was first validated against the centrifuge tests on the large-diameter monopiles. Parametric studies were performed on rigid piles with different diameters (D = 4–10 m) and aspect ratios (L/D = 3–7.5) under a wide range of loading heights (e = 5–100 m) in sands with different relative densities (Dr = 40%, 65%, 80%). The API and PISA p-y models were systematically compared and evaluated against the FE simulation results. The numerical results revealed a rigid rotation failure mechanism of the rigid pile, which is independent of pile diameter and aspect ratio. The computed soil pressure coefficient (K = p/Dσ′v) of different diameter piles at same rotation is a function of z/L (z is depth) rather than z/D. The force–moment diagrams at different deflections were quantified in sands of different relative density. Based on the observed pile–soil interaction mechanism, a simple design model was proposed to calculate the combined capacity of rigid piles.

Centrifuge Modeling of Monotonic and Cyclic Lateral Behavior of Monopiles in Sand

AbstractThis paper presents a program of centrifuge model tests (100g) on the monotonic and cyclic lateral loading behavior of monopiles. The diameter and burial depth of the model pile were 59 and...

From cyclic sand ratcheting to tilt accumulation of offshore monopiles: 3D FE modelling using SANISAND-MS

Serviceability criteria for offshore monopiles include the estimation of long-term, permanent tilt under repeated operational loads. In the lack of well-established analysis methods, experimental and numerical research has been carried out in the last decade to support the fundamental understanding of monopile–soil interaction mechanisms, and the conception of engineering methods for monopile tilt predictions. With a focus on the case of monopiles in sand, this work shows how step-by-step/implicit, three-dimensional (3D) finite-element (FE) modelling can be fruitfully applied to the analysis of cyclic monopile–soil interaction and related soil deformation mechanisms. To achieve adequate simulation of cyclic sand ratcheting and densification around the pile, the recently proposed SANISAND-MS model is adopted. The link between local soil behaviour and global monopile response to cyclic loading is discussed through detailed analysis of model prediction. Overall, the results of numerical parametric studies confirm that the proposed 3D FE modelling framework can reproduce relevant experimental evidence about monopile–soil interaction, and support future improvement of engineering design methods.

Centrifuge testing on monotonic and cyclic lateral behavior of large-diameter slender piles in sand

The existing studies have been primarily focused on the lateral behavior of large-diameter stubby pile or small-diameter slender pile in sand, with little attention paid to large-diameter slender pile. This study presents a unique series of centrifuge tests on monotonic and cyclic lateral behavior of heavily instrumented large-diameter slender piles in medium dense sand. Two typical length to diameter ratios (L/D) are considered with the same length (L = 60 m) but different diameters (D = 4 and 6 m). It is found that the lateral behaviors of large-diameter slender pile, including its monotonic p-y response, cyclic accumulation of lateral displacement and cyclic stiffness evolution, are marginally different from those of small-diameter slender piles, but significantly deviate from the large-diameter stubby piles. This may suggest the longstanding argument of ‘diameter effect’ is relatively minor, while the lateral behavior of monopile in sand is more significantly governed by the relative pile-soil stiffness. The API (2011) non-conservatively predicts both stiffness and capacity of the large-diameter slender piles, leading to development of a new p-y formulation. These centrifuge testing results form a unique database to support development of new design methods for large-diameter slender piles, and to verify advanced numerical analyses involving cyclic models.

Experimental study on the bearing capacity of large-diameter monopile in sand under water flow condition

This paper presents the results of a series of laboratory studies to explore the effects of water flow on the bearing capacity of monopiles in sand. The load-displacement relationship, axial force, normalized bending moment of the pile shaft, the p–y curves and the bearing envelope are obtained to quantify the effect of water flow on the bearing capacity of the monopile. The results demonstrate that water flow can result in a maximum scouring depth of 0.7 D (pile diameter), which reduces the lateral bearing capacity by about 4–10%. It is also found that the bearing capacity of the pile for resisting eccentric lateral load (equals to moment and horizontal loads at mudline) slightly increases at the initial stage with the rise of the applied vertical load. Finally, the bearing capacity envelopes of pile with and without water flow are proposed, which can provide guidance for the design of monopiles.

Three-Dimensional Modeling of Monopiles in Sand Subjected to Lateral Loading under Static and Cyclic Conditions

Here, the results of a three-dimensional finite element study of the complex interaction of horizontal and moment loads (HM) on offshore monopiles as failure envelope, are reported. A new design criterion is described which is based on critical length, ultimate limit states, load characteristics and Eigen-frequency to ensure stable behavior of laterally loaded monopiles. Numerical analyses were performed to examine nonlinear interaction of a soil-pile system for 10,000 load cycles. The resulting framework can predict angular rotation due to cyclic loading. According to the loading level and duration of a load, elastic strains accumulate in the vicinity of a pile. Fairly intermediate two-way cyclic loading induced the largest rotations irrespective of the analysis performed (i.e., drained versus partially drained). Based on the regression coefficients of the non-dimensional frameworks used, accumulating rocking deformations of a pile at seabed level appear to be dependent on cyclic load ratio, drainage condition, and duration of loading. For safe design, sensitivity of the natural frequency of offshore wind turbine (OWT) at a monopile critical length as well as shorter lengths were also examined. The analytical model proposed here for determining the natural frequency of an OWT considers that soil-structure interaction (SSI) can be represented by monopile head springs characterized by lateral stiffness, KL, rotational stiffness, KR, cross-coupling stiffness, KLR, and parabolic soil stiffness variation with depth.

Frequency effects in the dynamic lateral stiffness of monopiles in sand: insight from field tests and 3D FE modelling

With the offshore wind industry rapidly expanding worldwide, geotechnical research is being devoted to foundation optimisation – most intensively for large-diameter monopiles. The analysis and design of monopiles still suffers from significant uncertainties in relation to cyclic/dynamic loading conditions. The aim of this work is to shed new light on dynamic soil–monopile interaction, based on the results of unique full-scale experiments performed at the Westermeerwind wind park (Netherlands). The response of a 24 m long, 5 m diameter monopile to harmonic lateral loading of varying amplitude and frequency is inspected. The analysis of original field measurements (soil accelerations and pore pressures) enables the lateral stiffness observed at the monopile head to be linked to dynamic effects occurring in the surrounding soil. The interpretation of measured data is supported by three-dimensional finite-element studies, also looking at the influence of drainage conditions and monopile size. The set of results presented supports the need for dynamics-based monopile design, as higher frequencies gain relevance in the most recent offshore wind developments.

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.

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.

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.

Centrifuge Modeling of the Impact of Local and Global Scour Erosion on the Monotonic Lateral Response of a Monopile in Sand

Abstract The majority of offshore wind turbines are founded on large-diameter, open-ended steel monopiles. Monopiles must resist lateral loads and overturning moments because of environmental (wind and wave) actions, whereas vertical loads tend to be comparatively small. Recent developments in turbine sizes and increases in hub heights have resulted in pile diameters increasing rapidly, whereas the embedment length to diameter ratio (L/D) is reducing. Soil erosion around piles, termed scour, changes the soil strength and stiffness properties and affects the system’s load resistance characteristics. In practice, design scour depths of up to 1.3D are routinely assumed during the turbine lifetime; however, the impact on monopiles with low L/D is not yet fully understood. In this article, centrifuge tests are performed to assess the effect of scour on the performance of piles with low L/D. In particular, the effect of combined loads, scour type (global, local), and depth are considered. A loading system is developed that enables application of realistic load eccentricity and combined vertical, horizontal, and moment loading at the seabed level. An instrumented 1.8-m-diameter pile with L/D = 5 is used. A friction-reducing ball-type connection is designed to transfer lateral loads to the pile without inducing any rotational pile-head constraint, which is associated with loading rigs in tests of this nature. Results suggest that vertical and lateral load interaction is minimal. Scour has a significant impact on the lateral load-bearing capacity and stiffness of the pile, leads to increases in bending moment magnitude along the pile shaft, and lowers the location of peak pile bending moment. The response varies with scour type, with global scour resulting in larger moments than local scour. The size of the local scour hole is found to have a significant impact on the pile response, suggesting that scour hole width should be explicitly considered in design.

Quantifying the influence of pile diameter on the load transfer curves of laterally loaded monopile in sand

Although the load transfer curves (i.e., p-y curves) recommended by the current industry standards are regarded as questionable for the recently emerging large diameter monopile, the influence of pile diameter and bending stiffness on the p-y curves in sand has not been systematically quantified. Nor have the underlying mechanisms of the diameter dependent p-y curves been well understood. This study presents a series of well calibrated three-dimensional finite-element (FE) analyses, aiming to quantify p-y curves of laterally loaded piles with different pile diameters and bending stiffness in sand, and to reveal the underlying mechanisms. It is found that the p-y curves in the API [6] code, which are valid for piles with a small diameter (D = 1 m), overestimate the initial stiffness while underestimates the soil resistance of a large-diameter pile (D = 5 m) by more than 90%. The numerical analyses further reveal that the modification of the p-y curves with the increasing pile diameter is mainly associated with the altered soil deformation mechanisms around the increasingly stiff pile. On the other hand, the influences of the skin friction, base shear and base moment of the large-diameter pile (with D up to 5 m) on the p-y curves are almost negligible. New p-y curves considering the effect of failure mechanism of large diameter monopile in dense sand is proposed, which are shown to yield better predictions for large-diameter piles than the existing p-y curves.

Nondimensional parametric method for studying lateral cyclic response of offshore monopiles in sand

A monopile is the most widely used foundation type for offshore wind turbines. Long-term cyclic loading from wind and waves creates an accumulated displacement and changes in the stiffness, which are critical in the serviceability limit state. Design codes present little information in this regard. A series of model tests were conducted to further explore cyclic response of a stiff pile in drained sand subjected to 10,000 cycles. The improved nondimensional groups were applied to interpret the resulting data. The test results show that the accumulated residual displacement and cyclic stiffness increased with the number of cycles, depending on the load characteristics and sand densities. On this basis, two explicit models for predicting the accumulated residual displacement and unloading stiffness due to cyclic lateral loading were developed in a nondimensional form. The application of the improved models for laboratory tests and field-scale piles was also demonstrated.

Influence of scour depth and type on p–y curves for monopiles in sand under monotonic lateral loading in a geotechnical centrifuge

The influence of scour on the lateral response of monopile foundations for offshore wind turbines is investigated in this paper. Application of lateral load-displacement (p-y) curves to predict the lateral pile behaviour is subject to uncertainty as many of the presently used design approaches have been derived for long, slender piles. These piles, with typical length/diameter ratios (L/D) of greater than 10 behave differently compared to rigid monopiles, with L/D typically less than 6. In this paper, centrifuge tests are conducted on a monopile model under various scour scenarios and p-y curves are derived from strain gauges embedded along the model pile wall. Global scour and two different shapes for local scour holes are studied. Using the piecewise polynomial method for extraction of p-y curves from sparsely distributed strain measurements, it is recommended to use a 4th order polynomial for the moment profile to extract soil reaction and a 7th order polynomial for the moment profile to calculate pile deflection. Results indicate that the pile behaviour is significantly influenced by the nature (size, shape) of the scour holes affecting the pile–soil system and suggest that the p-y curves should be appropriately modified to account for this behaviour.

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.

Drainage conditions around monopiles in sand

Large diameter monopiles are typical foundation solutions for offshore wind turbines. In design of the monopile foundations in sand, it is necessary to understand the drainage conditions of the foundation soil under the design loading conditions as the soil performance (strength and stiffness) is highly dependent on the drainage conditions. This paper presents a numerical investigation into this issue, with a purpose to develop a simple design criterion for assessing the soil drainage conditions around a monopile in sand. It is found that for typical monopile foundations in sand, the drainage condition during a single load cycle is generally expected to be undrained. However, the current state-of-practice uses p-y springs derived for drained soil responses for monopile design. The impact of this discrepancy on monopile foundation design was evaluated and found to be insignificant due to the relatively low level of loading as compared to the capacity of the soil.

Un- and reloading stiffness of monopile foundations in sand

The eigenfrequency of offshore wind turbine structures is a crucial design parameter, since it determines the dynamic behavior of the structure and with that the fatigue loads for the structural design. For offshore wind turbines founded on monopiles, the rotational stiffness of the monopile-soil system for un- and reloading states strongly affects the eigenfrequency. A numerical model for the calculation of the monopile’s behavior under un- and reloading is established and validated by back-calculation of model and field tests. With this model, a parametric study is conducted in which pile geometry, soil parameters and load conditions are varied. It is shown that of course the rotational stiffness varies with mean load and magnitude of the considered un- and reloading span, but that for most relevant load situations the initial rotational stiffness of the monopile system, i.e. the initial slope of the moment-rotation curve for monotonic loading, gives a good estimate of the actual stiffness. Comparisons of different p–y approaches show that the ordinary API approach considerably underestimates the initial stiffness, whereas the recently developed ‘Thieken’ approach and also the ‘Kallehave’ approach give a much better prediction and thus might be used in the design of monopiles in sand.