Monopile Design Research Articles

The behavior of monopile embedded in sand under scour and cyclic loading conditions

The large-diameter monopile, as one of the most used foundations of offshore wind turbines, is subjected to severe sea conditions, such as the cyclic lateral loads induced by wind and waves, as well as the scour induced by the flushing currents. The combined cyclic lateral loads and scour introduces additional complexity and is encountered in offshore wind turbines as well as other offshore structures. In this study, the experimental tests and numerical simulation methods are adopted to investigate the behavior of monopile subjected to the combined cyclic lateral loads and scour. Firstly, the static and one-way cyclic lateral load tests are conducted to investigate the influence of scour conditions and the characteristics of cyclic lateral loads on the lateral displacement and bending moment of monopile. Subsequently, the three-dimensional numerical model is established by employing the stiffness degradation material model, reasonable comparisons between the numerical results and the experimental results suggest that the numerical model can be used as a reliable complement of the model tests. Finally, a series of parametric studies are carried out to investigate the effect of cycle numbers, scour conditions and cyclic load ratio on the accumulated deformation and lateral bearing capacity of monopile. It is found that the accumulated deformations are dependent on the cyclic load characteristics and scour depth, the one-way cyclic load and scour have opposite effects to the lateral capacity of monopile. A framework to predict the accumulated deformation of monopile subjected to the combined scour and long-term cyclic lateral load is developed, which can be served as a basis for the practical design of monopile.

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.

Site-specific soil reaction model for monopiles in soft clay based on laboratory element stress-strain curves

Large-diameter monopiles are the predominant foundation solution for supporting offshore wind turbines. They are conventionally designed using soil reaction curves developed for long slender piles used for supporting offshore oil and gas platforms (e.g., the API p-y model). However, due to the difference in the length/diameter ratio and the resulting soil mechanisms, the use of p-y curves alone can lead to significant under-prediction of the lateral stiffness and capacity of monopiles. To overcome the shortcoming, the authors have previously proposed a conceptual two-spring framework, i.e., the so-called ‘p-y + MR-θR’ model, to capture the monopile response in soft clay under lateral loading. The framework uses distributed p-y springs to consider the lateral soil resistance along the pile above the rotation point (RP) and a single moment-rotation (MR-θR) spring attached at the RP to capture the entire soil resistance below the RP, i.e., the distributed resistance along the pile, base shear and base moment at the pile tip. The proposed p-y and MR-θR springs were curve-fitted to the results of 3D numerical analyses. However, as the stress-strain response and the shear strength profile inevitably influence the p-y and MR-θR springs, the applicability of the empirical formulations to soil conditions other than those examined is uncertain. This study proposes an enhancement to the ‘p-y + MR-θR’ framework, in which the p-y and MR-θR springs are not tied to a specific soil and strength profile but fundamentally linked to the properties that can be measured directly in the site investigation and laboratory. This extension is achieved through analytical analyses and an extensive parametric numerical study. The predictive capabilities of the model are demonstrated by back-analyses of finite element analyses and centrifuge model tests. The proposed model provides practising engineers with a simple yet powerful approach to use site-specific soil reaction curves in the design of monopiles embedded in soft clay.

PISA design model for monopiles for offshore wind turbines: application to a stiff glacial clay till

Offshore wind turbines in shallow coastal waters are typically supported on monopile foundations. Although three-dimensional (3D) finite-element methods are available for the design of monopiles in this context, much of the routine design work is currently conducted using simplified one-dimensional (1D) models based on the p–y method. The p–y method was originally developed for the relatively large embedded length-to-diameter ratio (L/D) piles that are typically employed in offshore oil and gas structures. Concerns exist, however, that this analysis approach may not be appropriate for monopiles with the relatively low values of L/D that are typically adopted for offshore wind turbine structures. This paper describes a new 1D design model for monopile foundations; the model is specifically formulated for offshore wind turbine applications, although the general approach could be adopted for other applications. The model draws on the conventional p–y approach, but extends it to include additional components of soil reaction that act on the pile. The 1D model is calibrated using a set of bespoke 3D finite-element analyses of monopile performance, for pile characteristics and loading conditions that span a predefined design space. The calibrated 1D model provides results that match those obtained from the 3D finite-element calibration analysis, but at a fraction of the computational cost. Moreover, within the calibration space, the 1D model is capable of delivering high-fidelity computations of monopile performance that can be used directly for design purposes. This 1D modelling approach is demonstrated for monopiles installed in a stiff, overconsolidated glacial clay till with a typical North Sea strength and stiffness profile. Although the current form of the model has been developed for homogeneous soil and monotonic loading, it forms a basis from which extensions for soil layering and cyclic loading can be developed. The general approach can be applied to other foundation and soil–structure interaction problems, in which bespoke calibration of a simplified model can lead to more efficient design.

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.

Concept design of jacket foundations for offshore wind turbines in 10 steps

Simplified design procedures for offshore wind turbine (OWT) support structures are necessary in the concept and tender design stages for assessing the financial viability. Building upon the previous research on simplified monopile design, this paper presents a similar method to design jacket supported OWTs founded on piles or suction caissons. The method is based on the minimum and necessary data, namely the site characteristics (wind speed at reference height, wind turbulence intensity, water depth, wave height and wave period), turbine characteristics (rated power, rated wind speed, rotor diameter, cut-in and cut-out speed, mass of the rotor-nacelle-assembly) and ground profile (soil stiffness variation with depth and soil strength). A flowchart of the design process is also presented for visualisation of the rather complex multi-disciplinary analysis. Insights are given for 3 and 4 legged jackets. The calculation procedures that are required can be easily carried out either through a series of spreadsheets or simple hand calculations. An example problem emulating the design of a jacket on piles is taken from literature to demonstrate the proposed calculation procedure. The data used for the calculations are obtained from publicly available sources and the example shows that the simplified method arrives at a similar foundation to the one reported.

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.

Validation and Application of a New Software Tool Implementing the PISA Design Methodology

The PISA (Pile Soil Analysis) research project has resulted in a new methodology for the design of offshore wind turbine monopile foundations. A new software tool called PLAXIS Monopile Designer (MoDeTo) has been developed that automates the PISA design methodology. It facilitates the calibration of the so-called soil reaction curves by automated three-dimensional finite element calculations and it allows for a quick design of monopiles using the calibrated soil reaction curves in a one-dimensional finite element model based on Timoshenko beam theory. The monopile design approach has been validated for sand- and clay-type soils which are common in North Sea soil deposits. The paper presents a validation exercise based on the PISA research project proposal of a rule-based parametric model—General Dunkirk Sand Model (GDSM)—for Dunkirk sand as well as an application of the tool for a project involving an offshore wind turbine on a monopile foundation in sandy layered soil in which the PISA design is compared to the conventional API design. The paper concludes with a discussion of the results and the differences between the various methods.

A multi-spring model for monopile analysis in soft clays

The offshore wind industry in China has seen a rapid development in recent years and is projected to account for half of global yearly installed capacity in the years to come. Monopiles are a popular foundation solution for supporting offshore wind turbines. However, due to challenging seabed soil conditions, which often feature thick normally consolidated soft clays and harsh environmental loading (i.e. frequent occurrence of typhoons), extremely long monopiles are often designed. The large monopiles are costly to fabricate and install and sometimes kills the viability of the concept in cases where the bedrock is relatively shallow and expensive piling in rock is otherwise required. However, the state-of-practice for designing these monopiles in clays is typically by using the API p-y springs, which are widely known to underestimate soil-pile interaction stiffness and capacity for large diameter monopiles. Improvement to the design method can therefore have significant economic implications to the industry. The paper presents an effort toward this direction. A multi-spring beam-column model suitable for monopile design in soil conditions in China is proposed. The model features three soil spring components, namely the lateral p-y spring, the pile tip base shear s-y spring and rotational m-θ springs along the pile shaft. The validity of the model is verified by a comprehensive suite finite element parametric analyses. Guidance for incorporating the cyclic loading effect into design is also provided. The model proposed in this paper has large potentials for application in design practice.

Centrifuge modeling of the cyclic lateral behavior of large-diameter monopiles in soft clay: Effects of episodic cycling and reconsolidation

Large-diameter monopiles supporting offshore wind turbines (OWTs) are subjected to intermittent episodes of cycling and reconsolidation during the lifetime. The lateral soil-pile stiffness is likely to degrade during the cycling but tends to recover during the subsequent reconsolidation. The former effect has been widely acknowledged in monopile design, while the latter is less commonly recognized. This study aims to (a) investigate the effects of episodic cycling and reconsolidation on the evolution of cumulative displacement, stiffness and bending moment of large-diameter monopiles in soft clay, and to (b) analyze their consequences to the structural safety of the OWT (i.e., evolving natural frequency fN and thus likelihood of resonance). A series of centrifuge tests were performed to simulate laterally loaded monopiles subjected to multi-stage episodic cycling and reconsolidation. The experimental results show that the lateral soil-pile stiffness degrades during the initial cycling episode, but it entirely recovers during the subsequent reconsolidation to exceed the initial stiffness, with a percentage increase up to 50%. Consequently, the cumulative lateral pile-head displacement and maximum bending moment induced by the 3rd episode of cycling can be 63% and 15% smaller than that due to the 1st episode, respectively. The effects of episodic cycling and reconsolidation on the lateral stiffness, displacement and bending moment of the pile become more pronounced as the cyclic amplitude increases. Despite these observed beneficial effects, the stiffer lateral pile response after the reconsolidation would have increased fN of a typical 6 MW turbine founded on the monopile by up to 13% (i.e., from 0.206 to 0.232 Hz), forcing it to unfavorably approach the 3P frequency limit that could trigger resonance.

Study of typhoon impacts on the foundation design of offshore wind turbines in Taiwan

Taiwan is hit by an annual average of three to four typhoons with different intensities and routes. To consider the impacts of typhoon events on offshore substructure design, the historical metocean data for storm event-based frequency analysis are generally applied. However, whether a supertyphoon passes directly over an offshore wind farm has a great influence on the results of frequency analysis. Therefore, the American Bureau of Shipping has recently increased the return period of hydrodynamic loads from 50 to 100 years to address the higher variation in the severity of extreme storms in typhoon-prone offshore regions. In this paper, to assess the impacts of the increase in the return period (from 50 to 100 years) on the design of offshore substructures, case studies of different types of substructures (i.e. monopile, three-leg jacket and four-leg jacket) with different water depths were conducted for the offshore wind farms off the coast of Changhua County in central Taiwan. The quantitative differences in foundation design are presented by way of a comparison of substructure weights, overturning moments and pile lengths. The preliminary results of the case studies show that the monopile design is more obviously dominated by earthquakes than by typhoons and the jacket-type foundation is more significantly dominated by typhoons.

Foundations in Offshore Wind Farms: Evolution, Characteristics and Range of Use. Analysis of Main Dimensional Parameters in Monopile Foundations

Renewable energies are the future, and offshore wind is undoubtedly one of the renewable energy sources for the future. Foundations of offshore wind turbines are essential for its right development. There are several types: monopiles, gravity-based structures, jackets, tripods, floating support, etc., being the first ones that are most used up to now. This manuscript begins with a review of the offshore wind power installed around the world and the exposition of the different types of foundations in the industry. For that, a database has been created, and all the data are being processed to be exposed in clear graphic summarizing the current use of the different foundation types, considering mainly distance to the coast and water depth. Later, the paper includes an analysis of the evolution and parameters of the design of monopiles, including wind turbine and monopile characteristics. Some monomials are considered in this specific analysis and also the soil type. So, a general view of the current state of monopile foundations is achieved, based on a database with the offshore wind farms in operation.

Probabilistic analysis of offshore wind turbines under extreme resonant response: Application of environmental contour method

Offshore wind turbines can exhibit dynamic resonant behavior due to sea states with wave excitation frequencies coinciding with the structural eigenfrequencies. In addition to significant contributions to fatigue actions, dynamic load amplification can govern extreme wind turbine responses. However, current design requirements lack specifications for assessment of resonant loads, particularly during parked or idling conditions where aerodynamic damping contributions are significantly reduced. This study demonstrates a probabilistic approach for assessment of offshore wind turbines under extreme resonant responses during parked situations. Based on in-situ metocean observations on the North Sea, the environmental contour method is used to establish relevant design conditions. A case study on a feasible large monopile design showed that resonant loads can govern the design loads. The presented framework can be applied to assess the reliability of wave-sensitive offshore wind turbine structures for a given site-specific metocean conditions and support structure design.

Comparative Concept Design Study of Laterally Loaded Monopiles

Offshore wind turbine generators (WTG) are commonly founded on single large diameter piles, named monopiles. These monopiles are subjected to significant lateral loads and thereby sizeable overturning bending moments mainly due to action of wind and wave forces; thus the critical geotechnical design situation for monopiles supporting WTGs is often related to lateral loading conditions. The Pile Soil Analysis (PISA) joint industry research project [1] has recently proposed a monopile design method which encompasses finite element (FE) calculations under a specific design framework. Soil reaction curves that are crucial for monopile design (i.e. lateral force and moment reactions along the shaft and at the base of the pile) are derived from FE calculations, subsequently calibrated and entered into a 1D model which is then used for design optimisation. This method is implemented within the PLAXIS MoDeTo (Monopile Design Tool) software. This paper presents results of a concept monopile design study under lateral monotonic loading with the use of the PLAXIS MoDeTo method.

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.

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.

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.

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.

Force on the laterally loaded monopile in sandy soil

Large diameter monopile foundation is widely used in offshore wind farms. However, current design method, e.g. widely used American Petroleum Institute recommended P-y method, has been questioned during the design of monopile for this method is incapable to reasonably model the force on laterally loaded monopile. In this study, a method to derive the monopile–soil interaction force from finite element model is introduced, and the derived interaction force is validated through the force and moment equilibrium of monopile in sandy soil. Then, interaction force acting on the large diameter laterally loaded monopile in sandy soil is investigated in detail. The results show that: (1) the shear force which is tangent to the monopile circumference is an important component of soil resistance to laterally loaded monopile, (2) the distribution of normal force and shear force at a given depth can be generally described by parabola function, (3) the lateral friction force on monopile tip contributes another component of resistance and the peak value of this frictional resistance is up to 20% of the applied load at monopile head, (4) the moment resistance along the monopile shaft induced by shear force in vertical direction increases as the monopile deformation increases.

Experimental investigation of mechanical and fracture properties of offshore wind monopile weldments: SLIC interlaboratory test results

AbstractS355 structural steel is commonly used in fabrication of offshore structures including offshore wind turbine monopiles. Knowledge of mechanical and fracture properties in S355 weldments and the level of scatter in these properties is extremely important for ensuring the integrity of such structures through engineering critical assessment. An interlaboratory test programme was created to characterise the mechanical and fracture properties of S355 weldments, including the base metal, heat‐affected zone, and the weld metal, extensively. Charpy impact tests, chemical composition analysis, hardness tests, tensile tests, and fracture toughness tests have been performed on specimens extracted from each of the 3 material microstructures. The experimental test results from this project are presented in this paper, and their importance in structural integrity assessment of offshore wind turbine monopiles has been discussed. The results have shown a decreasing trend in the Charpy impact energy and Jmax values with an increase in yield stress from base metal to heat‐affected zone to weld metal. Moreover, the JIC fracture toughness value in the heat‐affected zone and weld metal is on average around 60% above and 40% below the base metal value, respectively. In addition, the average Charpy impact energy value in the heat‐affected zone and weld metal is around 5% and 30% below the base metal value, respectively. The effects of mechanical and fracture properties on the critical crack size estimates have been investigated, and the results are discussed concerning the impact of material properties on structural design and integrity assessment of monopiles.