Monopile Wind Turbine Research Articles

A simplified formula for calculating the limit load of cracked offshore wind turbine monopile under bending

A simplified methodology is proposed for calculating the plastic collapse (limit) bending moment load of a pipe with a circumferential flaw with an emphasis on its application for use in the assessment of cracked offshore wind turbine monopile using failure assessment diagrams. The proposed methodology is based on the theory of net section collapse (NSC) but differs from existing approaches in that it does not need idealisation and categorisation of the crack before assessment. The proposed methodology is validated against results presented in literature and also finite element analysis results. Although it is possible to obtain limit loads using FE analysis, this is computationally expensive and time consuming. The proposed approach allows for near-instantaneous calculation of limit load for any arbitrary crack configuration and loading direction. This is a significant development for the analysis of offshore wind turbine monopiles as it allows the suitability of the cracked structure to be assessed in pseudo-real time.

Countermeasures for local scour at offshore wind turbine monopile foundations: A review

Local scour at monopile foundations of offshore wind turbines is one of the most critical structural stability issues. This article reviews the contemporary methods of scour countermeasures at monopile foundations. These methods include armouring countermeasures (e.g., riprap protection) to enhance the anti-scour ability of the bed materials and flow-altering countermeasures (e.g., collars and sacrificial piles) to reduce downflow or change flow patterns around the monopiles. Stability number and size-selection equations for riprap armour layers are summarised and compared. Moreover, other alternative methods to riprap are briefly introduced and presented. A typical graph of the scour depth reduction with different collar sizes and elevations under specific test conditions is summarised and compared with a plot for a pile founded on a caisson. Reduction rates for different flow-altering countermeasures, including the collar, are listed and compared. A newly developed soil improvement method, namely microbially induced calcite precipitation (MICP), is also reviewed and introduced as a scour protection method. As a popular bio-soil treatment method, MICP has a good potential as a scour countermeasure method. Bio-soil treatment methods and traditional armouring methods are defined as active and passive soil enhancement scour countermeasures, respectively.

The Influence of Selected Strain-Based Failure Criteria on Ship Structure Damage Resulting from a Collision with an Offshore Wind Turbine Monopile

Abstract Offshore wind farms are developing well all over the world, providing green energy from renewable sources. The evaluation of possible consequences of a collision involves Finite Element computer simulations. The goal of this paper was to analyse the influence of selected strain-based failure criteria on ship damage resulting from a collision with an offshore wind turbine monopile. The case of a collision between an offshore supply vessel and a monopile-type support structure was examined. The results imply that simulation assumptions, especially the failure criteria, are very important. It was found that, using the strain failure criteria according to the minimum values required by the design rules, can lead to an underestimation of the ship damage by as much as 6 times, for the length of the hull plate, and 9 times, for the area of the ship hull opening. Instead, the adjusted formula should be used, taking into account both the FE element size and the shell thickness. The influence of the non-linear representation of the stress-strain curve was also pointed out. Moreover, a significant influence of the selected steel grade on collision damages was found.

Probabilistic fatigue strength modelling based on various statistical approaches for a double-side welded connection

S355 steel is currently used in the fabrication of most wind turbine monopile support structures and offshore structures. In these types of structures, most fatigue failures occur in the welded connections where cyclic loading is the main responsible for this phenomenon. For the high-cycle fatigue analysis, the stress life method utilizing S-N curves can be used to determine the strength of a welded joint under fatigue loading. In this research, the evaluation of design S-N curves for a double-side welded connection made of S355 steel is proposed. This study concludes with a comparison between the experimental fatigue curves obtained and the design S-N curves at 5% and 2.3% probability of failure proposed in design codes (EN1993-1-9, DNVGL-RP-C203 standards and IIW recommendations) for offshore structures and general steel structures. For the specimens under investigation, the hot-spot and nominal stress approaches are taken into consideration. The characteristic fatigue curves of the double-side welded connection are obtained using statistical analyses based on the ASTM E739, ISO 12107, Bayesian inference, and Weibull distribution. The hot-spot and nominal stress approaches yield very similar S-N results considering the specimens under study.

A numerical model of VPS for offshore wind turbine monopiles based on hypoplastic theory

AbstractThe current research on the penetration of offshore wind turbine monopiles primarily focuses on driving with a vibratory hammer, although there are several smaller studies that focus on the vibrating penetration mechanism. To further the body of research, this paper establishes a numerical model to calculate the vibration penetration speed (VPS) of an offshore wind turbine monopile that considers the soil plugging effect. In this model, a monopile is assumed to be a rigid body. The surrounding soil is divided into a series of circular cylinders of concentric axes. The shear stress at the external soil elements' interfaces and the soil's vertical stress at the monopile toe are calculated using the Gudehus–Bauer hypoplastic constitutive model. The VPS is obtained from the motion equation by compiling it in MATLAB R2012b. Finally, a finite element model is conducted to verify the results, showcasing the rationality of the proposed model. Parameter analyses are also conducted to study the influences of the soil void ratio, soil internal friction angle, vibration frequency, ratio of static load to dynamic load, and monopile diameter on VPS.

Effect of Turbine Weight on the Seismic Response of a Wind Turbine-Monopile System Located in Liquefied Multilayer Soil

The core objectives of sustainable development are to develop access to renewable, sustainable, reliable, and cost-effective resources. Wind is an essential source of renewable energy, and monopile wind turbines are one method proposed for harnessing wind power. Offshore wind turbines can be vulnerable to earthquakes and liquefaction. This numerical study defined the effects of wind turbine weight on the seismic response of a wind turbine-monopile system located in liquefied multilayered soil with layer thicknesses of 5, 10, 15, and 20 m using four far-field records. OpenSees PL analysis indicated that if the liquefied sand had a lower density or a thickness of more than 10 m, then an increase in the earthquake acceleration beyond 0.4 g caused the pile to float like liquefied soil and to lose its vertical bearing capacity. Moreover, increasing the wind turbine power from 2 to 5 kW had no significant effect on the soil-structure interaction response. As the earthquake acceleration increased, the bending moment of the pile-column also increased as long as liquefaction did not occur and the pile-column deformation remained rotational-spatial in shape. As the acceleration and liquefaction increased and the pile began to float in response to its transverse motion, there was no significant difference in the pile-column displacement along the length, but there was a decrease in the pile-column bending moments. As this phenomenon increased and the pile continued to float, transformation of the pile increased the difference between the displacement of the pile-column along its length and further increased the bending moments. These results were derived from multiple correlation analysis, the bending moment relations, and lateral displacement of the pile-column of the wind turbine.

Numerical Simulations of the Monotonic and Cyclic Behaviour of Offshore Wind Turbine Monopile Foundations in Clayey Soils

Most of the reported centrifuge tests available in the existing literature on offshore wind turbine foundations are focused on the behaviour of monopiles in sands, but very few studies on clayey soils can be found, due to the very long saturation and consolidation periods required to properly conduct experiments in such materials. Moreover, most of the reported numerical simulations using finite element analyses have been validated with monotonic centrifuge tests only. In this research, both monotonic and cyclic performance of offshore wind turbines in clay are validated and justified. The relationship between the monopile rotation in clays and the geometry and strength of the soil has been found and quantified. A prediction of the rotation for a high number of cycles of loading, based on the one experienced by the pile during the first cycle, can be obtained using the correlation derived in the paper. For those cases in which the rotation does not reach a steady value after a high number of cycles, the cumulative rate has been found significantly larger than the prediction conducted with standard analytical methods. A new design methodology for the design of offshore monopile foundations in clay is presented.

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.

Direct and fast probabilistic assessment of long term monopile load distribution from combined metocean data and fully nonlinear wave kinematics

A novel method is introduced to determine the long-term extreme value distribution of variables with an associated short-term distribution. The method is applicable to e.g. the extreme value with a return period of 10.000 years for wave height, crest height of waves as well as wave-induced response and loads on offshore wind turbine monopiles. The extreme values are determined by a combination of a joint probabilistic description of the met-ocean environment and of a recently developed database of fully nonlinear wave kinematics computations. For two benchmark monopile structures, representative for the conditions at Dogger Bank and the German Bight in the North Sea, the one-hour max value of crest height, inline force and overturning moment are computed via the Rainey slender-body force model. Convolution with the joint probability of the sea state parameters in the nondimensional space of wave steepness and Ursell parameter leads to the marginal probability distribution for these parameters up to a 10.000 year return period and properly accounts for the uncertainty of the extreme value parameters in also the short-term distributions. It is found that the best extreme value fit is generally obtained with a third-order polynomial fit within the space of steepness and Ursell parameter. Compared to the Forristall crest height distribution, the inclusion of full nonlinearity leads to larger crest heights at low return period levels. However, due to its realistic treatment of the breaking limitation, lower crests are predicted at the 10.000 year level for the two positions considered in the North Sea. With further verification of the wave kinematics in combination with load models and a thorough comparison to present engineering practice, the proposed methodology provides a robust future solution for directly estimating extreme value distributions of loads and response of offshore wind turbine monopiles.

Experimental Investigation of Lattice Deformation Behavior in S355 Steel Weldments Using Neutron Diffraction Technique

This study aims to investigate the influence of welding process on the elastic lattice deformation and its effects on fatigue and fracture behavior of S355 G10+M steel, which is widely used in fabrication of offshore wind turbine monopile structures. In situ neutron diffraction measurements were taken on cross-weld test samples at room temperature to monitor the evolution of intergranular strains under static and cyclic loading conditions. Both static and cyclic test results have shown that the {200} orientation exhibits the least load carrying capacity while {211} had the maximum stiffness. The hkl-specific response predicted using Reuss and Kröner model were found to agree well with experimental values obtained for the heat-affected zone for all the orientations; however, discrepancies between the experimental and model predictions have been observed for the base metal and weld metal. Moreover, the microstructural differences between the weld metal and heat-affected zone resulted in the maximum elastic–plastic strain mismatch at the interface of the two regions. The results from this experiment would be useful to understand the role of crystal-specific microstrains and lattice deformation on fatigue and fracture behavior of thick-walled monopile weldments.

Permanent accumulated rotation of offshore wind turbine monopile due to typhoon-induced cyclic loading

In order to study the effect of typhoons on the accumulated deformation of monopile foundations for offshore wind turbines, a series of 1-g laboratory model tests with a geometrical scale of 1:100 were carried out. Through the horizontal static and cyclic loading tests of a stiff pile embedded in a medium dense sand deposit, the relationship between the accumulated rotation of the pile and the number of loading cycles under different loading conditions was obtained. The results show that the final accumulated rotation is mainly caused by the typhoon load series and is not affected by the loading sequence. Based on these results, a method is presented to predict the accumulated rotation of the monopile foundation during its service life, and a case study of a 6 MW wind turbine supported by a monopile at a water depth of 30 m in sand is conducted by using the method. The results show that the permanent accumulated rotation of the monopile throughout the design life is mainly contributed by cyclic loading induced by typhoons and the contribution of cyclic loading with small amplitudes can be ignored.

A review of offshore wind monopiles structural design achievements and challenges

The taller and heavier OWT monopile structures has unique structural engineering design challenges which could lead to catastrophic under-conservative designs or more likely, expensive over-conservative engineering. The need for an efficient engineering design cannot be overemphasised given the current structural configurations and future OWT monopiles novel concepts due to the cost of materials for the increasing towers and foundations diameter and thickness, longer and bigger RNA, which directly impacts the manufacturing, transportation, and installation activities. This increasing trend is projected to continue as the industry utilises shallow to medium water depths for siting new concepts of Offshore Wind Turbine (OWT) monopile structures and development of wind farms. The engineering design through to installation challenges faced by the relatively new OWT industry are exacerbated by the shortcomings of the structural design techniques and practices adopted from the oil and gas industry and in accordance with design codes and standards. This paper presents a concise structural review of the current salient technical aspects, the recent improvements in offshore wind turbine monopile structural design, and the challenges of future OWT monopile concepts considering the increasing monopile structure size and turbine capacity. The review presented in this paper primarily focused on grouted connection between the foundation and tower, damping for monopile structural response analysis, soil scouring, soil-monopile interaction modelling, and corrosion. The aim of this paper is to critically assess, outline, and discuss the current OWT monopile structural design techniques achievements and identify future concepts structural challenges, and to provide structural design direction for OWT monopile research and development activities.

Material pre-straining effects on fracture toughness variation in offshore wind turbine foundations

S355 structural steel is a commonly used material in the fabrication of foundation structures of offshore wind turbines, which are predominantly supported using monopiles. During the manufacturing process of monopile foundations, S355 steel plates are pre-strained via a three point bending and rolling process, which subsequently changes the mechanical, fatigue and fracture properties of the material. The aim of this study is to investigate the variation in fracture toughness of S355 material by considering a range of pre-strain levels induced during the manufacturing process. Fracture toughness tests have been performed on compact tension specimens made of the as-received, 5% and 10% pre-strained S355 material. The test results have shown that the fracture toughness of the material decreases as the percentage of pre-straining increases. An empirical correlation has been derived between the yield strength of the material, the plastic pre-strain level and the fracture toughness values. The drawn relationship can potentially be utilised in the life assessment of offshore wind turbine monopile foundations to give a relatively accurate estimate of the remaining life by considering realistic values of fracture toughness post-fabrication, which results in better informed design and assessment.

On the performance of monopile weldments under service loading conditions and fatigue damage prediction

AbstractThick weldments used in offshore structures frequently act as fatigue crack initiation sites due to stress concentration at weld toe as well as weld residual stress fields. This paper investigates the cyclic deformation behavior of S355 G10+M steel, which is predominantly used in offshore wind applications. Owing to the vast size difference of monopile structure and weld cross‐section, a global–local finite element (FE) method was used, and the weld geometry was adopted from circumferential weld joints used in offshore wind turbine monopile foundations. Realistic service loads collected using supervisory control and data acquisition (SCADA) and wave buoy techniques were used in the FE model. A nonlinear isotropic–kinematic hardening model was calibrated using the strain controlled cyclic deformation results obtained from base metal (BM) as well as cross‐weld specimen tests. The tests revealed that the S355 G10+M BM and weld metal (WM) undergo continuous cyclic stress relaxation. Fatigue damage over a period of 20 years of operation was predicted using the local stress at the root of the weldments as the life limiting criterion. This study helps in quantifying the level of conservatism in the current monopile design approaches and has implications towards making wind energy more economic.

Environmental lumping for efficient fatigue assessment of large-diameter monopile wind turbines

Fatigue damage is one of the governing factors for the design of offshore wind turbines. However, the full fatigue assessment is a time-consuming task. During the design process, the site-specific environmental parameters are usually condensed by a lumping process to reduce the computational effort. Preservation of fatigue damage during lumping requires an accurate consideration of the met-ocean climate and the dynamic response of the structure. Two lumping methods (time-domain and frequency-domain) have been evaluated for a monopile-based 10 MW offshore wind turbine, both based on damage-equivalent contour lines. Fatigue damage from lumped load cases was compared to full long-term fatigue assessment. The lumping methods had an accuracy of 94–98% for the total long-term fatigue damage and 90% for individual wind speed classes, for aligned wind and waves. Fatigue damage was preserved with the same accuracy levels for the whole support structure. A significant reduction of computational time (93%) was achieved compared to a full long-term fatigue assessment. For the cases with 30° and 60° wind-wave misalignment, there was a mean underestimation of approximately 10%. Variations in penetration depth did not affect the selection of the lumped sea-state parameters. This work presents a straightforward method for the selection of damage-equivalent lumped load cases, which can adequately preserve long-term fatigue damage throughout the support structure, providing considerable reduction of computational effort.

Fitness-for-purpose assessment of cracked offshore wind turbine monopile

In this paper, the procedure for flaw acceptability assessment is examined through a case study of a semi-elliptical surface crack in an offshore monopile as it grows till it forms a through thickness crack. Using the procedure prescribed in an industrial standard (BS 7910), the fracture ratio, Kr is shown to increase monotonically with increasing crack depth. The load ratio, Lr, is initially insensitive to the crack depth. However, there is a rapid increase in Lr when the crack depth to thickness ratio exceeds 80%. Lr values obtained from detailed 3D FE limit analysis using elastic-perfectly-plastic material behaviour do not exhibit the asymptotic behaviour predicted by BS 7910 as the flaw transitions from deep crack to through-thickness crack. Furthermore, Kr predicted by BS 7910 is shown to be an over-estimation for the typical dimensions of offshore monopiles. The findings suggest that a structure with a deep flaw may be identified as unacceptable based on BS 7910 when it may still possess a non-trivial amount of structural residual life. This is a concern for monopiles where crack growth as a large flaw forms a significant part of the total life.

Simultaneous out-of-plane and in-plane vibration mitigations of offshore monopile wind turbines by tuned mass dampers

To effectively extract the vast wind resource, offshore wind turbines are designed with large rotor and slender tower, which makes them vulnerable to external vibration sources such as wind and wave loads. Substantial research efforts have been devoted to mitigate the unwanted vibrations of offshore wind turbines to ensure their serviceability and safety in the normal working condition. However, most previous studies investigated the vibration control of wind turbines in one direction only, i.e., either the out-of-plane or in-plane direction. In reality, wind turbines inevitably vibrate in both directions when they are subjected to the external excitations. The studies on both the in-plane and out-of-plane vibration control of wind turbines are, however, scarce. In the present study, the NREL 5 MW wind turbine is taken as an example, a detailed three-dimensional (3D) Finite Element (FE) model of the wind turbine is developed in ABAQUS. To simultaneously control the in-plane and out-of-plane vibrations induced by the combined wind and wave loads, another carefully designed (i.e., tuned) spring and dashpot are added to the perpendicular direction of each Tuned Mass Damper (TMD) system that is used to control the vibrations of the tower and blades in one particular direction. With this simple modification, a bi-directional TMD system is formed and the vibrations in both the out-of-plane and in-plane directions are simultaneously suppressed. To examine the control effectiveness, the responses of the wind turbine without control, with separate TMD system and the proposed bi-directional TMD system are calculated and compared. Numerical results show that the bi-directional TMD system can simultaneously control the out-of-plane and in-plane vibrations of the wind turbine without changing too much of the conventional design of the control system. The bi-directional control system therefore could be a cost-effective solution to mitigate the bi-directional vibrations of offshore wind turbines.

3D FE-Informed Laboratory Soil Testing for the Design of Offshore Wind Turbine Monopiles

Based on advanced 3D finite element modelling, this paper analyses the stress paths experienced by soil elements in the vicinity of a monopile foundation for offshore wind turbines subjected to cyclic loading with the aim of informing soil laboratory testing in support of monopile foundation design. It is shown that the soil elements in front of the laterally loaded monopile are subjected to complex stress variations, which gradually evolve towards steady stress cycles as the cyclic lateral pile loading proceeds. The amplitude, direction and average value of such steady stress cycles are dependent on the depth and radial distance from the pile of the soil element, but it also invariably involves the cyclic rotation of principal stress axes. Complementary laboratory testing using the hollow-cylinder torsional apparatus was carried out on granular soil samples imposing cyclic stress paths (with up to about 3 × 104 cycles) which resemble those determined after 3D finite element analysis. The importance of considering the cyclic rotation of principal stress axes when investigating the response of soil elements under stress conditions mimicking those around a monopile foundation subjected to cyclic lateral loading is emphasised.

Study on Hydrodynamic Characteristics of Wind Turbine Monopile under Nonlinear Wave

As wind power technologies become maturer, the monopile foundation of offshore wind turbine is widely used because of its simple structure, few occupied space and low cost. However, under severe sea conditions, the impact of nonlinear wave load applied against the monopile foundation on the system structure safety cannot be ignored. In this paper, the 5MW offshore wind turbine of the National Renewable Energy Laboratory (NREL) was taken as the research object, and the computational fluid analysis software ‘STARCCM +’ was used to study the hydrodynamic characteristics of the monopile foundation of the wind turbine under different wave parameters. This paper mainly analyzed the upper wave, pressure and wave forces around the monopile foundation of the wind turbine under the same period and different wave heights. And the wave force calculated by CFD was compared with the result based on potential flow theory. The research results showed that with the rise of wave height, the upper wave, pressure and wave force around the monopile foundation increase continuously, and the second-peak phenomenon appeared at some measuring points on the water surface of the monopile foundation. Because the CFD method considers the fluid viscosity and is more in line with the real sea conditions, it is more accurate to obtain wave forces based on this method.

Impact of scour on lateral resistance of wind turbine monopiles: an experimental study

The majority of offshore wind structures are supported on large-diameter, rigid monopile foundations. These piles may be subjected to scour due to the waves and currents that causes a loss of soil support and consequently decreases the pile capacity and system stiffness. The results of numerical models suggest that the shape of the scour hole affects the magnitude of pile capacity loss; however, there is a dearth of experimental test data that quantify this effect. This paper presents a series of centrifuge model tests on an instrumented model pile that investigates the effects of scour-hole geometry on the response of a laterally loaded pile embedded in sand. The pile instrumentation allowed load–displacement and p–y (soil reaction – displacement) curves to be derived. Three scour geometries (global, local wide, and local narrow) and three scour depths (1D, 1.5D, and 2D; where D is pile diameter) were modelled. For all three scour types, pile moment capacity decreased almost linearly with increase of scour depth. Simple empirical relations were proposed to evaluate the detrimental influence of scour on the pile moment capacity. A new method has been developed to allow designers to quantify the effect of scour-hole shape and severity of scour on the pile response.