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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, whe...
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Context 1
... CONSTANT-AMPLITUDE CYCLIC RESPONSE Table 4 summarises the unidirectional constant-amplitude tests presented in this paper. Sinusoidal cyclic loading was applied for all constant-amplitude cyclic tests. ...Context 2
... tests were conducted to 1000 cycles as the ratcheting behaviour evolves logarithmically and there are diminishing returns associated with performing longer-term tests. Figure 4 presents the monopile's ratcheting response for the tests presented in Table 4. The impact of ζ c on the ratcheting response is clear, impacting both the magnitude of rotation and the shape of the evolution of rotation with cycle number in both the very loose and dense samples. ...Context 3
... the monopile's post-cyclic response also informs interpretation of multiamplitude loading. Fig. 14 shows the post-cyclic response of tests in Tables 4 and 6 (except test L.C1 where reloading was not performed). The cyclic response is omitted from the plots for clarity, and the undisturbed monotonic response (backbone curve) is plotted dashed for comparison. ...Similar publications
The article presents the results of the study of the effect of the amount of large-size filler (fraction 5.0-0.65 mm), introduced into the fine sand at its angle of “internal friction”; the design of the developed pile with “pockets” on the side surface, which are filled with filler from the collar (funnel) on the surface of the soil and allows you...
Developing a new design method for offshore monopiles has been a big challenge for several authors for decades. The challenge is proposing an accurate design methodology to account for factors affecting the cyclic response of cyclic laterally loaded monopiles, which are poorly accounted for in current guidelines such as API and DNV. These factors a...
Due to the development of dedicated software and the computing capabilities of modern computers, the application of numerical methods to analyse more complex geotechnical problems is becoming increasingly common. However, there are still some areas which, due to the lack of unambiguous solutions, require a more thorough examination, e.g., the numer...
Advanced constitutive models that are capable of reproducing different aspects of sand behaviour have been an object of study for many years. The models developed in the critical state soil mechanics framework, with a formulation based on the state parameter, are of particular interest as they can predict the sand response over a full range of stre...
In this work, the authors have used 63 high strain dynamic pile load tests conducted by the authors in different offshore environments to develop neural network models, accurately predicting the total ultimate pile capacity and shaft resistance and end bearing. This model is developed explicitly considering the offshore pile driving scenario, where...
Citations
... Studies on single piles have been conducted to examine the lateral capacity, the impact of rigidity, and novel pile foundations like tapered piles [4] and fin piles, tripod, and helical piles are used [5][6][7]. Richards et al. [8] studied the rotational behavior of monopiles subjected to complicated cyclic lateral loading in sand and reported that large-diameter open-ended steel piles have grown in size over time to support greater OWTs and in deeper water. Sung-Ha Baek and Joonyoung Kim [9] studied the effect of soil properties, specifically the existence of silt in the sand matrix, on the response of piles to cyclic loading and investigated how variations in soil composition, density, and moisture content can affect the magnitude and distribution of lateral soil pressure along the pile shaft, ultimately influencing its structural response and stability. ...
... Offshore Wind Turbines Foundation/Platform Types(Richards, 2019). ...
... Lateral Load Acting on OWT Supported on Monopile Foundation(Richards, 2019). ...
The offshore wind energy sector is growing rapidly as a means to achieving target net zero GHG (Greenhouse gas) emission by 2050. This technology is been expanded to the offshore wind turbines (OWTs) both bottom-fixed and floating foundations. The foundations of such structures have to be optimized geometrically such that they are robust and resilient against environmental loads from wind, waves and currents which are usually cyclic, multidirectional and complex in nature.
Accurate modelling of the foundation behavior for bottom-fixed and floating OWTs is critical in predicting the global response of the system. This aspect of the integrated design of the entire OWT system is not very much understood in design software programs, typically resulting in empirical modelling to represent the constitutive behavior of the foundation or, and in extreme cases, models that assume the foundation is infinitely rigid (fixed) in many structural and geotechnical engineering applications.
The foundation modeling for OWTs requires considering accumulated rotational deformations due to combined cyclic and sustained loading, which affects the foundation stiffness. Additionally, it needs to account for the coupling of loads from different directions. Such effects have been rarely accounted for comprehensively with simple numerical models.
But, there exists certain specialized models like the Houlsby-Abadie Ratcheting Model (HARM) strongly rooted in the kinematic hardening principles within the hyperplasticity (thermo-mechanical) framework, which enables capturing of the accumulated deformations over many cycles (typically millions) of cyclic loadings. Then, there is the REDWIN model which is an acronym referring to “REDucing cost in offshore WINd by integrated structural and geotechnical design”, and it account for the multidirectional load coupling.
This research focuses on programming, improving, and developing a novel constitutive model called CLAP an acronym for “Cyclic Loading & Analysis of Piles”. The output of this work can be further enhanced and used for various applications and case studies on complex multidirectional cyclic loading in the offshore industry.
... This ratcheting behaviour is commonly observed when studying the cyclic loading response of foundations in dry sand (e.g. Leblanc et al. 2010, Abadie et al. 2019, Richards et al. 2020, Frick & Achmus, 2020. Figure 5(b) and (c) show the evolution of settlement and residual rotation with cycle number with respect to the cyclic moment sequence applied on top of the foundation (Figure 5(a)). The residual rotation is defined as the rotation at minimum load within a cycle after the foundation has effectively been unloaded. ...
... Monopiles which are widely used to support offshore wind turbines are subjected to complex lateral loading from wind, wave and current (Martínez-Chaluisant et al. 2010). The direction, magnitude and frequency of the lateral loading keep changing over time (Richards et al. 2020). The soils surrounding those monopiles may deform with increasing the number of loading cycles, leading to tilting of the whole structure. ...
... Only sinusoidal load can be achieved as the constant rotational velocity of the mass. In Richards et al. (2020), the monopiles are subjected to more complex multidirectional loading using two perpendicular electric actuators. ...
... Perpendicular cyclic loading tests were performed at 100g. As commented by Richards et al. (2020), perpendicular loading tests may provide fundamental insight and may also represent misaligned wind and wave loading in the field. As shown in Fig. 9, cyclic loading was applied in x-direction, while the loading in ydirection remained constant. ...
Offshore wind turbines are usually founded on monopiles. During the operation period, the structure is subjected to complex lateral loading from wind, wave and current. The soils surrounding those monopiles may deform with increasing the number of loading cycles, leading to tilting of the whole structure; hence, it is vital to carry out physical model tests to examine the long-term performance of monopiles. This study proposes an innovative experimental setup for centrifuge modelling of the response of monopiles under complex lateral loading. Hydraulic actuators are adopted to apply lateral loads on model pile, and electrohydraulic servo-valves and associated controllers are used to achieve a closed loop position or load control. A carefully designed spherical hinge and load bars are used to connect the model pile and actuator shafts. This enables that the pile can rotate freely and can move vertically freely. A centrifuge test on a winged monopole subjected to perpendicular lateral loading was carried out at 100g. The experimental results shed light on pile responses in the cyclic loading and constant loading directions.
... Despite their importance, the effects of drainage have not been systematically explored. The majority of experimental studies focus on dry or fully drained conditions (Ma et al., 2021;Rathod et al., 2021;Richards et al., 2020Richards et al., , 2021, while the associated numerical studies (Barari et al., 2017;Corciulo et al., 2017;Cuéllar et al., 2014;Liu and Kaynia, 2022) provide useful insights, however, due to significant computational demands, they could not be used, until recently, for systematic investigations. Takahashi et al. (2022) performed centrifuge tests involving monopiles in saturated dense Toyoura Sand (D r ≈ 80%). ...
... The rotational response under multi-directional loading of an OWT has been considered mainly for monopiles in sand, as reported by Su and Li (2013), Rudolph et al. (2014), Sheil and McCabe (2017), Nanda et al. (2017) and Richards et al. (2020). Equivalent studies for suction caissons have considered a layered seabed (sand underlain by clay), as reported in Zhu et al. (2018b). ...
... Consideration of scaling effects is required when interpreting laboratory floor single gravity experiments. Our approach follows that outlined in LeBlanc et al. (2010) and utilised in the experimental work of Zhu et al. (2013), Abadie et al. (2018), Zhu et al. (2018a) and Richards et al. (2020). This scaling framework builds on the stress-dilatancy relationships initially proposed in Bolton (1986), which recognises the importance of reducing the relative density of a reduced scale single gravity sand sample such that the soil dilatancy response is representative of that at full scale. ...
... Equation (2) for ζ c = 0.8, which provides a lower bound to the unidirectional data (see Fig. 13(c)), also bound the data from the multidirectional tests, such that predicting the accumulated rotation for the more complicated multidirectional loading can be simplified as lying within the range given by Equation (4) for ζ c = 0.1 and 0.8. This is similar to findings reported by Zhu et al. (2019) for multidirectional loading of caissons in sand over clay, but contrasts with findings for monopiles in sand subjected to continuous or random changes in loading direction (Rudolph et al., 2014;Nanda et al., 2017;Richards et al., 2020), where the accumulated rotation due to a varying load direction was higher than projections based on an appropriate single load direction due to the underlying micro-mechanical response as discussed earlier. Fig. 22 shows the normalised unloading stiffness in tests MD3 and MD4 together with that from unidirectional test SO2 (with the same ζ b and ζ c ). Evidently the normalised unloading stiffness in the multidirectional and unidirectional tests are similar over the first 45,000 cycles as the load direction is the same at ψ L = 0 • . ...
Cyclic loading features in many applications. Questions important for design include: Does the monotonic capacity increase or decrease following cyclic loading? How does foundation rotation, stiffness and damping evolve? This is investigated here for suction caissons in sand, looking to applications as foundations for offshore wind turbines where changing stiffness, capacity and accumulated rotation can be critical, and soil damping is being looked at more closely. The problem is investigated experimentally through a series of single gravity monopod caisson tests in saturated sand subjected to unidirectional or multidirectional cyclic loading with between 360,000 and 106 cycles applied in each test. Results from the unidirectional tests are consistent with previous experimental studies, whilst also demonstrating the expected changes in damping ratio during cyclic loading for a monopod caisson in sand. The multidirectional tests reveal more significant and potentially important findings, particularly on the very significant increases in unloading stiffness and damping ratio associated with load direction changes.
... In this study, due to the low effects of the Reynolds number, the wind speed was increased to maintain the level of soil strain [30]. Accordingly, dimensionless numbers including length, mass, frequency, and soil strain (see Table 1) were adopted and well-controlled in this study in accordance with the literature [31][32][33]. Details concerning the adopted dimensionless groups are demonstrated in Table 1. ...
A monopile is the most popular foundation type for wind turbines. However, the dynamic performance of monopile-supported wind turbines under different operating and ground conditions is not fully understood. In this study, an integrated monopile-supported wind turbine model in a wind tunnel was employed to jointly simulate the operating and ground conditions. A series of wind tunnel tests were designed and performed to investigate the dynamic performance of monopile-supported wind turbines. These tests included seven operating conditions (seven wind speeds and corresponding rotor speeds) and four ground conditions (one fixed ground condition and three deformable ground conditions with different soil relative densities). According to the test results, the structural responses and dynamic characteristics were analyzed and discussed. This shows that the assumption of fixed-base support significantly overestimates the natural frequency but underestimates the global damping ratio. With the increase in soil relative density, the natural frequency slightly increases, while the damping ratio decreases more significantly. With the increase in the wind speed and rotor speed, the increase in global damping is larger on softer ground. A regression analysis was performed to estimate the global damping ratio under different operating and ground conditions.
... The soil sample used in this study is Yellow Leighton Buzzard silica sand (LBS), which has an average particle size 50 of 0.81 mm, a specific gravity of 2.65, and maximum and minimum void ratios of 0.8 and 0.47, respectively (Richards, 2019). Two important scaling phenomena were considered to ensure relevance for field conditions. ...
Accurate prediction of soil settlements induced by open caisson construction in sand is essential for safe and reliable delivery of critical underground urban infrastructure. This paper presents a novel prescriptive design approach using a neural network (NN) constrained by empirical relationships, referred to as an 'empiricism-constrained neural network'. The proposed approach is benchmarked using a traditional closed-form empirical expression. Both methods are calibrated using experimental data from reduced-scale laboratory testing for the prediction of surface and subsurface settlement trough shape and magnitude. The outcomes demonstrate that while both methods accurately capture the primary effect of caisson depth on surface and subsurface soil settlements, the NN approach exhibits superior prediction accuracy. These methods are developed in a form amenable for routine design use in industry and have the potential for broader applicability in other design scenarios, such as building damage assessment and risk-assessment exercises.
... Using simplified models such as spring-dashpot models or p-y springs for modeling monopile (MP) and suction bucket foundations (SBF) is challenging, as it can be accompanied with remarkable error due to the effect of large diameter of these foundations in comparison to the conventional piles used in the oil and gas industry, from which they were originated, in terms of underestimating the small-strain stiffness of soil. The efforts made to overcome such issues led to the PISA project (Burd et al. 2020a, b;Byrne et al. 2020;Richards et al. 2020). More importantly, such models, in spite of their benefits such as the great simplification they make in the analysis and design process, are incapable of modeling pore water pressure generation and liquefaction (Kaynia 2019). ...
... Figure 7A illustrates the lateral displacements of the monopile subjected to different loading conditions, while Figure 7B The loading characteristics vary according to the significant spatial variations of the wind and waves in the application area, but the most frequently used cyclic loading frequencies are in this range. 39 In the initial model, loading was applied with a frequency of 0.16 Hz, and then parametric studies were carried out with different frequencies. ...
Offshore wind turbines play a critical role as a renewable energy source and are experiencing continuous growth in usage. Both the design and implementation phases of constructing these structures present difficulties. It is crucial to ensure these structures are built to resist such conditions, assuring their durability, as they are exposed to lateral external influences such as wind and wave loads. This study investigated how monopile foundations behave in saturated sandy soil under cyclic loading. Pore water pressure accumulations in saturated sandy soil, monopile head lateral displacements, and vertical settlements around the monopile are investigated using the hypoplastic material model and two‐phase element with the ANSYS finite element program. Analyses conducted in this study demonstrated that lateral cyclic loads could cause excessive pore water pressure accumulations around the monopile, leading to displacements in the monopile head and soil settlements around it, highlighting the importance of carefully considering loading characteristics during the design process to provide the security and longevity of offshore wind turbines.
