Monopile Offshore Wind Turbine Research Articles

Experimental Investigation of Nonlinear Forces on a Monopile Offshore Wind Turbine Foundation Under Directionally Spread Waves

Abstract Accurate prediction of nonlinear wave loading is crucial for designing marine and offshore structures, yet it remains a challenging task. Prior research has primarily focused on uni-directional extreme sea states, revealing that linear loading cannot accurately represent the total wave forces acting on offshore wind turbine foundations, with significant contributions from high-order harmonics. This study broadens the scope to include multi-directional and bi-directional wave interactions with monopile offshore wind turbine foundations. We use a phase-based harmonic separation method to isolate harmonic components in the presence of complex wave scenarios. This approach allows for the clear delineation of individual harmonics from the total wave force by controlling the phase of incident focused waves. Remarkably, this method shown effective even with multi-directional and bi-directional spreading. The clean separation of individual harmonics enables the estimation of contributions from each harmonic. Our findings are in line with previous research, showing that nonlinear loading can constitute up to 40% of the total under certain wave conditions. We have also observed that wider wave spreading reduces nonlinear high-order harmonics, and uni-directional waves induce the most severe nonlinear forces. These insights emphasise the importance of accounting for high-order nonlinear wave loading in offshore structure design.

Dynamic responses of monopile offshore wind turbines under different wind-wave misalignment angles

Wind-wave misalignment is a common occurrence for monopile offshore wind turbines during extreme typhoon conditions. This phenomenon has a significant impact on the structural dynamic response of turbine structures and poses a threat to their structural integrity. In this study, finite element analysis is conducted on 75 sets of working conditions of monopile offshore wind turbines to investigate their dynamic response under different wind turbine operating states, wind-wave misalignment angles, and randomness of wind and wave load conditions. The results of the analysis indicate that the impact of wind-wave misalignment angles on the dynamic response of the monopile offshore wind turbine varies depending on the operational conditions. Furthermore, the study reveals that there are variations in the dynamic response of the monopile offshore wind turbine under different combinations of wind and wave loads. During the parked state and yaw system fault of the monopile offshore wind turbine, the most adverse working conditions need to consider wind-wave misalignment angles of 0° and 180°. In contrast, the most adverse working conditions for operational states emerge when the wind-wave misalignment angle is 90°. These findings highlight the importance of considering wind-wave misalignment angles and the randomness of wind and wave load conditions when assessing the dynamic response and structural integrity of monopile offshore wind turbines.

Study on The Response Characteristics of Monopile Offshore Wind Turbines under Non-Gaussian loads in Typhoons

The fan structure is often seriously damaged under typhoon, so the study of the fan structure under typhoon is of great significance for its disaster prevention and mitigation. The actual measurement shows that the typhoon has obvious non-Gaussian characteristics, and the influence mechanism of this characteristic on the fan response characteristics is not clear. Therefore study the response characteristics of fans under non-Gaussian load. First, based on the autoregressive AR model, combined with the Johnson conversion model (JTM); second, the wind load is determined based on the specification; then the wind load. Finally, the response characteristics of 5MW fan are studied by characterizing the numerical cases of non-Gaussian fan with different strength of typhoon. The results show that the non-high characteristics of non-Gaussian wind field and the non-Gaussian characteristics become stronger with the strength of the non-Gaussian characteristics of wind field.

Development of a mini vibro-driver for pile testing in the centrifuge

Using vibro-installation to fully install monopiles for offshore wind turbines (OWT) has the potential to mitigate pile run, significantly reduce noise emissions to the marine environment, and decrease project costs. However, due to the inherent complexity of vibro-installation and the limited quantitative data available in the public domain, the mechanisms underpinning vibro-driving are not fully understood. This results in uncertainties in engineering predictions of vibro-drivability and the post-installation performance of monopiles. To address these issues, an innovative mini vibro-driver, designed specifically for use in a geotechnical centrifuge, was developed to study vibro-installation and the lateral loading response of monopiles in sand after installation. The paper outlines the challenges addressed in the design of the vibro-driver, its working principles, and its capabilities. Following this, the results from vibro-driven pile installation tests are discussed and provide an indication of the type and quality of data obtained. These data can contribute to improving the understanding of vibro-driving mechanisms and thereby improve future engineering predictions.

Practical considerations for numerical simulation of large-diameter monopiles for offshore wind turbines in overconsolidated sand: A case study of the VIBRO project in Cuxhaven, Germany

Monopiles are typically the preferred foundation for offshore wind turbines, which are becoming increasingly large. While 3D finite element modelling can aid the design process of monopile foundations, achieving reliable results requires precise constitutive soil models and input parameters derived from laboratory and field data, as well as careful consideration of soil sample preparation method. This study explores various soil preparation methods, including moist tamping, dry tamping, and air pluviation, along with two sample ratios (1 and 2) during triaxial tests. It uses an advanced soil model, the DeltaSand model, and a practical industry model, the Hardening Soil model with small strain (HSsmall). To evaluate model performance, the Vibro project performed in Cuxhaven, Germany, is used as a case study. Simulations using the DeltaSand model show good agreement with field data. The effects of soil preparation methods and sample ratios are more pronounced in HSsmall model simulations, in which better results are obtained when the model is calibrated using dry tamping data and a sample ratio of one.

Investigation of essential parameters for the design of offshore wind turbine based on structural reliability

The probabilistic-based design method is gradually gaining attention in the wind industry because it provides more accurate modeling of uncertainty variables than that of traditional methods. Unfortunately, the numerous uncertainty variables involved in structural design are major obstacles to the successful application of this method. Therefore, this study presents a sensitivity analysis (SA) of a benchmark monopile offshore wind turbine (OWT) to screen the top-ranking variables from the viewpoint of reliability. Primarily, a comprehensive reliability SA framework of OWT is proposed, in which a novel measurement of soil uncertainties is conducted using quantitative analysis from the perspective of soil structure interaction (SSI). Subsequently, a reliability SA is conducted to explore the crucial variables influencing the structural safety from the uncertain clusters. The results indicate that Young's modulus, structural geometry, and SSI have significant effects on the structural reliability of excessive vibration failure. The hydrodynamic and aerodynamic load variables exhibit the most prominent influence on excessive deflection failure. Additionally, the SSI uncertainties exhibit a non-negligible effect in affecting the structural reliability, i.e., the lateral bending stiffness shows more sensitivity to the normal operation cases, whereas the impact of joint stiffness is more remarkable in parked scenarios.

A semi-analytical approach for dynamic responses of monopile-supported offshore wind turbines subjected to accidental loads

Offshore wind energy is leading the way in sustainable energy generation. Offshore wind turbines (OWTs) need to be designed to withstand extreme or accidental loads in Ultimate Limit States (ULS) and Accidental Limit States (ALS), whereby high order eigenmodes may become significant. This paper introduces a semi-analytical approach for efficient and reliable analysis of the dynamic responses of monopile-supported OWTs subjected to accidental loads. A Rayleigh-Ritz solution is initially adopted to determine the high-order natural frequencies and eigenmodes of monopile OWTs, explicitly considering tapered towers and soil-pile interactions. Dynamic responses of OWTs are then calculated depending on the interaction schemes between accidental loads and the structures, e.g. soft contact loads (e.g. slamming, wind) and hard contact loads (e.g. ship collisions, ice impact). For soft contact loading conditions, the loads are considered independent of turbine responses, and the transient dynamic response is computed using the classical modal superposition method. While for hard contact loading conditions, the load actions depend on the contact stiffnesses and responses of the interacting bodies. A numerical contact algorithm is thus developed, and numerical iterations are performed to ensure convergence. The proposed approach is applied to the DTU 10 MW monopile-supported OWT subjected to extreme water slamming and ship collisions, respectively as example applications of soft and hard contact scenarios. The results are verified against nonlinear finite element analysis using USFOS and discussed with respect to the turbine natural frequencies and eigenmodes, contact forces and dynamic responses. A parametric analysis is conducted for ship-OWT collisions, exploring different impact scenarios by varying ship sizes, contact stiffnesses, and initial impact velocities. The proposed approach can serve as a promising tool for accidental load response analysis and the design of monopile OWTs.

Hydrodynamic performance of a monopile offshore wind turbine in extreme wave groups

Dynamic responses of offshore wind turbines, particularly under extreme wave excitations, are crucial to structural design. Extreme waves, e.g. breaking waves, are random processes, which induce uncertainty in the estimation of dynamic responses. This type of uncertainty has been accommodated through a safety of factor which is conservative and thus leads to cost inflation. To reduce the cost and enable economically viable development of offshore wind, it is essential to understand the dynamic process of offshore wind turbines under extreme wave excitation scenarios. However, it is challenging to predict these dynamic responses through numerical simulations, due to the nonlinearity and randomness in such a complex process. To overcome these challenges, this study conducted a series of physical model testing to better capture the extreme wave induced loads on a monopile, which are then served as input for numerical modelling. Through this hybrid model, different vibration mechanisms are revealed, which shed lights on further cost reduction in the design of offshore wind turbines.

Seismic vulnerability and performance-based assessment of monopile offshore wind turbine considering turbine blades

This paper investigated the seismic vulnerability and performance-based assessment of offshore monopile wind turbine (OWT) considering turbine blades. A three-dimensional finite-element model of NREL 5-MW OWT was established with detailed consideration of the nonlinear-behavior of turbine, monopile, blades, pile-soil interactions and pile-water interaction etc. Subsequently, incremental dynamic analysis and fragility evaluation were conducted via a great number of nonlinear time history analysis involving far-field and near-fault records with and without pulse characteristics. It is found that the ignorance of turbine blades in conventional lumped mass (LM) model would significantly underestimate the vulnerability of OWT's acceleration responses, and more importantly, underestimates the stress level at turbine top, misleading the failure pattern of OWT which may be probably damaged at both top and middle part of OWT rather than solely the middle part as estimated by the LM model ignoring the effect of turbine blades. Finally, it can be concluded that the modeling of turbine blades should be consider in the numerical studies.

Development and prediction of scour around offshore wind turbine monopile foundations in ebb and flow tides

Monopile foundations are commonly used to support offshore wind turbines (OWTs) in nearshore shallow waters, where the periodic ebb and flow of tides can cause severe seabed erosion around these foundations. Accurately predicting the progression of scour depth under these tidal conditions is crucial for the safety of OWTs. While a few empirical formulas for equilibrium scour depth in periodic tides have been proposed, they often overlook the backfilling effects during their derivation. Consequently, these methods fail to predict the progression of scour depth characterized by recurring scouring and backfilling processes. In this study, the scour progression under different tidal patterns, including square tides, symmetrical sinusoidal tides, and asymmetrical sinusoidal tides with various periods, is investigated using numerical methods. It is indicated by the results that deeper scour depths are produced by symmetrical tides compared to asymmetrical tides, while the highest scour depth is observed under square tides, potentially slightly exceeding that under unidirectional currents. Additionally, asymmetric scour depths at the front and back sides of the pile can result from the asymmetry of tides. Although scour progression is affected by tide periods, their impact on the equilibrium scour depth is limited. Furthermore, a new concept called normalized cumulative effective flow intensity is introduced, which plays a primary role in governing equilibrium scour depth. Based on this concept, prediction methods for both equilibrium scour depth and scour progression under sinusoidal tides, which closely resemble natural tidal patterns, are proposed. The accuracy of these methods is validated by experimental data from previous studies. These findings not only provide a method to optimize the design of monopile burial depth but also aid in the development of appropriate maintenance measures to prevent scour-induced damage to OWT foundations over their long lifespans.

Numerical study on the flow characteristics of an integrated fish cage based on the monopile offshore wind turbine foundation

The integration of net cages and offshore wind turbines can effectively develop and utilize marine resources and is one of the important ways of intensive use of the sea. The numerical simulation of the hydrodynamics of an integrated fish cage based on the monopile offshore wind turbine foundation in currents is carried out. The realizable k-ε model is adopted and the porous media model is used to simulate the net. To verify the validity of the numerical model, the numerical result is compared with the data of the corresponding literature. The results show that the numerical method is effective. On this basis, the flow field distribution results of the integrated structure at different velocities are calculated. At the center of the downstream zone of the integrated structure, the time-average velocity is lower than 80 % of the initial velocity. The effects of the net solidity and the cage draught on the flow field characteristics of the integrated structure are analyzed. The results show that the increase of net solidity will significantly reduce the velocity inside the net cage and in the downstream zone of the cage. As the net solidity increases from 0.14 to 0.32, the average velocity attenuation inside the net cage increased by 13.9 %. The change of cage draught will affect the velocity distribution in the horizontal and vertical directions. When the cage draught increases, the velocity on both sides in the downstream zone of the integrated structure increases more than 5 %. It is expected that the results of this study can provide data support for the design of integrated structure of net cages and offshore wind turbines.

Improving the scour-resistance performance of for offshore wind turbines: Experimental and computational fluid dynamics studies on macro and micro characteristics

To enhance the resistance to local scour around offshore wind turbine monopiles, 15 mixtures were designed based on Response Surface Methodology (RSM). Cement content, sodium silicate content, and rubber powder content were selected as independent variables. After determining their flowability, the compressive strength and shear strength were measured after curing in pure water and artificial seawater for 3 days, 7 days, 14 days, and 28 days. Experimental results indicate significant improvement in the mechanical properties of the modified soil, including increased Unconfined Compressive Strength (UCS), internal friction angle, and cohesion. The optimal mix ratio is identified as CSR40–10–15, consisting of 40 % cement, 10 % sodium silicate, and 15 % rubber powder. The strength variation mechanism is elucidated from both macroscopic and microscopic perspectives. Finally, numerical simulations using Computational Fluid Dynamics (CFD) software validate the scour resistance performance based on the optimal mix ratio of flowable solidified soil, offering a new approach for local scour protection around offshore wind turbine monopile.

Investigation of soil damping for offshore wind turbine monopile-soil system in stiff clay on elastoplastic-damage model

Damping plays a crucial role in the design of offshore wind turbine (OWT) monopile foundations. The soil damping of the monopile-soil system (MSS) represents the energy dissipation mechanism arising from the interaction between the pile and the soil. It is typically derived by back-calculating from the overall damping measured in the entire OWT structure. However, few studies have independently examined the soil damping in MSS, and the impact of key parameters such as pile diameter, pile embedded depth, cyclic load amplitude, and load eccentricity on the variation of soil damping in MSS remains unclear. This paper introduces an elastoplastic-damage constitutive model for the numerical simulation of the damping ratio variation in seabed soil and MSS. The model is implemented in ABAQUS software and validated against cyclic triaxial tests on stiff clay soil. On this basis, a three-dimensional finite element sensitivity study was conducted to elucidate the effect of these key parameters on the MSS damping ratio. The results of the study reveal that the MSS damping ratio exhibits a nonlinear and asymmetric trend as the loading cycles increase. The MSS damping ratio decreases with increasing pile diameter and embedded depth but increases with increasing lateral cyclic load amplitude and load eccentricity from the mudline.

Numerical analysis of the hydrodynamic response of an aquaculture-cage group in offshore wind farm

Offshore wind turbine and aquaculture cage integration represent a significant research theme for economic growth and establishing a marine ecological civilisation. To optimise offshore space utilisation, this study proposes a wind-aquaculture spatial integration strategy comprising four monopile offshore wind turbines and an aquaculture-cage group (OWT-ACG) with 32 cages in a 4 × 8 configuration. Based on the lumped-mass method, a mathematical model is established. In addition, the effect of the floating collar stiffness on the hydrodynamic response of the cage group and the nonlinear characteristics is analysed. The results indicate that the flexible collar significantly reduces the heave response of the cage but markedly increases the surge, pitch, and mooring tension of the cages relative to the rigid floating collar. Furthermore, the presence of current enhances the nonlinearity of the fairlead motion windward, reducing the nonlinearity of the fairlead motion leeward and increasing the fairlead depth into the water. The findings presented herein provide valuable insights and guidance for engineering practices regarding the design and analysis of wind-aquaculture integration.

Influence of precast microbial reinforcement on lateral responses of monopiles

Microbially Induced Calcium Carbonate Precipitation (MICP) provides an environmentally friendly solution for reinforcing large diameter monopiles for offshore wind turbines (OWTs). This study presents an investigation into the lateral responses of monopiles with precast microbial reinforcement using a low-pH one-phase method. Both static and cyclic loading tests were carried out. The results of static loading tests show that the failure mode of the bio-reinforced monopile was an overall overturn failure. The lateral bearing capacity was increased by 50% and the bending moment was reduced by about 25% with the bio-reinforcement. Further investigation was conducted on the secant stiffness, damping ratio, and accumulated deformation of the bio-reinforced monopile under various cyclic loadings. With the bio-reinforcement, the accumulated deformation under one-way cyclic loading can be reduced by 30%–60%. The influences of cyclic loading parameters and loading sequence on pile stiffness were clarified. The growth ratio of pile stiffness due to bio-reinforcement under one-way loading was between 1.65 and 2.82, and decreased with increasing load amplitude. The ratio was smaller under two-way loading. Three competing factors on pile stiffness were identified: cyclic compaction of the unreinforced sandy soil, weakening of the bio-reinforced soil and soil subsidence around the bio-reinforced soil.

Vibration control and stroke evaluation of bi-directional bistable nonlinear energy sinks applied in monopile offshore wind turbines during emergency shutdown

Monopile offshore wind turbines (OWTs) in complex wind and wave environments can generate more excessive vibrations in the fore-aft (FA) and side-to-side (SS) directions when emergency shutdowns occur, and therefore damping techniques to alleviate such vibrations are essential. Due to the potential frequency detuning (reduced damping effectiveness) of traditional tuned mass dampers (TMDs) and stroke issues (such as large strokes of TMDs and limited maximum strokes of unidirectional nonlinear energy sinks (NESs)) in existing absorbers, this paper proposes a bi-directional bistable nonlinear energy sink (BBNES) to mitigate the vibration of the OWT in the FA and SS directions during the emergency shutdown. First, the proposed device and its operation mechanism are elaborated, and an optimization method for the device is introduced. Then, the dynamic analysis model of the BBNES is developed for vibration control of the OWT and incorporated into a fully coupled numerical simulation tool with high fidelity, FAST v8. Subsequently, the responses of the OWT without and with the proposed BBNES and the bi-directional tuned mass damper (BTMD) are exhaustively investigated under diverse conditions (including mass ratios, wind-wave loadings, emergency shutdown times, wind-wave misalignments, and 1st mode natural frequencies of the OWT) to reveal their performances and compare them. The research results show that under most conditions, the BBNES and BTMD have a similar damping performance in the FA direction, whereas the BBNES has a slightly lower damping performance in the SS direction than the BTMD. In addition, except for the wind velocity of 8 m/s at the hub and the wind-wave misalignment of 90°, the FA stroke of the BBNES with a small mass ratio (i.e., 0.01∼0.035) is significantly smaller than that of the BTMD with the same mass ratio by more than 1.1 m, and even up to about 4 m. The SS stroke of the BBNES is slightly greater than that of the BTMD, and in most cases, the difference between both is less than 0.52 m. Therefore, the BBNES is superior to the BTMD in this regard. When the 1st mode natural frequency of the OWT decreases, the BTMD can lose its tuning efficacy and makes the tower more susceptible to resonance caused by 1p and wave loadings, while the BBNES is not affected by the 1st mode natural frequency variation of the OWT, demonstrating its stronger robustness.

Application of PISA method for monopile design in layered soil

The foundation design for offshore wind turbines (OWTs) is critical to ensuring their structural integrity and long-term stability. Large-diameter monopiles are widely used for OWTs owing to their proven effectiveness. The conventional design approach, derived from the oil and gas industry’s p-y model, has limitations when applied to monopiles for OWTs. Consequently, a novel design method based on the PISA joint industry research project was developed, which extends beyond the traditional lateral soil resistance (p-y curves) and incorporates additional soil reaction components from shaft friction, pile base shear, and base moment. Unlike the simplistic rule-based conventional approaches, the proposed method necessitates site-specific 3D finite element (FE) simulations for precise calibration of soil reaction curves. Although successfully applied in European offshore wind farms, the PISA method’s performance in regions with distinct seabed compositions, such as China, remains to be verified. Therefore, this paper presents a comprehensive case study conducted at the Xiangshan wind farm, a representative site characterized by layered soil. The study employed in-situ investigations (cone penetration test, borehole) and laboratory tests (direct simple shear, bender element) to delineate the seabed soil profile and derive essential soil parameters for accurate foundation design. To facilitate the design, an Abaqus-based design tool was developed, which automated the extraction of soil reaction curves from the 3D FE simulations, calibration of PISA springs, and execution of one-dimensional beam-spring analyses employing Timoshenko beam theory. Comparative analyses were performed on the computed responses of piles, encompassing load-deflection curves, pile deflections, and bending moment profiles. The outcomes from the 3D FE simulations, uncalibrated rule-based PISA model, and calibrated PISA model were compared to validate the PISA method for monopile design in layered soil conditions.

Numerical simulation of offshore monopiles with reinforcement in shallow soil layer

With the development of large-scale offshore wind turbines (OWTs), monopile foundations require larger diameter and greater embedded depth to satisfy the higher bearing requirements. Large-diameter monopiles for OWTs are mainly loaded horizontally, and the horizontal resistance provided by the shallow soil plays an important role in the loading of the monopile foundation. Therefore, shallow soil reinforcement (such as cement grouting and bio-grouting) is an effective method to improve the bearing capacity of the monopile. In this study, the lateral static loading mechanism of a shallow-layer reinforced monopile was investigated by finite element method (FEM). The effect of reinforcements with different stiffness values (elastic modulus), different strengths (cohesion and friction angle) and different reinforced ranges (width and depth) was analyzed on the lateral bearing capacity, deformations, soil resistances, and soil resistance per unit length to pile deflection (p-y) curves of monopiles. Based on the numerical results, an empirical formula was proposed for predicting the reinforcement coefficient of the bearing capacity. The effects of different reinforcement parameters on the distribution of soil resistance were analyzed, and the soil resistances obtained by FEM were compared with those from a theoretical formula. It was found that the resistance provided by the reinforced soil was clearly reduced at the interface with the unreinforced soil, and the resistance of the unreinforced soil was not significantly affected by the reinforced soil.

Numerical Investigation of Local Scour Protection around the Foundation of an Offshore Wind Turbine

The pile foundations of offshore wind turbines face serious problems from scour damage. This study takes offshore wind turbine monopile foundations as the research object and proposes an innovative anti-scour device for the protection net. A numerical simulation research method based on CFD-DEM was used to model the local scour of the pile foundation and protection net. The validity of the numerical model was verified by comparing the simulation results of the local scour of the pile foundation under the condition of clear water scour and the results of the flume test. The permeability rate was defined to characterize the overwatering of the protection net, and numerical simulations were performed for protection nets with permeability in the range of 0.681 to 0.802. The flow field perturbations, changes in washout pit morphology, and changes in washout depth development due to the protective netting were also analyzed. It was found that the protection net can effectively reduce the flow velocity around the pile, cut down the intensity of the submerged water in front of the pile, and provide scouring protection. Finally, the analysis and summary of the protection efficiency of the different protection nets revealed that the protection efficiency within the nets was consistently the highest. On the outside of the net, the protection efficiency is poor at a small permeability rate but increases with an increasing permeability rate.

Probabilistic analysis of lateral behaviour of offshore wind turbine monopile considering uncertainties of geological model and loads

Due to the complex geological, meteorological and hydrological conditions, along with the limited availability of field measurements, multiple uncertainties are related to the performance analysis of the offshore wind turbine. The main sources of uncertainty are the geological model and the environmental loads. However, the geological model uncertainty and loads uncertainty were often characterized separately in previous studies. To this end, this article proposes a novel framework for the coupled characterization of the geological model and loads uncertainties. With this new framework, the geological model is generated using a conditional random field method, in which the stratigraphic and geo-properties uncertainties are characterized simultaneously. Next, environmental parameters are considered as random variables, and the cross correlation between different environmental parameters is captured with the Pearson correlation coefficient. Then, multiple realizations of the geological model and these of environmental parameters are sampled with Monte Carlo simulation. With these generated realizations as inputs, the monopile foundation performance could then be analyzed probabilistically considering uncertainties of the geological model and loads. To demonstrate the effectiveness of the new framework, a case study of an offshore site in Taiwan is conducted. Further, a comparative study is performed to quantitatively reveal the influence of different uncertainty sources on site characterization and monopile behaviour assessment, through which the superiority of the presented framework is demonstrated.