Monopile Offshore Wind Research Articles

Behavior of PIP Slip Joint in the Offshore Wind Monopile under Combined Load Considering Local Buckling

Behavior of PIP Slip Joint in the Offshore Wind Monopile under Combined Load Considering Local Buckling

Time development of live-bed scour around an offshore-wind monopile under large current–wave ratio

Scour around a circular monopile in coastal regions has been investigated extensively over the past decades, but the time development of scour depth around an offshore-wind monopile under large current–wave ratio still lacks a predictive model. By considering the conservation of sand volume and adopting the conventional exponential law for temporal variation, a semi-empirical model, which has three parameters, i.e., an equilibrium scour depth, a shape coefficient and a scour characteristic time scale, is developed for predicting the time development of scour depth around a monopile under live-bed conditions of combined wave–current flows. A series of laboratory experiments was conducted in current-only and wave–current flows to obtain data for model calibration and validation. Experimental results indicate that adding weak waves on current accelerates scour development, which is successfully captured by the proposed model through using the far-field bottom shear stress as a key model input. The overall model inaccuracy is within 25%, and the model’s applicability is further confirmed by field measurements from an offshore wind farm in east China. This model can help to determine the timing of installing scour protection around offshore monopiles, especially for the circumstances with very strong local sediment transport (live-bed).

A case study of foundation damping in a piled offshore wind jacket structure

This paper presents an assessment of foundation damping in piled offshore wind jacket structures through a case study of the Block Island Wind Farm (BIWF). Foundation damping is one of several sources of energy dissipation which can improve fatigue performance in offshore wind structures. Damping was quantified through operational model analysis (i.e. system identification) using structural health monitoring data combined with foundation damping modeling. The results showed that for the 1st bending mode the hysteretic damping ratio of the pile under axial cyclic loading was estimated to range from 0.2% to 1.5%. This range was comparable in magnitude to the foundation damping ratios documented for operational offshore wind monopiles under lateral loading.

Re-evaluation of fatigue design curves for offshore wind monopile foundations using thick as-welded test specimens

The dominant majority of existing offshore wind turbines are supported using monopile foundations which are fabricated by welding thick steel plates. In the current fatigue design recommendations for welded steel structures, which are commonly used in the design of monopile structures, the initial inverse slope of the S-N curve is fixed to −3. The historic rationales for this assumption are the ease of fatigue life calculations, the effect of long-range residual stresses that were not originally captured in the test data on thin welded specimens, and analogy with fatigue crack growth. While this introduces an unquantified level of conservatism to deterministic fatigue calculations, it was an acceptable (and wise) assumption when the S-N design curves were generated a few decades ago; however, current designs and assessments require more accurate calculations and the need to more confidently quantify the likelihoods attached to these evaluations. In the present study, it is argued that unlike large braced structures (e.g. jackets) in which long-range residual stresses may remain in place as fatigue cracks propagate, in circumferential welds of monopile structures the natural relaxation of residual stresses, in the absence of long-range residual stresses, must be accounted for by performing fatigue tests on thick welded samples. This paper presents a thorough analysis of the fatigue test results obtained from 50 mm thick as-welded samples with representative profiles of residual stresses that exist in monopiles. Basic and advanced statistical approaches were employed in the analysis and higher values of inverse slope were found using the large thickness test data. The results from this study are compared with the recommended fatigue design curves available in international standards. The outcomes from this research draw important observations concerning the need to employ an appropriate value of inverse slope in the design of monopile structures by excluding the conservative approach imposed by a fixed value of −3 which was originally implemented for braced structures with the assumption of large and long-range residual stresses.

Integrated design and real-world application of a tuned mass damper (TMD) with displacement constraints for large offshore monopile wind turbines

Passive tuned mass dampers (TMDs) have been increasingly investigated for vibration control of wind turbines. However, TMD designs based on individual or a few operating conditions may result in insufficient consideration of the TMD's impact on wind turbines. For instance, the influence of end-stop collision forces on the main structural damage may be significantly underestimated, thereby affecting the overall lifecycle evaluation. In this study, focusing on large offshore fixed-bottom monopile wind turbines, we propose an integrated TMD design methodology taking the damper displacement constraints into full account. A comprehensive optimization of the passive omnidirectional pendulum-type TMD is conducted considering large number of operating conditions in the offshore environment. Next, a real-world application of the optimally designed TMD is performed for a 6450 kW monopile offshore wind on the Yellow Sea, China. We develop an on-site testing methodology and evaluation criteria for assessing the effectiveness of the TMD offshore. The results of resonance, emergency stop, operatingand reliability test conditions demonstrate the following: under the optimized TMD design taking into account all operating conditions, the TMD with a mass ratio of 1.56% can offer added damping in the fore-aft and side-side directions of the turbine, and provides the desired damping ratio of 1%; within a 4% frequency range around the design frequency, the TMD achieves effective vibration control; at rated power, the TMD achieves a reduction of more than 50% in the amplitude of the tower base side-side bending moment (Mx). The integrated TMD design with displacement constraints allows for the optimal TMD design across all operating conditions, thereby achieving superior overall vibration control and reduction in fatigue loads. The successful real-world application of the TMD based on integrated design methodology is of high value to the research community.

A New Ocean Rock Mass Rating and Its Application to Determine the Ultimate Bearing Capacity of an Offshore Wind Monopile Foundation

A New Ocean Rock Mass Rating and Its Application to Determine the Ultimate Bearing Capacity of an Offshore Wind Monopile Foundation

Experimental study on the sediment movement around monopile under vibration action

Offshore wind monopile foundations are subjected to strong wave current nonlinear effects and horizontal vibration loads caused by strong winds, waves, and rotating blades. In order to understand the rule of sediment movement around monopile under different vibration frequencies and amplitude, a cyclic loading test was carried out in the sand tank. The motion and migration rule of sediment particles around monopile under vibration action was observed by color tracer sand. The development process of the subsidence pit was qualitatively described by comparing the typical moment photos of sediment particle movement on the bed's surface with time. The transition zone of sediment particles was observed by vertical cutting of the sand bed. Then, the relationship between the relative time scale T0 and the vibration intensity under pure vibration load is analyzed, and the relationship between the equilibrium settlement depth and the initial settlement rate is obtained. The experimental results show that under vibration loading, the densification of the sand bed around the monopile foundation along the loading direction and the phenomenon of ratchet convective movement is relatively strong, and the relative time scale of settlement shows a tendency to decrease with the increase of vibration intensity; the multi-stage alternating loading of vibration and wave-flow loading will result in the backfilling and widening of the scouring pits; the rise in the amplitude and the frequency of vibration under the joint action of wave-current-vibration will decrease the depth of the equilibrium scouring, and the width of scouring pits will increase significantly with the increase of amplitude, and the backfilling of the sand will be more evident with the increase of the frequency of vibration.

Coupled stability of offshore wind monopile foundations and submarine slopes under periodic loads

Long-term cyclic loading on offshore wind monopiles will lead to a decrease in the mechanical properties of the seabed near the monopile's foundation, therefore, causing instability of the submarine slope and the monopile foundation. To capture the accumulation strain caused by high cycle numbers and plastic deformation of seabed sands, the high cycle accumulation (HCA) model is complied with Mohr-Coulomb constitutive models. The shear strength reduction (SSR) method is used to obtain the safety factor of submarine slopes. The coupled numerical model simulates a 6 MW offshore monopile foundation with a single periodic lateral load applied on the monopile at the sea level, which represents concentrated wind and wave loads in storm conditions. The results show that the plastic zones begin to form at the monopile foundation, then extends towards the slope toe and slope top, and eventually across the entire slope, leading to slope instability. In this process, the safety factor of the slope is significantly reduced with each stage of the process. It was found that both the sharp changes of load amplitude and large average load also decreases the safety factors. Moreover, at the beginning of the monopile installation, the safety factor will increase by nearly 10%, but it will drop by nearly 30% due to long-term periodic lateral loads. Therefore, these have important implications for the installation of monopile foundations for offshore wind turbines (OWTs) since the slopes we consider are within the range of slopes at planned sites for OWTs. An increase in monopile diameter and embedded depth can enhance the overall slope stability. However, an increase in embedded depth could potentially instigate more extensive seabed landslides despite enhancing overall stability.

The Effects of Biofouling and Corrosion Products on Impressed Current Cathodic Protection System Design for Offshore Monopile Foundations

The robustness of the cathodic protection systems utilized for offshore wind monopile foundations depends on the surface condition of the steel as well as the environmental conditions. This study investigated how preexisting biofouling and corrosion products on vertical uncoated steel surfaces extending from the intertidal zone to the buried zone affected the cathodic protection requirements when impressed current cathodic protection (ICCP) was applied under tidal conditions. The comparative results between initially clean and previously fouled and corroded panel sets showed that the fouling and corrosion products increased both the initial and mean current densities. They also altered the composition, slowed the formation, and reduced the protective properties of cathodic chalks during nine weeks of deployment in seawater at Port Canaveral, Florida.

Structural performance degradation identification of offshore wind turbines based on variational mode decomposition with a Grey Wolf Optimizer algorithm

The performance degradation assessment of offshore wind turbine (OWT) structures plays a crucial role in ensuring the safe operation of the structures. This paper presents a method for assessing the performance degradation of OWT structures based on an optimized variational mode decomposition (VMD) algorithm. The method introduces the technique of multisensor data fusion and the Grey Wolf Optimizer (GWO) algorithm to optimize the setting of VMD parameters and then enables the assessment of the structural performance degradation of offshore wind turbines by extracting the structural performance degradation signature of the structure from noise-reduction data. To demonstrate the correctness and advantages of the method in this paper, a 4-degree-of-freedom (4-DOF) system under the action of white noise is used. Numerical calculations show that the developed method can accurately identify the 5%, 10%, 15% or 20% reduction in the structural stiffness and the coefficient of variance of the optimal structure is decreased to 3.6% of the initial design. To further investigate the performance of the proposed strategy, physical tests of the offshore wind monopile structure are conducted. As the loss of the overall structural performance is simulated by removing different numbers of bolts from the connected device, it is demonstrated that the proposed method can effectively assess the structural performance. Finally, field measurements are carried out on a monopile OWT located near Rudong County, Jiangsu Province, in the Yellow Sea of China. The impact of typhoons on the performance of the structure is effectively assessed by analyzing measurement data before and three days after the typhoon In-Fa through the wind farm, the potential application of the method in the structural safety assessment and analysis of OWTs is validated.

Development of a semi-structured database for back-analysis of the foundation stiffness of offshore wind monopiles

Offshore wind turbines founded on monopiles are highly dynamic structures in which the stiffness of the soil adjacent to the monopile controls the natural frequency of the structure. As the loading regime and ground conditions surrounding the foundation are subject to considerable uncertainty, adaptable digital twins of the offshore structures are valuable as they allow the use of in-field monitoring data for model updating. As soil conditions and water depths are rarely uniform across a wind farm site, each structure is expected to behave differently. To back-analyse structural performance, geotechnical and structural data needs to be retrieved at every foundation location. A serverless cloud-based application was developed to allow quick and reliable storage and retrieval of geotechnical and structural data. The database was combined with an API layer to allow parametric data retrieval for back-analyses and digital twin updating across an entire wind farm. As the web application is hosted in the cloud, the data can be accessed through simple HTTP requests by authenticated users working offshore, in the office or remote. The performance of this solution is illustrated with a case study in which foundation stiffness across an entire wind farm site is parametrically calculated and updated.

Large-scale experimental study on scour around offshore wind monopiles under irregular waves

The Keulegan-Carpenter (KC) number is the main dimensionless parameter that affects the local scour of offshore wind power monopile foundations. This study conducted large-scale (1:13) physical model tests to study the local scour shape, equilibrium scour depth, and local scour volume of offshore wind power monopiles under the action of irregular waves with different KC numbers. Systematic experiments were carried out with the KC number ranging from 1.0 to 13.0. With a small KC number (KC < 6), and especially when the KC number was less than 4, the scour mainly occurred on both cross-flow sides of the monopile with a low scour depth. When the KC number exceeded 4, the shape of the scour hole changed from a fan to an ellipse, and the maximum scour depth increased significantly with KC. With a large KC number (KC > 6), the proposed method better predicted the equilibrium scour depth when the wave broke. In addition, according to the results of three-dimensional terrain scanning, the relationship between the local equilibrium scour volume of a single offshore wind power monopile and the KC number was derived. This provided a rational method for estimation of the riprap redundancy for monopile protection against scour.

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.

An accelerated corrosion-fatigue testing methodology for offshore wind applications

Offshore wind turbines are subjected to cyclic loading conditions during their operational lifespan which is typically between 20 and 25 years. An important issue in fatigue design and integrity assessment of offshore wind turbine foundations is the examination of the long-term fatigue and corrosion-fatigue behaviour of steel structures in the high cycle region. High cycle fatigue tests, particularly at low frequencies in a seawater environment, are time-consuming and costly. Therefore, there is an essential need to perform accelerated tests to predict the long-term behaviour of the structures under realistic operational loading conditions. In this work, the existing fatigue acceleration mechanisms have been reviewed and a novel methodology has been proposed for accelerated testing and analysis of fatigue data in different environments (i.e. air, salt-spray and seawater) at higher temperatures. Two distinct equations have been developed and proposed for the calibration and prediction of S-N fatigue life and crack growth behaviour of steels in different environments. The proposed methodology has been validated through comparison with the existing data in the literature and predictions have been made at operational temperatures using high temperature data. The proposed approach is relatively simple to calibrate for a material of interest and enables accelerating S-N fatigue and crack growth testing of the examined materials by a factor of two and three, respectively. The proposed methodology and the obtained results have been discussed in terms of the need for accelerated testing for fatigue design and integrity assessment of offshore wind monopiles, especially those which are close to the end of initial design life and need a comprehensive engineering analysis for life extension or decommissioning.

Equivalent linear pseudostatic and dynamic modelling of vertically vibrating monopile

To optimize offshore wind turbine (OWT) design, an engineering tool has been developed allowing for a detailed investigation of the effects of nonlinear soil stiffness and damping on foundation dynamics. We have studied the response of a vertically oscillating offshore wind monopile foundation in a realistic soil profile subjected to loads between 1 and 200 MN in the frequency range 0–10 Hz with pseudo-static and equivalent linear dynamic model. The non-linear soil behaviour is modelled with an equivalent linear method with shear modulus reduction and damping curves as input. The tool is verified and validated by comparison with elasto-dynamic model and experiments. With increasing load amplitudes foundation stiffness increases and damping decreases. For large load amplitudes the lower part of the pile foundation contributes more to foundation damping. The results indicate the nonlinear foundation stiffness and damping can be modelled rationally by combining stiffness and hysteretic damping from nonlinear static tools with apparent mass and radiation damping from elasto-dynamic analysis. The tool can be used to compute soil springs and dampers based on laboratory-based soil stiffness and damping.

Structural monitoring for lifetime extension of offshore wind monopiles: Verification of strain-based load extrapolation algorithm

Lifetime extension needs low-cost assessments that can identify the remaining useful life of offshore wind monopiles. A novel concept for load monitoring was developed that only needs strain gauges installed at one level of the support structure. Damage equivalent loads were calculated from strain measurements and extrapolated along a monopile using a regression algorithm. In this paper, the assumptions behind the load extrapolation algorithm were verified with two consecutive months of measurement data from an offshore wind park. The verification was performed separately for two offshore wind turbines. Both turbines had strain gauges installed at a distance of approximately 15 m and 25 m. Results show that monthly damage equivalent loads can be predicted with errors smaller than 4% based on measurement data only. Prediction using linear regression resulted in similar results for the total fatigue damage as a nonlinear k-nearest neighbor approach, but individual 10-min damage equivalent loads showed larger differences than for the more robust k-nearest neighbor algorithm, especially for small loads. These results are very promising and should motivate further research.

Shot peening effects on residual stresses redistribution of offshore wind monopile multi-pass weldments

In some industrial applications, welding is the only alternative to join different parts. However, the major problem in welded structures is the tensile residual stresses that are inevitably produced during welding process. Surface tensile stresses can threaten the performance of the weldments as they act as an accelerant in fatigue crack initiation and failure. Shot peening is a well-known method which can enhance weldments' fatigue performance and longevity by inducing surface compressive residual stresses in order to eliminate or limit tensile residual stresses.In this study, the effect of shot peening on redistribution of multi-pass welding residual stresses was numerically and experimentally investigated on very large components typical of welded joints used in offshore wind turbine monopiles. In experimental part of the study, the residual stresses were measured using the Incremental Centre Hole Drilling (ICHD) method before and after shot peening. Finite element studies were carried out using 3D welding models and random shot peening analyses. Moreover, extensive finite element analyses were conducted to study the effect of model dimensions and the number of passes on prediction of welding residual stresses. Interesting set of results obtained from both the numerical studies and the ICHD measurements were in good agreements and showed that shot peening can be advantageous even for large components with multi-pass welded joints. Additionally, reducing the number of weld passes in finite element models could considerably lower the computational time without affecting the accuracy of results at surface regions of the models.

Design optimization with genetic algorithms: How does steel mass increase if offshore wind monopiles are designed for a longer service life?

Knowledge about the scaling of steel mass of monopiles is needed to decide for which service life an offshore wind farm should be planned. A computer-aided method to optimize monopiles for different fatigue lifetimes was developed. The optimization was performed with a genetic algorithm. Fatigue constraints were evaluated with aero-hydro-elastic load simulations in the time-domain. Importance sampling was applied to reduce the required number of load cases to 120 (only 7% of total amount of load cases). The optimization was tested for an 8 MW offshore wind turbine. Results prove that the developed method using importance sampling is suitable to gain fast and accurate optimization results. Only 5% more steel is needed to raise the fatigue lifetime from 25 to 35 years for a design without inspections. The increase of steel mass flattens out towards longer fatigue lifetimes since the structure becomes stiffer and less prone to wave excitation. This is valuable input to decide on the ideal service lifetime and maintenance strategies.

Residual stress measurements in offshore wind monopile weldments using neutron diffraction technique and contour method

Reliable assessment of the fatigue life of offshore wind monopiles operating in harsh offshore environments relies on quantifying the level of residual stresses locked-in at circumferential weld regions. This study presents, for the first time, residual stress characterisation, using the contour method, on a large structural welded mock-up, typical of the weldment used in offshore wind monopiles. The contour method and neutron diffraction measurements were also conducted on a compact tension specimen extracted from the large mock-up. The extracted compact tension sample, typically used for fracture and fatigue crack growth tests, showed notably significant remnant residual stresses that could impact fracture and fatigue test results. In addition the measured 2D map of transverse residual stresses, acting normal to the crack plane, playing a key role in fatigue crack opening/closure, exhibited variations through the thickness of the compact tension sample. The key conclusion was that the residual stresses in small laboratory samples extracted from large scale weldments should be carefully characterised and taken into account in structural integrity tests. Besides, the measurement results on the welded mock-up showed that the level of damaging tensile residual stress in large-scale mock-ups and hence real size structural welded monopiles is considerably larger than residual stresses in extracted laboratory samples; hence will have more significant influence on structural integrity of offshore wind assets.

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.