Offshore Wind Turbine Support Structures Research Articles

A reliability-constrained optimisation framework for offshore wind turbine support structures

ABSTRACT A new approach that links finite element analysis (FEA) and a genetic algorithm (GA) with Monte Carlo simulation (MCS) for the reliability-constrained optimal design of offshore wind turbine (OWT) support structure is presented. This approach has been applied to optimise the NREL 5MW OWT on OC3 support structure. The objective function minimises the weight of the support structure, constrained to design and performance limits, by attaining the desired reliability level. A response analysis of the reference OWT is performed to estimate design loads. Then, deterministic optimisation (DO) is carried out to find the candidate design solutions. After that, a reliability assessment is conducted to apply the target reliability constraint. This study reveals that the design of a monopile support structure is mainly driven by fatigue limit state. Also, there is no guarantee that DO candidate designs always meet the structural reliability target level in all capacities.

Fatigue Analysis of a Jacket-Supported Offshore Wind Turbine at Block Island Wind Farm.

Offshore wind-turbine (OWT) support structures are subjected to cyclic dynamic loads with variations in loadings from wind and waves as well as the rotation of blades throughout their lifetime. The magnitude and extent of the cyclic loading can create a fatigue limit state controlling the design of support structures. In this paper, the remaining fatigue life of the support structure for a GE Haliade 6 MW fixed-bottom jacket offshore wind turbine within the Block Island Wind Farm (BIWF) is assessed. The fatigue damage to the tower and the jacket support structure using stress time histories at instrumented and non-instrumented locations are processed. Two validated finite-element models are utilized for assessing the stress cycles. The modal expansion method and a simplified approach using static calculations of the responses are employed to estimate the stress at the non-instrumented locations-known as virtual sensors. It is found that the hotspots at the base of the tower have longer service lives than the jacket. The fatigue damage to the jacket leg joints is less than 20% and 40% of its fatigue capacity during the 25-year design lifetime of the BIWF OWT, using the modal expansion method and the simplified static approach, respectively.

Effect of scour on the fatigue life of offshore wind turbines and its prevention through passive structural control

Abstract. Offshore wind turbine (OWT) support structures are exposed to the risk of fatigue damage and scour, and this risk can be effectively mitigated by installing structural control devices such as tuned mass dampers (TMDs). However, time-varying scour altering OWTs' dynamic characteristics has an impact on the TMD design and fatigue life, which has rarely been studied before. In this paper, a simplified modal model is used to investigate the influence of scour and a TMD on the fatigue life evaluation of a 5 MW OWT's support structure, and a traditional method and a newly developed optimization technique are both presented to obtain TMD parameters. This optimization technique aims at finding optimal parameters of the TMD which maximize the fatigue life of a hotspot at the mudline, and the effect of time-varying scour can be considered. This study assumes that the TMD operates in the fore–aft (FA) direction, while the vibration in the side–side (SS) direction is uncontrolled. Results show that scour can decrease the fatigue life by about 24.1 % and that the TMD can effectively suppress vibration and increase the fatigue life. When the scour depth reaches 1.3 times the pile diameter, the TMD with a mass ratio of 1 % can increase the fatigue life of an OWT's support structure by about 64.6 %. Further, it is found that the fatigue life can be extended by 25 % with the TMD optimized by the proposed optimization technique rather than using a traditional design method which does not take the change in dynamic characteristics into account.

An Overview on Structural Health Monitoring and Fault Diagnosis of Offshore Wind Turbine Support Structures

The service environment of offshore wind turbine (OWT) support structures is harsh, and it is extremely difficult to replace these structures during their operational lifespan, making their failure a catastrophic event. The structural health monitoring (SHM) of OWT support structures is a crucial aspect of operational maintenance for OWT support structures, aiming to mitigate significant financial losses. This paper systematically summarizes the current monitoring methods and technologies for OWT support structures, including towers and foundations. Through the review of monitoring content and the evolution of monitoring techniques for supporting structures, it delves deeper into the challenges faced by wind turbine monitoring and highlights potential avenues for future development. Then, the current damage identification techniques for OWT towers and foundations are analyzed, exploring various methods including model-based, vibration-based, artificial intelligence and hybrid fault diagnosis methods. The article also examines the advantages and disadvantages of each approach and outlines potential future directions for research and development in this field. Furthermore, it delves into the current damage identification techniques for OWT towers and foundations, discussing prevalent challenges and future directions in this domain. This status review can provide reference and guidance for the monitoring design of OWT support structures, and provide support for the fault diagnosis of OWT support structures.

A novel post-weld treatment using nanostructured metallic multilayer for superior fatigue strength

Welded joints exhibit fatigue failure potential from weld geometry and characteristics of the heat affected zone. In order to counteract fatigue, structures and components require larger thicknesses resulting in heavier designs exhausting the finite natural resources. We hereby introduce a novel post-weld treatment, which postpones or even prevents fatigue failure of the welded connection. A Cu/Ni nanostructured metallic multilayer (NMM) is applied via electrodeposition and a 300–600% increase in usable lifetime compared to the untreated weld is observed. A FAT class 190 with a slope of k = 6 is proposed for the design of NMM treated butt welds. Material mechanisms responsible for the fatigue strength increase are introduced herein. A case study shows that the design of offshore wind turbine support structures applying NMM post-weld treatment enables a lifetime extension as well as a 28% weight reduction compared to the structure without post-weld treatment.

Fatigue life assessment of offshore wind support structures in the presence of corrosion pits

Offshore wind turbine support structures, which connect the wind turbine transition piece and/or tower to the seabed, are located below the sea level and are in direct contact with seawater during the entire lifespan; therefore, they are highly susceptible to corrosion damage and cracking. In particular, the pitting corrosion is very crucial in these support structures, as it leads to local stress concentrations and thus affects the fatigue life. Although corrosion protection mechanisms are commonly implemented in offshore wind turbines, they have a finite life and therefore corrosion damage cannot be completely avoided during the entire life cycle, and this can lead to pitting corrosion on the steel surface. This paper aims to investigate the impact of pitting corrosion on fatigue durability of steel structures by performing tests on lab-scale coupons, made of S355 structural steel which is widely employed in fabrication of offshore wind support structures. For this purpose, cross-weld uniaxial samples were initially exposed to seawater for different time durations and then tested under cyclic loading condition. Furthermore, the durability analysis of corroded samples was carried out using a modified NASGRO equation. The results show that the pitting corrosion significantly reduces the fatigue life, and its level of life reduction is strongly dependent on the seawater exposure time. Moreover, the results show that employment of a linear trend for lower stress ranges would result in significant underprediction of the fatigue life, hence over-conservatism in the design life, at longer seawater exposure times.

Reliability-based design optimization of offshore wind turbine support structures using RBF surrogate model

Reliability-based design optimization of offshore wind turbine support structures using RBF surrogate model

Experimental study into the effect of wind-ice misalignment on the development of ice-induced vibrations of offshore wind turbines

The effect of misalignment between wind- and ice loading direction on the development of ice-induced vibrations of offshore wind turbines has been investigated experimentally. In the experiments a hybrid test setup was used to study the structural response to combined loading from physical model ice and numerically applied wind. The motivation for this study was the high uncertainty in the design of offshore wind turbine support structures in cold regions, caused by scarcity of full-scale and model-scale data on ice-structure interaction. Test results revealed that misaligned scenarios result in the development of sustained ice-induced vibrations in the ice load direction. The test results also showed that ice-induced vibrations can develop up to higher ice drift speeds for misaligned scenarios than for aligned scenarios. Both observations are considered to be related to low total damping in the ice drift direction for a misaligned scenario. Further comparison between a 90°-misaligned operational and an aligned idling scenario revealed that wind-induced structural displacements perpendicular to the ice drift direction do not cause the ice to fail. On the contrary, it was shown that the ice constrains the wind-induced motion for low relative velocities between ice and structure. For high relative velocities, wind-induced displacements approach those in open water as the ice fails in rapid succession at the sides of the structure during crushing. The analysis of a misaligned scenario with a smaller misalignment angle revealed that vibrations occur perpendicular to the ice drift direction and are characterized by relatively low amplitude and high frequency. The ice, being in contact with the structure, neither prevented those vibrations nor failed.

Fatigue Life Convergence of Offshore Wind Turbine Support Structure According to Wind Measurement Period

This paper investigated the fatigue life of offshore wind turbine (OWT) support structures. For this purpose, a 3 MW-capacity typical wind turbine is investigated using time-domain finite element simulations. In numerical simulations, different stochastic wind models corresponding to different accumulation periods are applied. Then, the stress-based fatigue life is estimated following the rain-flow counting algorithm and Palmgren-Miner linearly cumulative damage rule. The study also addresses the joint distribution of loads at the site of interest. Generally, the study emphasizes the significance of the long-term distribution of the applied environment loads and its influence on the fatigue life of OWT’s substructures. The results imply that the wind measurement period is directly linked to the fatigue life of offshore wind turbine support structures. Accordingly, its fatigue life is significantly reduced at the 25-year accumulative period of wind. Therefore, this study recommends that a sufficient number of accumulative periods of wind and other environmental loads should be considered appropriately.

Studying the Settlement of OWT Monopile Foundations Using a T-Z Spring with the Torsional Effect

The main purpose of this paper was to study the vertical settlement of offshore wind turbine (OWT) monopile support structures, where 5, 10, 15, and 20 MW OWT support structures were analyzed under power production, seismic, and tropical cyclone loads. Moreover, a t-z spring with shear and torsional degrees of freedom was developed to simulate the shear stress along the pile and soil surface under the combined effect of vertical loads and z-direction torsions. This t-z spring does not require excessive changes to the finite element program, where only a known factor is used to modify the traditional stiffness of the t-z spring. This paper, analyzing several kinds of OWT monopile foundations, indicates that the soil shear resistance may be less than the shear stress generated by the combination of vertical loads and torsions, which causes large vertical and rotational displacements resulting in the failure of monopile structures. This situation will be worse when the natural frequency of the first vertical-direction rotation is close to the integer multiples of the 3P frequency, which cannot be well-simulated using traditional t-z springs.

Requirements for the application of the Digital Twin Paradigm to offshore wind turbine structures for uncertain fatigue analysis

The Digital Twin (DT) paradigm offers an extension of simulation model utility into the operational phase of an engineering asset. The goal is a simulation “twinned” with observed data that reflects the actual performance of the asset. However, exploring sources of uncertainty for both the physical asset and the simulation model are a challenge. For example, random metocean conditions, and uncertainty on model parameters and response behaviour of offshore wind turbine (OWT) structures, contribute to uncertainty for predicted life under fatigue. In-service assessment of OWT structures will benefit from twinning simulations and observed data, where a framework to treat this uncertainty is defined. The DT needs to capture state, condition, and behaviour to a level that allows quantification and propagation of uncertainty for reliability analysis. Using a DT, built for fatigue assessment of bolted ring-flanges on OWT support structures, this paper explores the challenges and opportunities in defining uncertainties of interest. We propagate these uncertainties through the DT in a coherent manner using a Gaussian Process (GP) surrogate modelling approach, efficiently emulating a computationally expensive numerical simulator. The GP is an attractive surrogate model method given this computational efficiency, in addition to providing an estimate of prediction uncertainty at unobserved points in the output space. The use of the GP surrogate model is included within a definition of the Surrogate DT, a framework including “fast” and “slow” twinning processes. We define six requirements to apply DTs to OWT structures which provide practical guidelines for modelling this complex asset under uncertainty.

Interface debonding detection of grouted connection based on Lamb wave energy leakage

Grouted connections (GCs) are transitional elements that have been extensively employed in offshore wind turbine support structure to transfer loads from superstructure to foundation. However, interface debonding defects between steel tube and grout layer have an adverse effect on the durability and fatigue resistance of the GC. An effective interface debonding defects detection approach for GC is necessary. In this study, interface debonding defects detection approach based on Lamb wave energy leakage was proposed and validated numerically and experimentally. Numerical studies on Lamb wave energy leakage models and the GC models were performed to study mechanism of detection approach. Lamb wave is excited on the surface, and stress wave fields and time-history response of models were extracted and compared. Experimental investigations were carried out to verify the applicability of the proposed approach in practice. A scaled GC specimen was constructed and Lamb wave was excited by the PZT patches mounted on the surface of GC specimen. The output voltage of surface-mounted PZT sensors under different axial loads were compared. Both numerical and experimental results show that the proposed approach is effective to the detection of interface debonding defects. The damage evolution index (DEI) based on wavelet packet energy is defined and it can be adopted to qualitatively determine the initiation and evolution of the damage.

An efficient fatigue assessment model of offshore wind turbine using a half coupling analysis

Fatigue failure is an important issue for offshore wind turbine support structures. Especially, fully coupled time domain analysis will lead to high computational burden. Therefore, accurate and efficient fatigue evaluation methods have always been the common goal pursued by the industry and researchers. This paper evaluates fatigue damage under combined wind and wave loading, using a fixed jacket offshore wind turbine as an example. A half coupling model (HCM) with distinct advantage by considering aerodynamic damping and structural damping is developed for dynamic response analysis of offshore wind turbine. The precision and effectiveness of the developed HCM is verified to be reasonable by comparing with numerical simulated results. A linearized Morison equation is proposed to achieve frequency domain analysis of wave response which is embedded as a programming module in ANSYS. Based on the developed model, dynamic response analysis is carried out under wind and wave loading, and then combined stress spectra at the location of different tubular joints (X-joint and K-joint) are obtained. To consider the interaction of wind and wave, a spectral fatigue combination rule based on Han and Ma's method is used to calculate fatigue damage. The developed combination rule is assessed by compared with time domain fatigue result based on rainflow cycle counting. Analysis using the newly developed model and method demonstrates a high accuracy and efficiency for fatigue damage prediction.

A review of fatigue damage assessment in offshore wind turbine support structure

According to the climate crisis, offshore wind turbines can play an important role to improve the green energy sector. Due to the rough and stochastic metocean environment, the fatigue existence in marine structures is inevitable. Several probabilities and artificial intelligence models can predict the metocean environment for fatigue assessment. Fatigue analysis can be done in time and frequency domains, and for enhancing the efficiency and speed of computation the hybrid model is proposed. Furthermore, the multivariate linear statistical models of the simplified fatigue are also fast with high accuracy. For the Fracture Mechanics method, the geometry functions would be very useful to approximate the fatigue crake growth rate. Besides, J-integral is a criterion to determine the orientation and along with of the fatigue crack and define the plastic behavior of the fatigue crack tip zone. In the local scale, UniGrow and SWT probability models could bring proper fatigue lifetime approximation and recognize the fatigue resistance and residual fatigue. Multiaxial fatigue also can consider the effect of the in- and out-of-phase loading effects through the critical plane method. Furthermore, a survey of the specific offshore steel shows that the fatigue lifetime does not have good agreement with the S-N curve of guidelines due to the inhomogeneity of the material and different mechanical properties. The PhyBal method was used to reduce the cost of the fatigue test and improve the fatigue approximation accuracy.

Development of Combined Load Spectra for Offshore Structures Subjected to Wind, Wave, and Ice Loading

Fixed offshore wind turbines continue to be developed for high latitude areas where not only wind and wave loads need to be considered but also moving sea ice. Current rules and regulations for the design of fixed offshore structures in ice-covered waters do not adequately consider the effects of ice loading and its stochastic nature on the fatigue life of the structure. Ice crushing on such structures results in ice-induced vibrations, which can be represented by loading the structure using a variable-amplitude loading (VAL) sequence. Typical offshore load spectra are developed for wave and wind loading. Thus, a combined VAL spectrum is developed for wind, wave, and ice action. To this goal, numerical models are used to simulate the dynamic ice-, wind-, and wave-structure interaction. The stress time-history at an exemplarily selected critical point in an offshore wind energy monopile support structure is extracted from the model and translated into a VAL sequence, which can then be used as a loading sequence for the fatigue assessment or fatigue testing of welded joints of offshore wind turbine support structures. This study presents the approach to determine combined load spectra and standardized time series for wind, wave, and ice action.

Sensitivity analysis of design parameters for reliability assessment of offshore wind turbine jacket support structures

Offshore Wind Turbine (OWT) support structures are subjected to hostile environments, defined by highly stochastic loads and complex soil-structure interaction, and thus the need for a probabilistic approach towards design. The study carried out herein presents the sensitivity analysis of these inherent stochastic variables imposed on a complex OWT support structure via purpose-developed modular non-intrusive structural reliability assessment formulation. The results from this study reveal that the uncertainties in the wind speed is a structural design driving factor and the hydrodynamic load effects are secondary to this, for the ultimate (ULS) and Fatigue Limit States (FLS) while their relative sensitivities on the Serviceability Limit State (SLS) cannot be clearly distinguished but are seen to have a dominant impact. Also, it was inferred that incorporating correlation between the variables have a significant impact on the reliability of the structure in the ULS design.

Applicability of Concrete–Steel Composite Piles for Offshore Wind Foundations

Offshore wind-turbine support structures are largely made of steel since steel monopiles have accounted for the majority of installations in the last decade. As turbines become bigger, steel structures have led to an exponential increase in material and installation costs. From this point of view, the use of concrete for future support structures has been initiated. In this study, concrete–steel composite piles have been investigated. A pre-tensioned high strength concrete pile was placed in the lower part, mainly to support the axial load, and a steel pile in the upper part to resist the lateral load. A mechanical joint was adapted to connect the two different types of piles. Static axial, dynamic axial, and lateral load tests were performed to evaluate the load-displacement response of the composite pile, verify the integrity of the mechanical joint, and investigate its potential application to offshore wind foundations. This paper focused on the load-displacement response and the connection integrity; in particular, it investigated the lateral load-displacement response by comparing it to the results of beam-spring analysis. Based on the results from the field tests, a site-specific lateral load-displacement curve was suggested, and the connection integrity was verified.

Three-dimensional wind fields of tropical cyclones for wind turbine structures

An efficient unsteady turbulent wind model is developed to simulate the wind field of tropical cyclones for wind turbines. The method first establishes frequency-domain wind fields, which are then changed to time-domain wind fields using the inverse Fast Fourier Transfer. Finally, interpolation is used to determine the full field unsteady turbulent wind speed. The OpenMP scheme for parallel computing and the skyline method for efficient matrix decomposition sped up 34 times faster than that without these schemes using an eight-core computer. Then, offshore wind turbine (OWT) support structure analyses and designs under International Electrotechnical Commission (IEC) 61400-3 and realistic tropical cyclone loads were performed for three cases. The results indicate that IEC 61400-3 Design Load Cases (DLCs) 2.2, 6.1, and 7.1 control the steel design for the case without tropical cyclone loads, but with such loads, IEC 61400-3 DLC I.2 dominates almost all the steel designs, which was attributed to yaw misalignment. However, actual tropical cyclones may cause worse conditions, especially for the yaw always on the X axis. Nevertheless, this actual tropical cyclone condition does not excessively increase the weight of the steel design, so IEC 61400-3 DLCs I.1 and I.2 are sufficient to ensure the safety of the OWT steel design.

Numerically efficient fatigue life prediction of offshore wind turbines using aerodynamic decoupling

The fatigue life prediction for offshore wind turbine support structures is computationally demanding, requiring the consideration of a large number of combinations of environmental conditions and load cases. In this study, a computationally efficient methodology combining aerodynamic decoupling and modal reduction techniques is developed for fatigue life prediction. Aerodynamic decoupling is implemented to separate the support structure and rotor-nacelle assembly. The rotor dynamics were modelled using an aerodynamic damping matrix that accurately captures the aerodynamic damping coupling between the fore-aft and side-side motions. Soil-structure interaction is modelled using p-y curves, and wave loading calculated based on linear irregular waves and Morison's equation at a European (North Sea) site. A modal reduction technique is applied to significantly reduce the required number of degrees of freedom, allowing the efficient and accurate calculation of hotspot stresses and fatigue damage accumulation. The modal model was verified against a fully coupled model for a case-study, monopile supported offshore wind turbine in terms of response prediction and fatigue life evaluation. The modal model accurately predicts fatigue life (within 2%) for a range of parameters at a fraction of computational cost (0.5%) compared to the fully coupled model.

A fracture mechanics framework for optimising design and inspection of offshore wind turbine support structures against fatigue failure

Abstract. Offshore wind turbine (OWT) support structures need to be designed against fatigue failure under cyclic aerodynamic and wave loading. The fatigue failure can be accelerated in a corrosive sea environment. Traditionally, a stress–life approach called the S–N (stress–number of cycles) curve method has been used for the design of structures against fatigue failure. There are a number of limitations in the S–N approach related to welded structures which can be addressed by the fracture mechanics approach. In this paper the limitations of the S–N approach related to OWT support structure are addressed and a fatigue design framework based on fracture mechanics is developed. The application of the framework to a monopile OWT support structure is demonstrated and optimisation of in-service inspection of the structure is studied. It was found that both the design of the weld joint and non-destructive testing (NDT) techniques can be optimised to reduce in-service inspection frequency. Furthermore, probabilistic fracture mechanics as a form of risk-based design is outlined and its application to the monopile support structure is studied. The probabilistic model showed a better capability to account for NDT reliability over a range of possible crack sizes as well as to provide a risk associated with the chosen inspection time which can be used in inspection cost–benefit analysis. There are a number of areas for future research, including a better estimate of fatigue stress with a time-history analysis, the application of the framework to other types of support structures such as jackets and tripods, and integration of risk-based optimisation with a cost–benefit analysis.