Wout Weijtjens

Vrije Universiteit Brussel | VUB · Applied Mechanics (MECH)

The present contribution is part of the ongoing development of a fatigue assessment strategy driven purely on in-situ measurements on operational wind turbines. The primary objective is to estimate the remaining life time of existing wind farms and individual turbines by instrumenting part of the farm with a load monitoring setup. This load monitor...

In this contribution, first, the results in the development of a structural health monitoring approach for the foundations of an offshore wind turbine based on its resonance frequencies will be presented. Key problems are the operational and environmental variability of the resonance frequencies of the turbine that potentially conceal any structura...

Offshore wind turbines are exposed to continuous wind and wave excitation. The monitoring of high periodic strains at critical locations is important to assess the remaining lifetime of the structure. At some critical locations below the water level, direct measurements of the strains are not feasible. Response estimation techniques can then be use...

Offshore wind turbines are complex structures, and their dynamics can vary significantly because of changes in operating conditions, e.g., rotor-speed, pitch angle or changes in the ambient conditions, e.g., wind speed, wave height or wave period. Especially in parked conditions, with reduced aerodynamic damping forces, the response due to wave act...

Operational modal parameter estimation (OMA) techniques perform system identification without or with only limited knowledge of the operational inputs acting on the system. However, most of the current operational identification techniques impose multiple conditions on the spectral content of the unknown inputs. As a consequence, modeling errors oc...

The performance of offshore wind turbines is strongly influenced by the structural dynamics of the overall support structure. In situ measurements of the natural frequency of monopile-supported wind turbine structures have highlighted a mismatch between the as-designed and as-built natural frequencies of these structures. This design inaccuracy can...

In offshore wind, the modelled structural dynamics used in design do not fully describe the reality of build structures. For monopile supported turbines, previous work and other publications show an underestimation of eigenfrequencies. As fatigue and lifetime of a foundation is strongly dependent on using these eigenfrequencies, better models that...

Offshore wind turbines are dynamic structures that require an accurate estimate of the main eigenfrequencies to avoid resonance and, as a result, increased fatigue damage. This contribution focuses on the first and second mode eigenfrequencies measured over an entire offshore wind farm in the Belgian North Sea. The comparison between the field meas...

The task of evaluating deep learning algorithms in the context of Structural Health Monitoring (SHM) for damage detection is made particularly challenging by the limited availability of empirical data from damaged structures. Making it impossible to assert whether the trained algorithm would be able to pick up changes due to (unseen) damage. This s...

The fatigue limit state strongly drives the lifetime of offshore wind turbine support structures. Despite steady advancements in their design, the accumulation of fatigue damage over the lifetime of the structure remains uncertain; one way to improve certainty is to directly monitor structural loads at fatigue-critical locations, e.g., near or belo...

Structural Health Monitoring (SHM) plays a crucial role in assessing the integrity and performance of structures. Fibre-Bragg-Grating (FBG) technology has emerged as a valuable tool for strain measurements in SHM applications. However, FBG strain sensors are susceptible to temperature variations that can confound accurate strain measurements, poten...

The offshore wind turbine structure is designed to have the first natural frequency between the rotor frequency (1P) and blade passing frequency (3P), a so-called soft-stiff design. Degradation of the pile-soil interaction stiffness can lead to a reduction of the first natural frequency, bringing it closer to the 1P and wave frequency. This leads t...

The geotechnical characterisation of offshore wind farm sites requires measurement or estimation of the small-strain shear stiffness Gmax of the subsoil. This parameter can be derived from shear wave velocity Vs measurements if the bulk density of the soil is known. Since direct measurements of Vs are generally not available at all foundation locat...

This article presents a cost-effective method to monitor the structural health of transmission towers, a critical yet aging infrastructure that plays an important role in the overall reliability of the electrical grid. The method is validated experimentally on a real-world transmission tower which was subjected to several (exaggerated) damage scena...

Aging wind energy assets demand the development of methods able to effectively support informed decision-making. These needs have inspired the use of data-driven methodologies, which offer valuable insights to wind turbine owners and/or operators. Many approaches can be found in the literature for extrapolating fatigue damage measurements to estima...

Motivated by the mismatch of measured and computed eigenfrequencies for the second tower mode of offshore wind turbines as well as previous studies examining the influence of including flexible blades in structural models, the need to extend the current integrated model to include flexible blades became apparent. A basic modelling approach which ta...

The Smart Tower project, a collaboration between Vrije Universiteit Brussel (VUB) and Elia, aims to explore the application of Structural Health Monitoring (SHM) for transmission towers. The tower was equipped with a variety of sensors, including Strain Gauges (SG), Resistance Temperature Detectors (RTD), Piezoelectric Transducers (PZT), accelerome...

The build-up of ice on the blades is a known issue for wind turbine operators. In particularly in northernly countries continued operation under icing can lead to significant performance losses and increased wear of key components. Aside from the operational concerns the ice also poses a potential safety issue, as both falling ice or ice-throw can...

Fiber Bragg grating (FBG) based optical strain sensors have become more commonplace for fatigue monitoring applications of large structures. Despite many advantages above conventional strain gauges, in some cases FBG measurements can become corrupted by complicated failure modes, which might relate to fiber integrity or interpretation of optical si...

Modern wind turbines are large and slender dynamical structures with a fatigue loading profile of complex nature. The guarantee of their structural integrity is paramount for materializing cost efficient and more reliable wind energy. The measurement of the global dynamic response and loads of wind turbines is fundamental for achieving this goal. H...

The geotechnical characterisation of offshore wind farm sites requires measurement or estimation of the small-strain shear stiffness Gmax of the subsoil. This parameter can be derived from shear wave velocity Vs measurements if the bulk density of the soil is known. Since direct measurements of Vs are generally not available at all foundation locat...

Fatigue has become a major consideration factor in modern offshore wind farms as optimized design codes, and a lack of lifetime reserve has made continuous fatigue life monitoring become an operational concern. In this contribution, we discuss a data‐driven methodology for farm‐wide tower‐transition piece fatigue load estimation. We specifically de...

Offshore wind turbine support structures are fatigue-driven designs subject to a wide variety of cyclic loads from wind, waves, and turbine controls. While most wind turbine loads and metocean data are collected at short-term 10 min intervals, some of the largest fatigue cycles have periods over 1 d. Therefore, these low-frequency fatigue dynamics...

Fatigue has become a major consideration factor in modern offshore wind farms as optimized design codes and a lack of lifetime reserve have made continuous fatigue life monitoring become an operational concern. In this contribution we discuss a data-driven methodology for farm-wide tower-transition piece fatigue load estimation. We specifically tac...

It is common practice to install scour protection systems during the construction of offshore wind farms. This is because the guidelines allow for a more optimal structural design, as the potential effects of scouring may be ignored. However, multiple sources, as well as measurements, suggest that there is an additional gain in stiffness which is n...

In this paper, we introduce a novel deep learning technique for anomaly detection in the context of Population-Based Structural Health Monitoring (PB-SHM). The proposed method eliminates manual feature engineering by utilizing Power Spectral Density (PSD) as input, allowing examination of the entire spectrum. It is based on an auxiliary classificat...

The operational life of offshore wind turbines is in part driven by the fatigue life of key structural components, such as the substructure. In recent years, fatigue life management of operational assets has become evermore important, as older farms are closing on their design lifetime and newer farms are designed with tighter margins. To support d...

This paper presents a methodology for enhancing frequency-based Structural Health Monitoring (SHM) of Offshore Wind Turbines (OWTs) by incorporating digital twin models. The proposed methodology utilizes an automated operational modal analysis to extract modal frequencies from acceleration measurements. The environmental and operational variability...

Ice accretion on wind turbines poses a significant safety risk, especially in densely populated areas. Ice built up on the blades can be thrown towards people and infrastructure and cause potential damage and harm. This risk is known and, as such, several solutions exist in today’s market to detect ice accretion on the blades and timely shut down t...

Offshore wind turbines (OWTs) are dynamic structures, their stiffness plays a key role in their dynamic stability. Pore water pressure (PWP) evolutions around the monopile influence their stiffness, therefore models that can accurately capture these evolutions under various loading conditions are needed. A coupled pore-fluid element ( u-p formulati...

Offshore wind structures are exposed to a harsh marine environment and are subject to deterioration mechanisms throughout their operational lifetime. Even if the deterioration evolution of structural elements can be estimated through physics-based deterioration models, the uncertainties involved in the process hurdle the selection of lifecycle mana...

Offshore wind turbine support structures are fatigue-driven designs subjected to a wide variety of cyclic loads from wind, waves, and turbine controls. While most wind turbine loads and metocean data are collected at short-term 10-minute intervals, some of the largest fatigue cycles have periods over one day. Therefore, these low-frequency fatigue...

Monitoring accumulated data over years of operation of monopile-supported offshore wind turbines (OWTs) show a global mismatch between the as-designed and the actual performance of monopiles. This mismatch is due to the fact that design methodologies that were used are not suited for large-diameter piles. This mismatch is demonstrated, both in term...

Based on runs from PyWake, this contribution studies the applicability of graph neural networks (GNN) as a simultaneous wind turbine fleet, wind inflow and loads surrogate. As graphs are discrete mathematical sets of dependent objects, one can see how the layout-dependent interaction between wind turbines' aerodynamics, from which wake arises, lend...

Curtailment is a known phenomenon for wind turbine operators of both onshore and offshore wind turbine generators (WTG). Curtailment refers to the situation in which the power output of all WTG’s within a windfarm is forced below the expected power output at the occurring environmental conditions. A direct consequence of curtailment is the loss of...

Offshore wind turbines are exposed during their serviceable lifetime to a wide range of loads from aero-,hydro- and structural dynamics. This complex loading scenario will have an impact on the lifetime of the asset, with fatigue remaining the key structural design driver for the substructure, e.g. the monopile. The ability to monitor the progressi...

In recent years there has been an increased interest of the offshore wind industry to use structural health monitoring (SHM) data in the assessment of consumed lifetime and lifetime extension for an entire wind farm. In order for operators, certifying bodies, insurance entities and government agencies to agree on a lifetime extension, a commonly ac...

Structural Health Monitoring (SHM) represents the course of action of implementing a damage assessment strategy for engineering infrastructures. SHM systems can provide substantial aid towards the improvement of Offshore Wind turbines (OWTs) reliability, sustainability, and profitability. Usually, SHM system development is affected by three major c...

Railway bridges are key assets of a countries’ infrastructure,enabling transport of goods and people through freight and passengertrains. The studied structure is a steel railway bridge subjected to cyclicloading, equipped with 98 Fiber Bragg Gratings. A previous study iden-tified train passages as main drivers of damage, isolated and convertedthem...

Bridges are critical infrastructures subjected to cyclic loading and require fatigue monitoring to prevent high maintenance costs due to fatigue failure. This paper, part of OWI-lab’s research activities within the SafeLife-Infrabel project, presents a fatigue survey on four months of strain measurements of a steel railway bridge in Belgium, compri...

A Fernandes a , C Sastre Jurado ab , W Weijtjens a , B Stuyts ab , C Devriendt a a Vrije Universiteit Brussel (VUB)/Offshore Wind Infrastructure-lab (OWI-lab) b UGent, Geotechnical Laboratory, Technologiepark 68, Zwijnaarde, 9052, Inside the scope of the BOPTIC and Soiltwin projects, the Offshore Wind Infrastructure-lab (OWI-lab) is conducting rese...

Offshore wind structures are subject to deterioration mechanisms throughout their operational lifetime. Even if the deterioration evolution of structural elements can be estimated through physics-based deterioration models, the uncertainties involved in the process hurdle the selection of lifecycle management decisions. In this scenario, the collec...

Offshore wind turbines are designed to balance cyclic loading from wind, the rotating machinery and the waves against the fatigue limits of the materials used. A proper understanding of the structural dynamics of the completed structure is necessary for an accurate fatigue life calculation. Early designs underestimated the initial stiffness of the...

The current approach for detecting anomalies in acceleration signals relies extensively on feature engineering. Indeed, detecting rotor imbalances in wind turbines starts by first isolating and then assessing the energy of the 1P harmonic, leading to a feature that is efficient but not failure mode agnostic. While different engineered features can...

In the present contribution, data from a measurement campaign on XL monopiles (with 9.5 MW turbines and water depths of up to 36 m) is presented. This campaign is based on data collected by three types of sensors: strain gauges (installed at the TP-tower interface), accelerations (taken at bottom, mid and upper levels of the tower) and SCADA data,...

Structural Health Monitoring (SHM) has seen an explosion in data gathering in the last few years. This is illustrated in the offshore wind industry through an increase in the amount of placed offshore wind turbines (OWT), a higher rate of SHM instrumented OWTs and an increase in the sampling rate. The growing data gathering has led to the interest...

Offshore wind turbines (OWTs) are sensitive, dynamic structures that require accurate estimation of the natural frequency of the support structure. This requires an integrated analysis of the wind turbine structure, including a realistic representation of the foundation response. This paper studies how the natural frequency analysis is influenced b...

To date, despite advancements in the design of offshore wind turbines, the as-designed and identified as-built natural frequencies of offshore wind turbines still show discrepancies. These discrepancies are partially rooted in modelling uncertainties, as well as uncertain input parameters, related to e.g. aero-, fluid- or soil-structure interaction...

The mutual and simultaneous action of external variable forces on (offshore) wind support structures causes fatigue. Fatigue analysis in this context relies on 10-minute-long strain signals. Cycle-counting allows capturing the fatigue cycles nested within these signals but inevitably leaves some open loops called half-cycles or residuals. Some othe...

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...

The sustained development over the past decades of the offshore wind industry has seen older wind farms beginning to reach their design lifetime. This has led to a greater interest in wind turbine fatigue, the remaining useful lifetime and lifetime extensions. In an attempt to quantify the progression of fatigue life for offshore wind turbines, als...

The contribution of soil damping to the overall damping of offshore wind turbine structures has proven to be elusive. The strain-dependent soil stiffness and material damping can be quantified from small-scale element tests but applying the results from these tests in an integrated structural model requires a realistic assessment of the shear strai...

In this contribution we present a physics-informed neural net-work (PINN) approach for wind turbine fatigue estimation. This PINN in-corporates physical information of the structure’s fatigue profile in its loss function, referred to as Minkowski logarithmic error (MLE) - an extension of the log loss for any given Lp space. The function is mathemat...

In this contribution, the strain measurements of a railway bridge are used for anomaly detection, in the context of Structural Health Monitoring (SHM). The methodology used is a combination of a sparse convolutional autoencoder (CSAE) and a Mahalanobis distance. Due to the lack of labeled anomalous data, a simulated fault is used to evaluate the pe...

A Fernandes a , C Sastre Jurado ab , W Weijtjens a , B Stuyts ab , C Devriendt a a Vrije Universiteit Brussel (VUB)/Offshore Wind Infrastructure-lab (OWI-lab) b UGent, Geotechnical Laboratory, Technologiepark 68, Zwijnaarde, 9052, Inside the scope of the BOPTIC and Soiltwin projects, the Offshore Wind Infrastructure-lab (OWI-lab) is conducting rese...

Strain measurements using fibre Bragg grating (FBG) optical sensors are becoming ever more commonplace. However, in some cases, these measurements can become corrupted by sudden jumps in the signal, which manifest as spikes or step-like offsets in the data. These jumps are caused by a defect in the FBG itself, which is referred to as peak-splitting...

The design of monopile foundations for offshore wind turbines is most often driven by fatigue. With the foundation price contributing to the total price of a turbine structure by more than 30%, wind farm operators seek to gain knowledge about the amount of consumed fatigue. Monitoring concepts are developed to uncover structural reserves coming fro...

The sustained development over the past decades of the offshore wind industry has seen older wind farms beginning to reach their design lifetime. This has led to a greater interest in wind turbine fatigue, the remaining useful lifetime and lifetime extensions. In an attempt to quantify the progression of fatigue life for offshore wind turbines, als...

Offshore wind turbines are subjected to cyclic loads from wind and waves, ultimately resulting in a fatigue-driven design. These cyclic loads result in cyclic shell forces above the bolted ring flange and ultimately into the cyclic loading of the bolt tension forces. To model the transfer of the shell forces into the bolt, Load Transfer Functions (...

As older wind farms begin to reach their design lifetime, topics such as wind turbine fatigue, the remaining useful lifetime and lifetime extensions also start to be discussed with greater frequency. It is in this context that wind turbine fatigue assessment appears as crucial contribution. Its accurate portrayal can enable informed decisions regar...

The OWI-Lab Soiltwin project’s objective is to improve the fatigue design of offshore monopiles through better modelling the soil-pile interaction. The project aims to update these models based on in-situ measurements over an entire fleet of OWTs on monopile foundations. Therefore, the project aims to create an updatable ‘digital twin’ FE model for...