As with any strategic structure, vibration-based structural health monitoring techniques are often used to ensure the structurally safe operation of offshore wind turbines. Among such techniques, Operational Modal Analysis (OMA) methods allow the identification of modal properties, such as natural frequencies, mode shapes and damping, which variation might be caused by damage or operational/environmental factors. This paper investigates the application of OMA techniques on a two-bladed offshore wind turbine, which poses multiple challenges: fundamental OMA assumptions about the applied loads are violated by environmental and operational loads; the closely spaced modes of an offshore wind turbine are hard to identify; and an operative two-bladed offshore wind turbine is a time-variant system. Within this study, three OMA procedures to overcome some of the preceding challenges are discussed: (1) a standard frequency domain decomposition method; (2) a proposed enhanced transmissibility-based approach with a post-processing technique based on the Kurtosis index; and (3) a proposed refined hybrid OMA procedure that combines a transmissibility-based approach, the dedicated post-processing technique based on the Kurtosis index, and the frequency domain decomposition method. A numerical model representative of an operative two-bladed offshore wind turbine is used to compare the three procedures. Based on the comparison, the hybrid method is proven to be a promising new OMA-based procedure that outperforms the stand-alone transmissibility-based approach and the frequency domain decomposition method in identifying the modal properties of a two-bladed offshore wind turbine.

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This study investigates the sliding friction fluctuations through hysteresis cycles, observed between a steel sphere and wood specimens of beech and spruce, by using a tribometer and a profilometer. At first, the tracked wear profiles revealed a correlation between the normal force variation and the surface waviness. However, the normal force variation alone was not able to explain the observed friction force fluctuation. Hence, to explain most of the observed non-Coulomb friction hysteresis behavior, the wear-scar interaction is taken into account through an optimization framework used for data post-processing and leading to an optimal friction coefficient. A comparative study showed similar values between the identified optimal friction coefficient, the mean and energy-based friction coefficients.

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This work aims to develop a low-fidelity model for a lattice support structure for offshore wind applications. The proposed low-fidelity model consists of a sequence of regular Timoshenko beams, each of them characterized by homogenized mechanical and mass properties representative of the single bays of the reference space-frame structure. The homogenized elastic coefficients of the sequence of beams are then computed by means of two alternative procedures: case (a), via analytical expressions available in the literature and accounting for a partially isotropic behaviour; case (b) by means of an optimization procedure, with ad hoc calibration factors. The suggested methods to derive the homogenized elastic coefficients are then tested for both straight and tapered lattice structures. The prediction performance is evaluated in terms of estimation of the first five natural frequencies and mode shapes, response to dynamic loads, and ability to predict rotor-structure interaction phenomena. A parametric study is then performed to evaluate the potential and limitations of the proposed models. To bypass the optimization procedure (b), a data-driven approach is also proposed for the case of straight lattice structures. Overall, the developed low-fidelity model leads to a computational speed-up factor of at least 60. The prediction reliability of the low-fidelity model is discussed for a tapered and regular straight lattice structure. However, for the latter one, a more detailed comparative study between the various modelling assumptions is performed and discussed. With reference to the straight lattice tower, whenever an optimization procedure is used (case (b)), and with reference to a typical subset of the investigated geometrical parameter space, the mean prediction error of the first five natural frequencies is lower than 1%. On the other hand, for case (a) and for the same investigated subset, the mean prediction errors for the first two bending modes and the torsional mode are, 5.2%, 13.3% and 18.8%, respectively. These results are improved in case a data-driven regression model is used to predict the calibration factors, leading to mean prediction errors below 5% for the entire investigated parameter space.

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This study aims at shedding light on the mechanical behaviour of a prototype monopile–wind turbine tower connection, constituted by a slip joint. Selected examples of data set recorded during a long term monitoring campaign are illustrated and discussed. The data set encompass axial and hoop stresses measured over the slip joint area, relative displacements of the slip joint with respect to the monopile and acceleration levels recorded above the slip joint. In parallel, an ideal and simplified Finite Element model (FEM) of the slip joint is developed, in order to interpret the observed experimental data. Experiments first highlight the relevance of modelling the manufacturing imperfections of the overlapping steel sections. Subsequently, both experiments and FEM show that states of prestress need to be accounted for. Such prestress states first originate from the installation process, and subsequently from further loading events, triggering settlements of the slip joint. Finally, experiments and FEM showcase the force transfer mechanisms from the upper part to the lower part of the slip joint.

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Applying an oscillatory load is one of the most efficient ways to alter friction forces. Several theoretical and experimental studies on the influence of oscillatory loads on friction have been conducted, investigating the effect of both in-plane and out-of-plane oscillations for different tribological pairings. However, in the literature, the effect of an oscillatory load on the friction force has been studied with an emphasis on dynamic loads characterized by a high-frequency content, while a clear statement as to what is considered “high-frequency” is missing. Moreover, the effect of a combination of load directions on the friction reduction is not accounted for. Therefore, this study aims to determine the vibration-induced effect on friction regardless of the frequency range and direction of harmonic force for a single and multi-degree-of-freedom system. Analytical methods are used to obtain the friction modulation due to harmonic loads, considering a classical mass–spring–dashpot system on a moving belt and the Amontons–Coulomb law. It is found that, in the case of continuous slip, a general relation for the vibration-induced friction modulation is obtained utilizing the velocity response function of the investigated system. The latter is used to highlight a threshold from which the high-frequency regime starts and to determine the stick–slip boundaries. Moreover, through the velocity response function, the influence of different external harmonic forces is investigated and discussed. This includes considerations of phase, excitation frequency, system characteristics, and the choice of the normal contact force expression.@en

This study aims at assessing the predictive performance of the Amontons–Coulomb law to reliably predict the cyclic response, inclusive of stick–slip, of a single degree of freedom system in contact with the ground through two versions (steady-state and rate-and-state) of a regularized Dieterich–Ruina law. The assessment is carried out by defining a cost function and a physics-based constraint that enable the identification of the corresponding optimal coefficients of the Amontons–Coulomb law through a multi-start constrained non-linear optimization. The comparative study starts with a sensitivity analysis, aimed at first identifying the most meaningful model parameters for the Dieterich–Ruina law. Subsequently, the cyclic dynamic responses provided by both friction laws are analysed for varying model parameters, and characteristic features are observed within the dynamic forcing–displacement graph and the friction force–velocity plot, that could be directly linked to one friction model or the other. The sensitivity analysis led to the definition of a cost function expressed in terms of the displacement and velocity response differences and a constraint based on the phase difference. The optimization study identified areas of the Dieterich–Ruina's parameter space for which the Amontons–Coulomb law can reliably be used to predict a cyclic stick–slip response. The relevance of these results with respect to problems of modelling and identification of friction are discussed.

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Faults in complex systems such as Wave Energy Converters (WECs) are inevitable. WECs are expected to work in harsh environments to produce more power which increases the possibility of major failures. Increasing the reliability of WECs calls for the development of early Fault Detection (FD) algorithms which needs understanding of the enormous modes of failures to prevent the costly downtime of the device. Through this work, a methodology is provided to simulate the effect of malfunction with various levels of complexity dependent on time and velocity of the WEC. The fault signals are applied to an experimentally validated model of an array of flap-type WECs. The assumed failures range from the delayed responses to the complete blocked hinge. It is shown that even a signal of soft fault with 7% change in the response amplitude can change the period of the system. These changes are investigated for the development of a general FD algorithm for WECs.@en

The focus of this study is to characterize delamination using static and dynamic tests and to assess the interface failure mechanism of an innovative hybrid concrete beam, made out of conventional concrete and a special class of fibre reinforced material, known as Strain Hardening Cementitious Composite (SHCC). Three SHCC beams were subject to four-point bending tests, differing in the interface surface preparation and curing method. Damage and delamination were gradually induced due to increasing loads in steps of 2.5 kN, and their propagation was tracked by the use of linear variable differential transformers and Digital Image Correlation technique. Dynamic hammer tests were also carried out to identify the natural frequency variation due to progressive damage. The outcome of this comparison allowed us to assess the capability of using a frequency-based monitoring technique for possible early-stage delamination detection of hybrid civil structures. To understand the influence of delamination on the dynamic response, a simplified finite element modelling approach of delamination was adopted. The induced damage was modelled in a simplified manner by reducing the stiffness of the elements in the damaged area. This model can be potentially integrable into large-scale numerical models for Structural Health Monitoring purposes.@en

A radar equipment was used to measure the deflection response of bridge stay-cables induced by ambient and traffic excitation. After a concise description of the radar equipment and a summary of advantages and potential issues of the microwave technology, the paper focuses on the experimental tests performed on all stay-cables of the curved cable-stayed bridge erected in the commercial harbor of Porto Marghera, Venice, Italy. The bridge consists of an inclined concrete tower, single-plane cables and a composite deck; the curved deck has a centerline length of 231 m, with two different side spans and 9 cables supporting each side span. Three series of ambient vibration tests were performed (on July 2010, April 2011 and October 2019) on the two arrays of cables of the bridge by using conventional accelerometers and microwave interferometer. The availability of simultaneously collected radar and accelerometer data (which are usually regarded as reference data in dynamic tests) allowed to investigate the accuracy of the radar technique (in terms of natural frequencies and tensile force estimated from natural frequencies) and the errors/uncertainties in radar results. Furthermore, the tests allowed to verify the repeatability of radar survey, with SHM purposes.

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An alternative option to the traditional grouted joint for wind turbines is a direct steel-to-steel connection, also known as slip joint. In a recently published work, a proof of concept of a vibration-assisted installation and decommissioning technique of a slip joint was illustrated. Leveraging on the obtained results, the current study shows for the first time a decommissioning campaign carried out using a vibration-assisted technique applied on a prototype hydraulic wind turbine tower located in the North Sea, and connected to the monopile through a slip joint. The key aspect of the dismounting procedure is a priori knowledge of the resonance frequency clusters corresponding to the slip joint’s cross-sectional modes. Therefore, field hammer tests and experimental modal analysis were carried out inside the wind turbine tower. The identified frequencies and mode shapes were then compared with numerical ones estimated by a finite element model of the investigated structure. The comparison showed that a set of frequency clusters can be directly selected from a detailed numerical model. The preparatory work of the slip joint decommissioning was then executed by installing electric shaker devices, based on the dynamic identification results, and hydraulic jacks mounted inside the wind turbine tower. A first decommissioning trial was carried out in May 2019, while the final decommissioning was performed in August 2019. After analysing the measurements of the hydraulic pressures, displacements and excitation frequencies during the decommissioning campaigns, the results showed that it is possible to disconnect the slip joint if, in combination to a vertical static force, one of the identified cross-sectional mode shapes is excited. The vibration-assisted decommissioning proved to be a successful technique to dismount the connection in a controlled and straightforward manner.@en

The offshore wind market is developing towards exploiting wind resources in deeper water sites. Inevitably, this fuels new research and feasibility studies on alternative solutions for the wind turbine support structures. Within this context, this work aims at comparing three different support structure design concepts for a 14 MW two-bladed downwind wind turbine: an XXL monopile, a hybrid jacket-tower and a lattice tower. To ensure a fair comparability of the three design concepts, a load capacity analysis is first performed to assess the yield strength of each concept and guarantee a similar material utilization. The comparative analysis is then carried out in terms of total mass, dynamic behaviour of the interaction between rotor and support structure, soil-structure interaction and resulting hydrodynamic forces. Based on the given design constraints, the preliminary results of this study favour a lattice tower solution. The study also highlights peculiar dynamic phenomena such as veering, mode hybridization and mode coalescence for the dynamic interaction between the lattice tower and the two-bladed rotor which need to be taken into account during the design phase.@en

The structural failure of grouted connections for offshore wind turbines focused the industrial attention towards different and innovative solutions to guarantee a safe connection between the monopile foundation and the turbine tower. An alternative option to the traditional grouted joint is a direct steel-to-steel connection, also called a slip-joint which was sporadically used for onshore wind turbines. To such regard, a proof of concept is illustrated concerning a new installation and decommissioning technique of a slip-joint. The key aspect of the proposed method is to guarantee a proper fit and sound contact of the slip-joint by means of vibration-assisted settlements. Therefore, the effectiveness of applying a harmonic excitation during the installation and decommissioning procedure is experimentally investigated using a 1:10 scaled model of the slip-joint. During the dynamic tests, the applied static load and the settlements of the joint are monitored using load cells, displacement sensors and strain gauges placed both inside and outside the conical surfaces. For the installation tests, the results show that settlement occurs when applying a harmonic load at specific forcing frequencies. All the vibration-induced settlements tend to stabilize in time, indicating that a sound contact through vibration-assisted installation can be achieved. In a similar way, the decommissioning proved to be effective at certain forcing frequencies. According to all the tests performed during this experimental campaign, both the installation and decommissioning tests showed to be more sensitive to the forcing frequency rather than to the dynamic forcing amplitude.

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A challenging case study of dynamic friction was presented in a previous study (A. Cabboi, J. Woodhouse, Validation of a constitutive law for friction-induced vibration under different wear conditions, Wear 396–397 (2018) 107–125), concerning tests performed with a Polycarbonate pin sliding on a steel disc. Identifying and modelling the frictional frequency response for this system turned out to be rarely possible, since the measurements were affected by significant wear and by intermittent squeal occurrence. To shed light on the observed “capricious” behaviour, an “instantaneous” estimation of the frequency response of dynamic friction was developed, allowing the dynamic friction behaviour to be captured and tracked before, and for few cases during, squeal events. Each “instantaneous” frequency response of dynamic friction was fitted by a rate-and-state model, and variations of the model parameters for different sliding speeds, changing normal forces and at different wear stages were tracked. With direct relevance to squeal predictions, the model parameters identified through the proposed processing and fitting methodology could detect rapid transitions between velocity-strengthening and weakening behaviour. These transitions may occur at different sliding speeds, but they also occur during measurements carried out at a constant sliding speed. Based on the identified model parameters, a first qualitative attempt to predict squeal events by means of rate-and-state models is presented, and shown to give promising correlation with experimental results.

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