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The scientific community is devoting more attention to the wide scope of offshore wind turbine structures. Since such structures are subjected to high level of fatigue loads as well as a large number of load cycles caused by wind, waves and turbine operation, the fatigue performance of welded connections is usually a design driving criteria. In this paper, a brief review on experimental fatigue analysis of circular hollow section joints for jacket structures is presented. Special emphasis is given to full-scale experimental testing. In order to face some of the challenges in this area of expertise, an experimental research plan within the framework of the Innovative Training Network (ITN) AEOLUS4FUTURE is introduced, aiming to understand and validate the fatigue performance of circular hollow section joints produced by an automated process, using Tandem MIG/MAG welding.

With wind power establishing itself as a widely effective form of renewable energy source, a diverse range of maintenance schemes is being investigated towards life-time extension of existing wind farms. Among the numerous structural and mechanical parts of a Wind Turbine (WT), blades are the most critical and costly components. Exposed to a number of degradation mechanisms, such as cracks and fatigue, they may be well rendered structurally ineffective and unsafe. Detecting and localizing thus the existence of damage on WT blades is a crucial and essential task for planning optimal maintenance and assuring operational reliability of WTs. One of the main challenges for deploying Structural Health Monitoring (SHM) methodologies on in-service WT blades lies in the operational and environmental variability. It is widely reported that fluctuations in ambient temperature exert a strong impact on the vibration features of WTs, insinuating the damage induced structural changes may often be masked by changes due to temperature influences. With WTs operating in highly-varying climate conditions, it becomes thus imperative that temperature effects be properly taken into account within the context of damage detection and localisation. In this contribution, the most common vibration-based criteria for damage localization are examined and compared through a numerical application on a small- scale WT blade. The study is built around a 3-dimensional finite element model of the blade, which comprises an exterior laminate composite surface, modelled with shell elements, and interior foam represented by solid elements. The efficacy of localization is evaluated under varying environmental conditions and a critical assessment on the accessibility of sensing information and the severity of damage is presented. 9th European Workshop on Structural Health Monitoring (EWSHM 2018): Online Proceedings

Note: This is a preprint of paper #1914 presented at the 14th European Wave & Tidal Energy Conference (EWTEC) 2021 in Plymouth, UK. The final version of paper with the same title can be found in the EWTEC 2021 proceedings. Abstract: Lift-based Wave Energy Converters (WECs) have a number of attractive features, including the potential for unidirectional rotation, simplifying power take-off and reduction in wave loads by reducing generation of circulation, increasing survivability. The common assumption of small body, small amplitude response, together with the Haskinds Relationship is used to determine the optimum motion for a lift-based WEC to maximise power capture. It is shown that whilst for a 2D hydrofoil in deep water the optimum motion is circular, the optimum motion for a finite-width hydrofoil is generally elliptical due to differences in the hydrodynamic damping coefficients associated with the vertical and horizontal motions of the hydrofoil. It is shown that more circular hydrofoil motion can be achieved by utilising the elliptical motion of the water particles in shallow water. This occurs because the increased horizontal water particle motion in shallow water results in an increase in the wave-induced lift force associated with horizontal fluid particle motions, and thus a reduction in the optimum amplitude of motion in this direction. Preliminary calculations suggest that for a 30 metre wide hydrofoil in wave periods of about 10 seconds, the ideal water depth (where the optimum hydrofoil motion is circular) occurs at around 25 metres, which is a highly utilisable water depth. Other advantages of deployment in shallower water include an improvement in the alignment of the waves parallel to the hydrofoil and a reduction in the structural task associated with reacting against the seabed. This work was produced as part of the LiftWEC Project. This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 851885. This output reflects the views only of the author(s), and the European Union cannot be held responsible for any use which may be made of the information contained therein.

The fatigue limit state (FLS) of fixed offshore wind turbine structures is critical and difficult to handle. As it is the most common design driving criteria for offshore structures, the simulation and calculation of this phenomenon must be as accurate as possible. Research is needed to improve the current design. There are mainly two design approaches available: Integrated design approach (IDA) and Sequential design approach (SDA). The IDA, described in this paper, considers the coupled structural analysis of a whole wind turbine system exposed to wind- and wave-induced loads in an aero-hydro-elastic solver. The results given by solver are loads series, which are afterwards used for obtaining the stress series with stress concentration factors (SCF) included. The stresses are processed in terms of rainflow counting and finally, fatigue damage of a critical K-joint is obtained externally, to avoid the use of damage equivalent loads (DEL) as by default in the solver, but to calculate it by means of the Efthymiou principle. The whole procedure with methods is explained in this paper.

At the dawn of Industry 4.0, it has become apparent that assessment of engineered systems should be informed from the state of the system “as-is”. To this end, data needs to be fused with adequate and efficient system models. Such system models should account for the underlying physics and the possibly nonlinear dynamic processes involved. This paper introduces a physics-based parametric formulation for nonlinear structural systems. A Reduced Order Model (ROM) of the high fidelity system is developed, retaining the dependencies on system properties and on temporal and spectral characteristics of the excitation. The ROM formulation relies on i) Proper Orthogonal Decomposition applied to snapshots of the nonlinear response, and ii) manifold interpolation of the resulting projection bases. Its performance is evaluated on a 3D earthquake-excited shear frame with nonlinear couplings. The developed ROM can be exploited for a number of tasks including monitoring, diagnostics and residual life estimation of critical components.

One of the most important criteria in the design of fixed offshore wind turbine structures is fatigue resistance. There is an unabated need for research in order to improve and optimize current design methods. There are mainly two approaches for structural analysis available in the offshore industry: the Integrated Design Approach (IDA) and the Sequential Design Approach (SDA). Within the IDA, the entire wind turbine, consisting of the jacket structure including tower and the rotor nacelle assembly (RNA), is considered as a unique system exposed to wind- and wave-induced loads in an aero-hydro-elastic solver. In SDA, the jacket structure is converted into a superelement and implemented into an aero-elastic solver, where it is expanded by an RNA in order to obtain the wind-induced interface loads. The obtained interface loads are used for further analysis in a more advanced offshore code, where the wave-induced loads are simulated. The fatigue damage of the relevant K-joint in the support structure is afterwards compared to the one obtained in terms of IDA. Apart from the judgement about advantages and disadvantages of both approaches, this work benefits from confirming the reliability and applicability of both approaches.