Modelling damping sources in monopile-supported offshore wind turbines

Vibration damping in offshore wind turbines is a key parameter to predict reliably the dynamic response and fatigue life of these systems. Damping in an offshore wind turbine originates from different sources, mainly, aerodynamic, structural, hydrodynamic, and soil dampings. The difficulties in identifying the individual contribution from each damping source have led to considerable uncertainty and variation in the values recommended. This paper proposes simplified but direct modelling approaches to quantify the damping contributions from the aerodynamic, hydrodynamic, and soil interactions. Results from these models were systemically compared to published values and when appropriate with simulation results from the software package FAST. The range of values obtained for aerodynamic damping confirmed those available in the literature, and blade element modelling theory was shown to provide good results relatively efficiently. The influence of couplings between fore‐aft and side‐side directions on the aerodynamic damping contribution was highlighted. The modelling of hydrodynamic damping showed that this damping is much smaller than usually recommended and could be safely ignored for offshore wind turbines. Soil damping strongly depends on the soil specific nonlinear behaviour

Exploring the application domain of adaptive structures

Using a previously developed design methodology it was shown that optimal material distribution in combination with strategic integration of the actuation system lead to significant whole-life energy savings when the design is governed by rare but strong loading events. The whole-life energy of the structure is made of an embodied part in the material and an operational part for structural adaptation. Instead of using more material to cope with the effect of loads, the actuation system redirects the internal load-path to homogenise the stresses and change the shape of the structure to keep deflections within limits. This paper presents a systematic exploration of the domain in which adaptive two-dimensional pin-jo inted structures are beneficial in terms of whole-life energy and monetary costs savings. Two case studies are considered: a vertical cantilever truss representative of a multi-storey building supported by an exoskeleton structure and a simply supported truss beam which is part of a roof system. This exploration takes five directions studying the influence of: (1) the structural topology (2) the characteristics of the load probability distribution (3) the ratio of live load over dead load (4) the aspect ratio of the structure (e.g. height-to-depth) (5) the material energy intensity factor. Results from the main five strands are combined with those from the monetary cost analysis to identify an optimal region where adaptive structures are most effective in terms of both energy and monetary savings. It was found that the optimal region is broadly that of stiffness-governed structures. For the cantilever case, the optimal region covers most of the application domain and it is not very sensitive to either live-to-dead-load or height-to-depth ratios thus showing a wide range of applicability, including ordinary loading scenarios and relatively deep structures

Scour influence on the fatigue life of operational monopile-supported offshore wind turbines

Offshore wind turbines supported on monopiles are an important source for renewable energy. Their fatigue life is governed by the environmental loads and in the dynamic behavior, depending on the support stiffness and thus soil-structure interaction. The effects of scour on the short-term and long-term responses of the NREL 5-MW wind turbine under operational conditions have been analyzed by using a finite element beam model with Winkler springs to model soil-structure interaction. It was found that due to scour, the modal properties of the wind turbine do not change significantly. However, the maximum bending moment in the monopile increases, leading to a significant reduction in fatigue life. Backfilling the scour hole can recover the fatigue life, depending mostly on the depth after backfilling. An approximate fatigue analysis method is proposed, based on the full time-domain analysis for 1 scour depth, predicting with good accuracy the fatigue life for different scour depths from the quasi-static changes in the bending moment

Modelling wind turbine tower-rotor interaction through an aerodynamic damping matrix

Current wind turbine modelling packages mainly adopt a complex methodology in which aerodynamic forces are coupled with the motion of the wind turbine components at every time step. This can result in long simulation run times, detrimental for the large number of simulations required for fatigue or reliability analyses. This contribution presents an efficient wind turbine modelling methodology based on blade element momentum theory and a linearization of the aerodynamic forces. This allows the wind-rotor interaction to be reduced to static forces applied at the tower top, with additional terms proportional to the tower velocities expressed as an aerodynamic damping matrix. This aerodynamic model was implemented as part of a finite element model of the tower and was successfully verified against the fully-coupled modelling package FAST. The damping matrix components explain key features of the coupling between fore-aft and side-side vibrations of the wind turbine. This coupling causes energy transfers between the two directions, complicating aerodynamic damping identification. The aerodynamic damping matrix offers novel insights and an efficient method to describe the aerodynamic damping of wind turbines

Effect of canopy profile on solar thermal chimney performance

Solar thermal chimneys (STCs) are renewable energy power plants that require large-scale deployment to be economically competitive. This paper presents a steady-state analytical model developed to describe accurately the thermodynamics of the solar collector. The impact of different collector canopy designs on the performance is assessed. Results show that the height of the canopy has a significant effect on plant performance and that the canopy must be sufficiently high at the junction with the chimney to ensure maximum kinetic energy in the flow at the chimney inlet can be reached. A new collector profile with a partially sloped canopy is proposed. It was found to perform at similar levels of maximum power output to the best-performing existing canopy designs, and to be robust under varying environmental conditions. For ease of construction and reduction of associated costs this canopy can be built in stepped annular flat sections with only a minor loss in performance

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

Shape control and whole-life energy assessment of an 'infinitely stiff' prototype adaptive structure

A previously developed design methodology produces optimum adaptive structures that minimise the whole-life energy which is made of an embodied part in the material and an operational part for structural adaptation. Planar and complex spatial reticular structures designed with this method and simulations showed that the adaptive solution achieves savings as high as 70% in the whole-life energy compared to optimised passive solutions. This paper describes a large-scale prototype adaptive structure built to validate the numerical findings and investigate the practicality of the design method. Experimental results show that (1) shape control can be used to achieve 'infinite stiffness' (i.e. to reduce displacements completely) in real-time without predetermined knowledge regarding position, direction and magnitude (within limits) of the external load; (2) the whole-life energy of the structure is in good agreement with that predicted by numerical simulations. This result confirms the proposed design method is reliable and that adaptive structures can achieve substantive total energy savings compared to passive structures

Infinite stiffness structures via active control

Active control has been used in civil engineering structures for a variety of purposes. Although the potential for using deflection-control adaptation to save material has been investigated by a few other authors, little attention has been given to assessing whether these material savings outweigh the energy consumed through control and actuation. Our paper seeks to address this gap, presenting experimental work on a truss with effective infinite stiffness which builds on earlier theoretical studies. Senatore previously developed a design method that produces an optimum adaptive structure that minimises the total energy spent throughout the whole life of the structure (embodied in the materials + operational for the control) (Senatore, et al., 2013). The method was used to design a range of structures from trusses to space frames, both determinate and indeterminate, and it was shown that it allows energy saving up to 70% compared to state of the art optimisation methods. A large scale prototype structure has now been built to validate the numerical findings and investigate the practicality of the method. This paper discusses recent experimental findings and the making of the prototype. Using the insight acquired after the making and testing of the prototype the authors will discuss potential applications of adaptive structures in selection of different scenarios, ranging from cantilever seating tiers in sports stands to lightweight roofs to slender beams with 80:1 span/depth ratio

Understanding the behaviour of graphene oxide in Portland cement paste

This study reports on the effect of graphene oxide (GO) on the hydration of Portland cement (PC) and industrial clinker. GO accelerates PC hydration, whereas it temporarily retards that of clinker. This difference reflects a twofold behaviour of GO in cement pastes. Retardation is due to the interaction of GO with the surface of hydrating grains, while acceleration results from a seeding effect. Gypsum causes this difference. GO is shown to have little effect on the strength of hardened pastes, and this merely relates to the change of hydration degree, as opposed to reinforcing effect formerly assumed. Overall, GO is not particularly active as a nucleation surface, as it aggregates and behaves in a similar way to inert fillers (e.g. quartz). Polycarboxylate-ether copolymer could make GO an active seed in cement pastes, as it prevents GO from aggregating. Nevertheless, this was found to occur only in alite pastes but not PC pastes

Identification of aerodynamic damping matrix for operating wind turbines

© 2020 Elsevier Ltd Accurate knowledge of wind turbine tower vibration damping is essential for the estimation of fatigue life. However, the responses in the fore-aft and side-side directions are coupled through the wind-rotor interaction under operational conditions. This causes energy transfers and complicates aerodynamic damping identification using conventional damping ratios. Employing a reduced two-degree of freedom wind turbine model developed in this paper, this coupling can be accurately expressed by an unconventional aerodynamic damping matrix. Simulated time series obtained from this model were successfully verified against the outputs from the wind turbine simulation tool FAST. Based on the reduced system obtained, a matrix-based identification method is proposed to identify the aerodynamic damping for numerically simulated wind turbine tower responses. Applying harmonic excitations to the tower allowed the frequency response functions of the wind turbine system to be obtained and the aerodynamic damping matrix to be extracted. Results from this identification were compared to traditional operational modal analysis methods including standard and modified stochastic subspace identification. The damping in the fore-aft direction was successfully identified by all methods, but results showed that the identified damping matrix performs better in capturing the aerodynamic damping and coupling for the side-side responses