Instructional Efficiency in Asynchronous Online Discussions

Cognitive load mitigation strategies & community of inquiry framework are not discipline specific

Wave Load Mitigation by Perforation of Monopiles

The design of large diameter monopiles (8–10 m) at intermediate to deep waters is largely driven by the fatigue limit state and mainly due to wave loads. The scope of the present paper is to assess the mitigation of wave loads on a monopile by perforation of the shell. The perforation design consists of elliptical holes in the vicinity of the splash zone. Wave loads are estimated for both regular and irregular waves through physical model tests in a wave flume. The test matrix includes waves with Keulegan–Carpenter ( K C ) numbers in the range 0.25 to 10 and covers both fatigue and ultimate limit states. Load reductions in the order of 6%–20% are found for K C numbers above 1.5. Significantly higher load reductions are found for K C numbers less than 1.5 and thus the potential to reduce fatigue wave loads has been demonstrated

Cyclic Pitch Control for the Reduction of Ultimate Loads on Wind Turbines

In this paper we study the use of individual blade pitch control as a way to reduce ultimate loads. This load alleviation strategy exploits the fact that cyclic pitching of the blades induces in general a reduction of the average loading of a wind turbine, at least for some components as the main bearing, the yaw bearing, or the tower. When ultimate loads are generated during shutdowns, the effect of the use of cyclic pitch results in reduced peak loads. In fact, as the machine starts from a less stressed condition, the response to an extreme gust or other event will result in reduced loading on its components. This form of load mitigation can be seen as a preventative load mitigation strategy: the effect on load reduction is obtained without the need to detect and react to an extreme event, but by simply unloading the machine so that, in case an extreme event happens, the result will be less severe. The effect of peak load mitigation by preventative cyclic pitch is investigated with reference to a multi-MW wind turbine, by using high-fidelity aeroelastic simulations in a variety of operating conditions

Actuator fault tolerant offshore wind turbine load mitigation control

Offshore wind turbine (OWT) rotors have large diameters with flexible blade structures which are subject to asymmetrical loads caused by blade flapping and turbulent or unsteady wind flow. Rotor imbalance inevitably leads to enhanced fatigue of blade rotor hub and tower structures. Hence, to enhance the life of the OWT and maintain good power conversion the unbalanced loading requires a reliable mitigation strategy, typically using a combination of Individual Pitch Control (IPC) and Collective Pitch Control (CPC). Increased pitch motion resulting from IPC activity can increase the possibility of pitch actuator faults and the resulting load imbalance results in loss of power and enhanced fatigue. This has accelerated the emergence of new research areas combining IPC with the fault tolerant control (FTC)-based fault compensation, a so-called FTC and IPC “co-design” system. A related research challenge is the clear need to enhance the robustness of the FTC IPC “co-design” to some dynamic uncertainty and unwanted disturbance. In this work a Bayesian optimization-based pitch controller using Proportional–Integral (PI) control is proposed to improve pitch control robustness. This is achieved using a systematic search for optimal controller coefficients by evaluating a Gaussian process model between the designed objective function and the coefficients. The pitch actuator faults are estimated and compensated using a robust unknown input observer (UIO)-based FTC scheme. The robustness and effectiveness of this “co-design” scheme are verified using Monte Carlo simulations applied to the 5MW NREL FAST WT benchmark system. The results show clearly (a) the effectiveness of the load mitigation control for a wide range of wind loading conditions, (b) the effect of actuator faults on the load mitigation performance and (c) the recovery to normal load mitigation, subject to FTC action

Load mitigation method for wind turbines during emergency shutdowns

Wind turbines experience countless shutdowns during their lifetimes. A shutdown is a transient process characterised by a pitch-to-feather manoeuvre of three blades. Such a pitch manoeuvre is often collective, open-loop, and can substantially slow the rotor speed within several seconds. However, undesirable structural responses may arise because of the imbalanced aerodynamic loads acting on the rotor. To address this issue, this paper proposes a method that actively adjusts the individual pitch rate of each blade during an emergency shutdown. This method is founded on a minimal intervention principle and uses the blade-root bending moment measurements as the only inputs. The control objective is to minimise the differences in the blade-root flapwise bending moment among the three blades during the shutdown. Using a high-fidelity aeroelastic model, we demonstrate the controller performance under representative steady wind conditions with vertical wind shear. Compared with the baseline shutdown strategy, the proposed method effectively reduces the maximum nontorque bending moment at the main shaft and the tower bottom bending moment; the reductions vary between 10% and 40% under the investigated conditions. The present work can be further extended to reduce structural fatigue damages or to handle complex loading scenarios of offshore wind turbines during shutdowns.publishedVersio

High Specification Offshore Blades. Work Package: 1C – Blade Materials.

Blades are regarded as the only component unique to wind turbine blades. They represent only 10 – 15% of the total system cost so the perception is that a reduction in the cost of energy through blade cost improvements is constrained. However, the use of novel materials technologies is predicted to reduce design loading by 10 – 20%, which may indirectly lead to substantial cost savings. The aim of this report is therefore to identify materials technologies offering potential for improved blade performance and their potential for (patentable) intellectual property exploitation

Passively morphing blades for load alleviation of tidal turbines

Tidal turbines are exposed to time variable loading that can lead to premature failure [1,2]. The use of passive unsteady load mitigation technology, such as bend-twist coupling, is typically limited to low frequency fluctuations and is not suitable to large blades, due to structural rigidity requirements. Active control systems, such as actuated flaps, can respond to higher frequencies than whole-blade passive devices due to their smaller size [3]. However, active systems may reduce turbine reliability. Hence there is a need to develop reliable technology that mitigates unsteady loading in a varied range of frequencies, in order to prolong the fatigue life of tidal turbines. Analytically, it is possible to cancel the unsteady loading of a tidal turbine that rotates through the ocean shear layer with a fully chordwise highly flexible blade. Here, we demonstrate that under attached flow conditions, when a blade is rigid near the leading-edge and flexible near the trailing-edge, the unsteady load mitigation is proportional to the ratio of the flexible length to the full chord of the blade. We verify this relationship experimentally with a blade that has a passively morphing trailing-edge. The morphing trailing-edge extends 25% of the chord of the blade and it allows unsteady load mitigation of up to 25%, without any variation in the mean load -thus there is no penalty in terms of power extraction. In separated flow conditions, when the length-scale of vortical structures is similar to that of the flexible part of the blade, the load mitigation is about 15%. Hence, chordwise morphing blades alleviate loads in variable flow conditions and can contribute to tidal turbine survivability in a reliable way. [1] G.T.Scarlett, B.Sellar, T.van den Bremer,I.M.Viola, Unsteady hydrodynamics of a full-scale tidal turbine operating in large wave conditions, Renewable Energy, 143 (2019), 199-213. [2] G.T.Scarlett,I.M.Viola, Unsteady hydrodynamics of tidal turbineblades, Renewable Energy, 146 (2020), 843-855. [3] A. Young, J. Farman, R. Miller, Load alleviation technology for extending life in tidal turbines, Proceedings of 2nd International Conference on Renewable Energies Offshore, RENEW 2016, 521-530

Aerodynamic mitigation of extreme wind loading on low-rise buildings

Hurricanes and other extreme wind events cause immense devastation to our economy every year. Modern buildings should be designed to withstand extreme wind so that it reduces the financial strain on the economy. An experimental study was performed to compare aerodynamic performance of new roof designs. Traditional roof shapes were also included in this study to determine if the new designs had any merit in aerodynamic roof load mitigation. An atmospheric boundary layer wind tunnel was used with the characteristic wind of a suburban boundary terrain. The force results obtained from measurement of the roof loads showed that the largest reduction was achieved with the leading-edge spoiler, which resulted in 32.3% reduction of roof uplift. A few other methods also demonstrated adequate roof load mitigation. The aerodynamic modification of buildings provides a cost effective solution to reducing the economic impact of hurricanes and other extreme wind phenomena