Scale Offshore Wind Turbine Research Articles

Cross-wind fatigue analysis of a full scale offshore wind turbine in the case of wind–wave misalignment

Wind–wave misalignment is often necessary to consider during the design of offshore wind turbines due to excitation of side–side vibration and the low aerodynamic damping in that direction. The measurements from a fully instrumented 3.6MW pitch regulated-variable speed offshore wind turbine were used for the estimation of the side–side fatigue loads at the tower bottom. The joint wind–wave distribution and the distribution of the wind–wave misalignment angles were considered. The side–side fatigue at the tower bottom and the damping from site measurements are presented as function of the misalignment angles. A model of the same wind turbine was set-up and simulations with the aero-hydro-servo-elastic code HAWC2 were performed to investigate the effect of damping on the side–side fatigue. Turbulent wind field, irregular waves and flexible soil are used in the simulations based on site-measurements. The aim of the current study is to examine the sensitivity of the side–side fatigue to the wind–wave misalignment and different values of additional offshore damping in the system. It was found that the additional offshore damping of the physical system may be higher than what is typically used in offshore wind turbine sub-structure design, due to the low sensitivity of the measured side–side fatigue loads to the misalignment angle. Choice of an accurate damping value implemented in the model during the design of the wind turbine sub-structure can lead to material and cost savings.

Discrepancy study of modal parameters of a scale jacket-type supporting structure of 3.0-MW offshore wind turbine in water and in air

The discrepancy of modal parameters of a scale offshore wind turbine is studied by using the proposed assessment method. One theoretical development is that weak genuine modes can be separated from strong noisy modes; and the other is the size of the reconstructed Hankel matrix will not be changed to ensure the comparability of modal parameters from different scenarios. A numerical signal is synthesized to demonstrate the proposed method. Numerical results indicate that the approach can isolate the two genuine modal parameters respectively, by applying estimated pass band with a 2 by 2 Hankel matrix of the Eigensystem Realization Algorithm, which means it can be used as a criterion to assess modal parameters from different scenarios. An experiment with model scale 15 from a 3.0-MW offshore wind turbine is tested. Experimental results indicate that natural frequencies are considerably reduced and damping ratios are increased, with the rate of frequency change varies from 15.33% to 17.97%. The modal parameters obtained in water with waves are close to those obtained in still water even if the structure is excited by a hammer or waves. The modal parameters estimated from the reconstructed responses of different accelerometers are in excellent agreement.

Identification of support structure damping of a full scale offshore wind turbine in normal operation

The support structure damping of a 3.6 MW pitch controlled variable speed offshore wind turbine on a monopile foundation is estimated both in standstill conditions and in normal operation. The net substructure damping is identified from the parameters of an exponential curve fitted to the relative maxima of an impulse response caused by a boat impact. The result is used in the verification of the non aerodynamic damping in normal operation for low wind speeds. The auto-correlation function technique for damping estimation of a structure under ambient excitation was validated against the identified damping from the decaying time series. The Enhanced Frequency Domain Decomposition (EFDD) method was applied to the wind turbine response under ambient excitation, for estimation of the damping in normal operation. The aero-servo-hydro-elastic tool HAWC2 is validated with offshore foundation load measurements. The model was tuned to the damping values obtained from the boat impact to match the measured loads. Wind turbulence intensity and wave characteristics used in the simulations are based on site measurements. A flexible soil model is included in the analysis. The importance of the correctly simulated damping in the model is stressed for accurate load prediction. Differences in the identified damping between the model and the wind turbine are detailed and explained. Discrepancies between simulated and measured loads are discussed.

An overview of the state of the art technologies for multi‐MW scale offshore wind turbines and beyond

An overview of technological trends in the design of multi‐mega Watt wind turbines focused on the offshore sector is presented. The state of the art technologies for wind turbine design are multidisciplinary ranging from blade aeroelasticity, power transmission to the generator, to advanced control systems that ensure performance and the design of offshore support structures to minimize cost of energy. Light weight carbon fiber blades, aeroelastic tailoring using bend–twist coupling are discussed in coordination with a multitude of aerodynamic technologies for optimal power capture such as high‐lift airfoils, flaps, and flat‐back airfoil designs. The pitch control of the turbine responsible for rotating the blades about its axis to reduce loads is the primary load reducing mechanism of the wind turbine, for which technologies that enable wind sensing such as forward looking LIDARs and redundancy mechanisms such as individual blade pitch control offer promising advances. The lack of reliability of the gearbox has resulted in drive train technologies to move toward direct drives, whose benefits and liabilities are assessed in combination with generator concepts. The support structures are discussed within an offshore framework for shallow, moderately deep, and deep waters. Fixed substructures such as monopiles, tripods, jackets, as well as floaters such as spar buoys and tension leg platforms are brought forth. The advances in these different component technologies that enable the wind turbine system to be reliable and cost effective are described as a precursor to further in‐depth reviews.This article is categorized under: Wind Power > Science and Materials

Load analysis and comparison of different jacket foundations

As a country mostly surrounded by water, Korea plans to build a massive offshore wind farm off the west coast by 2019. Due to the different seabed condition, wind turbine with jacket foundation is considered as a suitable choice for the intermediate water depths so far. And preliminary work is necessary for the wind turbine manufactures. In this work, a 3-blade upwind, pitch controlled, variable speed 5 MW offshore baseline model is used. Two different Jacket foundations are designed for the water depth of 30 m. Using several load cases, the dynamic response of two kinds of jacket structures, are compared by modal analysis and dynamic analysis. The analysis results of the system natural frequencies, significant loads of different jacket members for both models are given in detail. This research gives a good knowledge of large scale offshore wind turbine in Korea for intermediate water depth.

Similitude relationships for physical modelling of monopile-supported offshore wind turbines

Offshore wind turbines are considered as an important element of the future energy infrastructure. There is currently a surge in the construction of such facilities in Europe, yet there is no track record of long-term performance of these structures. Offshore wind turbines are dynamically sensitive structures because of the very nature of the structural form (tall and slender) and the different types of dynamic and cyclic loading imposed on them. Lack of data concerning long-term performance indicates a need for detailed investigation to predict the future performance of such structures. Arguably this can be best carried out through small-scale well-controlled laboratory experimental investigation. In this paper, scaling laws are derived for the design of such model tests for studying the long-term performance. Non-dimensional groups that need to be preserved are identified while carrying out these tests. The effectiveness of these chosen non-dimensional groups is investigated by carrying out controlled tests on a 1:100 scale offshore wind turbine. Typical experimental data are presented.