Among the renewable energy technologies, offshore wind energy is expected to provide a significant contribution for the achievement of the European RE targets for the next future (passing from current 16 GW to 70 GW by 2030 [1]). Offshore wind has enormous potentialities: the resource is stable and abundant and the public acceptance is higher. Currently, almost all of the EU offshore wind farms are installed as bottom-fixed structures, mainly in the North Sea, at water depths below 50 m, which is the current technological limit. The development of Floating Offshore Wind Turbines (FOWT) would allow the exploitation of the 80% of total offshore wind resource in Europe. 
 
As highlighted in [2], the reduction of the levelized cost of energy (LCOE) may be achieved through several strategies. For instance, in the design process initial manufacturing and plant costs may be &nbsp;decreased through reduction of platform size and development of more more cost-effective mooring systems. However, also the increase of the generated power combined with the reduction of vibratory loads given by application of suitable advanced control systems may be effectively used for LCOE reduction purposes. 
 
In particular, in the operating region 3 (over rated speed) the maximum extracted power is limited for structural and electrical reasons, and one of the main concerns affecting FOWTs operations consists of motion and load perturbations arising for sea-wave/platform interactions and wind-gust effects yielding time-variant generated power and structural fatigue. 
In [3], for instance, induced sea-wave power fluctuations are alleviated acting on blade pitch and generator torque. 
 
The aim of this paper is the definition of a control strategy aimed at increasing wind turbine lifetime reducing vibratory blade and hub loads. The idea is to apply a Proportional-Integral (PI) controller that, fed by observed cyclic blade-root bending moments, provides the cyclic pitch of the blades to be actuated. 
The proportional and integral gain matrices are evaluated with an optimization procedure where the objective is the rejection of vibratory loads in the presence of a linear distribution of wind over the rotor disk. 
A linear, state-space, Reduced Order Model of the FOWT is identified and used for the controller synthesis, based on the aero-hydroelastic simulations provided by the open-source, commonly used, OpenFAST tool [4], which is also applied for validation of the control strategy. &nbsp; 
 
Computational investigations will examine the NREL 5MW wind turbine [5], supported by a spar buoy platform [6]. 
Preliminary results show significant reduction of the blade root bending moments at the multiples of the turbine revolution frequency (1P,2P) demonstrates the good potentialities of the proposed controller. 
In addition to a wide&nbsp;analysis of the results obtained by application of the developed control strategy, important issues like the influence of the controller on platform motions, the control effort and the predicted blade lifetime extension will be examined in the paper. 
 
REFERENCES 
1. Offshore Wind Temporary Working Group. Offshore wind implementation plan. Technical report, EU-SET Plan, 2018. 
2. James Rhodri and M Costa Ros. Floating offshore wind: Market and technology review. Carbon Trust Report Prepared for the Scottish Government, 2015. 
3. Lugni C. Bernardini G. Serafini J. Gennaretti M. Pustina, L. Control of floating offshore wind turbine response to sea waves. SDEWES 14th sustainable development of energy, water and environment systems conference, 2019. 
4. Jason Jonkman. The new modularization framework for the fast wind turbine cae tool. In 51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, page 202, 2013. 
5. Jason Jonkman, Sandy Butterfield, Walter Musial, and George Scott. Definition of a 5-mw reference wind turbine for offshore system development. Technical report, National Renewable Energy Lab.(NREL), Golden, CO (United States), 2009. 
6. Jason Jonkman. Definition of the floating system for phase iv of oc3. Technical report,National Renewable Energy Lab.(NREL), Golden, CO (United States), 2010.

Individual Blade Pitch Control for Alleviation of Vibratory Loads on Floating Offshore Wind Turbines

Paper Proceedings: https://asmedigitalcollection.asme.org/OMAE/OMAE2021/volume/85116

OMAE 2021

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