In order to reduce their cost, offshore wind turbines (OWTs) must have a powerful generator and a minimum overall weight. This has the consequence of making the structure of the OWTs sensitive to dynamic excitations (wind and wave loads) even at low frequencies. Indeed, modern multi-megawatt OWTs are composed of slender flexible and lightly damped components (blades and tower). The excessive vibrations of the OWT structural components can compromise the wind energy conversion to electricity, decrease the fatigue lifetime and even result in a total collapse of the OWT when exposed to harsh environmental conditions. It is therefore important to reduce the unwanted vibrations of an OWT by implementing a control device that enhances its structural safety.This paper proposes the use of a passive tuned mass damper (TMD) placed in the nacelle housing in the fore-aft direction to control the OWT vibrations due to wind and wave loads. A full three-dimensional model of a monopile-supported 10 MW DTU offshore wind turbine is developed using the finite element code Abaqus/Standard. This model simultaneously considers the real geometrical configuration of the OWT superstructure (blades, nacelle, tower, transition piece and monopile) and its interaction with the soil. The TMD mass is connected to the OWT structure by tuned spring and dashpot elements. The wind turbine superstructure (blades, tower, transition piece and monopile in water) is subjected to stochastic wind and wave loads generated using the NREL aero-servo-elastic simulator FAST.In addition to the passive TMD, an active control of the tower vibrations is investigated in this paper to improve the performance of the passive system. Indeed, an active vibration controller is added to the TMD within the developed 3D model in Abaqus. The modelled TMD is now an active tuned mass damper (ATMD). An optimal control scheme is chosen to obtain the optimal control force on the actuator. In this study, one makes use of the robust Linear Quadratic Regulator (LQR) feedback controller combined with a Linear Quadratic observer in order to reduce the required measurements of the different system states. The controller is implemented using a Fortran user subroutine, which is coupled to the Abaqus 3D model to test the capability of the ATMD in mitigating the turbine vibrations. It should be noted herein that almost all the previous studies on the active vibration control of OWTs are based on the application of the control scheme on a simplified wind turbine analytical model within Simulink/Matlab. The technique presented in this work is not meant to replace the conventional methods, as they are certainly still faster, and in particular, more appropriate for the selection and tuning of controller parameters. However, given the increasing complexity of wind turbines model, and the visualization capabilities included in Abaqus, this method could be particularly useful as a final development stage. As a first result, it is demonstrated that the vibration control using the proposed active control scheme has a promising prospect in reducing the excessive vibrations more than the passive TMD alone.

Vibration Control of an Offshore Wind Turbine

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

OMAE 2021

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.