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In modern MW-size machines it has become a common practice to introduce controllers that provide active damping of turbine components to reduce blade, tower and drive-train loads, whilst optimising energy capture. However, as wind turbines become larger and more flexible, these controllers have to be designed with great care as the coupling between flexible modes increases and so does the potential to destabilise the turbine. The most direct method to address the above issues has been to exploit the pitch control capabilities. Individual Pitch Control (IPC) has been proposed many times over the last few years for load mitigation. Bearing this in mind, this paper investigates two different approaches to design a controller to pitch each blade individually in the wind turbine operating region III. The first one is a decentralised control algorithm and the second one is an H ∞ loop shaping design. A controllability analysis of the wind turbine is also included in the paper. The investigation is conducted based on the NREL 5MW benchmark wind turbine. Turbine modeling and control is conducted in FAST and Simulink.Copyright © 2011 by ASME

Offshore wind turbines are becoming more common due to the scarcity of suitable land sites. By going offshore, maintenance costs become one of the driving expenses. Hence more reliable components should be implemented on offshore wind turbines. The down-wind rotor configuration does not require as powerful yaw drive as the upwind rotor configuration to align with the wind direction. Thus the yaw system can be simpler with fewer components that can fail and require maintenance. This paper presents numeric simulation studies of how the tower shadow impacts the blades when they pass through the wake behind the tower. The work concentrates on bottom-fixed offshore wind turbines designed for the specifications of the NREL offshore 5-MW baseline wind turbine. The blade response has been compared for a full truss tower and a conventional tubular tower to show how the different tower shadows influence the blades. The blades on the more transparent truss tower experience less root flapwise moment fluctuations due to the weaker tower shadow. The simulations were performed by means of GH Bladed, version 3.82.Copyright © 2010 by ASME

Recent and historical literature regarding hydrodynamics has been reviewed, with offshore wind turbine support structures in mind. Under conditions of separated flow, several relevant phenomena have been noted which are not covered by the commonly-used Morison equation: 1. damping of structural vibration or slow-drift motion; 2. the interaction of structural vibration and vortex shedding; 3. loads near the free-surface; and, 4. burst motions, caused by impulse-like loading from steep waves. References have been given to books and articles that describe the phenomena in more detail. A form of the Morison equation is proposed which has separate empirical coefficients for each of the velocity and acceleration terms. The coefficients can be determined from existing test data with use of least-squares error minimization. A simplified form of the equation provides a means to obtain conservative bias on both the applied load (bias towards a high drag coefficient) and damping (bias towards a low drag coefficient). Further investigation into free-surface and burst motion (ringing) phenomena is recommended, considering a slender wind turbine monotower in 20 to 50 m water depth.Copyright © 2009 by ASME

Wind turbines have grown tremendously during the last 20 years and it is of interest to investigate how large they can be expected to be. In the present paper several structural aspects have been studied in order to see how they develop as a function of size. It was found that as long as thrust and centrifugal forces dominate, then most aspects of the turbine are independent of scale, provided geometric similarity scaling is performed. The most important deviation from this rule is that selfweight of the blades results in stresses that grow linearly under geometric scaling. The effects of selfweight appear to become important for the largest machines currently available. (5MW)Copyright © 2007 by ASME

The paper describes how an existing computer code, VICoMo, see Moe & Arntsen (2002), has been extended to analyse Vortex Induced Vibrations (VIV) of SCR’s for situations in which the flow is neither in the plane of the riser, nor perpendicular to it. Various strategies were attempted in order to make the solution deal with flows at oblique angles. It was found that the best approach was to assume that the riser motions occurred in plane perpendicular to the flow, while motions in the flow direction were disregarded. One source of uncertainty is associated with the empirical database in VICoMo which is based on tests in which the flow was perpendicular to the riser elements. The timeseries from the experiment showed large temporal variations. Actually the average motions amplitudes at given points may be as low half of the maximum value. Also the largest amplitudes at different points on the experimental envelope on the riser will usually have been reached at quite different times. These features were not included in the theoretical model, and made direct comparisons between predictions and measurements difficult. We ended up comparing the envelopes of motion response. In several of the cases the predictions matched the experiments closely, for frequency, modeshape as well as amplitudes. In other cases the match was somewhat less satisfactory, however.Copyright © 2004 by ASME

The theory behind a computer programme for prediction of Vortex Induced Vibrations (VIV) of risers is summarised, and some experiences with its use are given. The programme in question is named VICoMo (VIV by Complex Modes) and is based on modal analysis by means of complex modes. The approach is iterative in nature because the added mass and damping forces depend nonlinearly on motion amplitude and frequency. Experiences with some runs to model a prototype Steel Caternary Riser (SCR) are summarised. A key element herein is the strategy for the choice of the parameters governing the iterations.Copyright © 2002 by ASME