Reducing Tower Fatigue through Blade Back Twist and Active Pitch-to-Stall Control Strategy for a Semi-Submersible Floating Offshore Wind Turbine

Abstract

The necessity of producing more electricity from renewable sources has been driven predominantly by the need to prevent irreversible climate chance. Currently, industry is looking towards floating offshore wind turbine solutions to form part of their future renewable portfolio. However, wind turbine loads are often increased when mounted on a floating rather than fixed platform. Negative damping must also be avoided to prevent tower oscillations. By presenting a turbine actively pitching-to-stall, the impact on the tower fore–aft bending moment of a blade with back twist towards feather as it approaches the tip was explored, utilizing the time domain FAST v8 simulation tool. The turbine was coupled to a floating semisubmersible platform, as this type of floater suffers from increased fore–aft oscillations of the tower, and therefore could benefit from this alternative control approach. Correlation between the responses of the blade’s flapwise bending moment and the tower base’s fore–aft moment was observed with this back-twisted pitch-to-stall blade. Negative damping was also avoided by utilizing a pitch-to-stall control strategy. At 13 and 18 m/s mean turbulent winds, a 20% and 5.8% increase in the tower axial fatigue life was achieved, respectively. Overall, it was shown that the proposed approach seems to be effective in diminishing detrimental oscillations of the power output and in enhancing the tower axial fatigue life.

The Impact of Pitch-To-Stall and Pitch-To-Feather Control on the Structural Loads and the Pitch Mechanism of a Wind Turbine

“…It was shown that the amplitude of the cyclic loads was reduced in pitch-to-stall control [13]. In [14], it is shown that back-twisted blades, together with pitch-to-stall control, improve the tower axial fatigue life and power output generation of a semi-submersible floating offshore wind turbine. In [15], Loza et al performed a comparative fatigue life assessment for a small wind turbine operating as a stall-regulated and pitch-regulated turbine.…”

Analysis of the Effect of a Series of Back Twist Blade Configurations for an Active Pitch-To-Stall Floating Offshore Wind Turbine

“…This is because, as found by Larsen and Hanson [6], this control method is not susceptible to 'negative damping' issues. The avoidance of negative thrust issues occurs since, as the wind velocity increases (in the above rated wind speed range), the rotor thrust increases for a stall controlled turbine, whereas for a feather controlled turbine the thrust diminishes, creating a negative derivative with respect to the wind speed [7]. Hence, active pitch-to-stall control enables a higher control frequency setting to be employed within the blade pitch controller, which improves both rotor speed regulation and power quality.…”

“…The design criteria of blade twist often focuses on maximising the turbine's power, as such, changing the setting of the constant pitch angle below rated wind speed has also been employed to optimize the energy output [9]. This work builds on the research presented on a VSVP-S FOWT by Ward et al [7], which investigated the effect on tower axial fatigue of a blade back twist that started at a distance of 75% along the blade radius, as measured from the root. For the semisubmersible platform analysed, an increase in the tower axial fatigue life of over 20% was achieved when operating in turbulent mean winds of 13mps.…”

“…Hence, the twists initiated have been confined to the last 25% of the blade (closer to the tip). Further details on the rotor thrust, power curves and pitch angles for the Feather Base Model and Stall Test Model, as derived through steady state analysis over the full range of operational wind speeds, are provided by Ward et al [7].…”

“…At wind speeds below rated, the controller did not change the pitch angle for either the feather or stall designs. More details of this controller and the process to obtain the gain settings can be found in Ward et al [7].…”