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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Floating wind turbines' taking place on Wednesday, 12 March 2014 at 16:30 -18:00. The meet-the-authors will take place in the poster area.

Roberts Proskovics University of Strathclyde, United Kingdom
Co-authors:
Roberts Proskovics (1) F P Julian Feuchtwang (1) Shan Huang (1)
(1) University of Strathclyde, Glasgow, United Kingdom

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Roberts Proskovics is a PhD student at the Centre of Doctoral Teaching in Wind Energy Systems at the University of Strathclyde. He is in the final year of his PhD in Dynamic Response of Spar-type Offshore Floating Wind Turbines.

Abstract

Unsteady aerodynamics associated with the offshore spar-type floating wind turbines

Introduction

Up-to-date, there are only 3 full-scale floating wind turbines installed in the world. All of these are based on the fixed bottom offshore wind turbines, which, in turn, are ‘marinised’ versions of the onshore wind turbines. There might come a point in time that these will need to be redesigned to meet the specific requirements of combined wind, wave and underwater current inputs. At that point it will be important to know what can be simplified in the initial designs.

Approach

FAST [1], a wind turbine simulation package developed by the NREL (National Renewable Energy Laboratory), was used together with the OC3-Hywind [2] model of a 5MW floating wind turbine to simulate the unsteady aerodynamic effects. Each of the 6 DsoF (degrees-of-freedom) associated with a floating wind turbine (surge, sway, heave, roll, pitch and yaw) was modelled separately by disabling the flag of the other 5 DsoF and setting acceleration of the floater to zero. Thrust, torque, blade root and tower base bending moments were simulated by using uniform wind across the rotor and varying the wave height and period. In total, 48 different combinations of wave heights and periods were used. These included wave heights from 1 to 6m, with a step size of one, and periods from 5 to 25sec.
The quasi-steady (quasi-steady aerofoil characteristics and equilibrium wake) and fully-unsteady (dynamic aerofoil characteristic and dynamic wake) results were obtained and compared. This also included looking at how the aerodynamic damping differs between the both cases, as well as different DsoF.
Frequency analysis (obtained using fast Fourier transforms) was performed on the results to better understand and identify the different components (wave frequency, rotational frequency, etc.) of the output signal.
To compare different DsoF exposed to multiple combinations of wave heights and periods, DELs (damage equivalent loads) were used. These allowed comparing each load case between all 6 DsoF to try to identify which is potentially the most destructive DoF.
Finally, the fully-unsteady aerodynamic results were compared to those of the fully-attached code written in MATLAB and based on the van der Wall and Leishman theory [3] that has been discretised in the time-domain.


Main body of abstract

In this chapter various outputs from the time marching simulation in FAST are analysed to try to identify the importance of the unsteady aerodynamics and to identify which DsoF are potentially the most detrimental for a floating spar-type offshore wind turbine.
The initial analysis was performed on pure surge DoF results. Linear waves and flexible structure (blade and tower DsoF set to ‘TRUE’) were used in all the simulations. Figure 1 shows surge displacement of the floating platform in 6 m waves with 25 sec period and uniform wind of 10 m/s for both the quasi-steady and unsteady aerodynamics. Both the mean displacement (wind input) and oscillatory motion (wave input) can clearly be seen. Simulation results are shown between 2750 and 3000 seconds to allow for the under-damped system to reach its ‘steady-state’ value (natural frequency of surge is ≈0.008 Hz).
The mean displacement in the fully-unsteady case is slightly higher (16.04 m for quasi-steady and 16.38 m for unsteady). In the same time, the amplitude of displacement is almost identical between the both cases (0.01 m difference).

Figure 2 shows thrust on the rotor for the same environmental conditions (H=6 m, T=25 sec and U=10 m/s). In this case both the mean and amplitude differ between the quasi-steady and unsteady simulation results. Unsteady aerodynamic assumption leads to a higher mean (712.72 kN for the unsteady versus 700.15 kN in the quasi-steady) and amplitude of loading (50.65 m for the unsteady versus 31.55 m for the quasi-steady). Higher mean thrust on rotor in the unsteady calculations leads to a higher mean displacement in Figure 1, however, the significantly larger amplitude of thrust loading does not lead to a higher displacement in surge.
Aerodynamic damping was investigated using tower top fore-aft (FA) displacement. Figure 3 shows the FA displacement of the tower top and the associated damping ratios calculated using the logarithmic decrement method. Because aerodynamic damping depends on the displacement (amplitude of motion) and frequency of oscillation, the tower’s mass and stiffness distribution was modified to match the surge motion of the platform in 18 sec waves.
Surge displacement, for waves of 6 m height and 18 sec period, is around 1.57 m in amplitude for both the quasi-steady and unsteady cases. This means that the aerodynamic ratio is ≈2.4 % for the quasi-steady and ≈3.8 % for the unsteady case. This difference in the aerodynamic damping leads to different amplitudes of response in thrust (Figure 2), but almost identical amplitude of surge displacement (Figure 1).
Structural damping was included in calculations. Exclusion of the structural damping would further increase the difference between the both damping ratios.
Torque response to surge motion in linear waves of 6 m height and 25 sec period is shown in Figure 4. There are significant differences between the both curves. Difference in the rotational velocities between the both cases makes an impression that the unsteady results are leading those of the quasi-steady. The fully-unsteady results show much larger mean and range values. Also, it is not a pure sine wave anymore, as it contains some high frequency components that produce these sharp peaks.
FFTs were used to analyse the torque response. Figure 5 shows single-sided amplitude spectrum of toque. While the quasi-steady torque contains only one harmonic, that of the wave frequency, the unsteady results contain more than 10 harmonics. These are result of using dynamic inflow which produces a highly non-linear response of axial induction factor to any changes in the angle-of-attack. The implications that these extra harmonics can lead to are shown in Figure 6.

Spectral response of the tower base roll (side-to-side) moment shows that these harmonics can potentially coincide with other frequencies (tower, blade, rotor etc.). In Figure 6 the 7th harmonic of the wave frequency has coincided with the tower’s natural frequency producing this large spike in the spectrum.

Conclusion

In this abstract the effects of fully-unsteady aerodynamics, compared to the quasi-steady, were studied in respect to floating spar-type offshore wind turbines. The OC3-Hywind turbine was simulated in FAST, and each DoF analysed.
Simulations performed using the fully-unsteady aerodynamic assumption often show very different load responses compared to the quasi-steady runs. In some cases the range of the loading can be more than 3 times the size of the quasi-steady (Figure 4). The quasi-steady assumption can lead to either under- or over-prediction of the loads on a turbine, depending on what DoF is being analysed.
Aerodynamic damping is always smaller when the quasi-steady assumption is used. While the unsteady method can lead to a larger load on a turbine, at the same time, it can also results in a smaller displacement of the floater, due to this extra aerodynamic damping.
An order-of-magnitude smaller aerodynamic damping in the side-to-side motion together with the highly non-linear response due to the dynamic inflow produce multiple wave harmonics in the in-plane loads (torque, tower base side-to-side moment). These can be very damaging to the turbine if they coincide with other natural frequencies of the turbine, such as in Figure 6.
The rotational velocity of the rotor and response frequency of the loads varies between different wave periods and DsoF. The amplitude of the tower base fore-aft bending moment is larger in the unsteady simulations, however, DEL results show that the quasi-steady case is much more detrimental to the turbine.
Using DEL, surge and pitch DsoF were identified to have the biggest difference between the quasi-steady and unsteady simulations. It also showed, that, while unsteady case has a much larger aerodynamic damping, it is still much more detrimental in pitch DoF for the tower base fore-aft movement.


Learning objectives
This abstract tries to answer the question of how important are the unsteady aerodynamics and do they need to be accounted for in the design of the floating offshore wind turbines.


References
[1] NWTC Computer-Aided Engineering Tools (FAST by Jason Jonkman, Ph.D.). http://wind.nrel.gov/designcodes/simulators/fast/. Last modified 4-October-2013; accessed 16-October-2013.
[2] Jonkman, Jason Mark. Definition of the Floating System for Phase IV of OC3. National Renewable Energy Laboratory, 2010.
[3] Van der Wall, B. G., and J. G. Leishman. "On the influence of time-varying flow velocity on unsteady aerodynamics." Journal of the American Helicopter Society 39.4 (1994): 25-36.