Floating offshore wind turbines

Transcription

1 Floating offshore wind turbines Michael Muskulus Department of Civil and Transport Engineering Norwegian University of Science and Technology 7491 Trondheim, Norway Stochastic Dynamics of Wind Turbines and Wave Energy Absorbers August 6 8, 2014

2 Part II Optimization and control of floating offshore wind turbines

3 Learning objectives Design criteria Wave forces on floating structures Basic equation of motion Mooring systems Floating wind turbine concepts Basic dynamic instability Recent work

4 Mooring systems

5 Mooring systems (Brown 2005, McCormick 2012) Unmoored structure free in surge, sway and yaw degrees of freedom (no restoring forces) mooring system required for stationkeeping Requirements Offset limitations (e.g. wind turbine power cable) Lifetime before replacement Installability Positioning ability Dynamic performance (e.g. yaw stability, floater accelerations)

6 Catenary equation (Faltinsen 1990) Basic relationship between horizontal anchor distance X and horizontal cable force T H : ( X = l h a ) 1 ( 2 + a cosh h ) h a where l is length of the chain, h is water depth, and a = T H /w. Needs to be numerically inverted.

7 Advanced mooring systems (Herbich 1990) Multi-component moorings common Include clump weights to increase restoring under extreme conditions (Luo 1992) More options: buoys, synthetic rope, etc.

8 Crowfoot mooring (Quallen et al. 2014) Increases yaw stiffness

9 Typical line characteristics (Laks 2014) Design criteria Fairlead tension (line breaking strength) No vertical loads at (drag) anchor General consideration: more influence on floater motion (higher stiffness) results in higher tension loads But limit on motion offsets (e.g. due to power cable)

10 Mooring system nonlinearity (Brown 2005) Mooring line characteristic typically nonlinear Complicates motion analysis Large dependence on mean environmental loads Typically more pronounced for shallower waters (higher stiffness)

11 Floating wind turbine concepts

12 Basic floating wind turbine concepts (Jonkman / NREL)

13 Stability triangle (Butterfield et al. 2007)

14 Wave forces for typical floaters Parameter Spar-buoy Semi-submersible TLP Hywind WindFloat Dimension D m m 6 10 m Low Seastate H = 1 m, T = 4 s, ω = 1.6 rad/s, λ = 24 m KC numbe D/λ Average Seastate H = 4 m, T = 10 s, ω = 0.6 rad/s, λ = 160 m KC number D/λ Extreme Seastate H = 12 m, T = 16 s, ω = 0.4 rad/s, λ = 350 m KC number D/λ

15 Overview of existing concepts (EWEA 2013)

16 Hywind / OC3 spar (Jonkman 2009) and (offshorewind.biz)

17 OC3 spar Added mass and damping from linear diffraction theory: M. Muskulus (Jonkman 2009) Norwegian University of Science and Technology

18 Radiation impulse response kernels: OC3 spar M. Muskulus (Jonkman 2009) Norwegian University of Science and Technology

19 OC3 spar Wave excitation (per unit wave amplitude): (Jonkman 2009)

20 Semi-submersible concepts (EWEA 2013) IDEOL floater and WindFloat

21 TLP concepts under development (EWEA 2013) Alstom Haliade 150 and Glosten Pelastar TLP

22 Hybrid concepts: Spar and point absorber M. Muskulus (Muliawan et al. 2012) Norwegian University of Science and Technology

23 Spar and point absorber Heave response (Muliawan et al. 2012) Significantly less dynamic heave above rated Other degrees of freedom show little change Power production wind turbine 6% higher due to less pitch Total power production estimated 10% higher

24 Control

25 Basic principles of wind turbine control Optimize power production through optimal C p tracking Additionally, the power needs to be limited when the wind speed is above rated Torque-generator control: adjust power take-off and accelerate/deaccelerate the rotor Blade pitch control: reduce aerodynamic forces (above rated wind speed) (Burton et al. 2001)

26 Generator torque control (Burton et al. 2001) Optimal power point tracking Special considerations in transition zone (around rated)

27 Blade pitch control (Gasch 2012) Pitching to feather: limit power by increasing pitch angle (reduced angle of attack) Smooth, good accuracy, but needs relatively large pitching actions for stronger winds

28 Basic instability in floating wind turbines (Skaare et al. 2007) Interaction with platform motion leads to a basic instability in floating wind turbines above rated wind speed Rotor thrust decreases with increasing wind speed due to blade pitching Negative damping occurs: Turbine moving towards the wind: thrust reduced Turbine moving out of the wind: thrust increased

29 Basic instability in floating wind turbines (Skaare et al. 2007) For fixed bottom turbines avoided by placing the bandwith of the pitch controller below the natural frequency of the first tower bending mode For floating turbines the periods in pitch and surge are much higher and this method would lead to large loads on the tower and rotor (and large variations in rotor speed and power output)

30 Estimator based control Response (Skaare et al. 2007) Hywind controller modified to include platform motion Wind velocity estimated such that the effect of tower motion is not contained in the estimate More than 50 percent reduction in fatigue damage

31 Optimization

32 Optimization of floating wind turbines (Tande et al. 2014) Optimization of total cost Wind turbine cost use steel mass as proxy variable Transport & Installation cost often neglected Operation & Maintenance cost often neglected

33 Five examples WINDOPT Computer-aided optimization of spar-buoy shape Short spar Alternative spar-buoy FOWT design TLB Structural optimization of splash zone Semisubmersible Mooring system optimization in the frequency-domain TLP Study of a parameterized family of TLPs

34 WINDOPT (Fylling & Berthelsen 2011)

35 WindOpt Results (Fylling & Berthelsen 2011) Comparison of initial design (black) and optimization under operational load (red) and survival load (blue) Example 1: Optimization with extreme conditions only Example 2: Optimization with fatigue life constraints Example 3: Optimization with power cable

36 WINDOPT Results (Fylling & Berthelsen 2011)

37 WINDOPT Results (Fylling & Berthelsen 2011)

38 WINDOPT Power cable (Fylling & Berthelsen 2011)

39 Short spar concept (Karimirad & Moan 2012)

40 Short spar Mooring systems (Karimirad & Moan 2012)

41 Short spar Reponse (Karimirad & Moan 2012) Surge response more favourable Tension response improved for higher frequencies

42 Short spar Structure performance (Karimirad & Moan 2012) Increased pitch motion (due to less mass / less meta-centric height) Less pretension but larger dynamic tension (stiffer mooring system) Smaller nacelle accelerations Increased bending moment due to increased tilt (gravity load)

43 Reduced wave loading for a TLB Baseline tension-leg-buoy model: (Myhr & Nygaard 2012)

44 Reduced wave loading for a TLB (Myhr & Nygaard 2012) Goal: minimize excess buoyancy (reduce mass and anchor cost, while avoiding snap loads)

45 TLB Results (Myhr & Nygaard 2012) Not possible to reduce excess buyancy but around 10 percent load reduction achieved for X3 Interestingly, X4 performs worse (40 percent more topside mass due to large no. braces)

46 TLP design (Bachynski & Moan 2012)

47 TLP designs Cost estimates (Bachynski & Moan 2012)

48 TLP designs Surge response (Bachynski & Moan 2012)

49 TLP designs Structure performance (Bachynski & Moan 2012) Tension dominated by wave excitation Bending moment significantly increased for TLPs 2-5 TLP design 3/4 optimal in this limited study

50 Mooring system optimization in the frequency domain (Brommundt 2012)

51 Mooring system optimization in the frequency domain (Brommundt 2012)

52 Mooring system optimization in the frequency domain (Brommundt 2012)

53 Mooring system optimization in the frequency domain Significant contribution from spectral wind loads: (Brommundt 2012)

54 Outlook

55 Outlook Complex design problem Complex physics Large uncertainties Many loadcases Certain potential for computer-aided optimization Research needs (cf. Efficient numerical models at different levels of fidelity Stochastic / statistical descriptions Reliability-based design optimization Life beyond potential flow and linear wave theory (e.g. CFD, wake structure behind floaters) Validation of numerical tools Alternative materials (concrete, composites) Asset management (e.g. scheduling of inspections and maintenance) Multi-unit moorings and multi-turbine units Ice-resistant designs

56 References S. Butterfield, W. Musial, J. Jonkman, P. Sclavounos: Engineering challenges for floating offshore wind turbines. Technical Report NREL/CP , National Renewable Energy Laboratory (2007). E.E. Bachynski, T. Moan: Design considerations for tension leg platform wind turbines. Marine Structures 29 (2012), M. Brommundt, L. Krause, K. Merz, M. Muskulus: Mooring system optimization for floating wind turbines in the frequency domain. Energy Procedia 24 (2012), D.T. Brown: Mooring systems. In: S. Chakrabarti (ed), Handbook of offshore engineering, Elsevier (2005). T. Burton, D. Sharpe, N. Jenkins, E. Bossanyi: Wind energy handbook. John Wiley & Sons (2001). EWEA: Deep water The next step for offshore wind energy. Report, European Wind Energy Association (2013). O.M. Faltinsen: Sea loads on ships and offshore structures. Cambridge University Press (1990).

57 References I. Fylling, P.A. Berthelsen: WINDOPT An optimization tool for floating support structures for deep water wind turbines. OMAE R. Gasch, J. Twele: Wind power plants. Springer (2012). J.B. Herbich (ed): Developments in offshore engineering Wave phenomena and offshore. Gulf Publishing (1999). J. Jonkman: Dynamics of offshore floating wind turbines Model development and verification. Wind Energy 12 (2009), M. Karimirad, T. Moan: Comparative study of spart-type wind turbines in deep and moderate water depths. OMAE Y. Luo: Optimum design of clump weights for offshore mooring systems. Proc. ISOPE Vol. 2 (1992), M.J. Muliawan, M. Karimirad, T. Moan, Z. Gao: STC (spar-torus combination): A combined spar-type floating wind turbine and large point absorver floating wave energy converter promising and challenging. OMAE (2012).

58 References A. Myhr, T.A. Nygaard: Load reductions and optimizations on tension-leg-buoy offshore wind turbine platforms. Proc. ISOPE Vol. 1 (2012), S. Quallen, T. Xing, P. Carrica, Y. Li, J. Xu: CFD simulation of a floating offshore wind turbine system using a quasi-static crowfoot mooring-line mode. ISOPE Journal of Ocean and Wind Energy, to appear. B. Skaare, T.D. Hanson, F.G. Nielsen: Importance of control strategies on fatigue life of floating wind turbines. OMAE J.O.G. Tande, K. Merz, U.S. Paulsen, H.G. Svendsen: Floating offshore turbines. WIREs Energy and Environment, to appear.