Aero-Hydro-Servo-Elastic Analysis of Floating Wind Turbines with Tension Leg Moorings Erin Bachynski, PhD candidate at CeSOS erin.bachynski@ntnu.no May 15, 2013 www.cesos.ntnu.no CeSOS – Centre for Ships and Ocean Structures

Floating wind turbine concepts studied at CeSOS We need to understand floating wind turbine behavior so that we can bring the cost down Make use of offshore wind resource Spar Semi-submersible TLP

Tension Leg Platform (TLP) Stability from tension legs, implying motions as an inverted pendulum Small motions (+) Flexible w.r.t. water depth (+) Smaller steel weight (+) Small footprint area on seabed (+) Challenging installation (-) Don’t spend too much time on the analysis challenges now

TLPWT Design Displacement Pontoon radius Tendons Shimada, 2011 MIT-NREL TLPWT (Matha, 2009) Botta, 2009 Shimada, 2011 Moon, 2010 Displacement Increases cost Decreases risk of slack Pontoon radius Increases stability Increases hull loads Tendons Inkluder et lysark som viser alternative design Kanskje også et kombinert WT+WEC konsept på samme lysark, samt kanskje resultater senere

Integrated aero-hydro-servo-elastic analysis control aerodynamics structural dynamics Challenges: -complexity -tight coupling -nonlinear -time domain -long term periods -transient (faults) hydrodynamics Jeg vil også vise denne og kommentere kompleksiteten i dynamisk analyse – samtidig som vindturbiner representerer den ultimate målsetting for CeSOS – på kombinere hydro., konstr. dyn. og reguleringsteknikk (”control”) Mention Software ? Source: NREL/Wind power today, 2010.

Aerodynamics J. de Vaal, 2012 Our approach: BEM Challenges: yawed flow, engineering approximations, wind input Mention Jabus’ work with more sophisticated models, Mahmoud with icing J. de Vaal, 2012

Control system Serves to Actions regulate rotor rotation speed regulate power output protect structure Actions Change generator torque Change blade pitch

Blade pitch mechanism failures Contribution to failure rate (failures/turbine/yr) (%) Jiang, 2012 PhD candidates at CeSOS studying the effects of control system failures on different platforms : Z. Jiang, M. Etemaddar, E. Bachynski, M. Kvittem, C. Luan, A. R. Nejad – many faults occur with an annual rate of 0.05- 0.25 per year Pitch system Wilkinson et al., 2011

What happens if one blade stops pitching? TLP, U=20m/s, Hs = 4.8m, Tp = 10.8s Continue operating with faulted blade Fault occurs Shut down turbine quickly

Comparison of controller fault effects on different platforms Spar TLP Semi-Sub 1 Semi-Sub 2

Environmental/ Fault Conditions Definition A No fault B Blade seize C Blade seize + shutdown D Grid loss + shutdown Environmental/ Fault Conditions EC U (m/s) Hs (m) Tp (s) Turb. Model Faults # Sims. Sim. length* (s) 1 8.0 2.5 9.8 NTM A, B, C, D 30 16 min. 2 11.4 3.1 10.1 3 14.0 3.6 10.3 4 17.0 4.2 10.5 5 20.0 4.8 10.8 6 49.0 14.1 13.3 A (idling) 3 hours 7 11.2 ETM A Max. thrust 50 yr. storm Ext. turb. * Simulation length after 200s initial constant wind period

No fault Blade seize Blade seize + shutdown Grid loss + shutdown Storm condition Extreme turbulence at rated speed Tower Top FA Bending Moment

Hydrodynamics Large volume structures: potential flow aerodynamics control Large volume structures: potential flow First order Second order sum-frequency Slender structures: Morison’s equation Tension-moored structures: ringing forces (3rd order)

structural dynamics hydrodynamics aerodynamics control Structural Modeling Flexible beam elements (tower, blades, mooring system) Rigid hull Global model – simplified generator

TLPWT Parametric Design Study: Pitch Tower Base Bending Diameter Water Depth Pontoon Radius Ballast Fraction 45 resulting designs 7 environmental conditions Responses at wave, wind, natural frequency, turbine frequencies Aerodynamic forces must be transferred to tower bending, inertia, or line tension Changes in diameter, pontoon radius, and ballast affect both stiffness and mass – complex results! Tendon tension variation decreases with pretension, but tower bending decreases with increased ballast Cost increases with displacement A design with three pontoons, large pontoon radius, and mid-range (4000 to 7000 tonnes) displacement may be reasonable Line Tension

Concluding remarks TLP wind turbines present complex, unanswered design and analysis challenges Numerical simulations require coupled aero-hydro-servo-elastic tools and expertise A wide variety of environmental and operational conditions must be considered In our studies of floating wind turbines at CeSOS we hope to provide insights that can help inform designers and regulatory bodies Time-domain simulations Modeling choices may affect results Storm conditions tend to drive hull, mooring system design Pitch fault conditions may drive blade, tower, gearbox designs

Thank you ! Thank you!

TLPWT + 3 Point Absorbers Preliminary results indicate no significant change in power output for WEC or WT by combining Reduced tendon tension variation (5-10%) and motions % difference calculated as [(TLPWTWEC) – TLPWT]/TLPWT

Simo-Riflex-AeroDyn Java: control AeroDyn: aerodynamic forces Nonlinear time domain coupled code (Riflex: MARINTEK) Single structural solver Aerodynamic forces via DLL Advanced hydrodynamics (Morison, 1st and 2nd order potential, ringing) (SIMO: MARINTEK) Control code (java) for normal operation and fault conditions Good agreement with HAWC2 (land-based and spar, including fault) SIMO: wave forces Riflex: structural deflections, time stepping