Offshore Wind TurbinesAero-Servo-Hydro-Elastic Computations of

Offshore StructuresEquinor 26.04.2019

Tor Anders Nygaard, Institute for Energy Technology (IFE), Norwaytor.anders.nygaard@ife.no

Outline• Motivation• IFE Wind Energy Research Focus• Load calculations

• 3DFloat• CFD

• Recent and Current Projects• Conclusions

Development of Landbased Wind Turbines

Source: https://northsearegion.eu/northsee/e-energy/future-energy-industry-trends/ , accessed 28.03.2019

Compared to rotor mass scaling with diameter **3, designers have gotten rid of more than 90% of the mass !

Cost of Energy

Source: R. WISER et al. Forecasting wind energy costs & cost drivers - The views of the world’s leading experts. Tech. rep. IEAWind Task 26, 2016.

Installed Capacity, Offshore Wind Power in Europe

Source: Floating offshore wind vision statement. Tech. rep. WindEurope, 2017.

Floater Examples, Steel Masses

Umaine Spar UMaine TLP

MIT TLB

SWAY

UMaine Semi-Sub OC4 Jacket

90 m

126 m

5 MW

Rotor+Tower: 490t steel

Platform steel [t]:1865 550 1250 3300 940 770

Raadal, H. L., Vold, B. I., Myhr, A. and Nygaard, T. A. (2014). GHG emissions and energy performance of offshore wind power. Renewable Energy, 2014, Vol. 66, 314-324

IFE R&D focus: Technology evaluation and development• Development of new wind turbine technologies for improved LCOE (Lifetime Cost of

Energy)• Develop and use state of the art numerical simulation tools customized to specific

needs• We have experience with most wind turbine concepts on land, bottom fixed or

floating.• Assist and enable industry partners so they can do most of the work themselves• Design detailed and realistic wind turbine rotors for the industrial that they can use

for research, development, engineering or operational purposes

The braceless concrete semi-submersible The tension leg buoy (TLB)

30.04.2019

IFE R&D focus: Wind-sea-structure simulations• We use CFD when higher accuracy or details is needed then what simplified

models can produce.• We are at a stage where we can reproduce experiments quite accurately

High-resolution URANS simulation of 5 MW rotor on supercomputer

IFE 3DWind simulation of the Vindeby offshore wind park

High-resolution DES simulation of stalled blade

Varying degree of stall for the NASA-Ames NREL 10m blade

Replicating the OO Star concept model tests

Load Computations• Frequency domain, useful approximation for:

• Bottom-fixed• Floating, if properly linearized around operating point below

rated wind speed, for fatigue calculations

• Concerns regarding floating wind turbine computations leading to time-domain:• Large deflections• Coupling between aerodynamics, structural rigid body and

elastic motions, hydrodynamics and pitch control system.• Design driving transients, such as emergency stop of rotor.

Time Domain Computations, 3DFloat

More Information about 3DFloat

• Nygaard, T. A., De Vaal, J., Pierella, F., Oggiano, L. and Stenbro, R. (2016). Development, Verification and Validation of 3DFloat; Aero-Servo-Hydro-Elastic Computations of Offshore Structures. Energy procedia 2016, Vol. 94, pg. 425-433

• Extra slides at the end of this presentation

3DFloat Verification/Validation Examples• OC3-HYWIND. Code-to-code (C2C) verification, IEA• OC4 Space-Frame «Jacket» C2C, IEA• OC4 Semi-Submersible. C2C, IEA • Tension-Leg-Buoys. Validation against wave tank experiments• OO-Star Semisubmersible. Validation against wave tank experiments• Bjørnafjorden Submerged Floating Tunnel and Floating Bridges

• More information at the end of this presentation

CFD: Rotors, Floaters, Bridge Girders

Oggiano, L., Pierella, F., Nygaard, T.A., De Vaal, J.B. and Arens, E. (2017). Reproduction of steep long crested irregular waves with CFD using the VOF method., Energy Procedia Vol 137. pg 273-281 (Deepwind 2017)

Oggiano, L., Pierella, F., Nygaard, T.A., De Vaal, J.B. and Arens, E. (2016) Comparison of Experiments and CFD Simulations of a Braceless Concrete Semi-submersible Platform. Energy Procedia 2016, Volume 94, pg. 278-289

Oggiano, L., Pierella, F., De Vaal, J. B., Nygaard, T. A., Stenbro, R., Arens, E. Modeling of 2D irregular waves on a sloped bottom using a fully nonlinear Navier-Stokes/VOF formulation. ISOPE - International Offshore and Polar Engineering Conference. Proceedings 2017 s. 622-629

Oggiano, L., De Vaal, J., Pierella, F., Arens, E. and Nygaard, T.A. (2016). Comparison of Experiments and CFD Simulations on a Rigid Monopile in Shallow Water under Regular Waves. Proceedings of the 26th (2016) International Offshore and Polar Engineering Conference (ISOPE), Rhodes, Greece, June 2016.

Oggiano, L., Arens, E., Myhr, A., Nygaard, T.A. and Evans, S. (2015). CFD Simulations on a Tension Leg Buoy Platform for Offshore Wind Turbines and Comparison with Experiments. Proceedings of the 25th (2015) International Offshore and Polar Engineering Conference (ISOPE), Kona, Big Island, Hawaii, June 2015.

Recent and Current Projects withStatoil/Equinor as Main Sponsor

• DIMSELO• Implementation of advanced hydrodynamics in 3DFloat, and

application to floating wind turbine computations• REDWIN

• Implementation of interface to NGI super-elements for soil-structureinteraction in 3DFloat, and validation against full-scale data for bottom-fixed wind turbines

• NEXTFARM• Wind farm wake models, and the consequences for wind farm

energy capture and loads (fatigue).• FIRM

• Implementation of advanced elements for fibre ropes in 3DFloat• Development of fibre rope mooring systems for floating wind

turbines

Bjørnafjorden Floating Bridge Phase VNorconsult/Olav Olsen Team

Dec 2018 – August 2019

• Support of time-domain computations with 3DFloat• Verification against ABAQUS/DynNo and SIMA• IFE is responsible (Competence/Time/Resources) for

aerodynamics/wind loads and validation of simulationmodels

Conclusions

• IFE contributes today to cost and risk reductions in offshore wind projects from research on computationmethods to applied research and engineering

• We have cooperated with industry, in particularStatoil/Equinor and engineering companies like Dr.techn. Olav Olsen, Norconsult and Aibel for manyyears, and are looking forward to continue to do so in the rapidly expanding and exciting field of offshore wind energy !

Extra Slides

About IFE

• Independent non-profit energy research foundation established in 1948

• 600 employees• Wind energy research for about 40 years• Software tailored for industrial processes based on

the Finite-Element-Method for about 40 years• 5 person research group with offshore wind turbines

as the primary focus• Many other relevant research areas, such as:

• Integrated operations• Manufacturing and analysis of metal parts• Corrosion, materials, mathematics, physics, chemistry,

numerical modeling, control systems, multi-phase flow, welding, batteries, hydrogen, energy systems…..

Installed Floating Wind Turbines

Source: R. JAMES et al. Floating Wind Joint Industry Project -Phase I Summary Report: Key findings from electrical systems, mooring systems, and infrastructure & logistics studies. Tech. rep. Carbon Trust, 2018.

3DFloat Simulation Model (IFE)• General Nonlinear Finite-Element-Model (FEM)

framework with 3 translational and 3 rotational Degrees-ofFreedom (DOF) at each node

• Co-rotated approach catches geometric nonlinearities(e.g. centrifugal stiffening of rotor blade or catenarymooring line behavior) with standard small-strain elements

• Euler-Bernoulli beam elements containing aero-and hydrodynamic information

• Cable elements tailored for mooring lines• Blade elements containing lift- and drag characteristics

tailored for wind turbine rotors and bridge girders• Linear Potential Thery bodies assigned to nodes• Springs, linear and quadratic dampers, constant or

harmonic point forces

3DFloat Simulation Model (IFE), cont• Blade pitch controllers:

• Generic PI control of pitch for variable-speed rotor• OC3/OC4 controllers• DLL interface to proprietary controllers (such as Statoils

controller for Spar-Buoy)

• Frequency domain solution (eigen frequencies and mode shapes)

• Time domain solutions• Implicit generalized-alpha scheme. This scheme can

suppress high-frquency vibrations without adding significantdamping to the frequencies of interest

• The implicit Newmark scheme is a special case of thegeneralized-alpha scheme

• Explicit central difference scheme

Optimization with IFE ALSIM Packeage• The optimizer manipulates the chosen design variables

and creates input files to 3Dfloat• The optimizer runs 3Dfloat, if desired one instance per

processor• The optimizer parses through 3DFloat output and

calculates cost function• New iteration if design variables and cost function are still

changing• Recent addition of algorithms:

• Efficient Global Optimization (EGO)• Genetic Algorithm• Bound Optimization BY Quadratic Approximation (BOBYQA) • DIviding RECTangles (DIRECT)

Hydrodynamic Loads• Buoyancy with respect to Still Water Line (SWL)• Buoyancy for nearly horizontal, partially submerged

elements with respect to the instantaneous water surface in swells.

• Wave kinematics:• Regular linear Airy or Streamfunction up to order 12. • Irregular linear or second order Airy• Import of kinematics generated externally (eg. CFD)

• Force Models• Morisons Equation, extended with axial loads• Dynamic pressure on cone sections and end caps• Linear Potential Theory, importing WADAM or WAMIT

frequency plane results

Aerodynamic Loads• Blade Element/Momentum (BEM) theory for rotors,

with extensions for yaw error and dynamic inflow.• Tower influence based on potential theory for upwind

rotors, and empirical tower wake model for downwindrotors

• Wind variablility in space and time accounted for by import of pre-generated turbulence files. «HAWC2-format» of the IEC/Mann model or TurbSimformat.

• Teknikgruppen turbulence generator tailored to thespectra defined in N400 for bridges.

• Cylinder elements with quadratic drag• Blade elements with lift-and drag lookup tables for

use in rotors, or for non-cylinder cross sections.

Bridge Aerodynamics Implementation in 3DFloat• Costa, C. and Borri, B. (2006). Application of indicial functions in

bridge deck aeroelasticity. Journal of Wind Engineering and Industrial Aerodynamics 94 (2006) 859–881.

• Verification against cases in paper is in progress.• Preliminary examples for illustration here: Step changes of pitch

angle and vertical velocity

OC3-HYWIND Verification

Jonkman, J. et al (2010). Offshore Code Comparison Collaboration within IEA Wind Task 23: Phase IV Results Regarding Floating Wind Turbine Modeling. European Wind Energy Conference & Exhibition, Warsaw, Poland, April 2010.

OC4 «Jacket» Verification

Popko, W. et al. (2014). Offshore Code Comparison Collaboration Continuation (OC4), Phase I, Results of Coupled Simulations of an Offshore Wind Turbine With Jacket Support Structure. Journal of Ocean and Wind Energy, 2014, Vol. 1, No. 1

OC4 Semisubmersible Verification

Robertson, A. et al. (2014). Offshore Code Comparison Collaboration, Continuation Within IEA Wind Task 30: Phase II Results Regarding A Floating Semisubmersible Wind System. 33rd International Conference on Ocean, Offshore and Arctic Engineering, OMAE2014, June 8-13, 2014, San Francisco, CA, USA

Tension-Leg-Buoy Validation

• Myhr, A. and Nygaard, T. A. (2015). Comparison of Experimental Results and Computations for Tension-Leg-Buoy Offshore Wind Turbines. Journal of Ocean and Wind Energy, 2015, Vol. 2, No. 1

OO Star Semisubmersible ValidationKillingstad, S. and Edfelt, K. (2014). Analysis of a Semi-Submersible Offshore Wind Turbine (in Norwegian). Master Thesis at NMBU

Engelsvold. A.(2015). Hydro-elastic Analysis of a Semi-submersible Offshore Wind Turbine (in Norwegian). Master Thesis at NMBU

Azcona, J., Bouchotrouch, F., González, M., Garciand, J., Munduate, X., Kelberlau, F. and Nygaard, T.A. (2014). Aerodynamic Thrust Modelling in Wave Tank Tests of Offshore Floating Wind Turbines Using a Ducted Fan. Journal of Physics: Conference Series 524 (2014) 012089

Submerged Floating Tunnel

3Dfloat was used by Dr.techn. Olav Olsen AS for design of a Submerged Floating Tunnel for Bjørnafjorden

3DFloat provides time domain solutions within the design loop.

Bjørnafjorden Floating Bridge Phase III

• 3Dfloat was used for time-domain computations withfull coupling within the design loop (Norconsult, Aker, Dr.Techn Olav Olsen, Aas-Jacobsen)

• Verification against Orcaflex (time-domain) and Nofaframe (frequency domain)