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Rick Damiani , PhD, PE Huimin Song, PhD Amy Robertson, PhD Jason Jonkman, PhD

A New Structural-Dynamics Module for Offshore Multimember Substructures within the Wind Turbine CAE Tool FAST. Rick Damiani , PhD, PE Huimin Song, PhD Amy Robertson, PhD Jason Jonkman, PhD 23 r d ISOPE - Anchorage, AK - July 3 , 2013. Outline. FAST Capabilities

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Rick Damiani , PhD, PE Huimin Song, PhD Amy Robertson, PhD Jason Jonkman, PhD

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  1. A New Structural-Dynamics Module for Offshore Multimember Substructures within the Wind Turbine CAE Tool FAST Rick Damiani , PhD, PE Huimin Song, PhD Amy Robertson, PhD Jason Jonkman, PhD 23rd ISOPE - Anchorage, AK - July 3, 2013

  2. Outline • FAST Capabilities • FAST8 Modularization Framework • SubDyn Module • Case Studies • Conclusions

  3. FAST Capabilities • FAST is a coupled aero-hydro-servo-elastic code that computes the loads and responses of both land-based and offshore wind turbines • FAST presently models: • HAWTs • Land-based wind systems • Monopiles • Floating systems • This presentation summarizes new additions available in FAST8 to model multi-member sub-structures within SubDyn • Jackets • Tripods

  4. FAST8 Modularization Framework AeroDyn Module Tight Coupling ServoDyn Module FAST Driver Code ElastoDyn Module HydroDyn Module SubDyn Module Loose Coupling Jonkman (2013)

  5. SubDyn – Substructure Structural-Dynamics Module Nonlinearities more important for tower behavior (Damiani et al. 2013) • Linear FEM • Euler-Bernoulli or Timoshenko beam elements • Constant or tapered cross-section • Craig-Bampton Dynamic LinearSystem Reduction • DOFs from 103 to 101 • Physical DOFs at boundaries + modal coordinates • Discard high-frequency content in the system dynamics • Time Integrator (Loose Coupling) • RK4, AB4, ABAM • Degree of Fixity: Clamped/Clamped (more in development, includingsoil-pile interaction module) ServoDyn Module ElastoDyn Module

  6. SubDyn – Main Input/Output TP - rigidly linked to interface nodes Other Modules Geometry, material properties, number of modes to retain (C-B), damping coefficients FAST Driver TP TP motions Hydro.loads TP loads Internal motions SubDyn Clamped restraints Member loads @ base restraints Soil module – future development

  7. Craig-Bampton Fundamentals • EOM: Separate boundary and internal DOFs R L • Transform internal DOFs from physical to modal: • Retain just m internal, generalized (modal) DOFs • Use transition piece (TP) node in place of interface nodes – since rigidly connected • Apply BC C-B reduced EOM:

  8. State-Space Formulation • Inputsu • States x • Outputs y • Integration w/ IC • Member Loads Other Modules FAST Driver TP TP motions Hydro.loads TP loads Internal motions SubDyn

  9. Case Study: OC4 Jacket – Full FEM vs. C-B ReductionModal Response • SubDynfull FE  1,014 DOFs • SubDyn C-B  10, 14, and 18 total DOFs 4 internal DOFs sufficient to capture first 4 jacket eigenmodes; higher numbers of DOFs improve higher mode predictions

  10. Case Study: OC4 Jacket – Full SystemModal Response • NREL 5-MW turbine (HH = 90 m) + OC4 Jacket (50-m water depth) + tower (68 m) • RNA mass = 350 tons • SubDyn connected to a full FE model of the tower 1 internal DOF sufficient to well capture first 6-9 structure eigenmodes (associated with tower, TP and RNA)

  11. Case Study: OC4 Jacket – Full SystemStatic Response • Rotor thrust loads  2,000 & 4,000 kN Excellent agreement with ANSYS; internal DOFs do not affect static deflections of tower and interface

  12. Conclusions • SubDyn – New FAST module for substructure structural dynamics • FEM + C-B Reduction  Linear model (limited loss in accuracy for typical offshore substructures) • System reduction dramatically reduces DOFs and computational time while retaining accuracy for lower frequency dynamics • SubDyn vs. ANSYS comparisons showed excellent agreement in both modal and static analysis • Little dependency on the number of internal modes retained on the overall turbine system dynamics at the dominant lower frequencies • To be developed: • Soil-structure interaction module • Higher order elements (non-axisymmetric members)

  13. References • Damiani, R.; Song, H.; Robertson, A.; Jonkman, J. (2013a). “Assessing the Importance of Nonlinear Structural Characteristics in the Development of a Jacket Model for the Wind Turbine CAE Tool FAST”. 32nd International Conference on Ocean, Offshore and Arctic Engineering (OMAE2013), Nantes, France; June 9-14, 2013 • Jonkman, J.M. (2013): The New Modularization Framework for the FAST Wind Turbine CAE Tool. 51st AIAA Aerospace Sciences Meeting and 31st ASME Wind Energy Symposium, Grapevine, Texas, Jan. 2013. • Schwartz, M.; Heimiller, D.; Haymes, S.; Musial, W. (2010). Assessment of Offshore Wind Energy Resources for the United States. 104 pp.; NREL Report No. TP-500-45889. • Tegen, S.; Lantz, E.; Hand, M.; Maples, B.; Smith, A.; Schwabe, P. (2013). 2011 Cost of Wind Energy Review. 50 pp.; NREL Report No. TP-5000-56266.

  14. For More Information Rick.Damiani@NREL.gov

  15. Offshore Wind Opportunities • Wind resource • Proximity to load centers • Reduced visibility • Reduced noise impact • US Offshore Potential: 600GW (≥Class 4 in transitional waters 30-60m) INSTALLED CAPACITY- Offshore/Onshore (end of 2012) • Europe 5 GW /106 GW • China 260 MW/75.5 GW • USA 0 MW/ 60 GW Schwartz et al. (2010)

  16. Offshore Wind Opportunities • Offshore costs primarily associated with BOS and O&M • Innovative foundation and installation methods • Land-based: 72 $/MWh • Offshore: 225 $/MWh • Evaluate Innovations and impacts on LCOE • Minimize LCOE Tegen et al. (2013)

  17. Nonlinearities • Types: • Material nonlinearity – NO • Steel members, which are common offshore operate far below yield strength • Axial shortening due to bending • Large displacement • Previous paper by Damiani (2013) showed nonlinear effects were negligible for multi-member structures

  18. Craig-Bampton Method • Approach: • Method for reducing size of FE model • Combines motion of boundary points with modes of the structure assuming the boundary points are held fixed • Similar to other reduction schemes • Advantages: • Allows for problem size to be reduced • Accounts for both mass and stiffness (unlike Guyan reduction) • Problem size defined by frequency range • Allows for different boundary conditions at interface (unlike model decoupling) • Summary: • C-B mass and stiffness matrices fully define system • Dynamics problem solved using C-B DOFs • C-B boundary DOFs provide location to apply BC’s and forces to couple with another structure • C-B transform is used to calculate physical response from C-B responses

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