OC 3 : Benchmark Exercise of Aero-elastic Offshore Wind Turbine Codes J A Nichols and T R Camp, Garrad Hassan and Partners Ltd. J Jonkman and S Butterfield, NREL T Larsen and Anders Hansen, Risø J Azcona, A Martinez and X Munduate, CENER F Vorpahl and S Kleinhansl, CWMT M Kohlmeier, T Kossel and C Böker, Leibniz University of Hannover D Kaufer, SWE University of Stuttgart

Outline Background and partners Objectives Project phases and approach Phase III: offshore tripod Results Future work

Background and Partners The Offshore Code Comparison Collaboration (OCCC) has been coordinated within the IEA Wind Annex XXIII by the National Renewable Energy Laboratory (NREL). Project group consists of research bodies, universities and partners from industry. Phase III includes contributions from: National Renewable Energy Laboratory (NREL) (USA) Endowed Chair of Wind Energy of the Universität Stuttgart (D) Garrad Hassan (UK) Risø National Laboratory (DK) National Renewable Energies Center (CENER) (ESP). Fraunhofer Centre for Wind Energy and Maritime Engineering (D) Leibniz University of Hannover (D) Simulation tools: Bladed, Flex5, FAST, HAWC2, ADCoS, WaveLoads and ANSYS

Objectives Establishment of a suite of benchmark simulations to test new codes and for training of new analysts Identification and verification of code capabilities and limitations of implemented theories Investigation and refinement of applied analysis methodologies Investigation on the accuracy and reliability of results obtained by simulations to establish confidence in the predictive capabilities of the codes Identification of further research and development needs

Project Phases Phase I Phase II Phase III Phase IV

Approach At each stage simulations are selected to highlight different areas of interest To start with, only basic models are used Then more features are added This facilitates identifying the differences between the different codes Basic StructureWind LoadsWave LoadsFull simulationDynamicsStatic Simulation

Phase III: Offshore Tripod Significant jump in complexity from monopile substructure. Statically indeterminate Loads influenced by relative deflection of members

Modelling – wave loads Importance of modelling the structure near the sea surface in detail Without a fine discretisation, sharp jumps are seen in load signals Axial Force (kN)

Modelling – overlapping members It is important to take account of the overlapping regions when structure members join at nodes In this case, the volume which could be double- counted would be 8% of the total volume below sea level having a significant effect on buoyancy and wave loads.

Modelling – shear deflection Bernoulli-Euler theory only considers pure bending of a beam. One side is compressed while the other is stretched. In real beams, there is some shear deformation of the material. This becomes important once relative deflection of joined members becomes important. x M P l

Modelling – shear deflection

Results - Eigenanalysis

Results – Output Locations

Results – bending moments due to wave loads

Results – shear forces due to wave loads

Results – axial forces due to wave loads

Motion of the dynamic support structure

Future Work Phase IV beginning Floating spar-buoy structure Stretching the limits of existing wind turbine codes Involvement of codes used by oil and gas companies to model offshore structures

Conclusions Identification of important issues for space-frame offshore support structures. Encouragement for the development of existing codes to incorporate these features. Establishment of baseline load calculations and results for new codes to be tested against. A number of engineers are now equipped with experience of modelling offshore structures with greater knowledge of the factors which influence loading results.