Offshore Code Comparison Collaboration, Continued, with Correlation and unCertainty
The Offshore Code Comparison Collaboration, Continued, with Correlation, and unCertainty (OC6) project focuses on validating offshore wind modeling tools through the comparison of simulated responses of select offshore wind systems to physical test data, based on issues identified in previous IEA Wind Task 23/30 projects (OC3, OC4 and OC5). OC6 will encompass the development and application of uncertainty quantification and a three-way validation procedure between engineering-level tools, higher-fidelity tools and measurements.
National Renewable Energy Laboratory
The objective of OC6 Phase I is to investigate the persistent under-prediction (about 20% on average) of the structural loads in the OC5-DeepCwind floating semisubmersible wind system, as observed within the OC5 Phase II project. The OC5 results indicate that much of this under-prediction originates from the low-frequency response of the system at the surge/pitch natural frequencies, resulting from nonlinear hydrodynamic loading. Therefore, this work will focus on better understanding this low-frequency response behavior, the applicability of computational-fluid-dynamics (CFD) models and engineering-level hydrodynamic models for predicting the nonlinear hydrodynamic loading, and the level of uncertainty in the measured response characteristics.
In 2017, two experimental test campaigns were developed and performed by a sub-group of the OC5 project to address the objective.
• Mooring test campaign: A simplified OC5-DeepCwind semisubmersible model (a rigid tower with no turbin) was moored to three taut-spring lines and subjected to motion decay and wave tests (regular and irregular waves), during which motion amplitudes of the model were measured. This test campaign examined the repeatability of the low-frequency response of the system to wave excitation and determine the level of uncertainty in the response behavior.
• Constrained test campaign: The same model without the tower, was fixed to a carriage and subjected to towing, forced oscillation and wave tests (regular and irregular waves), during which forces exerted upon the model were measured. This test campaign examined the hydrodynamic loading at a component level by separating the wave excitation and structural radiation forces.
Both model test campaigns were performed in the concept basin of the Maritime Research Institute Netherlands (MARIN) in Wageningen, the Netherland  and the first test was within the framework of the MARINET2 project . Both test campaigns will be examined within Phase I of the OC6 project.
Starting: November 2018
Summary of instrumentation
The experimental data was generated by various measurement equipment. Motions were recorded by optical measurement device, forces were measured using strain or six-component gauges, and wave elevation was measured with immersive bars.
Most of the data will stem from computational methods.
A guideline and definition document will be available through DAP.
Robertson A.N., F. Wendt, J.M. Jonkman, W. Popko, M. Borg, H.Bredmose, F. Schlutter, J. Qvist, R. Bergua, R. Harries, A. Yde, T.A. Nygaard, J.B. de Vaal, L. Oggiano, P. Bozonnet, L. Bouy, C.B. Sanchez, R.G. Garcia, E.E. Bachynski, Y. Tu, I. Bayati, F. Borisade, H. Shin, T. van der Zee, M. Guerinel. 2016. "OC5 Project Phase Ib: Validation of Hydrodynamic Loading on a Fixed, Flexible Cylinder for Offshore Wind Applications." Energy Procedia, 94:82-101. https://dx.doi.org/10.1016/j.egypro.2016.09.201.
Robertson A.N., F.F. Wendt, J.M. Jonkman, W. Popko, F. Vorpahl, C.T. Stansberg, E.E. Bachynski, I. Bayati, F. Beyer, J.B. de Vaal, R. Harries, A. Yamaguchi, H. Shin, B. Kim, T. van der Zee, P. Bozonnet, B. Aguilo, R. Bergua, J. Qvist, W. Qijun, X. Chen, M. Guerinel, Y. Tu, H. Yutong, R. Li, L. Bouy. 2015. "OC5 Project Phase I: Validation of Hydrodynamic Loading on a Fixed Cylinder: Preprint." 12 pp. https://www.nrel.gov/docs/fy15osti/63567.pdf.
Robertson A. 2017. "Uncertainty Analysis of OC5-DeepCwind Floating Semisubmersible Offshore Wind Test Campaign: Preprint." 10 pp. https://www.nrel.gov/docs/fy17osti/68035.pdf.
Robertson A.N., F. Wendt, J.M. Jonkman, W. Popko, H. Dagher, S. Gueydon, J. Qvist, F. Vittori, J. Azcona, E. Uzunoglu, C.G. Soares, R. Harries, A. Yde, C. Galinos, K. Hermans, J.B. de Vaal, P. Bozonnet, L. Bouy, I. Bayati, R. Bergua, J. Galvan, I. Mendikoa, C.B. Sanchez, H. Shin, S. Oh, C. Molins, Y. Debruyne. 2017. "OC5 Project Phase II: Validation of Global Loads of the DeepCwind Floating Semisubmersible Wind Turbine." Energy Procedia, 137:38-57. https://doi.org/10.1016/j.egypro.2017.10.333.
Robertson A., J. Jonkman, M. Masciola, H. Song, A. Goupee, A. Coulling, C. Luan. 2014. Definition of the Semisubmersible Floating System for Phase II of OC4. NREL/TP-5000-60601. https://www.nrel.gov/docs/fy14osti/60601.pdf.
Robertson A., J. Jonkman, F. Vorpahl, W. Popko, J. Qvist, L. Froyd, X. Chen, J. Azcona, E. Uzunoglu, C.G. Soares, C. Luan, H. Yutong, F. Pengcheng, A. Yde, T. Larsen, J. Nichols, R. Buils, L. Lei, T.A. Nygard, et al. 2014. "Offshore Code Comparison Collaboration, Continuation within IEA Wind Task 30: Phase II Results Regarding a Floating Semisubmersible Wind System: Preprint." 17 pp. https://www.nrel.gov/docs/fy14osti/61154.pdf.