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This paper provides a summary of the work done within Phase III of the Offshore Code Comparison Collaboration, Continued, with Correlation and unCertainty (OC6) project, under the International Energy Agency Wind Technology Collaboration Programme Task 30. This phase focused on validating the aerodynamic loading on a wind turbine rotor undergoing large motion caused by a floating support structure. Numerical models of the Technical University of Denmark 10¿MW reference wind turbine were validated using measurement data from a 1:75 scale test performed during the UNsteady Aerodynamics for FLOating Wind (UNAFLOW) project and a follow-on experimental campaign, both performed at the Politecnico di Milano wind tunnel. Validation of the models was performed by comparing the loads for steady (fixed platform) and unsteady (harmonic motion of the platform) wind conditions. For the unsteady wind conditions, the platform was forced to oscillate in the surge and pitch directions under several frequencies and amplitudes. These oscillations result in a wind variation that impacts the rotor loads (e.g., thrust and torque). For the conditions studied in these tests, the system aerodynamic response was almost steady. Only a small hysteresis in airfoil performance undergoing angle of attack variations in attached flow was observed. During the experiments, the rotor speed and blade pitch angle were held constant. However, in real wind turbine operating conditions, the surge and pitch variations would result in rotor speed variations and/or blade pitch actuations, depending on the wind turbine controller region that the system is operating. Additional simulations with these control parameters were conducted to verify the fidelity of different models. Participant results showed, in general, a good agreement with the experimental measurements and the need to account for dynamic inflow when there are changes in the flow conditions due to the rotor speed variations or blade pitch actuations in response to surge and pitch motion. Numerical models not accounting for dynamic inflow effects predicted rotor loads that were 9¿% lower in amplitude during rotor speed variations and 18¿% higher in amplitude during blade pitch actuations. This research has been supported by the office of Energy Efficiency and Renewable Energy (grant no. DE-AC36-08GO28308). Peer Reviewed Roger Bergua1, Amy Robertson1, Jason Jonkman1, Emmanuel Branlard1, Alessandro Fontanella2, Marco Belloli2, Paolo Schito2, Alberto Zasso2, Giacomo Persico3, Andrea Sanvito3, Ervin Amet4, Cédric Brun5, Guillén Campaña-Alonso6, Raquel Martín-San-Román6, Ruolin Cai7, Jifeng Cai7, Quan Qian8, Wen Maoshi8, Alec Beardsell9, Georg Pirrung10, Néstor Ramos-García10, Wei Shi11, Jie Fu11, Rémi Corniglion12, Anaïs Lovera12, Josean Galván13, Tor Anders Nygaard14, Carlos Renan dos Santos14, Philippe Gilbert15, Pierre-Antoine Joulin15, Frédéric Blondel15, Eelco Frickel16, Peng Chen17, Zhiqiang Hu17, Ronan Boisard18, Kutay Yilmazlar19, Alessandro Croce19, Violette Harnois20, Lijun Zhang21, Ye Li21, Ander Aristondo22, Iñigo Mendikoa Alonso22, Simone Mancini23, Koen Boorsma23, Feike Savenije23, David Marten24, Rodrigo Soto-Valle24, Christian W. Schulz25, Stefan Netzband25, Alessandro Bianchini26, Francesco Papi26, Stefano Cioni26, Pau Trubat27, Daniel Alarcon27, Climent Molins27, Marion Cormier28, Konstantin Brüker28, Thorsten Lutz28, Qing Xiao29, Zhongsheng Deng29, Florence Haudin30, and Akhilesh Goveas31 1National Wind Technology Center, National Renewable Energy Laboratory, Golden, CO 80401, USA 2Department of Mechanical Engineering, Politecnico di Milano, Milan 20156, Italy 3Laboratory of Fluid-Machines, Dipartimento di Energia, Politecnico di Milano, Milan 20156, Italy 4Wind Department, Bureau Veritas, Paris 92937, France 5Marine Division, Research Department, Bureau Veritas, Saint-Herblain 44818, France 6Wind Turbine Technologies, Centro Nacional de Energías Renovables, Sarriguren 31621, Spain 7Integrated Simulation Department, China General Certification Center, Beijing 100013, China 8Research Institute, China State Shipbuilding Corporation, Chongqing 401122, China 9Offshore Technology Department, DNV, Bristol BS2 0PS, UK 10Department of Wind Energy, Technical University of Denmark, Lyngby 2800, Denmark 11State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian 116024, China 12Département Electrotechnique et Mécanique des Structures, Électricité de France, Paris 91120, France 13Wind Energy Department, eureka!, Errigoiti 48309, Spain 14Department of Wind Energy, Institute for Energy Technology, Kjeller 2027, Norway 15Département Mécanique des Fluides, IFP Energies nouvelles, Rueil-Malmaison 92852, France 16Research and Development, Maritime Research Institute Netherlands, Wageningen 6708, the Netherlands 17Marine, Offshore and Subsea Technology, Newcastle University, Newcastle NE1 7RU, UK 18Aerodynamic Department, Office National d’Etudes et de Recherches Aérospatiales, Paris 92190, France 19Department of Aerospace Science and Technology, Politecnico di Milano, Milan 20156, Italy 20Floating Offshore Group, PRINCIPIA, La Ciotat 13600, France 21Wind Energy Group, Shanghai Jiao Tong University, Shanghai 200240, China 22Department of Offshore Renewable Energy, Tecnalia Research & Innovation, Donostia-San Sebastián 20009, Spain 23Wind Energy Department, Netherlands Organisation for Applied Scientific Research, Petten 1755, the Netherlands 24Wind Energy Department, Technische Universität Berlin, 10623 Berlin, Germany 25Institute for Fluid Dynamics and Ship Theory, Hamburg University of Technology, 21073 Hamburg, Germany 26Department of Industrial Engineering, University of Florence, Florence 50139, Italy

This paper provides a summary of the work done within the OC6 Phase II project, which was focused on the implementation and verification of an advanced soil?structure interaction model for offshore wind system design and analysis. The soil-structure interaction model comes from the REDWIN project and uses an elastoplastic, macroelement model with kinematic hardening, which captures the stiffness and damping characteristics of offshore wind foundations more accurately than more traditional and simplified soil?structure interaction modeling approaches. Participants in the OC6 project integrated this macroelement capability to coupled aero-hydro-servo-elastic offshore wind turbine modeling tools and verified the implementation by comparing simulation results across the modeling tools for an example monopile design. The simulation results were also compared to more traditional soil-structure interaction modeling approaches like apparent fixity, coupled springs, and distributed springs models. The macroelement approach resulted in smaller overall loading in the system due to both shifts in the system frequencies and increased energy dissipation. No validation work was performed, but the macroelement approach has shown increased accuracy within the REDWIN project, resulting in decreased uncertainty in the design. For the monopile design investigated here, that implies a less conservative and thus more cost-effective offshore wind design. The authors would like to thank the Norwegian Geotechnical Institute for their work in the REDWIN project to develop the capability being incorporated in OC6 Phase II and to provide the data to model the foundation as well as for their ongoing support. We would also like to thank the Norwegian University of Science and Technology for their support in developing the model for this project. This work was authored in part by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the US Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding is provided by the US Department of Energy Office of Energy Efficiency and Renewable Energy Wind Energy Technologies Office. The views expressed in the article do not necessarily represent the views of the DOE or the US Government. The US Government retains, and the publisher, by accepting the article for publication, acknowledges that the US Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work or allow others to do so, for US Government purposes.

This paper defines a methodology to compare different offshore renewable energy (ORE) mooring configurations in terms of the risk of entanglement they present to marine megafauna. Currently, the entanglement of large marine animals is not explicitly considered in environmental impact studies. Recommendations need to be developed, assessing the risk of entanglement of ORE mooring configurations at the beginning of their design process. Physical parameters of the mooring system affecting the relative risk of entanglement have been identified as tension characteristics, swept volume ratio and mooring line curvature. These have been investigated further through six different mooring configurations: catenary with chains only, catenary with chains and nylon ropes, catenary with chains and polyester ropes, taut, catenary with accessory buoys, taut with accessory buoys. Results indicate that the taut configuration has the lowest relative risk of entanglement, while the highest relative risk occurs with catenary moorings with chains and nylon ropes or with catenary moorings with accessory buoys. However, the absolute risk of entanglement is found to be low, regardless of the mooring configuration. This methodology can also be applied to other mooring configurations, arrays or power cables.

The vision of large-scale commercial arrays of floating marine energy converters (MECs) necessitates the robust, yet cost-effective engineering of devices. Given the continuous environmental loading, fatigue has been identified as one of the key engineering challenges. In particular the mooring system which warrants the station-keeping of such devices is subject to highly cyclic, non-linear load conditions, mainly induced by the incident waves. To ensure the integrity of the mooring system the lifecycle fatigue spectrum must be predicted in order to compare the expected fatigue damage against the design limits. The fatigue design of components is commonly assessed through numerical modelling of representative load cases. However, for new applications such as floating marine energy converters numerical models are often scantily validated. This paper describes an approach where load measurements from large-scale field trials at the South West Mooring Testing Facility (SWMTF) are used to calculate and predict the fatigue damage. The described procedure employs a Rainflow cycle analysis in conjunction with the Palmgren–Miner rule to estimate the accumulated damage for the deployment periods and individual sea states. This approach allows an accurate fatigue assessment and prediction of mooring lines at a design stage, where field trial load measurements and wave climate information of potential installation sites are available. The mooring design can thus be optimised regarding its fatigue life and costly safety factors can be reduced. The proposed method also assists in monitoring and assessing the fatigue life during deployment periods.

Mooring systems are among the most critical sub-systems for floating marine energy converters (MEC). In particular, the occurrence of peak mooring loads on MEC mooring systems must be carefully evaluated in order to ensure a robust and efficient mooring design. This understanding can be gained through long-term field test measurement campaigns, providing mooring and environmental data for a wide range of conditions. This paper draws on mooring tensions and environmental conditions that have been recorded (1) for several months during the demonstration of an MEC device and (2) over a period of 18 months at a mooring test facility. Both systems were installed in a shallow water depth (45 m and 30 m, respectively) using compliant multi-leg catenary mooring systems. A methodology has been developed to detect peak mooring loads and to relate them to the associated sea states for further investigation. Results indicate that peak mooring loads did not occur for the sea states on the external contour line of the measured sea states, but for the sea states inside the scatter diagram. This result is attributed to the short-term variability associated with the maximum mooring load for the given sea state parameters. During the identified sea states, MEC devices may not be in survival mode, and thus, the power take-off (PTO) and ancillary systems may be prone to damage. In addition, repeated high peak loads will significantly contribute to mooring line fatigue. Consequently, considering sea states inside the scatter diagram during the MEC mooring design potentially yields a more cost-effective mooring system. As such, the presented methodology contributes to the continuous development of specific MEC mooring systems.

The design of wave energy mooring systems is challenging: overdesign incurs a significant cost penalty, underdesign may lead to a premature failure and incorrect design could reduce the power production. Consequently, compliant mooring systems are being developed for wave energy applications. This paper presents tank test results for a scale model of the buoy and mooring used at the South West Mooring Test Facility (SWMTF), an offshore facility developed to conduct long-term sea trials for wave energy device moorings. A compliant three leg catenary mooring system using Nylon ropes in the water column is investigated. Preliminary static, quasi-static, decay, regular and irregular wave tests were conducted on the 1:5 scale model, using the Ifremer basin in Brest. A corresponding numerical model was developed with a time-domain mooring modelling tool, inputting hydrodynamic data from a radiation/diffraction potential modelling program. After the calibration of several hydrodynamic parameters, the numerical model demonstrated good agreement with the experiment. However, numerical results show large differences with the field test results, mainly because of unknowns in the anchor position. The methods and procedures presented will allow the effective validation of numerical models to enable the development of appropriate mooring systems in wave energy applications.