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This work presents the results of a test campaign for a Floating Offshore Wind Turbine (FOWT) at a scale of 1:36. The floater is a Reduced-Draft-Spar platform for the DTU 10 MW reference wind turbine. A multifan system creating the equivalent aerodynamic thrust forces is used to replace the scaled physical turbine model, which is known as hybrid hardware-in-the-loop testing. The tests focus on different motion reduction techniques: an anti-heeling ballast and control system, a real-time observer, and a tuned liquid multi column damper, which are tested in a wave tank for the first time. Various tests are carried out to identify and calibrate the FOWT system. Results for regular waves, irregular waves and turbulent winds are presented in both time and frequency domain. Among the key results are the performance indicators of the motion reduction strategies. Several clear examples are provided along with a detailed analysis of the dynamic response for irregular wave conditions. The comparison of the numerically reproduced dynamic responses to the tests are presented as well, where a good agreement is found.

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.

ABSTARCT: This work analyses the accuracy of large-scale experimental testing procedure in ocean basin facility involving real-time hybrid model testing (ReaTHM) techniques. The analysis is based on a scaled concept for a 15MW floating offshore wind turbine (FOWT) supported by a concrete semi-submersible platform (ActiveFloat) developed within the framework of the project COREWIND. The real-time hybrid model considered includes a multi-fan system located at the aero-rotor interface, which permits to generate the aerodynamic loads, reducing the limitations typically given by scaled problems. In order to assess the uncertainties in the hardware in the loop (HIL) implementation, firstly we define the quantities of interest to be evaluated from all the possible sources liable to inaccuracy identified. Then, we quantify the systematic and random discrepancies of the selected mooring, platform and HIL parameters. Finally, we propagate the previously quantified errors, running simulations in OpenFAST under extremal and severe environmental load cases in Gran Canaria Island (Spain) site. Comparing the platform response and mooring tensions of these uncertainty propagations with the ones of the unperturbed simulation as a baseline case, we analyse the effect of each representative parameter. Thus, the reliability of the results in ocean basin testing is numerically assessed, depending on the design load case. Raúl Guanche also acknowledges financial support from the Ramon y Cajal Program (RYC-2017- 23260) of the Spanish Ministry of Science, Innovation and Universities.

With the aim of evaluating the effects of the selection of the wind turbulence model on the dynamic response of a semi-submersible floater supporting the DTU 10 MW RWT, we ran aero-hydro-servo-elastic simulations employing the OASIS code developed at IHCantabria. For each of the 4 sea states selected, apart from the reference case with only wave loads, we analysed 30 different realizations with coupled wind and wave loads for each of the 7 full-field turbulent winds generated by the turbulence simulator TurbSim: Kaimal and von Karman spectral models with three different turbulence classes each and the API turbulence model. From the resulting platform motions and mooring line dynamics, it can be concluded that the selection of the wind turbulence model is not trivial. At U 0 = 5.83 m / s , the obtained results show a high dependence on the turbulence class. However, as the wind speed increases and the wind turbine achieves the rated speed, the impact of the turbulence intensity is drastically reduced, and the design tension is dominated by the thrust force and thus by the vertical wind speed profile. The highest design tension of the windward mooring lines is obtained with the API model at U 0 = 8.75 m / s (at least 357 Tm).

During the last decades the offshore wind energy sector has experienced large developments. Despite bottom fixed wind turbines have been widely used, some their limitations have brought to scout and develop concepts based on floating support structures. The behavior of such structures is affected by forces of different nature, so the analysis of these structural systems becomes complex and requires an accurate definition of their dynamics. This is one of the reasons for which the numerical simulations can be highly improved from experimental tests at reduced scale. The interaction between hydrodynamic forces and the structure is investigated experimentally by means of wave tank tests. In these circumstances the correct representation of the aerodynamic forces is not trivial due to laboratory scale law conflicts. These issues can be eased by using hybrid systems. This work aims to describe a hybrid system developed by IH Cantabria. The system is meant to define the most significant aerodynamic loads affecting the dynamic performance of a floating wind turbine by using an aerodynamic model (BEM), while their generation in the scaled model is obtained by using a multi-fan system. This approach successfully satisfies issues related to the scalability of the aerodynamic forces and their variability due to the turbine controller and wind variability. Nevertheless, some shrewdness have to be taken in order to comply with the following matters. The correct representation of the dynamic effects relative to the aerodynamic forces requires high frequency calculations. For this reason some simplifications on the aerodynamic model must be taken. This work explains the criteria used to define the simplifications to be adopted, showing the low impact they have on the tests results. On the present paper it will be demonstrated the capabilities of the multi-fan that was chosen to reproduce the rotor aerodynamics. Moreover, it will evidence the high fidelity of the forces developed by the multi-fan, both in terms of amplitude and reactiveness on the forces fluctuations. The final section will prove the ability of the hybrid system to reproduce with high fidelity and large flexibility the aerodynamic load conditions desired in lab scale wave tank tests.

Mooring systems are under a cyclic loading process caused by the randomness of metocean conditions, which could lead to a fatigue failure of the station keeping system. The present paper presents an innovative methodology for the assessment of floating offshore wind turbine mooring system fatigue considering the full lifetime of the structure. The method integrates the impact of the life cycle metocean conditions over the dynamic performance of the platform thanks to coupled numerical models, selection and non-linear data interpolation techniques and commonly accepted fatigue approaches. One of the benefits of using this methodology is that there are no uncertainties due to the selection of a reduced set of sea states. The methodology is applied to a set of moorings with different properties in the DeepCwind platform to evaluate the solution which offers the best compromise between size and fatigue damage. Results show that the best long-term mooring behaviour is achieved with a weight of approximately 300 kg/m. A comparison is conducted between the fatigue damage obtained through the life-cycle method and conventional methods. The mean differences observed between the standard and the new method proposed are between 13% and 49% depending on the use of the S–N or T-N curves.