Experimental study of the functionality of a semisubmersible wind turbine combined with flap-type Wave Energy Converters

Significance Statement

Offshore renewable energy systems, namely Floating Wind Turbines (FWTs) and Wave Energy Converters (WECs), are expected to contribute significantly to reach the sustainable energy targets in the world in an efficient manner.

It might be beneficial to combine offshore renewable energy systems of different technologies into one facility. This in turn implies new design, research and technological challenges. Novel and reliable combined energy systems should be developed, satisfying functionality (high energy effectiveness), survivability and endurance requirements, but at the same time at low costs. High fidelity computational tools for the integrated dynamic analysis of these solutions should be developed to assess the concepts, while appropriate laboratory experiments for demonstrating the potential and validating the developed numerical models should be conducted.

In the EU project MARINA Platform three combined concepts have been selected and studied both numerically and experimentally under operational and survival conditions. One of the selected concepts is the Semisubmersible wind energy and Flap-type wave energy Converter (SFC) developed at the Centre for Ships and Offshore Structures (CeSOS). The SFC concept  consists of a FWT of a braceless semisubmersible platform type, a 5 MW wind turbine placed on the central column of the platform, three fully submerged rotating flap-type WECs hinged at the pontoons of the semisubmersible and three catenary mooring lines for station keeping.

The functionality and survivability of the SFC concept has been examined experimentally in operational and extreme environmental conditions and the measured responses are compared with predictions obtained by a numerical analysis model. The tests were performed in the Hydrodynamic and Ocean Engineering Tank in Ecole Centrale de Nantes, France.

The laboratory model of the SFC was built in an 1:50 scale. The Power Take-Off system of each of the WECs was modelled using a linear mechanical rotary damper that provides a constant damping force. The wind turbine rotor is modelled with a redesigned small-scale rotor that was driven by a motor to rotate during the experiments and produces an equivalent thrust force in model scale.

Different test conditions have been considered in order to study the responses of the SFC in operational and extreme conditions. Quasi-static, motion decay, regular and irregular waves without and with aligned wind excitation tests have been conducted.

The responses measured during the experiments were compared with numerical predictions obtained by a fully coupled multibody numerical analysis model (Simo-Riflex-Aerodyn). The responses considered are the motions of the semisubmersible platform, produced power by one flap-type WEC, tension of mooring lines, internal loads of the arms that connect the rotating flap with the pontoon of the semisubmersible platform, acceleration of the nacelle and bending moment in wind turbines tower base. A very good agreement between experimental and numerical results is observed for the motions of the semisubmersible platform, rotation of WECs and internal loads of different parts of SFC (e.g. mooring lines, arms of WEC, tower of wind turbine) highlighting the accuracy of the numerical methods that were used. The combined operation of the WECs does not affect the acceleration of nacelle, the tower base bending moment and the tension of the mooring lines, while insignificantly affects the motions of the platform.

The validated results that are obtained confirm the good performance of the SFC concept in extreme environmental conditions and its survivability without the observation of strong nonlinear hydrodynamic phenomena. An indication about the relative contribution of the power from flap-type WECs for selected wind and wave environmental conditions is concluded. It is both experimentally and numerically justified that combining the flap-type WECs with the FWT has an insignificant effect on the wind power production but increases the total power production by 3~5% for the selected facility layout and environmental conditions. The functionality and survivability of the combined SFC concept was demonstrated. However, a further study to maximize the produced power by means of geometrical optimization of the flap-type WECs and an appropriate control scheme for the operation of the PTO configuration is required in the years to come.

The present studies serve towards demonstrating the feasibility of combined wind/wave energy systems and their future potential.

Journal References

Michailides C, Gao Z and Moan T. Experimental Study of the Functionality of a Semisubmersible Wind Turbine Combined with Flap-Type Wave Energy Converters, Renewable Energy, Volume 93, 2016, Pages 675-690.

Michailides C, Gao Z and Moan T. Experimental and numerical study of the response of the offshore combined wind/wave energy concept SFC in extreme environmental conditions, Marine Structures, Volume 50, 2016 Pages 35-54.

Centre for Ships and Ocean Structures (CeSOS), Centre for Autonomous Marine Operations and Systems (AMOS), Department of Marine Technology, Norwegian University of Science and Technology (NTNU), Otto Nielsens vei 10, NO-7491, Trondheim, Norway.

Figure Legend: Different views of the physical model of SFC during experiments.

functionality of a semisubmersible wind turbine combined with flap-type Wave Energy Converters

About the author

Constantine Michailides, senior lecturer, holds a BSc, MSc and PhD in Civil Engineering from Aristotle University of Thessaloniki, Greece. In the period of May 2013 – Dec. 2015, he was postdoctoral fellow in CeSOS and AMOS at NTNU mainly working on experimental and numerical investigation of the SFC concept as well as numerical analysis of floating submerged road tunnels. In January 2016 he joined Liverpool John Moores University, UK as a Senior Lecturer.

Constantine is performing research on numerical analysis, experimental testing and structural-field monitoring of offshore and coastal structures and systems. Constantine is a member of research/professional bodies in marine and offshore engineering technology (e.g. ISOPE, ISSC) and member of the Physical Sciences Research Council (EPSRC) Peer Review Associate College.


About the author

Dr. Zhen Gao is a professor of marine structures at the Department of Marine Technology, NTNU since 2015. His main research areas cover coupled dynamic analysis of offshore renewable energy devices (including offshore wind turbines, both bottom-fixed and floating, wave energy converters, floating tidal turbines and combined concepts); marine operations related to installation and maintenance for offshore renewable energy devices.

He is a member of the Specialist Committee V.4 Offshore Renewable Energy in the International Ship and Offshore Structures Congress (ISSC) for 2009-2012 (committee member) and 2012-2015, 2015-2018 (committee chair). He has participated and is now participating in several research projects and education programs on offshore renewable energy, including EU FP7 MARINA Platform Project (2010-2014) and EWEM (European Wind Energy Master) Program (2012- ).


About the author

Dr. Torgeir Moan has been Professor of Marine Technology at NTNU since 1977, has e.g. been Director of CeSOS in the period 2002-2013 and is currently senior adviser of AMOS. Professor Moan’s main disciplines are structural analysis and design and he has carried out research as well as engineering design and analyses of innovative concepts of high speed vessels, LNG and FPSO ships, oil and gas platforms, floating bridges as well as offshore wind turbines and wave energy converters.

He has delivered several keynote lectures at major conferences and received several international awards. He has been elected Fellow of several Scientific Academies and professional societies.


Journal Reference

Constantine Michailides, Zhen Gao , Torgeir Moan. Experimental study of the functionality of a semisubmersible wind turbine combined with flap-type Wave Energy ConvertersRenewable Energy, Volume 93, 2016, Pages 675–690.

Centre for Ships and Ocean Structures (CeSOS), Centre for Autonomous Marine Operations and Systems (AMOS), Department of Marine Technology, Norwegian University of Science and Technology (NTNU), Otto Nielsens vei 10, NO-7491, Trondheim, Norway.



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