marinet-tecnalia-infrastructure access report...this report discusses the experimental campaign...

Infrastructure Access Report

Infrastructure: HMRC

User-Project: NAUTILUS

Performance of Semi-Submersible Offshore Wind Platform

Goren Aguirre (Tecnalia R&I)

Marine Renewables Infrastructure Network

Status: Final Version: 02 Date: 07/04/2014

EC FP7 “Capacities” Specific Programme

Research Infrastructure Action

Infrastructure Access Report: [Insert the User-Project acronym]

Rev. [Version Number, e.g. 01], [Pick the version date] of 23

ABOUT MARINET MARINET (Marine Renewables Infrastructure Network for emerging Energy Technologies) is an EC-funded network of research centres and organisations that are working together to accelerate the development of marine renewable energy - wave, tidal & offshore-wind. The initiative is funded through the EC's Seventh Framework Programme (FP7) and runs for four years until 2015. The network of 29 partners with 42 specialist marine research facilities is spread across 11 EU countries and 1 International Cooperation Partner Country (Brazil). MARINET offers periods of free-of-charge access to test facilities at a range of world-class research centres. Companies and research groups can avail of this Transnational Access (TA) to test devices at any scale in areas such as wave energy, tidal energy, offshore-wind energy and environmental data or to conduct tests on cross-cutting areas such as power take-off systems, grid integration, materials or moorings. In total, over 700 weeks of access is available to an estimated 300 projects and 800 external users, with at least four calls for access applications over the 4-year initiative. MARINET partners are also working to implement common standards for testing in order to streamline the development process, conducting research to improve testing capabilities across the network, providing training at various facilities in the network in order to enhance personnel expertise and organising industry networking events in order to facilitate partnerships and knowledge exchange. The aim of the initiative is to streamline the capabilities of test infrastructures in order to enhance their impact and accelerate the commercialisation of marine renewable energy. See www.fp7-marinet.eu for more details.

Partners

Ireland

University College Cork, HMRC (UCC_HMRC) Coordinator

Sustainable Energy Authority of Ireland (SEAI_OEDU)

Denmark

Aalborg Universitet (AAU)

Danmarks Tekniske Universitet (RISOE)

France

Ecole Centrale de Nantes (ECN)

Institut Français de Recherche Pour l'Exploitation de la Mer (IFREMER)

United Kingdom

National Renewable Energy Centre Ltd. (NAREC)

The University of Exeter (UNEXE)

European Marine Energy Centre Ltd. (EMEC)

University of Strathclyde (UNI_STRATH)

The University of Edinburgh (UEDIN)

Queen’s University Belfast (QUB)

Plymouth University(PU)

Spain

Ente Vasco de la Energía (EVE)

Tecnalia Research & Innovation Foundation (TECNALIA)

Belgium

1-Tech (1_TECH)

Netherlands

Stichting Tidal Testing Centre (TTC)

Stichting Energieonderzoek Centrum Nederland (ECNeth)

Germany

Fraunhofer-Gesellschaft Zur Foerderung Der Angewandten Forschung E.V (Fh_IWES)

Gottfried Wilhelm Leibniz Universität Hannover (LUH)

Universitaet Stuttgart (USTUTT)

Portugal

Wave Energy Centre – Centro de Energia das Ondas (WavEC)

Italy

Università degli Studi di Firenze (UNIFI-CRIACIV)

Università degli Studi di Firenze (UNIFI-PIN)

Università degli Studi della Tuscia (UNI_TUS)

Consiglio Nazionale delle Ricerche (CNR-INSEAN)

Brazil

Instituto de Pesquisas Tecnológicas do Estado de São Paulo S.A. (IPT)

Norway

Sintef Energi AS (SINTEF)

Norges Teknisk-Naturvitenskapelige Universitet (NTNU)

Infrastructure Access Report: N

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DOCUMENT INFORMATION Title Performance of Semi-Submersible Offshore Wind Platform

Distribution Public

Document Reference MARINET-TA1-NAUTILUS

User-Group Leader, Lead

Author

Goren Aguirre Tecnalia Parque Tecnológico de Bizkaia C/ Geldo, edificio 700 E-48160 Derio Spain

User-Group Members,

Contributing Authors

Infrastructure Accessed: HMRC

Infrastructure Manager

(or Main Contact)

Florent Thiebaut

REVISION HISTORY Rev. Date Description Prepared by

(Name)

Approved By

Infrastructure

Manager

Status

(Draft/Final)

01 25/03/14 Goren Aguirre Florent Thiebaut Draft

02 07/04/14 Goren Aguirre Florent Thiebaut Final


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ABOUT THIS REPORT One of the requirements of the EC in enabling a user group to benefit from free-of-charge access to an infrastructure is that the user group must be entitled to disseminate the foreground (information and results) that they have generated under the project in order to progress the state-of-the-art of the sector. Notwithstanding this, the EC also state that dissemination activities shall be compatible with the protection of intellectual property rights, confidentiality obligations and the legitimate interests of the owner(s) of the foreground. The aim of this report is therefore to meet the first requirement of publicly disseminating the knowledge generated through this MARINET infrastructure access project in an accessible format in order to:

• progress the state-of-the-art

• publicise resulting progress made for the technology/industry

• provide evidence of progress made along the Structured Development Plan

• provide due diligence material for potential future investment and financing

• share lessons learned

• avoid potential future replication by others

• provide opportunities for future collaboration

• etc. In some cases, the user group may wish to protect some of this information which they deem commercially sensitive, and so may choose to present results in a normalised (non-dimensional) format or withhold certain design data – this is acceptable and allowed for in the second requirement outlined above.

ACKNOWLEDGEMENT The work described in this publication has received support from MARINET, a European Community - Research Infrastructure Action under the FP7 “Capacities” Specific Programme.

LEGAL DISCLAIMER The views expressed, and responsibility for the content of this publication, lie solely with the authors. The European Commission is not liable for any use that may be made of the information contained herein. This work may rely on data from sources external to the MARINET project Consortium. Members of the Consortium do not accept liability for loss or damage suffered by any third party as a result of errors or inaccuracies in such data. The information in this document is provided “as is” and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and neither the European Commission nor any member of the MARINET Consortium is liable for any use that may be made of the information.


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EXECUTIVE SUMMARY In recent years, some of the most important utilities have undertaken research projects aimed at producing electricity with Floating Offshore Wind Turbines. The four floater semi-submersible platform concept has been the one chosen by NAUTILUS for this initiative. The constitution of NAUTILUS FLOATING SOLUTIONS SL, an industrial and technological consortium made up of ASTILLEROS DE MURUETA, TAMOIN, VELATIA and VICINAY MARINE INNOVACIÓN, four leading companies in advanced technology with presence in international markets and the applied Research Centre, TECNALIA, has been signed, with the aim of becoming leaders in the development of floating platforms for the offshore wind. This report discusses the experimental campaign conducted by Tecnalia research group as request of NAUTILUS within the MARINET project framework. The purpose of the campaign has been the hydrodynamic performance of a semi-submersible platform to support a wind energy turbine. The turbine chosen in this study has been the 5MW reference WT of NREL, the location depth has been 60m and the operation area is the Basque coast. Experimental setup and results from decay tests, regular wave motion RAOs, irregular wave responses, fairlead mooring loads and accelerations are presented. Experimental results are useful to be compared with available numerical simulations and other numerical and experimental results found in literature.


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CONTENTS

1 INTRODUCTION & BACKGROUND ...................................................................................................................7

1.1 INTRODUCTION .................................................................................................................................................... 7 1.2 DEVELOPMENT SO FAR .......................................................................................................................................... 7 1.2.1 Stage Gate Progress .................................................................................................................................... 7 1.2.2 Plan For This Access ..................................................................................................................................... 9

2 OUTLINE OF WORK CARRIED OUT ................................................................................................................. 11

2.1 SETUP ............................................................................................................................................................... 11 2.2 TESTS ............................................................................................................................................................... 12 2.2.1 Test Plan .................................................................................................................................................... 12 2.2.2 Results ....................................................................................................................................................... 13 2.2.3 Dynamic behaviour .................................................................................................................................... 15

2.3 ANALYSIS & CONCLUSIONS................................................................................................................................... 18

3 MAIN LEARNING OUTCOMES ....................................................................................................................... 19

3.1 PROGRESS MADE ............................................................................................................................................... 19 3.1.1 Progress Made: For This User-Group or Technology ................................................................................. 19 3.1.2 Progress Made: For Marine Renewable Energy Industry .......................................................................... 19

3.2 KEY LESSONS LEARNED ........................................................................................................................................ 19

4 FURTHER INFORMATION .............................................................................................................................. 20

4.1 SCIENTIFIC PUBLICATIONS .................................................................................................................................... 20 4.2 WEBSITE & SOCIAL MEDIA ................................................................................................................................... 20

5 REFERENCES ................................................................................................................................................ 20

6 APPENDICES ................................................................................................................................................ 20

6.1 STAGE DEVELOPMENT SUMMARY TABLE ................................................................................................................ 20 6.2 ANY OTHER APPENDICES ..................................................................................................................................... 22 6.2.1 Tank test planning ..................................................................................................................................... 22


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1 INTRODUCTION & BACKGROUND

1.1 INTRODUCTION The constitution of NAUTILUS FLOATING SOLUTIONS SL, an industrial and technological consortium made up of ASTILLEROS DE MURUETA, TAMOIN, VELATIA and VICINAY MARINE INNOVACIÓN, four leading companies in advanced technology with presence in international markets and the applied Research Centre, TECNALIA, has been signed, with the aim of becoming the world leader in the development of floating platforms for the off-shore wind power market. Thus, the objective is to build and install a floating 5MW turbine, technically an economically feasible. To achieve this goal the structure must be easy building, wind turbine assembly onshore, make easier maintenance procedures and dynamically stable to improve power production. After benchmarking different options, a four floater foundations was selected. Its technical and economic feasibility process is summarized in this report.

1.2 DEVELOPMENT SO FAR After analysing the state of art and carried out different brain storming sessions and technical meetings, an initial design was defined. Based on geometrical parameters and location features, first predictions of semi-sub behaviour can be obtained through simple spreadsheet computations, following engineering assumptions and physical principles. Environmental loads on the structure, mooring main features, overturning moment, heeling angle, natural periods and capital expenditures can be computed in order to determine main dimensions. These spreadsheet calculations were used to identify candidate designs that meet the design criteria. Once our candidate complies with functional requirements and is validated for 6 DLC, it goes a step forward to conceptual design.

Fig 1 Design process work flow

Nowadays, mooring, hydrodynamics and aerodynamic are being checked. Mooring is designed by quasi-static methodology for 100 years return period. Hydrodynamic features are set by diffraction/radiation software and aerodynamic behaviour is checked with FAST. Furthermore stability and global strength have to be analysed. Stability is checked according to DNV standards and global strength is assessed by FEM modelling the structure as beams. The final design will be validated by coupling codes, analysing a wide range of design load cases. At the beginning of 2014 basic tank test will be carried out and main test campaign will be done mid-2014 checking wave, current and wind effects.

1.2.1 Stage Gate Progress

Previously completed: � Planned for this project: �


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STAGE GATE CRITERIA Status

Stage 1 – Concept Validation

• Linear monochromatic waves to validate or calibrate numerical models of the system (25 – 100 waves) �

• Finite monochromatic waves to include higher order effects (25 –100 waves) �

• Hull(s) sea worthiness in real seas (scaled duration at 3 hours) �

• Restricted degrees of freedom (DofF) if required by the early mathematical models �

• Provide the empirical hydrodynamic co-efficient associated with the device (for mathematical modelling tuning)

�

• Investigate physical process governing device response. May not be well defined theoretically or numerically solvable

�

• Real seaway productivity (scaled duration at 20-30 minutes)

• Initially 2-D (flume) test programme

• Short crested seas need only be run at this early stage if the devices anticipated performance would be significantly affected by them

�

• Evidence of the device seaworthiness �

• Initial indication of the full system load regimes

Stage 2 – Design Validation

• Accurately simulated PTO characteristics

• Performance in real seaways (long and short crested)

• Survival loading and extreme motion behaviour.

• Active damping control (may be deferred to Stage 3)

• Device design changes and modifications

• Mooring arrangements and effects on motion �

• Data for proposed PTO design and bench testing (Stage 3)

• Engineering Design (Prototype), feasibility and costing

• Site Review for Stage 3 and Stage 4 deployments

• Over topping rates

Stage 3 – Sub-Systems Validation

• To investigate physical properties not well scaled & validate performance figures

• To employ a realistic/actual PTO and generating system & develop control strategies

• To qualify environmental factors (i.e. the device on the environment and vice versa) e.g. marine growth, corrosion, windage and current drag

• To validate electrical supply quality and power electronic requirements.

• To quantify survival conditions, mooring behaviour and hull seaworthiness

• Manufacturing, deployment, recovery and O&M (component reliability)

• Project planning and management, including licensing, certification, insurance etc.

Stage 4 – Solo Device Validation

• Hull seaworthiness and survival strategies

• Mooring and cable connection issues, including failure modes

• PTO performance and reliability

• Component and assembly longevity

• Electricity supply quality (absorbed/pneumatic power-converted/electrical power)

• Application in local wave climate conditions

• Project management, manufacturing, deployment, recovery, etc


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STAGE GATE CRITERIA Status

• Service, maintenance and operational experience [O&M]

• Accepted EIA

Stage 5 – Multi-Device Demonstration

• Economic Feasibility/Profitability

• Multiple units performance

• Device array interactions

• Power supply interaction & quality

• Environmental impact issues

• Full technical and economic due diligence

• Compliance of all operations with existing legal requirements

1.2.2 Plan For This Access

A model of the NAUTILUS semi-submersible platform equipped with the NREL 5MW wind turbine has been tested in the hydrodynamic and ocean tank of Hydraulic and Maritime Research Center in Cork under wind and wave loads. This report aims at presenting the results obtained with numerical simulations with these experimental results. The numerical model is based on the FAST design code from NREL and ORCAFLEX for calculating hydrodynamic and mooring loads. This hydrodynamic model includes non linear hydrostatic and Froude-Krylov forces, diffraction/radiation forces obtained from linear potential theory and Morison forces to take into account viscous effects on the heave plates. Furthermore, the focus of this report is to summarize the work done by consortium members in analyzing the data obtained from the test campaign and its use for validating the offshore wind modelling tool, FAST. The floater was a 1/60th-scale model fitted with NREL 5-MW Reference Wind Turbine. Semi-submersible platforms for floating wind turbines are challenging for numerical tools. These floaters are generally made of an assembly of columns, braces and pontoons. Heave plates may also be placed at the bottom of the columns. Hydrodynamic loads have to be correctly modelled on these elements of various dimensions. The numerical model has been calibrated against the results of free decay tests. Comparison of floating wind turbine platform motions are realised for wave only load cases and for wind and wave load cases.

1.2.2.1 Wave tank facilities

The Ocean Wave Basin at the HMRC is 25m long, 18m wide and 1m deep. The wave generation system incorporates 40 flap type wedge shaped aluminium paddles attached to the 18m side of the tank. Each of these paddles has a hinge depth of 0.75m and is fitted with an active absorption system. The paddles are dry backed and sealed with a rolling fabric gusset seal, which allows each paddle to move independently. An electric servo-motor drives each paddle via a belt. The wave generating system is capable of directional irregular seas up to a significant wave height of 0.16m and period of 2.5s. Active absorption is incorporated into the paddle system, this uses a feedback signal to adjust paddle motion. To further minimise the amount of reflected waves in the tank an artificial Enkamat absorption beach is located at the opposite end to the paddles. Monochromatic, panchromatic and recorded time series can be applied to the device undergoing testing. This allows for the fundamental characteristics of the device to be obtained as well as modelling the device in real sea conditions.

1.2.2.2 Scaling of the model

For these model tests, the floating NAUTILUS semi-submersible has been designed. It is a column stabilized platform composed of 4 columns of 9.5 m diameter separated by 33 m. These 4 columns are connected with a rigid ring pontoon at its bottom and a X braces which also support the wind turbine. Heave plates are placed at the bottom of each column in order to reduce amplitude of motions. The wind turbine mounted on this platform is the reference 5 MW turbine from NREL. This wind turbine is designed to have a hub height of 90 m. The tower of the turbine has


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been adapted in order to fit this value and to connect with the NAUTILUS platform. The length of the new tower is 77.6 m; the base diameter is 6 m. The model tests have been performed at scale 1:60th. A major challenge in the scale-model testing of an offshore wind turbine that is excited by waves is deciding on the appropriate scaling approach for the experiment. For wind turbine testing, a scaling approach based on preserving the Reynolds number is typically used as this preserves the relationship of viscous and inertial forces for fluid flow. For offshore structural testing, however, a scaling approach based on preserving Froude number is more typical as this preserves the relationship between the gravitational and inertial forces of the waves. For the Nautilus tests, a Froude-based scaling approach was used both to create the scaled geometry of the structure and to scale the environmental conditions of the tests. The scale factor was defined taking into account next constrains:

• Model displacements, inertia and centre of gravity

• Wave generating system capacity of directional irregular seas up to a significant wave height of 0.16 m. • Wave tank deep up to 1 m. • Mooring footprint was limited by wave tank breadth up to 18 m

Fig 2 Model construction


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2 OUTLINE OF WORK CARRIED OUT

2.1 SETUP Once scale factor is defined, the model construction was requested to HMRC. It was dome at HMRC workshop taking care of achieving as much as possible model weight, center of gravity and gyration radius. Main properties of the system are given in table 1.

Full scale 1:60th

model scale

Mass 6961 (t) 33.10 (kg)

Draft 19 (m) 317 (mm)

VCG 12.32 (m) 205 (mm)

Rx Ry 29.56 (m) 493 (mm)

Rz 23.34 (m) 0.389 (mm)

Wind thrust 850 (kN) 3.94 (N)

Nacelle height above MSL 89 (m) 1486 (mm)

Mooring linear weigth 229 (kg/m) 63 (gr/m)

Table 1 Main features of the prototype and model

The methodology set up for this study was based on Froude similarity’s law. To properly simulate the behaviour of the system, lengths, weights, principal moment of inertia and position of the gravity centre of each subpart (floater, tower, nacelle, rotor) have been respected. Rotor thrust has been simulated/measured by a constant force applied in the nacelle. HMRC has an air fan which allows simulating variable loads but its weight and electrical cables are too heavy. So it was decided to simulate wind effects with a constant force so that the tests were as accurate as possible. Some designing parameters such as tension in the mooring lines (load cells) have also been measured as well as environmental conditions (wave characteristics). Motion capture has been used to obtain the 6 degrees of freedom of the model (figure 2). Sensors cables have been brought under water to terminal acquisition.

Fig 3 model set up and instrumentation

Due to basin dimension limits forward mooring lines were cut up to cover maximum footprint (see figure 4) and in order to be able to test 45 degree wave heading, the system was rotated so that the wave front impacted correctly on the floater.


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Fig 4 Tank test general arrangement

2.2 TESTS

2.2.1 Test Plan

A large number of tests were performed at the HMRC wave basin to characterize the behavior of the systems in a variety of conditions. The tests performed include:

2.2.1.1 Static Draft, Trim and Heel.

Purpose of Test: Record draft, trim and heel.

2.2.1.2 Inclining test.

Purpose of Test: To determine the metacentric height (GM) of the model.

2.2.1.3 Static offset and free-decay tests

Purpose of Test: To determine the natural periods and damping coefficients of the moored model in free oscillatory modes in six DOF including surge, sway, heave, roll, yaw and pitch.

2.2.1.4 Wave-only tests using regular waves

Purpose of Tests: To establish transfer functions for all measured responses in regular monochromatic wave conditions. To observe any non-linearity in the response transfer function by varying wave height at a few selected wave periods. To define steady state drift force for each regular wave of given period and height.


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2.2.1.5 Wave-only tests irregular waves (operational, survival)

Purpose of Tests: To establish the behaviour of the complete moored vessel in an irregular sea state with and without the influence of wind. Generally, the sea states experienced at the offshore field are simulated in these tests to study the operational and survival characteristics of the system.

The sampling frequency for most tests was 32 Hz at model scale, corresponding to a Froude-scaled sampling frequency at full scale of roughly 4 Hz. All data from the HMRC tests were converted to full scale using Froude scaling. The final amount of test was much higher than expected for 5 days test campaign. Full test plan can be seen at the appendix

2.2.2 Results

2.2.2.1 Free decay

The objective of the decay tests is to determine the damping coefficient and the natural periods in every degree of freedom. The focus will be on decay test results for surge, heave and pitch motions. Natural periods are presented in Table 2. It can be observed that all periods fall outside first order wave excitation.

(a) Decay test in surge direction

(b) Decay test in heave direction

(c) Decay test in pitch direction

Fig 5 Experimental results with free decay test (results full scale)


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Experimental (s)

Surge 97.7

Sway 14.6

Heave 20.7

Roll 26.3

Pitch 25.3

Yaw 102.2

Table 2 Natural periods

2.2.2.2 Response Amplitude Operators (RAOs)

Regular wave tests have been performed in order to measure motion and mooring tension. Results for three headings (0, 22.5 and 45 deg) are briefly discussed next. The reasons for conducting 22.5 and 45 deg heading tests were: 1. To study response signals for seas with non-zero incident angle. This will be interesting to study the behavior but also seas to 0 deg. 2. To excite non fore-back motions (sway and roll) more significantly. In the 0 deg test, these sway and roll motions are very low. For the correlation analysis, it will be easier to have a more significant signal.

(a) Surge RAO

(b) Heave RAO


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(c) Pitch RAO

Fig 6 Model test RAOs results for different wave direction with and without constant wind.

2.2.3 Dynamic behaviour

A range of combined sea states, headings and wind speeds have been considered. This report is presenting result for an operational and survival sea state without wind. The wave conditions analyzed in this paper are defined in Table X. All cases were run with no wind present and the direction of wave propagation was varying from 0 to 45 degrees. This excited the platform in the surge (X), heave (Z), and pitch (θ). For brevity, only surge, heave, pitch and mooring results are shown for two most representative cases.

Hs (m) Tp (s) γ Heading (º)

1.88 9.15 1.69 0; 22.5; 45

3 10.40 1.69 0; 22.5; 45

5 11.92 1.69 0; 22.5; 45

7 13.06 1.69 0; 22.5; 45

8 13.56 1.69 0; 22.5; 45

9 13.97 1.69 0; 22.5; 45

10 14.39 1.69 0; 22.5; 45

Table 3 Sea states


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2.2.3.1 Operational Case (Hs=1.88 m, Tp=9.15 s)

(a)

(b)

(c)

(d)

(e)

Fig 7 Full scale results for wave amplitude (a), fairlead tension (b), surge and heave (c) and pitch (d) time series for operational

case


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2.2.3.2 Survival case (Hs= 10 m, Tp=14.39 s)

(a)

(b)

(c)

(d)

(e)

Fig 8 Full scale results for wave amplitude (a), fairlead tension (b), surge and heave (c) pitch (d) and acceleration (e) time

series for survival case


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2.3 ANALYSIS & CONCLUSIONS

The two main purposes for the NAUTILUS testing campaign were to better understand the behavior of floating offshore wind systems and to obtain experimental data to be used for validating offshore wind system modeling tools. The tests were essential in meeting these goals, and were profitable in regard to producing data that will be analysed deeply. This section summarizes main conclusions from the experimental tests in regard to the scaled-model testing of these systems.

1. The natural periods and damping values are similar to those reported in literature. 2. All motion RAOs are smaller than expected, heave mainly. 3. Surge motion response in double frequency has been described, due to the coupling of surge and heave

motions. 4. Operational conditions motions and accelerations RMS values fall within turbine manufactures

operational limits. 5. Maximum accelerations values in nacelle are below admissible figures. 6. Fairlead tensions are below 50% of maximum breaking load.


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3 MAIN LEARNING OUTCOMES

3.1 PROGRESS MADE

Comparisons between model test and simulation results have been realised for a floating wind turbine system under wind and wave loading. The numerical model has been calibrated against the results of free decay tests. The motions of the floating wind turbine platform have been compared for regular waves loading only and for regular waves and constant wind loading. Cases of wave loading only have shown a good agreement for surge, heave and pitch motions. For the cases of wave and wind loading, a good agreement has been observed for the steady state surge and pitch offset. Surge and heave motions have also shown a good agreement. These degrees of freedom are not strongly influenced by wind loading. For pitch and heave motion, numerical simulations and tank test have shown differences.

3.1.1 Progress Made: For This User-Group or Technology

First experimental campaign with a semi-submersible platform for an offshore wind turbine has been carried out by NAUTILUS with HMRC wave basin research group assistance. The experimental setup, results from decay tests, regular wave motion RAOs, irregular wave responses, tendon loads and accelerations, have been tested and analyzed.

3.1.1.1 Next Steps for Research or Staged Development Plan – Exit/Change & Retest/Proceed?

Some future research lines are: 1. Wind loads have been introduced with a constant weight. It’s not a correct approximation to real wind loads, More accurate tests in regards to wind simulation should be done. 2. Second order effects have not been found significant in present campaign but further study is necessary. 3. Some issues in regards to lines tension spectra in survival conditions have arisen, whose analysis is left for future research. 4. Work is presently ongoing to improve FAST and ORCAFLEX’s modeling approach to overcome the limitations identified. When this work is complete, efforts will be redone to compare the NAUTILUS data with the new and improved FAST simulation tool.

3.1.2 Progress Made: For Marine Renewable Energy Industry

Most technology advanced semi-submersible platforms for offshore wind turbine are tri floater ones. Nautilus is a narrower and four-floater semi-submersible. This test campaign has validated stability and dynamic behaviour for a 5 MW wind turbine. Nautilus design allows building in a standard shipyard without the need of extraordinary facilities and providing the opportunity to easy mass production.

3.2 KEY LESSONS LEARNED

While a Froude-based scaling approach is, for hydrodynamic, a comprehensive methodology, the approach must be modified in regard to the rotor due to the aerodynamic loading being so dependent on Reynolds number.

The NAUTILUS tests highlighted the importance of having instrumentation that is light-weight and does not alter the dynamic behavior of the offshore wind system. At 1/60th scale, the systems are so small that the weight of the air fan and the cabling becomes significant. In addition, mooring line linear weight becomes very low and it is difficult to find a chain which fits with scaled linear weight. At 1/60th scale, mooring line load is so low that it is difficult to measure it due to load cell range.

For all these reasons, key lesson learned is to carry out tank test with a lower scale factor, and suitable scale factor might be 1 to 30th.


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4 FURTHER INFORMATION

4.1 SCIENTIFIC PUBLICATIONS List of any scientific publications made (already or planned) as a result of this work:

• Conference/journal paper planned.

4.2 WEBSITE & SOCIAL MEDIA

5 REFERENCES 1. Dominique Roddier, Christian Cermelli, Alexia Aubault. WindFloat: A floating foundation for offshore wind turbines. Journal of Renewable and Sustainable Energy. 2010 2. J. Jonkman, S. Butterfield. Definition of a 5 MW Reference Wind Turbine for Offshore System Development. National Renewable Energy Laboratory 3. Handbook of Offshore Engineering. Subrata Chakrabarti. Elsevier. 4. Amy N. Robertson, Jason M. Jonkman, Andrew J. Goupee. Summary of Conclusions and Recommendations drawn from the deepcwind scaled Floating Offshore Wind System Test Campaig. ASME 2013 32nd International Conference on Ocean, Offshore and Arctic Engineering OMAE2013 5. M. Philippe, A. Courbois, A. Babarit. Comparison of Simulation and Tank Test Results of a Semi-submersible Floating Wind Turbine under Wind and Wave Loads. ASME 2013 32nd International Conference on Ocean, Offshore and Arctic Engineering OMAE2013

6 APPENDICES

6.1 STAGE DEVELOPMENT SUMMARY TABLE The table following offers an overview of the test programmes recommended by IEA-OES for each Technology Readiness Level. This is only offered as a guide and is in no way extensive of the full test programme that should be committed to at each TRL.


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6.2 ANY OTHER APPENDICES

6.2.1 Tank test planning


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