WaveNET Hydrodynamic Analysis - MaRINET2 · The WaveNET is a unique modular wave energy conversion system that utilises the relative motion between a series of wave interacting bodies

Infrastructure Access Reports

Infrastructure: IFREMER Deep Seawater Wave Tank

User-Project: WHA

WaveNET Hydrodynamic Analysis

AlbaTERN Ltd.

Marine Renewables Infrastructure Network

Status: Draft Version: 01 Date: 22-Nov-2012

EC FP7 “Capacities” Specific Programme Research Infrastructure Action

Infrastructure Access Report: WHA

Rev. 01, 22-Nov-2012 Page 2 of 29

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)

http://www.fp7-marinet.eu/


Rev. 01, 22-Nov-2012 Page 3 of 29

DOCUMENT INFORMATION Title WaveNET Hydrodynamic Analysis

Distribution Public

Document Reference MARINET-TA1-WHA

User-Group Leader, Lead Author

FINDLAY David AlbaTERN Ltd. [Optional: Insert address and contact details]

User-Group Members, Contributing Authors

FINDLAY David AlbaTERN Ltd. FLECK Hector AlbaTERN Ltd. MAVEL Vivien AlbaTERN Ltd.

Infrastructure Accessed: IFREMER Deep Seawater Wave Tank

Infrastructure Manager (or Main Contact)

Christophe MAISONDIEU

REVISION HISTORY Rev. Date Description Prepared by

(Name) Approved By Infrastructure

Manager

Status (Draft/Final)

01 03/12/12 First Draft Vivien Mavel Draft


Rev. 01, 22-Nov-2012 Page 4 of 29

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 AlbaTERN (ABT) completed a series of tests over a two week period on the WaveNET wave energy system at the IFREMER (IFM) wave basin in Brest, France during October 2012. The WaveNET is a unique modular wave energy conversion system that utilises the relative motion between a series of wave interacting bodies arranged in an articulated hexagonal array. The WaveNET is a scalable technology currently being developed around a 7.5kW conversion module although future designs are anticipated in the 75-100kW range. This approach allows an arbitrary number of modules to be integrated into an extensible hexagonal network whereby the capacity of a particular deployment can be tailored to suit both the power demand and available wave resource. In these tests, a series of possible configurations were investigated including an isolated module (SQUID), a 1Hex array and a 3 Hex array. A scale of 1:10 of the 7.5kW module size (~1:20 of the 75kW size) was used for the model resulting in a maximum deployed extent of 4.8m for the 3Hex array. Tests were performed across a range of both regular and irregular sea states. The WaveNET can be considered as an array absorber. The 3Hex configuration has 16 distinct, yet mechanically coupled surface-piercing bodies while power is captured across 36 power take-off articulations. This approach – if correctly configured can lead to a system with exceptionally high levels of built-in redundancy while the smaller module size offers dramatic reductions in the associated OPEX costs of full scale deployments. These tests were designed to provide insight into the implications of coupling wave interacting bodies in this fashion as well as validating the overall performance of the design concept. A sophisticated multi-body motion tracking system provided by IFREMER allowed the dynamic response of the system - and all constituent bodies - to be monitored and recorded while additional instrumentation was used to monitor the applied power take off torque, surface elevation and mooring loads. The system was found to respond well (to or above expectations) in a wide range of sea states. The tests provided confidence in the dynamic response of the system prior to a full scale deployment scheduled for 2013 with no major structural modifications required. An extensive dataset detailing the performance of the 3 configurations across different sea states was gained and this continues to provide interesting insight into the coupled performance of the system as well as a baseline dataset for comparison with numerical modelling activity. Delays in the sourcing of critical instrumentation components and issues with the performance of the wave making equipment resulted in fewer tests being completed than would have been ideal. In addition, issues surrounding the performance of the power take off system resulted in lower than expected applied torques and considerable uncertainty in the developed power. These issues will be resolved during a further period of testing scheduled for February 2013.


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CONTENTS

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

1.1 INTRODUCTION .................................................................................................................................................... 7 1.2 DESCRIPTION OF THE TECHNOLOGY ......................................................................................................................... 7 1.3 DEVELOPMENT SO FAR .......................................................................................................................................... 8 1.3.1 Stage Gate Progress .................................................................................................................................... 8 1.3.2 Plan for This Access ..................................................................................................................................... 9

2 OUTLINE OF WORK CARRIED OUT ................................................................................................................. 12

2.1 SETUP ............................................................................................................................................................... 12 2.1.1 Description of Wave Basin......................................................................................................................... 12 2.1.2 Description of the Scale Model .................................................................................................................. 12 2.1.3 Instrumentation Systems ........................................................................................................................... 14 2.1.4 Operations on the device ........................................................................................................................... 16

2.2 TESTS ............................................................................................................................................................... 17 2.2.1 Available Sea states ................................................................................................................................... 17 2.2.2 Test Plan .................................................................................................................................................... 18

2.3 RESULTS ............................................................................................................................................................ 19 2.4 ANALYSIS & CONCLUSIONS................................................................................................................................... 20 2.4.1 Frequency response of the device ............................................................................................................. 20 2.4.2 Frequency responses across the array ...................................................................................................... 21 2.4.3 Mooring loads ........................................................................................................................................... 24

3 MAIN LEARNING OUTCOMES ....................................................................................................................... 25

3.1 PROGRESS MADE ............................................................................................................................................... 25 3.1.1 Progress Made: For This User-Group or Technology ................................................................................. 25 3.1.2 Progress Made: For Marine Renewable Energy Industry .......................................................................... 25

3.2 KEY LESSONS LEARNED ........................................................................................................................................ 25

4 FURTHER INFORMATION .............................................................................................................................. 26

4.1 SCIENTIFIC PUBLICATIONS .................................................................................................................................... 26 4.2 WEBSITE & SOCIAL MEDIA ................................................................................................................................... 26

5 REFERENCES ................................................................................................................................................ 26

6 APPENDICES ................................................................................................................................................ 27

6.1 STAGE DEVELOPMENT SUMMARY TABLE ................................................................................................................ 27


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1 INTRODUCTION & BACKGROUND

1.1 INTRODUCTION AlbaTERN are a Scottish wave energy developer that has been working on the development of ‘economically viable’ wave energy systems since 2007. The company employs 10 engineers working from hydrodynamics and resource assessment to electrical and control system design. Albatern’s main technology - the WaveNET –is more a system for wave energy conversion than a device. The WaveNET is described in more detail in the following section.

1.2 DESCRIPTION OF THE TECHNOLOGY The WaveNET design involves a mechanically coupled array of wave energy absorbing modules arranged in a hexagonal configuration. Modules are known as SQUIDs and are designed to be road transportable and straightforward to deploy. Each SQUID has a central buoyant riser and three link arms, the outer ends of which are provided with additional buoyant elements. The SQUID functions as a wave energy converter by converting the relative motions between the various buoyant elements through hydraulic conversion modules located in articulations at either end of the link arms. By connecting SQUID modules together at the outer buoyant elements – a deformable 2D hexagonal array can be assembled with the extent of the array defined by the number of interconnected modules. This design produces a close packed convertor array that benefits from near trapping effects and hydrodynamic coupling to enhance the yield from individual modules. The unique configuration allows energy to be captured from pitch, roll, surge, sway and heave motions of the individual modules with exceptionally high power to weight ratios and an efficient use of sea space. The modular design is ideally suited for mass production and can be readily reconfigured to suit alternative sites and power requirements. Figure 1 provides images of the WaveNET and SQUID technologies.

3 Hex WaveNET Side Profile (1/2 scale)

Isolated SQUID

3 Hex WaveNET Isometric

Figure 1: The WaveNET and SQUID Technologies


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1.3 DEVELOPMENT SO FAR WaveNET technology has been under development since 2007. Devices and modules have been previously tested at 100th, 50th, 20th, 8th and ½ scales both in tank facilities and open water environments. The design, while keeping the main feature of the extensible hexagonal array, has evolved considerably as a result of the various testing programs undertaken. A full scale first generation SQUID 7.5 kW module was successfully deployed in a low energy wave site for a period of 8 months during 2012. The device continued to generate power to a local dissipation sink throughout the duration of this deployment. AlbaTERN are currently involved in 2 projects to deploy full scale units based on the technology tested during these trials:

1. SQUID R & D Project – This project involves the deployment of a single SQUID 6.2 module (7.5kW) in a real sea environment.

2. WaveNET Project – This project, part funded through the Scottish Government’s WATERS2 program, will see a 3Hex WaveNET (up to 45kW) based on six SQUID 6.2 modules deployed in an open water test site.

1.3.1 Stage Gate Progress Previously completed: Planned for this project:

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


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STAGE GATE CRITERIA Status

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

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

1.3.2 Plan for This Access The broad objective for this access period is to build on the knowledge gained through previous testing campaigns and numerical modelling activities into the complex dynamic responses of fully coupled wave energy converter arrays to inform the design and implementation of the WaveNET system. These tests will augment previous trials by providing the necessary instrumentation capability to simultaneously track real time 6 DofF motions for up to 16 bodies in addition to implementing controllable applied forces at 36 powers take off articulations and the monitoring of mooring loads. The model includes a number of significant conceptual improvements over previous array model tests – notably the transition to an inclined link arm (as opposed to previous planer models), the system of nodes and anti-nodes as described in section 1.2 and the provision of a degree of out of plane compliance at the main articulations. These changes have been introduced into the system following previous model tests and extensive numerical modelling. A key objective, therefore, is to validate and qualify the scope of improvement in the dynamic response, structural loading and yield performance as a result of these changes.


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These tests will be used to inform the design and implementation of the system proposed for the company’s current full scale deployment projects. Consequently, the model and set up are, as close as practicably, accurate 10th scale replications of these systems. In some aspects – such as the water depth – it was not always possible to replicate the full conditions of the proposed site and the implications of this will be considered during the analysis stage. The WaveNET is an array based absorber and therefore the investigation of array interactions and multiple unit performance is implicit


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A schedule detailing the planned activities is given in Table 1.

Monday Tuesday Wednesday Thursday Friday Sat Sun

Week 1 Day 1 Arrival Kick Off Meeting Assemble Model (Config 1) Discuss testing plan Fit motion capture reflectors Test systems/ data acquisition Practice deployment procedures

Day 2 Initialisation Adjust Hydrostatics Configure Mooring System Deploy, Recover, Redeploy Calibrate Instrumentation Systems Check Positioning of Motion Capture System

Day 3 – Regular Waves Full Sweep of Regular Waves with 3 Hex Model Repeat with 3 levels of Damping Test Duration 3- 4mins Inspect Model

Day 4 – Irregular Waves Full Sweep of Irregular Waves with 3 Hex Model Test Duration 8-10mins

Day 5 – Irregular Waves Repeat selected tests from Day 4 with alternative PTO torque/damping values Recover Model and Reconfigure

Week 2 Day 6 Regular Waves Config 2 Full sweep of regular waves with 1 Hex configuration

Day 7 Irregular Waves Config 2 Full sweep of Irregular waves with 1 Hex System

Day 8 – Configuration 3 Tests – Regular and Irregular on isolated SQUID module

Day 9 – Survivability SQUID PTO tests and WaveNET Survivability Tests

Day 10 – Further survivability and conclusions Further Survivability and Conclusions

Table 1: Planned Test Schedule


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2 OUTLINE OF WORK CARRIED OUT

2.1 SETUP

2.1.1 Description of Wave Basin The IFREMER wave tank is 50 m long and 12.5 m wide. It is 10 m deep for the first 37.5 m and 20 m for the last 12.5 m before the beach. The wave generated could reach 0.50 m in regular wave and 0.30m specific heights in irregular waves.

Figure 2 - Wave Basin Schematic

2.1.2 Description of the Scale Model Based on the dimensions of the wave basin, the model was designed for 1/10th scale (of the ½ scale – or 1/20th of the full scale) allowing tests in waves equivalent to up to 5m (10m) height.

2.1.2.1 Structural Model

A WaveNET is defined as the combination of at least 3 Squids in a hexagonally shaped array. It can therefore have an arbitrary number of hexagons. A Squid is made of a riser, three link arms and three buoyancy modules. When assembled in a WaveNET the buoyancy module become the connection point between squids and are called anti nodes. The risers are then called nodes.

Figure 3: View of a 1Hex WaveNET onshore


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Figure 3 depicts the model out of the water. The WaveNET array is assembled by connecting SQUID modules together at common anti-nodes. A total of 7 SQUID modules were fabricated. This provides for a 3Hex WaveNET (6 modules) with a single The design was based as far as practical as a 10th scale realisation of the current design and care was taken to match the outer dimensions of the hydrostatic components and to scale the mass and buoyancy distributions accordingly. The design allowed a very compact folding up of the 3Hex configuration suitable for transportation and ready for deployment.

2.1.2.2 PTO System

A power take off (PTO) system comprising specially marinised gearbox motors integrated into each articulation axis (36 in total) was built into the scaled model. Prior to deployment the motors were characterized such that the relationships: voltage vs rotational velocity and current vs torque were known and understood Each motor had a rated working torque of 0.25 Nm. The output of the electrical motors were individually rectified and connected in series such that the DC voltages were additive around the circuit.

2.1.2.3 Control System

A dedicated controller was developed by AlbaTERN to permit the current, and therefore the torque, developed by each motor to be controlled. The control box was located at the instrumentation station and connected to the device by means of a single umbilical cable.

Figure 4 - Control box Top

Figure 5 - Control Box Side 1

Figure 6 - Control Box Side 2

The control box provided three means of operation: 1. Direct Load Mode – The combined output from all motors was connected to a variable resistor bank that

provided a linear relationship between the current (torque) and the developed voltage (speed). The

relationship is fully defined by the selected resistance and the resulting loading regime can be considered as

mechanical damping.

2. Controlled Torque Mode – A controller (buck converter) was used to provide a controlled current ensuring

that all motors received the same current (torque) regardless of voltage (speed).

3. Short Circuit Mode – Tall motors are effectively short circuited – providing the maximum possible torque

although with the limitation that voltage could not be recorded.

The controller was provided with a dedicated data acquisition system allowing real time monitoring and tuning of the system.


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2.1.3 Instrumentation Systems A range of instrumentation was provided to monitor the performance of the system under test. The Instrumentation system included:

Motion tracking system – A 6 camera Qualysis motion tracking system was used to record the trajectories of

the various bodies within the system. All surface piercing bodies (Risers and Antinodes – 16 on 3 Hex System)

were equipped with 4 passive reflectors (64 in total). This set up provided 6 DOF position data resolved and

translated into the body’s co-ordinate system for each tracked body. The positions of all remaining bodies in

the system (Links, Node Wings etc.) can then be derived numerically from this dataset. During operation,

position data was recorded at 100 Hz. Velocities and accelerations were calculated from the derivative and

second derivative of the position data respectively.

Mooring Tension – Mooring loads were recorded from load cells positioned on the ‘shore’ side of the

mooring set up. Although the system was equipped with 6 mooring lines, symmetry within the system

allowed the mooring loads to be accurately recorded by a set of 4 load cells – 2 on the up-wave mooring

lines and 2 on the near side mooring lines. The remaining 2 far side mooring lines were assumed to be equal

in magnitude of load to the nearside lines due to the symmetry of the system and low directional spreading

capability of the wave makers.

Water surface elevation was tracked using wave gauges at 2 positions – one directly up-wave of the device

and one in line with the equilibrium position of the device.

PTO voltage was monitored and recorded at 100 Hz by the main data acquisition system and simultaneously

(at 4 Hz) by AlbaTERN’s data acquisition and control system.

PTO Current was monitored and recorded at 100 Hz by the main data acquisition system and simultaneously

(at 4 Hz) by AlbaTERN’s data acquisition and control system.

Video recording equipment was used to provide a visual record of all tests as follows

o A standing camera recorded all tests from a consistent viewpoint (see figure 6 for example)

o Two GOPRO high definition cameras were used variously to film underwater and plan views of the

device.

o An underwater Spyball allowed monitoring and recording of the behaviour of each individual motor

or specific aspects of the device as required.

Figure 7: View of the 3Hex WaveNET from the side of the tank


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Figure 8: Underwater view of the 3Hex WaveNET from

Figure 9: View of the 3Hex WaveNET from the top


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2.1.4 Operations on the device Various operations were required to be performed before the device was configured and ready for testing; these operations are discussed briefly in this section.

2.1.4.1 Array Deployment and Retrieval

A deployment rig was specially designed to allow the lifting and handling of the device in its folded configuration. The array is suspended from 6 equally spaced tethers attached to six anti nodes. Tether length is such that once in the water the device opens naturally - due to hydrostatic forces- and expands to its operation configuration.

Figure 10: Deployment of the 3Hex WaveNET

Figure 11: Recovery of the 3Hex WaveNET

2.1.4.2 Mooring Arrangement and Calibration

Due to the depth of the tank, replicating a seabed mooring pattern was not considered to be appropriate for these tests. Instead, mooring attachment points were located on down stands from the sides of the wave basin (for the 4 side moorings) and from the movable bridge for the leading mooring lines. A pulley block at the mooring attachment point allowed the mooring pretension to be easily adjusted from the side of the tank while load cells were used to monitor the tension. The mooring lines were made form 6mm polyester. A degree of elasticity was provided using nylon bungee cords. The load extension profile of the mooring system was measured before and after the testing campaign. Once installed the array was connected to the pre-installed mooring system with the help of a small boat.


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2.1.4.3 Installation of Qualisys Reflectors

To provide 6 degree of freedom tracking, a minimum of 4 qualisys reflectors is required for each tracked body. Qualisys reflectors were mounted on specially commissioned lightweight carbon fibre ‘tree’ assembles. These were attached to the device from a small boat once the device was installed and connected to the mooring system. The flexibility of the structure in the water made it straightforward to access any part of the device from the boat.

2.1.4.4 Hydrostatic Calibration and Adjustment

Hydrostatic adjustment is possible by varying the amount of closed cell foam provided in buoyancy cavities in both the riser and anti-node components. Additional counteracting ballast weight is also provided in outlying anti-nodes and this can be tuned if required. While hydrostatic adjustment is possible when the device is installed, this was not found to be necessary. After the initial installation the device was removed once at which point fine tuning of the hydrostatics were performed.

2.1.4.5 SQUID Replacement

It is a requirement of the WaveNET concept that it is possible to remove and replace individual SQUID modules from the array without compromising the overall structural integrity or seaworthiness of the system. Achieving this means that a failure of an individual module does not necessarily require the recovery of the entire array and that the overall system has significantly higher levels of built in redundancy. While not necessarily identical to the procedure that would be carried out on the full scale device, the operation of replacing a SQUID was carried out successfully and the array shown to be hydrostatically stable and recoverable.

2.2 TESTS

2.2.1 Available Sea states Table 2 shows the range of regular sea states and the equivalent scales that the IFREMER wave basin is capable of generating. Table 3 provides the same information for irregular waves.

1 Period (s)

Wave height

(m)

1/2 4 5 6 7 8 9 10

20th

10 5 0.5 1

8 4 0.4 1 1 1 1

6 3 0.3 1 1 1 1 1 1

4 2 0.2 1 1 1 1 1 1

2 1 0.1 1 1 1 1 1 1 1

Table 2: Available Regular Wave Tables Sea states

1 Period Tp(s)

Wave height Hs(m)

1/2 4 5 6 7 8 9 10

20th

6 3 0.3 1 1 1

4 2 0.2 1 1 1 1 1

2 1 0.1 1 1 1 1 1 1 1

Table 3: Available Irregular wave Sea states


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2.2.2 Test Plan Various delays resulting from the supply of critical motion tracking components and issues with the wave making machines meant that the actual test schedule differed from the planned schedule. An overview of the actual test schedule is given in Table 4.

Monday Tuesday Wednesday Thursday Friday Sat Sun

Week 1 Day 1 Arrival Deployed Model, Recovered Model Hydrostatic calibration Redeployed Model

Day 2 Initialisation Tested Instrumentation Systems Configured Mooring System Trial Waves run

Day 3 Calibrated Instrumentation System

Day 4 Awaiting arrival of Qualisys tree attachments

Day 5 Attachment of Qualisys trees Calibration of Qualisys system

Week 2 Day 6 Full sweep of Regular Waves with 3 Hex Configuration. Test duration: 6min 3 alternative PTO settings per test. 26 runs in total

Day 7 Irregular wave sweep 3Hex system. Spectrum: JONSWAP gamma 3.3 Single PTO settings Replaced SQUID in-situ Recovered device 16 runs in total

Day 8 Regular wave sweep with Isolated SQUID – various PTO settings. Full Regular sweep and selected Irregular waves (JONSWAP 3.3) on 1Hex configuration. 31 runs in total

Day 9 Alternative Mooring Configuration with 1 Hex System. Wave makers not functioning correctly. Redeployed 3 Hex System 15 runs in total

Day 10 Selected Regular and Irregular waves on 3Hex system. Decommissioning 13 runs in total

Table 4: Actual Test Schedule


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2.3 RESULTS The tests produced a rich dataset detailing the motion of individual nodes within the array across a full range of both regular and irregular sea states. Raw data output for time series output was clean and consistent and presented no difficulties for post processing. Example 6 degree of freedom position datasets for a single body responding to a regular wave are presented in Figure 12.

Figure 12: Example of a time series of the 6 degrees of freedom of a body in regular waves

From the position information of the various floating bodies, the relative angle between floating bodies and link arms were calculated. Figure 13 shows an example of the derived relative angle time series. This angle variation is directly related to the power taken off the system, hence giving valuable information for yield and performance calculations.

Figure 13: Example of a time series of the angle between a riser and a link arm


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Another variable measured directly with IFM’s instrumentation was the tension in the mooring lines. Figure 14 shows an example of time series obtained for one mooring line. Here again the signal is very clean and shows the harmonic response of the mooring lines.

Figure 14: Mooring tension time series

2.4 ANALYSIS & CONCLUSIONS

2.4.1 Frequency response of the device From the regular wave tests it is possible to get the harmonic response of each body for a given frequency. Plotting the amplitude of the response against the period gives the Response Amplitude Operator (RAO). Example RAO plots are presented in Figure 15.

Figure 15: Example of the RAO of a floating body


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2.4.2 Frequency responses across the array RAO’s provide a convenient means of assessing how various response modes relate to the excitation frequency of the waves. This provides means of identifying the predominant energy capture modes and interpreting the relative motions between different bodies in the array.

2.4.2.1 Numbering Conventions

Figure 16 shows the numbering conventions used for the analysis of the bodies within the array. We can identify three different rows arranged perpendicular to the incident wave direction: Risers 1, 2 and 3 form the first row, Risers 4 and 6 the second row and Riser 5 the last row.

Riser Numbering Convention Anti-node numbering convention

Link arm numbering convention

Row 1 Row 2 Row3 Row 1 Row 2 Row3 Row 1 Row 2 Row3

Figure 16: Numbering Conventions

2.4.2.2 Variation within a Row

It is interesting to examine the response changes across the different ‘rows’ of the device. Figure 17 shows the derived angle RAO for the 3 Link arms in Row1. The excitation is provided by plane regular waves and therefore, as expected, we see broadly consistent responses for these 3 link arms. The flexibility within the array does allow for different motions however and we can observe that higher frequency waves exhibit a slightly larger response from the centre link arm than opposed to the outside links and that this trend diminishes as the frequency reduces.

Figure 17: Angle RAO of the front row link arms

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2.4.2.3 Variation between Rows

Figure 18 shows how the body RAOs for an anti-node in row 1 (Anti-Node 1) compare to an anti-node in row 2 (Anti-Node 6). Figure 18In both cases the predominant modes are surge (x), heave (z) and pitch. Significantly higher surge motions are exhibited by the body in the second row – particularly for the lower frequency excitations. This can be explained partially by restrictions due to the presence on the mooring attachment on the leading anti-node (1) and partially by a lengthening, ‘accordion’ effect within the array.

Figure 18: Comparison of RAOs of anti-nodes from front row and second row

Figure 19 shows how the RAOs vary between bodies in row 2 (Anti-Node 6) and row 3 (Anti-Node 5). It can be seen that at low periods both bodies demonstrate similar trends in surge although the second row appears to exhibit higher magnitudes until a transition point at around 8 seconds thereafter the additional compliance of the 3rd row results in increased surge motion of around 15% until a peak at around 9 seconds. The response of the third row in pitch is quite interesting with the appearance of multiple peaks at regular periodic intervals. This phenomenon appears to be due to coupling effects within the array. Again we see an increase in the response amplitude of the third row as the period increases.

Figure 19: Comparison of RAOs of anti-nodes from second row and third row


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The same trend showing the gradual emergence of multiple peaks within the response as the wave propagates through the array can be seen by considering the link angle as shown in Figure 20. The three traces correspond to 1st(1), 2nd (16) and 3rd (13) row outer link arms. From this we can see the largest response is evident in the middle row and we get one peak in the first row, two in the 2nd row and three in the third row.

Figure 20: Evolution of the angular RAO in the link arms in line with the waves across the front row, second row and third row.


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2.4.3 Mooring loads The two following tables show the maximum tension in two of the four monitored mooring lines. These two tables clearly show a clear trend that is consistent with modelled expectations. Table 5 shows the trend in the leading line while Table 6 shows the same information for a side line which as expected is considerably less.

Table 5: Maximum mooring tension in line 1

Table 6: Maximum mooring tension in line 3

Maximum tension mooring in line 1 (N)

Full Scale Period 4 5 6 7 8 9 10

Wave height Model Scale 1.26 1.58 1.90 2.21 2.53 2.85 3.16

5 0.5 18.5502

4 0.4

18.0615 14.6729 10.1766 8.93849

3 0.3

18.64797 13.20675 9.427224 7.43971 6.42967

2 0.2 15.87849 11.67539 8.710416 6.006099 5.191545 5.191545

1 0.1 8.971074 6.918399 5.810606 4.572484 3.790513 3.660184 3.660184

Maximum tension mooring in line 3 (N)

Full Scale Period 4 5 6 7 8 9 10

Wave height Model Scale 1.26 1.58 1.90 2.21 2.53 2.85 3.16

5 0.5 5.428368

4 0.4

5.743817 4.525036 4.585975 4.829731

3 0.3

4.234679 4.901424 3.822445 3.851122 3.797352

2 0.2 2.101813 3.406625 3.227393 2.897605 2.503294 2.101813

1 0.1 1.15905 1.979935 1.883149 1.951258 1.578454 1.739763 1.25942


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3 MAIN LEARNING OUTCOMES

3.1 PROGRESS MADE

3.1.1 Progress Made: For This User-Group or Technology This testing campaign has provided valuable information which will inform the on-going development of the WaveNET and SQUID technologies. In general, the results have been very positive with the model functioning well in most sea states. The data gathered is still undergoing complete analysis however at this stage the results have been encouraging and show broad agreement with modelled and anticipated outcomes. Video data gathered, in particular the underwater footage, has already proven vital in communicating the essence of the technology and its operation to the wider marine energy and investor communities.

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

AlbaTERN is in the preparatory stages of deploying a full scale implementation of the technology described in this report. Considerable confidence on the performance and dynamic response of the device was gained from this testing period and no specific design changes are anticipated as a result. The PTO implementation was a new and ambitious approach for AlbaTERN and indeed finding a cost effective solution to replicate the required torque in a controllable manner over small reciprocating angles and at the scale under test remains a considerable problem – not least when the requirement is to control 36 independent articulations simultaneously in a salt water environment. The approach taken in the test produced mixed results. Of the 36 motors (each of which had to be individually sealed and marinised) there were 6 failures for a variety of mechanical and electrical reasons. Design modifications to increase the developed torque and reduce the consequence and rate of failure of the PTO system are being implemented and further tank tests on the same model will be used to provide a more comprehensive of the power generation and yield of the device at a later stage. No significant structural changes to the model are required at this stage.

3.1.2 Progress Made: For Marine Renewable Energy Industry The set up used during the testing with 6 Qualisys cameras and 64 reflecting ball tracking 16 floating bodies has been a real success. It provides a good benchmark of the quantity and the quality of the data that can be gathered for multi-bodies WEC. The data gathered from this testing campaign represents a unique set of information providing baseline knowledge for the development of ABTs WEC. The study of the motion and dynamics of 16 mechanically coupled bodies arranged in a 2D array is probably a marine energy first and the resulting baseline data set represents a real added value for the marine renewable energy industry particularly with regards to the study of array interaction. The production and successful testing of a scaled model of a WEC made of multiple bodies mechanically coupled proves the validly of such an approach to the industry. This testing campaign has proven that the complexity of a modular concept can be dealt with, proving at the same time that all the benefits of a modular design are achievable.

3.2 KEY LESSONS LEARNED The particular flexible geometrical arrangement of the WaveNET responds well to a wide range of wave

climates and offers an attractive means of capturing energy from the full orbital motion of the incident waves.

Preparation and collaboration with the infrastructure provider is key for the success of the testing


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4 FURTHER INFORMATION

4.1 SCIENTIFIC PUBLICATIONS There are currently no plans to produce scientific publications based on the work described in this report.

4.2 WEBSITE & SOCIAL MEDIA Website: www.albatern.co.uk

5 REFERENCES


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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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Documents

WaveNET Hydrodynamic Analysis - MaRINET2 · The WaveNET is a unique modular wave energy conversion system that utilises the relative motion between a series of wave interacting bodies