Marine Renewables Infrastructure Network · Infrastructure Access Report: SPPCTP Rev. 02,...

Infrastructure Access Report

Infrastructure: QUB Shallow Water Wave Tank

User‐Project: SeaPower SPPCTP

SeaPower Platform Configuration Test Program

Sea Power Ltd.

Marine Renewables Infrastructure Network

Status: Final Version: 02 Date: 22‐Oct‐2014

EC FP7 “Capacities” Specific Programme Research Infrastructure Action

Infrastructure Access Report: SPPCTP

Rev. 02, 22‐Oct‐2014 of

ABOUTMARINETMARINET (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. �� 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)

NetherlandsStichting 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)


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DOCUMENTINFORMATIONTitle SeaPower Platform Configuration Test Program Distribution Public Document Reference MARINET‐TA1‐SPPCTP User‐Group Leader, Lead Author

Cian Murtagh SeaPower Ltd. Atlantic Sound, Lacken, Enniscrone, Co. Sligo, Ireland

User‐Group Members, Contributing Authors

Joe Murtagh SeaPowerTom Lyne SeaPowerRalph Burke FAL Ltd. Rachael Moore QUB

Infrastructure Accessed: QUB Shallow Water Wave Tank Infrastructure Manager (or Main Contact)

Bjoern Elsaesser

REVISIONHISTORYRev. Date Description Prepared by

(Name) Approved By Infrastructure

Manager

Status (Draft/Final)

01 09/2014 C. Murtagh JM DRAFT 02 10/2014 R. Moore JM FINAL


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ABOUTTHISREPORTOne 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

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.

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

LEGALDISCLAIMERThe 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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EXECUTIVESUMMARYThe SeaPower wave energy converter (WEC) is a floating ocean going attenuator primarily suited to deep water locations. The technology has been tested a number of times at wave laboratories including the HMRC basin in Cork, and the deep water basin at Strathclyde in Scotland. Survival loads were also measured in Lough Corrib site. Numerical modelling and Cost of Energy calculations have also been carried out prior to access to the beginning of this testing campaign. The scope of this testing was primarily to compare relative motion response amplitude operators (RAOs) for multiple configurations of a baseline device and secondarily to determine the power curves and performance of the best model(s) using an on‐board power take‐off (PTO) system. Numerical simulations have previously been carried out on these WEC configurations allowing the accuracy of this numerical analysis to be validated accordingly. Different types of mooring setups, ballast arrangements, draught changes and wave directionality were also included in the testing campaign under the same controlled environments for comparisons and for the production of comparative power RAOs. The performance of the SeaPower platform at this scale has once again proven itself to be excellent and higher than competitor technologies. The power results are published within this report. The type of PTO damping strategy implemented during testing also benefits the Levelised Cost of Energy (LCOE) for the device. It has shown itself to be a successful way of absorbing power and since the forces required are lower than for other damping strategies, the cost of the PTO components (whether they be direct drive or hydraulic pumps) should be lower. The forces and velocities were measured throughout these tests and serve as a good design tool to size an adequate PTO system.


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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 .................................................................................................................. 13

2.1 SETUP ............................................................................................................................................................... 13 2.2 TESTS ............................................................................................................................................................... 15 2.2.1 Test Plan ........................................................................................................................................................ 2.3 RESULTS ............................................................................................................................................................ 20 2.4 ANALYSIS & CONCLUSIONS ................................................................................................................................... 24

3 MAIN LEARNING OUTCOMES ......................................................................................................................... 24

3.1 PROGRESS MADE ............................................................................................................................................... 24 3.1.1 Progress Made: For This User‐Group or Technology ................................................................................. 24 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 .......................................................................................................................................................

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

6.1 STAGE DEVELOPMENT SUMMARY TABLE ................................................................................................................ 27 6.2 ANY OTHER APPENDICES ..................................................................................................................................... 29


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1 INTRODUCTION&BACKGROUND1.1 INTRODUCTIONThe SeaPower Platform WEC consists of two wave interacting bodies; a longer main body and a shorter hinged pontoon. The device is classified as a hinged barge attenuator device. At large commercial scale, the pontoons making up these two bodies would preferably be manufactured using concrete and both the shape and size of the pontoons were carried out with concrete construction in mind. The SeaPower Platform has been in development since 2008 with the main objective being to make a commercially viable wave energy convertor. The SeaPower Platform extracts energy from the ocean waves efficiently and safely whilst requiring minimum maintenance and producing maximum economic return. Cost of Energy is the primary focus of the development of the device. The SeaPower Platform WEC is a floating device designed to be situated in deep water with slack moorings. A PTO is actuated at the hinge and absorbs power from the moving bodies. The Scale=1x model is built from sheet aluminium and moored using light chains. The scaling factor for the model is designated a 1:20 scale for a future AMETS test site, 1:5 scale for Galway Bay “¼ scale” test site, and 1:36 for the commonly referenced Mollison site. Numerical analysis was carried out at full scale at the Mollison site which corresponds to the 1:36 scale. The dimensions for the device have not all been firmly established, for example the separation distances between pontoons. This dimension was the subject of a previous numerical analysis and results indicated improvements in the related cost of energy forecasts. A shorter overall WEC length for the same or better power capture is the sought goal. The effect of power production for different pontoon separation configurations was not tested in a wave basin prior to this testing period. As well as this, the effect of power production when altering the mooring stiffness, device orientation and wave directionality was also not tested prior to this campaign. During this testing period these unexplored parameters which could impact the performance of the device were addressed and selected model configurations were tested satisfactorily in real sea states in monochromatic and panchromatic seas with a working PTO on board allowing mechanical power to be accurately measured.

1.2 DEVELOPMENTSOFAR

1.2.1 StageGateProgressPreviously 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 (DoF) 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


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STAGE GATE CRITERIA Status 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 • 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


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1.2.2 PlanforthisAccess This plan details the correct procedure used for testing the SeaPower Platform WEC model in the QUB wave basin situated in Portaferry. The scope of testing was primarily to compare relative motion RAOs1 for multiple configurations of a baseline device and secondarily to determine the power curve and performance of the best model(s) using a PTO dynamometer. Numerical simulations have previously been carried out on these WEC configurations allowing a comparison of this analysis and therefore validation of the numerical model. Procedures described include

• Assembling, deployment and removal of the SeaPower Platform WEC model • Ballasting of the WEC model for each configuration • Calibration of the SeaPower Platform WEC model

The SeaPower Platform which consists of 3 pontoons was designed and built in‐house at SeaPower headquarters in Ireland. It was designed to be easily modified as the pontoon positions can be easily adjusted using quick release clamps allowing the bodies to be moved in a longitudinal direction. Foam extenders were also available to increase the widths of these pontoons but these were not used during this testing campaign. Instead the focus was on investigating the WEC behaviour as the pontoons were moved relative to each other thus creating devices with different configurations and different overall lengths. Another configuration involved increasing the draught –of the device by inserting more ballast into each pontoon and increasing the amount of the device that was submerged.

Each pontoon configuration required ]a hydrostatic calculation to be conducted in order to determine the related ballast configuration. Access hatches on the top of each pontoon allowed ballast blocks to be inserted and removed with ease decreasing the amount of idle time between tests A shakedown of the model in still water prior to the test also helped to determine a suitable waterline and ballast configuration. The ballast blocks were made up from small pieces of lead arranged in easily manageable blocks with each block weighing 5kg.

The aluminium WEC model had a final displacement of 180kg overall and approximately half of this mass was comprised of removable lead ballast blocks. The aluminium model was approximately 4 metres in length and was also designed to be disassembled into four light manageable components allowing two people to lift the heaviest of these components with ease. The WEC model was manually placed into the tank by three people and the absorbing beach area around three sides of the tank provided a working platform to assemble the WEC model into its first configuration. The aluminium WEC model without ballast was then towed out to the middle of the wave tank and moored into position. There is almost zero load on the WEC model due to the mooring arrangement which meant that the moorings did not affect the draught of the machine. The ballast blocks were then inserted through the hatches. The riser cable connecting the WEC device to the data acquisition equipment an approximate distance of 25m ran through the overhead gantry above the tank and was secured with cable ties. These were plugged into the PLC system in the control room. A camera was set up on a stand facing the WEC model in order to record video of each set of runs.

1 The Motion RAOs gave an indicator of performance of various machines, and is not considered as fully conclusive in a shape optimisation study.


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1.2.2.1 Plan

Figure SeaPower Platform 1x scale. Configuration (iii) End View

To facilitate each configuration change the pontoon spacing of the WEC model was to be changed as well as the orientation of the full device. Therefore it was essential for a person to enter the tank regularly during the testing period. Each time a configuration was changed the WEC was ballasted again and trimmed appropriately to ensure that the metacentres of both bodies were correct before proceeding. A master document detailing the scheduling of the entire testing campaign was created prior to testing and this document was adhered to throughout the testing period. Ramp up and ramp down times for the waves as well as a sampling length to gather enough data was known before testing. The initial estimated time to carry out the full testing campaign was 15 days.


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The SeaPower Platform WEC device was tested using a number of monochromatic tests on each of the first 5 pontoon configurations. Quick release moorings were provided for changing the orientation of the WEC model. Following the monochromatic tests, panchromatic tests were carried out on the two configurations that produced the greatest power output. The objective of these panchromatic tests was to establish the performance data for the newly configured WEC and validate the numerical simulation that had been established. The wave paddles were operated using the procedures specified in QUB document WP001‐10‐03. The monochromatic waves selected for this scale model are shown in Table below. Three wave heights were selected which were within the range of the capabilities of the tank basin and also the spectrum of a full scale site:

Table Monochromatic waves calibrated and used for testing

Height=80mm Height=120mm Height=150mm

Period [s] Wavelength

[m] Period [s) Wavelength

[m] Period [s] Wavelength

[m] 1.00 1.525 1.00 1.543 1.00 1.543 1.22 2.172 1.22 2.194 1.22 2.173 1.45 2.893 1.45 2.893 1.45 2.893 1.68 3.553 1.68 3.554 1.68 3.553 1.89 4.134 1.89 4.185 1.89 4.132 2.13 4.793 2.13 4.794 2.13 4.793 2.25 5.098 2.25 5.099 2.25 5.096 2.33 5.315 2.33 5.315 2.33 5.316 2.56 5.931 2.56 5.932 2.56 5.931

The panchromatic waves selected for this scale model are shown in Table below. Three wave heights were selected which were within the range of the capabilities of the tank basin and also the spectrum of a full scale site:

Table Panchromatic Bretschnieder waves calibrated and used for testing

Height=100mm Height=125mm Height=150mm

Tp [s] Tp [s] Tp [s]

1.20 1.20 1.20

1.40 1.40 1.40

1.60 1.60 1.60

1.80 1.80 1.80

2.00 2.00 2.00

2.56 2.56 2.56


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Table 3specifies the data that was recorded during the testing and with the minimum frequency with which it was recorded. The data acquisition (DAQ) system in Portaferry reads in the raw signals from the two sensors and produces results in a comma‐separated value (CSV) file which corresponded to a test number and sea state. The raw signals were initially converted into angular velocity and torque values about the hinge in an excel file and these torque, velocity and power figures were also displayed in real time while the test was running.

Table 3 Information regarding Data that was recorded

Data Recorded Units & Minimum Recording Frequency Water depth Daily or after every modification of depth

SeaPower Model positioning/ location in tank After every modification of model position eg. orientation of model.

SeaPower Model Ballast arrangement After every modification of model configuration

Mooring Stiffness Vertical and horizontal components of mooring

stiffness were measure after every modification of mooring configuration

Load Cell Continuously logged during each test. Output to a CSV file in the form of torque about hinge. Units [N m]

Position Sensor Continuously logged during each test. Output to a CSV file in the form of angular velocity about hinge Units

[rad/s]

Average Power Instantaneous values for angular velocity and torque were measured and averaged at the end of a test run

for a particular sea state.

Table 4 Uncertainty related to the Data recorded

Data recorded Uncertainty Water depth ±1 mm

CyanWave model position ±25 mm abs, ±5 mm rel Load Cell (for load and torque measurement) +‐ 5N , +‐ 1.175 N m

Position Sensor (for absolute position and velocity) +‐0.1mm, +‐0.0235 [rad/s]


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2 OUTLINEOFWORKCARRIEDOUT2.1 SETUP

Figure Shakedown of the device before the testing campaign.

Figure Sketch of Mooring Setup – for near zero vertical stiffness, and low horizontal stiffness.

“Mooring A”


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Location of WEC in the tank. Mooring lines not shown.

Figure 4 Wave Basin Dimensions and Topography


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Figure 5 SeaPower Platform on station. Monochromatic Waves H=80mm. “Mooring B”; softer mooring configuration


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y = 0.7016x ‐ 19.075

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2.2 TESTS

2.2.1 MooringsBefore the test began, the mooring anchors and lines were put in place using quick release couplings and connected to the device. There was a total of 4 lines used throughout this testing campaign. Two configurations of mooring layouts were tested. The first configuration “Mooring A” referred to the setting all four mooring lines to a mooring stiffness curve in Figure 6. The stiffness of the two aft and forward lines were measured experimentally using a measuring metre stick fixed to a reference point and a digital load scales. The front mooring lines refer to the lines closest to the wave paddles. The back lines refer to the lines furthest from the wave paddles. The vertical stiffness was set to zero by using a floating buoy arrangement – refer to Figure 3.

The forward mooring lines are the primary lines. They keep the device on station during heavy seas and they have the biggest effect on power production depending on the mooring line properties. Therefore it is these forward lines

that were changed creating a second mooring configuration for testing. By comparing monochromatic RAOs for both mooring configurations, it was found that the softer mooring arrangement was much better for power production.(referred to as “Mooring B”).

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Figure 6 Horizontal Stiffness – Two Front lines. “Soft”

Figure 7 Horizontal Stiffness ‐ Two aft lines "Stiff"


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2.2.2 PontoonConfigurations The previously tested “baseline” WEC model was modified so as to make it adjustable. This meant that the pontoons could be moved back and forth relative to the hinge allowing 5 different shape configurations with two orientations each to be tested. The first configuration was the original baseline machine as tested in the HMRC and Strathclyde basins. The four subsequent shapes, (ii) to (v), were variations of the first, and their initial selection coincided with a numerical analysis shape optimisation study carried out by Pure Marine Gen Ltd on behalf of SeaPower Ltd. The following shape configurations were each tested in the forward and aft configurations:

(i) Baseline Model, Baseline Mooring, 120mm Draught, Overall Length =3.9m. As tested previously2 (ii) Baseline Model, Hinge Half‐Way, Baseline Mooring, 120mm Draught, Overall Length=3.9m (iii) Hinge Half‐Way, 5m Aft Spacing, Baseline Mooring, 120mm Draught, Overall Length=2.9m (iv) Hinge Half‐Way (565mm to main), 0m Aft Spacing, Baseline Mooring, Overall Length=2.84m (v) 0m Aft Spacing, Baseline Mooring, 120mm Draught, Fwd Pontoon Moved Back to 820mm, 565mm from

main. Overall Length = 3.35m The mooring configurations were:

(i) Mooring A: 4 point, arrangement, Stiff moorings on all four lines (ii) Mooring B: 4 point arrangement, Soft moorings on front lines and stiff moorings on aft lines

The three draught configurations were:

(i) WEC draught 120mm (common across all pontoons) (ii) WEC draught 166mm (common across all pontoons) (iii) WEC draught 166mm on smaller “tail pontoon” and 120mm on main body pontoon

The wave directionality tests were:

(i) 0 degrees (facing into headed sea) (ii) 15 degrees (iii) 30 degrees

2 E.g. this baseline configuration was tested in HMRC (Panchromatic Power Results are included in the Appendix for Comparison)


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Each configuration was tested in both the forward and aft (reverse) orientation with the model turned around 180 degrees in each case. This gave a total of 10 RAO results in total during the monochromatic testing phase.

2.2.2.1 PontoonConfigurationsastested(inpictures)

Figure 8: Five shape configurations with two orientations each (Ten in total)

Each configuration was tested in the forward and aft orientations. These were designated as 0 degree orientation and 180 degree orientation respectively. RAOs for each configuration were established and the best configuration was selected as the candidate for further tests. By using the configuration with the best results changes to the mooring configuration, device draft and wave directionality could be carried out using monochromatic waves. The configuration that gave the best monochromatic power capture was configuration (v) with 180 degree orientation(the bottom right image in Figure 8). Configuration (i) represents the baseline model.


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Figure 10 Close up of PTO: Stepper Motor Brake, Load cell, Position Sensor (Dynamometer)

The best candidate shape was then used to test the effect of varying draughts. The default baseline draught was 120mm and this was then set to 166mm overall draught by the addition of ballast however this deep draught setting did not improve power capture. Following this test the 120mm draught was adjusted back by removing ballast in the small pontoon whilst the 166mm draught was maintained on the small ‘tail’ pontoon only. The highest power capture result occurred during this test. The overall length of the device was 3.9 metres for the baseline compared to 3.35 metres for the improved configuration.

Figure 9 Best configuration: Scale=1x Overall length = 3350mm. 180deg orientation


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2.3 RESULTS The initial testing focused on motion RAOs for the five main configurations. Following on from the configurations study the next step was to quantify the effects on power of a deeper draught (heavier) device. It was decided to use configuration (iii) for this. A deeper 166mm draught would lend itself to the manufacture of concrete pontoons at smaller scales. It was found that a deeper draught did not produce better power results however a deeper draught on the tail pontoon only did produce better power capture results for configuration (v).

Figure 11 – Graph showing the Impact of a Deeper Draught (166mm) on Power Output

Figure 12 ‐ Graph showing the Impact of a Shallower Draught (120mm) on Power Output

As can be clearly seen from the graphs on Figures 11 and 12the shallower draught device was better for power production than the deeper draught. The third configuration of draught which involved leaving the deeper draught on the smaller “tail” pontoon and reverting to a more shallow draught for the other larger body gave the best results

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Power RAO Monochromatic, Config (iii) "Deep Draught" (166mm)

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Power RAO Monochromatic, Config (iii) Shallow Draught (120mm)


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overall for power production and these are shown on Figure 13. It is assumed that in terms of Cost of Energy the deeper draught in this situation would not contribute to higher capital expenditure (CAPEX) of device since the draught mainly comes from ballast material which is not expensive. This deeper draught on the smaller body would also suit concrete construction because the walls of a concrete vessel would be thick and heavy and require a relatively deep draught to begin with. The following graph shows the mechanical power captured on the final device in monochromatic sea states. The power is reported here at Scale=1x and can be interpolated to larger scales quiet easily. This graph was for the best configuration with soft moorings, 166mm draught small pontoon only and 180 degree orientation to the waves meaning the small pontoon would face away from the wave paddles. This is shown in Figure 9. The graph of power RAO was plotted against wavelength and these wavelengths correspond to wave periods as outlined in Table . The peak period occurs at between T=1.45s and T=1.68s.

Figure 13: A Power RAO graph for the Device that gave optimum Power Output during Monochromatic testing

The next step was to look at the variation of power as the wave directionality changed. It was noted that the power did not drop off significantly at small angles, especially up to 15 degrees. At 30 degrees the power noticeably dropped whilst still remaining relatively high. The peak power dropped from 900 W/m2 H to 600 W/m2H over the 30 degree range. This is equivalent to power drop offs as indicated by the following graphs:

0100200300400500600700800900

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Power RAO Monochromatic, New Configuration, Shallow Draught Main, Deep Tail Draught, 180 degree orientation

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Power RAO Monochromatic: 15 deg Directional Wave


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Figure 14: Power RAO graphs showing the Impact of Wave Directionality

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Figure Panchromatic Power Capture SPPCTP testing ‐ Coulomb Damping, not fully optimised

Figure Panchromatic Power Capture HMRC testing 2012 for comparison


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2.4 ANALYSIS&CONCLUSIONS

• The WEC performance is sensitive to changes in length. • According to the graph on Figure there is a very wide power capture bandwidth associated with this WEC. • The rectilinear mechanical brake on board served as an excellent PTO system emulator and allowed the

engineers to apply various levels of constant force damping and measure power accurately in real time in a sea state.

• Despite the shallow nature of the facilities, the power was comparable to previous tests in deeper water. The use of Coulomb or constant force damping settings as opposed to optimised linear damping settings did not result in much reduction of power even though a approximate reduction of 20% was expected.

• The SeaPower Platform is designed to operate as a deep water floating platform, not because it is a deep draught device but because of the higher occurrences of energetic waves further offshore than in shallower waters near shore. In relative terms the water depth in the basin used during this testing campaign was not ideal. Although the power in a wave is approximately the same whether deep or shallow, it was estimated that there may have been a 10% loss of power due to friction effects on the tank basin floor. Also the aft parts of the WEC were operating very close to the sloped floor of the tank were the water depth would have been beginning to decrease and some degree of hydrodynamic losses would also be associated with the motions of the WEC in these more shallow conditions.

• It is thought that this WEC device is not sensitive to small wave directionality changes although a site with consistent wave direction would be preferred. Certainly, between the +‐15degrees range there is no measurable degradation in power. Further testing of the device regarding wave directionality would be required to verify this crude prediction.

• A softer mooring system is preferred in terms of power capture when compared to a more stiff system. This implies low stiffness in the vertical and horizontal component directions from the mooring points on the WEC.

3 MAINLEARNINGOUTCOMES3.1 PROGRESSMADE

3.1.1 ProgressMade:ForThisUser‐GrouporTechnology An assumption has been made that changes and improvements to the model (reduced cost or increased power) can be determined easier using monochromatic waves. This made the testing of multiple configurations quicker as monochromatic seas were developed much quicker in the tank and average powers were very quickly determined for each configuration change. Optimising power in panchromatic runs would involve many weeks more of testing, analysis and learning time in order to optimise the damping to the WEC to provide the best power output.

Although the performance of the device is obviously different in more realistic, irregular panchromatic waves, running the tests in monochromatic waves proved to be a simple and effective way to compare designs and discover possible improvements. Although SeaPower Ltd have their own wave flume for testing, such discoveries are only possible in a larger basin where realistic moorings can be used and wave reflections from side walls are minimised .


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3.1.1.1 NextStepsforResearchorStagedDevelopmentPlan–Exit/Change&Retest/Proceed?

3.1.2 ProgressMade:ForMarineRenewableEnergyIndustry The marine renewable energy industry, in particular ocean wave energy, could benefit from the power take‐off (PTO) method deployed in this small scale model. This method of measuring power could potentially prove to be an inexpensive method of determining average and rated powers for any scale of device. SeaPower intend to employ a similar method of measuring power for the larger “¼ scale WEC” device planned. In order to measure power from the SeaPower Platform, a novel and simple PTO system was deployed on board the model consisting of an adjustable linear brake dynamometer. A stepper motor connected to a calliper brake was controlled from the control room via Ethernet cable. A constant force also known as stepped Coulomb Damping (Falcao, 2007) was applied in each sea state with the magnitude of this damping load applied to the WEC being known in advance due to a numerical simulation. It was found during testing however that there were differences of about +‐20% in the ideal damping torque required for obtaining best average power. Many of the runs were repeated in order to find the optimum Coulomb damping torque for the system. The control strategy for such a PTO system would be far easier to implement than a linear damping strategy. Monochromatic test runs in different wave heights allowed the damping torque to be found per metre wave height. This method of applying damping settings and measuring power out from a load cell and position sensor was the first time such a PTO was attempted within the company. It proved to be very successful method of measuring average mechanical power in each sea state and estimating rated power for Hsdesign3. Previous attempts at measuring power involved using a linear damper valve which consisted of a hydraulic ram and adjustable valve along with load and position sensor. Comparing results with previous tests showed that there was no discernible difference between linear damping and Coulomb damping when it came to average powers as predicted. (Falcao, 2007). Power RAO graphs plotted were obtained by using this Power take off system / mechanical dynamometer with the mechanical power being measured in each case and in each configuration.

3.2 KEYLESSONSLEARNED

• A thorough mechanical, electrical and data logging dry test (shakedown) of the model before the arrival, deployment and recovery of the model is very helpful.

• Wave slamming was observed to occur on the wall of the pontoon facing the oncoming waves. These slamming loads could be avoided if that face was tapered into a convex shape which would also lower drag forces and hence cost when towing the WEC out to a deployment site.

3 Hs,design. Maximum significant wave height [metres] that the PTO is operational. Rated Power would occur here.


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• A shallow tank maybe suitable for testing deep water WEC models as the results are similar to deeper water with some evidence of degradation in power arguably due to the shallowness and therefore floor friction present.

• Adjustment of the PTO damping settings to achieve optimum power output proved challenging but possible. It was necessary to repeat some test runs to arrive at the optimum levels.

• In the extreme short period and high wave height waves, the mooring lines and anchors seemed to be under the highest loading conditions with snatching observed. The design of the moorings for a larger device will be challenging. Although the mooring lines and anchors showed a great deal of endurance at this scale, there was some movement observed and resetting of the anchors in the shorter period waves.

• The safe/survival mode for the machine may indeed be when the bodies are unlocked. This is because the largest relative angles observed and measured were below the design threshold even in the largest waves. The PTO strokes were never exceeded although the largest wave produced was 150mm which is equivalent to 5.4 metres at very large scale. Larger waves exist so although the machine definitely does not require end stops, a different PTO type with no end stops such as rotational PTOs would be beneficial.

• Where motion response is best, lower torque values are required for power capture. • Lower magnitudes of WEC hinge torque result from the Coulomb force PTO damping strategy tested here,

compared with higher values of torque requirement from a linear damping strategy tested previously4 • The WEC configuration has slightly changed from the previous baseline model. The WEC is now shorter at 1x

scale, with an overall length of 3.35m for the approximately the same amount of power. This means that the Cost of Energy has been improved.

• The next phase would be to build a larger “¼ scale machine” for a suitable ¼ scale test site off shore. (scale=5x). The overall length of such a SeaPower Platform would be 16.75m long for such a machine and the power could be measured using a similar dynamometer brake over a full year of testing to confirm the powers measured at smaller scale. The torques and velocities could be scaled according to Froude scaling laws to size the PTO adequately.

4 FURTHERINFORMATION4.1 SCIENTIFICPUBLICATIONSList of any scientific publications made (already or planned) as a result of this work:

None

4.2 WEBSITE&SOCIALMEDIAwww.seapower.ie Please see SeaPower Platforms YouTube channel for videos of this testing campaign in Portaferry.

5 REFERENCESFalaco, A. d. (2007). Modelling and Control of an Oscillating . Marine Renewable Energy Guide. (2013). Tank Testing of Wave Energy Conversion Systems. EMEC. Retrieved from

EMEC.

4 in HMRC 2012


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6 APPENDICES6.1 STAGEDEVELOPMENTSUMMARYTABLEThe 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 ANYOTHERAPPENDICES

6.2.1 MaximumTorquesandVelocities

Figure No PTO damping (survival mode). Largest rotational velocities and swept angles

Figure PTO locked Largest PTO torques measured per metre Hs wave height

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

0.00.10.20.30.40.50.60.70.80.91.0

1.00 1.50 2.00 2.50

Hinge Rel. V

elocity

rad

s‐1/m

Hs

Tp [s]

Panchromatic Waves ‐ No PTO DampingMax velocities per metre Hs

Mean Hinge Rotational VelocityMax Hinge Swept Angle

0

50

100

150

200

250

300

1.00 1.20 1.40 1.60 1.80 2.00 2.20 2.40 2.60

Hinge Torqu

e Nm/m

Hs

Tp [s]

Panchromatic Waves ‐Maximum Torques per metre HsPTO locked up

Peak Torque about Hinge


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6.2.2 Dimensionsofbestconfiguration

6.2.3 Variationofwavedirection–plottedonsamegraph

Figure 20 Impact of Wave Directionality on Power Output

Figure Dimensions of Best Configuration. Boxed dimensions were altered during the test

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