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Infrastructure Access Report

Infrastructure: UCC-HMRC Ocean Wave Basin

User-Project: OWEL Marinet Testing UKP0619

OWEL Marine Demonstrator

IT Power Ltd. / Offshore Wave Energy Limited

Marine Renewables Infrastructure Network

Status: Final Version: 01 Date: 12-Nov-2012

EC FP7 Capacities: Research Infrastructures Grant Agreement No: 262552, MARINET

OWEL Marinet Testing UKP0619

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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/


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DOCUMENT INFORMATION Title OWEL Marine Demonstrator

Distribution Public

Document Reference MARINET-TA1-OWEL Marinet Testing UKP0619

User-Group Leader, Lead Author

Dr Ned Minns OWEL

User-Group Members, Contributing Authors

Mark Leybourne ITP

Infrastructure Accessed: UCC-HMRC Ocean Wave Basin

Infrastructure Manager (or Main Contact)

Brian Holmes

REVISION HISTORY Rev. Date Description Prepared by

(Name) Approved By Infrastructure

Manager

Status (Draft/Final)

01 12/11/12 Issued M. Leybourne N. Minns 1.0


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

progress the state-of-the-art

publicise resulting progress made for the technology/industry

provide evidence of progress made along the Structured Development Plan

provide due diligence material for potential future investment and financing

share lessons learned

avoid potential future replication by others

provide opportunities for future collaboration

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

ACKNOWLEDGEMENT The work described in this publication has received support from the European Community - Research Infrastructure Action under the FP7 “Capacities” Specific Programme through grant agreement number 262552, MaRINET.

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 OWEL is a deep water, floating wave energy converter that uses wave crests to compress a volume of air and drive a uni-directional air turbine. The concept has been under development for approximately 10 years and has undergone a number of phases of research encompassing physical and numerical modelling at a variety of scales. In the current phase of development, IT Power are leading a project to design and deploy a marine demonstration device, part funded by a £2.5m grant from the Technology Strategy Board (TSB). A consortium of nine companies has been formed to deliver the project and provide all of the necessary knowledge and expertise. Initial, small scale tank testing of the demonstrator design had been carried out prior to the Marinet project in a towing tank at the University of Southampton. This had proved the design and tested a number of geometry and buoyancy configurations to maximise the power capture. The testing was however, limited to simplified, regular wave conditions and conducted in a relatively narrow facility. This therefore necessitated the testing of the device in more realistic wave conditions with a configuration that closely represented that of the large scale demonstrator being designed for deployment at the Wave Hub test site. The testing at the HMRC Ocean Wave Basin proved the design of the marine demonstrator to efficiently convert wave energy to pneumatic power in realistic conditions, based on those of the Wave Hub site. Statistical analysis of the long-term wave data for the site was used to predict likely extreme waves and these were included in the testing. Motions and mooring loads were recorded in the scaled extreme seas and used to validate a 3 degree of freedom CFD model. Furthermore, the results from these tests have directly influenced the design of the large scale demonstrator, particularly by confirming the naval architecture, providing air flow data for the PTO specification, informing the control requirements and presenting the power conversion characteristics.


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CONTENTS

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

1.1 INTRODUCTION .................................................................................................................................................... 7 1.2 DEVELOPMENT SO FAR .......................................................................................................................................... 8 1.2.1 Stage Gate Progress .................................................................................................................................... 8 1.2.2 Previous Testing Phases .............................................................................................................................. 9

2 OUTLINE OF WORK CARRIED OUT ................................................................................................................. 10

2.1 AIMS ................................................................................................................................................................ 10 2.2 SETUP ............................................................................................................................................................... 10 2.3 TESTS ............................................................................................................................................................... 11 2.3.1 Test Conditions .......................................................................................................................................... 11 2.3.2 Test Plan .................................................................................................................................................... 15

2.4 RESULTS ............................................................................................................................................................ 16 2.4.1 Performance .............................................................................................................................................. 16 2.4.2 Motions ..................................................................................................................................................... 18 2.4.3 Mooring loads ........................................................................................................................................... 19

2.5 CONCLUSIONS .................................................................................................................................................... 20

3 MAIN LEARNING OUTCOMES ....................................................................................................................... 21

3.1 PROGRESS MADE ............................................................................................................................................... 21 3.1.1 For This Technology ................................................................................................................................... 21 3.1.2 For Marine Renewable Energy Industry .................................................................................................... 21

3.2 KEY LESSONS LEARNED ........................................................................................................................................ 22

4 FURTHER INFORMATION .............................................................................................................................. 22

4.1 SCIENTIFIC PUBLICATIONS .................................................................................................................................... 22 4.2 WEBSITE & SOCIAL MEDIA ................................................................................................................................... 22

5 REFERENCES ................................................................................................................................................ 22


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1 INTRODUCTION & BACKGROUND

1.1 INTRODUCTION Offshore Wave Energy Limited, (OWEL) began developing a new wave energy conversion concept, named OWEL, in 2001. It has novel method of operation and as such, is intended to overcome the some of the disadvantages inherent in other devices and to provide an efficient and economic method for the conversion of wave energy. OWEL is a floating, moored device that uses incident, deep water waves to compress air and drive an air turbine. The schematic in Figure 1.1 shows the operating principle of the device. As waves enter into the device, the floor, which ramps upwards, induces a rise in wave height which causes the wave crests to create a seal with the roof and trap a volume of air ahead of the wave. As the wave progresses along inside the duct it forces the trapped air out of the duct and through a power take-off (PTO) comprising a uni-directional air turbine connected to a generator.

Figure 1.1, A schematic of the OWEL operating principle.

A grant was awarded by the UK’s Technology Strategy Board (TSB) in 2011 to design, construct and deploy a large scale, marine demonstration device and advance the commercialisation of OWEL. The initial design of the demonstrator was based upon previous, small scale physical modelling studies carried out in idealised conditions. Access to HMRC wave basin by the MaRINET project enabled the optimised design to be investigated in realistic conditions in order to confirm its suitability for the marine demonstrator. An artist’s impression of the configuration of the large scale demonstrator is shown in Figure 1.2.

Figure 1.2, An artist's impression of the large scale, demonstration device.


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1.2 DEVELOPMENT SO FAR

1.2.1 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

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 - The TSB project is Stage 4

Hull seaworthiness and survival strategies

Mooring and cable connection issues, including failure modes

PTO performance and reliability


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STAGE GATE CRITERIA Status

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

1.2.2 Previous Testing Phases

1.2.2.1 Phase 1

- Initial, proof of concept in 2001 by QinetiQ at the Southampton Solent towing tank, tested a number of different geometry arrangements and identified the most suitable design to develop. - 2D, narrow wave flume testing in 2008 at the University of Southampton (UoS). - 3D, wave basin tests at HMRC, UCC in 2009 with a multi-duct model. - Further, single duct testing in 2011 at the UoS for the optimisation of the design and to inform the design of the large scale, single duct marine demonstrator.

1.2.2.2 Phase 2

Intermediate scale, ~1:4, testing at Narec in 2005 with a single duct measuring 12m in length. This investigated scaling parameters and showed that the concept is viable at large scale.

The 2011 physical modelling investigations at the UoS focused on the further refinement of the geometry and buoyancy/ballast arrangement of the single duct configuration. The facility used however, had significant limitations; only regular waves were achievable, it was relatively narrow, had limited motion capture capabilities and fairly poor quality waves. That said, the performance of the floating, single duct had been significantly improved over that from the baseline design and peak pneumatic efficiencies exceeding 60% were recorded. It was therefore, deemed both necessary and prudent to re-confirm the new design in realistic conditions before progressing with the remainder of the development of the large scale demonstrator.

PHASE 1 – Small Scale, Physical Modelling (~1:50) PHASE 2 – 1:4 Scale Testing

Figure 1.3, Example images taken from some of the physical modelling investigations that have been carried out at Phase 1 and 2.


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2 OUTLINE OF WORK CARRIED OUT

2.1 AIMS The primary objective of this testing was to finalise the design of the marine demonstrator to be deployed at Wave Hub with the key aims to: - Confirm the naval architecture – buoyancy and ballast distribution - Investigate the effects of device trim on performance to inform the control systems design. - Assess the mooring forces and motions in scaled, extreme wave conditions. - Provide data to validate the theoretical models being used to design the demonstrator. - Make estimations of performance and mooring loads in off-axis waves and directional sea states. - Predict likely full scale output at the Wave Hub site based on detailed power matrices from site specific spectra tests.

2.2 SETUP The model geometry that was tested was a 1:50 scale representation of the marine demonstrator and is summarised in the drawings in Figure 2.1. The vertical pipe at the stern of the duct contained an orifice acting as a turbine PTO simulator. This was raised above the water to prevent water ingress into the pressure sensors that were connected across it to measure the air flow. The buoyancy and ballast arrangement was based on the conclusions from previous physical modelling studies at the UoS. This was refined during the preliminary design work of the current TSB project to determine a suitable naval architecture design that would be achievable at large scale. The moorings, specified by project partner Mojo Maritime, were also scaled down from the preliminary specifications for the demonstrator and comprised three catenaries made from thin, metal chain as shown in Figure 2.2.

Figure 2.1, CAD drawings of the 1:50 scale model, including the buoyancy sections (all dimensions in mm).


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Figure 2.2, An overview of the mooring arrangement with dimensions of the principal lengths.

2.3 TESTS

2.3.1 Test Conditions The wave conditions were determined in order to meet the objectives set out for the testing. The majority of the waves run were irregular, uni-directional sea states intended to represent the conditions at deep water wave sites. These were used to evaluate the performance in a realistic environment and assess the behaviour of the device in extreme conditions. The long term wave statistics for the Wave Hub site were analysed from both measured data, from a Waverider buoy deployed at the site, and modelled data, from the Met Office’s UK waters wave model at the U04 calculation point. The scaled distribution of sea state occurrences from both data sets is plotted in Figure 2.3 and shows that the majority of the seas are associated with waves with a steepness of 1:40. This distribution was used to determine the range of irregular wave test conditions intended to examine the performance characteristics that could be expected at the site. In order to determine the extreme “load case” conditions that a demonstrator would be required to withstand, the statistical maximum wave that could occur in each sea state was calculated and the 50 year return wave calculated by Smith et al. (2011) was incorporated. The conditions for each three hourly sample can be characterised by a value of the significant wave height, Hs. This gives an indication of the predominant wave height but does not state the maximum wave height, Hmax. As a conservative rule of thumb, Hmax can be up to 2*Hs, but this is not always the case. However, a statistical relationship can be used to determine the maximum expected wave height, Hmax, occurring during an event lasting time, T (seconds) with a significant wave height Hs, and average zero crossing period Tz

Hmax = kHs[½ln(N)]½, where N = T/Tz.

Tucker and Pitt (2001) recommend k = 0.9 for the maximum wave and k = 1.0 for more frequent waves (fatigue waves).The period, Tmax, that is associated with the maximum wave height Hmax, can take a range of values. Tucker and Pitt suggest that the period can be between 1.05Tz < Tmax < 1.4Tz. Both extremes of wave period were considered when determining the extreme load cases:

- the minimum period, Tmax =1.05Tz, was associated with Hmax to evaluate the steepest 50 year wave. - the maximum period, Tmax = 1.4Tz, was associated with Hmax to find the most extreme surge 50 year wave.


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The resulting extreme waves in each sea state from this analysis with the minimum associated wave periods are plotted in Figure 2.4. The load case conditions are denoted by green markers and were chosen for their steepness and height. It was ensured that these wave events occurred within the Bretschneider spectra that were run in the basin.

Figure 2.3, The distribution of wave occurrence at the Wave Hub site taken from recorded wave buoy data and the Met Office's wave model.

Figure 2.4, The statistical extreme wave events in each sea state from the wave buoy and Met Office model data, with the load cases identified by the green markers.


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The forty irregular sea states that were selected for testing are shown in Figure 2.5. Time was available to measure 24 of the conditions with an empty tank and undisturbed wave field to determine the actual characteristics of each sea state. The height and periods of the remaining 16 seas were predicted based upon calibration curves of input and measured data from the facility.

Figure 2.5, The irregular wave conditions that were used during the tests, with the blue markers denoting the conditions that were measured as undisturbed wave fields in the empty basin. The red conditions were predicted from calibration curves.

In addition to the irregular waves, regular monochromatic waves were also used in the testing to study the fundamental characteristics of the design, in particular the floating dynamic response to single frequency waves. The regular wave conditions chosen were intended to be at the same as the irregular seas but were limited by the idiosyncrasies of the wave generation system. Once again, time was available to measure some but not all conditions as shown in Figure 2.6.

Figure 2.6, The regular wave conditions that were used during the tests, with the blue markers denoting the conditions that were measured as undisturbed wave fields in the empty basin. The red conditions were predicted from calibration curves.


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Further analysis of the characteristic conditions at Wave Hub was carried out to investigate the types and directionality of the sea states. The spectral shapes, particularly for the lower energy seas are not well represented by a classic distribution such as Bretschneider. In many of the low energy seas, the spectra comprised two wave components; a locally generated wind sea and a long distance, storm swell. These conditions are known as bi-modal seas as they have two spectral peaks at different wave periods as shown in the example by Smith et al. (2012) in Figure 2.7. Summary sea state statistics, Hs and Ts, neglect the spectral shape and can be misleading as the peak period may be different to that of either of the twin peaks. It was intended to investigate the performance of the model with some site specific spectral profiles however, to calibrate such a specific and complicated condition would have required a significant amount of time. It was therefore decided to only use standard Bretschneider spectra [Bretschneider (1959)] which are suitable for representing more energetic seas. This meant that the lower energy seas of Wave Hub may not have been well represented in the tank however, seas with higher energy are likely to have been approximated well. Conditions with short wave periods and low heights contribute only a small percentage of the available energy per annum and it is therefore not too critical if the predicted performance in these waves is different to what it would actually be. Some, short crested, irregular wave tests were run to assess the effect of directional spread on the conversion performance of the device. Figure 2.8 is a plot of the mean directional spread values over the period January 2005 – October 2005 recorded by a buoy deployed at the Wave Hub site. This shows that a spread of 45° provides a good approximation of typical, average conditions at the site. The spreading function used by the wave generation system was based on that originally proposed by Longuet-Higgins et al. (1963). This features the term 𝑐𝑜𝑠2𝑠𝜃 to provide a distributed weighting to the directionality of the sea state. s = 8 was used to provide a directional spread of +/- 45°.

Figure 2.7, Example sea state spectra for the Wave Hub site, from Smith et al. (2012).

Figure 2.8, The variation in average directional spreading in sea states at the Wave Hub site from January to October, from Smith et al. (2012).

The prevailing wave direction at the Wave Hub site was also considered as this was deemed to have the potential to affect both the performance and station keeping by the three point mooring system. It had been anticipated that the device would ‘weathervane’ to face the predominant direction of wave propagation. Analysis of the two data sets to find the mean wave directions, shown in Figure 2.9, indicated that waves typically approached from a Westerly direction and with the majority of the spread being with +/- 10° of that.


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Waverider data Met Office data

Figure 2.9, Wave roses for the Wave Hub site showing the distribution of wave direction and height for the measured Waverider buoy data and Met Office model data.

2.3.2 Test Plan The test plan, shown in Figure 2.10, evolved as the investigations progressed. The model set up and measurement of the undisturbed wave field for the specified test cases took the majority of the first week of allotted time. This therefore reduced the amount of testing time available and so the number of conditions for each test was reduced accordingly.

Figure 2.10, The test plan for the 10 days of testing, including the additional, desirable tests.

Significant Wave Height, Hs

Significant Wave Height, Hs


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Tests 1-12 were critical and had to be completed in order that the test time produced a useful output. Tests 13-15 were “like-to-have” to give information for validating our numerical models and to give information on operational modes. Tests 16-19 were included in the list to be carried out if time allowed. By reducing the number of waves in each test, tests 1-15 and 17 were completed in the test period. In addition, a number of alternative naval architectural designs were tested and gave valuable insight into the critical motions of the machine.

2.4 RESULTS

2.4.1 Performance The first tests carried out were to investigate the relationship between damping and the conversion efficiency in order to determine the optimum orifice size to maximise power capture. Prior to the tests, 4 orifices, with diameters of 17, 20, 22 and 24 mm were calibrated to find their discharge coefficients. The model was then tested across a range of sea states with each of the orifices installed. Figure 2.11 shows the results for three of the orifices and that the 24 mm diameter orifice generated the highest efficiencies although the 22 mm orifice was slightly better for T < 0.75 sec. Whilst varying the orifice size to ensure a smaller diameter in shorter wave periods could have been slightly beneficial in subsequent tests, a 24 mm orifice was used throughout the testing in order to reduce the time spent modifying the model between runs.

Figure 2.11, The effect of orifice diameter - flow damping, on the conversion performance.

In order to efficiently capture the incident wave crests, the height between the water surface and underside of the duct roof at the bow should be optimally set. Varying this duct entry height alters the trim of the duct and so the amount of air volume within that can be compressed. Previous testing in regular waves suggested that a larger bow up trim in shorter wave periods and lower trim for longer waves was beneficial. However, in these tests, in irregular seas, a trim with 25 mm freeboard at the bow maximised the conversion efficiency over the majority of conditions as shown in Figure 2.12.


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Figure 2.12, The effect of the model trim on the conversion performance across a range of sea states.

With a 24 mm orifice and 25 mm bow up trim, the model was tested in 15 irregular sea states. The coutour plots in Figure 2.13 show the conversion efficiency and pneumatic power, normalised by their maximum values, for the significant wave height and peak wave period scaled up to full scale (1:84). This shows the reduction in efficiency with wave periods greater than 10 s and a peak at approximately 8.5 s.

Figure 2.13, The characteristics of conversion efficiency and power for full scale conditions.

Performance characteristics in long crested conditions were then compared with directional and off axis seas and regular waves. The performance in irregular, long crested seas was greater than that in regular waves for peak periods, Tp > 1 sec. This was due to the modification of the motions through excitation from different wave frequencies and phases causing a reduction in their magnitude. An average reduction in mean power capture of approximately 10% was observed in directional, short crested seas in comparison to long crested, uni-directional irregular waves. The conversion performance in 10° off-axis waves was slightly reduced for short period conditions (T < 0.9 s), however, the difference was negligible for longer periods. It was observed that the duct had a tendency to passively weathervane to align with the predominant wave direction, particularly for longer wavelength waves.


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2.4.2 Motions

Figure 2.14, Response Amplitude Operators for the 6 degrees of freedom across the range of regular wave periods tested.

The model was tested in 40 regular wave cases and both the performance and motions recorded for each. The Response Amplitude Operators (RAOs), which are the average motion response normalised by the wave height, for each of the 6 degrees of freedom were calculated and then averaged for each wave period tested. The resulting response curves are shown in Figure 2.14 and show the relationship between the wave period and the translation and rotation behaviour of the model. Although not obvious on the plot, due to the scales, the heave response peaked at T = 1.3 s and was maintained at a high level for periods greater than that. This was a result of the “contour following” nature of the hull that tends to occur for wavelengths greater than twice of the duct length. It has been seen in other testing, when comparing static and floating models, that this characteristic is detrimental to performance. Despite attempts to reduce the heave of this model, the performance was still reduced by its effect and therefore the reduction of heave should be a design driver for subsequent device configurations. Other notable features include the large peak in susceptibility to surge at T = 0.9 s and the peak in pitch at T = 1 s. An animation based on the wave and motion data gathered was created to visualise in real time, the free surface profile; the motions of the model and the air power developed. This allowed the phases between the forcing of the wave and the response of the model to be investigated. An example of this animation and resulting graphical output is provided in Figure 2.15.


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Figure 2.15, Visualisation of the model motion and power capture through real time animation from the results.

2.4.3 Mooring loads A lightweight, small load cell was attached to the model between the bow connection point and the mooring chain on the port mooring. This provided force data applied to the moorings and an example of which is plotted in Figure 2.16. The four largest load case, sea states identified in Figure 2.4 were run with the model in power production trim and in the survival trim where the duct was lowered so that only the reserve buoyancy on the deck protruded above the waterline. It was found that, contrary to the conceived survival mode strategy, the mooring loads were reduced when the device was in power production mode rather than with the lid resting on the water surface. It is likely that, despite the slightly increased mooring loads, the survival strategy will result in lower structural loads in extreme conditions. This is because the duct roof would be supported by the water below it if the duct were filled with water. This would reduce the loads on the deck due to the weight of the green water (overtopped water on the deck) being supported from beneath and also likely alleviate slamming on the underside of the duct roof. It was identified however, that this requires further investigation with load models such as CFD.

Figure 2.16, An example time series from the sea state that generated the greatest mooring load.


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Summary results for the mooring loads of the four greatest load cases are given in Table 2.1 and have been scaled up according to Froude similitude. Load case #3 had the largest peak mooring loads with #2 also being very similar. This highlights that despite case #3 having the largest wave height, a sea state with a much lower Hmax may also result in mooring loads of a similar magnitude. It is therefore important to test a range of extreme conditions as the greatest loads may arise in waves that are not necessarily the largest as the phase between the device motions and the wave field have a large effect.

Load Case # Tp [s] Hmax [m] Mean Mooring [Tonnes]

1 7.3 9.6 11

2 8.7 11.0 13

3 10.0 14.5 10

4 11.5 14.3 12

Table 2.1 Scaled mean mooring loads for the four maximum load cases.

In addition to testing the original mooring, with the two forward chains attached to the bow, the connection points of the forward moorings were moved a quarter of the duct length aftwards along the duct. This was to investigate the influence of the moorings on the motions and power capture however, it was found that this alternative configuration had little influence on the dynamics or conversion efficiency. The mooring loads and motions of the device with the simple three point, catenary arrangement validated the suitability of the design. This system also enables the duct to weathervane slightly towards to prevailing wave direction which is beneficial to the annual energy output.

2.5 CONCLUSIONS This testing provided necessary learning about the behaviour of the performance of the single duct configuration of the OWEL device. The results and findings have fed directly into the design of the large scale, marine demonstrator. This has led to the progression of the design and provided confidence that the configuration is well suited for deployment at Wave Hub. Furthermore, predictions made from the performance results of the small scale model demonstrate that a commercial scale machine will generate electricity economically. The motion time series results for pitch, heave and surge have been used to adjust and validate a 3 degree of freedom CFD model that was developed to aid the design of the demonstrator. Comparison of the experimental and computational results shows a good degree of agreement as shown by the example in Figure 2.17. The refinement of the CFD model is on going and will be used to predict full scale pressures exerted on the structure in addition to the motions and mooring loads in regular waves.

Figure 2.17, Comparisons between the pitch, heave and surge motions of the CFD (Blue) and Experimental (Red) models.

This report will be made publically available on the OWEL company website in order to provide information to interested parties, disseminate the findings of the testing and provide an account of the lessons learned.


Rev. 01, 12-Nov-2012 of 22

3 MAIN LEARNING OUTCOMES

3.1 PROGRESS MADE

3.1.1 For This Technology The development of OWEL is continuing through the progression of the TSB funded project. Detailed design work of the marine demonstrator is currently on-going with numerical modelling providing loads analysis to inform the structural design. The continuation of the project is dependent on securing investment to co-finance the TSB grant and therefore fund the construction of the demonstrator.

It is anticipated that the successful demonstration of the device at large scale in a marine environment will prove the WEC concept and lead to further investment to continue the development of OWEL with the subsequent phase to deploy a prototype device at a more energetic site. Future concepts of the device will comprise multiple ducts combined on the same floating platform to improve the economics and power output of a single unit, an artist’s impression of which is shown in Figure 3.1.

Figure 3.1, An artist's impression of a proposed, future, multi-duct device with a multi-MW output.

3.1.2 For Marine Renewable Energy Industry A significant amount of testing time was used to provide additional calibrated regular and irregular wave conditions in the HMRC Ocean Basin. Furthermore, the repeatability of a number of test conditions was examined to ensure that the measured characteristics could reliably be reproduced. This work will be of future benefit to those testing at the facility as it provides a range of calibrated input conditions for testing. This report presents the predictions made of the extreme and operational wave conditions at the Wave Hub site and will be of use to other developers interested in deploying at the site. The typical directional spreading within sea states has also been presented with a suggested spreading distribution factor that can be used for input into wave tank wave makers and numerical models. This work has shown that a 3 degree of freedom, rigid body CFD model may be developed and validated by physical results. This has proved the applicability of methodology and capability of the software to usefully model floating WECs where complex flow physics cannot be accurately simulated using other techniques. This may be useful for other WEC developers considering using a CFD approach to model motions and pressures applied on a large scale device.


Rev. 01, 12-Nov-2012 of 22

3.2 KEY LESSONS LEARNED - Confirmed that the naval architecture design for the large scale demonstrator was suitable. - Provided time series air flow data on which the turbine PTO specification could be made. - Motions data has been used in the validation of numerical models to predict structural loading. - Mooring loads have indicated that a three point, catenary system is appropriate for the device and have

been used to validate a numerical model to estimate full scale mooring loads. - The intended survival strategy was found to increase mooring loads rather than reduce them. This strategy

therefore requires further investigation to test its worth, particularly for reducing structural loads. - A single duct entry freeboard was found to maximise the power conversion over the majority of wave

periods tested and so varying the trim to match the sea state may not be as critical as previously thought. - The reduction of heave is necessary to increase power capture and should therefore be a design driver for

future devices. - Identified that trim strategies to benefit power capture were not as essential as previously thought and this

therefore has implications on the requirements for the control system and active ballast.

4 FURTHER INFORMATION

4.1 SCIENTIFIC PUBLICATIONS Presentation of the results and findings of this work and other recent OWEL physical modelling is planned to be disseminated at the European Wave and Tidal Energy Conference (EWTEC) in 2013.

4.2 WEBSITE & SOCIAL MEDIA Website: www.owel.co.uk; www.itpower.co.uk YouTube Link(s): http://www.youtube.com/watch?v=SS73_DDZ5KQ LinkedIn/Twitter/Facebook Links: Online Photographs and Articles: http://www.owel.co.uk/2012/08/27/eu-marinet-testing-%E2%80%93-further-owel-modelling-at-hmrc-in-cork/ http://www.itpower.co.uk/node/245 http://www.fp7-marinet.eu/access_user-projects_OWEL.html http://www.mediacontact.ie/mediahq/uccpress/39103/hydraulics-and-maritime-research-centre-nets-an-owel.html

5 REFERENCES BRETSCHNEIDER, C. L. (1959). Wave Variability and Wave Spectra for Wind-Generated Gravity Waves. Technical

Memorandum No. 118. US Army Corps of Engineers. LONGUET-HIGGINS, M. S., CARTWRIGHT, D. E. & SMITH, N. D. (1963). Observations of the Directional Spectrum of

Sea Waves Using the Motions of a Floating Buoy. Ocean Wave Spectra. Prentice-Hall. SMITH, H. C. M., HAVERSON, D., SMITH, G. H., CORNISH, C. S. & BALDOCK, D. (2011). Assessment of the Wave and

Current Resource at the Wave Hub Site. PRIMaRE Report for Wave Hub, UK. SMITH, H. C. M., PEARCE, C. & MILLAR, D. L. (2012). Further analysis of change in nearshore wave climate due to an

offshore wave farm: An enhanced case study for the Wave Hub site. Renewable Energy, 40, 51-64. TUCKER, M. J. & PITT, E. G. (2001). Waves in Ocean Engineering, Elsevier Science.

http://www.owel.co.uk/

http://www.itpower.co.uk/

http://www.youtube.com/watch?v=SS73_DDZ5KQ

http://www.owel.co.uk/2012/08/27/eu-marinet-testing-%E2%80%93-further-owel-modelling-at-hmrc-in-cork/

http://www.itpower.co.uk/node/245

http://www.fp7-marinet.eu/access_user-projects_OWEL.html

http://www.mediacontact.ie/mediahq/uccpress/39103/hydraulics-and-maritime-research-centre-nets-an-owel.html

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