OCEANS of CLEAN RENEWABLE ENERGY; (Stage 1)

Transcription

1 Marine Renewables Infrastructure Network Infrastructure Access Report Infrastructure: Hydraulics & Maritime Research Centre User-Project: O.C.R.E. OCEANS of CLEAN RENEWABLE ENERGY; (Stage ) OCRE Energy Ltd. Status: Final Version: Date: March 24 EC FP7 Capacities Specific Programme Research Infrastructure Action

2 Infrastructure Access Report: O.C.R.E. 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 25. The network of 29 partners with 42 specialist marine research facilities is spread across EU countries and 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 7 weeks of access is available to an estimated 3 projects and 8 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 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) 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) Belgium -Tech (_TECH) Rev. [], [March 24] Page 2 of 24

3 DOCUMENT INFORMATION Title Distribution Document Reference User-Group Leader, Lead Author User-Group Members, Contributing Authors Infrastructure Accessed: Infrastructure Manager (or Main Contact) Ocean of Clean Renewable Energy Public MARINET-TAOCRE Brian SHAUGHNESSY Brian HOLMES OCRE Energy Ltd Brian SHAUGHNESSY OCRE Kerry GORMAN OCRE Seamus COUGHLAN OCRE William O BRIEN OCRE Mark COUGHLAN OCRE Brian HOLMES Independent Adviser Hydraulics & Maritime Research Centre, University College Cork, Ireland. Florent THIEBAUT REVISION HISTORY Rev. Date Description Prepared by (Name) Approved By Infrastructure Manager Status (Draft/Fina l) March 24 Technical report Brian HOLMES Final Page 3 of 24

4 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. Page 4 of 24

5 EXECUTIVE SUMMARY This technical investigation concerned the physical model testing of a wave energy converter (WEC) designed to extract energy in the three rotational modes of motion, pitch, roll and yaw. The device is an attenuator type principle based on a segmented hull with longitudinal hinge connections enabling one degree of freedom per joint. There are nine hull segments configured to allow two pitch, three roll and three yaw power take-off connections. The OCRE is a shallow draft, surface floating WEC conceived to operate in an array in intermediate depth, near-shore waters providing significant levels of power to the neighbouring electrical grid. To achieve this overriding objective a solo unit was expected to produce over MW in operational type seas. The O.C.R.E. wave energy converter Testing was conducted in the ocean basin at HMRC, University Collage Cork so all types of waves could be experienced during the evaluation process, that is; monochromatic waves and long and short crested panchromatic seaways. During the configuration optimisation trials the device design variables investigated were; the power takeoff (PTO) damping and the mass distribution of the water ballast within the subdivided hull compartments. An interesting conclusion from the test programme was that in short crested seas the roll PTOs contribution brought the overall power conversion back to the same level as that produced by the pitch only in long crested waves. Page 5 of 24

6 CONTENTS INTRODUCTION & BACKGROUND...7. INTRODUCTION DEVELOPMENT SO FAR Stage Gate Progress Plan For This Access OUTLINE OF WORK CARRIED OUT SETUP TESTS Test Plan RESULTS ANALYSIS & CONCLUSIONS MAIN LEARNING OUTCOMES PROGRESS MADE Progress Made: For This User-Group or Technology Progress Made: For Marine Renewable Energy Industry KEY LESSONS LEARNED FURTHER INFORMATION SCIENTIFIC PUBLICATIONS WEBSITE & SOCIAL MEDIA REFERENCES APPENDICES STAGE DEVELOPMENT SUMMARY TABLE ANY OTHER APPENDICES Page 6 of 24

7 INTRODUCTION & BACKGROUND. INTRODUCTION Many independent wave energy converter developers have differing views for what constitutes the best possible specification for an operational device. These design choices include the optimal rated output for a WEC, from as small as a few kilowatts (circa 5kW) to over a Megawatt per device. For economical reasons some developers are attempting to combine the smaller, individual devices to increase the electrical output from one cluster of units. The deployment sites also vary, from onshore (often attached to an artificial structure), through the inshore (<2m water depth, possibly bottom standing)) to the nearshore (circa 5-5m water depth and moored). The OCRE design team have identified that buoyant, moored, large output devices, greater than MW, located in the nearshore zone are the most appropriate to suppling the significant amount of electricity that governments, utilities and power companies are basing economic viability on. To realise large scale energy production the OCRE device design statement specifies a shallow draft floating unit with a rated output between -2MW. To achieve this a 9 segment attenuator line array, capable of extracting wave energy from the 3 rotational degrees of freedom (DoF), pitch, roll and yaw, was specified. The pitch and roll periods should be in the 5-7 seconds range to suit the commonly occurring wave periods for open ocean deployment stations. A lightship hull can be ballasted, as required, with sea water such that a selected sea state tuning facility would be available. The 9 segments are combined by the power take-off systems as shown in Figure. This results in 2 pitch sections, 3 roll sections and 3 yaw sections. The configuration, fundamentally in groups of 3 segments, is not, however, symmetrical along the longitudinal axis of the vessel. DIRECTION OF WAVES 6mm 6mm 6mm 6mm 6mm 6mm 6mm 6mm 6mm YAW (Bow) ROLL (Bow) PITCH (Bow) YAW (Mid) ROLL (Mid) PITCH YAW (Mid/Stern) (Stern) ROLL (Stern) Figure.: Basic PTO Configuration of the O.C.R.E. A standard 3 line catenary mooring supported by weight bearing outlying buoys with light surface tethers to the WEC is proposed for station keeping and directional heading control. A mathematical model of the device was constructed by an external consultant company and used to investigate the fundamental operation and performance of the OCRE. This model adopted a frequency domain approach and was constrained to pitch motion only. The excitation forces were restricted to monochromatic waves. The output from the mathematical model guided on the initial device and segment dimensions. From these theoretical results an idealised physical model was manufactured and deployed in an outdoor lake to observe and film the typical motions when under attack by real waves. A 3 day private contract was then undertaken with the Hydraulics and Maritime Research Centre at University Collage Cork for a more structured investigation and defendable proof of concept validation trials. The power take-off simulator was a friction brake type damper. The test programme consisted of excitation by both monochromatic waves, to produce the device response amplitude operators (RAOs), and panchromatic seaways, to generate the power matrix with reference to Bretschneider spectra seaways. Page 7 of 24

8 Following encouraging results from both the mathematical and physical models an application for a more extensive test programme was made to MaRINET..2 DEVELOPMENT SO FAR.2. Stage Gate Progress Previously completed: Planned for this project: STAGE GATE CRITERIA Stage Concept Validation Linear monochromatic waves to validate or calibrate numerical models of the system (25 waves) Finite monochromatic waves to include higher order effects (25 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 2-3 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. Status Page 8 of 24

9 STAGE GATE CRITERIA 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 Status.2.2 Plan For This Access A schematic of the test plan drawn up for this Stage device optimisation and performance programme is shown in Figure 2. The schedule is divided into three fundamental branches; mass distribution; hull profile and mooring, that would be tackled in separate access units. An evaluation period was allowed for between the sections to accommodate model modification and programme adjustment if required. Figure.2: Stage Test Plan Page 9 of 24

10 Based on the number of wave conditions and device configurations outlined in the test plan the time required for this stage of the development was estimated to be 4 weeks. This included set-up, with sensor and data acquisition system calibration. Standard HMRC wave conditions and seaways were used so no additional time was required for basin calibration. The mooring trials were reserve tests that could be omitted if more time than scheduled was required to complete the two main investigations. The graphs presented in this report are in model scale, unless otherwise stated Objectives The first test set re-investigates and extends the limited previously obtained empirical data. It had been evident from the earlier trials that the friction brake power take-off simulator had not functioned satisfactorily so they would be replaced. The problem was a mechanical difficulty to set the damping accurately which resulted in three related sources of error: Firstly, the absolute required value for the damping (Ns/m) had proven difficult to set for an individual PTO; Secondly, it had not been possible to set each individual PTOs to the same value for selected tests; Thirdly, because the brake was located in the splash zone water on the friction pads lead to changes in the damping during, and between, test runs. The second set of trials involved changes to the design that might encourage better performance by enabling improved hull motions and excitation forces. 2 OUTLINE OF WORK CARRIED OUT 2. SETUP The OCRE testing was conducted in the HMRC Ocean Basin. This is a 25*8* metre wave tank capable of producing single period, monochromatic waves and selected multi-period, panchromatic seaways of any chosen spectral mix. In this instant the standard Bretschneider profile was used. Long and short crested seas can be produced on demand. The workable wave range is between.5 seconds and 2.5 seconds with a maximum height of 25mm. Combined with the m water depth this suggested an open ocean site equivalent scale of λ= :5. This equates to a full size wave climate of approximately 3.5 to 7.5 seconds periods and 8 meters significant height with irregular wave. These sea states can be regarded as the production range of the OCRE. High energy survival seaways would not be included at this stage. The OCRE model was designed accordingly. 2.. Model Re-fit and Sensors Before the test programme commenced three main model modifications were undertaken: The PTO simulators were replaced; The internal hull bulk heads were strengthened; The load cells were replaced by more sensitive sensors. The PTO simulator; the friction brake was replaced by an Airpot Airpel anti-stiction air cylinder. These are usually employed as very low friction pneumatic actuators so in this instant they are used in the reverse direction. The wave loading force acting on each 3 segment sector oscillates the associated Airpel drive rod and an air outlet valve (tap) can be adjusted to apply varying levels of damping on the piston motion. A Futek load cell is attached between one end of the actuator and the hull to monitor the force exerted by the OCRE. This configuration, showing each of the 3 motion modes, is shown in Figure 3. The relative motion between any pair of segments is measured Page of 24

11 with a Gill non-contact Blade 25 position transducer, as shown in Figure 4. The blade, shown in the diagram, passes through a fixed U shaped activator such that the linear position of the pair is recorded. A special calibration rig is supplied by the manufacturer to convert the position to an angular record. Internal bulkheads; During the first series of trials water leaked between internal chambers to make adjustments of the water ballasting difficult. OCRE undertook to shore up the offending bulkheads to ensure the test investigating the changes to the roll moments of inertia would be more successful. Although the model had been constructed from transparent polycarbonate it had then been painted so it was not possible to observe any leaks visually. Figure.3: PTO Simulators Figure.4 Motion Sensor The configuration of the internal chambers is shown in Figure 5. All chambers are the same volume and can be filled independently. Each model segment had 4 vertical transverse bulkheads and one horizontal sub-deck spanning the full breadth. This resulted in 8 chambers per segment, 4 upper and 4 lower. 6mm WATER IN 23mm WATERTIGHT CHAMBER Figure.5: Bulkhead Arrangement Page of 24

12 Load cell replacement: The original Futek LCM 2 load cells had a range of ±,lbf. Although these sensors worked the resulting time series of the loads was somewhat noisy. Based on the maximum loads measure in the first test series they were replaced by the same unit series but with a range of ±25lbf (±56N). Although the manufacturer s stated the sensors were splash/light immersion proof experience had show this was not always the case. Prior to attachment to the model the connection of the data transfer cable to the sensor was encased in shrink wrap tubing Model Deployment HMRC have pre-calibrated a set of standard regular and irregular waves in the Ocean Basin at a fixed station approximately halfway between the wave generation paddles and the start of the energy absorption beach slope, in the deep section of the tank. The OCRE WEC was located there using the Centre s standard catenary mooring. [because of the size of the model it was later discovered that the buoyancy of the surface weight bearing floats had to be increased due to the wave loading on the bow]. A photograph of the device on station is shown in Figure 6. Figure.6: OCRE on Station For handling purposes the model was launched as a lightship and ballasted with water on station, as shown in Figure 7. Although only the power take of simulator force and motion were being monitored this still resulted in 6 sensor connections and an extensive wiring harness, as shown in Figure 8. A rubber bungie was used to hold the weight of the data cables whilst having minimal effect on the movement of the model. Figure.7: Ballasting Model Figure.8: Data Cable & Harness Page 2 of 24

13 Transvers Motion; Y axis (mm) Transvers Motion; Y axis (mm) Infrastructure Access Report: [Insert the User-Project acronym] 2..3 Model Verification The configuration of the model did not allow for the usual dry rig approach for validation of the model physical properties, especially mass distribution and damping levels, so the secondary wet tests were conducted as the primary verification. These are in the form of free of forced oscillations to measure the response of the model against the design specification Natural Periods Because there is a restoring force on the device hull for pitch and roll, free oscillation tests reveal the natural periods under investigation. This was done for each 3 segment set related to one PTO connection. The applied damping was set to zero. It was known that the flat keel hull would have heavily natural (hydrodynamic) damping so the Centre s more sensitive Qualisys motion monitoring system was used. This is an active infra-red digital camera instrument that measures the position of passive reflective markers. If these are attached to a body its movement are tracked. The following time history examples are the results for the roll motion. The hull is given an initial displacement (Y axis) and then released such that it returns to rest under natural action (X axis). The resonant (Eigen) period can then be measured from the free oscillations recorded. The trace on the left (Figure 9) shows the response of the model when the ballasting is evenly distributed in all the lower chambers. A natural period of oscillation of.9 seconds is recorded. The trace on the right (Figure ) shows the response when the water ballast is distributed as shown in Figure. A natural period of oscillation of.2 seconds is recorded. It should be noted that the trace is from the segment group with the water in the outer chambers of hull. This will increase the polar moment of inertia and hence the resonant period. 94 FREE ROLL OSCILLATIONS: Even Ballast; Flat Keel 9 FREE ROLL OSCILLATIONS: Roll Ballast; Flat Keel Roll Period =.9sec. 88 Roll Period =.2sec Time (Seconds) Figure.9: Time History; Even Ballast; Flat Keel Time (Seconds) Figure.: Time History; Roll Ballast; Flat Keel Also indicated on these time traces in the natural, hydrodynamic, damping of the system. The actual value is obtained from the ratio of adjacent peaks in the time trace, also referred to as the exponential decay. The primary cause of this reduction in motion is the wave making of the hull which must extracts energy from the system to create the waves. The level of damping can be influenced by the quality of the model construction. Sharpe edges can induce vortex shedding which is also an energy loss. This is proportional to the velocity the body moves through the fluid and leads to the second order motion effects described later in this report. These losses are often exaggerated in small models relative to a full size devise so care must be taken when interrogating and interpretation the results. Page 3 of 24

14 KEY: Upper and lower chambers empty Upper and lower chambers full Figure.: Water Arrangement for Roll Ballast Configuration Applied Damping The ability to set the PTO damping accurately and consistently was checked by manually oscillating the model sections and also by wave activation. The results for this are shown in Figures 2, 3, 4 & 5 for sea state B22. Figure.2: Initial PTO Damping Figure.3: Optimal PTO Damping Figure.4: Low PTO Damping Figure.5: High PTO Damping Page 4 of 24

15 For reasons explained in the Analysis section the pitch motion is used for this demonstration, together with a high sea state of Tp = 4 seconds & Hs = 6m. This was to extend the length of the damping line and improve the comparison. These graphs plot the instantaneous force measured on the load cell with the corresponding velocity calculated from the time differential of the position sensor. The damping level is indicated by the slope of the line in units of Ns/m (or kg/s). Figure 2 shows the initial guesstimate for the PTO piston tap setting at.5 turns. This produced a damping of 6Ns/m. The tap was then opened another full turn for a lower resistance to produce the characteristics shown in Figure 3. The damping value was then Ns/m. For high damping the valve was closed to half a turn and produced a slope equivalent to 2Ns/m. The corresponding power calculations (force * velocity) indicated the.5 turn setting absorbed the most wave power so this was reset as the optimum and re-tested, as shown in Figure 2. The resulting slope was exactly the same as the original test and proved the tap setting was a reliable tool for obtaining the PTO damping on request. The damping plots also show a consideration that is important to adhere to for this work. Since a gas is used as the PTO pressure resistance medium the air compressibility must be guarded against. The high damping plot, Figure 5, shows the effect of compensability in that the plot line is no longer linear, though the average is still as expected. The damping settings per motion type were also checked, as shown in Figures 6 & 7. For these tests a short crested sea had to be use to stimulate the roll motion. Figure 6 shows the pitch damping which confirms the previous results and that they are independent of the type of wave front encountered. Figure 7 shows the 3 roll damping results (only 2 can be identified in the graph). This plot corroborates that all 3 ROLL dampers are the same and that the setting is as per the pitch damper. Figure.6: Pitch PTO Damping Figure.7: Roll PTO Damping 2.2 TESTS The general structure of the test programme is shown in Figure 2. The mass distribution options, together with the other parameter combinations, were the main series to be run and the first ones undertaken. TABLE details the actual monochromatic wave set used to generate the magnification response, or transfer function, of the machine to a single excitation frequency. Because regular waves can only be long crested in form the pitch motion alone could be recorded by this approach. Non head sea waves from the port quarter (i.e. down a mooring line) were attempted to provide excitation forces in roll but the OCRE did not respond at all in these conditions. [this was probably due to the model hinge and bearing construction rather than hydrodynamic reasons]. Page 5 of 24

16 Monochromatic Waves /5 Scale Full Scale Equivalent Frequency Period Wave Height Frequency Period Wave Height Name (Hz) (s) (mm) (Hz) (s) (m) f f f f f f f f f f Table Monochromatic Wave Table The standard practice at HMRC for the generation of a WEC power matrix is to run at least 5 real sea states, as recommended by the Offshore Engineering Industry standard. In the absence of an actual site specific bi-variate wave climate scatter diagram these are based on an open ocean site with Bretschneider type spectral profiles and distributed around the seaways in a way that includes iso-hight and iso-period combinations. In this way the uncertainty when extrapolating and interpolating the results to complete a full array of the power matrix is reduced. Due to sensor restrictions the number of seaways was reduced to, the characteristics of which are shown in Table 2. Panchromatic Waves /5 Scale Full Scale Equivalent Name Tp (s) Hs (mm) Tp (s) Hs (m) B B B B B B B B B B B Table 2 Panchromatic Seaway Table All sea states were run with long crested and two types of short crested waves. The latter were based on a cosine squared spreading (Cos 2 ), which produces an extreme short crested sea, and the more realistic Cos 8, generally regarded as those found in near-shore waters. Page 6 of 24

17 Instantaneous Power (watts) Infrastructure Access Report: [Insert the User-Project acronym] 2.2. Test Plan The raw data sets were acquired as instantaneous time histories from each sensor. This produced power take off simulator paired values of force and position. There were 6 sensors in all. The first step in the analysis was to convert the angular locations (θ) into an angular velocity (dθ/dt) by numerical differentiation of adjacent records. This was then transformed to a linear velocity (dx/dt) at the damper shaft using the moment arms shown in Table 3. Damper name Yaw (Bow, Mid, Stern).3m Roll (Bow, Mid, Stern).3m Pitch (Bow, Mid/Stern).7m Distance between brake and hinge Table 3 Distance from position sensor Since the corresponding force is measured directly the instantanious power absoption could then be obtained from the product of the two values: Power [W] = force [N] * velocity [m/s] A time series plot of this instantaneous power from irregular wave excitation is shown in Figure 8. From these instantaneous power time series any required statistical value can be obtain. In this instance, for both the monochromatic and panchromatic test sets, the average power was calculated in order to evaluate the performance of the various device configurations. An average power value for each of the 8 PTOs is obtained separately and combined to show the conversion of the OCRE as a unit Time (data points) Figure.8: Instantaneous power for one of the pitch power take-offs Page 7 of 24

18 Pitch Power (w) Roll Power (W) Infrastructure Access Report: [Insert the User-Project acronym] 2.3 RESULTS The results presented here are a small sample of the full test series taken from the Mass Distribution section. They represent the superior device combinations. As shown in the Test Plan flow diagram, Figure 2, the main variables evolved to include: Parameter Ballast PTO Damping Waves Options 6 distributions 4 settings (plus infinite) Monochromatic (6 2 heights) Panchromatic (5 sea 3 spreadings) Spectral profile (frequency composition) Table 4 Test variables 2.3. Monochromatic Waves Single period regular waves are used to investigate the device behaviour when excited by an external force. They produce the transfer function of the applicable degrees of freedom motions to an appropriate range of wave frequencies, or the Response Amplitude Operators (RAOs) as required. Figure 9 shows the response of the device power absorption for the pitch and roll PTOs for the roll ballast and optimal damping configuration. [Note that the roll power is approximately zero since this motion is not excited in long crested waves]. Roll Ballast Opt.5Turn H6.2.4 Pitch Pitch2 Roll Roll2 Roll Period (s) Figure.9: Regular wave transfer function for the power absorption parameter As can be seen the peak performance occurs at a wave period of approximately.2 seconds for both motions. This is in agreement with the free oscillation results shown in Figure. The other main factors that can be observed in these summary results are: The two pitch sectors exhibit similar characteristics across the wave periods; The three roll sectors perform quite differently. The mid PTO is more active than the other two and the stern segment contributes no power to the system; The roll power values are extremely low since the incident wave was a long crested head sea, which offers little excitation force in this mode; Page 8 of 24

19 Power (w) B B2 B4 B5 B6 B7 B9 B2 B5 B22 B23 B B2 B4 B5 B6 B7 B9 B2 B5 B22 B23 Power (w) Power (w) Infrastructure Access Report: [Insert the User-Project acronym] The pitch power conversion is equivalent to 5kW full scale Panchromatic Waves Based on the results from the regular wave trials the irregular seaway analysed data was compared, as detailed in Table Crest Length Wave energy devices have the potential to be deployed in any ocean, or sea, around the world. They will, therefore, be exposed to a wide variety of wave conditions which vary in both time, at a particular site, and space, between different locations. Since most wave energy devices are oscillators that have resonant periods of excitation with an associated frequency dependant magnification response, as shown in Figure 8, the performance of the WEC may vary at different stations. In the case of the OCRE, which is attempting to extract energy in 3 modes of motion, the sensitivity to waves will include the seaway energy spreading, or multi modal wave systems, that produce the local conditions. In particular this will influence the crest length of the seaways. Since this sea state parameter is not well defined, or archived, a range of options were used to evaluate the performance sensitivity of the WEC. These ranged from the two extremes of long crested seas, with no spreading, to the shortest crested seas, with Cos 2 spreading. A Cos 8 condition, generally regarded as the most commonalty occurring seaways, was added. A comparison of the OCRE in each of these seas is shown in Figures 2, 2 and 22. [NB power values are in model λ=5] Roll Ballast: Opt.5Turns: LongCrested Roll3 Roll2 Roll Pitch Pitch Roll Ballast: Opt.5Turns: Cos2 Roll3 Roll2 Roll Pitch2 Pitch Figure.2: Power conversion in long crested seaways Figure.2: Power conversion in short crested seaways Roll Ballast: Opt.5Turns: Cos8 B B2 B4 B5 B6 B7 B9 B2 B5 B22 B23 Roll3 Roll2 Roll Pitch2 Pitch Figure.22: Power conversion in mixed seaways The irregular wave trials display the following device characteristics across the range of seaways tested: Page 9 of 24

20 B B2 B4 B5 B6 B7 B9 B2 B5 B22 B23 B B2 B4 B5 B6 B7 B9 B2 B5 B22 B23 Power (w) Power (w) B B2 B4 B5 B6 B7 B9 B2 B5 B22 B23 B B2 B4 B5 B6 B7 B9 B2 B5 B22 B23 Power (w) Power (w) Infrastructure Access Report: [Insert the User-Project acronym] The maximum power conversion in pitch occurs in long crested seas; The forward pitch section performs slightly better than the stern section; In short crested seas the pitch conversion drops by approximately 4%; The roll segments perform the best in short crested seas; The overall power conversion is lowest in short crested seas; The roll contribution is less than 5% of the pitch power conversion; The performance for the mixed seas are proportional to both the long and short crested waves results; The roll contribution in mixed seas makes up for the loss of the pitch power in these conditions Power Take-Off Damping To identify the optimal applied damping a series of PTO resistance trials were conducted. The dashpot damper tap was adjusted to different settings and the power conversion measured in the same seaways. Examples of these tests are shown in Figures 23, 24, 25 & Roll Ballast: High.Turns: Cos2 Roll3 Roll2 Roll Pitch2 Pitch Roll Ballast: Opt.5Turns: Cos2 Roll3 Roll2 Roll Pitch2 Pitch Figure.23: Power with high PTO damping Even Ballast: Light 2.5Turns: Cos2 Roll3 Roll2 Roll Pitch2 Pitch Figure.24: Power with medium PTO damping Even Ballast: vhigh.5turns: Cos2 Roll3 Roll2 Roll Pitch2 Pitch Figure.25: Power with low PTO damping Figure.26: Power with very high PTO damping The results show that the optimal damping, in all motion modes was obtained with a dashpot tap setting of.5 turns. From the flow velocity calibration graphs this was equivalent to an applied damping of 2Ns/m. There are indications that the optimal pitch and roll damping may not be the same but model problems were producing inconsistencies during these tests so no confirmed interpretation can be drawn. Page 2 of 24

21 B B2 B4 B5 B6 B7 B9 B2 B5 B22 B23 B B2 B4 B5 B6 B7 B9 B2 B5 B22 B23 Power (w) Power (w) B B2 B4 B5 B6 B7 B9 B2 B5 B22 B23 B B2 B4 B5 B6 B7 B9 B2 B5 B22 B23 Power (w) Power (w) Infrastructure Access Report: [Insert the User-Project acronym] Hull Ballast As shown in the test plan, Figure, the third main parameter investigated was to change the mass distributed within the device hull. The purpose was this was to change the natural period of roll between the different hull sections. In order to keep the overall draft the same the amount of change was limited, as shown in Figure, but in all four options were tested. The results are presented in Figures 27, 28, 29 & 3. It should be noted that some of these results were contaminated due to internal hull bulk heads leaking during the 256 second irregular seaway test run. This was particularly problematic when the upper chambers were filled with water and the lower air filled. An attempt was make to use lead weights instead of the water ballast but it was then discovered that the outer hull joints were also not water proof..2 Even Ballast: Opt.5Turns: Cos8.2 Roll Ballast: Opt.5Turns: Cos Roll3 Roll2 Roll Pitch2 Pitch Roll3 Roll2 Roll Pitch2 Pitch Figure.27: Power with lower tanks filled Figure.28: Power with inner and outer tanks filled Roll Ballast: Even Upper Chamber:.5 Turns: Cos 8 Roll3 Roll2 Roll Pitch2 Pitch Roll Ballast: 3c roll2+heavy.5 Turns: Cos 8 Roll3 Roll2 Roll Pitch2 Pitch Figure.29: Power with upper tanks filled Figure.3: Power with mixed tanks filled As can be seen from the ballast tests a reasonable improvement in power conversion can be achieved with the different configurations, and across all sea states. In all wave conditions the pitch contribution was always contributed considerably more power than the roll motion Keel Profile The free oscillation trials had shown the viscous damping of the model in roll mode was very high. An attempt was made to jury rig a curved keel to reduce the vortex shedding from the model submerged edges to investigate if this would enable additional motion, and hence velocity. This had to be achieved without changing the device draft so an open semi-circular profile made of thin polycarbonate sheet was used. Since this flooded with water the mass of the Page 2 of 24

22 B B2 B4 B5 B6 B7 B9 B2 B5 B22 B23 Power (W) Power (w) Infrastructure Access Report: [Insert the User-Project acronym] model was increased. Increased oscillations for the free motion trials were recorded but not a significant change so the investigation was discontinued Mooring Arrangement The results had shown that the contribution from the yaw PTOs was negligible. Different mooring configurations were investigated to evaluate if this motion mode could be enhanced. No significant improvement could be achieved so the examination was discontinued Wave approach direction As specified above the primary test programme was based on head seas. A set of trials with quarter and beam seas were conducted to establish if the roll motion could be enhanced. Unfortunately due to the model construction these tests were not very successful when the hinge and bearing joints kept jamming, so the investigation was discontinued. 2.4 ANALYSIS & CONCLUSIONS All the analysis for this series of trials is based on the comparison of the absorbed power, primarily that obtained from the irregular seaways tests. Figures 3 & 32 show comparative results from the former, friction PTO, with those from this set of test using the pneumatic PTO. It can be seen that the power conversion is more evenly distributed for the pneumatic PTO, Figure 32, whilst the overall device output remains the same across all sea states tested. This comparison has significant implications with respect to the motions of the individual segments and the design of the OCRE. Although each mode is individual it may not be independant..2 Short Crested 8 Roll Ballast High Damping Power Contributions.2 Roll Ballast: Opt.5Turns: Cos Roll - Stern Roll - Mid Roll - Bow Yaw - Stern Yaw - Mid Yaw - Bow Pitch - Mid/Stern Pitch - Bow Roll3 Roll2 Roll Pitch2 Pitch Figure.3: Power with friction PTO Figure.32: Power with pneumatic PTO The crest length trials show, as expected, that all the conversion is achieved by the pitch motion in long crested seas, since there is no excitation force in the roll direction for head seas. The results also demonstrate that the OCRE WEC has a reasonable strong association to the seaway crest length. In short, Cos 2, seas the drop off of the pitch performance is noticeable whilst the roll contribution is at a peak. The overall performance, however, is lower than the long crested, Cos, waves. In the mixed Cos 8 seas the overall performance returns to the same level as in long crested seas due to the combination of pitch and roll PTO output, whilst each individual motion if below the optimal output for the two other wave front combinations. The main conclusion from the PTO applied damping set of results, Figures 23 to 26, is that all the individual modes have the same optimal value of 6Ns/m. There is some scatter on the results which may mask fine adjustment but Page 22 of 24

23 in general a single PTO setting suits all conditions. More investigation of this parameter utilising an improved model and damping simulator could be warranted but the indications are that improvements would be minimal. It can be seen from the mass distribution trial results, Figures 27-3, that some improvement can be achieved by shifting the hull ballast around the internal compartments. Increases in power absorption of between 25% & 5% are possible by changing the ballast from the initial evenly distributed case. Unexpectedly the biggest increase is due to the for ard pitch PTO rather than the roll mode PTOs. From the combined sea state tests a power matrix for each of the OCRE configurations was produced. An example (showing a limited number of results) is shown Figure 33 for the measured values. The full table is generated by extrapolating and interpolating the strategically located empirical results and from this the annual energy output is obtained. Hs Proto :4 :2 ½ :5 : MAIN LEARNING OUTCOMES Tp [s] Proto : :2½ : : [m] [mm] BRETSCHNEIDER OCRE Power (Short Crested 8 Roll Ballast High Damping) : : Tz [s] :2½ : Proto Figure.33: Power matrix for OCRE WEC 3. PROGRESS MADE During the previous, limited duration, test programme it had been noted that the defective power take-off simulator had influenced the natural movements of the OCRE WEC and, therefore, the device power conversion capacity. This problem was correct for the MaRINET project such that the current potential of the design could be evaluated. Some problems were encountered with the model which can now be corrected in future construction operations. 3.. Progress Made: For This User-Group or Technology The various members of the OCRE development team had no previous experience of physically testing wave energy converters or working in a wave tank environment. The experience gained operating in close alliance with the facility provider was of considerable benefit to each of the company personnel. Technically it was noted that further investigation is required if the roll and, in particular, the yaw motion modes are to contribute a more significant quantity of power conversion. Page 23 of 24

24 3... Next Steps for Research or Staged Development Plan Exit/Change & Retest/Proceed? Following this successful completion of the Stage test programme the technical development will follow a twin parallel path. A mathematical time domain model that can deal with short crested irregular wave excitation will be investigated. This will be validated against the MaRINET empirical results prior to the model s application to optimise the OCRE design. A larger scale model with a more realistic power take-off mechanism will be designed and constructed to begin the Stage 2 process Progress Made: For Marine Renewable Energy Industry The most significant generic contribution of the OCRE trials towards the marine renewable technology came from the fact that the WEC operated in multiple modes of motion, rather than the single degree of freedom most devices attempt to capture. How these differ was an essential aspect of the test programme. 3.2 KEY LESSONS LEARNED Be prepared prior to arrival at the facility, especially the model suitability; Work closely with the infra-structure personnel before and during the access period; Do not arrive with extensive additional requests that will consume important test time; Ensure the test plan is feasible in time and complexity for the infra-structure capability; Video & photograph as much of the operation as feasible for later reference; Validate the raw data on a regular basis; If possible conduct pre-liminary data analysis during the test period; Ensure all achieved data files can be accessed away from the infra-structure if specialist acquisition systems have been used. 4 FURTHER INFORMATION 4. SCIENTIFIC PUBLICATIONS List of any scientific publications made (already or planned) as a result of this work: 4.2 WEBSITE & SOCIAL MEDIA Website: YouTube Link(s): LinkedIn/Twitter/Facebook Links: Online Photographs Link: 5 REFERENCES O Callaghan J; OCRE Tank Testing Report; HMRC, UCC, April 23. Brewster P; Technical Note -Preliminary design issues; PreMarine, October 2. Page 24 of 24