DEVELOPMENT OF EVOPOD TIDAL STREAM TURBINEG C Mackie, Ocean Flow Energy Limited, UK SUMMARY The paper describes the development of a floating tethered platform for supporting a turbine which extracts energy from bi-directional tidal stream flows. The author initially describes the special environmental conditions and operational constraints for tidal stream turbines that led to the proposed design solution – a semi-submerged platform connected to a spread moored midwater buoy. The proof of concept of the solution is described involving simulations and small (1/40th) scale model tests carried out at Newcastle University’s combined wave and current flume tank. This led to the further testing of a 1/10th scale device in the real bi-directional tidal flow conditions in Strangford Narrows, Northern Ireland and preliminary findings from these trials are presented. 1. INTRODUCTION Turbines placed in the middle of the water column tend to be supported on monopiles or tripod structures. The greater height off the seabed means that significant bending moments are experienced by the support structures and proportionally greater pile diameters need to be used. Gravity bases and monopile support structures are commonly deployed for oil and gas installations in the North Sea, particularly in the shallower and less extreme environment of the Southern North Sea. However the deeper waters and more extreme wave environment of the Central and Northern North Sea and Atlantic Margins have seen the adoption of compliant moored floating platforms as providing more economically attractive solutions. A study carried out for The Carbon Trust [1] identified that the majority of the UK’s tidal stream resource is in deeper waters that are exposed to Atlantic wave systems. Deeper water in this context means waters over 40m deep and not the thousands of meters of water from which oil and gas is now being recovered in the Gulf of Mexico and off the coasts of Brazil and south west Africa. Nevertheless anything over about 30m water depth is beyond the capacity of the jack-up barges currently used for the installation of pile and gravity base support structures for offshore wind and tidal turbines. If the turbine device is not rigidly fixed to the seabed then the further options available are a midwater device constrained by tension tethers or a semi-submerged or surface floating platform supported by tension tethers or catenary mooring systems. Figure 1 shows the generic options available. The commitment of governments to reduce carbon emissions and the requirement placed on energy producers to generate a certain percentage of their output from renewable sources has led to a resurgence of interest in tidal stream and ocean current energy sources. The majority of the UK’s tidal stream resource is in areas of harsh wave environment and in water depths where jack-up installation vessels will have problems operating. This paper presents the concept development of a moored floating platform for deploying a tidal stream turbine - EVOPOD - that has application to most deep water energetic tidal stream and ocean current sites. 2. 2.1 OPTIONS TURBINE OPTIONS A number of technologies are currently being developed for recovering the kinetic energy in free flowing tidal streams and ocean currents. The majority of solutions employ a turbine that delivers torque through its shaft axis that can be coupled to a rotating generator machine. With open channel flow there is very little differential pressure head involved so turbine solutions generally employ either horizontal axis axial flow lifting surface devices or vertical axis lifting surface (Darrieus) or drag (Sevonius) devices. 2.2 FIXING / ANCHORING OPTIONS In order to recover energy from the flow these turbines will experience a significant amount of drag which needs to be reacted against a fixed point, normally the seabed. To a degree the fixing solution is dictated by the position of the turbine in the water column. Some bottom mounted devices use gravity bases or post tensioned rock anchors to load the contact surface between the base structure and the seabed in order to use friction to resist the horizontal force. Other bottom mounted devices use piles grouted or hammered into drilled holes in the seabed to use the shear resistance of the steel piles to react the turbine horizontal forces. Figure 1: Options for constraint of tidal stream turbines If the anchor points experience uplift then gravity and drag-in anchor solutions are ruled out and only grouted-in pile or rock anchors can be used.57 Mull of Galloway 80 2.2 5 12 . though not quite as strong.57 Channel Islands Race of Alderney 39 4. 25 6. If the UK is to exploit these areas it must develop technologies that can be installed and maintained in a tidal stream environment that is also exposed to severe wave climates. Good flow speeds. 3.1 THE ENVIRONMENT TIDAL STREAMS Pentland Firth and the English Channel particularly around the Channel Islands.89 Hoy.e.e.2 WAVE ENVIRONMENT Figure 2 gives expected wave conditions for the Pentland Firth area exposed to Atlantic wave systems.15 S.64 2.20 2.39 Pentland Firth Areas Pentland Skerries 59 6. Pentland Firth 76 4. 25 10 . 25 4. Typical flows and water depths at these sites are shown in Table 1 [2]. exist where the Atlantic meets the Irish Sea at Rathlin Head and the Mull of Galloway.000% 0. 25 1. 3.80 1.000% 20. 25 9.93 Rathlin Island 80 2. Pentland Skerries 63 4.79 1.46 1.15 Duncansby Head 65 5.41 1.2 5 Hs (m) Figure 2: Percentage occurrence of wave heights – Pentland Firth Wave headings will be predominantly from the W to NW sector but occasional storms can also be expected from the N to NNE direction as shown in Figure 3.57 Big Russel 48 2. Pentland Firth 58 4.20 2.000% 10. 25 2. Pentland Firth 71 5. The change in buoyancy with submergence balances the increase in the vertical component of the mooring line tension.44 1. which permits gravity and drag-in type anchors to be used. The concept of tethering a tidal stream turbine to the seabed using a catenary mooring solution forms the subject of this investigation. With this solution it is feasible to use a catenary mooring system and with such a system the load applied to the seabed anchors is always horizontal. 25 7.If a submerged tethered turbine is to maintain constant elevation above the seabed it must be positively buoyant otherwise it will descend in the water column as the flow speed and horizontal drag force increases until the vertical component of the mooring line tension is balanced by the device’s buoyancy force.38 Irish Sea Areas Rathlin Sound 40 2. A floating tethered turbine on the other hand has a reserve of buoyancy (freeboard) that can come into play as the current speed increases. i.000% 0. the Location Water depth (m) V-spring (m/s) V-neap (m/s) 2. These are formed because the phase difference between tidal elevations in the Atlantic and North Sea leads to strong flows in the channels to the north and south of our mainland.39 1. 25 3. no uplift. % occurrence 25. 25 5.44 2. Together these sites represent about 60% of the UK’s tidal stream resource.000% 5. 3.000% 15.18 Stroma.05 1. Ronaldsay.57 Table 1: Location of UK deep water tidal stream sites .38 S Ronaldsay. Therefore a positively buoyant submerged device must rely on a tension tether mooring system which will impart uplift into the anchor points on the seabed.38 Casquets 115 2.2 5 11 . i. 25 8. Britain is fortunate in having some of the world’s best sites for strong tidal streams. Alternatively the number of bottom mounted devices installed will have to be larger than the number of surface or midwater devices installed for the same tidal stream farm output.0m. Shaft centre below WL 15m Kinetic power flux through disc 9.60 m/s device Bottom 45m 5.Wave direction 0 30.00% 10.00% 270 0.00 3.82 m/s device Midwater 30m 7.5m which relates to a limiting sea state with a significant wave height of about 1.00 5.00 MW Corresponding flow speed over disc 50% exceedance Figure 4: Wave exceedance data expected for Pentland Firth Typically jack-up barges used for marine operations can only jack themselves up on their legs when the wave heights are less than 1. Figure 4 shows expected wave height exceedance data for the Pentland Firth. Fe Se pt e N .00 1.00% 330 25.3 CONSTRAINTS ON MARINE OPERATIONS The alternative to a jack-up would be to use a floating installation vessel. 3.00 2.00 Hs (m) 4.00 0. The alternative would be to rely on dynamic positioning to maintain station but vessels constructed for the offshore oil industry rarely experience currents greater than 1.25 m/s mounted device Table 2: Decay in steady current flow speed with water depth As the amount of energy flowing through the turbine disc per unit time reduces with immersion this requires that a larger bottom mounted turbine would be required to generate power output equal to that generated by turbines closer to the sea surface. Breakdown of a turbine during the winter months would lead to significant generating down-time if it required intervention by a jack-up barge to change out components.54 MW 3.00% 5.00 ry ry e t be r ov em be D r ec em be r ch ly M ay ril gu s br ua M ar O ct o Ja n Au m be Ju n ua Ap Ju r The horizontal speed of flow is generally assumed to decay in accordance with a 1/7th power law over the depth of the water column [2].54 MW 3.0m/s and would have difficulty maintaining station in anything approaching the 3m/s to 4m/s flow rates where tidal devices are to be deployed. Additionally kinetic power increases with speed cubed whereas drag increases with speed squared therefore greater anchoring loads are required to constrain a device generating the same power in slower moving water. even assuming that a jack-up capable of operating in water depths of 60m and resisting the drag forces associated with peak flows of 3. this effectively rules out marine operations involving jack-up barges for the months of September through to May in exposed areas of the Pentland Firth.5m/s (7 knots) (mid flow rate between spring and neap tides) could be found.0m/s would experience the kinetic flux through a 20m diameter disc and corresponding flow rates as given in Table 2.00% 20. As Figure 4 shows. For this type of vessel to maintain position it would have to deploy a mooring spread which is difficult given that the seabed in regions of strong tidal flow will be predominantly swept of all sediment cover leaving bare rock which rules out the use of conventional drag-in anchors. Surface 3.4 FLOW DECAY WITH WATER DEPTH The marine operations involved in installing tidal stream turbines will inevitably be weather constrained. Therefore a horizontal axis turbine placed in a 60m deep tidal stream site with a surface current of 4.00% 300 15. Any device that requires precise positioning such as the mating of a turbine with its pre-positioned seabed foundation will require even more benign conditions to carry out operations and such operations will only be feasible when brief periods of slack water coexist with periods of calm weather. Wave Exceedance 10% exceedance 6.00% 90 60 30 240 120 210 180 150 Figure 3: Percentage occurrence of different wave headings expected in the Pentland Firth 3. 3. A floating solution can be disconnected from its moorings and removed off site for maintenance. switchgear and transformer. wave period and water depth and will also vary in direction according to the prevailing sea conditions.5 WAVE PARTICLE MOTION • Onto the steady tidal stream current must be added the oscillatory motion of the wave particles which will vary in magnitude with wave height. 4 EVOPOD SOLUTION Evopod is a semi-submerged. i. similar equipment to that fitted in a wind turbine nacelle. DEVICE DESCRIPTION 4. A 10m wave is an extreme event with a one year return period. The device is moored off to a sub-surface buoy that is held on a geofixed location by a 4 point catenary spread mooring system. The decay of the peak horizontal component of wave particle velocity with water depth for different wave conditions is shown in Figure 5. The reduced motion responses of a semisubmerged platform with small waterplane area surface piercing struts lowers the risk of platform motions destroying turbine performance.1 DESIGN RATIONALE Figure 6: Evopod with midwater buoy The required dimensions of the submerged pod were determined for a full scale 1800kW device fitted with a horizontal axis mono-turbine. An adaptive control system that can cope with wave frequency fluctuations in shaft input torque level will be able to capture some of this kinetic energy in the waves. A catenary mooring connected to a subsurface buoy can be deployed using anchor handling boats that are less weather sensitive than jack-ups and in advance of hooking-up the turbine to its mooring. A variable speed drive coupled to an induction generator is one possible solution as modern PWM (pulse width modulation) drives have a fast response time. The pod was sized to provide sufficient volume and length to house the gearbox. Figure 5: Variation in peak wave particle velocity with wave height and water depth The circle drawn on Figure 5 shows the location of Evopod’s turbine. Ocean current sites are generally in water depths greater than 120m. Peak horizontal wave particle velocity 0 Distance below WL (m) • • -3 -2 -1 -10 -20 -30 -40 -50 -60 0 1 2 3 H=2m / T=5s H=4m / T=8s H=10m / T=12s than 40m. A semi-submerged platform was selected in preference to a surface floating platform. 4.e.5 to 3 times turbine diameter). generator.2 Horizontal velocity (m/s) The Evopod solution is illustrated in Figure 6 and described below. The buoyant strut layout selected consists of a single vertical strut forward and twin V-struts aft. inverter drive. floating platform that is designed to support a turbine or turbines in a bidirectional tidal stream or a unidirectional ocean current. The transverse separation of the struts provides roll damping and a hydrostatic restoring moment to resist The rationale behind the selection of this particular solution is: • A moored or tethered solution requires reasonably deep water (2. The majority of the UK’s tidal stream energy sites are in water depths greater . Any near-surface horizontal axis turbine operating in exposed sea areas will have to cope with fluctuations in flow with a passing wave that could be as much as 25% to 50% of the steady flow rate.0m wave will be exceeded 50% of the time while a 4m wave will only be exceeded 10% of the time. This requires a disconnect system that is relatively sea state insensitive and can be carried out quickly during periods of slack water. In an area such as the Pentland Firth a 2. The work was carried out with the support of the Regional Development Agency in the north east of England. 5 VALIDATION OF THE CONCEPT Figure 7: 1/10th scale Evopod • The yaw stability when subject to current eddies and fluctuating waves. The waterplane area was kept to a minimum but made sufficient to limit the change in draft to not more than 2. wave and current (WWC) flume tank.0m when subject to the change in vertical component of force from the mooring system between zero (slack water) and maximum (mid tide) flow rates. The 1/40th scale model tests were carried out by Newcastle University’s Department of Marine Science and Technology (MAST) using initially their towing tank and then their combined wind. namely: • • The ability of the device to avoid being dragged under by the vertical component of the mooring line forces The ability of the device to align with the current rather than wave direction. While the tests were limited to unidirectional currents and could therefore not validate the ability of the device to rotate about its mooring system. The semi-submerged nacelle or pod has surface piercing struts so that there is sufficient reserve of buoyancy to resist vertical component of drag force reacted by the moorings.the generator torque reaction. ONE North East. While the above concept offers a potential solution to the requirement for a floating tethered turbine. . The device is moored such that it is free to yaw (weathervane) into the predominant current direction which allows the use of simple fixed pitch downstream turbine. The success of these tests led to the construction of a 1/10th scale device (Figure 7) that is currently undergoing trials in real tidal flow conditions in Strangford Narrows. A programme of numerical simulations. The baseline concept is to use a submerged power export swivel mounted in the buoy as used in floating production platforms but an alternative solution involves deleting the swivel and fitting a transverse thruster to the device to allow the umbilical cable to be unwound at periods of slack water to avoid more than 270 degrees of rotation. The device is moored off to a geo-fixed spread moored midwater buoy that is sufficiently immersed to avoid the worst of wave action and has positive buoyancy to help support the weight of the catenary mooring lines. Table 3 also shows the key particulars for the scale test devices. An advantage of tethering the floating body to a midwater buoy is that the floating body maintains constant freeboard with change in tide level. Power is exported to the seabed via an umbilical cable running from the Evopod device to the centre of the mooring buoy and from there to the seabed. a number of risk issues were identified. they did confirm the feasibility of the semi-submerged hull geometry and the midwater buoy spread mooring solution. A semi-submerged nacelle or pod houses the turbinegenerator equipment. The numerical modelling carried out by Ocean Flow Energy is described later. The transversely separated surface piercing struts provide a righting lever to resist the heeling moment induced by the generator torque reaction from the single turbine. A mono-turbine is used for simplicity. wind and current forces. In the 1/10th scale device it also houses the control. A target set of parameters for a full scale device deployed at a tidal stream farm in the Pentland Firth is given in Table 3. instrumentation and data logging equipment. small (1/40th) scale model tests and 1/10th scale device trials was structured to validate the concept. adjusting only the angle of connection of its yoke to the midwater buoy. The longitudinal separation of the struts provides some pitch damping. These trials are complementary to a programme of fundamental investigations into tidal stream energy conversion being carried out by Queen’s University Belfast as part of the ‘Supergen’ programme. Northern Ireland adjacent to Queen’s University’s Portaferry Marine Laboratory. The surface piercing struts have a small waterplane area so that the motions of Evopod in waves are minimised and do not destroy the turbine performance. For the tests carried out in the WWC tank the wind tunnel facility was not used. 375 0. though lags.5s 1/40th Scale (Newcastle University test tank) 0.21s Hs = 14m Hs =1.37 375 1.538 0. OrcaFlex is a marine dynamics simulation program developed by Orcina for the static and dynamic analysis of flexible pipeline and cable systems in an offshore marine environment.5 13. The drag force is fairly constant over the range of operating turbine speeds but falls off rapidly if the blade has stalled so it is important to avoid stall when developing the load control system.0 1800 4. For this full scale model simulation the heading lag is corrected after about 10 seconds.4m Tz = 14s Tz = 4.0 Hs = 3m Tz = 8s 1/10th Scale (Strangford Narrows) 2.35m Tz = 2.000 15.343 5.004 0.075m Tz = 1. Evopod heading prediction in beam seas with tidal direction change 200 150 100 50 0 0 100 200 300 Time (sec) Evopod resiprocal heading Flow direction Flow speed x 40 Figure 8: Analysis of Evopod heading in beam seas An appropriate turbine blade geometry was selected for fitting to the physical models in order to apply the correct drag to the mooring system. The hydrodynamic loads on the pod are calculated using Morison’s equation which is acceptable for a body that is small in relation to the applied wave length.57 1.26 Hs = 0.26s Hs = 0. OrcaFlex has the facility to add wing sections to the struts so that the correct lift and drag forces are applied to the device according to the angle of incidence to the combined current and wave particle velocity.86 0. Typical performance . A programme of simulation work was identified to address this issue using the OrcaFlex package. drag and moment coefficients for the struts were derived using the Java-Foil fluid dynamics package. The lift. Evopod follows.Full Scale (Pentland Firth) Length overall (m) Breadth across struts (m) Displacement (t) Turbine diameter (m) Rated output (kW) Rated flow speed (m/s) Average operating sea state Survival sea state 21. Modelling the floating body and its mooring system in OrcaFlex when subject to seas (Sea State 6 which is only exceeded for 10% of the time at the Pentland Firth site) at right angles to the tidal flow demonstrates that the device aligns with the current direction even as the flow speed decays. It is a fully 3D non-linear time domain finite element program and is eminently suitable for modelling the loads and excursions of a catenary mooring system. The vertical surface piercing struts are represented as pipe elements to give the correct weight.5 0. It can also be used to generate applied forces on the mooring system from Evopod and its midwater buoy. A risk area with the concept of a swing moored device is that when waves are not aligned with current the device heading will deviate from alignment with the current vector to the detriment of power capture performance.63 Hs = 0.3m Tz = 2. The submerged pod is represented in OrcaFlex as a cylindrical towed body and the turbine as a disc with an appropriate drag coefficient. WT_Perf uses blade-element momentum theory to predict the performance of wind turbines and is a descendent of the PROP code originally developed by Oregon State University.15 1. the change in flow direction as the tide changes from flood to ebb.43s Table 3: Evopod key parameters 5. WT_Perf software made publically available by NWTC (USA’s National Wind Technology Centre) was used to estimate blade efficiency and turbine performance in order to select an appropriate gearbox and generator for the 1kW 1/10th scale device. buoyancy and waterplane area but with zero drag properties.7 375.1 NUMERICAL MODELLING Newcastle University’s WWC tank can only create an environment where waves are in line with the current. 3 1/10th SCALE TRIALS Drag force in WWC tank Flat water With Waves Turbine Stalled Trend line for With Wave Trend line for still water While the 1/40th scale tests confirmed the results obtained from OrcaFlex that the model was stable and was not dragged under by the mooring forces the unidirectional flow in the WWC tank did not allow understanding of the ability of Evopod to swing around its geofixed mooring system as the tide direction changes. Northumberland. 30 25 Drag Force (N) 20 15 10 5 0 0 0.2 0.40 1.8 Figure 10: Total drag force on Evopod Figure 12: 1/10th scale Evopod in Strangford Narrows . Drag measurements were made with the device pulled along by a carriage in the University’s towing tank and repeated with the device held in position by its mooring system in the flume tank (with and without waves). 35 Period (sec) Figure 11: Heave motion response of Evopod 5.5m/s was identified. Turbine performance curves 1200 1000 800 600 400 200 0 0 50 RPM 100 150 Power (W) Drag (N) Torque x 10 (Nm) An optical tracking system was fitted to the model in the WWC tank in order to measure first order motion responses. The 1/10th scale Evopod was constructed in steel and fitted out with a comprehensive instrumentation package by NaREC (New and Renewable Energy Centre) at Blyth. While the flow in the deepest part of the Narrows can peak at 4.00 10.00 RAO (m/m) 0.8.80 0.60 0.6 0.00 25.40 Strip theory Model Tests Figure 9: WT_Perf turbine performance curves 5. The 1/40th scale model was fitted with an electromagnetic brake that generated a resistive torque when an electrical current was applied.4 Velocity (m/s) 0. By adjusting the applied electrical current it was possible to home in on the optimum power operating point for the small turbine. This equates to a form drag coefficient for a disc equal to the swept area of the turbine of about 0. Figure 10 shows a plot of the drag forces in the WWC tank and clearly demonstrates the much reduced drag on the system when the turbine is in stall mode.00 15. Heave RAO 1.7 0.5 0.20 0.00 0.1 0.5m/s as you move inshore the flow rate reduces and a site near the Portaferry Marine Laboratory in 6m water depth (mean tide level) where the flow peaks at 1. The peak heave response at 14 seconds is well removed from average operating sea conditions but would coincide with extreme events which may be an issue in terms of vertical loads imparted into the midwater buoy through the yoke connection. Figure 12 shows Evopod operating at the Portaferry site. It is noted that the turbine drag reduces to about 80% of its peak value when operating at its point of peak efficiency.00 5. This would require placing the device in real tidal flow conditions all be it at a scale flow rate.00 The key objective of these tests which were carried out by MAST was to confirm viability of the concept and to firm up on the turbine drag properties used in the OrcaFlex model.3 0. Figure 11 compares the measured heave response with that computed using a simple strip theory model developed by Ocean Flow Energy.2 1/40th SCALE TESTS 0. A suitable site for carrying out affordable 1/10th scale trials was identified as Strangford Narrows.curves for the 1/10th scale turbine are shown in Figure 9.20 1. By measuring the turbine speed of rotation and combining this with the applied resistive torque it was possible to find the shaft power generated by the scale turbine.00 20. The flow vectors are fairly consistent in heading during the flood and ebb periods and Evopod was found to maintain a reasonably consistent alignment to the flow. Future trials will provide measurement of the wave excitation that may be present during the trials and which might be causing the yaw motion. However Figure 14 overlays the power generation curve on the plot of heading angle and there does not appear to be any link between power fluctuations driven by unsteady flow conditions and heading angle.0m. This is of the same absolute value at Portaferry as it would be for a full scale device in the Pentland Firth. Another possible cause is the coupled roll and yaw motion linked to the changing heel angle of the device as it counteracts the generator torque reaction. 1/10th scale unit trials data 350 300 250 200 150 100 50 0 16 18 20 Time (sec) 22 Head (deg) Shaft Power (W) Figure 14: Yaw motion of 1/10th scale Evopod Figure 13: Sample tidal stream flow data at Portaferry site .5m is equal to more than half the mean water depth of 6. On parameter that has not scaled correctly is tidal range. This shows that at the Portaferry site the flood flow is more consistent than the ebb flow and the duration of the flood tide is longer than the duration of the ebb tide. However there is some evidence of a yaw oscillation with a period of about 2 seconds and amplitude of about 5 degrees (see Figure 14). Figure 13 shows the measured flow characteristics at the test site during a period of neap tides. Surveys carried out at other potential tidal stream sites have revealed that the real flow conditions frequently deviate from theoretical sinusoidal tidal flow assumption. At the Portaferry site the tidal range of 3.The Portaferry site is exposed to waves from the NW and SE directions generated over a reasonable fetch and will provide scale sea conditions. Nevertheless the mooring system works successfully though with greater variation in the excursions and submergence of the midwater buoy. J.. the results to date (October 2008) have demonstrated the yaw stability of Evopod under real tidal flow conditions. REFERENCES 1. 1/10th scale device trials can help to de-risk full scale prototype tests for a fraction of the cost provided that a suitable scale environment can be found. 7. BLACK & VEATCH. While the 1/10th scale trials are still ongoing. CONCLUSIONS A high percentage of the strongest tidal stream sites around the UK coast are in areas of relatively deep water exposed to harsh wave climates. The trials have also demonstrated that the device can safely weathervane about its mooring system as the tide direction changes. The Carbon Trust. G. 9. A.. I.G. 2. The analysis indicated that Evopod’s strut and turbine geometry ensures that it aligns with the predominant current direction even when exposed to beam seas. ‘Phase II UK Tidal Stream Energy Resource Assessment’. The OrcaFlex numerical modelling tool can provide useful performance analysis of a floating tethered tidal turbine in non-aligned wave and current conditions. . 2005. April 2006. S. Such an environment exists at the Strangford Narrows. His previous experience includes managing the design process on a number of FPSO projects and supporting wave energy companies in the development of model scale and full scale prototypes. Novel solutions requiring installation and maintenance techniques that can cope with moderately deep water and harsh weather conditions are needed to exploit this resource. MELVILLE. 8. Proceedings of the I MECH E. AUTHORS BIOGRAPHY Graeme Mackie holds the current position of Managing Director at Ocean Flow Energy Limited.. OWEN. ‘Tidal Current Resource Assessment’. BRYDEN. COUCH. Newcastle University and Queen’s University Belfast. The 1/40th scale model tests carried out in Newcastle University’s WWC flume tank provided confirmation that Evopod’s semi-submerged floating platform connected to its midwater buoy mooring could cope with the high drag forces developed by the tidal turbine.6. ACKNOWLEDGEMENTS The author would like to thank the support provided by ONE North East..
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