Proceedings of the HYDRALAB III Joint User Meeting, Hannover, February 2010
WAVE AND CURRENT TESTING OF AN ARRAY OF WAVE ENERGY CONVERTERS Ian Bryden (1) & Brian Linfoot (2) (1) The University of Edinburgh, UK, E-mail:
[email protected] (2) Heriot-Watt University, Edinburgh, UK, E-mail:
[email protected]
This paper briefly describes the physical model testing of an array of generic wave energy devices undertaken in the NTNU Trondheim basin during 8th to 20th October 2008 funded under the EU Hydralabs III initiative. The aim of the tests was to provide data for the validation of numerical models of the device motions, power recovery and mooring components when moored in a closely spaced array. The tests were not intended to be proof-of -concept tests for a particular device and none of the tests were designed to study survival response under extreme loading. Tests were completed at 1/20 scale on a single oscillating water column device and on close-packed arrays of three and five devices following calibration of instrumentation and the wave and current test environment. The arrays were tested under similar environmental loading with partial monitoring of mooring forces and motions. A total of ninety-five tests were undertaken including wave and current calibrations and damping tests. Analysis of the data indicates that close–packed arrays of energy converters may be more efficient in energy capture that the same number of similar devices. No significant current induced-motions on the WECs were observed in any array configuration.
1.
INTRODUCTION
The scientific aim of the tests was to provide data for the validation of numerical models of the device and mooring components and device motion and power interactions when moored in closely spaced arrays; the tests were not intended to be proof-of-concept tests for a particular device and none of the tests were intended to study survival response under extreme loading. The data gathered during the experiments contributes to the objectives of four workstreams of Phase 2 of the UK SuperGen Marine Energy Research Consortium Project which has the aim of increasing knowledge and understanding of the device-sea interactions of energy converters from model-scale in the laboratory to full size in the open sea. 2.
DESCRIPTION OF TEST SET-UP AND DEVICES
Tests were completed at 1/20 scale on a single oscillating water column device and on close-packed arrays of three and five devices following calibration of instrumentation and the wave and current test environment. One WEC was fully instrumented with mooring line load cells, optical motion tracker and accelerometers and tested in regular waves, short-and long-crested irregular waves and current. The wave and current test regimes were measured by six wave probes and a current meter. Arrays of three and five similar WECs, with identical mooring systems, were tested under similar environmental loading with partial monitoring of mooring forces and motions. There where five phases of experiments. In chronological order they were: 1. Basin calibration 2. Tests on WEC 1 in regular and irregular waves and current 3. Tests on WECs 12345 in regular and irregular waves and current 4. Tests on WECs 123 in regular and irregular waves and current 5. Tests on damping and mooring stiffness of WEC1 A total of ninety-five tests were undertaken including wave and current calibrations and damping tests.
Proceedings of the HYDRALAB III Joint User Meeting, Hannover, February 2010
2.1
Ocean Basin, NTNU (Marintek)
The experiments were carried out in NTNU Ocean Basin, Trondheim, Norway. The rectangular basin has sides 80m by 50m and a total depth of 10 m (variable). The basin has 144 flap generators on the 80m side and a double- flap generator on the 50m side. The 80m long multi-flap generators were used in these experiments and current was generated across the basin parallel to these flaps. Two sides of the basin have fixed beaches. The floor of the basin was lowered to 2.8m below the surface for all tests to correspond to the depth in the 12m by 11m basin at Heriot-Watt University. The water in the basin is chlorinated.
2.2
Instrumentation and Data Acquisition
Instrumentation included 6 wave height meters, 5 internal water level sensors, 5 pressure transducers, an electromagnetic current meter, 10 mooring line load cells, 5 heave accelerometers, 5 pitch sensors and a motion capture unit. The time-dependent data from the instruments was acquired synchronously using the Marintek CATMAN data acquisition system, filtered, scaled and output at 80 Hz real-time (corresponding to 17.8889Hz at full scale). The experiments were conducted according to Froude scaling laws at 1:20 scale.
2.3
Wave Energy Converters Tested
The wave energy devices tested were generic oscillating water column devices each fitted with an adjustable damping orifice plate. The wave energy converters are shown in Figure 1. Each WEC has a displacement weight of 850N. Two mooring line load cells numbered n.a and n.b were located on each of lines n= 1, 2, 3, 4 and 7 in the positions shown in Figure 2 and 3.
PLAN
800 mm
A
200.00
SECTION A-A Variable orifice plate and wind shield
100 mm
Access hole to ballast and instruments
800.00
200 mm
Water level
120
300.00
Demountable damping plate Bolted to fins and bolted to buoy. PVC
A DATE TITLE
MODEL SCALE
WEC MODEL (Three Column)
DRAWN BY
1: 20
26/02/2008 B Linfoot
REVISED
Figure 1. Wave Energy Converter Model showing principal dimensions
21/09/2009
Proceedings of the HYDRALAB III Joint User Meeting, Hannover, February 2010
2.4
Mooring Layout
The mooring system is shown in Figure 2. WAVE GENERATORS
X-Axis
Data co-ordinate frame origin Ocean Basin Centre WEC 1 at rest
(2.02,-3.50)
(2.02,0.00)
(2.02 3.50) 13
1
Y-Axis
4 50 3.
5
1
14
2
2
15
(-1.01,5.25)
3
5
(-1.01,1.75)
(-1.01,-1.75)
10
7
Dimensions in metres Anchor Mooring point coordinates in metres referenced to the data coordinate system
4
Schematic shows location of WECs identified by reference numbers
3
11
12
(-4.04,3.50)
Mooring lines 1 to 15 BOLD with active load cells ITALIC with passive link components
6
9
8
(-4.04,-3.50)
(-4.04,0.00) 3.50
Figure 2. Layout of mooring points showing WEC and mooring line numbers
ELEVATION
PLAN
Load cell n.b P 35
00
w T
Load cell n.a
T
T
T
2800
500
Mooring line number n (1-15)
85
2850
500 70 280
Dimensions of typical mooring line in mm
Figure 3. Arrangement and dimensions of mooring line
Proceedings of the HYDRALAB III Joint User Meeting, Hannover, February 2010
3.
REMARKS ABOUT THE EXPERIMENTAL CAMPAIGN
The original plan had been to measure the full 6 degree-of-freedom motions of all five WECS. Unfortunately it was confirmed that the optical motion monitoring system could only cope reliably with a single floating body so that the motion measuring system had to be augmented with strap-down heave accelerometers and pitch sensors on all five devices. Despite best efforts and support from Marintek staff, insufficient time was awarded to complete the full test plan as originally envisaged and requested. Three of the models will therefore be tested in a much smaller facility equipped with absorbing wavemakers, but without current. The prime purpose of these tests will be to duplicate key tests in the NTNU campaign to provide inter-tank comparisons and extend the range of parameter variations studied - including the effects on motions and mooring loads of variations in mooring stiffness and geometry - and allow the wave-WEC interactions to be studied in more detail. These tests are also required because our analysis of the NTNU data has shown relatively large variability in the regular wave test environment in the NTNU basin. This has complicated the analysis and interpretation of the mooring load data, motions and power recovery. In parallel with this basin activity three of the 1:20 scale WECs will be instrumented and moored at an open-water site on Strangford Lough, Northern Ireland. (scheduled for May 2010 - subject to permissions from local regulatory bodies). 4.
RESULTS & CONCLUSIONS
All data, results, photographs, videos and documentation are available on the SuperGen Marine web site http://www.supergen-marine.org.uk The data have been analysed to compare measurements of wave power and wave heights within the array (Ashton et.al., 2009) and mooring system damping (Vickers & Johanning, 2009). The data have also been used to calibrate Orcaflex numerical models. A comprehensive comparison of wave power, device motions and mooring forces in currents and regular and irregular waves is being prepared for an IAHR paper. Analysis of the data indicates that close–packed arrays of energy converters may be more efficient in energy capture than the same number of individual devices. No significant current-induced largeamplitude motions of the WECs were observed in any array configuration. ACKNOWLEDGEMENTS This work has been supported by European Community’s Sixth Framework Programme through the grant to the budget of the Integrated Infrastructure Initiative HYDRALAB III within the Transnational Access Activities, Contract no. 022441. The SuperGen Marine II consortium is supported by the UK Engineering and Physical Sciences Research Council under grant EP/E040136/1. The team members wish to place on record their gratitude to Ms A Neyts, Department of Biological Sciences NTNU for her excellent administrative assistance and to thank Mr Csaba Pákozdi, project manager, Marintek for his expert scientific advice and day-to-day assistance during the campaign. REFERENCES Ashton I., Johanning, L. and Linfoot, B. 2009. Measurement of the Effect of Power Absorption in the Lee of a Wave Energy Converter, OMAE2009-79793 . Vickers, A., and Lars Johanning. 2009. Comparison of damping properties for three different mooring arrangements. EWTEC2009.
