Aalborg Universitet Overtopping Measurements on the Wave Dragon Nissum Bredning Prototype Frigaard, Peter; Kofoed, Jens Peter; Rasmussen, Michael R. Published in: The Proceedings of The Fourteenth (2004) International Offshore and Polar Engineering Conference Publication date: 2004 Document Version Publisher's PDF, also known as Version of record Link to publication from Aalborg University Citation for published version (APA): Frigaard, P., Kofoed, J. P., & Rasmussen, M. R. (2004). Overtopping Measurements on the Wave Dragon Nissum Bredning Prototype. In J. S. Chung, K. Izumiyama, M. Sayed, & S. W. Hong (Eds.), The Proceedings of The Fourteenth (2004) International Offshore and Polar Engineering Conference (14 ed., pp. 210-216). The International Society of Offshore and Polar Engineers. 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Downloaded from vbn.aau.dk on: December 31, 2023 Proceedings of The Fourteenth (2004) International Offshore and Polar Engineering Conference Toulon, France, May 23−28, 2004 Copyright © 2004 by The International Society of Offshore and Polar Engineers ISBN 1-880653-62-1 (Set); ISSN 1098-6189 (Set) Overtopping Measurements on the Wave Dragon Nissum Bredning Prototype. Peter Frigaard1, Jens Peter Kofoed1 & Michael R. Rasmussen1 1 Depart. Civil Engineering, Aalborg University Aalborg, Denmark ABSTRACT The paper describes the methods used to estimate (calculated from some indirect measurements) the overtopping of the wave energy converter Wave Dragon placed in a real sea environment. The wave energy converter in quistion is the 237-tonne heavy Wave Dragon Nissum Bredning Prototype. Comparisons are made with laboratory measurements of the overtopping of a laboratory-scale model. KEY WORDS: Wave Energy; Overtopping; Prototype Testing; Wave Dragon; Low-Pressure Turbine; Scale E ects; INTRODUCTION Figure 2: Location of the Nissum Bredning Prototype waves towards a ramp, a oating reservoir for collecting the overtopping water, and some hydroturbines for converting the pressure head into power. Over the period 1998 to 2001, extensive testing on a scale 1:50 model was carried out at Aalborg University. During recent months, testing has started on a prototype of the Wave Dragon in Nissum Bredning, Denmark (scale 1:4.5 o the North Sea). Figure 1: The Wave Dragon Nissum Bredning Prototype. The Wave Dragon is an o shore wave energy converter of the overtopping type. 4-11 MW if placed in the North Sea. For the Nissum Bredning 18.2 kW is installed. Figure 2 shows the location of the Nissum Bredning Prototype, indicated on the map by the ellipse. The Nissum Broads (Nissum Bredning) are just o the North Sea, separated from it by two tongues of land. Basically it consists of two oating wave re ectors focusing the 210 Figure 3 shows the average wave energy density in the Broads. The upper arrow on the gure indicates the present location of the Nissum Bredning Prototype; the lower arrow shows the most exposed location in the Broads, to which the machine will be moved during early summer 2004. (calculated from indirect measurements) of the Nissum Bredning Prototype with overtopping rates measured in a hydraulic laboratory using a scale 1:50 model (scale 1:11.1 relative to the Nissum Bredning Prototype). THE WAVE DRAGON CONCEPT The present location was chosen to test the functionality of the machine, as easier access to the machine makes it possible to sort out the inevitable 'teething troubles' before deploying it in the most exposed position. The Wave Dragon consists of three main elements:  Two wave re ectors focusing the waves towards the ramp, linked to the main structure. The wave re ectors have the veri ed e ect of increasing the signi cant wave height substantially, and thereby increase the energy captured by approx. 100% under typical wave conditions.  The main structure consisting of a doubly curved ramp and a reservoir.  A set of low head Kaplan-propeller turbines for converting the hydraulic head in the reservoir into electricity. The Nissum Bredning Prototype was grid connected at its rst location (upper arrow on gure 3), thus making it the world's rst o shore wave energy converter. Figure 4: The Basic principle of the Wave Dragon. When the waves overtop the ramp, this water is lled into the oating reservoir at a higher level than the surrounding sea, and this hydraulic head is utilized for power production through the specially designed hydroturbines. Figure 3: Energy density in Nissum Bredning. The gure also shows the 2 di erent locations where the machine will be placed. The Wave Dragon Nissum Bredning Prototype has a total weight of 237 tonnes, still it is only a scale 1:4.5 of the eventual North Sea Prototype. Through an advanced phenumatic system it is possible to adjust the oating level. The entire main body of the Wave Dragon can be raised or lowered, and in this way the crest freeboard can be varied in order to maximize energy output from the Wave Dragon under the sea conditions prevailing at any time. This level adjustment happens continuously. The used time scale for the operation corresponds to approx. 250 wave periods. Over the next 2 years, an extensive measuring program will establish the background for optimal design of the structure and regulation of the power take-o system. Planning for a full scale deployment within the next 2-3 years is in progress (Srensen et. al. 2003). Such a structure will probably be placed somewhere in the North Sea, or in the North Atlantic Ocean. Knowledge about overtopping rates is privotal for establishing the optimal regulation strategy for the Wave Dragon and for the eÆciency achievable from the Wave Dragon. What is the optimal oating level for a given sea condition? When is the optimal time to switch on or switch o the turbines? The Nissum Bredning Prototype is instrumented to monitor power production, wave climate, forces in mooring lines, stresses in the structure, and movements of the Wave Dragon reservoir and the re ecting arms. Equations predicting average overtopping rates for the Wave Dragon were rst established through tank tests at Aalborg University on a scale 1:50 model in 1998-1999 (Martinelli and Frigaard 1999). The present paper compares the estimated overtopping rates 211 Hald and Frigaard 2001 reported overtopping from the tests with the second-generation model, and presented a modi ed overtopping equation: Martinelli and Frigaard 1999 tested the Wave Dragon in longcrested seas as well as in short-crested seas with some standard JONSWAP spectra. They presented the following equation to predict overtopping (linear ramp inclination 45 deg.): qS R = 0:017cd
exp( 48 Hcs q where, c = = L = q d op p = = s c = = S T H R op ) 2 p L gH 3 p q s Sop 2 discharge due to overtopping reduction coeÆcient accounting for directional spreading e ects, cd = 0:9 length of structure ramp; a length of 86.6 meter was assumed (21.3 meter for the Nissum Bredning Prototype) g 2 Hs =Lop ; whereLop = 2 Tp peak period; a constant ratio 1.2 between the peak and the mean period was assumed signi cant wave height crest freeboard Rc = 0:025cd
exp( 40 H s qS p p op L gHs3 2 ) Sop 2 Figure 6: Dimensionless overtopping relation. The Wave Dragon model tested by Martinelli and Frigaard 1999 turned out to have some rather large movements; consequently the dynamic behavour of the model was slightly changed. Figure 6 shows the test data from Hald and Frigaard 2001. Tests rst carried out using the original model layout in Martinelli and Frigaard 1999 were repeated by Hald and Frigaard 2001; these tests are labelled Test series A and Test series B in the gure. A reasonable t with the Martinelli and Frigaard 1999 equation (full line) is seen. Test series C and D represent minor modi cations made to the re ector draught; resulting in a small increase in the overtopping. Test series E to J represent data measured on the model with the doubly curved overtopping ramp implemented. The e ect of the doubly curved ramp is clearly seen. The Hald and Frigaard equation (dotted line) is also shown on the gure together with the original data points. Kofoed 2003 tested various slope layouts in order to nd a structure producing the maximum overtopping e ect. Kofoed's tests veri ed a doubly curved ramp to have a signi cantly positive e ect. Incorporating these changes, a second-generation model was constructed and tested at Aalborg University in 2001. A photo of the model and the prototype can be seen in gure 5. All data shown in gure 6 represent 5 standard wave conditions. Each data point corresponds to a test including approx. 1000 waves in a controlled wave environment. In table 1 the wave conditions are listed in North Sea scale. The reason for choosing the Nissum Broads location for the 1:4.5 scale model of the Wave Dragon was indeed the fact that the wave climate in Nissum Broads corresponds very well with the wave climate in the North Sea, scaled by 4.5. Situation [-] 1 2 3 4 5 s H m 1.0 2.0 3.0 4.0 5.0 T p sec. 5.6 7.0 8.4 9.8 11.2 Frequency % of time 46.7 22.6 10.8 5.1 2.4 Energy MWhmyear 8.2 23.8 30.2 29.4 24.3 Tabel 1: Tested wave situations. In addition, an overtopping simulation software tool had to be developed, allowing real time simulations of overtopping rates Figure 5: Photo showing the doubly curved ramp. 212 The regulation strategy for the Wave Dragon consists of 2 steps. First, the optimal crest freeboard ( oat level) for the actual sea condition is calculated. For this calculation a time scale corresponding to approx. 250 wave periods is used. Second, the 'work span' for the turbines needs to be de ned. The 'work span' is de ned as the the accepted variation in the water level in the reservoir. Once the water level reaches the top of the 'work span' in question, all turbines will be switched on. And correspondingly, when the water level reaches the lowest level of the 'work span' all turbines will be switched o . (Jacobsen and Frigaard 1999). The design of the reservoir and the turbine con guration of the Wave Dragon had to be based on a reasonable input discharge history. The time history was considered a random process, and it was generated according to a distribution of overtopping volumes per wave consolidated in literature and mean values obtained by the tests previously described. The probability of occurrence of wave overtopping for vertical structures, cf. Franco et al. 1994 and for rubble mound dikes, cf. van der Meer and Janssen 1995, can be given in the form of a Rayleigh type distribution: ov P Rc )2 ) = exp( ( cH s where, = c a constant; set to 1.21 c can be interpreted as a roughness factor. According to van der Meer and Janssen 1995, the distribution of the overtopping volumes of the individual waves, given that overtopping takes place, is given by a Weibull distribution with shape parameter 0.75: V jov P = P (V  mean V j 0:75 ( ( Vmean ) a ) )=1 o exp where, m = = = ov = a q T P mean V j o = qTm Pov mean overtopping discharge mean wave period; a constant ratio 1.2 between the peak and the mean period was assumed. the probability of overtopping given in the equation above mean overtopping of wave, given wave is overtopping Note that the scale factor a used for the quanti cation of the overtopping, given that overtopping takes place, is the average overtopped volume, magni ed due to the 'average' overtopping probability. The probability Pv , that a generic wave (the comming wave) is associated to a overtopping volume V less or equal to Vmean is: P v = P (V  mean ) = PV jov V
ov P Kofoed and Burchart (2000) veri ed the equations giving the time variations of the overtopping for smooth slopes TURBINE CONFIGURATION In order to maximize energy output from the Wave Dragon, the machine is equipped with several small turbines rather than one larger turbine. In this way it is possible e.g. to switch on only a part of the installed power in sea conditions producing relatively small amounts of overtopping water. Furthermore this construction allows a single small turbine to be switched on or switched o depending on the actual amount of water coming from a single wave. Figure 7: Kaplan turbine with cylinder gate. A number of di erent strategies to control this mechanism will be tested in the coming years in order to maximize energy output. For the time being, the 'work span' is simply divided into a number of distances. 213 Ideally, the Wave Dragon would be equipped with several similar small turbines; however, the actual turbine con guration is a result of compromises brought on by nancial constraints rather than logical, optimal technical solutions. Like most other wave energy projects, the Wave Dragon project had to adjust the turbine con guration according to the available funds. Therefore, the Wave Dragon is equipped with 3 di erent types of turbines:    A Kaplan turbine with Siphon inlet, see gure 9. The turbine was developed through the EU CRAFT project: Low{ Pressure Turbine and Control Equipment for Wave Energy Converters (Wave Dragon). Diameter of the Siphon turbine is 0.34 meter, and area of cross section is 0.0908 m2 . Rated power output is 2.6 kW (500 kW for the North Sea Wave Dragon). Calibrated ow is 0.22 M 3 =sec: at 0.5 meter head. 3 Dummy turbines, see gure 8. The turbines are not able to produce power, but simply let the overtopped water run back into the sea through a set of calibrated valves. Diameter of the valves are 0.43 meter, and area A of cross section is 0.147 m2 . The discharge Q through the dummy turbines was calibrated to follow the equation Q = k
p2gh, (h=pressure A head). For the 3 dummy turbines the k-values were found to be 1.05, 1.07 and 1.09 (Knapp and Riemann 2003). 6 Kaplan turbines with cylinder gates, see gure 7. These turbines have data similar to the 'Siphon' Kaplan turbine. Diameter of these turbines is 0.34 meter. Rated power output is 2.6 kWatt. Calibrated ow 0.22 m3 =sec at a head of 0.5 meter. The turbines were fabricated in Austria by Kossler, and were installed in September 2003. Installation of gear and generators were nished February 2004. Figure 9: Kaplan turbine with Siphon inlet some uncertainty. In particular, it must be stressed that the actual amount of water passing the turbines will be slightly lower than calculated, as the ow through the turbines is not linearly depending on the head. However, relative to the typical duration of the operations of the turbines (open valves), the duration of the opening of and closing processes of the valves are insubstantial. WAVE MEASUREMENTS IN NISSUM BREDNING Waves are measured indirectly through a pressure transducer placed approx. 2 meter under the sea surface. The pressure transducer is mounted on an arm attached to the mooring pile of the Wave Dragon. The transformation from pressure to surface is done by linear wave theory. Wave parameters are calculated continuously in a 17 minutes long time window. The length of the time window corresponds to 250 - 300 waves. Zero down crossing analyses are performed and the average period Tm is used to characterize the waves. The peak periods Tp of the spectra are assumed to be Tp = 1:2
Tm . PROTOTYPE OVERTOPPING The Wave Dragon oats on air chambers to make it possible to adjust the oating level of the machine. Therefore, scale e ects have to be considered, and tank tests are not directly scalable. Scale e ects will mainly be presented on the movements of the Wave Dragon, and laboratory tests have shown that the overtopping is very strongly depending on the movements. It must be mentioned that movements have been measured in all laboratory tests, and that the Nissum Bredning Prototype is equipped with 6 accelerometers in order to measure movements. Analyses of the movements have still to be performed. Figure 8: The 3 'dummy' turbines. Based on measurements of the instantaneous pressure head, and knowing the number of currently open turbines, it is possible to calculate the ow through the turbines. The opening period is de ned as the period from starting time of the opening process to starting time of the closing process. Obviously the periods of the opening and closing operations will be accompanied by During the winter of 2003/2004 and over the coming winter, overtopping results have been and will be collected continuously, in periods without down time of the instruments. 214 Obtaining measurements in a real sea condition is diÆcult. And it has indeed turned out to be even more diÆcult and timeconsuming than expected. The sea is extremely rough on the instruments. Nevertheless, throughout the months of November and December 2003 average overtopping rates have been successfully monitored on the Nissum Bredning Prototype, both under ordinary conditions and under storm conditions (Kofoed and O'Donovan 2003). Figure 10 also demonstrates the frequency for the turbines to be switched on (and later switched o ). The situation seen on the gure corresponds to a turbine being switched on almost every 10 second in average. It is assumed that all water overtopping the ramp passes through the turbines. This means that no spill is expected. Visual inspections support this assumption; at least in calmer wave conditions. where, The outlet is calculated as: Q overtop F = = = = G = i N h Therefore, an estimate of the overtopping amount can be calculated, knowing the characteristics of the turbines, the head of the free surface and the opening time of the turbines. Figure 10 gives an example of the measurements. Unfortunately,it is diÆcult to see the measurements clearly in the gure; however, the purpose of including the plot is to give an idea of the variations in the signals. ' Q out = R PN i time =1 i (h)Gi dt F turbine number number of installed turbines instantaneous head in reservoir calibrated function describing ow out through a turbine function describing whether the turbine valve is opened or closed. At present this function can only take the values 0 or 1. The overtopping data from the Nissum Bredning Prototype show good agreement with the laboratory overtopping data, although slightly more overtopping is seen on the Nissum Bredning Prototype than expected from the laboratory. The x-axis represents the time. The length of the axis is 5 minutes corresponding to a little less than 100 waves. Figure 11 gives an example of such a comparison. 0.003 Prototype data 3 2 Q*=q*sqrt(s op/(2 p ))/(sqrt(g H s L ) The y-axis shows 5 curves: 'work span' (in this example app. 10 cm), water level within 'work span' (in this example 0% 100%), number of running turbines (0-10 in the example), wave height (approx 50 cm in the example) and water level in reservoir (50-60 cm in the example). Hald & Frigaard (2001) 0.002 0.001 0.000 0.00 0.05 0.10 0.15 R*=Rc/Hs*sqrt(s op/(2p)) Figure 11: Example of Overtopping results In the near future, much more data covering a larger parameter range can be expected. Hopefully, this will lead to more detailed conclusions. For the time being, however, we nd it surprising, that the data do in fact correspond so well. Normally, overtopping data show an enormous scatter, even for laboratory data. A factor of 5-10 is not unusual in literature. However, for the conditions with large amounts of overtopping less scatter is normally seen. Figure 10: Measured data as shown in the control program. In the controlled laboratory environment, the waves were generated, Rayleigh distributed, with a standard JONSWAP spectrum and a standard groupiness factor. The sea conditions were kept constant for a period corresponding to 1000 waves. In the real sea in Nissum Broads, the wave conditions are much more scattered. From gure 10 each individual wave can be recognized as a change in the water level in the reservoir. This can be seen most clearly when studying the curve for the water level within the 'work span' (red curve). 215 It seems like, very few scale e ects can be observed. The Wave Dragon oats on an air cushion, which means that in order to establish a correct model, the sti ness of this air cushion has had to be scaled. Such a scaling is very diÆcult to implement correctly, and a certain amount of scale e ects was to be expected. Hald, Tue and Frigaard, Peter (2001): Forces and Overtopping on 2. generation Wave Dragon for Nissum Bredning. Phase 3 project, Danish Energy Agency. Project no: ENS-51191/000067. Hydraulics & Coastal Engineering Laboratory, Aalborg University. Jacobsen, K.P. and Frigaard, P. (1999): User's manual for the Program Wave Dragon { Power Simulation. Aalborg University. Measurements of the movements of the Nissum Bredning Prototype have not been analysed yet, but based on several visits and a preliminary look at the data, we think that some scale e ects are present on the movements (larger movements in laboratory). Knapp, W. and Riemann, S. (2003): Measurements on Wave Dragon, Nissum Bredning on 24th of May 2003: Dummy turbine calibration. Laboratorium fur Hydraulische Maschinen, Technische Universitat Munchen. CONCLUSION Overtopping has been measured on the Nissum Bredning Prototype of the Wave Dragon. The functionality of the Wave Dragon overtopping concept has been proven. Kofoed J. P. and Burcharth H. F. (2000): Experimental veri cation of an empirical model for time variation of overtopping discharge. 4. European Wave Energy Conference (EWEC 2000), Aalborg, Denmark, Dec. 2000. Good agreement to laboratory-based overtopping equations was found, although some more overtopping was measured in the Nissum Bredning Prototype. Kofoed J. P., Frigaard P., H. C. Srensen and E. Friis-Madsen (2000): Development of the Wave Energy Converter - Wave Dragon. ISOPE-2000, Seattle, USA, May, 2000. The extra overtopping is assumed to originate from wind e ects, and from scale e ects on the movements of the Wave Dragon. Kofoed J. P.(2003): Wave Overtopping of Marine Structures - Utilization of Wave Energy. Ph. D. Thesis, defended January 17, 2003 at Aalborg University. Department of Civil Engineering. ACKNOWLEDGEMENTS Kofoed, J. P. and O'Donovan, E. (2003): Status report First o shore experiences, Wave Dragon, Nissum bredning. Hydraulics & Coastal Engineering Laboratory, Aalborg University, September 2003. (Con dential). The Wave Dragon has recieved support from The European Commission, The Danish Energy Agency and several private companies and foundations. Please see homepage for a full list. MORE INFORMATION Martinelli, L. and Frigaard, P. (1999): The Wave Dragon: 3D overtopping tests on a oating model. Low-Pressure Turbine and Control Equipment for Wave Energy Converters (Wave Dragon), contract JOR3-CT98-7027. Aalborg University. Futher information on the project can be found at one of the Web pages: www.wavedragon.net or www.civil.aau.dk for further information on the project. H.C. Srensen, E. Friis-Madsen, W. Panhauser, D. Dunce, J. Nedkvinte, P. Frigaard, J.P. Kofoed, W. Knapp, S. Riemann, E. Holmen, A. Raulund, J. Prst, L.K. Hansen, L. Christensen, T. Nohrlind, T. Bree, P. McCullen (2003): Development of Wave Dragon from Scale 1:50 to Prototype. Proc. 5th European Wave Energy Conference, Cork Ireland. REFERENCES Franco, L., Gerlone, M.D. and Van der Meer, J.W. (1994): Wave overtopping at vertical and composite breakwaters. Proc 24th International Conference on Coastal Engineering, Kobe, Japan. ASCE. Van der Meer, J.W. and Janssen, J. P. F. M. (1995): Wave Run Up and Wave Overtopping at Dikes. Task Committee Reports, ASCE. 216