9TH Ewtec 2011

Aalborg Universitet Characterization of Wave Climate at Hanstholm Location with Focus on the Ratio between Average and Extreme Waves Heights Margheritini, Lucia; Frigaard, Peter; Stratigaki, V. Published in: 9th ewtec 2011 Publication date: 2011 Document Version Publisher's PDF, also known as Version of record Link to publication from Aalborg University Citation for published version (APA): Margheritini, L., Frigaard, P., & Stratigaki, V. (2011). Characterization of Wave Climate at Hanstholm Location with Focus on the Ratio between Average and Extreme Waves Heights. In A. S. Bahaj (Ed.), 9th ewtec 2011 : Proceedings of the 9th European Wave and Tidal Conference, Southampton, UK, 5th-9th September 2011 University of Southampton. General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. - Users may download and print one copy of any publication from the public portal for the purpose of private study or research. - You may not further distribute the material or use it for any profit-making activity or commercial gain - You may freely distribute the URL identifying the publication in the public portal Take down policy If you believe that this document breaches copyright please contact us at vbn@aub.aau.dk providing details, and we will remove access to the work immediately and investigate your claim. Characterization of Wave Climate at Hanstholm Location with Focus on the Ratio between Average and Extreme Waves Heights L. Margheritini1, P. Frigaard1, V. Stratigaki 2 1 Department of Civil Engineering, Aalborg University, 2Department of Civil Engineering, Ghent University Department of Civil Engineering · Sohngaardsholmsvej 57 · 9000 Aalborg, Denamrk Technologiepark 904, Ghent, Zwijnaarde, B-9052, Belgium 1 lm@civil.aau.dk pf@civil.aau.dk 3 Vicky.Stratigaki@UGent.be 1 The wave energy sector is in need of showing positive experience from the real sea trial in order to prove its feasibility. For this purpose, an accurate knowledge of wave conditions at the selected location of installation is fundamental. A design challenge for wave energy devices is the large differences between the extreme wave conditions in which the device is designed to survive and the average wave conditions for which the device is to be optimised. Indeed, the ratio between extreme loads and operational loads has been identified to be a fundamental factor for the design and the cost analysis of the wave energy units. The present paper provides an estimate of everyday wave conditions at Hanstholm location, by mean of scatter diagrams for different locations within the harbour vicinity. Results on wave heights transformation from offshore to shore realized with numerical model for operational and extreme waves are then presented and compared in different strategic points for WECs installation in the proximity of Hanstholm harbour within the Danish Wave Energy Centre (DanWEC). Deployment of WECs, Design Waves, Operational Conditions. I. INTRODUCTION The Danish Wave Energy Centre (DanWEC) has been realized because of participated desire to market the trial wave energy projects which are already on the way to Hanstholm, namely WaveStar, Waveplane and Dexa [1]. Additionally, the center will contribute at creating a local base for knowledge, education and possibly a workplace which will be leased out to trial projects. It is therefore likely that different developers will deploy their wave energy devices during the next years in this location. The DanWEC is a part of Hanstholm harbour in the North-West of Denmark (Fig. 1). The location is particularly challenging for the construction of a harbour and very interesting for the establishment of a wave energy centre due to its wave conditions. It is indeed because of the proximity of the Harbour, inaugurated in the 1967, that long time wave measurements are available. The areas South of Hanstholm is characterized by the beaches and sand dunes on the West coast. East of Hanstholm the landscape consists of reefs and cliffs. The main part of the harbour is characterized by the fishing industry. Today Hanstholm harbour began a process of expansion and modernization that includes the creation of the DanWEC. Successful demonstration of full scale wave energy converters is one of the primary challenges of the sector, especially when discussing floating devices. The loads that devices have to survive during storm conditions are much greater than the loads during operation conditions to such an extent that some WECs have been developed to include one “storm mode” by either removing the most delicate parts out of the water [2], or by submerging entirely for protection and accepting a reduced or nil power production when the significant wave height exceeds specific threshold [3]. Both wave loads and wave power are proportional to the wave height. It is also important to consider wave transformation going from offshore conditions to close to shore that can result in decreased wave power and loads. The objectives of the present paper are to describe the wave conditions in the proximity of Hanstholm harbour where the DanWEC is located and to highlight the ration between operation and extreme wave heights in three representative points for potential WECs installations. Indication on the wave resource will also be provided. Fig. 1 Hanstholm harbour location. II. WAVE DATA AND OFFSHORE WAVE CLIMATE Wave data used in the present study come from different buoys and time records [4][5]: 1) Operational conditions. Hanstholm (buoy 3110; 474 700 E, 6 332 100 N, 20 m. water depth. Duration: 01/11/0525/02/09). Fjaltring (buoy 2031; 441976 E, 6 259 466 N, 17.5 m. water depth. Duration: 11/08/99-25/02/09). Hirtshals (buoy 1041; 524 559 E, 6 381 744 N, 17 m. water depth. Duration: 11/12/91-25/02/09). 2) Extreme conditions. Hanstholm (buoy 3110; 474 700 E, 6 332 100 N, 20 m. water depth. Duration: from 19/5/1998 to 25/2/2009). The buoy outputs are Hm0 and Tm01 calculated over 30 minutes: Hm0 = 4 ⋅ m 0 Tm 01 = Wave directions in Hanstholm have been distributed in directions of 45°. The main wave direction results to be West direction with 52.0% probability of occurrence, followed by North-West and North directions with 27.10% and 10.28% (Fig. 2). Wave power has been calculated in deep water assumption as: Pwave = 0.49 ⋅ H S2 ⋅ (TP / 1.15) (4) (1) m0 m1 (2) +∞ m0 = ∫f n ⋅ E ( f ) df (3) 0 Where f is the frequency [s-1] and E(f) is the spectrum energy density depending on the frequency [m2s]. A. Operational wave conditions Hanstholm buoy is not directional and therefore the data from the other buoys provided the information on directionality. The angle of the incoming waves in Hanstholm has been interpolated by a vectorial addition of the wave directions in Hirtshals and Fjaltring. In occasions, the vectorial addition could not be executed because the times of the extracted data in the different buoys were not equal. In those cases, a weighed mean has been calculated between the next and the previous recordings from the specific buoy (Fjaltring or Hirtshals buoy) as:  timeHai − timeX _ previ DirX i = Dir _ timeX _ next i ⋅   timeX _ next i − timeX _ previ  timeHai − timeX _ previ + Dir _ timeX _ previ ⋅ 1 −  timeX _ next i − timeX _ previ      +  (3) Where: DirXi is the direction of the waves in the location (Hirtshals or Fjaltring) in the time i (time from Hanstholm). Dir_timeX_nexti is the direction of the waves in the location (Hirtshals or Fjaltring) in the successive time to time i from Hanstholm. timeX_nexti is the time of the location successive to the time i from Hanstholm. timeHai is the time i from Hanstholm. timeX_previ is the time of the location previous to the time i from Hanstholm. Dir_timeX_previ is the direction of the waves in the location (Hirtshals or Fjaltring) in the previous time to time i from Hanstholm [7]. Fig. 2 Directional offshore wave climate at Hanstholm buoy. B. Design wave conditions To determine the design wave height for specific return period, the general procedure is from Liu and Frigaard [5] is adopted. This makes use of “Peak Over Threshold method” to select the storm events over the record and the choice of Gambel or Weibull theoretical distributions for the extreme wave height distribution by mean of Maximum Likelihood Method or Least Square Method. A 90% fractal of the confidence interval provides a design wave height = 8.0 m for 50 years return period. In accordance to the standard the range of the wave peak period TP is given by:
130   
280  (5) Corresponding to 10.3 s. < TP < 15.1 s. III. NUMERICAL SIMULATIONS The bathymetry at location comprehends an area going from the Hanstholm buoy to shore, therefore including water depth from 20 m. to 0 m. Transformation of waves from offshore to three different locations (Fig.3) has been done using the computer model MILDwave [8] for specific combinations of Hm0 and Tp representative of offshore operational and extreme conditions. The three close to shore locations have coordinates: • 1: 475400E, 6331700N; 9.26 m water depth. • 2: 475900E, 6332100N; 11 m water depth. • 3: 476500E, 6332500N; 14 m water depth. be considered parallel to the top edge the figures, several meters away out from them. The waves have been generated in order to obtain the required wave height at the buoy location (considered here to be the offshore incoming wave). For incoming waves of 3 m and higher, points 1, 2 and 3 are bottom limited. Fig. 3 Bathymetry and reference points 1, 2 and 3. The waves are modelled as long crested irregular waves, characterized by a JONSWAP spectrum (γ = 3.3) defined by significant wave height Hmo and Tp. The wave directions West (270°), North-West (315°) and North (0°) have been simulated. The operational conditions were calculated for water level at +0.00 m, while the extreme wave conditions for a water level at +1.20 m. In each simulation, a uniform rectangular grid is created with cell spacing dx = 1.5 m and dy = 1.5 m. For the applications of the “long” grid for West direction a cell spacing of dx = 3.0 m is used. Several grids have been constructed, according to each wave direction. Grid areas situated above water level (dry land) are simulated as fully transmitting, considering ‘land grid cells’ as ‘water grid cells’. Fig. 4. Contour plot of the calculated Hm0 (W, Hm0 = 2.0 m, Tp = 5.9 s). C. Numerical settings No lateral sponge layers were used. At the top and bottom boundaries, sponge layers have been added for absorption of the generated waves. The width of each of the sponge layers is equal or larger than: 3x “Wave Length”. For each of the wave directions runs have been performed, for operational and extreme conditions. In all simulations the possibility that wave breaking occurs is taken into account. The land area at the coastline is simulated as not reflecting, in order to achieve a negligible amount of reflection from the land. The waves that are propagating towards the coast are being absorbed by the downwave sponge layer. D. Contour plots Results are here graphically presented by mean of contour plots generated by the MILDwave program for few representative cases (Fig.4-8). The model generation line is to Fig. 5 Contour plot of the calculated Hm0 (W, Hm0 = 4.4 m, Tp = 9.1 s) Fig. 6 Contour plot of the calculated Hm0 (N, Hm0 = 7.0 m, Tp=11.9 s) E. Transformation matrices For each direction, results are presented for various “7– gauges” arrays, placed at distances according to the limits suggested by the Coastal Engineering Manual (CEM) [9]. In the matrices (Tables 1 and 2) only the results of significant heights and peak periods for the third gauge of each array (i.e. the central location) are given, after an analysis using WaveLab. There is a weak tendency for longer waves to lose less in height than waves of the same height and shorter periods. The point with the highest reduction in wave height when waves are coming from West direction is point 2, located at 11.0 m water depth. This is not straightforward considering that point 1, despite being at a shallower location, has the least wave heights reduction for waves coming from the same direction. Indeed point 1 is approximately 0.5 Km further West from point 2 and 1 Km from point 3 and that could partially justify the results. What is probably explaining the results better is the shoaling, appreciable regularly in the studied wave conditions only for point 1, for waves coming from West direction. It is anyways difficult to explain local phenomena dominated by bottom interactions of different incoming waves and wave periods, such as the one under discussion now but there is a noticeable difference between the three locations under exam despite the relative small distance among them. With regard to changes on the directionality, this has been studied only for point 2. It appears that there is not a big change in directionality when waves approach the shore from Hantholm buoy to point 2: only in few cases the difference from the incoming wave direction is above 15˚ and anyways never more than 18˚. TABLE 1. TRANSFORMATION MATRIX OF SELECTED OPERATIONAL CONDITIONS FROM THE HANTHOLM BUOY TO POINTS 1, 2 AND 3 IN FIG.3, WEST, NORTH-WEST AND NORTH DIRECTIONS. WATER LEVEL +0.0M Fig. 7 Contour plot of the calculated Hm0 (NW, Hm0 = 3.5 m, Tp = 8.2 s). Hmo [m] 0.9 0.9 1.5 1.5 2.0 2.0 2.5 3.0 3.5 4.4 0.9 1.5 2.0 2.5 3.5 0.9 1.5 Fig. 8 Contour plot of the calculated Hm0 (NW, Hm0 = 8.5 m, Tp=13.1 s). Buoy Tp [s] 5.4 7.8 5.7 7.3 5.9 7.0 7.1 7.6 8.2 9.1 5.2 5.7 5.9 7.1 8.1 5.1 5.6 θ [˚] 271 278 271 283 273 282 280 283 286 280 316 316 315 316 317 1 1 Point 1 Hmo Tp [m] [s] 0.9 5.4 1.1 7.8 1.0 5.7 1.9 7.3 1.9 5.9 2.2 7.0 2.8 7.1 3.3 7.6 3.3 8.2 2.9 9.1 0.7 5.2 1.2 5.7 1.7 5.9 2.5 7.1 3.0 8.1 0.6 5.1 1.0 5.6 Hmo [m] 0.6 0.5 1.0 1.0 1.4 1.5 2.0 2.0 2.5 3.3 0.8 1.4 1.9 2.5 3.4 0.8 1.4 Point 2 Tp [s] 5.4 7.8 5.7 7.3 5.9 7.0 7.1 7.6 8.2 9.1 5.2 5.7 5.9 7.1 8.1 5.1 5.6 θ [˚] 279 289 285 298 288 290 291 300 300 302 318 319 319 322 327 359 357 Point 3 Hmo Tp [m] [s] 0.7 5.4 0.7 7.8 1.0 5.7 1.1 7.3 2.0 5.9 1.5 7.0 2.0 7.1 2.5 7.6 3.0 8.2 3.5 9.1 0.8 5.2 1.4 5.7 1.9 5.9 2.4 7.1 3.4 8.1 0.9 5.1 1.4 5.6 All the extreme waves tested are bottom limited for point 1. The highest wave reaching point 1 is 5.0 m in extreme conditions corresponding to an incoming wave of 8.6 m from North-West, Tp=13.1s with a water set-up of +1.2 m in a total water depth of 10.26 m. In general, for the same offshore conditions, the highest extreme waves arrive in point 3 where under these circumstances some shoaling may occur. For the mentioned offshore wave condition, the wave reaching point 3 is 8.1 m in a total water depth of 15.2 m. TABLE 2. TRANSFORMATION MATRIX OF SELECTED EXTREME CONDITIONS FROM THE HANTHOLM BUOY TO POINTS 1, 2 AND 3 IN FIG.3, NORTH-WEST AND NORTH DIRECTIONS. WATER LEVEL +1.2M. Hmo [m] 7.0 8.6 7.0 8.5 Buoy Tp [s] 11.9 13.1 11.9 13.1 θ [˚] 321 322 3 2 Point 1 Hmo [m] 4.9 5.0 4.3 4.4 Point 2 Hmo θ [m] [˚] 6.7 329 7.4 329 7.0 349 7.0 348 Point 3 Hmo θ [m] [˚] 7.0 316 8.1 328 7.2 348 7.7 348 IV. CLOSE TO SHORE WAVE CLIMATE It was then possible to extrapolate the scatter diagrams for points 1, 2 and 3; in particular, directional scatter diagram of point 2 is presented (Fig. 9), while for point 1 and 3 nondirectional results are reported (Tables 3, 4). The average wave power associated to the different points is 5.86 kW/m for pint 1, 4.20 kW/m for point 2 and 4.56 kW/m for point 3. Again, the highest wave climate in point 1 may surprise as this point is featuring the shallower water depth. Nevertheless point 1 is the one further West among the three (and the west direction is the main wave direction covering 52% of all offshore incoming waves), but most of all because of the occurrence of shoaling effects for operational conditions, resulting in an increased wave height compared to offshore conditions. If this is indeed the main reason, wave length should be decreased as consequence of shoaling. This has not been deeply investigated in the present report. TABLE 3. WAVE CLIMATE IN POINT 1 = 5.86 KW/M. Hs [m] Tp [s] Prob Pwave [kW/ m] P* prob 0.51.0 1.01.5 1.52.0 2.02.5 2.53.0 3.03.5 >3.5 0.83 1.18 1.83 2.3 2.81 3.29 3.78 5.55 7.11 6.74 7.04 7.55 7.79 9.39 0.60 0.14 0.10 0.06 0.05 0.03 0.00 1.65 4.24 9.60 15.86 25.43 35.84 57.15 0.99 0.59 0.93 0.97 1.33 0.90 0.16 TABLE 4. WAVE CLIMATE IN POINT 3 = 4.56 KW/M. Hs [m] Tp [s] Prob Pwave [kW/ m] P* prob 0.51.0 1.01.5 1.52.0 2.02.5 2.53.0 3.03.5 >3.5 0.80 1.29 1.72 2.16 2.63 3.13 3.75 5.83 6.46 6.78 6.79 7.61 4.12 9.21 0.67 0.101 0.081 0.08 0.03 0.01 0.01 1.59 4.60 8.5 13.46 22.47 17.22 55.3 1.06 0.46 0.69 1.05 0.63 0.19 0.47 V. RATIO BETWEEN AVERAGE AND EXTREME WAVE HEIGHTS From the results collected until now, it is possible to present a ratio between the average and the extreme wave heights at the three locations selected within the DanWEC in the proximity of the Hantholm harbour and at the buoy. The average wave height is defined for each wave climate as:  Figure 9. Wave Climate in Point 2 = 4.20 kW/m.  
 0.49  (6) Where Tm is the average wave climate period (= 5.67s, 7.31 s, 6.19 s and 6.61 s for the buoy, point 1, 2 and 3 respectively) and Pwave the wave power calculated like in Eq. 4 [kW/m]. The extreme wave heights are the ones already presented in Table 2. In Table 5 the results are summarized for the buoy and points 1, 2 and 3 considering two different extreme wave heights. Based on this definition, we can see that for the specific case the extreme wave heights are between 3.9 and 6.8 times higher than average wave heights for the 4 points taken into consideration in this study. If we consider the Hpmean to be related to the income that a developer may expect from the performance of its device while Hextreme related to the cost of the device, we could assume that we want the ratio Hpmean/ Hextreme to be as big as possible. In this optic, it seems reasonable to prefer point 1 or the buoy location to points 2 and 3. It is indeed possible to say that passing from point 3 to point 1 there is 73% gain considering 1stHextreme as the design wave height and 77% when considering 2ndHextreme. A gain of 73% on this ration could with some reason be considered as an economical gain equal to 73%. TABLE 5. SUMMARY OF AVERAGE AND TWO EXTREME WAVE HEIGHTS FOR WAVE CLIMATES AT POINT 1, 2 AND 3. Hpmean [m] 1st Hextreme [m] 2nd Hextreme [m] Hpmean/1st Hextreme Hpmean/2nd Hextreme Buoy 1.54 8.6 7 0.18 0.22 Point 1 1.28 5.0 4.3 0.26 0.30 Point 2 1.18 7.4 7.0 0.16 0.17 Point 3 1.19 8.1 7.2 0.15 0.17 VI. CONCLUSIONS The wave climate at location has been extensively studied to taking the directionality into account. The main wave direction is West direction. The wave climate offshore the Hanstholm harbor is 6.55 kW/m with 98% of the energy flux associated to the sectors N, NW and W. The offshore extreme wave analysis has been performed on direct wave measurements available from a buoy (non directional) outside Hanstholm harbour at d=20 m. water depth. The study resulted in a Hs50=8.0 m. Wave transformation from offshore to 3 different close to shore locations has been conducted with MILDwave for wave conditions representative of the scatter diagram and of the extreme waves, with different wave periods and a set up of +1.2 m for extreme conditions. The average wave power associated to the different points is 5.86 kW/m for pint 1, 4.20 kW/m for point 2 and 4.56 kW/m for point 3. The average wave height = Hpmean has been defined as the wave height that provides the average wave power when multiplied by the average period of a specific wave climate in the wave power equation. It is suggested that Hpmean is proportional to the expected income while Hextreme is proportional to the cost of the installation. It is therefore suggested the ration Hpmean/ Hextreme is a valuable indicator of the convenience of a location for wave energy installation. In the case understudy, after comparison of the 4 different Hpmean/ Hextreme it can be concluded that the choice of point 1 and 2 can result in a gain up to 73% on point 3. It is suggested that the gain is proportional to an economic gain. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] The DanWEC website [online]. Available: http://www.danwec.com/en/home.htm L. Marquis, M. Kramer, P. Frigaard, “First Power Production Figures from the Wave Star Roshage Wave Energy Converter”. In Proc. of the 3rd International Conference and Exhibition on Ocean Energy: ICOE 2010. S. Parmeggiani1, J. P. Kofoed, E. Friis-Madsen ”Extreme Loads on the Mooring Lines and Survivability Mode for the Wave Dragon Wave Energy Converter”, in Proc. of World Renewable Energy Congress 2011 – Sweden Marine and Ocean Technology (MO) 8-11 May 2011, Linköping, Sweden. L. Margheritini, J.P. Kofoed, V. Stratigaki, P. Troch, “Estimation of Wave Conditions for SSG Breakwater at Hanstholm Location” Department of Civil Engineering, Aalborg University, DCE Contract Reports; 90, 78 s. 2010. L. Margheritini1, A. Figueras1, J. P. Kofoed1, V. Stratigaki2, P. Troch “Available wave power in the vicinity of Hanstholm harbor and Danish Wave Energy Center (DanWEC)”. 2011 Submitted to Renewable Energy Journal. Liu and Frigaard, 2001. Zhou Liu and Peter Frigaard. “Generation and analysis of random waves.”ISBN: None. Department of Civil Engineering at Aalborg University, 2001. A. Figueras Alvarez “Estrimation of available wave power in the near shore area around Hanstholm harbour”. M. Eng. thesis, Civil Engineering department, Aalborg University, Denmark 2010. V. Stratigaki, P. Troch, L. Margheritini, Jens Peter Kofoed: “Estimation of wave conditions along a new breakwater for Hanstholm harbour (Denamrk), using the numerical model MILDwave” accepted to Coastal Structures 2011. The CEM website. Available: http://chl.erdc.usace.army.mil/cem