Transmission studies on horizontal interlaced multi-layer moored floating pipe breakwater (HIMMFPB) with three layers of pipes

Transcription

1 Indian Journal of Geo-Marine Sciences Vol. 42(6), October 2013, pp Transmission studies on horizontal interlaced multi-layer moored floating pipe breakwater (HIMMFPB) with three layers of pipes Maltesh V Hoolihalli 1 & Arkal Vittal Hegde 2 Department of Applied Mechanics and Hydraulics, National Institute of Technology Karnataka, Surathkal , Karnataka, India [ m_hoolihalli@yahoo.com; arkalvittal@gmail.com] Received 22 May 2012; revised 13 August 2012 Present study consists part of the series of physical model scale experiments conducted for the study of the transmission characteristics of horizontal interlaced multi-layer moored floating pipe breakwater (HIMMFPB). Studies were conducted on physical models of the floating breakwater with three layers of PVC pipes, wave steepness, H i /gt 2 (H i =incident wave height, g=acceleration due to gravity and T=wave period) varying from to 0.936; relative width, W/L (W=width of the breakwater) varying from 0.4 to 2.65 and relative spacing of pipes, S/D=2 (S=horizontal c/c spacing of pipes and D=diameter of the pipes). Transmitted wave height is measured and the collected data is analyzed by plotting nondimensional graphs depicting the variation of K t (transmission coefficient) with H i /gt 2 for various values of d/w (d=water depth) varying from to and K t with W/L for different values of H i /d which was varied from 0.06 to It is observed that K t decreases as H i /gt 2 increases for the range of d/w values used in the study. It is also observed that K t decreases with an increase in W/L for the range of H i /d values used. Maximum wave attenuation achieved in the study with the present breakwater configurations is 78.62%. [Keywords: Transmission coefficient; Wave steepness; Relative breakwater width; Relative wave height; Ratio of spacing to diameter]. Introduction Harris 1 conducted studies on a model breakwater which consisted of a floating slab of breadth comparable to the length of the wave to be attenuated. It was observed that performance was most sensitive to the area of solid slab per meter length of breakwater, but is sensitive to a lesser degree on overall breadth. They concluded that the performance [defined by 100 (1 K t )] improved as the ratio of wavelength to solid breadth became 1. Performance approached to 98% when the ratio of wavelength to solid breadth was 0.4. Chen 2 designed three rigid floating breakwaters and added two more later, each making use of a different mechanism or combination of mechanisms of wave energy dissipation and reflection. Basic concept of model A was to dissipate the energy by breaking waves on a sloping board beach. The results showed that for relative width=2.7, the transmission coefficient was less than about 0.20 for Model B. Model C was a type of perforated breakwater, designed to decrease the direct striking force by decreasing the reflecting area and to dissipate wave energy by the flow of water through the holes. Model D consisted of a platform ballasted sufficiently to cause it to be immersed with its bottom beneath the water surface. Model E was a modified version of Model D. To avoid the transmission of energy through the gates, Model E was adopted based upon the concept of a fixed energy dissipater similar to those used at the front of a spillway, rather than using the flapping gates. Harms 3 presented design curves for the Goodyear floating tire breakwater that are based on 1:4 and 1:8 scale laboratory tests, and have additionally been substantiated by the available full scale data. Two important floating tire breakwater design parameters have been assessed over a practical range of conditions, the breakwater size required for a desired level of wave attenuation and the associated peak mooring force. Experimental data for the Goodyear floating tire breakwater compared to that for the vertical plate indicate that the Goodyear floating tire breakwater with beam size equal to twelve times draft, offers approximately the same level of wave attenuation as a fixed vertical plate of equal draft. Murali 4 studied the conventional floating breakwaters and the feasibility of developing a cage

2 723 HOOLIHALLI & HEGDE: TRANSMISSION STUDIES ON HIMMFPB WITH THREE LAYERS OF PIPES floating breakwater was explored. Experiments were conducted to study the performance under wave and wave-current environments. Variation of water surface oscillation and velocities within the cage, the effects of mooring line stiffness and initial tensions on transmission characteristics were discussed. It was concluded that for a wide range of W/L (i.e., between 0.14 and 0.6), gap-to diameter ratio equal to 0.22, relative draft of 0.46, and width of float equal to 1.0, it was possible to achieve a transmission coefficient below 0.5. Transmission coefficient increased by about 30% for waves influenced by the following currents. Location or alignment of the floating breakwater should be selected so that current effects are minimal. Based on the experimental and theoretical investigations on the behavior of pontoon type floating breakwaters, Sannasiraj 5 concluded that theoretical and experimental measurements show good agreement except at the roll resonance frequency. Transmission coefficient is not significantly affected by the mooring configurations studied. However, the tests showed higher transmission coefficients for a floating breakwater with cross moorings. The mooring at the water level and at the bottom of the floating breakwater yielded significantly smaller mooring forces than those obtained with crossed moorings. Sundar 6 studied the hydrodynamic performance characteristics of a floating pipe breakwater model (row of pipes separated by a distance equivalent to the pipe diameter) moored to the flume floor with a slack mooring. Investigations were carried out with random waves in an experimental program. Tests were conducted on three models with pipes of different diameter. Average reflection and transmission coefficients were evaluated as a function of relative breakwater width (W/L). Statistical analysis was carried out to prove that the heave and surge motions, as well as peak mooring forces follow the Rayleigh distribution. Hegde 7 tested performance characteristics of horizontal interlaced multi-layer moored floating pipe breakwater. Experiments were conducted in a regular wave flume for physical models with S/D = 5, W/L = 0.77 to Transmission and mooring forces were measured. The transmission coefficient K t decreased with an increase in relative breakwater width W/L and wave steepness H i /L. Dong 8 conducted two-dimensional physical model tests to measure the wave transmission coefficients of the three types of breakwaters under regular waves with and without currents. Based on the initial comparison of the wave transmission coefficients, they proposed the use of the board-net floating breakwater for use with fish cages. It was observed that the wave transmission coefficients of two sizes of boards (32 m and 100 m wide); the protection of the breakwater was much improved by the wider board. Krishnakumar 9 studied the hydrodynamic performance of single and double wave screens. Peak pressures on the screen are found to reduce with a decrease in its relative depth of submergence (d s /h). Percentage reduction in K r increased with decrease in d s /h. At the same time the percentage increase in K t for the lowest depth of submergence (d s /h = 0.25) is found to be about 10% to 15%. Murali 10 carried out a wave flume study to investigate the hydrodynamic interaction with a model offshore deck in regular and random wave fields. Model deck consisted of a Perspex sheet of size 2 m 1.95 m 0.01 m. Perspex sheet was connected to the longitudinal and transverse stiffeners for rigidity. It was concluded that K t in general decreases with an increase in relative length of the deck slab B/L and the relative clearance between the still water level (SWL) and the deck slab e/h. K r is found to marginally increase with an increase in B/L. Further, K r is found to increase with a decrease in e/h (draft) both for regular and random waves. Minimum force occurs when the value of B/L is about 1.0. Wang 11 presented the configuration of a floating breakwater, i.e. a structure consisting of perforated diamond-shaped blocks assembled with shafts. Transmission coefficient is observed which increased from 0.04 to 0.94 as B/L (B is the breadth of the breakwater) decreased. This reverts to the fact that larger the widths of the floating breakwater, more is the stability of the breakwater and therefore smaller is its oscillation. When the water depth or mooring line angle is increased, transmission coefficient decreases more gently as B/L increases from0.132 to In the present study attempts were made to assess the hydrodynamic performance of horizontal interlaced multi-layer moored floating pipe breakwater (HIMMFPB) in the regular wave flume of the Department of Applied Mechanics and Hydraulics at National Institute of Technology Karnataka (N.I.T.K), Surathkal, Mangalore, India for n = 3, S/D=2, d/w = to This breakwater model

3 INDIAN J. MAR. SCI., VOL. 42, NO. 6, OCTOBER was tested and found to be stable for waves of 18 cm, which is equivalent to 5.4 m wave height occurring in the monsoon season (model scale 1:30) in the Arabian Sea, off Mangalore coast, India. Breakwater is assumed to be flexible and intended to be economical as the material involved in its construction is poly vinyl chloride (PVC) pipes, which are relatively inexpensive and easily available compared to materials used for the construction of other types of breakwaters. The dimensional analysis is carried out using Buckingham's π theorem. The variables considered under the present investigations are: W = width of the breakwater; d = depth of water; L = wavelength; H i = incident wave height; H t = transmitted wave height; T = wave period; ρ = mass density of sea water; γ = weight density of sea water; g = acceleration due to gravity. Considering L, H i and ρ as repeating variables, the dimensional analysis yielded following non-dimensional π terms: H t /H i (transmission coefficient, K t ), W/L, H i /gt 2, d/w, H i /d. Materials and Methods A pictorial representation of the breakwater model in plan and section is shown in Fig. 1. Model consists of PVC (poly vinyl chloride) pipes of 40 mm diameter. Pipes are placed parallel to each other with spacing S between them in each layer, and the adjacent layers are oriented at right angles to each other so as to form an interlacing of pipes. In the flume longitudinal pipes are placed along the direction of propagation of waves and transverse pipes are placed and tied perpendicular to the longitudinal pipes. Length of the longitudinal pipes defines the width W of the floating breakwater. It is well known that with appropriate number of pipe layers n, spacing of pipes S, and relative breakwater width W/L it is possible to achieve a considerable and effective attenuation of the incident wave energy. Levels of pre-tension in the leeside and seaside moorings were kept at zero before the wave structure interaction. To measure the incident, reflected and transmitted wave heights, the calibrated capacitance type wave probes were used. They have long and insulated rod, which is partially submerged in water. The sensors undergo changes in capacitance and the signals are processed there itself for conversion which aides in easy transmission to distant point without being affected by ambient noise. An electronic meter is kept at a convenient place and all the sensors are connected to it. The meter processes the signals and displays them in four barographs provided on the front panel of the meter. The data is then fed continuously to the computer through a RS232 port. Present study deals with the analysis of the variation of transmission coefficient with the wave steepness and relative breakwater width. Experiments In the present work regular waves of different periods and heights as mentioned in Table 1 were generated for W/L ratios of 0.4 to The water depths considered are 400 mm, 450 mm, 500 mm. A spacing to diameter ratio of S/D = 2 was adopted. Based on the W/L ratios used the range of prototype breakwater widths arrived were from 1.6 m to 5.4 m. Waves are generated in bursts of 5 waves only in order to avoid wave distortion due to reflection and re-reflection from the breakwater structure and the wave paddle. After each burst, wave generation was stopped till tranquility was achieved in the flume. Thereafter, next burst was generated. The breakwater model was placed in the flume at a distance of 28 m from the wave generator flap as shown in Fig. 2. Table 1 Details of wave-specific and structure-specific parameters considered in the present study Wave-specific parameters Experimental range Incident wave height, H i (mm) 30, 60, 90, 120, 150, 180 Wave period, T (sec) 1.2, 1.4, 1.6, 1.8, 2.0, 2.2 Depth of water, d (mm) 400, 450, 500 Fig. 1 Floating pipe breakwater model setup used in present work Structure-specific parameters Experimental range Diameter of the pipes, D (mm) 40 Ratio of spacing to of pipes, S/D 2 Relative breakwater width, W/L 0.4 to 2.65 Number of layers, n 3

4 725 HOOLIHALLI & HEGDE: TRANSMISSION STUDIES ON HIMMFPB WITH THREE LAYERS OF PIPES Scale Factor The floating breakwater model was constructed to suit the prototype maximum wave height of 5.4 m and a maximum water depth of 15 m. A geometrically similar scale of 1:30 was adopted and hence the range of model wave heights was 30 mm to 180 mm for water depths of 400 mm-500 mm. Based on this scale ratio, the model to prototype scale factors were obtained using Froude s model law. Fig. 2 Regular wave flume setup for the present investigation Results and Discussion Effect of Wave Steepness on Transmission Coefficient Figures 3 to 9 show the variation of K t with H i /gt 2 and d/w as parameter for n= 3, S/D=2. It was found that the exponential variation gives the largest R-squared value as compared with logarithmic, quadratic, power and linear fit. Figures also show a decrease in the value of K t with an increase in H i /gt 2 for a range of d/w=0.082 to This tendency is obvious since, as steepness increases the wave heights are large (and wavelengths are short), the waves tend to become sharp crested and hence become unstable. As these steep waves meet an obstruction, large amount of energy is dissipated due to instability and hence more attenuation of wave height is achieved. Figures 3 to 9 also show a decrease in K t with an increase in H i /gt 2 for a range of d/w=0.082 to For low steepness waves such as H i /gt 2 =0.063 considered in present studies, the observed maximum wave attenuation was 2.61% and for waves of higher steepness Fig. 3 Variation of K t with H i /gt 2 for n=3, S/D=2 for d/w=0.082 to Fig. 4 Variation of K t with H i /gt 2 for n=3, S/D=2 for d/w=0.095 to H i /gt 2 =0.936 the maximum attenuation was found to be 78.62%. These Figures also indicate that d/w has significant influence on K t. It can be easily derived from the graph that there is an increase in K t with an increase in d/w. This means that as d increases relative to W,

5 INDIAN J. MAR. SCI., VOL. 42, NO. 6, OCTOBER Fig. 5 Variation of K t with H i /gt 2 for n=3, S/D=2 for d/w=0.108 to Fig. 6 Variation of K t with H i /gt 2 for n=3, S/D=2 for d/w=0.125 to Fig. 7 Variation of K t with H i /gt 2 for n=3, S/D=2 for d/w=0.149 to Fig. 8 Variation of K t with H i /gt 2 for n=3, S/D=2 for d/w=0.185 to Fig. 9 Variation of K t with H i /gt 2 for n=3, S/D=2 for d/w=0.244 to more energy flows beneath the breakwater and also since the submergence of the breakwater is constant, the percentage of water column intercepted by the breakwater decreases as depth increases. The maximum attenuation found was 78.62%. The graphs also reveal that for d/w=0.082, the maximum attenuation of wave height was78.51% and for d/w=0.276, the maximum attenuation of wave height is 58.51%. Hence, there is a significant influence of d/w on K t. Effect of Relative Breakwater Width on Transmission Coefficient Relative breakwater width (W/L) plays a significant role in the wave transmission characteristics, because of the viscous as well as turbulent dissipation of wave energy as the waves propagate past the breakwater. It is observed from the graphs plotted that the transmission coefficient K t decreases with the increasing relative breakwater width. This is due to the fact that, for a given wavelength as W/L increases the width of the breakwater increases and hence, large amount of wave structure interaction takes place which results in greater wave attenuation. Figures 10 to 15 show the variation of K t with W/L and H i /d as a parameter. It was found again that the exponential variation gives the largest R-squared value as compared with linear, quadratic, power and logarithmic fit. The graphs indicate that as W/L increases K t decreases for the range of H i /d = to used in the study. The graphs also reveal that as H i /d increases a decrease in the value of K t is observed. Higher values of H i /d indicate either wave height is larger or the depth of water is lesser. In view of this, if d is constant and H i is varying, an increase in H i /d indicates an increase in H i value and hence, the steepness and this trend is obvious in the graphs. The

6 727 HOOLIHALLI & HEGDE: TRANSMISSION STUDIES ON HIMMFPB WITH THREE LAYERS OF PIPES Fig. 10 Variation of K t with W/L for n=3, S/D=2 for H i /d=0.06 to Fig. 14 Variation of K t with W/L for n=3, S/D=2 for H i /d=0.300 to Fig. 11 Variation of K t with W/L for n=3, S/D=2 for H i /d=0.120 to Fig. 12 Variation of K t with W/L for n=3, S/D=2 for H i /d=0.180 to Fig. 13 Variation of K t with W/L for n=3, S/D=2 for H i /d=0.240 to Fig. 15 Variation of K t with W/L for n=3, S/D=2 for H i /d=0.375 to transmission coefficient decreases with the increase in relative breakwater width for the range of H i /d values from to considered in the present study. The maximum wave attenuation achieved in the present study is 78.62% for H i /gt 2 =0.637, H i /d=0.20 and W/L=2.40. Conclusions Based on the results obtained and subsequent discussion carried out, following conclusions are drawn: K t decreases with an increase in H i /gt 2 values from to for a range of d/w= to For low steepness waves H i /gt 2 =0.063 considered in present studies, a maximum attenuation of 2.61% and for higher steepness waves H i /gt 2 =0.936 maximum attenuation of 78.62% is achieved. This clearly shows the influence of wave steepness on the transmission coefficient. K t decreases with an increase in d/w values from to For d/w=0.082 the maximum attenuation of wave height is 10.64% and for d/w=0.276 the maximum attenuation of wave height is 78.62%. Hence, there is a significant influence of d/w on K t.. K t decreases with the increase in W/L values form 0.40 to 2.65 for the range of H i /d=0.060 to K t decreases with the increase H i /d values from to The maximum wave attenuation achieved in the present

7 INDIAN J. MAR. SCI., VOL. 42, NO. 6, OCTOBER study is 78.62% for H i /gt 2 =0.637, H i /d=0.200 and W/L=2.4. Acknowledgements Authors express their sincere thanks to the Director, National Institute of Technology Karnataka, Surathkal, Mangalore, India and to the Head, Department of Applied Mechanics and Hydraulics, National Institute of Technology Karnataka, Surathkal, Mangalore, India for their encouragement and for providing all the necessary infrastructural facilities required to carry out the present study. References 1 Harris A J & Webber N B, 1968, A floating breakwater, Proceedings of 11 th Coast Engrg Conf, pp London, England. 2 Chen K & Wiegel R L, 1970, Floating breakwaters for reservoir marinas, Proceedings of 12 th Coast Engg Conf, Volume III, , Washington, D C. 3 Harms V W, 1979, Design Criteria for Floating Tire Breakwater, Journal of the Waterway, Port, Coast & Ocean Division, ASCE, Vol. 106, No. WW2, Murali K & Mani J S, 1997, Performance of cage floating breakwater, J Waterway, Port, Coast & Oc Engg, ASCE, 123(4), Sannasiraj S A, Sundar V & Sundaravadivelu R, 1998, Mooring forces & motion response of pontoon-type floating breakwaters, Ocean Engg, 25(1), Sundar V, Sundaravadivelu R & Purushotham S, 2003, Hydrodynamic Characteristics of Moored Floating Pipe Breakwater in random waves Proceedings of Institution of Mechanical Engineers, Journal of Engineering for Maritime Environment, Vol. 217, Part M, Hegde A V, KiranKamath & Magadum A, 2007, Performance Characteristics of Horizontal interlaced multilayer moored floating pipe breakwaters Journal of WPCOE, ASCE, July/August, 2007, Volume 133, No 4. 8 Dong G H, Zheng Y N, Li Y C, Teng B, Guan C T & Lin D F, 2008, Experiments on wave transmission coefficients of floating breakwaters Ocean Engineering Journal, Elsevier Publications 35 (2008) Krishnakumar C, Sundar V & Sannasiraj S A, 2009, Hydrodynamic performance of single & double wave screens, Journal of WPCOE, ASCE. 10 Murali K, Sundar V & Kannayya Setti, 2009, Wave-Induced Pressures & Forces on Deck Slabs near the Free Surface, Journal of WPCOE, ASCE, Vol. 135, No Wang H Y, Sun Z C, 2010, Experimental study of a porous floating breakwater, Ocean Engineering Journal, Elsevier Publications, 37 (2010)