STUDIES ON THE TRANQUILITY INSIDE THE GOPALPUR PORT

Transcription

1 STUDIES ON THE TRANQUILITY INSIDE THE GOPALPUR PORT INTRODUCTION Sundar. V 1, Sannasiraj. S. A 2 and John Ashlin. S 3 Gopalpur port is an artificial harbor located in Odisha state. The geographical location of the Gopalpur coastal site is N and E. The continuous problem in not achieving desired tranquility under the proposed phase I of the development of the port and an increase in the necessity of handling mass cargo export and import lead to phase II of Port. In order to maintain the tranquility and feasibility in the operation of berths inside the harbor, Southeast breakwater of 2170 m long and a North breakwater of length 375 m was proposed of which the Southeast breakwater up to 1600 m has been done. In the meanwhile, it is indeed to know the operation capability of berths located southern side of Harbor. The level of work completion in constructing Port in phase II is shown in Fig. 1. N N Fig. 1. The level of completion in construction of Gopalpur Port in Phase II In this paper, the results on the tranquility conditions near the berth, turning circle and the navigation channel inside the harbor to facilitate the prediction of the number of operable days for the vessels operating the port obtained through a numerical study are presented and discussed. WAVE TRANSFORMATION MODEL The Department of Ocean Engineering, IIT Madras has developed numerical models on the diffraction/refraction of waves due to the presence of near-shore structures. The model is developed using the mild slope equation because of its generality in dealing with complex wave fields. The mild slope equation is solved by generalized conjugate gradient method as it has a fast convergence rate. The present numerical model is used to predict the wave transformation as it propagates from deep water towards shallow water over complex bathymetric conditions. 1 Professor, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai , India, vsundar@iitm.ac.in (Member, IAHR). 2 Professor, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai , India, sasraj@iitm.ac.in (Member, IAHR). 3 Project Associate, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai , India, johnashlins@gmail.com. 1

2 WAVE TRANSFORMATION The offshore wave characteristics derived from NIO wave atlas for different months are adopted for the study. The nearshore bathymetry was digitized from the naval hydrographic chart. The GCG numerical model is applied for the various wave directions to predict the wave transformation in the presence of breakwaters up to the level of completion in construction as southern breakwater length 1600 m, berth and harbor basin. The results on wave height and wave phase contours for the different wave directions, wave height and wave period are presented in Fig. 2. Fig. 2. height and wave phase distribution for ( H = 0.75 m, T = 5 sec, = 83 0 ) The waves approaching with θ = 83, show that the tranquility along the approach channel and a few locations on the south of the northern breakwater looks rather questionable as occasionally the waves are up to even about 80 the incident waves. Further, the phase contour diagram exhibits clearly the phenomena of diffraction of waves due to the presence of the breakwaters. WAVE TRANQUILITY AT DIFFERENT LOACTIONS The wave tranquility at six different locations has been explored from the probability of wave occurrences. The percentage of operable days of berth during different months are: March - 98%; April - 71%; May - 96%; June - 98 %; July % and August -100 %. CONCLUSION Under the present conditions, the tranquility is found to be satisfactory at the berth location from March to the month of October. The tranquility near the berth would be questionable during the months November to February, thus making the operations difficult. In order to overcome this difficulty, the possibility of extending the eastern side of the southern breakwater at an earliest possible date is absolutely essential. REFERENCES Berkhoff, J.C.W Computation of combined refraction-diffraction. Proceedings of 13th International Conference on Coastal Engineering, ASCE, 1, Ganesh, G. and Sundar, V. (2001) Numerical modeling of combined refraction and diffraction of wave and its applications, ICTACEM, 2001, IIT Kharagpur, India, paper no:

3 STUDIES ON THE TRANQUILITY INSIDE THE GOPALPUR PORT Sundar. V 1, Sannasiraj. S. A 2 and John Ashlin. S 3 Abstract: Gopalpur port ( N and E) is an artificial harbor located in Odisha state. The continuous problem in not achieving desired tranquility under the proposed phase I of the development of the port and an increase in the necessity of handling mass cargo export and import lead to phase II of Port. Under the phase II, Gopalpur port will have southern break water (SBW) length of about 2170 m and northern break water (NBW) of about 375 m. During construction, it is envisaged to operate the port by opening a few berths on the south side of the harbor. The SBW has been completed upto a length of about 1600 m. Under this situation, as it is essential to know the wave penetration characteristics inside the harbor for bringing in the vessels in to the harbor, a numerical model study has been carried out. Mild slope equation with the appropriate boundary conditions is adopted in the solution process. The studies were carried out for different wave characteristics, viz., its height, period and its direction approaching the harbor. The main objective of this study is to evaluate the tranquility near the berth, turning circle and the navigation channel inside the harbor to predict the number of operable days for the vessels operating the port in relation to the incident wave condition. The detailed analysis of the results from the numerical model are presented and discussed in this paper Keywords: Mild slope equation; breakwater gaps; harbor development; operable days; tranquility. INTRODUCTION The Gopalpur Port is located along the Bay of Bengal along the east coast of India. It is situated 22 km south to the Rushikulya River mouth. This Port is one of the most environmental friendly; because it is established on a 4 km stretch of coastline that has no vegetation and is not a home for any endangered species in its region. This place is well connected by railway line and road line to other states such as Jharkhand, Chhattisgarh to export mineral resources and forest produces. Under the phase I development, i.e., at present, only 1600 m out of the proposed 2170 m of the SBW has been completed, which may be provide only a certain degree of desired tranquility particularly from the waves approaching from the south east. As the port is expected to operate an all-weather port, it becomes mandatory to check for the tranquility and arrive at the number of favorable days for uninterrupted port operations. Further an increase in the necessity of handling mass cargo export & import leads to the development of the port under phase II. The phase II in developing Port designed for a navigation channel will have a water depth of 18.5 m 1 Professor, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai , India, vsundar@iitm.ac.in (Member, IAHR). 2 Professor, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai , India, sasraj@iitm.ac.in (Member, IAHR). 3 Project Associate, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai , India, johnashlins@gmail.com. 3

4 and a width of 200 m. The turning circle will have a depth of 17.5 m and a diameter of 600 m. The 120,000 DWT vessels handling berths are designed for water depth of 18.5 m. In order to maintain the tranquility and feasibility in operations at the berths inside the harbor, the SBW of length 2170 m and a NBW of 375 m long are proposed for all weather operations. As stated earlier, the present situation is the existence of the SBW for a length of 1600 m which is shown in Fig.1. In this paper, the results on the tranquility conditions near the berth, turning circle and the navigation channel inside the harbor to facilitate the prediction of the number of operable days for the vessels operating the port obtained through a numerical study are presented and discussed. PREDICTION OF OFFSHORE BOUNDRY DATA Snell's law (also known as the Snell Descartes law and the law of refraction) that governs the wave refraction is given as, sin θ 0 (1) sin θ = C 0 Where, C o is the deep water wave celerity (deep water wave length ) and C is the wave celerity. The angle conventions for the seabed contours, as well as for the wave direction used for the numerical model are shown in Fig. 2. N y Normal to Shore Line Parallel to shore Line Deep water Direction sp Parallel to X -axis Shore Line Fig. 1. The level of completion in construction of Gopalpur Port in Phase II Fig. 2. Angle conventions used in the numerical model x The deep-water wave angles were converted to that corresponding to shallow waters through the Snell s law, taking into consideration of the inclination of shoreline with respect to geographic North. These shallow water wave angles were then expressed with reference to shore normal according to the angles conventions employed in the present numerical model. The conservation of energy, while, the wave propagates from deep to shallow waters is adopted to obtain the wave height at -21 m contour, i.e., the offshore boundary for the wave transformation model. NUMERICAL MODEL General The Department of Ocean engineering, IIT-Madras has developed a numerical model to investigate the transformation of waves as it propagates from deep waters towards the nearshore [Ganesh and Sundar (2001)]. The model is developed using the mild slope equation because of 4

5 its generality in dealing with complex wave fields. The mild slope equation is solved by generalized conjugate gradient (GCG) method as it has a faster convergence rate. The combined refraction-diffraction equation derived by Berkhoff (1972) describes the propagation of periodic, small amplitude, surface gravity waves over an arbitrarily varying mild sloped seabed. The equation is of the form, Cg CCg CCg 2 0 (2) x x y y C Where, is the wave potential, is the wave angular frequency, C is the wave celerity and C g is the group celerity. The above equation is transformed into Helmholtz equation of the form, 2 K 2 ( x, y) 0 Using the following relations ( CCg) ( CCg) and K k 0.5 (4) ( CCg) In this formulation, is the modified wave potential function and K is the modified wave number. Boundary conditions have to be imposed all along the boundary including land, breakwaters and open boundaries. On the offshore boundary, the radiation boundary condition is used so that the reflected waves are truly outgoing. in ik (5) n n The other boundaries represent a structure like breakwaters or groins, an open boundary, or a coastline. In such condition, the boundary condition is, ik 0 (6) n Where n is the direction normal to the boundary and α is the reflection coefficient that varies with the type of boundary. Finite difference scheme is employed for the numerical discretization of Helmholtz equation. The derivatives are approximated using centered difference scheme. Writing the discretized form of above equation for each grid in the domain and applying suitable boundary conditions, the resulting system of algebraic equations can be written in a matrix form as [A]{ } = {f} (7) Where [A] is the coefficient matrix, {Φ} is the nodal value of velocity potential and {f} is a vector obtained from the boundary conditions. The numerical solution of the above system of equations is arrived using Generalized conjugate gradient method. The method successfully estimates new approximations to the solution, considering the direction of residual error vector, till the prescribed accuracy is achieved. The offshore boundary is modeled as an open boundary in which case only incident waves and reflected waves are allowed to propagate. The lateral boundaries as well as the shore are considered to absorb the partially the wave energy. The groin or any other obstructions are treated (3) 5

6 as partially reflecting boundaries by prescribing the reflecting coefficients. The model requires the wave characteristics (wave height, wave period and its direction) at the offshore boundary and the water depths at all the grid points. It also requires the location of the breakwaters and harbor alignment. The model gives the waves characteristics inside the harbor basin. RESULTS AND DISCUSSION Transformation The offshore wave characteristics derived from NIO wave atlas for different months are adopted for the study. The wave conditions at the numerical offshore boundary are shown in Table 1. Month Table 1: characteristics for the tranquility study Deep water direction w.r.t North (Ѳ o ) period T (sec) direction from shore normal (Ѳ) in d=21m January February March April May June July August September October November December The nearshore bathymetry was digitized from the naval hydrographic chart. The GCG numerical model was executed for the various wave directions to predict the wave transformation in the presence of breakwaters up to the level of completion in construction as southern breakwater length 1600 m, berth and harbor basin. The study area considered here is 7.7 km along shoreline and 4.7 km normal to the shoreline up to a water depth of -21 m. The study area is discretized with a grid size of 1150 mm along the shoreline and 700 mm normal to the shoreline. After interpolation, each the above said grid size was reduced to 6.67 m in the both directions for the purpose of computation. The numerical model results for the wave tranquility studies are represented in the form plots showing the wave phase distribution and wave disturbance coefficient distribution for the computational domain. The results on wave height distribution (on the left side) and wave phase contours (on the right side) for the different wave directions, wave height and wave period as per classified in Table 1 are presented in Fig. 3 (a) to (d). The wave phase distribution shows the wave approach and the combined effect of diffraction and refraction inside the harbor basin with 6

7 the presence of the breakwaters. The wave diffraction pattern, due to wave interception by the breakwater is clearly visualized near the tip of the breakwaters. Fig. 3.a height and wave phase distribution for ( H = 0.75 m, T = 5 sec, = 83 0 ) Fig. 3.b height and wave phase distribution for ( H = 1 m, T = 6 sec, = ) Fig. 3.c height and wave phase distribution for ( H = 1.75 m, T = 7 sec, = ) Fig. 3.d height and wave phase distribution for ( H = 1.75 m, T = 6 sec, = ) 7

8 The waves approaching with θ = 83, Fig. 3 a show that the tranquility along the approach channel and a few locations on the south of the northern breakwater is rather questionable as occasionally the waves are up to even about 80 the incident waves. Further, the phase contour diagram for the same wave conditions exhibit clearly the phenomena of diffraction of waves due to the presence of the breakwaters. The results on the tranquility within the harbor basin for the south easterly waves with θ = -75, -59 and -38 are shown in Fig. 3 b, c and d respectively. The results clearly demonstrate that the south breakwater is quite effective in attenuating these waves and the phenomena of diffraction due to breakwaters are faithfully reproduced by the numerical model. As observed earlier, a few locations on the south of the NBW is seen to be a zone of waves upto about 85 the incident waves. As of now under the present conditions, the tranquility is not found to satisfy at least during the months November to February as the harbor basin is directly exposed to the waves approaching from the east. In order to overcome this difficulty, it is strongly advised to complete the construction of the eastern side of the SBW as envisaged in the original proposal. WAVE TRANQUILITY AT DIFFERENT LOACTIONS The wave tranquility at six locations (1: At berth; 2 & 3: western and eastern end of Turning circle; 4: at the harbor mouth; 5 & 6 along the navigation channel outside the harbor) has been explored from the probability of wave occurrences. The location of these points on the bathymetry is shown in Fig. 4. The limiting wave heights are defined as 0.8 m at the berth and turning circle. The percentage of occurrence of wave heights at various points from March to August is shown in Tables 2 to 7. The percentage of operable days of berth during different months are: March - 98%; April - 71%; May - 96%; June - 98 %; July % and August -100 %. However, the wave climate in the turning circle might be slightly being less during some of the months and the ship can be maneuvered during fair period of the day to avoid the unfair wave occurrence. CONCLUSIONS As of now under the present conditions, the tranquility is found to be satisfactory at the berth location from March to the month of October. The tranquility near the berth would be questionable during the months November to February, thus making the operations difficult. In order to overcome this difficulty, the possibility of extending the eastern side of the southern breakwater at an earliest possible date is absolutely essential. REFERENCES Berkhoff, J.C.W Computation of combined refraction-diffraction. Proceedings of 13th International Conference on Coastal Engineering, ASCE, 1, Ganesh, G. and Sundar, V. (2001) Numerical modeling of combined refraction and diffraction of wave and its applications. Second international conference on Theoretical, Applied, Computational, and Experimental Mechanics, ICTACEM, 2001, Indian Institute of Technology, Kharagpur, India, paper no: 050. CERC (1984) Shore Protection Manual. Coastal Engineering Research Centre, US. 8

9 N LOCATION POINTS 1 LOCATIONS 1. BERTH 2.WESTERN END OF TURNING CIRCLE 3. EASTERN END OF TURNING CIRCLE 4. HARBOUR MOUTH 5. NAVIGATION CHANNAEL 6. NAVIGATION CHANNAEL Fig. 4. The location points shown on the bathymetry 9

10 Points operable days Points operable days Proceedings of HYDRO 2013 INTERNATIONAL, Table 2: Operability in %ge at various points as per requirements, for the month of March H incident = 1.0 m; Deep water direction = ; Period (T) = 7 sec occurrence Allowable wave at the points Amplitudes at different wave heights Table 3: Operability in %ge at various points as per requirements, for the month of April H incident = 1.0 m; Deep water direction = ; Period (T) = 6 sec occurrence Allowable wave at the points Amplitudes at different wave heights

11 Points operable days Points operable days Proceedings of HYDRO 2013 INTERNATIONAL, Table 4: Operability in %ge at various points as per requirements, for the month of May H incident = 1.75 m; Deep water direction = ; Period (T) = 7 sec occurrence Allowable wave at the points Amplitudes at different wave heights Table 5: Operability in %ge at various points as per requirements, for the month of June H incident = 2 m; Deep water direction = ; Period (T) = 7 sec occurrence Allowable wave at the points Amplitudes at different wave heights

12 Points operable days Points operable days Proceedings of HYDRO 2013 INTERNATIONAL, Table 6: Operability in %ge at various points as per requirements, for the month of July H incident = 2.25 m; Deep water direction = ; Period (T) = 7 sec occurrence Allowable wave at the points Amplitudes at different wave heights Table 7: Operability in %ge at various points as per requirements, for the month of August H incident = 2.75 m; Deep water direction = ; Period (T) = 8 sec occurrence Allowable wave at the points Amplitudes at different wave heights