METHOD OF DRYDOCK BALLASTING TO MATCH TIDAL FLUCTUATION

CROSS-REFERENCE TO RELATED APPLICATIONS: NONE

FIELD

[0001]The discussion below relates generally to a method of drydock ballasting to match local tidal fluctuation during a slow heavy transfer between land and drydock.

BACKGROUND

[0002]When using air bearings, such as casters or air slides, to transfer a ship between land and drydock, the process is very slow due to the massive loads on the air bearings. It usually takes 2 to 5 hours to complete the transfer. The tidal fluctuation, however, can be significant during the transfer, especially in northern coastal areas, where the tidal fluctuation is normally 10 ft plus. To make the drydock flush with the land throughout such a tidal fluctuation, the conventional way is the drydock master just passively ballasting or de-ballasting the drydock while watching an inclinometer. It's trial and error, so a thorough drydock ballasting method is to be developed to assist any costly transfer.

[0003]An air caster or air slide is a square-shaped device of air bearing that is preset under a ship docking cradle, one bearing each of the starboard and port sides of the cradle. A ship typically sits on about 10 docking cradles arranged along the ship's longitudinal direction. Once all air bearings inflated, the docking cradles together with the ship carried are lifted 2 or 3 inches so that the air bearings with the cradles and the ship can slide along prepared surfaces under control of the transfer winches. The surface must be continuous between land and drydock, no step or significant slope change, to allow the air bearings go through and avoid bending damage to the ship transferred. The present invention is to keep the surface continuous between land and drydock.

SUMMARY

[0004]Embodiments of the present invention are directed to a method of drydock ballasting to match local tidal fluctuation during a slow heavy transfer between land and drydock. The apparatus for a slow ship transfer between land and drydock is a group of ship docking cradles lifted by air bearings like air casters or air slides, sliding along prepared ramp surfaces on land, on drydock and between, under control of transfer winches. The drydock is added a knee structure on each of starboard side and port wing tank end walls to allow it to pivot on the bulwark at the land ramp end. The present method of drydock ballasting are to keep it zero the total moment around the pivot of all forces on the drydock during the tidal fluctuation of the entire transfer process.

[0005]As discussed in greater detail below, to keep the drydock still on the edge of the land bulwark, the amount of ballasting water in each tank or the time duration to flush in or pump out water is determined using a spreadsheet analysis, involving firstly a tidal height prediction then calculations of moments of all forces on the drydock during each cradle move.

[0006]In an embodiment, the control room of a drydock has indicators of the amount of the ballast water for each tank, so the spreadsheet analysis only involves the tidal prediction and the moment calculations. The moment divided by the known lever arm of each tank gives the amount of the ballast water for each tank.

[0007]Another embodiment employs the additional calculations of the water flush-in rate to determine the ballasting time duration of each move for the drydock master to keep the drydock still, when the drydock control room has no indicator of the amount of water for each tank.

[0008]Other embodiments and their feathers will become apparent to those of ordinary skill in the art from the following detailed description which discloses, in conjunction with the accompanying drawings, examples that explain features in accordance with embodiments. This summary is not intended to identify key or essential features, nor is it intended to limit the scope of the invention, which is defined solely by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]The attached drawings help explain the embodiments described below.

[0010]FIG. 1 is a flow diagram 100 illustrating an example of a process for determining the amount of the ballast water for each tank when the control room of a drydock has indicators of tank capacity.

[0011]FIG. 2 is a graphical plot of the tidal height prediction formula for a tide-in and tide-out cycle on a specific date.

[0012]FIG. 3 is a flow diagram 300 illustrating an example of a process for determining the additional information besides those on FIG. 1, when drydock control room has no indicator of tank capacity.

DETAILED DESCRIPTION

[0013]A couple of examples or embodiments of the present invention are described, and it should be appreciated that the present invention provides many applicable inventive concepts that can be embodied in a variety of ways. The embodiments discussed herein are merely illustrative of ways to make and use the invention and are not intended to limit the scope of the invention. Rather, as will be appreciated by one of skill in the art, the teachings and disclosures herein can be combined or rearranged with other portions of this disclosure along with the knowledge of one of ordinary skill in the art.

[0014]Embodiments of the invention are directed to a method of drydock ballasting to match tidal fluctuation during a slow ship transfer between land and drydock. The transfer apparatus mainly comprises a drydock with a strong knee structure added to each side of the wing tank end walls, a land ramp prepared with painted surface, a pair of cribbing ramp fixed on drydock pontoon deck with the painted surfaces having the same slope as the land ramp, a group of ship docking cradles lifted by air bearings like air casters or air slides underneath to slide along the prepared surfaces carrying the ship transferred, and the transfer winches to control the movement of the combination of air bearings, cradles and the ship. The added knee structures allow the drydock pivoting around the contact points of the knees and the land ramp end bulwark. The present method of drydock ballasting are to keep the drydock still on the pivot by maintaining both the equilibriums of the forces on drydock and of their moments around the pivot during the tidal fluctuation of the entire transfer process.

[0015]FIG. 1 is a flow diagram 100 illustrating an example of a process for determining the amount of the ballast water for each tank to keep the drydock still on the knee pivot during the tidal fluctuation of the ship transfer. It is an example of the present invention when the control room of a drydock has indicators of tank capacity.

[0016]Step 102 specifies a three consecutive tidal heights above Mean Lower Low Water (MLLW) predicted by NOAA for the nearest station on the planned transfer day: 1^st low height L₁@time T₁, 1^st high height H₁@time T₂, and 2^nd low height L₂@time T₃. Assuming the tidal fluctuation is a sine curve and the x variable is the total minutes after midnight 00:00, the tide-in period tidal height prediction formula can be derived from the 1^st low (L₁@T₁) and the 1^st high (H₁@T₂), as shown in Step 104; and the tide-out period tidal height prediction formula can be derived from the 1^st high (H₁@T₂) and the 2^nd low (L₂@T₃), as shown in Step 106. Simply putting both formulae together forms the combined formula y=f(x) in Step 110 for a full cycle containing the tide-in period with the Step 104 formula and the tide-out period with the Step 106 formula. FIG. 2 shows a graphical plot of the combined formula y=f(x) for a tide-in and tide-out cycle on a specific date.

[0017]Step 120 specifies a start time t₀ of the transfer. It is back calculated from the tide formula y=f(x) in Step 110 with a known tide height L₀ at the start. One condition for the transfer to start is that the drydock must be able to let the bay water flow through the flush-in holes on the shell, so the drydock draft, i.e., the start tide level, must be at least at the top of the flush-in holes, which gives that the minimum draft at start d₀ equals drydock pontoon deck height H_pd minus the distance between the pontoon deck and the top of the flush-in holes D_h, i.e., d₀=H_pd−D_h. Since the drydock is flush with the land surface at the end of the cribbing ramp, the start tide height L₀ equals the land ramp end height above the MLLW, H_L, minus the drydock cribbing ramp end height, H_c, then minus D_h, i.e., L₀=H_L−H_c−D_h. The land ramp end height H_L can be measured and calculated using the NOAA tide prediction data then adjusted by the NOAA real tide data.

[0018]Step 130 specifies the drydock Longitudinal Center of Buoyancy (LCB) measured from the knee end, and Step 140 specifies the drydock Long Tons of water displaced Per Inch Immersion (TPI) at the average draft. Both LCB and TPI are the properties of the drydock. Step 150 specifies the transfer pace, Δt minutes per cradle. The pace is decided by the control winch speed and the total length of the cable, is typically 3-10 minutes per cradle. Step 160 specifies the lever arm (r) of each drydock tank. It is the longitudinal distance from the drydock knee end to the middle of each tank.

[0019]Step 170 specifies each cradle load and distances between them. For simplicity, the average cradle load P in long tons is used. The distances between the total N cradles are {1^st to 2^nd: Δc₁, 2^nd to 3^rd: Δc₂, . . . Δc_n−1} in ft, and the first move distance from the stop position of the 1^st cradle safely on edge to that of the 1^st cradle just crosses the gap between land and drydock, is Δ₀ ft. A moment to rotate the drydock counterclockwise is taken as positive in the present invention. For ship launch examples (transfers from land to drydock), the 1^st move, i.e., the 1^st cradle travels quite a distance then just crosses the D_gap ft gap of land to drydock, gives the drydock a positive force moment, M_c1=P*D_gap; 2^nd move, i.e., 2^nd cradle just crosses the gap with 1^st cradle proceeded Δc₁ ft on drydock, gives the drydock a positive moment M_c2=P*D_gap+1*P*Δc₁; 3^rd move with the 1^st and 2^nd cradles proceeded Δc₂ ft on drydock, M_c3=P*D_gap+2*P*Δc₂; and so on till the last move, the Nth cradle just crosses the gap with the other N−1 cradles proceeded Δc_n−1 ft on drydock, gives M_cn=P*D_gap+(N−1)*P*Δc_n−1. For ship hauling examples (transfers from drydock to land), the 1^st move, i.e., all N cradles travel Δ₀ ft with the 1^st cradle at the end of the move just getting off the drydock onto the land, gives the drydock a negative moment, M_c1=−(N*P*Δ₀); 2^nd move, all the N−1 cradles on the drydock travel Δc₁ ft with the 2^nd cradle at the end off the drydock, gives M_c2=−((N−1)*P*Δc₁); and so on till the last move, the Nth cradle travels Δc_n−1 ft off the drydock, gives M_cn=−(1*P*Δc_n−1).

[0020]The main box 180 has inputs of the start time t₀ from step 120, the drydock LCB and TPI from Step 130 and Step 140, the transfer pace Δt from Step 150, lever arm of each tank r from Step 160, and each cradle load and distances between them from Step 170. The main box 180 has 10 columns. Column 1 is numbering the end of each move, i.e., a specific cradle traveling quite a distance on land or drydock and ending at just across the gap between land and drydock. Column 2 is the time when each move ends. It is the total minutes after midnight 00:00, the time x in the formula y=f(x) in Step 110. Column 3 is the tide height at the time in Column 2. Column 5, moment by each cradle move M_c, is defined in Paragraph [0019]. The other columns are named obviously by the names. At the transfer start, the start time x is t₀ from step 120, the tide height y is y₀=f(x₀) calculated by the formula in Step 110, and the sum of selected tank water is set to zero (usually the farthest tank is selected for ballasting during transfer, not for setting the transfer start draft d₀ defined in Paragraph [0017]).

[0021]For the first two moves (n=1, or 2), calculations for Cradle#1, or 2 transfer end are performed to show the pattern for each move. The time of the Cradle #N transfer end, t_n, is calculated by adding the transfer pace Δt to the previous cradle transfer end time t_(n−1): t_n=t_(n−1)+Δt. The tide height y_n at the time of t_n is calculated by the formula in Step 110: y_n=f(t_n). The n-th cradle move's tide move Δy_n is calculated by subtracting the current move tide height by the previous move tide height: Δy_n=y_n−y_n−1. The n-th move's moment by buoyancy M_bn is calculated based on the n-th move's tide move Δy_n: M_bn=−Δy_n*12*TPI*LCB. The n-th moment by cradle move M_cn is defined in Paragraph [0019]. The n-th move's desired ballast moment M_n is calculated based on the other two moments of this move: M_n=−(M_bn+M_cn). If this M_n is positive, water is to be flushed in through the flush-in holes; If M_n is negative, ballast water is to be pumped out. The n-th move's selected tank ballast water weight W_n is calculated by dividing the desired ballast moment M_n by the lever arm of the selected tank r: W_n=M_n/r. A negative W_n means this amount of ballast water pumped out. The sum of the selected tank ballast water after the n-th move Σ_n is calculated by adding W_n onto the previous sum Σ_(n−1): Σ_n=W_n+Σ_(n−1). The drydock master watches the indicator of the selected tank capacity to get this Σ_n to keep the drydock still on the knee pivot.

[0022]The process described in FIG. 1 works for either a land to drydock transfer (ship launch) or a drydock to land transfer (ship hauling). When hauling a ship from drydock to land, the transfer start draft d₀ is prepared with the knees pressed on land by the weight of drydock and ballast water. A ship launch transfer does not necessarily preload the knees. The load on knees for each cradle move, Pk_n, completes the force equilibrium to the drydock and can be easily calculated by subtracting the tide buoyancy from the cradle load P and the ballast water weight W_n for a launch transfer: Pk_n=P+W_n−Δy_n*12*TPI; But Pk_n=−P+W_n−Δy_n*12*TPI for a hauling transfer. A negative Pk_n means the preloaded or loaded knees are taken some load off by the tide buoyancy.

[0023]Another embodiment/example of the present invention is when the control room of a drydock has no indicator of tank capacity. FIG. 3 is a flow diagram 300 illustrating an example of a process for determining the additional information besides those in the main box 180 to obtain the ballasting time duration of each move for the drydock master to keep the drydock still on the knee pivot during the tidal fluctuation of the ship transfer.

[0024]Step 301 specifies the same drydock draft at the transfer start d₀ as defined in Paragraph and mentioned in the first example of the present invention. Step 302 specifies the total pressure loss coefficient in a flood pipe and at its flush-in hole end and its hard-edge exit end, K_total, which is a property of the manufactured pipe (K=1 for the hard-edge exit, K=0.5 for the flush-in entrance, and K=0.3 in the pipe: K_total=1.8 typically for the entire pipe). Step 303 specifies the dimensions of each tank: Length (L) and Breadth (B). Step 304 specifies the flush-in hole height above the tank floor, h. Step 305 specifies the number of the flooding valves of each tank, N. Step 306 specifies the diameter of the flush-in hole, φ. Step 307 specifies the constant drydock pump-out rate, gpm_c. These are the 7 inputs used in the supplement box 310.

[0025]The supplement box 310 has 11 columns, but the first 4 and the 2 shaded ones are borrowed from the main box 180. The other 5 columns are the drydock draft d at each move end, the selected tank water depth D, water head (pressure) Z, flood rate (gallon per minute) gpm, and the selected tank ballasting time duration T_b. At the transfer start, the drydock draft is d₀ from Step 301 and the other columns were already set in the main box 180. Calculations for the first two moves are performed to show the pattern for each move. The drydock draft at the end of the n-th move d_n is calculated by adding the n-th move's tide move Δy_n onto the previous draft d_(n−1): d_n=Δy_n+d_(n−1). The selected tank water depth at the n-th move end D_n is calculated based on the sum of the selected tank water after the n-th move Σ_n in the main box 180: D_n=Σ_n*35/L/B. The n-th move's water head Z_n is approximated based on whether the tank water reaches the flush-in hole: if it hasn't reach (D_n<h), Z_n=d_n−h; otherwise, Z_n=d_n−D_n. The n-th move's flood rate gpm_n is calculated based on the water head Z_n, the flush-in hole diameter φ and the pressure loss coefficient K_total: gpm_n=2826*φ²*sqrt(Z_n/(1+K_total)). Finally, the n-th move's ballasting time duration of the selected tank T_bn is calculated out, based on the n-th move's selected tank ballast water weight W_n in the main box 180, for the drydock master to use to keep the drydock still on the pivot: if W_n is positive, flush-in T_bn=W_n*35*7.48/(gpm_n*N) minutes; otherwise pump-out T_bn=|W_n|*35*7.48/(gpm_c*N) minutes, here N is the number of the flooding valves of each tank from Step 305 and gpm_c is the input from Step 307.

[0026]Due to the differences between real tidal heights and the predicted ones, during transfer in the drydock control room, an accurate digital inclinometer must be provided to the drydock master in addition to a program implementing the present method. The drydock should be preset level, verified by the inclinometer, at the transfer start draft d₀ defined in Paragraph [0017]. At each move with the master's adjustments to the ballasting plan per the inclinometer readings, the drydock should be kept level. Compared to the conventional trial-and-error way of the drydock master just passively ballasting or de-ballasting the drydock while watching the inclinometer, the present invention of thorough drydock ballasting method is scientific, proactive and detail-adjustable. It provides safe operations to shipyards, ship owners and marine insurers.

[0027]The claims define the invention and form part of the specification. Limitations from the written description are not to be read into the claims.

[0028]To the extent the subject matter has been described in language specific to structural features and/or methodological steps, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or steps described. Rather, the specific features and steps are disclosed as example forms of implementing the claimed subject matter. To the extent headings are used, they are provided for the convenience of the reader and are not to be taken as limiting or restricting the systems, techniques, approaches, methods, devices to those appearing in any section. Rather, the teachings and disclosures herein can be combined, rearranged, with other portions of this disclosure and the knowledge of one of ordinary skill in the art. It is the intention of this disclosure to encompass and include such variation.

[0029]The indication of any elements or steps as “optional” does not indicate that all other or any other elements or steps are mandatory. The claims define the invention and form part of the specification. Limitations from the written description are not to be read into the claims.