US3670682A

US3670682A - Method and apparatus for handling ships

Info

Publication number: US3670682A
Application number: US9A
Authority: US
Inventors: Clyde M Leavitt; Walter C Johnson; Don C Hall
Original assignee: Litton Systems Inc
Current assignee: Ingalls Shipbuilding Inc
Priority date: 1970-01-02
Filing date: 1970-01-02
Publication date: 1972-06-20
Anticipated expiration: 1989-06-20
Also published as: JPS4940516B1; FR2073686A5; ES384677A1; GB1276606A; CA953118A

Abstract

A launching and retrieving facility for handling ships with an open ended, sectionally constructed drydock having one side of removable wingwalls. In an inshore position the drydock is ballasted to rest upon a firm foundation and a track system on the drydock floor aligns with a similar shore-based track system thereby allowing the ship to be rollably or slidably transferred between the drydock and shore. After the ship is transferred to the drydock, the wingwalls are set in place and the drydock deballasted to float off the foundation. The drydock and ship thereon are moved into the launching area where the drydock is ballasted down until the ship is set afloat. Ship retrieval from water to shore is achieved by a reverse procedure.

Description

United States Patent Leavitt et al.

1 June 20, 1972 [54] METHOD AND APPARATUS FOR HANDLING SHIPS [72] Inventors: Clyde M. Leavitt; Walter C. Johnson, both of Pascagoula; Don C. Hall, IV, Ocean [21] Appl.No.: 9

FOREIGN PATENTS OR APPLICATIONS 1,103,171 3/1961 Germany ..114/45 Primary ExaminerMilton Buchler Assistant ExaminerGregory W. O'Connor Attorney-Alan C. Rose, Michael L. Wachtell, Walter R. Thiel and Alfred B. Levine [57] ABSTRACT A launching and retrieving facility for handling ships with an open ended, sectionally constructed drydock having one side of removable wingwalls. In an inshore position the drydock is ballasted to rest upon a firm foundation and a track system on the drydock floor aligns with a similar shore-based track system thereby allowing the ship to be rollably or slidably transferred between the drydock and shore. After the ship is transferred to the drydock, the wingwalls are set in place and the drydock deballasted to float off the foundation. The drydock and ship thereon are moved into the launching area where the drydock is ballasted down until the ship is set afloat. Ship retrieval from water to shore is achieved by a reverse procedure.

18 Claims, 7 Drawing Figures PATENTEDmzo m2 SHEET 2 OF 3 u D D D D D D D D D U T /0a I L METHOD AND APPARATUS FOR HANDLING SHIPS BACKGROUND OF THE INVENTION Because no new shipyard has been built in the United States since World War II, American built ships have been unable to compete economically within the World Market. As an example, in 1966 of 700 commercial ships built throughout the world only 13 were built in UnitedStates shipyards. Because a constant demand is present and ever growing that bigger and better ships be built at lower cost per ton, production techniques are being adopted which bring to the production of ships the know-how and manufacturing advances of our free enterprise system.

In order to compete in the World Shipbuilding Market, new shipyards are needed in the United States which provide a volume manufacturing flexibility designed for the series production of vessels whether they be of identical or different designs. This volume manufacturing capability will bring to the shipbuilding industry the same economic advantages now uniformly enjoyed by the automobile industry. It has been proven beyond a doubt that the conventional stationary shipway method of constructing ships is obsolete and cannot be modified to be competitive, especially in the construction of a plural number of ships having identical or substantially similar designs.

An example of a new innovative shipyard designed for utilization of a mass production systems approach, is the one recently built by Litton Systems, Inc., in Pascagoula, Mississippi. At this yard ships are manufactured on continuous ways and are made out of modules. The modules are manufactured from assemblies, and the assemblies are manufactured out of still smaller assemblies. The modules are mounted on cradles which are in turn rollably mounted on a track system so that the whole ship grows as it passes through fixed work stations. Thus, the controlled flow of material through men and machines are applied throughout and the economics of these production techniques are thereby utilized in the shipbuilding field.

The above discussed production techniques enables the yard to handle the construction of a number of ships simultaneously on continuous ways and allows the utilization of a single launching facility rather than a separate launching facility for each shipbuilding way. The orientation of the track system and the production technique which entails moving the ships in a transverse rather than a longitudinal direction, dictates a design for a launching facility with the capability of launching the ships being constructed from a horizontal rather than an inclined shipway. Building the ships in a horizontal mode greatly lessens construction costs because the sub-assemblies can be aligned perpendicular to a base-line without a declivity board.

Due to the lengths and sizes of ships which might be constructed in the above described manner, the stresses and strains which might occur during conventional end-launching could be too great and might cause severe damage to the ship being launched. Similarly, the use of a standard graving dock launching facility has proven impractical because of the problems in getting the completed vessel from the continuous horizontal ways into the dock. Because of the systems approach, construction of the vessel within the graving dock becomes impossible.

The most practical facility for launching the ships is a floating drydock which can be oriented with the track system to accept the completed vessel for launching. In designing such a floating drydock facility for a shipyard located along a riverbank or channel, it is desirable to have a drydock with the capability of having ships moved onto the drydock sideways rather than longitudinally so that the drydock need not jut out into the water for any great distance. Because of the size of the ships, it is also important that the floating drydock have sufficient stability so that the ship will not be spilled into the water which might result in capsizing, nor be put under excessive strains during launching, which might lead to structural damage of the hull. As the single launching facility would be used for ships of varying lengths and designs, it is also desirable that the drydock be designed in such a way that its length and capabilities can be easily adapted to the particular type of vessel being launched. Also, because of the single facility aspect, it is important that the drydock not be out of operation for any extended period of time due to repairs or modifications.

Because shipyards not only build ships but also repair and refurbish ships, a necessary feature of a single launching facility is that it be equally adaptable for retrieving vessels from the water to the shore facility where they can be repaired or modified out of the water.

It is therefore an object of this invention to provide a floating drydock for launching ships which are constructed simultaneously on continuous horizontal ways.

Another object of the invention is to provide an improved method for launching and retrieving ships from horizontal rather than an inclined shipyard ways involving a single launching facility.

Another object of the invention is to provide an improved floating drydock facility adaptable to have ships moved thereupon sideways and which will have improved stability in a floating condition with a ship thereon, so as to avoid any damage to the ship during launching or retrieval.

Still another object of the invention is to provide a floating drydock whose length can be easily adjusted to accommodate the launching or retrieving of ships of varying sizes and which would not be subject to long periods of being out of service due to repairs which might have to be made upon the drydock.

SUMMARY OF THE INVENTION In accordance with a primary feature of the invention, a floatable drydock is provided having removable wingwalls along the side of the drydock adjacent the shore when the drydock is in a position for receiving the ship to be launched. The wingwalls are set in place prior to deballasting the drydock and moving it to a place having sufficient depth to allow ballasting of the drydock an amount sufficient to launch the ship.

BRIEF DESCRIPTION OF THE DRAWINGS The specific nature of the invention as well as other objects, aspects, use, and advantages thereof, will clearly appear from the following description and from the accompanying drawings, in which:

FIG. 1 is a perspective view of a floatable drydock in accordance with our invention supported by a launching grid and aligned with the track transfer system of a shore facility. The removable wing walls are shown attached to the drydock with a portion of one wing wall cut away.

FIG. 2 is a plan view schematically illustrating how the sectionalized construction of the floating drydock makes it adaptable for self-repair of the individual drydock sections.

FIG. 3 is a sectional view of a dogging device which can be used as one means of fastening the removable wing walls to the floating drydock shown in FIG. 1.

FIG. 4 is a schematic representation taken from a side view of the launching facility illustrating the location of the drydock when it is in position to receive the completed hull structure and further illustrating one means of winching the drydock from the shoreline to a position where the ship is launched.

FIG. 5 is a schematic representation of a side view of the launching facility after the completed hull has been moved onto the drydock and showing the installation of the removable wingwalls.

FIG. 6 is a perspective view which shows the floating drydock and the completed vessel thereon as it is being moved from the shoreline to the launching position.

FIG. 7 is a schematic view showing the floating drydock and the completed vessel after the drydock has been ballasted down and the completed vessel set afloat.

In the following description of the drawings, the same reference numeral is used to designate the same element in the different figures whenever practical.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates a preferred embodiment of a floating drydock constructed in accordance with our invention in an on-shore position. The drydock 10 is sectionally constructed and includes a series of

drydock sections

12, 14, 16 and 18. These sections can be fastened together to form the drydock l and the length of the dock can be adjusted to accommodate a particular ship by either adding on additional drydock sections or removing one or more of the sections. Each section has a respective removable wingwall 20. The wingwalls 20 are securely fastened to the drydock sections by a plurality of fastening devices 22 located along the inside and outside walls of the removable wingwalls and adapted to be fastened to appropriate receptacles located on the drydock sections. Each wingwall has a hollow interior (shown at the cutaway portion 23) and when fastened to the drydock by the fasteners 22 will form a water tight seal so that the wingwalls can function as ballast tanks during the operation of the drydock l0. Depending upon the particular dimensions of the drydock sections, it may be desirable to divide the removable wingwall for each section into several sections. This would simplify the task of attaching and removing the wingwalls because of the reduced weight of each wingwall section.

In an on-shore position, the drydock rests upon a supporting structure which, depending upon the particular bottom contours and geological characteristics at the shoreline, may consist of a series of pilings with a platform thereon as shown in FIG. 1. The shore installation 30 necessarily includes a transporting means for the ships, such as a series of parallel aligned tracks 28 upon which rollable cradles supporting the ship can be moved. In a proper on-shore position, a series of similarly disposed tracks 25 on the floor of the drydock 10 will be aligned with the tracks on the shore thereby allowing transfer of the ship from the shore installation 30 to the drydock 10 along the track system by means of the rollable cradles.

In addition to the advantages of utilizing a sectionalized drydock, which provides a simple means of adjusting the length and capacity of the launching/retrieving facility by the addition and subtraction of sections, proper design of the drydock also gives the advantage of allowing the drydock to be self-repairable. As an example, in FIG. 2, the

drydock sections

12, 14 and 16 are constructed such that the length of each drydock section is less than the width between the interior sides of the wingwalls with the wingwalls mounted. Where it becomes necessary to repair a section of the drydock such as section 12, that section need only be detached from the

remaining sections

14 and 16. The drydock is then ballasted down so that the section 12 can be floated from position A to a position B which is rotated 90 degrees and above the drydock. The drydock can then be de-ballasted and floated, until section 12 is raised out of the water. After the necessary repairs are made, the drydock is again ballasted down and the repaired section 12 floated off, rotated in a reversed 90 and reattached to the drydock.

Because the wingwalls must maintain a watertight seal with the drydock and provide longitudinal strength to the drydock, it is necessary that the fastening means 22 which are used to clamp the wingwalls 20 to the drydock sections provide a powerful clamping and holding action between the two adjacent members. An example of such a fastening device is the dogging device shown in FIG. 3. In the figure a projecting lug 40 with a sloping top is attached near the bottom of the wingwall 20 to be fastened. Above the lug 40 is a drop bolt 43 having a threaded end thereon. Attached to the top of a drydock structure 10 is a swinging link 42 which can mate with the projecting lug 40. On the top of the link are two projecting ears which can mate with the drop bolt 43. During operation after the wingwall has been properly placed on the drydock, the

link 42 is rotated upwards until it mates with the lug 40. The drop bolt 43 is then engaged with the link ears. Tightening the nut on the drop bolt 43, pulls the link over the sloping top surface of the lug 40, thus exerting a powerful downward force on this lug. Because the screw thread on the link 42 is an application of the principle of an inclined plane, the dogging device therefore is an example of the multiplication of the action of two inclined planes. The result is a tight connection between the wingwall 20 and the respective drydock section.

FIG. 4 schematically illustrates how a completed ship hull 8 is moved to the drydock 10 in preparation for a launching. The vessel 8 is constructed on the horizontal ways at the shore installation 30 and is supported by a series of cradles 32 each of which are mounted on a plurality of rolling carriages 34. Preferably, these carriages have self-powered locomotive means to facilitate movement of the ships. In preparation for launch, the drydock 10 is ballasted down so that it securely rests on the launching grid 15. As explained previously, the tracks 25 on the drydock 10 are aligned with the respective tracks 28 along the ship way 30 and the gap between these tracks along the portion of the drydock on which the removable wingwalls will be fastened is temporarily bridged with removable tracks 27. The ship can then be moved along the tracks sideways until it is centrally located on the drydock 10 as shown in FIG. 5.

FIG. 4 additionally shows a possible means of moving the drydock and the ship from a position on the grid to a position off-shore which is deep enough to allow sufficient ballasting down of the drydock 10 so that the ship 8 can be set afloat. Such a system may entail a chain reeving and anchorage system employing a chain or cable 50 and a Windlass 51. One end of the chain 50 is attached and securely fastened to the shore and the other is attached to a secure anchorage somewhat off the shoreline. In order to move the drydock with or without the vessel to a position otT-shore or from an offshore to on-shore position, it is only necessary to have the powered Windlass 51 rotate and wind itself along the chain or cable 50 in the appropriate direction. Such a system has been conventionally used for ferry boats where two chains are laid parallel across the river bed so that the ferry boat by means of a similar type Windlass winds itself along the chains from one bank to another. It should, of course, be apparent that conventional block and tackle 53 and/or howser lines 54 could also be used for guiding movement of the drydock. After the drydock has been moved off-shore and ballasted down, the reeving chain would merely lie slack as shown by dotted line 55.

Further describing operation of the drydock, FIG. 5 shows the ship 8 after it has been moved on cradle 32 to the appropriately centered position on drydock 10 which is resting on the launching grid 15. After the connecting rails 27 have been removed the removable wingwalls 20 can be lowered into place on the drydock and securely fastened thereon. The wingwalls can be easily handled and positioned on the drydock with the aid of cranes 21 which are rollable mounted on the track system along the horizontal ways 30.

Once the wingwalls 20 have been securely fastened the drydock 10 is deballasted by pumping water out of the ballast tanks within the drydock sections with conventional pumps (not shown for purposes of clarity), until the drydock 10 with the ship 8 thereon is raised in the water to a floating position such that the drydock l0 and ship 8 will clear the launching grid 15.

FIG. 6 shows the drydock l0 and the ship 8 as they are moving from a position adjacent the shore facility 30 to deeper water where the drydock can be lowered to float the ship. Aside from the chain reeving and anchorage system described in FIG. 4, the drydock can be moved away from the shoreline by means of lines 53 attached to two fixed buoys 56 and 57 which are anchored out in deeper water. Full control is maintained on the floating drydock at all times with the use of lines 59 connecting the drydock to the shore. These lines are paid out as the drydock is moved away from the shoreline thereby always maintaining positive control on the floating drydock.

After the drydock has been moved away from the shore facility 30 and the launching grid 15 into water sufficiently deep to allow ballasting down of the drydock, the tanks a, 10b, 10c and 10d within the drydock 10 are flooded and the drydock is lowered into the water until the ship 8 floats free and clear of the cradles 32 as illustrated in FIG. 7. it then remains only to move the ship 8 longitudinally out of the drydock either under its own power or under tow.

The same procedure can be carried out in reverse order to retrieve a ship from the water to the shore facility. In retrieving a ship, the vessel would first be moved over the drydock which has been ballasted down in deep water and aligned with cradles 32 which support the ship when it is in drydock. ln retrieving ships, it is important that the cradles 32 be properly placed under the ship as the drydock is deballasted and floated, otherwise severe structural damage can result. Once the cradles 32 are properly set, the drydock can then be deballasted and floated until it fully supports the ship being retrieved. After the drydock has been sufiiciently deballasted to raise its draft to a level higher than the surface of the launching grid the drydock 10 and the ship 8 thereon is aligned in the appropriate position above the grid and then ballasted down until it rests securely on the grid. The wingwall can then be removed and the ship 8 moved from the drydock 10 back into the shipyard or other shore installation where the necessary repair work and/or modifications can take place. After any work to be done on the ship has been completed, the reverse process would be followed in order to re-launch the ship.

The result of the launching and retrieval system described above, is a versatile, inexpensive, self-repairable and easily operated single drydock facility which can be used for launching and retrieving a large number of ships without necessitating the use of more than one drydock facility. The drydock described is easy to maintain and inexpensive to build when compared with a conventional horizontal way manufac turing facility and/or graving dock repair yards.

What is claimed is:

l. A floatable drydock for retrieving and launching vessels to and from a waterfront comprising:

a. a rectangularly shaped bottom portion to support a vessel during launching and retrieving, said bottom portion having a longitudinal direction parallel to the longitudinal axis of a vessel to be supported therein and a transverse direction parallel to an athwartships axis of said vessel, said bottom portion including at least a portion of a first buoyancy chamber;

. a first wingwall located along one longitudinally directed side of said bottom portion, said first wingwall including at least a portion of a second buoyancy chamber;

a second wingwall located along the other longitudinally directed side of said bottom portion, said second wingwall being removable from said bottom portion, said second wingwall including at least a portion of a third buoyancy chamber; and

means for transferring a vessel between said bottom portion and the waterfront comprising a plurality of tracks, a plurality of rolling carriages, and a plurality of means for supporting a vessel on said plurality of rolling carriages, said plurality of tracks including a first number therefrom mounted on the waterfront, a second number therefrom mounted in said transverse direction on said bottom portion and a third number therefrom removably mounted between said first and second number for temporarily bridging the gap between said first and second number of tracks along the portion of said drydock which said removable wingwall mounts, each track in said first number being aligned with the respective track in said second number and in said third number, said plurality of rolling carriages and means for supporting a vessel on said carriages rollably mounted on said tracks for transporting a vessel in an athwartships direction along said plurality of tracks until the vessel is moved between the waterfront and said drydocks.

2. The floatable drydock of claim 1 further including means for adding and removing ballast to said drydock to vary the degree the flotation of said drydock.

3. The floatable drydock of claim 1 further including means for moving said drydock between the waterfront and a position in a body of water removed from the waterfront.

4. The floatable drydock of claim 1 wherein said drydock includes a plurality of detachable sections, each section including a bottom portion, a first wingwall and a second removable wingwall.

5. The floatable drydock of claim 4 wherein each detachable section has identical dimensions.

6. The floatable drydock of claim 4 wherein each detachable drydock section has a length smaller than the distance between the first and second wingwalls of any of said sections when they are fully mounted on the respective bottom portions, thereby allowing any one section to be drydocked for repairs and maintenance by any other section.

7. The device as claimed in claim 1 wherein said first buoyancy chamber in said first wingwall communicates with at least a portion of one buoyancy chamber in said bottom portion to form a first ballast tank and wherein said third buoyancy chamber and said second removable wingwall communicates with at least a portion of one buoyancy chamber in said bottom portion to form a second ballast tank.

8. The device as claimed in claim 7 and further including means for selectively maintaining a watertight seal between said second wingwall and said bottom portion for forming said second ballast tank.

9. A method of retrieving vessels floating in a body of water onto a fixed dock, employing a floatable drydock having two wingwalls longitudinally disposed along the sides of said drydock, one wingwall being removable, comprising the steps of:

a. moving the vessel to a position above a submerged floatable drydock;

b. changing the flotation of said drydock an amount sufi"- cient to raise said vessel out of the water and float the drydock with the vessel thereon;

c. moving said drydock with the vessel thereon to a position adjacent to the fixed dock;

d. removing the wingwall from the drydock;

e. bridging the portion of the drydock from which the removable wingwall has been removed with removable tracks;

f. transporting the vessel in an athwartships direction on carriages rollably mounted on tracks from the drydock to the fixed dock.

10. The method of claim 9 wherein the step of transporting the vessel on carriages is followed by the step of removing the removable track from a portion of the drydock.

l l. The method of claim 9 wherein the step of moving said drydock with said vessel thereon to a position adjacent to said fixed dock includes the step of aligning the drydock with said fixed dock by changing the floating position of said drydock until it firmly rests on a firm foundation adjacent to said fixed clock.

12. The method of claim 11 wherein the flotation of said drydock is changed by adding ballast to the drydock to decrease flotation to align said drydock with said fixed dock.

13. A method of side launching vessels from a fixed dock to a body of water employing a floatable drydock having at least two wingwalls longitudinally disposed along the sides of said drydock, at least one wingwall being removable, comprising the steps of:

a. moving the vessel in an athwartships direction relative to said fixed dock and said drydock from said fixed dock onto said drydock when said drydock is aligned with the fixed dock and from which the removable wingwall has been removed;

b. attaching said removable wingwall to said drydock;

c. putting said drydock and vessel thereon in a floating condition;

d. moving said drydock and said vessel from a position aligned with said fixed dock to a position removed therefrom in said body of water;

e.-changing the flotation of said drydock thereby causing it to attain a submerged position sufiicient to enable said vessel to float free.

14. The method of claim 13 wherein said drydock and vessel thereon is placed in a floating condition by removing ballast from said drydock until said drydock and vessel float free of a fixed foundation on which they rest when in an aligned position with said fixed dock.

15. The method of claim 14 wherein the step of changing the flotation of said drydock comprises adding ballast to said drydock to increase its draft to enable said vessel to float free.

16. The method of claim 13 wherein the step of moving the vessel in an athwartships direction from the fixed dock onto said drydock comprises transporting the vessel on carriages rollably mounted on tracks and wherein this step is preceded by the step of bridging the portion of the drydock from which the removable wingwall has been removed with removable tracks.

17. The method of claim 16 wherein the step of transporting the vessel on carriages rollably mounted on tracks is followed by the step of removing the removable tracks from the portion of the drydock from which the removable wingwall has been removed.

18. The method of claim 17 wherein the step of bridging a portion of the drydock with removable tracks is followed by the step of ballasting the drydock down on a launching grid so that the tracks on the drydoclcare aligned with tracks on the fixed clock.

Claims

1. A floatable drydock for retrieving and launching vessels to and from a waterfront comprising: a. a rectangularly shaped bottom portion to support a vessel during launching and retrieving, said bottom portion having a longitudinal direction parallel to the longitudinal axis of a vessel to be supported therein and a transverse direction parallel to an athwartships axis of said vessel, said bottom portion including at least a portion of a first buoyancy chamber; b. a first wingwall located along one longitudinally directed side of said bottom portion, said first wingwall including at least a portion of a second buoyancy chamber; c. a second wingwall located along the other longitudinally directed side of said bottom portion, said second wingwall being removable from said bottom portion, said second wingwall including at least a portion of a third buoyancy chamber; and d. means for transferring a vessel between said bottom portion and the waterfront comprising a plurality of tracks, a plurality of rolling carriages, and a plurality of means for supporting a vessel on said plurality of rolling carriages, said plurality of tracks including a first number therefrom mounted on the waterfront, a second number therefrom mounted in said transverse direction on said bottom portion and a third number therefrom removably mounted between said first and second number for temporarily bridging the gap between said first and second number of tracks along the portion of said drydock which said removable wingwall mounts, each track in said first number being aligned with the respective track in said second number and in said third number, said plurality of rolling carriages and means for supporting a vessel on said carriages rollably mounted on said tracks for transporting a vessel in an athwartships direction along said plurality of tracks until the vessel is moved between the waterfront and said drydocks.

9. A method of retrieving vessels floating in a body of water onto a fixed dock, employing a floatable drydock having two wingwalls longitudinally disposed along the sides of said drydock, one wingwall being removable, comprising the steps of: a. moving the vessel to A position above a submerged floatable drydock; b. changing the flotation of said drydock an amount sufficient to raise said vessel out of the water and float the drydock with the vessel thereon; c. moving said drydock with the vessel thereon to a position adjacent to the fixed dock; d. removing the wingwall from the drydock; e. bridging the portion of the drydock from which the removable wingwall has been removed with removable tracks; f. transporting the vessel in an athwartships direction on carriages rollably mounted on tracks from the drydock to the fixed dock.

11. The method of claim 9 wherein the step of moving said drydock with said vessel thereon to a position adjacent to said fixed dock includes the step of aligning the drydock with said fixed dock by changing the floating position of said drydock until it firmly rests on a firm foundation adjacent to said fixed dock.

13. A method of side launching vessels from a fixed dock to a body of water employing a floatable drydock having at least two wingwalls longitudinally disposed along the sides of said drydock, at least one wingwall being removable, comprising the steps of: a. moving the vessel in an athwartships direction relative to said fixed dock and said drydock from said fixed dock onto said drydock when said drydock is aligned with the fixed dock and from which the removable wingwall has been removed; b. attaching said removable wingwall to said drydock; c. putting said drydock and vessel thereon in a floating condition; d. moving said drydock and said vessel from a position aligned with said fixed dock to a position removed therefrom in said body of water; e. changing the flotation of said drydock thereby causing it to attain a submerged position sufficient to enable said vessel to float free.

18. The method of claim 17 wherein the step of bridging a portion of the drydock with removable tracks is followed by the step of ballasting the drydock down on a launching grid so that the tracks on the drydock are aligned with tracks on the fixed dock.