US3894503A

US3894503A - Method for converting a floatable barge into a semi-submersible vessel

Info

Publication number: US3894503A
Application number: US348594A
Authority: US
Inventors: Alan C Mcclure
Original assignee: Brown and Root Inc
Current assignee: Brown and Root Inc
Priority date: 1973-04-06
Filing date: 1973-04-06
Publication date: 1975-07-15
Anticipated expiration: 1992-07-15
Also published as: CA990588A; NL7404577A; GB1438916A; AU6741674A; DE2416357A1; AU472884B2

Abstract

Methods and apparatus are provided for converting a floatable barge into a semi-submersible vessel. An independently floatable cradle unit is provided which has at least two submersible hulls, a plurality of support columns, transverse braces, and a buoyancy mechanism for selectively ballasting and de-ballasting the submersible hulls. The cradle unit is sunk or at least partially submerged and a barge is floated thereabove, whereupon the cradle is raised to lift the barge from the water. The barge is rigidly connected to the upper portions of the support columns. An inclined lay ramp and/or derrick may be mounted on the cradle to convert the barge into a semi-submersible lay or derrick vessel.

Description

United States Patent [1 1 McClure 1 1 METHOD FOR CONVERTING A FLOATABLE BARGE INTO A SEMI-SUBMERSIBLE VESSEL [75] Inventor: Alan C. McClure, Houston, Tex.

[73] Assignee: Brown & Root, Incorporated,

Houston, Tex.

[22] Filed: Apr. 6, 1973 [21] Appl. No; 348,594

Lloyd 1 14/115 D X 1 1 July 15, 1975 3,797,438 3/1974 Fayren .1 114/05 D Primary Examiner-Trygve M. Blix Ari-(slum E.raminerCharles E. Frankfort Altorney, Agent. or Firm-Burns. Doane, Swecker & Mathis 5 7] ABSTRACT Methods and apparatus are provided for converting a floatable barge into a semi-submersible vessel. An independently floatable cradle unit is provided which has at least two submersible hulls, a plurality of support columns, transverse braces, and a buoyancy mechanism for selectively ballasting'and de-ballasting the submersible hulls. The cradle unit is sunk or at least partially submerged and a barge is floated there above, whereupon the cradle is raised to lift the barge from the water. The barge is rigidly connected to the upper portions of the support columns. An inclined lay ramp and/or derrick may be mounted on the era die to convert the barge into a semi-submersible lay or derrick vessel.

7 Claims, 13 Drawing Figures ymgmqmm 1 5 ms 3 a 94'. 503

SHEET AW .ii' i METHOD FOR CONVERTING A FLOATABLE BARGE INTO A SEMI-SUBMERSIBLE VESSEL BACKGROUND AND OBJECTS OF THE INVENTION This invention relates to semi-submersible vessels, and more particularly concerns the conversion of a conventional floatable barge into a semi-submersible vessel.

Various offshore naval operations are being performed in increasing frequency in rough water areas throughtout the world. In the North Sea and Australia, for example, extensive oil drilling and pipeline laying operations are being conducted. The severe wave conditions typically encountered in areas like these greatly limit effective vessel utilization.

Vessels having hull configurations which are characterized by large water plane areas, such as conventional floating barges, for example, are subjected to excessive motion in heave, pitch, and roll directions during rough sea conditions. Consequently, vessels such as these might encounter substantial difficulties during the performance of operations such as pipeline laying, ship to ship material transfer, hoisting operations, platform erection, and other operations which require steady working decks. The dangers produced by such unstable conditions can threaten the safety of the crew as well as result in costly delay.

Vessels of the semi-submersible type, on the other hand, typically exhibit a high degree of stability and thus are being widely employed in rough waters. Semisubmersible vessels typically utilize a pair of buoyant hulls connected below a working platform by means of vertically disposed support columns. The buoyancy of the hulls may be varied to enable the working platform to be selectively raised and lowered in relation to the water surface. In this manner, the buoyant hulls can be submerged to a position below the water surface while maintaining the working platform above the water surface at a higher level than the maximum expected wave heights.

Thus, contact of the semi-submersible with the water surface, where wave action is most severe, is of a limited nature, occuring solely at the vertically projecting support columns. Consequently, the semi-submersible offers minimal obstruction to wave action such that the natural periods of motion of the semi-submersible, during heave, pitch, and roll, are longer than similar periods of normally encountered waves. The resultant stability of the working platform facilitates the performance of sea-based operations, in spite of the presence of a rough water environment.

Numerous problems are involved in satisfying the growing need for semi-submersible vessels to carry out the tasks required in rough-water operations. Among these problems are the considerable time and expenses encountered in constructing a sufficient number of semi-submersibles. It would, therefore, be highly advantageous to provide semi-submersible vessels quickly and economically.

One proposed form of semi-submersible is illustrated in US. Pat. No. 3,575,005, issued on Apr. 13, 1971. This patent proposes a method of transforming a conventional ship-type vessel into a semi-submersible. It would be desirable, however, that a conventional vessel be transformed into a semi-submersible in a manner which requires less vessel modification while providing maximum vessel stability and utilization.

It is, therefore, a general object of the invention to provide novel methods and apparatus for obviating or substantially alleviating problems of the types previously discussed.

It is a particular object of the invention to devise novel methods and apparatus for providing semisubmersible vessels at considerable savings of time and cost.

It is another object of the invention to provide novel methods and apparatus for modifying the hull characteristics of conventional floatable barges to minimize the effects of wave action thereon.

It is still a further object of the invention to provide novel methods and apparatus for converting conventional floatable barges into semi-submersible vessels in a manner requiring minimal vessel modification while providing optimum vessel stability.

It is still another object of the invention to provide such novel barge conversion methods and apparatus wherein substantial portions of the conversion operation may be accomplished independently of a physical presence of the barge.

It is yet another object of the invention to provide novel methods and apparatus for converting conventional floatable barges into semi-submersible lay vessels in a manner requiring minimal vessel modification while providing optimum vessel stability.

BRIEF SUMMARY OF PREFERRED EMBODIMENTS OF THE INVENTION At least some of the foregoing objects are intended to be accomplished by the provision of barge conversion methods and apparatus in accordance with this invention.

In one preferred method aspect of the invention a floatable barge is converted into a semi-submersible vessel. In effecting this conversion, a cradle unit is provided which has a pair, or more, of preferably parallel submersible hull members, a plurality of columns extending transversely upwardly from each hull, transverse bracing structures preferably interconnecting columns of one hull with opposing columns of the other hull, and a buoyancy mechanism for selectively ballasting and de-ballasting the hull members.

The buoyancy mechanism is actuated to cause the cradle unit to sink, at least partially in a body of water, such that the hulls are submerged, the bracing structures define support surface means which may be horizontal and be disposed below the water surface, and the opposing columns define a barge-receiving zone therebetween.

With the cradle thus disposed, the barge is floated on the body of water to a position adjacent the bargereceiving zone, with the underside of the barge located above the horizontal support surface means. The buoyancy mechanism is then actuated to bring the horizontal support surface means into supporting contact with the underside of the barge and lift the barge from the surface of the body of water.

Rigid connection is made between the barge and the upper portions of the columns.

The bracing structures include a plurality of brace beams interconnecting opposing columns, and diagonal brace elements.

The buoyancy mechanism includes ballast pumping equipment in each hull which communicate by a conduit extending through a brace beam.

The barge and cradle are interconnected by extension frames. These frames may include upper surfaces which define extensions of the barge hull and working platforn. and may serve to support at least part of an offset rotary crane.

During the conversion operation, the barge may be floated to a position adjacent the barge-receiving zone, yet off-set from the center thereof. An extension frame may be provided to serve as an auxiliary working deck.

In an independently significant aspect of the invention the cradle may include an inclined or downwardly curved lay ramp which is suitable for the laying of an elongate flexible structure, such as pipeline, from the vessel. The lay ramp may be supported by columns at one side of the cradle, the columns having rampsupporting portions of varying height in accordance with the ramp inclination.

The after-most column adjacent the ramp may be provided with a hitch mechanism for supporting a conventional stinger mechanism Further aspects of the invention relate to combinations of structural elements for implementing the previously described methods.

DRAWINGS Other objects and advantages of the present invention may be understood from the following detailed description of preferred forms thereof with reference to the accompanying drawings in which like numerals refer to like elements and in which;

FIG. 1 is a schematic sectional view of a barge conversion assembly taken along the line 2-2 in FIG. 2 with the tower portion removed, according to one preferred embodiment of the subject invention;

FIG. 2 is a schematic side elevational view of the barge conversion assembly including the tower portion;

FIG. 3 is a schematic front elevational view of the barge assembly depicted in FIG. I;

FIG. 4 is a schematic front elevational view of another preferred barge conversion assembly according to the invention, with the tower portion removed;

FIG. 5 is a schematic front elevational view of a further preferred embodiment according to the invention, with derrick and tower portions removed;

FIG. 6 is a schematic sectional view of the barge conversion assembly taken along line 66 in FIG. 7;

FIG. 7 is a schematic side elevational view of the barge conversion assembly depicted in FIGS. 4 and 6, including the derrick and tower portions;

FIG. 8 is a schematic longitudinal cross-sectional view through a submersible buoyant hull member forming part of the barge conversion assembly of the invention;

FIG. 9 is a transverse sectional view through a support column, taken along the line 9-9 in FIG. 2', and

FIGS. 10 through 13 are schematic front elevantional views sequentially depicting various steps in the barge conversion operation of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the present invention a barge conversion assembly is provided, comprising a conventional floatable barge and a buoyant cradle unit attachable thereto to form a semi-submersible vessel.

One form of the barge conversion assembly is illustrated schematically in FIGS. 1, 2, and 3 and comprises a barge l0 and a buoyant cradle unit 12. The barge 10 is a conventional floating derrick barge having front, rear, and side portions 16, I8, and 20 and a working deck or platform 22 carried thereby.

Operably disposed on the working deck 22 is a conventional derrick or crane unit 24. The crane includes a boom 25 which is rotatably mounted on a tub 26 that is secured to the deck 22. Only a tub portion of the crane is depicted in FIG. 1 to indicate the relative location thereof. In this connection, the rotational axis of the crane is arranged so as to intersect the centerline of the barge. Located at the front of the barge is a control tower 28 from which various barge activities are directed.

It will be appreciated that the barge, when in a floating condition, exhibits a relatively large water plane area, thereby rendering the barge highly sensitive to wave action. Thus, in rough water areas, or where long swells are prevalent, motions of the barge in heave, pitch, and roll directions will tend to restrict the usefulness of the barge.

The cradle unit 12 includes twin submersible hulls, preferably in the form of buoyant caissons or pontoons 3UP and 308 which are oriented on port and starboard sides in parallel relationship. The pontoons include ballast chambers and equipment for selectively ballasting and de-ballasting these chambers with sea water, as will be subsequently described.

More specifically, each of the pontoons carries a plurality of

support columns

32 and 34 which extend transversely relative to the longitudinal axes of the pontoons. While the preferred embodiment described in connection with FIGS. 1, 2, and 3 discloses three columns associated with each pontoon, it will be appreciated that any suitable number of columns may be employed.

Between corresponding, or opposite, ones of the fore, aft, and intermediate (amidships)

support columns

32F, 32A, 32!, and 34F, 34A, 34] are provided horizontal transverse bracing

assemblies

35F, 35A and 35I. Each bracing assembly includes a lower horizontal brace beam 36 and an upper horizontal brace beam 38 secured between oppositely disposed ones of the

columns

32 and 34. A plurality of

diagonal brace members

40,41,42 and 43 preferably four in number, are secured at their upper ends to the upper brace beam 38. The two inner

diagonal braces

41 and 42 are connected at theirlower ends to the lower brace beam 36. The two outer

diagonal braces

40 and 43 are connected at their lower ends to the

opposed support columns

32 and 34, respectively.

With the pontoons 30, the

columns

32 and 34, and the bracing assemblies 35 in an assembled condition, the cradle constitutes a unitary floatable structure.

Although the manner of assembling the barge and cradle unit together will be described subsequently in greater detail, it is noted that the

columns

32, 34 are secured to the barge by means of steel frame extension structures 46 which are rigidly secured between the barge and the columns. In their assembled condition, the frame structures 46 define extensions of the barge hull.

Referring particularly to FIG. 8, it will be understood that the pontoons 30S and 30A are substantially identical. Each pontoon is provided with a pump room 48. Within the pump room are disposed a plurality of

pumps

52 and 54 which are suitably connected, via appropriate piping and valve manifolds, between a sea chest 56 and a number of water-tight ballast chambers 58. The

pumps

52 and 54 are operable to regulate the transfer of sea water to the ballast chambers to alter the buoyancy of the pontoons. Additionally, the pump 54 is arranged to transfer ballast water from one pontoon to the other through a conduit 60 extending through the lower horizontal brace beam 36 of the intermediate bracing assembly. in this fashion, the pump mechanism of one pontoon may, if necessary, be utilized to control the buoyancy of both pontoons.

The control room of the control tower 28 is provided with a main ballast control console (not shown) for remotely operating the ballast pumps and their associated valves. Auxiliary ballast controls are desirably disposed in the pontoon pump rooms and are capable of being operated independently of the main control console if such necessity arises.

The pontoons also include chambers 59 and 61 for the storage of fresh water and fuel, respectively. Suitable pumps may be provided for conducting this water and fuel to the barge. Such an arrangement tends to maximize usage of the barge storage facilities.

As can be observed in FIG. 9, the intermediate ones 321 and 341 of the support columns include central tubes 62. These tubes provide shear strength across the columns and accommodate access to the pontoon pump rooms via appropriately arranged ladders. The fore and

aft support columns

32F, 32A, and 34F, 34A are also provided with central tubes to facilitate passage of piping between the barge and the pontoons.

The tubes are held in fixed relation within the col umns by means of fore-aft bulkheads 64 and transverse bulkheads 66.

CONVERSION OPERATION Conversion of a floating barge into a semisubmersible vessel in accordance with the invention is initiated by fabricating the cradle unit 12. In this connection, and as can be viewed in FIGS. through 13, the pontoons S and 30F are constructed and are provided with stub portions 70 of the

support columns

32 and 34.

The pontoons are then launched into a floating condition. While afloat, the pontoons are interconnected by installation of the lower horizontal brace members 36. The remaining portions of the support coluns 32 are then erected, defining a barge-receiving zone 72 therebetween. Installation is completed of the remainder of the transverse bracing

assemblies

35F, 35A, and 35l, including the upper transverse brace beams 38 and the

diagonal braces

40, 41, 42 and 43.

Portions of the hull. extension frames 46 are then welded onto the upper ends of the support columns.

As the above-described construction of the extension frames is being carried out, the piping systems, machincry, and wiring are installed within the pontoons, support columns, and hull extension frames.

The cradle unit is then transported to a deep water location where the

pontoons

30S and 30P are ballasted causing the cradle unit to sink to a depth sufficient to permit the barge 10 to be floated into the bargereceiving zone 72. With the barge 10 in position, the pontoons will be de-ballasted to raise the cradle unit and thus the brace beams 38 into engagement with the underside of the barge (FIG. 12). The upper surfaces of the brace beams 38 thus define a generally horizontal barge support surface means. The above-water portions of the extension frames 46 are then connected.

Finally, the pontoons are further de-ballasted to raise the barge out of the water to a position where installation of the hull extension frames 46 may be completed.

As an alternate procedure, portions of the extension frames 46 may be initially constructed on the barge and then extended outwardly to the support columns.

To complete the conversion, the piping and wiring systems between the barge and the cradle unit are interconnected.

Operation of the vessel as a semi-submersible will be substantially the same as that of conventional semisubmersible vessels. For a detailed summary of the operation of vessels of this type, reference may be had to a paper entitled Development of the Project Mohole Drilling Platform drafted and presented by the present inventor at an annual meeting of the Society of Naval Architects and Marine Engineers on Nov. 11-12, 1965.

During semi-submersible operation of the vessel, the cradle unit provides optimum support for the barge by means of the cross bracing assemblies 35. The bracing assemblies themselves, however, offer minimal obstruction to wave action and thus do not impair vessel stability and movement.

LAY BARGE CONVERSION In another preferred embodiment of the invention a conventional floatable barge is converted into a semisubmersible lay vessel suitable for the offshore laying of an elongate flexible element, such as pipeline (FIGS. 4, 6, and 7).

in accomplishing this conversion a cradle unit 79 is constructed. This cradle unit 79 includes a pair of port and starboard pontoons 8UP and 808, a plurality of

upstanding support columns

82F, 82A and 821, and 84F, 84A, and 841 attached to the pontoons. Bracing assemblies 86, similar to 35 extend between corresponding, opposite support columns. Extension frames 88, 89, and 90 extend between the support columns and the sides of the barge 78.

Significantly, the support columns 84 and the extension frame 90 are arranged to accommodate a lay ramp 92. The lay ramp 92 is downwardly inclined in a foretoaft direction and is carried at the upper ends of the starboard support columns 34. The top outer edges of the support columns 84A and 84l are disposed at levels of progressively diminishing height in a fore-to-aft direction in accordance with the inclination of the ramp 92. That is, the support columns 84A and 84l are recessed at their top outer edges to define shoulders 94 which support the lay ramp 92. The forward, upper end of the lay ramp 92 is substantially flush with the extension frame 90 and the deck of the barge. The lower end of the ramp terminates above the starboard pontoon S adjacent the expected water surface level.

The ramp may be of any suitable width, and may extend outwardly of the support columns 34. Accordingly, appropriately sized bracing flanges 98 may be utilized to support the outwardly extending portion of the ramp.

The hull extension frame 90 extends along substantially the entire length of the barge to define an auxiliary deck surface 95, which is flush with the working deck of the barge 78. This auxiliary deck can be used to carry equipment associated with pipeline laying, such as pipe storage units and welding equipment and other apparatus such as described in the previously cited patents.

Known pipelaying techniques and equipment may be utilized in conjunction with a converted lay barge according to this invention. Exemplary thereof are the techniques and equipment described in Lawrence US. Pat. Nos. 3,390,532 (issued July 3, 1968), 3,472,034 (issued Oct. 14, 1969), 3,487,648 (issued Jan. 6, 1970) and 3,585,806 (issued June 22, 1971), Rochelle U.S. Pat. No. 3,507,126 (issued June 13, 1972), and Jones et al. US. Pat. No. 3,668,878 (issued Apr. 9, 1969). The afore-mentioned patents are assigned to the assignee of this invention and the disclosures of these patents are incorporated herein by reference.

Moreover, additional below-deck quarters may be built into the hull extension frames to accommodate additional personnel, supplies and equipment.

Preferably, the aft starboard support column 34A is provided with a hitch structure 100 suitable for carrying a conventional stinger mechanism (not shown) dur ing laying operations. Such a stinger and hitch are shown in Lochridge US. Pat. No. 3,606,759, issued on Sept. 21, 1971 and assigned to the assignee of this invention. The disclosure of this Lochridge patent is incorporated herein by reference.

The procedure for converting the barge into a semisubmersible lay barge is similar to that described in accordance with FIGS. 10 through 13. In this instance the pontoons 80S and 801 are constructed and floated. The remaining portions of the cradle 79, including the lay ramp 92, are then installed. Subsequently, the conventional barge 78 is floated into position between the support columns, and is raised by the cradle unit such that the

frame extensions

88, 89, and 90 may be connected between the barge and the support columns. In this fashion, the barge is rendered capable of semisubmersible operation for the off-shore laying of elongated flexible elements, such as pipeline.

If desired, conventional auxiliary pipe-laying equipment including a stinger unit and pipe-tensioner unit may be mounted on the cradle prior to the initiation of a pipe-laying operation.

It may be desirable to re-locate the crane 102 from its center-line position to a location P off-set toward the port side in order to compensate for the added starboard weight imposed by the lay ramp and lay equipment. In order to provide maximum compensation, the crane may be partially located on the frame extension 89 which is of a length sufficient to accommodate the crane. Such an arrangement also serves to increase the effective reach of the crane over the port side.

An offset arrangement of the crane may also be utilized in connection with the barge conversion assembly described in connection with FIGS. 1, 2, and 4, should it be desired to increase the outward reach of the crane. Compensation for the offset forces imposed by the crane during lifting operations may be provided by suitable ballasting of the submersible hulls 30S and 30F.

Although the preceding embodiments of the invention have been discussed in connection with a barge which is positioned substantially centrally between the support columns, it will be appreciated that the other arrangements are possible in accordance with the teachings of this invention. One alternate arrangement is shown in FIG. 5 in which a barge is seated upon shoulders 112 formed in the starboard support columns 114. The barge is also supported by a plurality of brace assemblies 116 and is connected to a frame extension 118 projecting from the port support columns 120.

The frame extension 118 defines an auxiliary working deck on which various equipment, such as a crane, can be mounted. Also, additional crew quarters and storage compartments may be installed in the belowdeck portion of the frame extension 118. Assemblage of the structure shown in FIG. 5 is accomplished in a manner similar to the procedure discussed in connection with FIGS. 10 through 13. In this instance, however, the barge will be floated to an offset position adjacent the barge receiving zone, e.g., wherein its starboard side is situated over the shoulders 112. Subsequent to being lifted by the cradle unit, the barge is connected to the starboard support columns 1 14 and to the frame extension 118.

SUMMARY OF MAJOR ADVANTAGES AND SCOPE OF THE INVENTION As is apparent, the present invention provides novel methods and apparatus for enabling a conventional floatable barge to be effectively converted into a semisubmersible vessel. In this fashion, a pre-existing barge which meets structural and design standards required of sea-going vessels may be transformed into a semisubmersible with only minimal amounts of barge modification.

Significantly, most of the conversion efforts, i.e., construction of the cradle unit, may be accomplished independently of a physical presence of the barge. Thus, work may be advanced while the barge is engaged in other marine operations. Because of this and the rapid manner in which the final conversion operation is accomplished, use of the barge is lost for only a minimal period.

The cradle unit is constructed in a manner which produces optimum strength and stability, while presenting minimal obstruction to free travel through the water. This is the result of the compact, yet effective, bracing assemblies which rigidify the cradle elements into a unitary, independently floatable structure capable of withstanding highly severe wave environments. The relatively skeletal arrangement of the bracing assemblies, however. assures that the stability and maneuverability of the vessel will not be unduly impaired by wave action.

The bracing assembly also provides a guide for enabling the ballasting systems of the submersible hulls to be conveniently interconnected.

The present invention is also highly advantageous in affording rapid conversion of a conventional barge into a semisubmersible lay barge. By means of a unique design, a lay ramp is carried by the cradle unit and is arranged to be operably associated with the barge working deck.

The provision of frame extensions which are rigidly connected between the barge and the support columns enables the effective working deck area of the barge to be extended, as well as enabling extra below-deck quarters to be added. This added area may be utilized to mount an offset crane, a feature which is particularly advantageous in counterbalancing the barge.

Although the invention has been described in connection with a preferred embodiment thereof, it will be appreciated by those skilled in the art that additions. modifications, substitutions and deletions not specifically described may be made without departing from the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

l. A method of converting a floatable barge having a working deck into a semi-submersible vessel comprising the steps of:

providing an independently floatable cradle unit having spaced horizontally elongate submersible hull members defining port and starboard submersible hull members,

a plurality of columns extending vertically upwardly from each of said hull members to define port and starboard columns,

transverse bracing means interconnecting said port columns with said starboard columns, said bracing means comprising lower transverse bracing, upper transverse bracing, and intermediate diagonal bracing, and

buoyancy means for selectively ballasting and deballasting said submersible hull members;

actuating said buoyancy means to cause said cradle unit to partially sink within a body of water while maintaining said cradle unit positively buoyant, such that said submersible hull members are completely submerged,

said upper transverse bracing defines a horizontally disposed support surface means disposed below the surface ofthe body of water between the port and starboard submersible hull members, and

the upper ends of at least some of the columns of said submersible hull members extend above the water surface to define a barge receiving zone therebetween;

floating said barge on the body of water into said barge receiving zone between said port and starboard columns to orient the underside of said barge over said support surface means, while maintaining said cradle unit positively buoyant;

actuating said buoyancy means for de-ballasting said hull members and increasing the postive buoyancy thereof to:

raise said cradle unit in a manner bringing said support surface means into supporting contact with the underside of said barge, with said barge being located entirely between said port and starboard columns, and

lift said barge from the surface of the body of water, so that said barge is carried above the water solely by buoyant forces acting on said support surface with said submersible hull members being floatingly disposed in said body of water; and

prior to the termination of said method, rigidly weld ing a plurality of extension frames between the upper portions of at least some of said port and starboard columns and the port and starboard sides of said barge, respectively, with portions of said extension frames being substantially flush with said working deck of said barge to define an extension of opposite sides of said working deck.

2. A method according to claim 1 wherein said step of providing said buoyancy means includes the steps of providing a plurality of ballast chambers and ballast pumping means in each hull, and communicating the ballast pumping means of each hull by conduit means extending through one of said brace beams.

3. A method according to claim 1 wherein the step of floating said barge includes the step of floating a barge having a rotary crane mounted on a working deck thereof with the rotary crane axis extending generally through the centerline of said barge.

4. A method according to claim 1 and further including the step of mounting a pivotal crane at least partially on the upper surface of one of said extension frames.

5. A method according to claim 1 wherein the step of floating said barge includes the step of floating said barge such that the centerline thereof is offset within said barge receiving zone; and said step of rigidly welding includes the steps of welding a frame extension between the barge and the support columns at one side of said cradle unit and arranging an upper horizontal surface of said frame extension to extend along a substantial portion of the length of the barge generally flush with a working deck portion of said barge.

6. A method of converting a floatable barge having a working deck into a semisubmersible lay vessel suitable for laying an elongate flexible means into a body of water, said method comprising the steps of:

a plurality of columns extending vertically up wardly from each of said hull members to define port and starboard columns,

transverse bracing means interconnecting said port columns with said starboard columns, said bracing means including upper and lower transverse bracing and intermediate diagonal bracing, and

an inclined lay ramp section supported by rampsupporting portions of the columns of one of said submersible hull members. the ramp-supporting portions being arranged at levels of rearwardly diminishing height in accordance with the inclination of said ramp, and

actuating said buoyancy means to cause said cradle unit to partially sink within a body of water while maintaining said cradle unit positively buoyant. such that said submersible hull members and said transverse bracing are completely submerged.

floating said barge on the body of water into said barge receiving zone between said zone-defining port and starboard columns to orient the underside of said barge over said support surface means,

while maintaining said cradle unit positively buoyant;

actuating said buoyancy means for de-ballasting said hull members and increasing the positive buoyancy thereof to:

lift said barge from the surface of the body of water, so that said barge is carried above the water solely by buoyant forces acting on said support surface means with said submersible hull members being floatingly disposed in said body of water; and at least prior to the termination of said method:

rigidiy welding upper portions of said rampsupporting columns to one side of said barge so as to establish a lay ramp that extends from an upper location substantially flush with said working deck to a lower position disposed below the underside of said barge for guiding elongate flexible means from said barge, and rigidly welding a plurality of extension frames between the upper portions of at least some of the columns located opposite said ramp supporting columns and the other side of said barge, with portions of said extension frames being substantially flush with said working deck of said barge to define an extension of said working deck. 7. A method according to claim 6 including the step of attaching a stinger mechanism to said cradle unit at the aft-most column supporting said ramp.

Claims

1. A method of converting a floatable barge having a working deck into a semi-submersible vessel comprising the steps of: providing an independently floatable cradle unit having spaced horizontally elongate submersible hull members defining port and starboard submersible hull members, a plurality of columns extending vertically upwardly from each of said hull members to define port and starboard columns, transverse bracing means interconnecting said port columns with said starboard columns, said bracing means comprising lower transverse bracing, upper transverse bracing, and intermediate diagonal bracing, and buoyancy means for selectively ballasting and de-ballasting said submersible hull members; actuating said buoyancy means to cause said cradle unit to partially sink within a body of water while maintaining said cradle unit positively buoyant, such that said submersible hull members are completely submerged, said upper transverse bracing defines a horizontally disposed support surface means disposed below the surface of the body of water between the port and starboard submersible hull members, and the upper ends of at least some of the columns of said submersible hull members extend above the water surface to define a barge receiving zone therebetween; floating said barge on the body of water into said barge receiving zone between said port and starboard columns to orient the underside of said barge over said support surface means, while maintaining said cradle unit positively buoyant; actuating said buoyancy means for de-ballasting said hull members and increasing the postive buoyancy thereof to: raise said cradle unit in a manner bringing said support surface means into supporting contact with the underside of said barge, with said barge being located entirely between said port and starboard columns, and lift said barge from the surface of the body of water, so that said barge is carried above the water solely by buoyant forces acting on said support surface with said submersible hull members being floatingly disposed in said body of water; and prior to the termination of said method, rigidly welding a plurality of extension frames between the upper portions of at least some of said port and starboard columns and the port and starboard sides of said barge, respectively, with portions of said extension frames being substantially flush with said working deck of said barge to define an extension of opposite sides of said working deck.

6. A method of converting a floatable barGe having a working deck into a semi-submersible lay vessel suitable for laying an elongate flexible means into a body of water, said method comprising the steps of: providing an independently floatable cradle unit having spaced horizontally elongate submersible hull members defining port and starboard submersible hull members, a plurality of columns extending vertically upwardly from each of said hull members to define port and starboard columns, transverse bracing means interconnecting said port columns with said starboard columns, said bracing means including upper and lower transverse bracing and intermediate diagonal bracing, and an inclined lay ramp section supported by ramp-supporting portions of the columns of one of said submersible hull members, the ramp-supporting portions being arranged at levels of rearwardly diminishing height in accordance with the inclination of said ramp, and buoyancy means for selectively ballasting and de-ballasting said submersible hull members; actuating said buoyancy means to cause said cradle unit to partially sink within a body of water while maintaining said cradle unit positively buoyant, such that said submersible hull members and said transverse bracing are completely submerged, said upper transverse bracing defines a horizontally disposed support surface means disposed below the surface of the body of water between the port and starboard submersible hull members, and the upper ends of at least some of the columns of said submersible hull members extend above the water surface to define a barge receiving zone therebetween; floating said barge on the body of water into said barge receiving zone between said zone-defining port and starboard columns to orient the underside of said barge over said support surface means, while maintaining said cradle unit positively buoyant; actuating said buoyancy means for de-ballasting said hull members and increasing the positive buoyancy thereof to: raise said cradle unit in a manner bringing said support surface means into supporting contact with the underside of said barge, with said barge being located entirely between said port and starboard columns, and lift said barge from the surface of the body of water, so that said barge is carried above the water solely by buoyant forces acting on said support surface means with said submersible hull members being floatingly disposed in said body of water; and at least prior to the termination of said method: rigidly welding upper portions of said ramp-supporting columns to one side of said barge so as to establish a lay ramp that extends from an upper location substantially flush with said working deck to a lower position disposed below the underside of said barge for guiding elongate flexible means from said barge, and rigidly welding a plurality of extension frames between the upper portions of at least some of the columns located opposite said ramp supporting columns and the other side of said barge, with portions of said extension frames being substantially flush with said working deck of said barge to define an extension of said working deck.

7. A method according to claim 6 including the step of attaching a stinger mechanism to said cradle unit at the aft-most column supporting said ramp.