US3802137A

US3802137A - Offshore well rig

Info

Publication number: US3802137A
Application number: US00240525A
Authority: US
Inventors: J Armstrong
Original assignee: GULF OFFSHORE CO
Current assignee: GULF OFFSHORE CO
Priority date: 1972-04-03
Filing date: 1972-04-03
Publication date: 1974-04-09
Anticipated expiration: 1991-04-09

Abstract

A portable well rig for mounting on offshore platforms including a telescoping mast mountable on a base which also has the engine and drawworks mounted thereon, a drain pan and splash guard on the base, a pair of longitudinally extending beams with jacks for moving the base longitudinally of said beams, a pair of transverse support members, detachable clamps for attaching the support members to platform floor beams, and jacks for moving the entire rig laterally along the floor beams.

Description

United States Patent 1191 Armstrong 1 51 Apr. 9, 1974 OFFSHORE WELL RIG Inventor: James Eubank Armstrong, Houston,

Tex. 1

[73] Assignee: Gulf Offshore Company, Houston,

Tex.

Filed: Apr. 3, 1972 Appl. No.: 240,525

US. Cl 52/118, 52/115, 173/57, 173/151, l75/207 Int. Cl E2lb 15/00, E04h 12/34 Field of Search 1. 173/28, 37, 23, 1; 175/85, 175/8, 9; 52/111, 113, ll5-l18, 632; 61/465 References Cited UNITED STATES PATENTS 4/1952 Bates et a1. 173/37 4/1966 Wolff 1 61/465 X 2/1961 McLean et a1 61/465 X 2/1960 Wilkinson et al 52/115 3,033,525 5/1962 Johnson 61 /46.5

FOREIGN PATENTS OR APPLICATIONS 232,896 1/1969 U.S.'S.R 173/37 Primary Examinerl'lenry C. Sutherland Assistant Examiner-William F. Pate, lII

Attorney, Agent, or Firm-Ned L. Conley; Murray Robinson; David Alan Rose 5 7 ABSTRACT 16 Claims, 6 Drawing Figures OFFSHORE WELL RIG BACKGROUND OF THE INVENTION 1. Field of the Invention Tis invention relates to rigs of the type used for drilling and working over wells, such as oil wells, and particularly to those specifically designed for offshore well locations.

2. Description of the Prior Art Offshore oil wells are often drilled from a stationary platform set in the water and having a floor 30 to 40 feet above the surfaceof the water. Because of the expense of such platforms and drilling operations, and to facilitate servicing of completed wells, often several wells, up to 40 or more, are drilled from the same platform, through holes in the platform floor spaced several feet apart.

Following completion of the wells it is often necessary to do workover operations on the wells. Such workover operations may, include, for example, replacing down-the-hole equipment, acidizing, fracturing, and wash-out operations. Such workover operations are performed with a rig similar to a drilling rig, although it is usually smaller because it is not required to carry as heavy weights of pipe. Many workover rigs may also be used for the drilling of relatively shallow wells.

Such rigs are well known, having been used for many years on land wells. These rigs are commonly made portable, being carried on the bed of a truck or a trailer. A number of different companies manufacture and sell such portable rigs for both workover and shallow drilling operations. Examples of rigs heretofore used may be found in the Composite Catalog of Oil Field Equipment and Services, 1970-7l Edition, at pages 1203 to1211,1282,1283,1323to1338,1612 to 1615, 1999 to 2019, 2050 to 2062, 3422 to 3427, 4079 to 4082 and 5023 to 5034.

In recent years some of these workover rigs have been adapted for use on offshore platforms, as shown, for example, at page 2018 of the aforesaid Composite Catalog. However, it has been found that such rigs are not readily adaptable to offshore platform operations. In the first place, when used on land portability from one well to the other was readily accomplished due to the fact that the rigs are mounted on trucks or trailers. However, this is not feasible in offshore operations, where the rig must be moved from one platform to another, and where, after installation on a platform, it must be moved a number of times to service all of the wells on the platform. The land basedrigs may weigh 100,000 pounds or more. In order to move such a unit to an offshore platform it must be dis-assembled into smaller components and then reassembled on the platform. Another problem encountered on offshore platforms is the limited amount of space available for the workover rig. Land based rigs are seldom compact enough to be moved to all the positions necessary to service all of the wells on a platform.

SUMMARY OF THE INVENTION The invention comprises a rig, primarily for use as a workover rig, which is particularly adapted to solve the problems heretofore encountered in operations on offshore platforms. Preferably the rig has adjustable supports allowing mounting directly on the floor beams of the platform, and means for readily moving the mast and rotary table of the rig to line up with a plurality of wells served by the platform. In the preferred embodiment, the rig is'assembled on the platform from a plurality of relatively light weight units, so that transfer of the rig to and from the platform, and assembly on the platform, can be accomplished without the need for any lifting equipment except the platform crane. Preferably many of the units are clamped together, for versatility in positioning with respect to each other, thereby avoising the necessity of time-consuming operations formerly needed to fit various units at desired 10- cations. Compactness is preferably achieved by utilizing structural elements as storage tanks, and by mounting'some units on top of others.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a plan view of an offshore platform showing one embodiment of a rig in accordance with this invention thereon;

FIG. 2 is an elevational view of the embodiment of the rig shown in FIG. 1, taken at line 2-2 of FIG. 1;

FIG. 3 is another elevational view of the embodiment of the rig shown in FIG. 1 of the drawing, taken at line 33 of FIG. 1;

FIG. 4 is an elevational view of one embodiment of a mast which may form a part of the rig of this invention;

FIG. 5 is a plan view of the embodiment of the mast shown in FIG. 4, with the mast sections shown nested; and

FIG. 6 is a plan view of a cargo vessel, showing how one embodiment of the rig of this invention may be carried on a vessel, together with other equipment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. I shows an offshore platform having equipment thereon such as may be required for workover operations. One embodiment of a rig in accordance with this invention is depicted at 12. The platform may also support the tool pushers quarters 14, a platform crane 16,

a hydraulic accumulator 18, generators, air compressors, and other utility equipment at 20, and high pressure pumps and

tanks

22 and 24. A pipe rack 26 may be mounted on either side of the rig.

As shown in the drawing, the rig is positioned with its rotary table 28 positioned over one of a plurality of oil wells 30. Each oil well has a christmas tree (not shown) mounted below the platform surface shown in FIG. 1. The rig rests upon

platform floor beams

32 and 34 which extend the full length of the platform. In the design of offshore platforms such beams are provided with sufficient strength to support heavy equipment which must be mounted on the platform during drilling, production and workover of the wells. These beams may be as close together as 30 feet, normally where no platform beam.

Structural truss members

42 and 44 rest upon the

beams

38 and 36 respectively and may be secured thereto by clamps (not shown) similar to clamps 40. The

support members

42 and 44 are preferably enclosed box type structures which may be used for storage of water or drilling mud. The support members include upper horizontal beams 46 and brace members 48 to provide support for other equipment to be described. A conventional shale shaker 49 may be mounted on the member 42.

Hydraulic jacks 41 are mounted on the

beams

36 and 38, being connected to the beams and arranged to push against the

members

42 and 44.

A pair of strongback beams 50 are mounted on the beams 46, extending longitudinally thereof, and being clamped thereto by C-clamps 52, the

members

42, 44 and 50 forming a truss to support other equipment, as hereinafter described. The strongbacks are preferably made of I-beams or other structural members, boxed in to form tanks which may be used as fuel tanks for the engines.

The space between the

elements

42 and 44, and beneath the strongbacks 50, provides clearance space for blowout preventers which will be required for working on the wells.

A base member 54 resting on the strongbacks 50 comprises beams 56 extending longitudinally of the strongbacks, being clamped thereto by means of C- clamps 58. At least two hydraulic cylinders or jacks 55 are mounted on the strongbacks 50 with their rods connected to the base member 54. More than one connection location may be provided on the base and the strongbacks to move the base to various positions relative to the strongbacks, as will hereinafter be explained.

The base 54 comprises a base member for the tool house 60, the mast 62, and the drawworks 64. To conserve space on this base, the winch 66 is mounted on top of the tool house, and the engines 68 which provide power to the drawworks aremounted on top of the drawworks.

Stairways

70 and 72 are provided for access to the floor of the base. A hand rail 74 preferably made of sheet metal, not only provides a safety device, but is connected to the pan 76, best seen in FIG. 3, which substantially covers the bottom of the base, to provide a catch basin to catch spilled liquids, such as mud, oil and other liquids which might contaminate surrounding waters. The pan 76 preferably extends the full length of the base 54 and is closed at both ends for this purpose.

In the embodiment of the invention shown in the drawing the mast is made in three sections, a lower section 80, a center section 82 and a crown section 84.

Each section is U-shaped in cross-section, and the sections are tapered slightly, so that the base of the U is narrower than the distance between the legsof the U at their extremities, thereby facilitating nesting of one section within the other, as shown in FIG. 5.

The lower section of the mast comprises four legs 86 joined together by

cross members

88 and 90 and including angular braces 92. As shown in the drawing, FIGS. 3 and 4, the middle section 82 is telescoped within the lower section 80, being overlapped a short distance for increased strength and rigidity. The crown section 84 is telescoped within the middle section 82 and is also overlapped therewith. The middle and crown sections are of structures similar to the lower section, except that a platform 94 is provided at the top of the crown section for mounting the crown block 96. Also a tubing board 98 is mounted on the open side of the middle section and supported at its outer edge by a cable 100 extending upwardly to the top of the crown section. A drilling line anchor 101, as well known in the art, may be mounted on the lower section of the mast.

Elevation of the middle section is accomplished by means of hydraulic cylinders 102 which extend to the base member 54, and elevation of the crown section is accomplished by means of the hydraulic cylinder 104 which is mounted on the lower end of the middle section and extends upwardly with its rod engaging the upper end of the crown section. After the sections are elevated to position and bolted in place, the rods of the hydraulic cylinders may be disconnected from the extended sections of the mast and retracted within the cylinders.

The rig of this invention is particularly well suited for transport to and erection on offshore locations. As shown in FIG. 6, all of the equipment required for the workover operation may be shipped on a l45-foot cargo vessel, whereas offshore workover rigs heretofore available had to be shipped on a barge. The rig may be shipped broken down into relatively small units, and preferably is broken down into units weighting not more than 15,000 pounds each, so that the rig units are readily hoisted onto the offshore platform by means of the platform crane 16 which is normally available. Previously, because of the heavy weight of elements of such rigs, it has been necessary to provide an additional heavy duty crane or other special facilities to place the rig onto and remove the rig from the platform.

When the cargo vessel reaches the platform on which the rig is to be installed, the individual elements are lifted onto the platform by means of the platform crane 16. The jacking beams 36, 38 are then laid along the platform beams 32 and 34, respectively, in such a position that the well to be worked on is positioned between the beams 36 and the beams 38. These jacking beams are then clamped to the platform beams by means of the C-clamps 40.

The

tanks

42 and 44, which, it will be recalled, constitute truss members, are then placed on the jacking

beams

36, 58, as shown in FIG. 2, with the inner edges of the tanks supported by temporary blocks in substantially level position. The tanks are then clamped to the jacking beams. The strongbacks 50 are then placed on the

tanks

42, 44, spanning them and lying along the beams 46, as seen in FIG. 2, and the C-clamps 52 are applied to secure the strongbacksin position. The temporary blocks under the

tanks

42 and 44 may now be removed.

The base 54 is then placed on the strongbacks, in such a position that the rotary table will be substantially directly over the opening in the platform which provides access to the well which is to be worked. The base is then clamped to the strongbacks in this position, by means of the C-clamps 58. The tool house, drawworks and engine may then be placed on the base, and clamped or bolted into position. Preferably the drawworks is made in at least two parts so that it can be lifted by the relatively low capacity platform crane, and the engines are hoisted separately from the drawworks and placed on top of the drawworks.

The rotary table is also placed on the base, and the lower section of the mast hoisted into place and connected at its lower end to the base. The middle section and crown section of the mast are then nestedinside of the lower section, the

hydraulic cylinders

102 and 104 are connected, and the middle'and crown sectionsof the mast are then elevated by means of the hydraulic cylinders. When these sections have reached their positions, they are bolted or pinned in place, and the hydraulic cylinders may then be disconnected and retracted. Other equipment, such as the stairways and the pipe racks may then be put in place, various hoses and other lines run, and the rig is ready for operation.

After completion of operations upon one well, the rig is readily moved to another well. For example, looking at FIG. 2, in order to move from the well below opening 110 to the well below opening 112, the C-clamps 58 which holdthe base to the strongbacks 50 are removed, and the jackingmechanism within the base which is connected to the strongbacks is operated to slide the base longitudinally of the strongbacks to reach the desired position. The C-clamps are then reapplied to hold the rig in this position.

To move the rig laterally, i.e. longitudinally of the platform as shown in FIG. 1, the C-clamps holding the

tanks

42 and 44 to the jacking

beams

36 and 38 are removed, and the hydraulic jacks 41 are operated to move the entire rig along the jacking

beams

36 and 38. As shown, the jacking

beams

36 and 38 are preferably each made in three sections so that when the rig has been moved a distance equal to the length of one section, that section may be picked up and moved over to the other side of the rig. to prepare a further skid on which the rig'may be moved. When the rig has been moved the desired distance, beams 36 and 38 are again clamped to the

tanks

42 and 44.

It will be apparent that the use of C-clamps to hold the various elements in place is highly advantageous, because it is not necessary to accurately position every unit in order to line up bolt holes, or to burn or drill new holes every time the rig is moved. The C-clamps may be of any suitable design and structure having sufficient strength to accomplish their purpose. The primary requirement is that a movable fastener be used which does not require modification of the structures it is holding when the fastener is moved from one location to another.

The novelty and superiority of the rig of this invention become most apparent upon a consideration of the time saved by the use of this rig as opposed to rigs previously available. In many instances, the present rig can be disassembled, removed from a platform, shipped to another platform, and reassembled on the other platform in 24 hours. Previously it has taken from 3 days to a week to accomplish such a move.

Furthermore, the novel structure of the rig of this invention allows it to be readily moved from one well to another on the platform. The rig base and support structure can even be set at an acute angle to the platform floor beams, while still being supported by the beams and being clamped rigidly in place, if this is necessary because of the positioning of other equipment on the platform.

The use of structural members as tanks, and the placement of the engines on top of the drawworks makes the rig unusually compact, so that it takes up much less platform space than rigs heretofore used.

The entire rig may occupy a space onthe platform as little as 12 feet wide and 30 feet long. In addition, since the rig is readily broken down into comparatively small units, transporting between platforms is facilitated.

The rig of this ivention is readily adjustable to various spacings of platform floor beams. For example, the jacking

beams

36, 38 may be spaced to lie on floor beams 45 feet apart when positioned as shown in FIG. 2. If the floor beams are closer together, down to 30 feet, the

members

42, 44 and the jacking beams are placed closer together. At 30 feet spacing, the strong backs may cover the entire span. No modification of any structure is required for different floor beam spacings. Thus, the

elements

42, 44 and 50 form an adjustable length truss.

Although preferred embodiments of the invention have been shown and described, the invention is not limited to these embodiments, since many variations thereof within the scope of the invention will be apparent to those skilled in the art. Thus the limits of the invention are those defined by the claims.

I claim:

1. A rig comprising a lower support structure,

an upper base structure on said support structure,

a mast, drawworks and power source on said base structure,

a plurality of C-elamps holding said base structure to said support structure, and

solid handrails connected to a drip pan substantially covering the bottom of said base, whereby spillage can be controlled.

2. A rig for mounting on an offshore platform comprising i a pair of parallel jacking beams, each comprising a plurality of beams in end-to-end relation,

C-clamp means for fastening said jacking beams to floor beams of said platform,

a support element resting on each jacking beam, with the outboard edge of the elements resting on the beams and the inboard portions extending toward each other,

C-clamp means fastening said support elements to said beams,

a truss beam resting on said support elements,

at least one of said truss beam and said support elements comprising a container for liquid,

C-clamp means fastening said truss beam to said support elements to form a truss,

a base member resting on said truss,

C-clamp means fastening said base member to said truss,

a mast, drawworks and power source on said base member,

jacking means connected to said support elements and to said jacking beams, whereby the truss and the apparatus supported thereby may be moved horizontally relative to the platform upon release of the C-clamps fastening the support elements to the jacking beams and operation of said jacking means,

further jacking means connected to said truss and to said base member substantially at right angles to the first-mentioned jacking means, whereby the base member and the apparatus supported thereby may be moved horizontally relative to the truss upon release of the C-clamps fastening the base member to the truss and operation of the further jacking means, and means on said drawworks for supporting the power source thereon, said power source being mounted above said drawworks by such means.

3. A method for assembling a rig on an offshore plat form having parallel, spaced-apart floor beams, comprising clamping a jacking beam to each floor beam, with each jacking beam lying on top of and extending longitudinally of a floor beam,

placing a support element on each jacking beam,

with each support element resting on a jacking beam and cantilevered toward the other support element,

supporting the inboard portions of the support elements with temporary block means,

clamping the support elements to the jacking beams,

placing a truss beam on the support elements to span the space between them, I clamping the truss'beam to the form a truss, and

then removing the temporary blocks.

4. A method as defined by claim 3 and including connecting a jack between each jacking beam and the support element resting thereon.

5. A method as defined by claim 4 and including clamping a base member to the truss, with the truss lying on top of the truss, and

mounting a mast, drawworks and power source on said base member.

6. A method as defined by claim 5 and including connecting a jack between each jacking beam and the support element resting thereon, and connecting another jack'at right angles to the first jacks between the base member and the truss.

7. A rig comprising an adjustable length truss, comprising a plurality of elements held together by clamping means,

a base member mounted on said truss,

a mast, drawworks and power source mounted on said base member, and jacking meansconnected to said truss and to said base,'whereby said base may be moved horizontally along said truss. h

8. Arig as defined by claim 7, and including jacking beams supporting said truss, and

jacking means connected to said truss and to said jacking beams, whereby said truss may be moved horizontally along said jacking beams.

9. A rig comprising a lower support structure, including an adjustable length truss comprising at least two spaced-apart support elements and a beam, and clamping means holding said beam to said support elements,

an upper base structure on said support structure,

a mast, drawworks and power source on said base structure, and

clamping means holding said base structure to said support structure.

10. A rig as defined by claim 9 and including a horizontal beam under the outboard edge of each support element, so that said support elements are cantilevered toward each other.

11. A rig as defined by claim 10 and includingjacking means connected to said support structure and to said support elements to base structure,.whereby the base structure and the apparatus thereon may be moved horizontally relative to said support structure.

12. A rig as defined by claim 11 and including further jacking means connected to said support structure and to said horizontal beams at right angles to the firstmentioned jacking means, whereby said base structure and the apparatus thereon may be moved horizontally in any direction.

13. A rig comprising at least two parallel elongate elements, each element being releasably fastened at each end to a support element,

said support elements being spaced-apart and comprising enclosed box-type structures suitable for storage of liquids,

each support element resting on its outermost edge on a beam extending horizontally substantially perpendicularly to said elongate elements, so that the support elements are cantilevered toward each other when resting on said beams, and the inner edges of the support elements are supported by said elongate elements, and

a mast, drawworks and power source mounted on said elongate elements.

14. A structure comprising a pair of spaced-apart, horizontally extending, substantially parallel elongate beams,

an enclosed, box-type structure suitable for storage of liquids supported on each beam and having an inner portion extending laterally therefrom toward the other beam,

at least two elongate structural elements lying on and spanning said box-type structures,

said structural elements being fastened to said boxtype structures to form a truss, whereby the structural elements are supported by the box-type structures and the adjacent sides of the box-type structures are supported by the structural elements, and

a mast, drawworks and power source mounted on said structural elements. 15. A method for assembling a rig on an offshore platform having at least two parallel, spaced-apart floor beams, comprising placing an enclosed box-type structure suitable for -the storage of liquids on each of two floor beams, with each such structure resting on a floor beam and cantilevered toward the other floor beam,

supporting the inboard portions of the said structures with temporary block means,

placing a truss beam onsaid structures to span the space between them,

fastening the truss beam to said structures to form a truss, so that the truss beam supports the inboard portions of said structures, and

then removing the temporary blocks.

16. A rig comprising an adjustable length truss, comprising a plurality of parallel elongate elements releasably clamped on each end to spaced-apart support elements,

each support element including a beam extending horizontally substantially perpendicular to said elongate elements, positioned to support saidsupport elements,

said beams being located at the outermost edges of the support elements, so that the support elements are cantilevered toward each other when resting on said beams, and the inner edges of the support elements are supported by said elongate elements,

a base member mounted on said truss, and

a mast, drawworks and power source mounted on said base member.

Claims

1. A rig comprising a lower support structure, an upper base structure on said support structure, a mast, drawworks and power source on said base structure, a plurality of C-clamps holding said base structure to said support structure, and solid handrails connected to a drip pan substantially covering the bottom of said base, whereby spillage can be controlled.

2. A rig for mounting on an offshore platform comprising a pair of parallel jacking beams, each comprising a plurality of beams in end-to-end relation, C-clamp means for fastening said jacking beams to floor beams of said platform, a support element resting on each jacking beam, with the outboard edge of the elements resting on the beams and the inboard portions extending toward each other, C-clamp means fastening said support elements to said beams, a truss beam resting on said supPort elements, at least one of said truss beam and said support elements comprising a container for liquid, C-clamp means fastening said truss beam to said support elements to form a truss, a base member resting on said truss, C-clamp means fastening said base member to said truss, a mast, drawworks and power source on said base member, jacking means connected to said support elements and to said jacking beams, whereby the truss and the apparatus supported thereby may be moved horizontally relative to the platform upon release of the C-clamps fastening the support elements to the jacking beams and operation of said jacking means, further jacking means connected to said truss and to said base member substantially at right angles to the first-mentioned jacking means, whereby the base member and the apparatus supported thereby may be moved horizontally relative to the truss upon release of the C-clamps fastening the base member to the truss and operation of the further jacking means, and means on said drawworks for supporting the power source thereon, said power source being mounted above said drawworks by such means.

3. A method for assembling a rig on an offshore platform having parallel, spaced-apart floor beams, comprising clamping a jacking beam to each floor beam, with each jacking beam lying on top of and extending longitudinally of a floor beam, placing a support element on each jacking beam, with each support element resting on a jacking beam and cantilevered toward the other support element, supporting the inboard portions of the support elements with temporary block means, clamping the support elements to the jacking beams, placing a truss beam on the support elements to span the space between them, clamping the truss beam to the support elements to form a truss, and then removing the temporary blocks.

5. A method as defined by claim 4 and including clamping a base member to the truss, with the truss lying on top of the truss, and mounting a mast, drawworks and power source on said base member.

6. A method as defined by claim 5 and including connecting a jack between each jacking beam and the support element resting thereon, and connecting another jack at right angles to the first jacks between the base member and the truss.

7. A rig comprising an adjustable length truss, comprising a plurality of elements held together by clamping means, a base member mounted on said truss, a mast, drawworks and power source mounted on said base member, and jacking means connected to said truss and to said base, whereby said base may be moved horizontally along said truss.

8. A rig as defined by claim 7, and including jacking beams supporting said truss, and jacking means connected to said truss and to said jacking beams, whereby said truss may be moved horizontally along said jacking beams.

9. A rig comprising a lower support structure, including an adjustable length truss comprising at least two spaced-apart support elements and a beam, and clamping means holding said beam to said support elements, an upper base structure on said support structure, a mast, drawworks and power source on said base structure, and clamping means holding said base structure to said support structure.

11. A rig as defined by claim 10 and including jacking means connected to said support structure and to said base structure, whereby the base structure and the apparatus thereon may be moved horizontally relative to said support structure.

12. A rig as defined by claim 11 and including fuRther jacking means connected to said support structure and to said horizontal beams at right angles to the first-mentioned jacking means, whereby said base structure and the apparatus thereon may be moved horizontally in any direction.

13. A rig comprising at least two parallel elongate elements, each element being releasably fastened at each end to a support element, said support elements being spaced-apart and comprising enclosed box-type structures suitable for storage of liquids, each support element resting on its outermost edge on a beam extending horizontally substantially perpendicularly to said elongate elements, so that the support elements are cantilevered toward each other when resting on said beams, and the inner edges of the support elements are supported by said elongate elements, and a mast, drawworks and power source mounted on said elongate elements.

14. A structure comprising a pair of spaced-apart, horizontally extending, substantially parallel elongate beams, an enclosed, box-type structure suitable for storage of liquids supported on each beam and having an inner portion extending laterally therefrom toward the other beam, at least two elongate structural elements lying on and spanning said box-type structures, said structural elements being fastened to said box-type structures to form a truss, whereby the structural elements are supported by the box-type structures and the adjacent sides of the box-type structures are supported by the structural elements, and a mast, drawworks and power source mounted on said structural elements.

15. A method for assembling a rig on an offshore platform having at least two parallel, spaced-apart floor beams, comprising placing an enclosed box-type structure suitable for the storage of liquids on each of two floor beams, with each such structure resting on a floor beam and cantilevered toward the other floor beam, supporting the inboard portions of the said structures with temporary block means, placing a truss beam on said structures to span the space between them, fastening the truss beam to said structures to form a truss, so that the truss beam supports the inboard portions of said structures, and then removing the temporary blocks.

16. A rig comprising an adjustable length truss, comprising a plurality of parallel elongate elements releasably clamped on each end to spaced-apart support elements, each support element including a beam extending horizontally substantially perpendicular to said elongate elements, positioned to support said support elements, said beams being located at the outermost edges of the support elements, so that the support elements are cantilevered toward each other when resting on said beams, and the inner edges of the support elements are supported by said elongate elements, a base member mounted on said truss, and a mast, drawworks and power source mounted on said base member.