US3613381A

US3613381A - Offshore structures and their installation

Info

Publication number: US3613381A
Application number: US48060A
Authority: US
Inventors: Dwight J Cox
Original assignee: J Ray McDermott and Co Inc
Current assignee: McDermott Inc
Priority date: 1970-06-22
Filing date: 1970-06-22
Publication date: 1971-10-19
Anticipated expiration: 1988-10-19
Also published as: CA946630A; FR2099758A5; NO135375C; NO135375B; JPS516962B1; GB1346086A; NL7107905A

Abstract

JACKET COLUMNS FOR OFFSHORE, STRUCTURES, RENDERED INITIALLY BUOYANT BY REMOVABLE UPPER AND LOWER CLOSURE MEMBERS, ARE INSTALLED BY BEING UPENDED AND THE LOWER CLOSURE MEMBER IS THEN COMPLETELY REMOVED BY TEARING IT AWAY FROM THE SURROUNDING JACKET COLUMN WALL AND WITHDRAWING THE TORN REMAINS UPWARDLY THROUGH THE JACKET COLUMN.

Description

Oct. 19, 1971 I cax 3,513,38!

OFFSHORE STRUCTURES AND THEIR INSTALLATION Filed June 22. 1970 4 Sheets-Sheet 1' 6 3 u 2 t I Z \L i I I 1 I i 3! L 14 :3 i J, I in 9 i l -l z 1 1 ill" 26 a4 z W 6 If i I"! i I z 20% I F t I A I l i a i z/ 25 5F I 1' 36 i 24 l I /7 l .r v 1 R 4 %& i F 13].

mvr wmrz DWIGHT J COX Oct. 19, 1971 n. J. .cox

OFFSHORE STRUCTURES AND THEIR INSTALLATION 4 Sheets-Sheet z Filed June 22. 1970 INVENTOR DWIGHT J COX B W 4 ATT NEW Oct. 19,, 1971 o. J. cox 3,613,381

OFFSHORE STRUCTURES AND THEIR INSTALLATION Filed June 22. 1970 4 Sheets-Sheet 5 Q v Y J E yE-B- .INVENTOR DWIGHT J COX ATTOR EYS' Oct. 19, 1911 O. J. COX

OFFSHORE STRUCTURES AND THEIR INSTALLATION Filed June 22. 1970 4 Sheets-Sheet &

I N VENTOR OW/GHT J COX United States Patent ()1 ice US. Cl. 61-465 19 Claims ABSTRACT OF THE DISCLOSURE Jacket columns for offshore structures, rendered initially buoyant by removable upper and lower closure members, are installed by being upended and the lower closure member is then completely removed by tearing it away from the surrounding jacket column wall and withdrawing the torn remains upwardly through the jacket column.

This invention relates to offshore structures, typically platforms, of the type comprising at least one hollow column through which a member of smaller transverse dimension extends in the completed installation. The invention also relates to methods for installing such struc tures.

While of more general applicability, the invention is particularly useful in connection with offshore platforms of the type having legs comprising jacket columns through which piles are driven during installation. Such platforms are usually constructed on shore, with temporary closure plates applied to the ends of the jacket column to render the structure buoyant. The platforms are transported, usually by barge, to the offshore installation site and are off-loaded by launching or by use of a barge crane. Once off-loaded, the floating attitude depends upon the particular platform design, the flotation characteristics of the jacket columns, etc., and the structure may be in any of various initial positions ranging from horizontal to upright but upside down. It is accordingly necessary to rotate the platform to proper upright position, and this is accomplished by use of a barge crane and selective flooding of the jacket columns. When the platform is in proper upright position, it is then necessary to remove the temporary closure plate. While removal of the closure plate at the upper end has posed no particular difliculty, prior-art workers have encountered problems inn accomplishing removal of the lower closure plate after the platform has been upended. According to a common prior-art practice employed when the platform is to be secured in place by internal piles driven through the jacket column, the lower closure plates are secured to the extreme lower end of the respective jacket columns by a peripheral seal weld and removal is accomplished by dropping a pile downwardly through the jacket column with sutlicient force to break the seal weld of the closure plate. This procedure has proved troublesome, particularly when the underlying earth formation is hard, because the closure plate tends to hang up on the end of the pile and makes it diflicult, if not impossible, to drive the pile.

Prior-art workers have proposed various procedures for overcoming such difficulties, including particularly the use of closure plates made of rigid polymeric materials designed to shatter under the force of a pile, and the use of relatively heavy rubber diaphragms capable of being pierced by a pile or the like. Polymeric plates have proved generally unsuitable, principally because they require bolted flange plates and gasket seals. The use of rubber diaphragms has been unduly expensive and also has had the disadvantage that the diaphragms must be secured by peripheral clamping rings which, to adequately secure the diaphragm, must be thicker radially of the jacket column 3,613,381 Patented Oct. 19, 1971 than the annular space normally available between the pile or other internal member and the jacket column wall.

A general object of the invention is to provide a structure of the type described in which the lower closure member or members can be disconnected by remote operation and removed completely from the jacket column, so that the interior of the column is left substantially entirely free for passage of a pile, conductor pipe, or other elongated member.

Another object is to devise an improved closure member for jacket columns and the like, and means for remotely both disconnecting the plate and completely removing the same.

A further object is to provide a closure plate of the general type described which can be made of relatively thin, light weight material and is therefore more easily removed than are the closure plates heretofore employd.

Yet another object is to devise an improved method for installation of offshore structures of the type embodying at least one hollow columnar member which is initially buoyant and which is submerged by selective flooding during installation. In this regard, the invention makes it possible to install the lower closure at any point within the columnar member, rather than being confined to the lower end, with the result that the flotation characteristics can be improved and the problem of rotating to vertical position is simplified.

Generally stated, in offshore structures according to the invention, the jacket columns or other columnar structures are provided with lower closure members which, though initially sealed to the wall of the column against passage of water, can be peripherally broken away by an upward strain and then withdrawn upwardly through the columnar structure. The closure member itself is constructed of relatively thin material, the construction and configuration being such that adequate strength is provided to withstand the hydraulic pressures involved, yet the plate has the capability of being severed or broken away from the surrounding wall of the columnar structure under upward forces which can be applied remotely, e.g., from the top of the columnar structure. In typical apparatus embodiments, the closure member is in the form of a truncated cone welded at its periphery to the wall of the jacket column or the like and equipped with a tearing arm rigidly secured to the closure member near the periphery and extending upwardly to a point above the closure member, the arm there being connected to a pull line which extends through the column to the upper end thereof. A tearing strip, typically a length of heavy chain, extends around the periphery of the closure member on the lower side thereof, and is welded at spaced points to the closure member, one end of the tearing strip being secured to the tearing arm. An upward strain of adequate magnitude applied to the pull line first operates the arm to provide an initial tear in the closure member, from an inner point toward the periphery, and then to pull the tearing strip to complete the tear around the periphery of the closure member, the entire member thus being freed for upward withdrawal through the jacket column or the like.

In order that the manner in which the foregoing and other objects are attained according to the invention can be understood in detail, particularly advantageous embodiments thereof will be described with reference to the accompanying drawings, which form a part of the original disclosure of this application, and wherein:

FIG. 1 is a view, partly in vertical section and partly in side elevation, of a jacket column for an offshore platform according to one embodiment of the invention;

FIG. 2 is a vertical sectional view taken on line 2-2, FIG. 3;

FIGS. 3 and 4 are transverse sectional views taken respectively on lines 3-3 and 44, FIG. 1;

FIG. 5 is a fragmentary vertical sectional view taken on line 55, FIG. 3;

FIG. 6 is a fragmentary sectional view taken on line 66, FIG. 3;

FIG. 7 is a top plan view of the upper closure plate of the jacket of FIG. 1;

FIGS. 88D are sequence diagrams illustrating successive stages of removal of the lower closure plate of the jacket of FIG. 1 according to the invention;

FIG. 9 is a vertical sectional view, similar to FIG. 2, illustrating a modified form of the structure shown in FIG. 2;

FIG. 10 is a vertical sectional view similar to FIG. 1 but showing a lower closure plate in accordance with another embodiment of the invention; and

FIG. 11 is a transverse sectional view taken on line 1111, FIG. 10.

FIGS. 1-7 illustrates a typical, and particularly advantageous, embodiment of the invention applied to a columnar leg structure, indicated generally at 1, which is to form part of an offshore platform (not shown) of any of the various constructions well known in the art. Leg 1 comprises a right cylindrical tubular jacket column 2 of conventional construction. Such jacket columns are quite large, with diameters of several feet being common, so that it is desirable to substantially complete the structure on shore, transport the same horizontally to the offshore installation site, and then install the jacket in vertical position. In some cases, the jacket column may be launched at the construction site and then floated to the installation site. More frequently, the jacket column is transported on a suitable vessel and launched at the installation site. In either case, once launched, the jacket column must float with a predetermined attitude. Accordingly, to render the jacket column properly buoyant, removable upper and lower end closures, indicated respectively at 3 and 4, FIG. 1, are provided. In most installations, the jacket column must accommodate a second elongated element, such as a pile (not shown), and it is particularly desirable that, in installed condition, the interior of the jacket column be essentially free so that the pile or the like can have a diameter only slightly smaller than the inner diameter of the jacket column. Thus, for example, the jacket column 2 may have an inner diameter of 43% in. and the pile to be installed therethrough may have an outer diameter of 42 in., so that there is an annular clearance with a radial Width of less than 1 in. between the pile and the jacket column.

In this embodiment of the invention, the upper closure plate 3 is in the form of a flat circular plate 5 having a diameter slightly larger than the outer diameter of jacket column 2, the plate 5 being secured to the upper end of the jacket column only by a continuous seal weld 6, FIG. 1, at the juncture between the bottom face of the plate and the outer surface of the jacket column, weld 6 being effective to secure the plate to the jacket column in liquid-tight fashion. A short bar 7 extends axially through an opening at the center of plate 5 and is welded thereto, with the weld adequate to seal against liquid. A portion 8 of the bar extends within the jacket column and is provided with an aperture 9. A like portion 10 of the bar projects outwardly of the jacket column and is provided with an aperture 11. Plate 5 is reinforced by parallel stiifeners 12 disposed each on a different side of bar 7 and extending chordwise across the upper surface of the plate. Short cross members 13 extend between the respective stiifeners 12 and the edges of bar 7, the two members 13 and the Width of bar 7 being aligned diametrically of the plate and at right angles to stiffeners 12. Stilfeners 12 are of right angle transverse cross-section, an edge of one Web being welded to the upper surface of the plate, the other web of each stiffener projecting parallel to the plate. Cross members 13 are welded to plate 5 as well as to stiffeners 12 and bar 7.

Lower closure member 4 comprises a frusto-conical member 15 formed of relatively thin metal. Member 15 is disposed in a location near the bottom end of the jacket column 2 and is arranged so as to taper inwardly and toward the upper end of the jacket column. The outer periphery of member 15 is secured to the wall of jacket column 2 by a continuous seal weld 16, FIG. 5, with the weld being at the juncture of the upper or lower surface of member 15 and the inner surface of the wall of the jacket column and being effective to join member 15 and jacket column 2 in liquid-tight fashion. At its center, member 15 is closed by a fiat circular plate 17 having a diameter slightly larger than the diameter of the central opening of member 15. Plate 17 is secured to member .15 by a continuous seal weld 18, FIG. 2, between the peripheral portion of the plate and the upper surface of member 15.

Member 15 is provided with a radially extending opening snugly accommodating end portion 20 of an arm 21, end portion 20 being secured to member 15 by a weld 22, FIG. 6, which extends continuously around the arm at the lower surface of member 15. End portion 20 of arm 21 extends completely through the opening in member 15 and, below that member, is provided with a notch 23, FIG. 2, which extends at right angles to member 15 and opens away therefrom. Arm 21 is made from fiat metal stock and includes a longer straight portion 24 and a shorter straight portion 25. Portion 24 slants inwardly of jacket 2 toward closure member 3. Arm portion 25 extends parallel to plate 17, is spaced a significant distance therefrom, and is provided at its end with an aperture 26 spaced inwardly from the wall of the jacket column.

A length of chain 27 extends about the outer peripheral portion of member 15 on the lower side thereof, one end link 28 of the chain being disposed in notch 23 in end portion 20 and arm 21 and welded thereto. The major portion of the length of the chain extends circularly along a line concentric with the wall of jacket column 2 and spaced just inwardly from the outer edge of member 15. As seen in FIG. 5, the outer portion of every other link of the chain is welded directly to the lower surface of member 15, the welds 29 being such that the welded links 30 slant downwardly and inwardly away from member 15, the alternate unwelded links 31 thus being allowed to freely engage the lower surface of member 15 and slant downwardly and outwardly therefrom, as will be clear from FIG. 5. A short portion 32, FIG. 4, of chain 27 is left free and slack, between the first welded link 30 and end link 28. At its other end, the chain is run to a point immediately adjacent to arm 21, end portion 33 of the chain then extending along beside arm 21, with the end links of portion 33 extending along the lower surface of plate 17. Every link of end portion 33 of the chain is welded to

members

15 and 17, respectively.

Bar 7 and arm 21 are interconnected by a pull line assembly comprising a wire rope 34, FIG. 1, provided with

anchor shackles

35 and 36. Shackle 35 is connected to lower end 8 of bar 7 by a suitable pin extending through the shackle and aperture 9. Similarly, shackle 36 is connected to shorter portion 25 of arm 21 by a pin extending through the shackle and aperture 26. The effective length of wire rope 34 and

shackles

35 and 36 is greater than the axial distance between apertures 9 and 26 so that, in the assembled condition seen in FIG. 1, there is a significant amount of slack in rope 34.

Bar 7, with its apertured outer end 10, constitutes means by which a strain can be applied to upper closure 3 to remove the closure after weld 6 has been removed. The combination of bar 7 and rope 34 then constitutes means by which a strain can be applied, to arm 21, axially of jacket column 2. The combination of arm 21 and chain 27 constitutes tearing means for removal of lower closure member 4.

The apparatus embodiment just described provides one typical means for carrying out the method of the invention. Structure 1 is constructed on shore. Lower closure member 4 is installed, with arm 21 connected to rope 34, the location of member 4 being calculated, in view of the weight and size of the jacket column, to provide the desired buoyant condition when the platform is launched. Upper closure member 3 is then installed, with rope 34 attached to bar 7 and arm 21. It will be understood by those skilled in the art that the platform embodying jacket column 2 can be transported to the offshore site either by vessel, with launching occurring at the installation site, or by being launched at, e.g., the construction site and towed to the installation site. When the platform is to be towed for any significant distance, the location of lower closure member 4 is selected to provide not only adequate buoyancy but also a proper buoyant attitude for towing. When transport is by vessel, with launching of the platform at the installation site, and assuming that the size and weight of the jacket columns are such as to provide sufiicient excess buoyancy, lower closure member 4 is installed at a point within the jacket column spaced from the lower end thereof by a distance such that the buoyancy resulting from the internal space sealed off by the upper and lower closure members is offset axially toward the upper end of the jacket column, so that the platform, when launched, rotates automatically toward the desired upright position. In all embodiments, final positioning and sinking of the platform or other structure is accomplished by selective flooding of the columns, employing the usual valving systems (not shown) common for that purpose.

With the jacket column in upright position, a pulling line (not shown) is attached to the outer end of bar 7. Weld 6 is then removed, as by a welder using a cutting torch. The pulling line is then operated to take up the slack in rope 34, and then to apply an upward strain to arm 21. Under such strain, arm 21 is caused to swing outwardly toward the wall of the jacket column, with the result that a generally triangular portion 40, FIG. 8, of the relatively thin member is torn away. The shape and orientation of this initial triangular tear is determined by the rigidity of arm portion and its associated weld, and by the presence of chain end portion 33 adjacent the arm. The resulting initial tear is such that the apex of the triangle, located at the inner edge of arm portion 20, occurs first, further outward swinging movement of the arm causing the two sides of the triangular tear to progress, one along a generally radial line lying between the location of arm 21 and chain end portion 33, the other slanting generally chordwise of the jacket column and away from the side of arm 21 which is opposite chain end portion 33, as seen in FIG. 8.

When arm 21 has been swung to a position in which longer straight portion 24 thereof extends along the inner surface of the wall of the jacket colum, further pulling of the arm by rope 34 causes weld 16 to be ruptured generally along the basic of the triangle of the initial tear, portion 40 thus being separated from the remainder of member 15 and the corresponding portion of chain 27, including initially slack portion 32, now extending generally lengthwise of the jacket column in taut condition, as seen in FIG. 8A. Continuing of the strain via rope 34, and now also via chain 27, causes member 15 to be tore away progressively, both along weld 16 and along lines commencing adjacent plate 17 and running outwardly to the wall of the jacket column, with the result that a series of segments 41 are formed with each such segment remaining attached to chain 27 by the corresponding welds 29, FIG. 5, between the chain links and member 15, as will be clear from FIG. 813. Finally, as the tearing action progresses along weld 16 until, e.g., about half of member 15 has been torn away, a last segment 42 is torn away, segment 42 including plate 17 which remains attached to the remaining part of member 15 by reason of weld 18 and the fully welded chain end portion 33, as will be clear from FIG. 8D. With segment 42 now torn free, and with all of the torn segments of lower closure member 4 remaining attached to chain 27, the entire residue of the lower closure member is withdrawn upwardly through the jacket simply by pulling rope 34, as will be clear from FIG. 8D. Only a negligible internal annular ridge, the residue of weld 16. remains on the wall of the jacket column and, accordingly, the interior of the jacket column is left virtually unobstructed for free passage of the pile or other elongated element to be installed within the jacket column.

It will thus be seen that, after the jacket column has been installed in upright position, the method of the invention comprises the steps of removing the upper end closure, then applying a continuing upward strain to the lower closure member by pulling means extending through the jacket column, progressively tearing the lower closure member away from the jacket column by such upward strain, and then withdrawing the remains of the lower closure member upwardly through the jacket column, with the torn lower closure member remaining attached to the pulling means which caused it to tear. As in the method embodiment illustrated in FIGS. 8-8D, it is particularly advantageous to tear the closure member into a plurality of pieces, e.g., segments 4042, with the pulling means thus preserving a substantially straight condition throughout the operation, and with tearing steps including tearing along a peripheral weld so that, in effect, no residue of the lower closure member remains on the wall of the jacket column.

FIG. 9 illustrates another apparatus embodiment of the invention with which the method can be carried out in essentially the same fashion just described with reference to FIGS. 8-8D. In FIG. 9, the lower closure member 4a for jacket column 2 comprises a relatively thin dished plate 45 of generally spherical configuration disposed within the jacket column so as to be concave with respect to the lower end of the jacket column and convex with respect to the upper end, plate 45 being secured directly to the inner surface of the wall of the jacket column by a peripheral seal weld 46. Near its periphery, plate 45 is provided with a radially extending opening which accommodates a flat bar 47, bar 47 being welded rigidly to plate 45 by a weld which closes the opening in liquid-tight fashion. Bar 47 lies in a plane which extends axially of jacket column 2, the width of the bar being such that the bar projects a small but significant distance both above and below the plate 45. To the straight top edge of bar 47 is welded a flat plate 48. Plate 48 is spaced slightly above plate 45, by reason of the width of bar 47, and slants downwardly and outwardly.

Tearing arm 49 comprises an elbow 50 and a straight piece of pipe 51 Welded to one end of the elbow. The free end of elbow 50 is welded to the top face of plate 48 in such a position that the elbow extends into contact With, or into immediate proximity to, the inner surface of the wall of jacket column 2, as shown. Straight piece 51 thus projects inwardly and upwardly, terminating in a location just short of the central axis of the jacket column. Arm 49 is completed by a cap 52 which is telescoped over the free end of straight pipe portion 51 and is welded rigidly thereto. Cap 52 includes an end plate to which is welded a flange 53, the flange projecting away axially from pipe 51 and terminating above the center of plate 45. Flange 53 is provided with an aperture to accept the pin of the lower shackle 36 of the pulling means, as hereinbefore descibed.

Tearing chain 27 extends about the peripheral portion of the lower surface of plate 45 and is welded thereto. and to bar 47, in the same manner hereinbefore described with references to FIGS. 2, 4 and 5.

From the foregoing description, it will be clear that arm 49 and bar 47 function in the same fashion as herein before described with reference to arm 21, so that a tri- 7 angular portion of plate is first torn away as wire rope 34 is pulled upwardly to pivot the arm outwardly. Further pulling via arm 34 causes chain 27 to tear plate 45 in successive segments, generally as described with reference to FIGS. 8-8D.

In the embodiment shown in FIGS. 10 and 11, the lower closure member 4b comprises a flat circular plate which is of slightly smaller diameter than the inner diameter of jacket column 2. Jacket column 2 is provided with conventional, axially extending pile spacers 61 interconnected by an annular ring 62 at the lower ends of the pile spacers, ring 62 being welded directly to the inner surface of the jacket column. A circular tearing ring 63 is welded to the inner face of ring 62, tearing ring 63 being of square cross-section and the ring being formed of one length of metal bar, with the ends of the bar being in juxtaposition, but not mutually connected, in the completed ring. The dimensions of jacket column 2, plate 60, pile spacer ring 62, and tearing ring 63 are such that the peripheral edge of plate 60 just overlaps the lower surface of ring 63, plate 60 being secured to ring 62 by a seal weld so that the entire assembly constitutes a liquid-tight closure for the jacket column.

A generally triangular pulling plate 64 is welded to one end only of the metal bar which forms tearing ring 63. Plate 64 projects upwardly from the tearing ring and lies in a plane which extends chordwise of the tearing ring, the dimensions of plate 64 being such that one apex 65 thereof is spaced significantly above plate 60 and significantly inwardly of the jacket column wall. At apex 65, plate 64 is provided with an aperture to accept the connecting pin of lower shackle 36 of the pulling means.

Connecting eyes 66 and 67 are secured respectively to the inner surface of plate 64 and the upper surface of plate 60, the latter connection being at the center of the plate, and an insurance cable 68 is attached to the two eyes 66, 67.

The assembly is completed by stiffeners 69 which are welded to the lower surface of plate 60 and extend chordwise of the plate, being equally spaced on opposite sides of the center of the plate, a cross-member 70 being welded between the two stiffeners at the center thereof.

When lower closure member 412 is to be removed, the upper closure member of the jacket column already having been removed, an upward strain is applied to plate 64 via rope 34 and shackle 36, with the result that tearing ring '63 is torn away from pile spacer ring 62 progressively. Assuming a stronger weld between plate 60 and ring 63 than is provided between ring 63 and ring 62, the outer portion of plate 60 is deformed progressively as the tearing ring is separated from the pile spacer ring. The action continues until ring 64 is torn completely away, plate 60 remaining attached to the ring, or being retained by the insurance cable 68. The entire residue of lower closure 4b is then withdrawn upwardly through the jacket column, as hereinbefore described.

It will be understood by those skilled in the art that the apparatus embodiments illustrated and described are representative of structures which make it possible to carry out the method of the invention. It will also be understood that, aside from the method advantages, the closure members 4, 4a and 4b have the distinct advantage of making it possible to easily install the lower closure member at any predetermined location, spaced upwardly from the lower end of the jacket column, which is called for by the flotation requirements of the particular jacket column. In this regard, the embodiments of FIGS. 1-6 and 9, due to their upwardly and inwardly tapering configuration, provide the maximum strength obtainable with the relatively thin metal employed to provide ease of removal of the closure member. It will be understood that various changes and modifications can be made without departing from the scope of the invention. as defined in the appended claims.

What is claimed is:

1. A columnar structure adapted to be floated to an offshore site and there immersed by flooding for installation in upright position, comprising in combintaion:

an elongated hollow columnar member;

a removable upper closure member attached to one end of said columnar member in fluid-tight relation;

a lower closure member attached to said columnar member at a location along the length thereof which is remote from said one end;

tearing means comprising an arm secured to said lower closure member and projecting therefrom on the side of said lower closure member which faces said one end; and

pull line means secured to said arm and extending through said columnar member for manipulation from said other end to apply a strain to said arm effective to cause said lower closure member to be broken away about its periphery from said columnar member, said pull line means being operative for removal of said lower closure member, after the same has been thus broken away, by pulling the remains of said lower closure member through said columnar member.

2. A columnar structure according to claim 1, wherein:

said tearing means further comprises an elongated tear member extending about the periphery of said lower closure member and attached to said lower closure member at least at a plurality of points spaced throughout the length of said elongated tear member, said elongated tearing member also being attached to said arm.

3. A columnar structure according to claim 2, wherein:

said lower closure member is of relatively thin material; said arm comprises a base portion rigidly attached to said lower closure member along a line extending from a point on the body of said lower closure member to a point at the periphery thereof, said arm extending from said base portion to a point which is spaced inwardly a substantial distance from the wall of said columnar member and also spaced from said lower closure member a substantial distance toward said one end of said columnar member, and

said pull line means is secured to said arm at said point, whereby a strain applied to said arm via said pull line means is effective initially to swing said arm outwardly toward the wall of said columnar member and thereby cause an initial tear of the relatively thin material of said lower closure member, which initial tear commences at a point inwardly of said base portion and progresses outwardly to the periphery of said lower closure member.

4. A columnar structure according to claim 3, wherein:

the relatively thin material of said lower closure member is metal, and

said elongated tear member is a flexible metal member welded to the periphery of said lower closure member.

5. A columnar structure according to claim 4, wherein said flexible metal member is located on the side of said lower closure member which faces away from said one of said columnar member.

6. A columnar structure according to claim 4, wherein said flexible metal member is a chain and only some of the links of said chain are welded to said lower closure member.

7. A columnar structure according to claim 6, wherein only alternate links of said chain are welded to said lower closure member.

8. A columnar structure according to claim 3, wherein:

said lower closure member comprises a portion which includes the periphery thereof and which is of relatively thin metal,

said portion of said lower closure member having an opening extending generally radially of the longitudinal axis of said columnar member,

said base portion of said arm being of metal, extending through said opening to effectively fill the same, and being welded to said portion of said lower closure member in liquid-tight fashion,

said elongated tear member is a flexible metal member extending along the side of said portion of said lower closure member which faces away from said one end of said columnar member, and

one end of said flexible metal member is secured directly to a part of said base portion which projects through said opening.

9. A columnar structure according to claim 8, wherein a straight portion of said flexible metal member at the' other end thereof extends generally beside said opening, said flexible metal member being welded to said portion of said lower closure member at spaced points along the full length, including said straight portion, of said flexible metal member.

10. A columnar structure according to claim 9, wherein said flexible metal member is a chain.

'11. A columnar structure according to claim 8, wheresaid portion of said lower closure member is generally frusto-conical and is so disposed as to taper inwardly and toward said one end of said columnar member, and said lower closure member further comprises a central metal plate welded to the small end of said frustoconical portion to close the same. 12. A columnar structure according to claim 11, wheresaid central metal plate is thicker than said frustoconical portion and, the other end of said flexible metal member is secured to said central plate. 13. A columnar structure according to claim 2, wheresaid lower closure member is of metal; said tear member comprises a metal strip welded to said columnar member,

said lower closure member being welded along its periphery to said metal strip, the combination of said lower closure member and said metal strip constituting a liquid-tight closure for said columnar member; and said arm is secured to said metal strip. 14. In an offshore structure of the type described, thecombination of:

an elongated hollow columnar member which is to be installed offshore in an upright position; an upper closure member removably secured to one end of said columnar member in liquid-tight fashion; a lower closure member disposed within said columnar member in a location remote from said one end,

said lower closure member bridging the interior of said columnar member and being welded along its periphery to said columnar member to provide a liquid-tight closure; and pulling means connected to said closure members and extending longitudinally through the interior of said columnar member. 15. In an elongated structure adapted to be installed upright at an offshore site, the combination of:

an elongated hollow columnar member fabricated of metal;

a first closure member secured in liquid-tight fashion to the end of said columnar member which is the upper end when the columnar member is installed; and

a second closure member disposed within said columnar member in a location remote from said upper end,

said second closure member comprising a metal member which is generally convex with respect to said upper end of said columnar member and includes a relatively thin outer peripheral portion welded in liquid-tight fashion to a surrounding part of said columnar member,

said second closure member completely bridging the interior of said columnar member to close the same in liquid-tight location and being completely removable from said columnar member by rupturing the weld between said peripheral outer portion and said surrounding part.

16. A structure according to claim 15, wherein:

said metal member of said second closure member is at least generally frusto-conical,

said second closure member further comprising a fiat metal plate welded to and closing the smaller end of said frusto-conical member.

17. A structure according to claim 15, wherein said metal member of said second closure member is dished and extends completely across the interior of said columnar member.

18. The method for installing an elongated hollow columnar member at an offshore site, comprising:

equipping the columnar member with upper and lower closure members to render the same buoyant;

floating the columnar member, so equipped, at the installation site;

upendingand submerging the columnar member, lower closure member down, with the columnar member then occupying at least approximately the desired position for installation;

removing the upper closure member; and

pulling upwardly on the lower closure member to first tear the same away from the columnar member and then withdraw the same upwardly therethrough.

.19. The method defined in claim 18, wherein:

said step of equipping the columnar member with a lower closure member is accomplished by welding a peripheral metal part of the lower closure member to a surrounding part of the columnar member, and

said step of pulling upwardly on the lower closure member is carried out to rupture the weld between the peripheral metal part and the surrounding part of the columnar member progressively along the extent of the weld as the pulling step proceeds.

References Cited UNITED STATES PATENTS 2,913,880 11/1959 Rechtin et al. 6146.5 3,036,438 5/1962 Sims 61-465 X 3,533,241 10/1970 Bowerman et a1 6l46.5

JACOB SHAPIRO, Primary Examiner US. Cl. X.R. 61--69, 82