US3485050A - Marine structures - Google Patents

Description

Dec. 23, 1969 w. M MARTINOVICH MARINE STRUCTURES Filed Oct. 12, 1967 FIG.

FIG. 2

FIG. 3

INVENTOR:

WILLIAM M. MARTINOVICH FIG. 4

HIS AGENT US. Cl. 61--46 United States Patent 3,485,050 MARINE STRUCTURES William M. Martinovich, San Francisco, Calif., asslgnor to Shell Oil Company, New York, N.Y., a corporation of Delaware and Earl and Wright, San Francisco, Calif a corporation of California, as tenents in common Filed Oct. 12, 1967, Ser. No. 674,909 Int. Cl. E0211 17/00; E04h 12/10; E04c 3/32 9 Claims ABSTRACT OF THE DISCLOSURE A marine structure or offshore platform having large diameter legs and interconnecting cross-bracing members of smaller diameter in which the cross-bracing members are provided with bifurcated ends forming spread-apart arms that are connected to the edges of the legs. The resultant structure smoothly transfers the loading from the cross-bracing to the legs while minimizing stress concentrations which tend to cause the legs to collapse or radially deform.

Background of the invention This invention relates to marine structures and more particularly to offshore platforms having a working deck arranged above the surface of the water supported by legs resting on or driven parrtially into the floor of the body of water.

Conventional platforms are generally constructed with a plurality of legs that are interconnected below the water surface by cross-bracing members to give the paltform more rigidity. The cross-bracing members usually consist of tubular members arranged either horizontally or diagonally between the legs when viewed in elevation. In addition, the cross-bracing members are generally of a smaller diameter than the legs. Even when single leg structures are employed to support a platform, the leg is generally provided with sloping supports or braces extending between the leg and the floor of the body of water.

Regardless of the type of structure or the number of legs employed, at least one end of the brace must be attached to the platform leg. In the case of a tubular cross-brace being attached to a tubular leg, the conventional practice is to shape the end of the cross-brace to conform to the curved surface of the leg and then attach the brace to the leg by suitable means such as welding. The resultant structure is the same as the geometrical intersection of two cylinders whose axes are in the same plane. In the case of a cross-brace attached to a leg of a right angle, the structure is similar to that of a chair leg and rung.

When the cross-bracing member has a smaller diameter thanthe platform leg, as is typically the case, a problem arises in connecting the members in a manner to Withstand operational loading without failure. Since the crossbrace is smaller, the load carried by it is transmitted to only a small portion of the leg. The natural tendency of the leg, being a tubular member, is to resist the loading imposed by the cross-brace at the edges of the leg rather than directly in the center. Thus, a convevntional cross-brace which intersects a platform leg in the center will tend to collapse the leg or cause large radial deformation of the leg. Merely increasing the size of the leg to withstand the loading is unsatisfactory since the increase adds appreciably to the Weight of the structure which makes it more difficult to handle. Furthermore, the cost of the structure Would be considerably increased.

The platform leg cannot be adequately reinforced or 3,485,050 Patented Dec. 23, 1969 stiffened internally due to space limitations since the legs contain piles, drilling tubes, diver access tubes and the like which consume the majority of the interior of the tubular leg. Gusset plates and the like have been employed on the exterior of the leg so that. the load transfer takes place from the small diameter cross-brace to the gusset plate and then to the large diameter leg, However, this indirect transfer causes stress concentrations which can drastically reduce the fatigue life of the joint and structure.

The aforementioned problems are particularly aggravated in offshore platforms since these structures are always being dynamically loaded by wave, current, ice and wind forces which produce many cycles of loading. Accordingly, there is a need for structural tubular joint for use with an offshore platform which is extremely efficient and relatively free of stress concentrations in order to provide a safe structure.

Summary In view of the shortcomings of the prior art as discussed above, it is a primary object of this invention to provide an improved platform structure capable of withstanding the bending and torsional moments and loading produced in a marine structure under operating conditions.

Another object of this invention is to provide a tubular joint for use in offshore platforms that transfers load in a manner that minimizes stress concentrations.

It is another object of this invention to provide an improved cross-bracing member for use on offshore platforms.

It is still another object of the invention to provide a small diameter tubular cross-bracing member having a bifurcated end which is attached to a large diameter tubular platform leg.

It is a further object of the invention to provide a tubular cross-bracing member for a platform leg in which the bifurcated arms are formed of a unitary piece with the main body of the cross-bracing member.

It is a still further object to provide a cross-bracing member in which the bifurcated arms are formed of separate pieces rigidly attached to the cross-bracing member and the leg.

In accordance with these objects and others which may become apparent on further reading, the subject invention provides an improved platform structure having superior strength over prior art structures and is capable of Withstanding the operating conditions encountered offshore. More specifically, the invention provides an offshore structure in which the cross-bracing members interconmeeting the legs of the structure are formed with special ends which distribute the loading between the leg and the brace over a wider area resulting in a more rigid joint. More specifically, the special ends are in the form of spread-apart arms which engage the platform leg near the edges of the leg so that the transfer of load from the small diameter cross-brace to the large diameter platform leg is a smooth transition, thereby reducing stress concentrations. This results in a joint having va long fatigue life for the cyclic or continuous loading encountered in an offshore environment.

Brief description of the drawings FIGURE 1 is an elevational view of a typical offshore platform incorporating the subject invention;

FIGURE 2 is a cross-sectional view of a leg of the platform taken along the line 2-2 of FIGURE 1.

FIGURE 3 is a cross-sectional view of a leg of the platform taken along the same line but showing an alternative construciton; and,

FIGURE 4 is a cross-sectional view of a cross-brace taken along the line 44 of FIGURE 3.

Description of the preferred embodiments The subject invention may be suitably employed on several different types of offshore installations. One such installation shown in FIGURE 1, is a so-called platform positioned in a body of water and used for drilling oil or gas wells in the water floor 11. The platform includes a plurality of legs 12 (generally four) supported on or partially driven into the floor 11 and adapted to support a working deck 13 from which the drilling operations may be performed. Located on the deck are the normal complements of a drilling operation including crew quarters 14, derrick 15 and the like. The legs 12 are of a tubular construction and are interconnected by tubular bracing members including a horizontal bracing member 16 and diagonal bracing members 17. In the case of a four-legged structure the bracing members and legs form a generally box-like or rectangular structure.

The cross-sectional view of one of the legs 12 shown in FIGURE 2 also shows the underside of the crossbracing member 16 and is directed toward one of the embodiments of the subject invention. As seen in FIG- URE 2, the cross-brace 16 has a diameter smaller than the platform leg 12 and is connected to the leg by means of a pair of spaced arms 18 and 19. Each arm is in the form of a half-round cylinder made from a tubular piece of approximately the same diameter as the cross-brace 16 which has been split longitudinally. The arms converge at the cross-bracing member 16 where they are attached thereto by suitable means, for example, welding, so that the outer surface of the arms is substantially a continuation of the outer surface of the cross-brace 16. The arms are preferably shaped to conform with the outer surface of the leg 12 where they are suitably attached to the leg 12, e.g., by means of welding. The spread between the arms 18 and 19 is preferably designed so that at the widest point of divergence the outer edges of the arms are approximately equal to the diameter of the leg 12. The spread of the arms may, of course, be less but in no event should the spread be greater than the diameter of the leg.

The V-shaped space formed between the arms 18 and 19 is preferably closed by means of a flat plate 20 welded to the upper and lower sides of the arms and also to the leg 12. The plates 20 serve to add additional rigidity to the structure and also prevent sea Water from entering the interior of the cross-brace 16.

Another embodiment of the invention is shown in FIGURE 3. In this arrangement, cross-bracing member 16a corresponds to bracing member 16 of the FIGURE 2 embodiment and includes arms 21 and 22 corresponding to arms 18 and 19. However, the bracing member 16a and arms 21 and 22 are constructed from the same piece rather than being constructed of separate parts as in the embodiment shown in FIGURE 2. The end of bracing member 16a is bifurcated to form arms 21 and 22 by splitting the tubular brace 16a longitudinally and spreading the ends apart. Arms 21 and 22 are cut to conform to the leg 12 in the same manner as discussed above with reference to arms 18 and 19 and are welded to the leg 12. Suitable plates 23 may be Welded to the upper and lower sides of the bracing member 16a and also to the leg 12 to close the opening formed by the two legs 21 and 22. These plates are similar in design and function to the plates 20 disclosed with reference to the embodiment of FIGURE 3.

The composite structure of both the embodiment of FIGURE 2 and the embodiment of FIGURE 3 is the same. In each instance, a bracing member is bifurcated at the point of juncture with the platform leg so that the brace is attached to the platform by means of spaced arms rather than being attached directly to the platform.

The interior of the cross-bracing member in either embodiment may be provided with suitable reinforcing at the point of convergence of the arms. For example, 7

as shown in FIGURES 3 and 4 a generally cylindrical, longitudinally extending flange 24 is positioned concentrically inside the brace 16a by means of an annular fiat web 25 attached to the flange and the brace 16a. Longitudinally extending gusset plates 26 may be suitably welded into place at the top and bottom of the flange 24 as shown in FIGURE 4 and extend between the web 25 and the leg 12 as shown in FIGURE 3. Flange 24 and web 25 serve to reinforce the cross-brace 16a. Gusset plates 26 are attached to the leg 12 to add further reinforcement and also serve as additional support for the plates 23 which close the openings formed by the arms.

Although not shown, the embodiment of FIGURE 2 may employ similar reinforcing means. In addition, the reinforcing means may be omitted or other forms of spirit of the invention.

It will be readily apparent that the construction shown in FIGURE 2 and the construction shown in FIGURE 3 may be readily interchanged with each other. Therefore, either embodiment or a combination of both may be adapted to this cross-bracing used when constructing a platform. In addition, the subject invention is equally suited for diagonal bracing members such as the members 17 shown in FIGURE 1. It will also be readily apparent that although the invention is shown as applied to a platform, it is equally suited for other marine structures where bracing is employed; for example, docks, bridges, dolphins and the like.

In operation, that is, when the platform is positioned in a body of water and subject to the forces of wind, ice, waves, current and the like, the subject invention provides greatly improved cross-brace-to-leg joint which transfers the load from the cross-brace out to the edges of the platform leg while minimizing or eliminating harmful stress concentrations that could produce structural failure. The bifurcated bracing member described also possesses a greater resistance to twisting or torsion in the leg as well as lateral bending of the bracing member due to the spaced arms which distribute the loading over a wider area on the platform leg.

I claim as my invention:

1. In a marine structure of the type having leg means supported by the floor of a body of water, a platform carried by said leg means and structural bracing attached to said leg means, an improved structure comprising:

at least one substantially vertical tubular leg:

an elongated, tubular, structural, bracing member having a smaller diameter than said leg; and,

at least one end of said bracing member terminating in a pair of divergent spaced tubular arms partially surrounding and conforming to said leg and connected directly thereto;

said arms having a spread greater than the diameter of said bracing member and no greater than the diameter of said leg to thereby form a substantially V-shaped opening between said arms when viewed vertically.

2. A structure as defined in claim 1 including plate means welded to said arms and said leg to close said V-shaped opening.

3. A structure as defined in claim 2 wherein said arms and said bracing member comprise a unitary element.

4. A structure as defined in claim 2 wherein said arms comprise separate elements welded to said bracing member.

5. A structure as defined in claim 2 wherein both ends of said bracing member terminate ina pair of spaced arms.

6. A structure as defined in claim 5 wherein said arms comprise half-round cylindrical members having substantially the same diameter as said bracing member and welded to said bracing member so that the outer surface of said arms is a continuation of the outer surface of said bracing member.

7. A structure as defined in claim 2 including reinforcing means carried internally of said arms and said bracing member.

8. A structure as defined in claim 7 wherein said reinforcing means comprise:

concentric, longitudinally extending flange means positioned at the point said arms intersect said bracing member;

annular web means carried between said flange means and said bracing member and secured to said flange means and said bracing member; and,

gusset plate means secured to said flange and Web means and extending to said leg.

9. A structure as defined in claim 2 wherein said leg means comprise four legs and said structural bracing comprises cross-braces inter-connecting said legs, said composite structure having a generally closed box-like configuration.

References Cited UNITED STATES PATEN'L S 692,681 2/ 1902 Leonard 52697 1,822,389 9/ 1931 Blakely. 2,189,201 2/ 1940 Flader. 3,253,417 5/ 1966 Manning 61-465 3,021,159 2/1962 Back 287189.36 X

JACOB SHAPIRO, Primary Examiner US. Cl. X.R.

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