US3479828A

US3479828A - Platform leg

Info

Publication number: US3479828A
Application number: US714657A
Authority: US
Inventors: Rafael Fernandez Luque
Original assignee: Shell Oil Co
Current assignee: Shell USA Inc
Priority date: 1967-04-28
Filing date: 1968-03-20
Publication date: 1969-11-25
Anticipated expiration: 1986-11-25
Also published as: GB1129723A; NL6805940A

Description

Nam-25,, 1.969 R; F. LUC QUE 3,479,828

PLATFORM LEG Filed Marh 20, 1968 2 Sheets-Sheet l INVENTORZ RA FAEL, FERN'ANDEZ LUQUE HIS ATTORNEY Nov. 25, 1969 R. F. LUQUE 3,479,828

PLATFORM LEG Filed March 20, 1968 2 Sheets-Sheet 2 RAFAEL FERN'ANDEZ LUQUE HIS ATTORNEY United States Patent 3,479,828 PLATFORM lLEG Rafael Fernandez Luque, Rijswijk, Netherlands, assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed Mar. 20, 1968, Ser. No. 714,657 Claims priority, application Great Britain, Apr. 28, 1967, 19,618/67 Int. Cl. E02b 17/02; B63!) 35 44 US. Cl. 6146.5 8 Claims ABSTRACT OF THE DISCLOSURE A leg for supporting a structure, such as an offshore drilling platform, on a marine bottom, said leg including an upstanding framework terminating at its lower end in a foot member, said foot member being of a design that diminishes the turbulence resulting from water current flow" in the vicinity of the leg wherethe framework is connected to the foot member thereby reducing scour of the marine bottom.

The present invention relates to a leg suitable for supporting a structure on a marine bottom. More particularly, the invention comprises a leg for supporting a structure, such as an offshore drilling platform, on a sea bed, said leg including an upstanding framework terminating at its lowermost end in a foot member which diminishes water current flow turbulence in the vicinity of the leg where the framework is connected to the foot member thereby reducing scour of the marine bottom.

In an attempt to locate new oil fields, an increasing amount of well drilling has been conducted at offshore locations, such for example, as off the coasts of California, Louisiana and Texas, and, more recently, in the North Sea. Since fixed drilling platforms are very expensive and cannot be readily recovered after drilling operations have been completed, many drilling contractors and oil well companies are employing self-contained mobile platforms for drilling and other operations, such as well completion and workover operations, whenever possible. One of the more popular mobile units is the jack-up type in which a floating hull or platform serving as a work deck is floated into position over the desired well site on the ocean floor or sea bed. The platform is then lifted above the water into drilling position by actuating a suitable mechanism to force columns or legs associated with the platform downwardly to the sea bottom. To prevent the legs from sinking too deep into the marine bottom, a foot normally is provided at the lower end thereof to distribute the load carried by each leg over an area which is sufiiciently large to hamper the penetration of the foot into the sea bed. Water currents passing along such foot, however, are apt to scour the marine bottom in the neighborhood of the foot and form an erosion pit into which the foot will sink. Unequal sinking of the legs supporting the platform will thus result in a tilt of this structure which is very undesirable.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a leg for a marine structure, said leg including a foot which is of such design that scour of the marine bottom in the neighborhood of the foot is minimized.

This object has been attained in the present invention by providing in an offshore platform a support leg including an upstanding framework and a foot member attached to the end of the framework, the foot member including a footplate and a downward extension. The footplate has an upper surface of a configuration that will reduce turbulence of water flowing thereover, thereby preventing or substantially reducing scour of the marine bottom in the neighborhood of the foot. The downward extension, which is generally in the form of a solid body member, extends into the marine bottom and prevents lateral displacement of the leg. Other objects, purposes, and characteristic features of the present invention will be obvious from the accompanying drawings and from the following description of the invention.

DESCRIPTION OF THE DRAWING In describing the invention in detail, reference will be made to the accompanying drawings in which like reference characters designate corresponding parts throughout several views, and in which:

FIGURE 1 shows a side view of a marine structure carried by legs according to the invention.

FIGURE 2 shows on a scale larger than in FIGURE 1 a side view of the foot of one of the legs of the marine structure according to FIGURE 1.

FIGURE 3 shows (on a smaller scale) a section taken in the direction of arrows 33 in FIGURE 2 of a quadrangularly shaped footplate.

FIGURES 4 and 5 are plan, cross-sectional views of alternative shapes of footplates.

FIGURE 6 is a bottom view in reduced scale of the footplate according to FIGURE 2.

FIGURES 7, 8 and 8A are bottom views illustrating alternative forms of downward extensions.

FIGURES 9 and 10 are diagrammatic, cross-sectional views illustrating alternative ways to attach the foot to the remainder of the leg.

The marine structure shown in FIGURE 1 comprises a platform including a work deck 1, carried by four open framework legs 2 of which only two have been shown. The legs 2 are connected to the deck 1 by mechanisms 3 of any known type which are suitable to vary the relative position between the legs 2 and the deck 1, in order to lift the deck 1 and to support this deck in the position as shown above the water 4, or to lower the deck 1 to a floating position. Since the mechanisms 3 are known per se in various forms, they are not described here in detail. Such mechanisms may be winches, hoists, air jacks, etc.

Each leg 2, includes at its lower end, a foot 5 comprising a footplate 6 and a downward extension 7, integrally attached to the footplate. Each foot preferably is made at least partially of reinforced concrete although other materials may be used. The downward extension 7, generally in the form of a solid body member, is suitable to penetrate into the marine bottom 8 to such an extent that the lower surface of the footplate 6 is flush with or partly submerged in the marine bottom or sea bed 8.

As may be seen with reference to FIGURE 2, the upper surface 9 of the footplate 6 is of a shape so that the height of the footplate is greater in its center than at its periphery. In the embodiment as shown, this surface 9 is conically shaped. The upper surface, however, may be curved, as for example in the embodiment shown in FIGURE 9 which will be described in greater detail below.

The footplate 6 includes a lateral wall 11 having a flat bottom surface. The lateral wall 11 of the footplate 6 has, as shown in the drawing, an upper periphery which preferably coincides with the periphery of the upper surface 9 of the footplate 6. The height h of this lateral wall is as small as possible. The height it will vary according to the type of materials employed in the construction of the foot and those materials which are more liable to damage by impact, such as reinforced concrete as applied in the construction as shown in FIGURE 2, will require a thicker lateral wall 11. The height h of the lateral wall 11 may then be between and ,6 of the greatest dimension of the footplate 6.

Connected to the upper surface of the footplate 6 is a framework 10 of the leg including columns 12, which are interconnected by a bracing 13 of which only a horizontal component is shown. The columns 12 are connected to the reinforcements (not shown) of the concrete footplate 6 by any suitable expedient. The particular shape of the footplate 6 forms a con struction which is extremely suitable for transferring the load supported by the leg to the marine bottom. Moreover, this construction will create a minimum resistance to water currents passing therealong, which results in only small turbulence, if any, in-the flow of water around the foot. Consequently, the scour of the marine bottom by the water current, as well as the formation of low pressure zones which would give rise to the formation of quicksand by flow of pore space fluid out of the marine bottom to the low pressure zone thereabove is prevented. Thus, the marine bottom remains stable and competent to support the foot resting thereon.

Lateral displacement of the legs 2 is prevented by the downward extension 7 of the foot. The extension 7 forms an integral unit with the footplate 6. In the embodiment shown in FIGURE 2, the downward extension is, as is the footplate 6, preferably formed of reinforced concrete. Downward extension 7 comprises four tapered wings or ribs 12 which are arranged in the form of a cross as can be seen most clearly in FIGURE 6, which is a bottom view of the foot according to FIGURE 2.

The shape of the footplate when viewed in the direction of section 33 of FIGURE 2 is substantially quadrangular as can be seen from FIGURES 3 and 6, respectively. However, the application of the invention is not restricted to footplates of this shape and, for example, alternative shapes are circular footplate 6' and triangular footplate 6" as shown in FIGURE 4 and FIGURE 5, respectively. It is, however, necessary that the height of the center of the upper surface of the footplate be sufficiently low so as to prevent or minimize the formation of turbulence in the flow of water passing thereover. It has been found that this height at its maximum should be one tenth of the greatest dimension of the footplate.

Further, the invention is not restricted to the particular design of the downward extension as shown in FIGURES 2 and 6. If desired, the corners between the wings 12 of the extension as shown in FIGURE 6 may be filled up as indicated in FIGURE 7 which is a view taken in the same direction as FIGURE 6. Thus, a pyramid-shaped downward extension is formed of cement in a shape which is for the greater part thereof bounded by triangular planes or surfaces 13, which planes are interconnected by rectangular planes or surfaces 14 and a small quadrangular plane 15 formed by or at the ends of the wings or ribs.

In another embodiment the downward extension may consist of three wings or ribs 16 as shown in FIGURE 8 which is a view of the bottom of the footplate 6 taken in the same direction as FIGURE 6. In this embodiment, wings or ribs 16 are arranged in the form of a three pointed star. If desired, the extension may also be formed in the shape of a three-sided pyramid (as indicated in FIGURE 8 by the reference numeral 17).

The width of the wings of the downward extensions employed in the various illustrated embodiments is constant over the height thereof. However, the lateral planes of the wings 12 need not be parallel, but may converge in a vertical direction and/or in a horizontal direction as desired. The wings need not extend in a straight line but may be slightly curved as indicated with respect to the embodiment illustrated in FIGURE 9. As is clearly shown in the drawing, however, all of the alternative forms of disclosed downward extensions have a plurality of surfaces progressing from the periphery of the footplate toward a central position spaced from the bottom surface thereof.

FIGURES 9 and 10 illustrate not only alternative configurations that can be assumed by the foot, but also alternative means that may be employed to attach a foot to the remainder of the leg. In FIGURE 9, the upstanding framework of leg 20 is constructed of tubular steel and comprises columns 21 which are interconnected by bracings, 22, 22', 22". The footplate 23 is of reinforced concrete and cast around the lower parts of the columns 21 and the bracing 22" thus forming an integral unit with the framework. The downward extension comprising curved wings or ribs 24 is also made of reinforced concrete and forms an integral unit with the footplate 23.

The construction as shown in FIGURE 10 is entirely of metal. The columns 26 are interconnected by bracing 27 and welded to the footplate 28 at their lower ends. A plate 29 is welded to the interior surface of the cone formed by the footplate 28 and the plates 30 of the downward extension 31 are welded to the plate 29.

For maximum effectiveness of the operation of the footplate in reducing fluid flow turbulence, it has been found that the upstanding framework associated therewith should be open over its side surfaces for more than 50 percent over the part thereof adjoining the foot and extending over a height which is at least equal to the greatest lateral dimension of the framework. In FIGURE 3, for example, this latter dimension is indicated by D. The height over which the leg is open for at least 50% is indicated in FIGURE 1 by H, which height is equal to or greater than D. Furthermore, the distance between a column of the framework and the periphery or outer rim of the footplate must not be too small, since otherwise the slight turbulence which may be created in the water current on passing along a. column will scour the sea bottom. Preferably this distance is at least one fourth A) of the greatest dimension of the cross-section of the framework, this cross-section being taken just above the footplate. Thus, the distance A between the outside of the column 12 of the leg 2 (FIGURE 3) and the outer rim of the footplate 6 is at least one fourth the dimension D of the leg 2.

While this invention has been described with particular reference to preferred embodiments thereof, it should be understood that the particular forms disclosed have been selected to facilitate explanation of the invention rather than to limit the number of forms which it may assume. For example, the invention may be used not only with drilling platforms but with a wide variety of other marine structures such as offshore radar stations, lighthouses, etc. In addition, the legs according to the invention may be used with drilling platforms of other than the jack-up type, such as submersible units. Further, it should be understood that various modifications, alterations, and adaptations may be applied to the specific form described to meet the requirements of practice without in any manner departing from the spirit of the invention or the scope of the subjoined claims.

I claim as my invention:

1. An offshore drilling platform adapted to be positioned on the ocean floor, said platform comprising:

work deck means;

a plurality of legs depending downwardly from said deck means to the ocean floor, said legs supporting said deck means at a predetermined distance above the surface of the ocean, and each leg comprising:

framework means attached to the work deck means and including upstanding columns interconnected by bracing means;

a foot fixedly attached to the lower end of said framework means, said foot comprising:

a footplate including a substantially flat bottom surface and an upper surface which varies in height from a maximum at the center of the footplate to a lesser height at its periphery,

whereby fluid fiow turbulence will be reduced as water passes thereover, said maximum height of the center of said footplate being no greater than one tenth of the greatest dimension of the footplate; and

an extension projecting downwardly from said footplate and attached thereto, said extension being in the form of a solid body member having a plurality of surfaces progressing from the periphery of said footplate toward a central position spaced from said bottom surface whereby said extension will assist in the penetration of the marine bottom and resist lateral displacement of the leg after such penetration has been made.

2. In an offshore platform adapted to be positioned on a marine bottom, leg means comprising:

upstanding framework means;

a footplate including a substantially fiat bottom surface and an upper surface which varies in height from a maximum at the center of the footplate to a lesser height at its periphery, whereby fluid flow turbulence will be reduced as water passes thereover, said maximum height of the center of said footplate being no greater than one tenth of the greatest dimension of the footplate; and

3. Leg means according to claim 2 wherein said footplate further includes a lateral wall terminating at its lower end in a flat bottom surface, the lateral wall having a height varying in proportion to the impact strength of periphery of said lateral wall corresponds to the periphery of said upper surface of the footplate, the height of said wall being at a maximum one one-hundredth of the greatest dimension of the footplate.

7. Leg means according to claim 2 wherein the framework means includes upstanding column means affixed to said footplate, the smallest distance between said column means and the periphery of the footplate being at least one fourth A) of the greatest dimension of the framework as applied to that portion of the framework just above the upper surface of the footplate.

8. Leg means according to claim 2 wherein the framework is open over its side surfaces for more than 50 percent over the portion thereof adjoining the foot and extending over a height that is at least equal to the greatest lateral dimension of the framework.

References Cited UNITED STATES PATENTS 6/1952 Halliburton 6146.5 5/1965 Le Tourneau 6l46.5

JACOB SHAPIRO, Primary Examiner U.S. C1. X.R. 6 l5 3; 3773