US 3805534 A
A platform anchor and conductor silo for a slide resistant offshore platform comprises at least two large diameter concrete cylinders joined end to end, said concrete being longitudinally prestressed by a plurality of high strength steel tendons, said platform anchor having removable end closures. The anchor is towed to location and upended with the large diameter end down. When the internal pressure is increased to an amount equal to the external hydrostatic pressure at the lower end, the lower end closure is removed. The anchor is then positioned over the desired location, the internal pressure is reduced, causing an increase in effective weight forcing the anchor into the ocean floor. When the upper closure is submerged and the internal pressure further reduced below hydrostatic, the resulting pressure imbalance provides additional force to drive the anchor into the ocean floor to a substantial depth.
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Description (OCR text may contain errors)
United States. Patent [191 Brasted SLIDE RESISTANT PLATFORM ANCHOR CONDUCTOR SILO  U.S. Cl 6l/46.5, 6l/53.5, 61/82, 1 14/206 R  Int. Cl... E02b 17/00, E02d 23/02, B63b 21/26 [58,] Field of Search 61/46, 46.5, 52, 50, 53.5, 61/53.56, 53.68, 53.64; 52/725, 229, 423; 1 14/206 R  References Cited v I UNlTED STATES PATENTS 3,643,447 2/1972 Pogonowski.... 61/46 3,496,900 2/1970 Mott et a1 114/206 R 3,620,026 11/1971 Mallard 6l/53.5 3,512,811 5/1970 Bardgette et a1. 61/46 1,069,328 8/1913 Griffin 52/423 844,294 2/1907 Winslow 61/53 3,624,702 1l/1971 Meheen 61/46 3,613,381 10/1971 Cox.... 61/46.5 3,483,708 12/1969 Marshall 6l/53.5 3,380,256 4/1968 Rebikoff 6l/46.5 3,138,932 6/1964 Kofahl et a1. 6l/46.5 1,706,002 3/1929 Sipe 61/53.62
OTHER PUBLICATIONS Caisson Piers Designed For Offshore Oil Drill Plat- 5] Apr. 23, 1974 Concrete Pile Supports A reprint of an article from Offshore Magazine Furnished By Raymond Inc. 140
Cedar St., N.Y., NY. 10006 Primary Examiner-W.C. Reynolds Assistant Examiner-Alexander Grosz [5 7] ABSTRACT A platform anchor and conductor silo for a slide resistant offshore platform comprises at least two large diameter concrete cylinders joined end to end, said concrete being longitudinally. prestressed by a plurality of high strength steel tendons, said platform anchor having removable end closures. The anchor is towed to location and upended with the large diameter end down. When the internal pressure is increased to an amount equal to the external hydrostatic pressure at the lower end, the lower end closure is removed. The anchor is then positioned overthe desired location, the internal pressure is reduced, causing an increase in effective weight forcing the anchor into the ocean floor. When the upper closure is submerged and the internal pressure further reduced below hydrostatic, the resulting pressure imbalance provides additional force to drive the anchor into the'ocean floor to a substantial depth.
6 Claims, 9 Drawing Figures pmmmmza 1914 3805584 sum 1 or 2 FIG] BACKGROUND OF THE INVENTION The invention relates to a method and apparatus for anchoring an offshore drilling or production platform in deep water where soil slides frequently occur making producing operations extremely hazardous due to the shearing action of the soil acting on the platform support legs and well conductors.
DESCRIPTION OF PRIOR ART The oil industry in its efforts to locate and produce hydrocarbons at offshore locations, particularly inthe Gulf of Mexico, is continually confronted with conducting drilling operations in greater water depths as well as in areas where underwater mud slides occur causing extensive damage to offshore structures anchored in the floor of the body of water.
The prior art suggests several methods of sinking caissons in the floor of the ocean. US. Pat. jNo. 2,475,888 illustrates a pier capable of being embedded in the ocean floor by means of rotation, the pier pulling itself down into the ocean floor through the use of enormous screw threads disposed on the outer diameter of the pier. A'method such' as this would be unwieldy and unworkable at water depths of 300 feet. Another method used in installing a caisson in the ocean floor is illustrated by US. Pat. No. 3,380,256 which shows a large diameter caisson used in the ocean floor having minimal penetration into the ocean floor and extending above the ocean floor a substantial distance, i.e., at least the height of a drilling rig.
It will be noted that both of these patents illustrate caissons that either extend to the surface of the body of water or at least extend a substantial distance above the ocean floor.
It-will be further noted that US. Pat. No. 3,380,256 also discloses sinking of the caisson by having the upper end closed and partially evacuating the inside volume of the caisson thus utilizing the hydrostatic head of the water to aid both in sinking the caisson and thereafter to aid in driving it into the ocean floor a minimal depth whereupon drilling operations are carried out on the ocean floor within the caisson.
SUMMARY-OF THE INVENTION without having to assemble component parts after reaching the offshore location.
DESCRIPTION OF THE DRAWING The nature of the present invention will more fully be appreciated with reference to the drawing in which:
FIG. 1 illustrates the slide resistant anchor and conductors as used with an offshore platform;
FIG. 2 shows an overall view of the combined platform anchor and conductor silo; FIG. 3 is a crosssectional view taken along line 33 of FIG. 2;
2 FIG. 4 is a cross-sectional view taken along line 4-4 of FIG. 2; and
FIGS. 5-9 illustrate a preferred method of installing the anchor and conductor silo of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, and more particularly to FIGS. 24, one embodiment of the combination anchor and conductor silo 10 is shown. The anchor 10 is fabricated in individual cylindrical sections of approximately 10 to 30 feet in length and then aligned end to end for assembly. The small diameter or upper end section 2 is provided with a plurality of conduits 41 about its circumference through which high strength steel tendons 3 are installed and suitably anchored at one end (FIG. 4) to a connector 1 for changing diameters. A tensioning force is then applied to the opposite end to maintain the tendons 3 and the members 2 at the desired levels of stress before securing the tendons 3 to the uppermost concrete member 2. The larger diameter or lower cylindrical members 5 are similarly provided with a plurality of conduits 42 through which high strength steel tendons 6 are installed and tensioned as described for members 2. The diameters of the members 2 and 5 are large compared to the wall thickness of the cylindrical concrete members 2 and 5. For example, the internal diameter of cylindrical member 2 may be from 19 to 20 feet while having a wall thickness of approximately 2 to 3 feet and the cylindrical member 5 may have an internal diameter of 30 to 40 feet and a wall thickness of roughly 3 to 5 feet.
Prior to being launched the removable end caps 7 and 8 are installed closing the ends of anchor 10 enabling it to float in water 15 as shown in FIG. 5. Removable straps 9 are installed so that buoyant floats 11 may be suitably attached to provide stability to the anchor 10 while it is towed to location by a conventional means 12. The floats 11 and straps 9 are removed from the anchor 10 upon arrival at location.
The anchor 10 is then upended using the derrick barge 12 (FIG. 6) such that the large diameter end of anchor 10 is nearest the ocean floor20. The cable means 16 connected to end closure 8 and barge 12 is used to suspend the anchor 10 relative to the floor 20 as shown in FIG. 6. A pump means 13 on barge 12 is in fluid communication with the inner bore of the anchor 10 through a flexible hose means 14 and is used to pressurize the inside of the anchor 10, thereby forcing removable end closure 7 from the lower end (FIG. 7). After removal of the end closure 7 the anchor 10 is positioned over the final location and lowered into contact with floor 20. The hose means 14 is then utilized to bleed pressure from the inside of the anchor 10 to increase its effective weight aiding the anchor 10 in penetrating the floor 20. Further reduction of pressure inside the anchor 10 below the hydrostatic head above the anchor 10 provides a penetrating force which in combination with the dead weight of the anchor 10 drives the anchor 10 to the desired depth D (FIG. 9), which may be on the order of 200 to 350 feet below the sea floor 20. Penetration of the anchor 10 into the sea floor 20 is preferably to a depth corresponding to approximately the full length of the anchor 10. The total length of the anchor 10 can be readily adjusted as con- 4 ditions of soil instability dictate. Preferably the method taught by the present invention is used in water depths equal to or exceeding 250 feet. Penetrating to a depth D as shown in FIG. 9 greatly improves the ability of an offshore platform (FIG. 1) to withstand the forces developed in areas where soil slides are a potential hazard. In areas such as this, soil instability may be prevalent to depths of 1'00 feet or more below sea floor 20 making it essential that any anchoring means extend to a substantially greater depth.
After the anchor 10 has been installed to the desired depth D, the end closure 8 is removed through the use of cable 16 and barge 12. A protective steel cap sleeve 21 is then installed at the upper end of anchor 10 using cable 16 and barge 12. The protective steel sleeve 21 is subsequently grouted to the anchor 10. Additional anchors 10, if required, are installed using the previously described procedure prior to installing a platform structure 30, such as that shown in FIG. 1. Such a platform structure 30 is well known in the art and has a plurality of legs 25 extending substantially vertically upwardly from the ocean floor 20 with accompanying structure. Suitably attached to the protective sleeve 21 are horizontal support members 26 that extend outwardly and are connected to legs 25 and other cross members 27. After the platform assembly 30 has been completely installed and connected to the anchors 10, the inside of anchors 10 then act as conduits for subterranean wells drilled into the ocean floor 20. Thus, the anchor 10 also serves to protect the conductor pipes 31 from being subjected to shear forces incurred during soil slides.
in one embodiment of the invention the anchor 10 would have the following nominal properties:
Outside Diameter Inside Diameter Length Upper End 25 feet 19.5 feet 105 feet Lower End 35 feet 27.0 feet 270 feet An anchor 10 having these dimensions would have a total weight of approximately 10,035 tons and a total displacement of 10,082 tons. By evacuating the inside of the anchor a force of 5,700 tons due to its own weight plus the hydrostatic head acting upon the cross sectional area of the anchor 10 provides the driving force required to penetrate the sea floor 20 to a substantial depth, D.
If more force is required to drive the anchor into the sea floor than the combined anchor weight and hydrostatic head, additional temporary length of anchor may be attached to the primary anchor. These temporary lengths may extend to the surface of the water or to a substantial distance above it. The temporary anchor extension could be bridged-off at the bottom and filled with a weighting material such as drilling mud. Depending on the volume of the extension and the specific gravity of the weighting material, the driving force can be controlled and varied over a wide range.
In a second embodiment of the present invention, steel cylinders are substituted for the concrete members 3 and 5. An anchor using steel cylinders would be of a constant internal diameter and would have a wall thickness of approximately 2-5 inches. The same method would be utilized to install an anchor made from steel cylinders as is used for an anchor made from concrete members.
While only one embodiment of the invention has been illustated it is not limited to only one but is intended to disclose both a method and apparatus for anchoring an offshore platform in areas where soil instability is prevalent.
I claim as my invention:
1. A method of installing an anchor having removable end closures in the floor of a body of water to anchor an offshore platform, said method comprising the steps of:
upending said anchor such that said anchor is in a substantially vertical position;
removing the end closure adjacent the floor of said body of water; without removing the other end closure;
positioning'said anchor so that the bottom of said anchor is in contact with the floor of said body of water;
evacuating the interior portion of said anchor creating a pressure differential between said interior portion and the exterior of said anchor; forcing the anchor into the floor to a depth approximately equal to the full length of the anchor;
removing the top end closure from said anchor; and installing protective sleeve means at the upper end of said anchor.
2. The method of claim 1 wherein the end closure adjacent the floor of said body of water is removed by suitably attaching said platform to said anchors.
6. The method of claim 5 wherein said anchors act also as conductor silos for wells drilled into the floor of said body of water.