US3483708A

US3483708A - Method of anchoring marine structures

Info

Publication number: US3483708A
Application number: US694243A
Authority: US
Inventors: Peter W Marshall
Original assignee: Shell Oil Co
Current assignee: Shell USA Inc
Priority date: 1967-12-28
Filing date: 1967-12-28
Publication date: 1969-12-16
Anticipated expiration: 1986-12-16

Description

Dec. 16, 1969 P. w. MARSHALL METHOD OF ANCHORING MARINE STRUCTURES Filed Dec. 28, 1967 FIG. 2

FIG.

FIG. 4

all III lnn nunnu FIG. 3

FIG. 6

INVENTOR P. W. MARSHALL BY. H

HIS AGENT nited States Patent ()fiice 3,483,708 Patented Dec. 16, 1969 ABSTRACT OF THE DISCLOSURE A method of installing a marine structure offshore wherein a marine structure with pile guides carried on the lower end is positioned on the floor of a body of water and a pile is lowered into axial alignment with one of the guides by pulling the lower end of the pile from a point on the guide; after which the pile is driven into the water floor through the guide to thereby anchor the structure.

This invention relates generally to the anchoring of structures of various types to the floor of a body of water and more particularly to the method of anchoring marine structures of the type having a base with a plurality of skirt pile sleeves for receiving concentric stubpiles therethrough.

BACKGROUND OF In drilling and producing ells at offshore locations large marine structures are employed which are positioned on the fioor of a body of water and extend upwardly above the water surface. These structures or socalled platforms generally comprise a lower base element including a plurality of substantially vertical legs which extend above the surface of the water and an operating deck secured to the upper end of the legs above the water surface. If the platform is to be used for the drilling of wells, drilling equipment including a drill rig and auxiliary equipment, as well as living quarters for the crew and storage facilities, are usually mounted on top of the platform. If the platform is to be used for oil and gas production purposes, various manifolding equipment, storage tanks, meters and treating apparatus are generally mounted on the platform deck.

After a marine structure or platform of the abovedescribed type has been built on land, transported to a desired offshore location and positioned on the floor of a body of water, it is the general practice to anchor the structure to the Water floor by driving piles down through the vertical legs of the structure so that they penetrate up to 300 feet or more in the ocean floor. In general, the depth to which these piles are sunk depends on the nature of the soil on the ocean floor. Thus, the harder the formation beneath the structure the shorter the piles that are used. In addition, it is also a practice at times to install stub-piles through suitable sleeves or guide means attached to the lower end of the marine structure to further anchor the structure to the water floor. Stub-piles are piles that do not extend to the surface of the water and usually only extend up to the top of the sleeve or guide attached to the base of the marine structure, thereby providing support for the structure without causing an obstruction to wave action or impairing access to the platform deck.

In relatively shallow water stub-piles may be satisfactorily located in sleeves or guides attached to the lower end of the structure and then driven into the floor of the body of water by employing conventional methods. However, in deep water in the order of hundreds of feet in depth, the problem of aligning the piling in the sleeve or guide becomes increasingly difiicult if not impossible,

THE INVENTION especially at depths exceeding deep sea diver capabilities. Accordingly, there was a need for a method of anchoring marine structures in relatively deep water by remotely locating a stub-pile in a sleeve or guide attached to the lower end of the marine structure which method could be performed entirely from the Water surface without the need of employing deep sea divers and it is to this end that the subject invention is directed.

SUMMARY OF THE INVENTION In view of the above, it is a primary object of the present invention to provide a method of installing a sub-pile to anchor a marine structure which method can be performed entirely from the water surface.

It is another object of the present invention to provide a method of locating and aligning a stub-pile and a guide or sleeve attached to the lower end of the marine structure and outboard thereof by suspending the pile in the Water in the vicinity of the sleeve and moving the pile laterally into axial alignment with the sleeve by pulling the lower end of the pile from a point on the sleeve or guide.

Still another object of the present invention is to provide a method of the subject type in which the lower end of the pile is pulled into registry with the upper end of the sleeve or guide.

A further object of the present invention is to provide a method of locating and aligning a stub-pile and a guide or sleeve attached to the lower end of the marine structure by connecting a drawline to the lower end of the pile and extending the drawline past and in sliding contact with the sleeve or guide and then extending the drawline up to the water surface where the drawline is tensioned to thereby pull the pile into aligned registry with the sleeve or guide.

These and other objects of this invention will become apparent from a reading of the ensuing description of the invention taken in conjunction with the drawings.

In summary, the present invention provides a method of installing a marine structure at an offshore location by means of stub-piles which are lowered in a substantially vertical position in the vicinity of skirt pile sleeves or guides attached to the lower end of the marine structure adjacent the water floor, moving the pile laterally into substantial axial alignment and registry with the sleeve by pulling the lower end of the pile from a point on the sleeve, lowering the pile through the sleeve into engagement with the water floor and finally driving the pile into the water floor through the sleeve.

The pile is preferably anchored to the sleeve after it is driven into the water floor. The pile may be pulled into alignment with the sleeve by connecting a drawline to the lower end of the pile, extending it past and in sliding contact with the sleeve and continuing to extend the drawline up to the water surface where it may there be tensioned to pull the pile into axial alignment with the sleeve and preferably into registry with the upper end thereof.

DESCRIPTION OF THE DRAWINGS FIGURES 1 through 4 are elevational views of a typical marine structure to which the subject invention may be adapted and sequentially illustrate the subject method;

More specifically, FIGURE 1 is an elevational view showing a stubpile being lowered into the water in the vicinity of a skirt pile sleeve attached to the marine structure;

FIGURE 2 is the same elevational view showing the pile pulled into alignment with the skirt pile sleeve;

FIGURE 3 is the same elevational view showing the pile being driven into the water floor;

FIGURE 4 is the same elevational view showing the completed installation with the pile driven into the water floor to thereby anchor the marine structure;

FIGURE 5 is a plain view of the upper end of one of the skirt pile sleeves; and

FIGURE 6 is a cross-sectional view of a skirt pile sleeve taken along the lines 6-6 to FIGURE 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGURE 1 of the drawing, a marine structure is shown as comprising a plurality of support members or legs 10 interconnected by suitable bracing 11 and positioned on the floor 12 of the body of Water 13. It will be understood for the purposes of this invention that the structure could employ any number of legs which are usually arranged in a configuration having a closed perimeter or it could even be a single leg structure. The legs 10 are generally tubular and constructed of large-diameter pipe but may also be solid members such as I-beams. A plurality of skirt pile sleeves 14 are attached to the lower ends of the legs 10 and carried outboard thereof by means of suitable bracing 15. Each skirt pile sleeve 14 is a tubular structure open at each end with the upper end having a flared mouth portion 16 for receiving a pile. The lower end of the sleeve 14 is adapted to rest against the floor 12 of the body of water 13. Mounted on top of the legs 10 at a suitable distance from the surface 17 of the body of water 13 is an operating platform or deck 18 which carries suitable well drilling or servicing equipment, or alternately, platform 18 may be a temporary platform carrying construction equipment. Although the skirt pile sleeves 14 are shown as continuous tubular members it will be understood for the purposes of this invention that any form of guide means for guiding the pile may be employed, for example, vertically spaced rings attached to the marine structure of a sufiicient size to allow a pile to pass therethrough.

A barge 20 positioned on the surface 17 of the body of water 13 adjacent the marine structure includes a deckmounted derrick 21 which serves to lower a pile 22 in a substantially vertical position into the water 13 near one of the skirt pile sleeves 14. A drawline 23 extending from a winch 24 located on the operating deck 18 passes over suitable pulleys 25 carried on the deck on leg 10, past pulley 26 carried by the skirt pile sleeve 14 and then to the pile 22 where it is connected in a manner to be hereinafter described.

It is preferable for the purpose of this invention that the drawline 23 be positioned over the

pulleys

25 and 26 prior to locating the offshore structure on the floor 12 of the body of water 13. Accordingly, when the marine structure is sunk and positioned in its desired location the drawline 23 would not be connected to the pile 26 as shown but rather would extend back up to the water surface where it would be suitably tied off on the operating deck 18. Thus, when the structure has been positioned on the water floor and it is desired to drive a pile through the skirt pile sleeve 14 the free end of the drawline 23 would be transferred to the barge 20 where it would be attached to the lower end of the pile 22 at the water surface. The pile 22 may be fabricated by adding lengths to the upper end of the pile and successively lowering it to the position shown in FIGURE 1. The drawline 23 is preferably connected to the lower end of the pile 22 by means of a pulling ring 27 attached to the pile 22 in a manner that the pulling ring will release from the pile under impact and yet is capable of withstanding the pulling force exerted by drawline 23. For example, the pulling ring may surround the pile 22 and be attached thereto by any suitable tangible means such as shear pins.

I Referring now to FIGURES 5 and 6 one suitable arrangement for mounting the pulleys 26 employed at the skirt pile sleeve 14 is shown in great r det il- Th pul y .4 26 is carried on a shaft 28 which is retained in the interior wall of the skirt pile sleeve 14 by means of inverted U-shaped brackets 29 which are rigidly attached to the inner wall of the sleeve 14. The brackets 29 prevent the shaft 28 from moving upwardly when the drawline 23 is tensioned. After the shaft 28 is positioned in the brackets 29 shear pins 30 are inserted through suitable holes in the sleeve 14 to thereby prevent downward movement of the shaft 28. The upper surface 31 of the brackets 29 is tapered so that when the pile 22 is lowered into the sleeve 14 the pile will be guided and centered between the brackets 29. It will be apparent that the diameter of the pile 22, the interior diameter of the skirt pile sleeve and the thickness of the brackets 29 must be selected to provide suflicient clearance to permit the pile 22 to pass between the brackets 29.

As the pile 22 is lowered to the position as shown in FIGURE 1, the slack in the drawline 23 is taken up by means of the winch 24. Alternatively, the pile may be first lowered into the vicinity of the sleeve 14 and then tension applied to the drawline. Since the drawline 23 acts around pulley 26 on skirt pile sleeve 14 the pulling ring 27 and hence the pile 22 is actually being pulled from a point on the skirt pile sleeve, i.e., from the pulley 26. Therefore, continued tension in the drawline 23 serves to pull the pile 22 into axial alignment with the skirt pile sleeve 14 as shown in FIGURE 2. The exact location of the pulley 26 is not critical; however, it is desirable to have the pulley located within the interior of the skirt pile sleeve as shown in FIGURES 5 and 6 so that tension on the drawline 23 will not only serve to slign the pile 22 with the skirt pile sleeve 14 but will also bring the pulling ring 27 into register with the flared upper end 16 of the skirt pile sleeve 14.

In relatively deep waters the unsupported span of the pile 22 extending between the skirt pile sleeve 14 and the derrick 2 1 will be quite long and its is therefore desirable to support the pile 22 near its upper end to prevent the pile from buckling. This may be accomplised by employing a suitable pile guide 32 attached to a truss member 33 carried by the operating deck 18 as shown in FIGURES 2 and 3. A similar guide 34 carried by the barge 20 may be used when the pile is first lowered into the water as shown in FIGURE 1. The guide 34 preferably incorporates suitable pipe-holding means, for example, slips, so that the pile may be held stationary when the lift cable of the derrick 21 is removed from the upper end of the pile so that successive lengths can be added to fabricate the pile 22. The slips may be gimbal-mounted so that wave induced motion of the pile can be accommodated. Alternately, the barge mounted guide 34 may be used throughout the pile installation procedure, in which case the barges anchor system must provide adequate lateral support for the top of the pile, in addition to holding the barge on location.

After the pulling ring 27 is positioned in the flared upper end 16 of the sleeve 14, the pile may then be lowered into the top of the sleeve 16 by any suitable means, for example, a steam pile driver 34 as shown in FIGURE 3, or by force of the weight and mass of the pile. When the pile is driven pulling ring 27 releases from the pile 22 and the pile is driven against shaft 28 causing shear pins 30 which hold the pulley shaft 28 in place to break thereby permitting the pile to pass through the sleeve 14. The shaft 28 and pulley 26, having served their function are expendable and merely fall to the bottom of the sleeve 14. Although in the arrangement disclosed the pulley 26 is located in the interior of the sleeve 14, it will be appreciated that the pulley 26 could be attached to any portion of the sleeve 14 or flared mouth 16 without departing from the scope of the invention. For example, the pulley 26 could be attached to a bracket afiixed to the upper edge of the flared mouth 16. However, it is preferable to locate the pulley edge of the flared mouth 16. However, it is preferable to locate the pulley interiorly since this location assures better axial alignment between the pile 22 and the sleeve 14.

The piling may then be driven through the sleeve 14 to a desired depth in the floor 12 of the body of water, which may typically be in the order of several hundred feet. After driving the piling is cut off just above the flared mouth 16 by underwater divers or by a pipecutting tool of any suitable type and the piling suitably secured to the sleeve 14 by suitable means well known in the art. Typically, the pile is secured by filling the annulus between the pile 22 and the sleeve 14 with cement.

An alternative arrangement which may be used when driving the pile 22 is to employ a follower pile or section or pile which extends from the top of the pile 22 up to the driving equipment on the surface. The socalled follower is employed until the pile 22 has been driven to the position as shown in FIGURE 4 after which the follower is removed and brought to the surface. This arrangement eliminates the necessity of employing divers to cut off the pile at the top of the sleeve 14. When an underwater pile driving hammer is developed, the follower pile above a stub pile can be eliminated since the hammer would remain on the top of the pile until it has been driven to its final elevation. It will be apparent that in view of the above discussion the particular means employed to drive the pile 22 into the position shown in FIGURE 4 is not essential to nor does it form a part of the present invention. For example, it may even be desirable to eliminate pile driving equipment altogether and drill a hole in the ocean fioor for the pile as shown in the US. patent to Siebenhausen 3,115,755. The guide 32 and the arrangement of the pulleys 25 while preferably employed as shown in the drawings and as described above may, of course, be altered Without departing from the invention. In this regard, it will be noted that the guide 32 might be eliminated provided there is suificient rigidity or buoyancy in the pile 22 to permit it to be accurately driven from the surface. This, of course, will depend on the size of the pile selected and the depth of the water. Similarly, the arrangement of the pulleys 25 may be varied or they may even be eliminated so that the guide line 23 is pulled directly from the winch 24 without any intermediate pulleys.

In the course of planning for the pile installation described above it is necessary to determine that the pile will not buckle under its own weight plus the installation forces imposed upon it. This determination is made by analyzing the pile as a beam-column, which must be capable of supporting the static weight of the pile itself plus that of the pile driver or other equipment mounted on the pile. Additional installation forces, for example, blows from the ram of the steam pile driver or impulses from a resonant pile driver, may be applied which, although they would exceed thev static buckling strength of the pile, do not cause buckling because of their transient nature. These transient forces may be predicted by analyzing the pile as a composite of discrete elastic elements subject to a longitudinal stress wave, and air upper limit on the buckling tendencies of these transient forces, which do not affect the entire pile at once, may be obtained from a beam-column analysis.

Having thus described my invention what I claim is:

1. A method of installing at an offshore location a marine structure having a plurality of annular pile guide means attached to the lower end of said structure and outboard thereof for receiving concentric stub-piles therethrough, said method comprising:

positioning said marine structure on the floor of a body of water;

lowering a pile into said water in a substantially vertical position and in the vicinity of one of said pile guide means;

moving said pile laterally into substantial axial alignment with said guide means by pulling the lower end of said pile under tension from a point on said guide means;

laterally restraining said pile near the upper end thereof and outboard of said marine structure;

lowering said pile through said guide means into contact with the fioor of said body of water;

driving said pile through said guide means into the fioor of said body of water; and

securing said pile to said sleeve.

2. The method as defined in claim 1 including the step of cutting off any portion of said pile extending above said guide means after said pile is driven into said fioo-r.

3. The method as defined in claim 1 wherein the steps of lowering said pile into said water and moving said pile into alignment with said guide means are performed at substantially the same time.

4. The method as defined in claim 1 including the step of pulling said pile into registry with the upper end of saidguide means.

5. The method as defined in claim 4 wherein said pile is pulled from a point located near the upper end of said guide means.

6. The method as defined in claim 5 wherein said point is located interiorly of said guide means.

7. A method of installing at an offshore location a marine structure having a plurality of skirt pile sleeves attached to the lower end of said structure and outboard thereof for receiving concentric stub-piles therethrough, said method comprising:

positioning said marine structure on the floor of a body of water; extending a drawline from the water surface downwardly into contact with the upper end of one of said skirt pile sleeves and back up to the water surface;

connecting one end of said drawline to the lower end of a pile to be driven through one of said skirt pile sleeves;

lowering said pile into said water in a substantially vertical position;

tensioning the other end of said drawline to thereby pull the lower end of said pile into substantial alignment and registry with the upper end of said skirt pile sleeve;

lowering said pile through said sleeve into contact with the floor of said body of water;

driving said pile through said skirt pile sleeve into the floor of said body of water; and,

securing said pile to said sleeve.

3, The method as defined in claim 7 including the step of cutting off any portion of said pile extending above said sleeve after said pile is driven into said floor.

9. The method as defined in claim 7 wherein the steps of lowering said pile into said water and tensioning said drawline are performed at substantially the same time.

10. The method as defined in claim 1 wherein said pile is laterally restrained from the upper end of said marine structure.

11. The method as defined in claim 7 wherein said pile is laterally restrained from the upper end of said marine structure.

References Cited UNITED STATES PATENTS 7/1962 Postlewaite 7 12/1965 Siebenhausen 6l-46.5

JACOB SHAPIRO, Primary Examiner