US3646770A

US3646770A - Method and apparatus for stabilizing an offshore drilling platform structure

Info

Publication number: US3646770A
Application number: US43875A
Authority: US
Inventors: Francois Van Daalen
Original assignee: Shell Oil Co
Current assignee: Shell USA Inc
Priority date: 1970-06-05
Filing date: 1970-06-05
Publication date: 1972-03-07
Anticipated expiration: 1989-03-07
Also published as: CA950217A; NL7107604A; FR2095881A5; GB1351992A; DE2127615A1

Abstract

A method and apparatus for stabilizing an offshore drilling platform structure adapted to rest on the floor of a body of water by perforating at least one of the piles of the structure, thereby establishing fluid communication between the soil surrounding the pile and the interior of the pile. The fluid level in the pile is then maintained at a relatively constant low level.

Description

United States Patent Van Daalen [is] 3,646,770 1 Mar.7,1972

[54] METHOD AND APPARATUS FOR 2,895,301 I 7/1959 7 Casagrande 61 46. 5

STABILIZING AN OFFSHORE DRILLING PLATFORM STRUCTURE Francois Van Daalen, The Hague, Netherlands V e lnventor:

Assignee: Shell Oil Company,Ne yy York, NY,

Filed: June 5, 1970 Appl. No.: 43,875

U.S. Cl ..6l/46.5, 61/ 11 Int. Cl. ..E02d 21/00, E02d 3/10 Field ofSearch ..6l/46.5, 46, 11,13, 63, 53.74;

References Cited UNITED STATES PATENTS 3,550,384 12/1970 Bardgette et al ..6l/46 3,274,782 9/ 1966 Landau 7 2,919,552 1/1960 Hayward ..61/465 Primary Examiner-Jacob Shapiro AttorneyHarold L. Denkler and Theodore E. Bieber [57] ABSTRACT A method and apparatus for stabilizing an offshore drilling platform structure adapted to rest on the floor of a body of water by perforating at least one of the piles of the structure, thereby establishing fluid communication between the soil surrounding the pile and the interior of the pile. The fluid level in the pile is then maintained at a relatively constant low level.

METHOD AND APPARATUS FOR STABILIZING AN OFFSHORE DRILLING PLATFORM STRUCTURE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to offshore drilling platform structures, and, more particularly, to a method and apparatus for stabilizing an offshore platform structure.

2. Description of the Prior Art When drilling offshore in great water depths, piles having a higher load-carrying capacity are required in order to stabilize the offshore platform. However, pile dimensions cannot be increased arbitrarily since, at a certain depth, the penetration resistance of the pile exceeds the driving capacity of the largest pile-driving hammers presently available.

SUMMARY OF THE INVENTION It is an object of this invention to provide a method and apparatus for stabilizing a bottom supported offshore structure.

It is a further object of this invention to provide a method and apparatus for stabilizing an offshore platform structure by increasing the axial load bearing capacity of the friction piles of the offshore drilling platform structure.

It is a still further object of this invention to provide a method and apparatus for increasing the effective stress of the soil surrounding such friction piles.

These and other objects are accomplished by establishing fluid communication between the soil surrounding at least one pile and the interior of the pile below the floor of the body of water. The fluid level in the pile is then maintained at a relatively constant low level.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical sectional view of preferred apparatus for carrying out the technique of my invention;

FIG. 2 is a detailed vertical sectional view of a portion of the apparatus of FIG 1; and

FIGS. 3 and 4 are vertical sectional views, partly schematic, of an example of the technique of my invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The axial capacity of friction piles depends on the adhesion that is developed along the surface of the pile. This adhesion is related to the shear strength of .the soil surrounding the pile. TI-Ie relation between soil/pile adhesion and soil shear strength is complicated and not fully understood. Except for very stiff clays at shallow depths, an increase in shear strength generally corresponds to increased adhesion. Therefore, if the shear strength of the surrounding soil can be increased, pile capacity can be increased without altering the dimensions of the piles.

Referring now to the drawing, FIG. 1 shows an offshore drilling structure having a plurality of legs (only legs 11 and 12 being shown in FIG. I) for supporting a platform 13 above the surface 14 of a body of water, such as ocean 15. Legs 11 and 12 rest on the mudline or floor 16 of the body of water 15, the legs of structure 10 having friction pile means associated therewith extending into the soil of floor 16 for maintaining the legs in position on the floor l6 and thus stabilizing structure 10.

For example, as shown in FIG. 1, pile means 17 and 18, shown in dottedlines, are associated with legs 11 and 12, respectively. Suitable bracing means 19 preferably extend between the legs of structure 10 for bracing structure 10 for bracing structure 10 as is well known in the art.

Referring now to FIG. 2, at least one of the legs of structure 10 includes drainage pile means in accordance with the teachings of my invention. For example, the piling means 18 of leg 12 includes a drainage pile 20 which is driven therethrough into the ocean floor 16. Pile 20 includes at its lower end a penetrating portion 21 for penetrating the ocean floor 16. Pile 20 is further perforated as at

perforations

22 and 23 for reasons to be discussed further hereinbelow. Although only two such perforations are shown, obviously one or more perforations may be formed in pile 20 in accordance with the teachings of my invention, preferably at a significant distance below the floor 16.

The drainage pile 20 exemplifies one means for establishing fluid communication between the interior of a pile and the surrounding soil at a depth below the floor of the body of water. In general, such fluid communication can be established by driving an unperforated pile and then perforating it in situ, by driving a preformed porous concrete pile, by casting such a concrete pile in place, or the like. Such a pile can be substantially any axial load-bearing elongated hollow structure and substantially any means can be used to establish fluid communication between its interior and the surrounding soil.

A tubing string 24 preferably carries at its lower end a pump 25, or similar pumping means, for pumping fluid entering pile 20 through

perforations

22 and 23. A spider 26 or similar device is preferably connected to pump 25 to maintain the pump 25 in place, i.e., in spaced relation from the wall of the pile 20. Suitable holes 27 are formed in pump 25 for permitting the aforementioned fluid from entering pump 25. A conventional sucker rod 28 extends downwardly through leg 12 into fluid communication with pump 25 for removing fluids pumped by pump 25 to a remote location, such as platform 13 where the fluid is discharged onto water surface 14.

The drainage pile 20 of structure 10 is preferably applied thereto during the construction of structure 10. In operation, fluid is withdrawn from the soil surrounding the pile 20 through

perforations

22 and 23 formed therein until the soil is compacted to a shear strength equaling that of soil at a deeper depth.

In this manner, the effective stress of the soil surrounding the pile 20 is increased by lowering the pore water pressure in the surrounding soil. The perforated pile 20 acts as a sand drain since gravity drainage of water occurs from the soil through

perforations

22 and 23. The water being drained into pile 20 is kept at a constant low level by pumping it through pump 25 and sucker rod 28 to he water surface. The pressure of the pore water in the soil decreases as a function of time and place and thus a consolidation process begins. This decrease in pore pressure results in an increased effective stress and thus in an increased shear strength.

The capacity of pump 25 and the number of perforations are preferably so arranged that substantially all of the water present in the soil that drains into pile 20 during a period of at least several weeks is removed. Where the soil surrounding pile 20 is sandy, the water-removing equipment (i.e., pump 25, perforations 22 and23, sucker rod 28, etc.) is preferably arranged for continuous or intermittent operation throughout the period in which a high load-bearing capacity is important.

The techniques of my invention may also be applied to offshore structures that are already in place so as to adapt them to handle loads exceeding those for which they were previously designed. In this application of my invention, the drain hole or holes may be formed in the piles already in place and pumping units similar to unit 25 and sucker rod 28 may be installed therein in the manner disclosed in a copending applic t qntp l lmsontSsr-figufi. 1 fiot l n: 1968- t h application, a drain hole is prepared by sonically driving a hollow, bore-forming tool downwardly into the earths formation, removing the core of earth material out through the center of the bore-forming tool, placing an eduction or jet-pumping unit through the bore of the tool into the bottom of the borehole, and filling the borehole with a granular material such as sand either prior to or simultaneously with withdrawal of the boreforming tool.

EXAMPLE Leg 12 with its associated drainage pile 20 is shown schematically in FIG. 3. Here, WI refers to the water level in pile 20. In this example, a plurality of perforations are formed in pile 20, the total extent of the perforations being indicated as a permeable part L of pile 20. Gravity drainage of water occurs from the soil into the pile 20 through perforated part L'. The water in the pile is kept at a constant low level, i.e., W1 by pumping it to the surface (not shown in FIG. 3). The pressure of the pore water in the soil decreases as a function of time and place, or, in other words, a consolidation process starts. The rate at which a steady state is approached is relatively high in sand. In clay, the coefficient of consolidation is much lower than in sand and, therefore, a much slower approach towards a steady state is expected. In any case, a decrease in pore pressure results in an increase in effective stress and thus is an increased shear strength. In sand, almost no consolidation occurs and, therefore, a beneficial effect is expected only as long as pumping is continued. In clay, however, the increase in shear strength due to consolidation never completely disappears after the pore pressure has come back to its original value. This means that, even if pumping is stopped after a while, a stronger clay is still obtained.

An estimate of the maximum possible improvement may be obtained by considering the vertical stress at the pile wall for steady state conditions. The effective stress in the soil at an arbitrary depth, 2, is:

{hp-u where z is the coordinate in the vertical direction;

p is the total stress;

[I is the effective stress; and

u is the neutral stress. The total normal stress on a horizontal plane at that depth is:

h is the water depth;

y, is the water density; and

'yis the soil density (saturated). At the boundary of pile and soil (R,z) (in the perforated part), the neutral stress equals:

-y,, is the air density. Hence, the effective vertical stress at that place is:

Ay is the density difference between water and air; and

Ay is the density difference between soil and water (submerged density of soil).

This effective stress may then be compared to one at the wall of a nonperforated pile fo which:

So. in the case of a perforated pile, an increase in effective pressure is obtained of:

A} A'Yrr-HU where .1 A 3 is the difference in effective stress between a drainage and nondrainage situation.

where C is cohesion; F is an improvement factor; K is the lateral earth pressure coefficient; and

1b is the angle of shearing resistance.

in equation (9), it is assumed that the lateral earth pressure coefficient, K, on a vertical plane of failure, is unaltered by the consolidation process. Dealing with sand and normally consolidated clay (C=0), the formula reduces to:

Aw. u

'Yu For an average sand, A7,, is approximately equal to A'y so that even if h=0, a percent increase in shear strength may be expected along the perforated part L of the pile. If the water depth, h, is equal to the embedded length, L, the ratio becomes more favorable. For sand, a ratio of at least three is obtained.

To obtain an order of magnitude for normally consolidated clay, an average may be assumed to be:

This leads to a ratio of:

For a water depth Ir 0. this gives a triple increase in shear strength. lfh L, a value of at least five is obtained.

An experiment was carried out as shown in FIG. 4. Here, a pile 29 was formed comprising a 1-inch outer diameter pipe 30 perforated over a 6-inch length and covered with a screen 31. The pile 2 9 was placed in a S-gallon bucket 32 after which the bucket 32 was filled with water and soil, either sand or clayi.e., tests were carried out in both sand and clay. A load was transferred to the pile 29 by a string 33 connected to pipe 30 acting over two

pulleys

34 and 35. A weight 36 was attached to the free end of string 33. The deflection at the pile head was measured by a dial gauge 37 at the pile head. In these tests, the water level in the bucket 32 was at the top of the soil; h=0. The results of the tests are shown in Table I.

In the tests with sand, the pile 29 was pulled out immediately after installation. in the tests with clay, time between installing the pile 29 and pulling it out was about one day.

The ratio [.96 obtained with sand agrees with the theoretically expected value of 2. For clay, a ratio of 4.05 was obtained. Since the submerged density of the clay was 0.286 g./cm. this gives a ratio of l+l/0.286 or 4.5 which is in reasonable agreement with the observed value of 4.05. Form the foregoing, it can be seen that drainage influences the pullout capacity of a model pile in a very favorable way.

I claim as my invention:

1. A method for stabilizing an offshore platform structure adapted to rest on the floor of a body of water and having a plurality of friction piles extending into the floor of said body of water, said method comprising the steps of:

establishing fluid communication between the soil surrounding at least one of the piles and the interior of said pile below the floor and of said body of water;

maintaining the fluid level in said pile at a relatively constant low level by pumping fluid from the interior of the pile to said body of water.

2. The method of claim 1 wherein the step of maintaining the fluid level in said pile at a relatively constant low level includes the step of intermittently pumping fluid from said soil to said body of water.

3. Apparatus for stabilizing an offshore drilling platform structure adapted to rest on the floor of a body of water comprising:

an offshore drilling platform structure having a plurality of legs adapted to rest on the floor of a body of water;

pile means extending into the floor of said body of water associated with each of said legs adapted to maintain said legs in a stable position on said floor of said body of water;

fluid drain means formed in at least one of said pile means below said floor of said body of water communicating with fluid present in the soil forming said floor of said body of water; and

pumping means associated with said latter-mentioned pile and in fluid communication with said fluid drain means for maintaining the fluid level in said latter-mentioned pile means at a relatively constant low level.

4. The apparatus of claim 3 wherein the fluid drain means includes a plurality of perforations extending along said pile means a substantial distance below said floor of said body of water.

5. The apparatus of claim 3 wherein said pumping means includes a pump in fluid communication with said fluid drain means;

a sucker rod in fluid communication with said pump:

a tubing string coupled to said pump; and

all of said pump, said sucker rod and said tubing string extending through both said pile means and its associated leg.

6. The apparatus of claim 5 wherein said sucker rod extends through said tubing string and said tubing string extends through said pile means.

7. The apparatus of claim 5 wherein said sucker rod extends to said water surface 8. The apparatus of claim 5 including spacer means associated with said pump for spacing said pump from the wall of said pile means.

* i i t

Claims

1. A method for stabilizing an offshore platform structure adapted to rest on the floor of a body of water and having a plurality of friction piles extending into the floor of said body of water, said method comprising the steps of: establishing fluid communication between the soil surrounding at least one of the piles and the interior of said pile below the floor and of said body of water; maintaining the fluid level in said pile at a relatively constant low level by pumping fluid from the interior of the pile to said body of water.

3. Apparatus for stabilizing an offshore drilling platform structure adapted to rest on the floor of a body of water comprising: an offshore drilling platform structure having a plurality of legs adapted to rest on the floor of a body of water; pile means extending into the floor of said body of water associated with each of said legs adapted to maintain said legs in a stable position on said floor of said body of water; fluid drain means formed in at least one of said pile means below said floor of said body of water communicating with fluid present in the soil forming said floor of said body of water; and pumping means associated with said latter-mentioned pile and in fluid communication with said fluid drain means for maintaining the fluid level in said latter-mentioned pile means at a relatively constant low level.

5. The apparatus of claim 3 wherein said pumping means includes a pump in fluid communication with said fluid drain means; a sucker rod in fluid communication with said pump: a tubing string coupled to said pump; and all of said pump, said sucker rod and said tubing string extending through both said pile means and its associated leg.

7. The apparatus of claim 5 wherein said sucker rod extends to said water surface

8. The apparatus of claim 5 including spacer means associated with said pump for spacing said pump from the wall of said pile means.