US3855656A

US3855656A - Underwater buoy for a riser pipe

Info

Publication number: US3855656A
Application number: US00346529A
Authority: US
Inventors: K Blenkarn
Original assignee: BP America Production Co
Current assignee: BP America Production Co
Priority date: 1973-03-30
Filing date: 1973-03-30
Publication date: 1974-12-24
Anticipated expiration: 1991-12-24

Abstract

This describes an underwater buoy for a riser pipe which extends from a deep water subsea well to a vessel supported by a body of water. A hollow shell is attached to the riser to give an upper buoyant force. The water depth may be as deep as 10,000 feet and the buoy may be 600 feet or more beneath the surface of the body of water. Non-vertically directed hydrofoils are attached to the outer wall of the shell. The chord of each hydrofoil makes an angle of attack with the vertical such that should the riser pipe break beneath the buoy, the buoy would be propelled or rise at an angle and surface away from the vessel instead of rising vertically and having a collision with the vessel.

Description

[ Dec. 24, 1974 United States Patent [191 Blenkarn UNDERWATER BUOY FOR A RISER PIPE Primary ExaminerTrygve M. BliX [75] Assistant ExaminerGregory W. OConnor Inventor: Kenneth Blenkam Tulsa Okla' Attorney, Agent, or Firm.lohn D. Gassett; Paul F. [73] Assignee: Amoco Production Company, Tulsa,

Hawley Okla.

[57] ABSTRACT This describes an underwater buoy for a riser pipe [22] Filed: Mar. 30, 1973 [21] Appl. No.: 346,529

which extends from a deep water subsea well to a ves- [52] US. 9/8 R, l14/0.5 D, 166/05, sel supported by a body of water. A hollow shell is at- 175/7 tached to the riser to give an upper buoyant force. E2lb 17/00 The water depth may be as deep as 10,000 feet and [51] Int. [58] Field of Search the buoy may be 600 feet or more beneath the surface 175/6, 7; 166/.5 of the body of water. Non-vertically directed hydrofoils are attached, to the outer wall of the shell. The

chord of each hydrofoil makes an angle of attack with References Cited UNITED STATES PATENTS the vertical such that should the riser pipe break beneath the buoy, the buoy would be propelled or rise at an angle and surface away from the vessel instead of rising vertically and having a collision with the vessel. 166/5 12 Claims, 5 Drawing Figures D 5 mm 1.4 1

3,204,708 3,540,398 11/1970 Gimbel.....

3,563,043 2/1971 Nelson............. 3,643 751 2/1972 Crickmer,..............................

IV /V/ 74 IDITITEIIHEUUECZAIIUYII 3.855.656

SIIEEI 2 0F 2 RISER PRESSURE SIGNAL 2 I ACONTROLLER d: I-b

To 50 54 BUOY STINGER DRIVE MOTOR DRIVE TI 32 CONTROL MOTOR 39 39 BLEED VALVE 52 FRM AIR SOURCE FIG. 5

/f RISER INTERIOR PRESSURE SENSOR UNDERWATER BUOY FOR A RISER PIPE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to drilling wells in the bottom of water covered areas from a drilling vessel supported by the body of water and in which a riser pipe extends from the underwater well site to the vessel. It relates especially to a method of using a special buoy for applying an upward force to the riser pipe. Special means are provided so that should the riser break beneath the buoy, the buoy would rise at an angle and surface away from the vessel. Means are provided so that the air inside the buoy can be at ambient water pressure so that relatively thin walls can be used for the buoy.

2. Prior Art One method which has been suggested in the prior art to avert a catastrophe is to provide a plate over an opening in the top of the shell of a buoy and have it attached by explosive bolts. Suitable sensors were suggested to explode the bolts in the event the riser should break. Removal of the plate is supposed to result in fast flooding of the buoy. Ifthe buoy is flooded fast enough, it would lose its buoyancy before it hits the ship. The search for oil and gas has in recent years moved more-and more to water covered areas. At first, these wells at offshore locations were drilled from fixed platforms, which were in reality a deck held 50 feet or more above the surface of the water by a plurality of legs which extended from the deck into the primary body of water. In these operations, drilling was more or less merely an extension of dry land techniques. However, as the search for oil moved more and more into deeper and deeper water, it became impractical and in some cases impossible to use fixed platforms. Thus, it has become more and more desirable to use a floating vessel from which to drill wells in marine locations having deep water. In such operations the floating vessel is usually connected to a submerged wellbore by a long tubular member through which drilling tools, drilling fluid, etc., passed between the vessel and the wellbore. This long tubular member is commonly referred to as a riser pipe.

The submerged wellhead usually includes a blowout preventer and other control equipment. In one embodiment the upper part of the wellhead assembly includes a ball connector which provides a flexible connector between the wellhead assembly and the riser pipe. The lower end of the riser pipe is connected to this ball joint and is free to pivot thereabout. This is commonly called a flexjoint. Other types of flexjoints are commercially available. However, the ball and socket joint is enjoying increasing popularity. Although a vessel is anchored, it can have vertical movement from a few feet up to to 30 feet or more. To compensate for this vertical movement, a slip or telescopic joint is provided in the riser pipe. If the conventional riser pipe is supported solely at the lower end, its own effective weight, i.e., weight in water, causes it to be in a state of axial compression increasing from zero at the top to maximum at the bottom support. When drilling in deep water, the compressive stress in the wall of the riser pipe from this source alone is sometimes sufficient to buckle the riser pipe. This causes failure of the riser pipe. To counteract this buckling effect it has become a practice to apply a tensile force to the top of the riser pipe. Special tensioning devices are mounted on the ship and have their cables attached to the upper end of the riser pipe but below the slipjoint. These tensioning devices are commonly referred to as constant tensioning devices so that they can maintain a constant tension on the riser pipe although the ship may rise and fall with respect to the riser pipe. These constant tensioning systems are helpful but are also costly and must be maintained.

Function of these constant tensioning devices has the effect of pulling the riser pipe taut. Tensioners have worked well, but as drilling depths increase their limits are being approached. As the length of the riser pipe gets longer, e.g., 1,000 feet or more, the riser has an increasing tendency to act like a string of spaghetti. In order to supplement the tensioners, it has been suggested that flotation systems or jackets be fitted around the riser pipe at selected depths to help support the riser pipe and relieve the tensioning system of a considerable portion of the pipe weight.

For a discussion of using flotation jackets around riser pipes, attention is directed to an article in the Offshore magazine, June 5, 1971, p. 33, entitled Floating Riser Helps Extend Drilling to 1,700-ft Depths, by David Cook.

The present flotation systems apparently have reasonable engineering basis, but are subject to difficulty. For one thing, most of them to date are rather costly and their life expectancy is shorter than desired. A problem with attaching a buoy to the riser underwater is that if the riser should break beneath the buoy, the buoy would rise vertically due to its large upward buoyant force. The buoy with a large part of the riser pipe still attached would then have a catastrophic collision with the drilling vessel. My present invention provides underwater buoyancy to riser pipes which (I) is believed to be less costly than present flotation systems through its unique design and (2) is provided with means so that if the riser pipes break beneath the buoy, the buoy is caused to rise at an angle and surface away from the vessel,

BRIEF DESCRIPTION OF THE INVENTION This invention relates to an underwater buoy for a riser pipe which extends from an underwater well to a vessel supported by the body of water which comprises a hollow shell which is attached to the riser pipe to give an upper buoyancy force thereto. Non-vertically di rected hydrofoil means are attached to the outer wall of the shell, so that should the riser pipe break beneath the shell, the shell is directed upwardly at an angle and surfaces away from the vessel. A unique system is provided so that relatively thin walls can be used to fabricate the shell. The air in the shell is at ambient water pressure. This is accomplished by having a port in the lower portion of the shell so that sea water can enter this shell, but having an air source provide air to the interior of the shell above the port at ambient water pressure so that the shell will be buoyant. Liquid level means are provided so that the interface between the seawater and the air in the shell is known. If this is known then the buoyant force of the shell can be accurately determined.

BRIEF DESCRIPTION OF THE DRAWING Various objects and a better understanding can be had of the invention by the following description taken in conjunction with the drawing, in which:

FIG. 1 illustrates schematically the noval buoy connected to a riser pipe.

FIG. 2 shows a vertical view of my novel buoy connected to a riser pipe.

FIG. 3 is a section taken along the line 33 of FIG. 2.

FIG. 4 is simular to FIG. 1 but illustrates a riser interior pressure sensor used to control the amount of buoyancy in the buoy.

FIG. 5 illustrates schematically means for automati cally bleeding air from its buoy in response to pressure within the riser pipe.

DETAILED DESCRIPTION OF THE INVENTION Attention is first directed to FIG. 1 which shows drilling vessel supported at the surface 12 of a body of water 14. A wellhead 18 is indicated on the bottom 16 of the body of water. A riser pipe 20 extends from vessel 10 to subsea wellhead 18. My novel buoy 22 is positioned about riser pipe 20. Typically the depth of water 14 for which my invention is especially useful is 1,000 t0 10,000 feet or more in depth. Typically my buoy 22 is positioned sufficiently deep so that it is not subjected to surface currents and waves to any appreciable degree. Typically buoy 22 can be 600 feet or more from the surface. Although I have shown only one buoy 22 positioned along riser pipe 20, additional buoys can be placed at different depths if desired.

Attention is next directed to FIG. 3 which shows details of my buoy. Shown thereon is riser pipe 20 which has shell 24 of my buoy 22 attached thereto. Shell 24 can be attached to the riser by welding or any other way so that it is fixed to the riser pipe at the required depth. I provide port 26 in the lower portion of shell 14. Air is in the interior portion designated 30 which is above the water in the buoy.

For purpose of illustration, I show four hydrofoils 28 which are positioned at 90 spacing around shell 24, and attached to the shell 24 or to adjacent sections of riser 20. These hydrofoils 28 are supported by arms or brackets 29. Each hydrofoil 28 has a chord 31 which makes an angle 6 with a vertical. The angle 6 is selected such that if riser pipe 20 should break beneath the shell, that the shell would rise at an angle and surface away from ship 10. Many factors would be considered in determining the actual value of the angle 6 such as the depth at which shell 24 is positioned and the known current. However, typically the angle 6 wound normally be in the range from 30 to 60.

Attention will now be directed toward those means for supplying air to interior 30 of shell 24. This includes an air hose 44 which extends to an air source, not shown, on vessel 10. Air hose 44 is connected to a vertical stinger pipe 32 which has an upper end 34 which opens into shell 24. Stinger 32 is connected to a rack 36 which is driven by a motor means 39 having pinion gears 38. This is controlled by means 40 which can be either electric or hydraulic lines which extend to the surface. Vertical stinger 32 can be moved so that it takes the position of dotted line 32A having opening 32A which is at the top space 30 of shell 24.

Means are also provided so that the interface between the air in space 30 and the water in the lower part of shell 24 can be accurately determined. This includes liquid level indicator or sensor having a float 27 and lead wires 27A which extends to vessel 10.

When the buoy is placed at its required depth, the shell 24 is filled with water. Then when the buoy is in position on the riser pipe and it is desired to give it buoyancy, all that is necessary to do is to supply the necessary volume of air at the proper pressure to line 44. The proper pressure would be the ambient pressure or slightly in excess of the ambient pressure so that the water would be driven out of the shell through port 26. Then the pressure could be maintained at about the ambient water pressure so that the interface between the water and the air would be at or above port 26. After I have achieved the desired buoyancy, I move pipe 32 to where it is slightly above the desired air water interface. My reason for this is that should hose 44 break, I would maintain the captured air in the upper part of the shell 24 and thus retain my buoyancy until repairs could be made. Also by observing the readings from liquid level indicator 25 I can determine if there are any excessive leaks of air anywhere in the system. If the interface rises too rapidly, I can maintain buoyancy by adding more volume of air to hose 44. I then can check for leaks.

Sometimes it may be desired to reduce the buoyancy of buoy 22. This can be accomplished by raising the stinger pipe 32 so that its end 34 is at the desired level of the air-water interface and then releasing air through hose 44.

As[ mentioned above, if there were no provisions made to protect against it, a riser break below shell 24 would result in a catastrophe by the buoy and a part of the riser, moving rapidly upwardly, hitting the vessel. As I have explained, I provide hydrofoils so that in such event my buoy would surface at a position away from the vessel.

While I have not given any specific dimensions to shell 24 and size of hydrofoils 28, these can be calcu lated by marine design engineers in relation to the environment in which they are placed. Typically though, shell 24 may be 20 feet in diameter and 30 feet high. Typically the wall thickness can be one-half inch or less. Typically the chord of the hydrofoil 28 can be to about 5 feet to about 15 feet.

The weight or density of the drilling fluid within riser 20 is an important factor in determining the amount of tension which would be applied to riser 20 by the subsurface buoy and/or surface tensioning devices. We can adjust the upward force which buoy 22 will apply by varying the liquid level therein so that there is either a greater or lesser amount of air.

During drilling operations, the density of the drilling mud will ordinarily be varied considerably. If I know in advance when the density is being varied, I can adjust the amount of air in buoy 22 to compensate for change in tensioning required. I can either bleed off air from the buoy or add air thereto as necessary. However, there are situations which arise rather rapidly which necessitate a quick change in the amount of buoyancy applied to riser 20. For example, if a drilling crew is using a very heavy drilling mud, I would ordinarily have applied a relatively high tensioning value of the pipe 20 through the buoyancy means 22. If the drilling proceeds through a theft zone, it is possible that a large amount of my heavy drilling fluid will be lost, and its loss may occur rather quickly. The loss will be reflected by greatly reduced pressure at the lower end of the riser pipe. If this occurs, I would wish to change the upward buoyancy force rather quickly so that excessive stresses are not applied to the riser pipe. Modifications of my device, as shown in FIGS. 4 and 5, permit this rapid compensation of upward force on the riser pipe 20.

Shown in FIG. 4 is ship connected by riser pipe to the subsea wellhead 18 with the riser pipe buoyancy support 22 positioned on the riser pipe 20. This is just like that shown in FIG. 1. However, I modify FIG. 1 by adding a pressure sensor 46 at the lower end of riser pipe 20. This sensor detects the pressure within the riser pipe. Such sensors are well known and will not be described in detail here. The pressure detected by this sensor is transmitted to a controller 48 located on the ship, as shown in FIG. 5. Pressure control means 48 has two outputs. The first output 50 controls bleed valve 52 which bleeds air from buoy through air line 44. The second output'54 from controller means 48 is connected to pinion-drive motor 39. As I taught earlier, pinion means 38 raises or lowers vertical input pipe or stinger 32 into the interior of the buoy means 22. Control means 48 is so calibrated and programmed so that any pressure within a selected pressure range will cause the motor of the pinion means 38 to raise or lower vertical pipe 32 to a selected level for that particular pressure.

If there is a sudden drop in pressure detected by pressure sensor 46 in the bottom of the riser pipe 20, then control means 48 will have an output on line 54 to activate drive control motor 39 and raise the top 34 of stinger pipe 32 to the new desired elevation of the airwater interface in buoy 22, then output on line 50 opens valve 52, which bleeds the air off rather rapidly from air hose 44. Valve 52 is made sufficiently large so that it is nearly like cutting the hose 44 in two insofar as venting operations are concerned.

In the event pressure in the riser increases at sensor 46, the controller 48 is programmed to lower the elevation of stinger 32, and to supply air to the buoy 22 to increase buoyancy.

While the above invention has been described in detail, it is to be understood that various modifications can be made thereto without departing from spirit or scope of the invention.

I claim:

1. An underwater buoy for a riser pipe which extends from an underwater well to a vessel supported by a body of water which comprises:

a hollow shell attached to said riser to give an upward buoyancy force thereto;

non-vertically directed hydrofoil means attached to said shell for directing said shell upwardly through said water in a non-vertical direction to avoid striking said vessel in event said riser pipe fails below said shell.

2. A buoy as defined in claim 1, including:

a port in a lower portion of said shell,

means to supply gas to the interior of said shell above said port.

3. A buoy as defined in claim 2, including means to detect loss of gas from said shell.

4. A buoy as defined in claim 3 in which said means to detect loss of gas comprises a liquid level indicator positioned on the inside of said shell above said port and including means to transmit information concerning the position of said level to said vessel.

5. A buoy as defined in claim 4 in which said means to supply gas includes variable position discharge means to change the level in said shell at which said gas is discharged into or released from the interior of the shell independent of vertical movement of said liquid level indicator.

6. A buoy as defined in claim 5 in which said variable position discharge means includes a hollow pipe vertically disposed within said shell and means to move said pipe vertically therein.

7. A buoy as defined in claim 1 in which said hydrofoil means includes a plurality of hydrofoils in which the chord of each hydrofoil makes an angle of attack 6 with the vertical which is in the range of between about 30 and 8. An apparatus as defined in claim 1 including pressure sensing means for detecting pressure of fluid inside said riser pipe near the bottom of said body of water,

means to adjust the buoyancy of said shell in re sponse to pressure detected by said pressure sensing means.

9. In an apparatus as defined in claim 2 including a hollow stinger pipe vertically disposed within said shell, the lower end of said stinger pipe connecting said means to supply gas,

pressure sensing means for detecting the pressure of fluid inside said riser pipe at a level near the lower end of said riser pipe,

means to adjust the level of the top of said stinger pipe in response to the pressure detected by said pressure sensing means.

10. An underwater buoy as described in claim 1 wherein said hydrofoil means is attached to the outer wall of said shell.

11. A buoy system for a riser pipe which extends from an underwater well to a vessel supported by a body of water which comprises a hollow shell attached to said riser pipe to give an upward buoyancy force thereto,

pressure sensing means for detecting pressure of fluid inside said riser pipe,

means to adjust the buoyancy of said shell in response to the pressure detected by said pressure sensing means.

12. A buoy system as defined in claim 11 including hydrofoil means for directing said shell away from said vessel in event said riser pipe breaks beneath said shell when said shell is positioned beneath the surface of said body of water.

Claims

1. An underwater buoy for a riser pipe which extends from an underwater well to a vessel supported by a body of water which comprises: a hollow shell attached to said riser to give an upward buoyancy force thereto; non-vertically directed hydrofoil means attached to said shell for directing said shell upwardly through said water in a nonvertical direction to avoid striking said vessel in event said riser pipe fails below said shell.

2. A buoy as defined in claim 1, including: a port in a lower portion of said shell, means to supply gas to the interior of said shell above said port.

7. A buoy as defined in claim 1 in which said hydrofoil means includes a plurality of hydrofoils in which the chord of each hydrofoil makes an angle of attack theta with the vertical which is in the range of between about 30* and 60*.

8. An apparatus as defined in claim 1 including pressure sensing means for detecting pressure of fluid inside said riser pipe near the bottom of said body of water, means to adjust the buoyancy of said shell in response to pressure detected by said pressure sensing means.

9. In an apparatus as defined in claim 2 including a hollow stinger pipe vertically disposed within said shell, the lower end of said stinger pipe connecting said means to supply gas, pressure sensing means for detecting the pressure of fluid inside said riser pipe at a level near the lower end of said riser pipe, means to adjust the level of the top of said stinger pipe in response to the pressure detected by said pressure sensing means.

11. A buoy system for a riser pipe which extends from an underwater well to a vessel sUpported by a body of water which comprises a hollow shell attached to said riser pipe to give an upward buoyancy force thereto, pressure sensing means for detecting pressure of fluid inside said riser pipe, means to adjust the buoyancy of said shell in response to the pressure detected by said pressure sensing means.