US3858401A

US3858401A - Flotation means for subsea well riser

Info

Publication number: US3858401A
Application number: US420678A
Authority: US
Inventors: Bruce J Watkins
Original assignee: Regan Offshore International Inc
Current assignee: Baker Hughes Holdings LLC
Priority date: 1973-11-30
Filing date: 1973-11-30
Publication date: 1975-01-07
Anticipated expiration: 1992-01-07
Also published as: DE2444243A1; JPS577276B2; FR2252947B1; JPS5085502A; GB1455958A; FR2252947A1; DE2444243C2

Abstract

Flotation is provided for a subsea well riser conduit. A plurality of open bottom but otherwise airtight buoyancy gas receiving chambers are mounted about the riser conduits. A gas conduit connects each chamber with a source of compressed gas on the drilling vessel. Valves are provided in the gas conduit for introducing gas in selectable amounts into the chambers to displace water entrained therein out the open bottom of the chamber to provide a selectable amount of buoyancy to the riser conduit. The gas valve includes an associated float for holding open the valve when the water level is above a predetermined level in the chamber and for closing the valve when the water level falls below the level to avoid loss of gas out the open bottom of the chamber. The flange which forms the top wall of the chamber is adapted to allow adjacent riser conduits to be fit together. Additionally, the chambers and the associated flanges provide a passageway for associated drilling control apparatus.

Description

[lite atet [191 [111 $53,401 Watkins Jan. 7, 1975 FLOTATION MEANS FOR SUBSEA WELL Primary Examiner]acob Shapiro RISER [57] ABSTRACT Flotation is provided for a subsea well riser conduit. A plurality of open bottom but otherwise airtight buoyancy gas receiving chambers are mounted about the riser conduits. A gas conduit connects each chamber with a source of compressed gas on the drilling vessel. Valves are provided in the gas conduit for introducing gas in selectable amounts into the chambers to displace water entrained therein out. the open bottom of the chamber to provide a selectable amount of buoyancy to the riser conduit. The gas valve includes an associated float for holding open the valve when the water level is above a predetermined level in the chamber and for closing the valve when the water level falls below the level to avoid loss of gas out the open bottom of the chamber. The flange which forms 13 Claims, 6 Drawing; Figures Patented Jan. 7, 1975 3,858,401

2 Sheets-Sheet l Patented Jan. 7, 1975 I 3,858 401 2 Shasta-Sheet 2 FLOTATION MEANS FOR SUBSEA WELL RISER BACKGROUND OF THE INVENTION In subsea drilling, the long drilling riser conduit formed from conduit sections extends from the wellhead'to the vessel or floating platform. The riser conduit which is very heavy creates significant problems.

Wave action tends to move the vessel both vertically and horizontally. This is minimized by securely anchoring the vessel, but in deep water drilling, the vessel still undergoes rather large movement because of slack and elasticity in the anchoring mechanism. If the vessel moves too far horizontally, the elastic limit of the riser conduit may be reached and it may break. Vertical oscillation of the vessel is usually compensated for by having a device which allows for relative movement between the riser conduit and the vessel. The vertical oscillation of the water also effects the upper portion of the riser conduit which may result in strain or ultimate breaking of the riser conduit. As the length of the conduit and subsequently the weight thereof is increased for deeper drilling operations, these problems become more acute. For example, when the vessel moves horizontally and is no longer directly over the wellhead, the riser conduit will not be directly vertical. Therefore, the weight of the upper sections of the riser conduit will tend to pull the vessel farther, horizontally, from its position directly above the wellhead. As the vessel moves farther out and the conduit is farther from the vertical, the horizontal component of force will increase which will tend to drive the vessel even farther away. Clearly, if this were to continue, the elastic limit of the riser conduit could be reached and it could be deformed or break.

During storms or for routine maintenance, the vessel may have to leave its associated conduit. It is necessary at that time to provide a force to maintain the upper portion of the riser conduit near the surface of the body of water. 7

These problems associated with the riser conduit have been recognized, and it has been proposed to add flotation to a riser conduit to buoy it. Some early flotation systems utilized closed pressure vessels which were attached to the riser conduit by attaching the top part of the buoyant vessel to the top part of the riser conduit section and the bottom part of the vessel to the bottom part of the riser conduit section. However, because of the temperature differential between the warm drilling mud inside the riser conduit and the cold sea water out side of it which cause weakened sections of the riser conduit, the flotation method just discussed would cause non-uniform forces to act on the riser conduit. Some sealed buoyant vessels had to be extremely heavy to resist collapse and the high pressure in deep subsea wells, and they were extremely difficult to handle. Third, the closed pressure vessels gave a non-adjustable amount of buoyancy which created problems in balancing the flotation along the entire length of the riser conduit. It has also been proposed to use foamed plastic around the riser conduit. However, these plastics are very expensive, and at great depth, they can absorb water. Certain other improvements have been made in providing buoyancy for a riser conduit. For example, see U.S. Pat. No. 3,572,041 to Graaf, dated Mar. 23, 1971, in which a plurality of buoyancy tanks are disposed internally of a marine riser. These tanks are only placed in the top portion thereof so that the center of buoyancy is as high as possible above the center of gravity. In U.S. Pat. No. 3,667,240, dated June 6, 1972, to Vilain, buoyancy is provided by a storage tank which is filled with gas and hydrocarbons which creates a buoyant device. An external buoyant can arrangement is discussed in U.S. Pat. No. 3,720,066 to Vilain dated Mar. 13,1973.

It has also been proposed to create chambers around the riser conduit to add buoyancy thereto. See Rhodes, U.S. Pat. No. 3,017,934, dated Jan. 23, 1962 in which buoyant cans which are opened at the bottom are filled from a supply of gas leading to the can nearest the ocean floor. A gas conduit allows the gas to flow from a full buoyant tank to the tank immediately above it until all such tanks are filled. However, no gas will flow to higher tanks until the bottom one is full. Therefore, there is no adjustability in the system because no gas can enter an upper tank until the one directly below it is full. Also, in the aforementioned patent the tanks are especially formed to fit around the riser conduit, and their sole function is buoyancy.

It is an object of the present invention to provide flotation for a subsea riser conduit which can be easily handled and would be more reliable then prior art flotation systems. It is a further object of this invention to have a low cost system and to have a system where al most any desired buoyancy may be provided. A further object of the invention is to provide a buoyancy system which will also support auxiliary lines and conduits associated with the riser conduit. It is also an object of this invention to provide a device wherein a malfunction in part of the buoyancy system does not effect the rest of the buoyancy system. Another object of this invention is to provide a buoyancy device which will be of uniform size for each riser conduit section used to ease handling and assembling the buoyancy means.

BRIEF SUMMARY OF THE INVENTION Flotation is provided for a subsea well riser conduit run between a floating vessel provided with a source of compressed gas or air and a subsea wellhead. Such flotation means includes a plurality of open bottom buoyancy gas receiving chambers and means for mounting them about and along the riser conduits. Air or gas conduit means connected between the source of compressed gas and each of the chambers introduces gas in selectable amounts into the chamber, displacing water entrained therein out the open bottom of the chamber to provide a selectable amount of buoyancy to the riser conduit. Gas valve means are provided at one or more of the chambers. The gas valve means include associated float means for holding the valve means open when the water level is above a predetermined level in the associated chamber and for closing the valve when the water level falls below the level to avoid loss of gas out through the open bottom of the chambers. A restricted orifice is provided between each chamber and the gas conduit means to provide a generally equal distribution of gas to each chamber from the gas conduit. The opened bottom buoyancy chamber includes an annular airtight flange formed integrally of and extending radially outward from each of a plurality of riser conduit sections and a plurality of generally cylindrical airtight shells. Mounting means mount the shells about the riser conduit in an airtight and depending relationship to an associated annular flange to thereby provide airtight top and side walls to the open bottom chambers.

A centralizer is provided at the bottom of each chamher to generally maintain the desired spacing between the shells and the conduit sections.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevation depicting a floating platform or vessel over a subsea well site or formation with guide means interconnecting the vessel and the wellhead.

FIG. 2a is a sectional view taken along line II-II of FIG. 1 and shows the bottom portion of one section of the riser conduit connected to the top portion of another section of riser conduit.

FIG. 2b is a continuation of the bottom portion of the lower section of riser conduit of FIG. 20 showing the connection between that section of riser conduit and a third section of riser conduit.

FIG. 3 is a sectional view taken along line IIIIII in FIG. 2b.

FIG. 4 is a sectional view taken along line IVIV in FIG. 2b.

FIG. 5 is a detailed view of the gas conduit, the float, and the associated valve.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A subsea well riser or conductor conduit 20 extends from a floating vessel or platform through the body of water 12 to a wellhead 17. The vessel is suitably anchored on the surface of the water. The subsea well riser conduit is run from the vessel or platform of slot 11, which is below the derrick 19, to the wellhead indicated generally at 17 which is mounted on the well ternplate 15 above the conductor pipe 14 in the formation 13. Conventional blowout preventer apparatus 16 and riser coupling apparatus 18 may be additionally provided at the wellhead.

The subsea well riser conduit 20 is formed ofa plural ity of conductor conduit sections 21. Conductor conduit section 21a is the conduit section nearest the vessel while conduit section 21d is nearest the wellhead. These conduit sections are generally approximately 40 to 50 feet long. It is contemplated that the riser conduit of the instant invention could be used in a drilling operation conducted at a depth of 6,000 feet below the water surface.

Each conduit section has a cylindrical wall 22 having a top portion 23 and a bottom portion 24. At the end of the top of the conduit section 23 is a portion of larger inside diameter 25 which accepts the bottom portion of the conduit section mounted directly above it. O-rings 26 seal the junction of the two adjacent conduit sections. An annular section 27 is provided into which locking dogs clamp (FIG. 3) to hold the adjacent conduit sections together. Bottom flange 28 and top flange 29 project radially outward from the respective bottom and top portion of each conduit section. These flanges serve to guide the electrical or

hydraulic lines

31 and 32, and a pipeline 51 carrying pressurized gas whose function along with additional functions of the top flange will be discussed hereinafter. In the preferred embodiment,

tubing

31 and 32 are the hydraulic control lines for the choke and the kill functions. These hydraulic lines are in sections of approximately the same length as a riser conduit section and are connected by

connectors

33 and 34. Additionally, connector 34 functions to seal the opening in the top flange through which hydraulic line 31 passes.

flange 29 by mounting means 41 to form an airtight seal therebetween. Preferably, the mounting means includes a plurality of bolts around the periphery of flange 29 connecting the flange to the annular shell 42. Centralizer ring 43 mounted near the bottom of the shell services the maintain the annular shell a fixed distance from the cylindrical wall of the riser conduit. The contemplated centralizer is a ring 45 with radial fins 46 extending from the outer surface of the riser conduit to the inside of the shell. To add more support to the shell, other centralizers 47 may be spaced along each riser conduit section. There is no'seal to the bottom of the chamber and water is free to rise inside the chamber.

The chambers run almost the total length of each riser conduit section and all are substantially the same size. This aids in handling them. Because the shells are of uniform dimensions throughout their length, the cost per shell is decreased and handling is facilitated.

Air or gas supply means, including a gas line or gas conduit means, is connected between a source of compressed air or gas on the vessel and each chamber for introducing gas in selectable amounts into the chamber displacing water entrained therein out the open bottom of the chamber to provide a selectable amount of buoyancy to the riser conduit. In the preferred embodiment such gas supply means 50 includes a gas line or gas conduit means 51 and gas valve means 52. The gas valve means includes an associated float means for holding the valve means open when the water level is above a predetermined level in the associated chamber and for closing the valve means when the water level falls below the level to avoid loss of gas out through the open bottom of any chamber. Float means 55 is connected by a stem or connector members 56 to a valve member 57 above the valve seat 54 so that as the level of water 12 rises within the chamber 40, float means 55 causes the valve member 57 to move upward from valve seat 55 in FIG. 5 and allow gas into the chamber. The gas will displace water 12 inside the chamber until the level of water reaches a low enough level so that float 55 is no longer supported on the water and drops a sufficient distance to close the valve seat 54 of valve 52. Therefore, even though there is more water pressure on the lowermost riser conduit sections than on the uppermost riser conduit sections, when the upper riser conduit sections fill with gas, the valve will then close and gas can flow into the chambers associated with the lower conduit sections instead of having gas leaking from the bottoms of the chambers associated with the upper conduit sections.

Additionally, a means for restricting gas flow is provided between each chamber and the gas conduit means for providing a generally. equal distribution of gas from the gas conduit to the chambers. In the preferred embodiment, such means includes the provision of a restricted passage 53 formed between stem 56 and the orifice in valve seat 54. The gas flow restricting chamber to balance the flow of air between the respective chambers.

The gas conduit means passes through an aperture in the top flange 29. This aperture is sealed by the seal means 59. It should be noted that

seals

57 and 34 and the means 41 mounting the shell 42 to the top flange 29 must cause an airtight top portion because the gas pressure within each chamber, especially those farther beneath the surface of the water will be extremely high. In addition, spacer element 58 is provided to fit against the bottom flange.

By severly limiting the capacity of the valve or by the use of a very small orifice and by using very high pressure gas in the conduit, the failure of a valve to close in one or two chambers would not cause all chambers to cease filling with gas. Some gas would simply be discharged at the bottom of the chambers with a valve failure until the other chambers were filled with gas. The overall gas injection rate would not be reduced because many chambers would normally be filling simultaneously even through the chambers nearer the surface fill faster because the water pressure is lower there.

The failure of a single valveto close after all the chambers are filled is also not a significant problem. If the source of compressed gas is no longer delivering gas to the chamber, the water level will rise as gas flows back through the valve. However, the gas above the water is necessarily at the same pressure as the water. Therefore, the water level cannot rise above the valve. If flotation is to be completely adjustable (buoyancy addable or subtractable) a bleed line or other gas removing means (not shown).should be provided near the top of each chamber.

It should be recognized also that if each chamber is partially filled with gas to allow for a certain amount of buoyancy, the chamber will hold that amount of gas and maintain that amount of buoyancy.

When all chambers are full, there will be less buoyancy for the chambers nearer the wellhead because the 3 5 gas in those chambers is under higher pressure and therefore is heavier per unit volumn. There will be no tendency for a more buoyant lower portion of the riser conduit to float to a higher position than the upper por' tion of the riser conduit. The slightly more buoyant upper chambers have a tendency to maintain the conduit vertically. This limits horizontal movement.

As the buoyancy to the system is increased, the riser can become free-standing to allow the vessel to leave its associated conduit. The top portion should then terminate a sufficient distance below the water level to not become a hazard to navigation. A marker buoy would be attached to the riser to facilitate in relocating it.

Buoyancy means for use with subsea well apparatus have been shown which includes a marine conductor conduit 20 run from a floating vessel to a subsea well and which is formed of conduit sections 21 connected together in an end-to-end relationship. The buoyancy means comprises flange means 29 extending radially outwardly from and about one or more of the plurality of the conduit sections in an airtight relationship thereto. There are a plurality of airtight shells 42 and mounting means 41 for mounting the shells positioned about the one or more of the conduit sections and extending downwardly from the associated flange means to form a plurality of buoyancy chambers which are open at their bottoms. Gas supply means supply selectable amounts of gas from a source on the vessel to each chamber, the introduction of gas into the chambers displacing water therein out through the open bottom of the chambers.

I claim:

l. Flotation means for a subsea well riser conduit run between a floating vessel provided with a source of compressed gas and a subsea wellhead, said flotation means comprising:

a plurality of open bottomed buoyancy gas receiving chambers and means for mounting them about and along said riser conduit; and

gas supply means including gas conduit means connected between said source of compressed gas and each of said chambers for introducing gas in selectable amounts into each of said chambers independently of the others, displacing water entrained therein out open bottoms of said chambers, to pro vide selectable amounts of buoyancy to said riser conduit.

2. The flotationmeans of claim 1 wherein gas valve means are provided at one or more of said chambers, each of said gas valve means including a valve member and float means mechanically connected thereto for 20 holding said valve means open when the water level is above a predetermined level in the associate chamber and for closing said valve means when the water level falls below said level'to avoid loss of gas out through said open bottom of said chambers.

3. The flotation means of claim 1 wherein means for restricting 'gas flow is-provided between each chamber and the gas conduit means to provide for a generally equal and simultaneous distribution of gas from said gas conduit to said chambers.

4. The flotation means of claim 1 wherein one or more of said plurality of open bottomed buoyancy chambers comprises: i

an annular flange formed integrally of and extending radially outwardly of each of a plurality of riser conduit sections;

a plurality of generally cylindrical shells; and

mounting means for mounting said shells about said riser conduit in airtight and depending relation to an associated annular flange to thereby provide airtight top and side walls of the open bottomed chambers.

5. The flotation means of claim 4 further including aperture means through said annular flange for passing said gas conduit means therethrough.

6. The flotation means of claim 1 wherein said gas conduit extends into one or more of said chambers through the upper ends of said chambers closer to the floating vessel, and gas in said conduit flows in a direction from the source of compressed gas through said upper ends of said chambers.

7. The flotation means of claim 6 wherein gas valve means are provided in one ormore of said chambers near said open bottom of said chambers to allow gas to 55 pass only from said gas conduit into said chambers.

8. Buoyancy means for use with subsea well apparatus which includes a marine conductor conduit run from floating vessel to subsea well and which is formed of conduit sections connected together in an end-to-end relationship, said buoyancy means comprismg:

flange means extending radially outwardly from and about one or more of said plurality of conduit sections in airtight relation thereto;

a plurality of airtight shells and mounting means for mounting said shells positioned about said one or more of said sections and extending downwardly from the associated flange means to form a plurality of annular buoyancy chambers which are open at their bottoms; and 7 gas supply means for supplying selectable amounts of gas from a source on said vessel to each of said chambers; introduction of gas into said chambers displacing water therein out through said open bottoms of said chambers.

9. The buoyancy means of claim 8 wherein said gas supply means includes a gas line run from said vessel to said chambers and gas valve means within one or more of said chambers.

10. The buoyancy means of claim 9 wherein each of said gas valve means includes means for restricting gas flow associated with its gas line to regulate the rate of gas flow between the gas line and respective chamber to provide for a substantially equal pressurizing of said chambers.

11. The buoyancy means of claim 9 wherein each of said gas valve means comprises a valve seat and associated valve member for blocking the flow of gas through the valve means when said member is seated on said valve seat and float means connected to said valve member to open said valve means when said float is buoyed by surrounding water in said chamber, said valve means closing to stop the flow of gas into said chamber when the water level within said chamber recedes under the pressure of gas introduced therein so that said float no longer supports said valve member above said valve seat.

12. The method of providing buoyancy to a marine conductor conduit run from a floating vessel to a subsea well comprising the steps of:

providing a plurality of gas receiving buoyancy chambers with openings at their bottom ends in a vertical array along said marine conductor conduit; and

after running of said conductor conduit to said well from said floating vessel, introducing gas from a source thereof on said vessel into each of said chambers independently of the others and forcing water entrained therein out through the openings at the chamber bottoms to provide a selectable amount of buoyancy therefor.

13. The method of claim 12 including the additional steps of:

providing water level activated gas valves in said chambers and introducing said gas through said valves, and

closing said valves by the lowering of the water level in said chambers during said step of forcing water out of said chambers.

Claims

1. Flotation means for a subsea well riser conduit run between a floating vessel provided with a source of compressed gas and a subsea wellhead, said flotation means comprising: a plurality of open bottomed buoyancy gas receiving chambers and means for mounting them about and along said riser conduit; and gas supply means including gas conduit means connected between said source of compressed gas and each of said chambers for introducing gas in selectable amounts into each of said chambers independently of the others, displacing water entrained therein out open bottoms of said chambers, to provide selectable amounts of buoyancy to said riser conduit.

2. The flotation means of claim 1 wherein gas valve means are provided at one or more of said chambers, each of said gas valve means including a valve member and float means mechanically connected thereto for holding said valve means open when the water level is above a predetermined level in the associate chamber and for closing said valve means when the water level falls below said level to avoid loss of gas out through said open bottom of said chambers.

3. The flotation means of claim 1 wherein Means for restricting gas flow is provided between each chamber and the gas conduit means to provide for a generally equal and simultaneous distribution of gas from said gas conduit to said chambers.

4. The flotation means of claim 1 wherein one or more of said plurality of open bottomed buoyancy chambers comprises: an annular flange formed integrally of and extending radially outwardly of each of a plurality of riser conduit sections; a plurality of generally cylindrical shells; and mounting means for mounting said shells about said riser conduit in airtight and depending relation to an associated annular flange to thereby provide airtight top and side walls of the open bottomed chambers.

7. The flotation means of claim 6 wherein gas valve means are provided in one or more of said chambers near said open bottom of said chambers to allow gas to pass only from said gas conduit into said chambers.

8. Buoyancy means for use with subsea well apparatus which includes a marine conductor conduit run from a floating vessel to a subsea well and which is formed of conduit sections connected together in an end-to-end relationship, said buoyancy means comprising: flange means extending radially outwardly from and about one or more of said plurality of conduit sections in airtight relation thereto; a plurality of airtight shells and mounting means for mounting said shells positioned about said one or more of said sections and extending downwardly from the associated flange means to form a plurality of annular buoyancy chambers which are open at their bottoms; and gas supply means for supplying selectable amounts of gas from a source on said vessel to each of said chambers; introduction of gas into said chambers displacing water therein out through said open bottoms of said chambers.

12. The method of providing buoyancy to a marine conductor conduit run from a floating vessel to a subsea well comprising the steps of: providing a plurality of gas receiving buoyancy chambers with openings at their bottom ends in a vertical array along said marine conductor conduit; and after running of said conductor conduit to said well from said floating vessel, introducing gas from a source thereof on said vessel into each of said chambers independently of the others and forcing water entrained therein out through the openings at the chamber bottoms to provide a selectable amount of buoyancy therefor.

13. The method of claim 12 including the additional steps of: providing water level activated gas valves in said chambers and introducing said gas through said valves, and closing said valves by the lowering of the water level in said chambers during said step of forcing water out of said chambers.