GB2344841A - Self-supporting riser system with flexible vessel-connecting section - Google Patents

Abstract

A subsea riser system 10 de-couples movement of a surface vessel 30 from a riser 20 extending from a production tree 12 embedded in the ocean floor. The lower riser 20 is connected to the production tree and extends upwards toward the surface where it is supported by buoyancy modules 28. A flexible section 32 connects section 20 to the surface vessel 30 such that end 34 of the flexible section is the lowest point to allow a gravity-fed lowering of downhole tools through the risers and in to the well.

Description

SELF-SUPPORTING DE-COUPLED RISER SYSTEM croas-Refer ce to R.ted Aclfcations This application claims the benefits of provisional patent application Serial Number 60/112,165, filed on December 14, 1998, in the United States Patent & Trademark Office.

Tec This invention relates to a riser system used for workover operations of a subsea oil and gas well. More particularly, this invention relates to a de-coupled riser system for workover operations that eliminates the transference of motions from a vessel floating at the surface to a subsea riser.

Background of the Invention A conventional subsea wellhead assembly includes a wellhead housing that supports one or more casing hangers located at upper ends of strings of casing extended into the well. A tubing hanger lands in the wellhead casing above the casing hanger and supports a string of production tubing that extends through the smallest diameter casing.

After the well has been completed, a subsea production tree is mounted to the wellhead housing. The riser used to complete the well and run the tree is removed.

In the prior art, if a workover operation is needed, a completion riser with a blowout preventer (BOP) is affixed to the production tree. Access through the riser, BOP and production tree to the tubing may be made for various workover operations that are needed.

Completion risers in the prior art are preferably dual pipe or monobore assemblies that are time consuming and expensive to run. Proposas have been made to employ a light weight monobore riser for workover operations. One difficulty encountered during workover operations is the transference of motions from a vessel at the surface to the riser system. Various proposals have been made to decouple vessel motions from the riser systems in general.

For example, U. S. Patent No. 4,448,266 to Potts for a Deepwater Riser System for Offshore Drilling"utilizes a buoyant riser system for use in a deepwater offshore drilling environment. The buoyant riser system is anchored by a system of compliant guys below the active weather zone of the sea. A controllably buoyant housing of the system is submerged at a depth that is readily accessible to divers and includes a blowout preventer from which a subsea riser extends to a wellbore. Above the buoyant housing, a rigid marine riser suspended from a floating drill rig is coupled to the blowout preventer in the buoyant housing, thereby communicating the drill rig directly with the wellbore for drilling and well completion operations. The 266 system is designed for drilling, not workover operations with a light weight riser.

U. S. Patent No. 4, 448,568 to Gentry et al. for a 'Marine Surface Facility work Station for Subsea Equipment Handling"teaches an offshore process vessel for connecting service and production hoses to a deepwater free-standing production riser system. The vessel is provided with a pair of longitudinally aligned moonpools. Hoses are joined together in a linear array along the length of a flowline, which hangs as a catenary from a selectively ejectable plug while being attached to a riser system. Gentry does not teach running wireline through the hoses. Workover operations are performed through a service moonpool, not through the hoses.

Another example is U. S. Patent No. 5,046,896 to Cole for an"Inflatable Buoyant Near Surface Riser Disconnect System". The riser system of Cole includes inflatable buoyancy bladders and a near surface disconnect so that a drilling vessel can rapidly disconnect from the riser to leave the riser in a free standing buoyant position. y of the Invention The riser system of the invention is a lightweight alloy or composite monobore that may be lowered by a workover vessel into engagement with a subsea production tree. The riser system has a lower riser package having a blowout preventer that connects to the production tree. A lightweight monobore riser system is connected to the lower riser package with a lower connector. The riser extends upwards towards the surface. A buoyancy support is connected to the riser at its upper end to provide support and maintain the riser in a substantially vertical orientation. The vessel on the surface of the body of water is connected at an upper end of the riser by a flexible upper section. Preferably, the upper end of the riser is approximately 150-300 feet below the surface of the body of water. The flexible upper section has an outer conduit and a compliant inner liner that is encased within the outer conduit. The riser system of the invention decouples vessel motions from the main riser section.

Downhole tools are lowered through the flexible section into the well or wireline or coiled tubing.

By virtue of the free standing design of the riser, the workover riser package may be permanently parked on a subsea well for future workovers to eliminate running and retrieval of the riser package for each workover operation.

Brief Descript ; on of the Dravincs Figure 1 is a schematic view of the de-coupled riser system of the invention.

Figure 2 is a schematic view of the de-coupled riser system of the invention shown in use with a wireline or similar work string such as coiled tubing extending therethrough.

Figure 3 is an enlarged cross-sectional view of a connection between tubular sections of the riser system at Figures 1 and 2.

Detailed nacription of the Invantifla Referring now to Figure 1, a schematic of a de-couple riser system 10 is shown. Preferably, the de-couple riser system 10 is not used for production, but is used for workover operations. De-coupled riser system 10 releasably connects to a production tree 12 imbedded in ocean floor 14. Riser system 10 includes a lower riser package/blowout preventer 16 that is affixed to tree 12.

A lower connector 18 is affixed to the lower riser package 16. Riser system 10 also includes a riser 20 extending upward from blowout preventer 16.

Riser 20 is adjustable in length to accommodate different water depths. Riser 20 is preferably made up of ninety foot sections, which are typically monobore sections. The sections of pipe are lighter in weight than a conventional drilling riser, utilizing either light alloys, composite materials or a combination of these, with buoyant materials. Preferably, riser 20 is a type made up of pipe sections 21 connected together, having rapid makeup threads (Figure 3). Riser 20 has a lower end 22 that is affixed to lower connector 18, which is turn connects to blowout preventer 16. Riser 20 extends upwardly and terminates at an upper end 24 below sea level. The inner diameter of riser 20 need not be large enough for tubing hangers and tubing of the well to be pulled through. Riser 20 has a single bore that communicates with the interior of the tubing in the well.

A variable buoyancy support system, designated generally 26, is provided proximate upper end 24 of riser 20 for supporting riser 20. Support system 26 is preferably made up of variable buoyancy modules 28. The variable buoyancy modules 28 are attached to the upper end 24 of riser 20, which is preferably located 150-300 feet subsea.

Extending downward from light intervention vessel 30 is a flexible jumper or upper section 32. Flexible upper section 32 is a flexible conduit, having a compliant inner liner, of material such as an elastomer, encased within high strength outer layers, such as metal 33 (Figure 2).

An example of such a conduit is shown in U. S. Patent No.

5,645,110 to Nobileau. The upper section 32 is for decoupling vessel motions from riser 20. Upper section 32 has a laver end 34 for connection to upper end 24 of riser 20. Upper section 32 also has an upper end 36 that communicates with light intervention vessel 30. The flexible upper section 32 is typically 150 to 350 feet in length. Although it need not be suspended vertically from the vessel, the lower end 34 of upper section 32 will be the lowest point of flexible upper section 32 such that downhole tools can be lowered by gravity through upper section 32. There will be no curved portion in flexible upper section 32 that is lower than upper end 24 of riser 20. The inner diameter of the upper section 32 is preferably approximate the same as the inner diameter of riser 20.

In operation, vessel 30 will be positioned over tree 12. Lower riser/blowout preventer 16 and riser 20 will be made up and lowered into engagement with tree 12, using a remote operated vehicle, if necessary, for guidance.

Buoyancy module 28 and flexible upper section 32 will also be made up to riser 20 while it is being lowered from vessel 30. Once installed, variable buoyancy module 28 is adjusted to provide efficient tension to riser 20 to enable it to be free standing. The flexible upper section 32 flexes to isolate movement of vessel 30 from riser system 20. Downhole tools secured to wireline or coiled tubing may be run down through flexible or rigi-flex upper section 32, through riser system 20, and into the tubing in the well to perform operations, such as logging well characteristics or perforating. As shown in Figure 2, wireline 38 passes through flexible upper section 32, riser 20, lower riser portion/blow out preventer 16 and into the well.

After the workover operation has been completed, flexible upper section 32 may be disconnected from upper end 24 of riser 20 and retrieved to the workover vessel 30.

If desired, riser 20 may be left free-standing. If so, riser 20 may be filled with a corrosion inhibitor to allow it to remain installed and free-standing above subsea well 12 over a long term. For subsequent workover operations, a vessel smaller than vessel 30 may be employed because the smaller vessel will not need to have the capability to run riser 20. The smaller vessel would only need to be able to lower flexible upper section 32 into engagement with upper end 24 of riser 20. This could be performed with the assistance of a remote operated vehicle.

The invention has several advantages. One advantage is that the intervention riser system or jointed riser system is de-coupled from vessel motions at the surface by use of the flexible or semi-flexible jumper between the top of the riser and the vessel. Further advantages include eliminating the need for wave compensators, and eliminating the need to tension the riser with the vessel. The ability to allow the riser to remain free-standing with the well as a permanent installation can eliminate future running and retrieval operations of the riser, except for planned maintenance activities on the riser. This reduces the cost of a subsea well workover operations, because the workover vessel can be much smaller in size than typical vessels capable of running conventional risers. The smaller workover vessel need only be capable of running the flexible upper section.

While the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limite, but is susceptible to various changes without departing from the scope of the invention.

Claims (13)

1. In the Claims : 1. A riser system for releasably coupling to a subsea wellhead assembly of a well and extending to a floating vessel comprising: a lower riser package having a blowout preventer and adapted to be connected to said subsea wellhead assembly ; a riser formed of tubular sections secured together, affixed to said lower riser package and extending upwards towards a surface of a body of water; a buoyancy support connected to said riser for providing support to said riser; and a flexible upper section connected from said vessel to said riser and adapted to extend to said vessel, wherein a bottom end of said flexible upper section is below all of said flexible upper section so that a downhole tool may be lowered by gravity through said flexible upper section, through said riser and into said well.
2. 2. The riser system according to claim 1 wherein said riser comprises tubular sections of said riser are secured to each other by threads.
3. 3. The riser system according to claim 1 wherein said buoyancy support is attached to an upper end of said riser and said riser is substantially rigid.
4. 4. The riser system according to claim 1 wherein said flexible upper section is comprised of: an outer conduit; and an elastomeric inner liner encased within said outer conduit.
5. 5. The riser system according to claim 1 wherein said riser is a monobore.
6. 6. A riser system for workover operations with a workover vessel on a subsea well having a production tree and a string of production tubing comprising: a blowout preventer adapted to releasably connect to said production tree; a monobore riser affixed to said blowout preventer and extending upwards towards a surface of a body of water, said riser being made up of a plurality of sections of threaded pipe screwed together; a buoyancy support connected to said riser for providing support to said riser; a tubular flexible upper section secured to said riser and adapted to extend to said vessel, wherein a lower end of said flexible upper section is below all of said flexible upper section so that a downhole tool may be lowered by gravity through said flexible upper section, said riser and into said tubing.
7. 7. The riser system according to claim 6 wherein said upper section has an inner diameter that is substantially the same as an inner diameter of said riser.
8. 8. A method of connecting a subsea wellhead assembly to a floating vessel comprising the steps of: (a) providing a riser made up of joints of pipe; (b) coupling a lower end of the riser to the subsea wellhead assembly ; (c) supporting an upper end of said riser below sea level; (d) connecting said riser with said vessel by a flexible upper section; and (e) running downhole tools through said flexible upper section and said riser into said subsea wellhead assembly.
9. 9. The method according to claim 8 wherein said step (e) comprises: lowering said downhole tools through said flexible upper section by gravity on a flexible line.
10. 10. The method according to claim 8 wherein said step (c) comprises: securing buoyant sections to said upper end of said riser.
11. 11. The method according to claim 8 wherein said step (e) comprises: positioning a lower end of said flexible upper section below all portions of said flexible upper section.
12. 12. The method according to claim 10, further comprising : after step (e), retrieving said downhole tools back to said floating vessel ; then disconnecting said flexible upper section from said riser and retrieving said flexible upper section back to said floating vessel ; then moving said floating vessel away from said subsea wellhead assembly, while leaving said riser connected to the subsea wellhead assembly and said buoyant sections secured to said riser for subsequent workover operations.
13. 13. A method of performing workover operations on a subsea wellhead assembly, comprising: (a) from a first floating vessel, running a string of riser made up of joints of pipe into engagement with said subsea wellhead assembly ; (c) with buoyant sections attached to the riser, supporting an upper end of said riser below sea level; (d) moving said first vessel away from said subsea wellhead assembly, allowing said riser to remain connected to said subsea wellhead assembly, with said upper end of said riser remaining suspended below sea level; then (e) connecting said riser with a second vessel by a flexible upper section ; (f) running downhole tools on a flexible line through said flexible upper section and said riser into said subsea wellhead assembly to perform said workover operations; then (g) retrieving said downhole tools, disconnecting said flexible upper section from said riser and retrieving said flexible upper section back to said second vessel; and (h) moving said second vessel away from said subsea wellhead assembly, and allowing said riser to remain connected to said subsea wellhead assembly, with said upper end of said riser remaining suspended below sea level for future workover operations.