AU8950298A

AU8950298A - Stress relief joint for risers

Info

Publication number: AU8950298A
Application number: AU89502/98A
Authority: AU
Inventors: Lyle David Finn
Original assignee: Deep Oil Technology Inc
Current assignee: Deep Oil Technology Inc
Priority date: 1997-10-27
Filing date: 1998-10-26
Publication date: 1999-05-13
Also published as: NO984975L; FI982309A; NO319687B1; EP0911482A3; EP0911482A2; FI982309A0; EP0911482B1; NO984975D0; OA10906A; BR9804215A

Description

P/o/I1 1 Regulation 3.2

AUSTRALIA

PatentsI Act 1990

ORIGINAL

COMPLETE

SPECIFICATION

STANDARD

PATENT

STRESS RELIEF JOINT FOR RISERS Invention Title: The following statement is a full description of this invention, including the best method of performing it known to us: GH REF:

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1: *1 CASE 5956

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BTRESS RELIEF JOINT FOR RISERS BACKGROUND OF HE INVENTION Field of the Invention The invention is generally related to risers used in the production of hydrocarbons offshore and more particularly to the stress joint used for risers in conjunction with floating structures.

2. General Backround In the drilling and production of hydrocarbons offshore, the development of deep water operations from floaing vessels has included the use of tendons and risers under tension extending Such floating vesel have from the vessel to the sea floor. Such floating vessels have included tension buoyant towers and spar structure in which the floating structures extend well below the surface of the water S. and are subjected to heave, pitch, and roll motions.

SThe lower ends of the tendons and risers are connected to the sea floor by means of additional pipes or risers embedded in and grouted to the sea floor. The upper ends of the tendons and risers pass through openings in the keel or bottom portion of the vessels and are supported vertically by tension means located near the water surface.

When the vessel moves laterally in responseto nvironmental in the region just above the point where the riser is attached to the additional pipes at the sea floor. A current approach to Sthe situation has been to use thicker than nomal pipes.

Disadvantages of this approach are higher manufacturing costs and n-i CASE 5956 lower quality consistency in manufacturing of the thicker pipe.

This presents a need for a better arrangement for handling the bending stresses induced in the riser at the sea floor.

SUMMARY OF THE INVENTION The invention addresses the above need. What is provided is a stress relief joint for use with riser pipe in floating systems wherein a vessel is subject to variable motion caused by wind, currents, and wave action. The riser pipe has one end connectable to the sea floor and an upper portion adapted to pass "p through an opening at the bottom of the vessel. The lower end, Swhich is connectable to other pipes at the sea floor, is provided S with at least two concentric pipes or sleeves around the riser.

The lower ends of the pipes and riser are'welded to a flange.

S The upper end of each of the concentric pipes extends beyond the upper end of the pipe immediately surrounding it. The annulus S between the concentric pipes and the riser is preferably filled with a durable and pliable material. Also, shims may be inserted S in the annulus at the end of each pipe segment.

BRIEF DESCRIPTION OF THE DRAWINGS For a further understanding of the nature and objects of the present invention reference should be had to the following description, taken in conjunction with the accompanying drawings in which like parts: ,are given like reference numerals, and wherein: 25 a Fig:s a schematic view of a floating vessel, sea floor, and riser r ineconnecting the vessel and sea floor.

Fig. 2 is aenlarged detail view of a portion of Fig.. 1 g. an en arge

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i I .i Alr CASE 5956 showing the invention.

Fig. 3 is a longitudinal sectional view of the invention.

DETAILED DESCRIPTION OF THE PREFERRD

-EMBODIMENT

Fig. 1 generally and schematically shows a vessel 20 of spar or tension buoyant tower type with a pipe 22 exiting from its bottom or keel as indicated by numeral 24 and having a suitable connection at 26 to the sea floor 28. Lateral horizontal excursion of the vessel 20 is indicated by its position at Bending stresses occur on the pipe 22 where it exits the vessel at 24 at the keel and at the sea floor connection at 26, the dotted lines 22' e.qggerating such bending.

SFig. 2 illustrates the preferred embodiment of the invention, generally indicated by numeral' 10. Stress relief joint 10 is generally comprised of sleeves or pipes 30, 32 and 15 flange 34.

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1.: As seen in Fig. 3, pipes 30, 32 are received around the riser 22 such that they are concentric with the riser 22. Pipe 30 has an inner diameter larger than the outer diameter of the S riser 22 such that an annulus is formed between the riser 22 and 20 first pipe 30. Pipe 32 has an inner diameter larger than the outer diameter of pipe 30 such that an annulus is formed between pipes 30, 32. The upper end of pipe 30 extends beyond the upper end of pipe 32 immediately surrounding it.

The annulus between the concentric pipes 30, 32 and the riser 22 is preferably filled with a durable and pliable material, indicated by numeral 36. The pliable material 36 helps to insur that the riser 22 and pipes 30, 32 all deflect

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CASE 5956 -4laterally substantially the same amount. A suitable material for the operating environment, such as cement grout, is used as the pliable material 36. The lower ends of the pipes 30, 32 and riser 22 are welded to a flange 34.

Flange 34 is specially formed so as to effectively form short segments of the riser 22 and each pipe 30, 32, respectively indicated by numerals 22A, 30A, and 32A. Each segment extends upwardly from the base of the flange. This feature places the SI weld between the riser, pipes, and flange some distance away from X: 0 the most severe fatigue location, where the riser and pipes meet the base of the flange. It can also be seen that the weld point of the riser 22 and each pipe 30, 32 to the flange 34 are spaced apart longitudinally. This allows the flange to be thicker at "Z the base where needed for handling the loads occurring during normal conditions. The lower end of the annulus formed between each segment 22A, 30A, and 32A may be provided with rounded smooth surface profiles as illustrated to reduce the stress concentration associated with sharp corners in the surface Sprofile.

Shims 38 may be inserted in the annulus at the upper end of I each pipe 30, 32 and rigidly held in place by any suitable means such as welding. Shims 38 may be formed of metal, such as steel, to provide durability and serve as retainers for less rigid materials 36, such as polyurethane.

E 25 Although only two concentric pipes are shown, it should be understood that more may be used, depending upon the application and conditions. It should also be understood that although the 'I CASE 5956 preferred embodiment locates the stress relief joint at the lower end cf the riser, the joint may be positioned at other locations on the riser where necessary to accommodate appreciable bending loads.

The invention provides the advantage of lower costs because it may be formed from standard pipe sections that are readily available or can be manufactured using readily available equipment. The design is more reliable than the very thick i single pipe designs presently in use because it is easier to "0 control the quality of the thinner pipe material. The concentric pipe design also is more reliable because there is redundancy in the pipe sections. The inner pipe normally has the highest hoop stresses since this pipe is required to contain internal and external pressures. However, with the invention, the bending i"t stresses in the inner most pipe are significantly less than in

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the outer pipes. Thus, the inner most critical pipe is less likely to experience fatigue damage.

Because many varying and differing embodiments may be made i within the scope of the inventive concept herein taught and because many modifications may be made in the embodiment herein 4l detailed in accordance with the descriptive requirement of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.

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Claims

1. A stress relief joint for use with a riser in floating systems wherein a vessel is subject to variable motion caused by wind, currents, and wave action, the riser having one end connectable to the sea floor and an upper riser portion adapted to pass through an opening at the bottom of the vessel, the Sstress relief joint comprising: a. a plurality of pipes received around the riser such that an annulus is formed between each pipe and the riser, I said pipes being concentric with the riser and having first and second ends, with the first end of each pipe extending beyond the first end of the immediately surrounding pipe; g and a flange rigidly connected to the riser and the second end of said pipes.

2. The stress relief joint of claim further comprising a pliable material received in-the annulus formed between said pipes and the riser.

3. The stress relief joint of claim 1, further comprising a shim rigidly attached between each of said pipes and the riser at the first end of each of said Plurality of pipes.

4. A stress relief joint for use with a riser in floating systems wherein a vessel is subject to variable motion caused by 2 Lnd, currents, and wave action, the riser having one end connectable to the sea floor and an upper riser portion adapted to pass through an opening at the bottom of the vessel, the T stress relief joint comprising: CASE 5956 -7- a. a plurality of pipes received around the riser such that an annulus is formed between each pipe and the riser, said pipes being concentric with the riser and having first and second ends, with the first end of each pipe extending beyond the first end of the immediately surrounding pipe; b. a flange rigidly connected to the riser and the second end of said pipes; c. a pliable material received in the annulus formed between said pipes andthe riser; and "0 d. a shim rigidly attached between each of said pipes and the riser at the first end of each of said plurality of pipes. is

5. A stress relief joint for use with 'a riser in floating i: systems wherein a vessel is subject to variable motion caused by 15 wind, currents, and wave action, the riser having one end "f connectable to the sea floor and an upper riser portion adapted to pass through an opening at the bottom of the vessel, the stress relief joint comprising: a. a plurality of pipes received around the lower end of the riser such that an annulus is formed between each pipe and the riser, said pipes being concentric with the riser, and the upper end of each of said pipes extending beyond :the upper end of the immediately surrounding pipe; and b. a flange rigidly connected to the lower end of the riser and said pipes.

6. The stress relief joint of claim 5, further comprising a pliable material received in the annulus formed between said i4$ CASE 5956 -8- pipes and the riser.

7. The stress relief joint of claim 5, wherein said plurality of pipes comprises first and second pipes.

8. The stress relief joint of claim 5, further comprising a shim rigidly attached between each of said pipes and the riser at the upper end of each of said plurality of pipes.

9. A stress relief joint for use with a riser in floating systems wherein a vessel is subject to variable motion caused by wind, currents, and wave action, the riser having one end S'IOd connectable to the sea floor and an upper riser portion adapted |to pass through an opening at the bottom of the vessel, the stress relief joint comprising: a. first and second pipes received around the lower end of the riser such that an annulus is formed between each pipe and the riser, said first and second pipes being concentric with the riser, and the upper end of said first pipe A gextending beyond the upper end of the second pipe; i b. a flange rigidly connected to the lower end of the riser and said pipes; and c. a pliable material received in the annulus formed between said pipes and the riser. The stress relief joint of claim 9, further comprising a shim rigidly attached between each of said pipes and the riser at the upper end of each of said plurality of pipes.

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11. A stress reliefL joint for use with a riser in flotin sytem, te jintbeing substantiallY as herein described with reference toth ccmanly~ drwnS DATED this 26th day of October 1998 DEEP OIL~ TECH-NrqLO(GY, INCORPORATED By its Patent Attorneys GRIFFITH HACK