GB2176547A

GB2176547A - Retrievable packoff

Info

Publication number: GB2176547A
Application number: GB08609008A
Authority: GB
Inventors: Martin Bern Jansen; John Robert Pettit
Original assignee: Vetco Offshore Industries Inc
Current assignee: Vetco Inc
Priority date: 1985-04-26
Filing date: 1986-04-14
Publication date: 1986-12-31
Also published as: GB8609008D0; CA1255208A; NO172603B; AU5657186A; AU591485B2; GB2176547B; BR8601862A; NO172603C; FR2581125A1; SG14289G; FR2581125B1; NO861590L

Abstract

A packoff (10) with an elastomeric seal ring (16) interposed so between two metallic seal rings (12) and (14) with retrievable characteristics in the form of a relatively thin apertured curved cylindrical band (50) extending between the two metallic seal rings (12) and (14). The metallic band (50) is such that it does not interfere with the deformability of the elastomeric material when the packoff is set, but provides a mechanical connection between the upper and lower metallic rings (12) and (14) thus providing the packoff with retrievable capabilities. <IMAGE>

Description

SPECIFICATION Retrievable packoff BACKGROUND OF THE INVENTION This invention relates to seals and seal assemblies, often referred to as packoffs and packoff assemblies, respectively, for sealing the annular space, often referred to as a gland, between a wellhead casing hanger and the surrounding cylindrical wall of a wellhead. This invention is specifically directed to improving such packoff and packoff assemblies so that packoff may be retrieved from the gland after having been set, i.e., placed in position of sealing engagement between the casing hanger wall and the surrounding cylindrical wall.

For an example of a type of packoff and packoff assembly to which this invention relates, attention is directed to the U.S. Patent No. 3,797,864 of Hynes and Ortolon, and to a pending U.S. Patent Application of Slyker and Pettit, Serial No. 419,270, filed September 17, 1982, now under Allowance. These patents illustrate a packoff comprising an elastomeric ring interposed between two metallic seal rings. Each metallic seal ring has a pair of lips extending toward the elastomeric material so that on compression of the elastomeric material the lips are forced outwardly toward the surrounding walls.

It is recognized that a packoff of the type disclosed in these patents perform as anticipated when set, but often their retrieval is required. To retrieve such packoffs, the packoff assembly is pulled out of the gland, i.e., pulled upwardly back out of the well by pulling on the upper metallic seal ring. This pulling often resulted in a rupturing or tearing of the elastomeric material leaving half, or some part thereof, together with the lower metallic seal ring within the gland. When this happens, when the remaining parts of the packoff cannot easily be retrieved, other sealing arrangements must be made.

Recognizing also that the use of the elastomeric type seal is the best approach to packoff off the gland, numerous attempts have been made in the past to provide a means by which such a packoff can be retrieved.

One such attempt comprised a plurality of vertically disposed bolts extending through the upper and lower metallic seal rings and through the vertical center portion of the elastomeric material with sufficient play between the lower seal ring and the bolts to allow the bolts to move axially relative to the lower metallic seal ring when the packoff was set in the gland. This allowed the elastomeric material to deform and allowed the distance between the metallic seal rings to change. When the packoff was set, however, it was found that, in the areas immediately surrounding the bolts, the elastomeric material differed in its deformation than in the areas between the bolts. This resulted in different sealing capabilities near the areas of the bolts from that of the other areas.Thus, while this prior art attempt solved the problem of retrievability, the sealing capability of the packoff was compromised, i.e., the sealing capabilities around the packoff varied circumferentially around the packoff and provided leak paths.

Another attempt to make such a packoff retrievable was to use wires instead of bolts, but the result was the same.

Another approach to providing a packoff with retrievable capabilities is the use of a solid, cylindrical band disposed vertically midway of the elastomeric material and connected in some suitable manner between an upper and a lower metallic seal ring so that a pull on the upper metallic ring, pulled the lower metallic ring without stress on the elastomeric material. This approach divided the elastomeric material into two separate elastomeric seals: one adjacent the casing hanger and one adjacent the surrounding well housing wall.In this approach, in addition to having variable elastomeric distortion since the elastomeric material was no longer a single body of material, the inner solid metallic ring provided four possible leak paths instead of two in the gland; one leak path adjacent surrounding the wellhead wall, one on each side of the metallic band and one adjacent the sealing wall of the casing hanger. Again, while the solid band approach solved the retrievable problem, it compromised the capabilities of the packoff to seal the gland.

Still another approach for providing a packoff with retrievable capabilities is shown in the U.S. Patent No. 4,324,422, of Rains and Reimert in which a pair of metallic rings with dog-like portions, i.e., overlapping fingers, were imbedded in the elastomeric material and arranged in a lost motion connection between the fingers. The lost motion connection allowed deformation of the elastomeric material when the packoff was set and became interlocking when the packoff was to be retrieved by providing a mechanical connection between the upper and lower metallic seal rings when the fingers engaged one another in an upward pull on the upper seal ring. This approach, however, while again solving the retrieval problem, compromised the sealing capability of the packoff.The metallic sleeves and fingers, together with the lost motion connection, interfered with the deformation characteristics of the elastomeric material. Stated another way, the elastomeric material "saw" the metal and did not deform as a single solid body of elastomeric material would have deformed, when set.

Another type of packoff which is part of the prior art because it has retrievable capabilities is one utilizing upper and lower metallic rings with a deformable metallic band therebetween and surrounded by a plurality of elastomeric rings. The deformable metallic band is in the form of a plurality of V's laid sidewards having the elastomeric rings at the apex of the V's. When set, the ends of the legs and the apices of the V's and, together with the elastomeric rings, engage the gland side walls to form a seal. This packoff, however, relies heavily on the metal-to-metal sealing engagement against the gland walls. The use of several elastomeric rings, instead of one body of elastomeric material with different deformation characteristics, may provide several leakage paths.Nonetheless, a pull on the upper metallic ring will retrieve the packoff since the deformable metallic band is connected to the lower metallic ring. Other disadvantages in this type of packoff are the severe difficulties in coping with even small eccentricities between the casing hanger and wellhead housing. This type of seal also needs a very high amount of force to set, which is supplied by undesirable torque multiplication gears.

Another packoff which has retrievable capabilities is that shown in the U.S. Patent 3,561,527 of Nelson which an elastomeric ring between two upper and lower seal rings included a centrally located, vertically oriented serpentine shaped band imbedded in the elastomeric material. The connection of the serpentine band to both the upper and lower seal rings made the packoff retrievable, but in view of the size and thickness of the serpentine band, like the packoff of the U.S. Patent No.

4,324,422 of Rains and Reimert, supra, the metallic serpentine band interfered with the deformation characteristics of the elastomeric material and reduced the sealing capabilities of the packoff.

It is therefore an object of this invention to provide a packoff similar to the prior art utilizing two metallic seal rings with a ring of elastomeric material therebetween, but with retrieval capabilities which will not interfere or compromise the sealing capabilities of the elastomeric material.

SUMMARY OF THE INVENTION The packoff and packoff assembly of this invention which obtains the foregoing object comprises a packoff with an elastomeric seal ring interposed between two metallic seal rings with means for providing the packoff with retrievable characteristics in the form of a relatively thin curved cylindrical band provided with long, narrow slots which form a plurality of vertical bands extending between the two metallic seal rings. In the embodiment disclosed, the elastomeric ring is thicker than both the metallic rings and the gland to be sealed and when forced into a gland, the gland is sealed by the elastomeric ring. The two metallic seal rings have lips which overlap the elastomeric material and are forced outwardly by the deformation of the elastomeric material.The metallic band is such that it does not interfere with the deformability of the elastomeric material, but provides a mechanical connection between the upper and lower metallic seal rings thus providing the packoff with retrievable capabilities. The vertical bands provide flexibility in a radial direction only and reaction contact surfaces against which the elastomeric material reacts.

It will be apparent to those skilled in the art after a study of the following drawing and Detailed Description that the thin vertical bands which provide flexibility in a radial direction only, provide a tension member function and, due to large circumferential reaction surfaces, allow the deformation of the elastomeric material to drive the deflection of the vertical bands, thus not diminishing the sealing capability of the elastomeric material.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view in elevation of a packoff constructed in accordance with the invention.

Figure 2 is cross-sectional view in elevation of the metallic band or ring for imbedding in the elastomeric material of the packoff, Figure 3 is both a top and sectional view of the band, taken along 3-3 of Figure 2, to more clearly illustrate the details of the band, Figure 4 is a cross-sectional view in elevation of a packoff assembly in unenergized condition shown in a typical casing hanger and surrounding wellhead housing, Figure 5 is a view similar to Figure 2, but with the packoff in a pressure energized or set condition, and Figures 6 and 7 are schematic illustrations of a cross-section in elevation of the packoff with an overlay showing the deformation of the elastomeric material by movement and distortion of the overlay and the location of the metallic band with respect thereto;Figure 6 illustrates the free state of the packoff and Figure 7 illustrates the energized state of the packoff in the gland.

DETAILED DESCRIPTION Figure 1 illustrates a packoff 10 constructed in accordance with this invention and shown to comprise an upper metallic cylindrical seal ring 12, a lower metallic cylindrical seal ring 14 and a ring 16 of elastomeric material interposed therebetween.

The upper metallic seal ring is provided with a pair of integral downwardly extending seal/anti-extrusion lips 20. The lips are tapered with the thickest portions adjacent the main portion of the seal ring and tapering to a relatively thin wall at their lower end.

Similarly, the lower metallic seal ring has upwardly extending tapered seal/anti-extrusion lips 22 of the same contour, in cross section, as those of the upper metallic seal ring.

The elastomeric seal ring 16 extends into the cavities 24 and 26 between the lips of both the upper and lower seal ring and is provided with an outer wall 30 which is coextensive with the outer walls of the seal rings and an inner wall 32 which extends outwardly beyond the inner walls of the metallic seal rings. The inner wall 32 is formed with two tapered portions 34 and 36 immediately adjacent the termination of the upper and lower lips and a flat intermediate portion 40 parallel to the inner surfaces of the metallic seal rings and thus form a bulge in the wall 32 midway between the upper and lower lips.

The upper and lower metallic seal rings 12 and 14 each have vertical slots 42 and 44 located midway the thickness of the metallic seal rings into which the upper and lower section 46 and 48 of a metallic band 50 is positioned and suitably fixed, as by welding, to form a permanent bond to the upper and lower metallic seal rings, respectively. In the cross-section of Figure 1 the metallic band 50 is straight a distance substantially equal to the length of the tips as at 52 and 54, but then curves toward the inner wall as at 56 and 58 to form a bulge 60 with its maximum extent adjacent the central portion of the inner wall 32.

This band 50 gives the packoff its retrievable capabilities by forming a tension member between the upper and lower metallic sealing rings and is illustrated in more detail in Figures 2 and 3.

The left portion of Figure 2 is a crosssectional view similar to the view of Figure 1, but the remainder of the Figure shows the band 50 as having a plurality of spaced apart, relatively narrow, vertical slots 62 which form vertical metallic bands 64 therebetween to allow the elastomeric material to flow therethrough when the elastomeric material is molded into the configuration as shown in Figure 1 and extend essentially the width of the band 50 and into both straight portions 52 and 54 to also form solid bands 66 and 68 of metal at both the top and bottom of the band.The vertical metallic bands 64, on the other hand, give the band 50 flexibility in its middle and in the radial direction as represented by the arrow 70 and, thus, allow the vertical metallic bands to be deflected by the elastomeric material so that the elastomeric material can assume its normal "optimal-forsealing" configuration. This material of the band in one application is a high strength, low alloy ASTM-4130, approximately 0.030 thick.

Figures 4 and 5 illustrate a typical arrangement within a wellhead housing 72 with a casing hanger 74 supported on a profile (not shown). Typically, the casing hanger supports a string of casing, as shown by way of example in the U.S. Patents Nos. 3,492,026 and 3,797,864, as well as in some of the patents, supra. A packoff assembly 76 typically includes the packoff 10 and a packoff drive nut 80 which supports the packoff 10.

The packoff drive nut 80 is shown with internal threads 82 engaging external threads 84 on the upper end of the casing hanger such that rotation of the packoff drive nut 80 will thread the packoff assembly downward into the gland 86 formed between the inner bore wall 90 of the wellhead housing and the sealing surface 92 on the casing hanger. To allow the packoff drive nut 80 to rotate without rotation of the packoff 10, the packoff assembly 80 has a swivel connection 94 and a thrust bearing 96. The swivel connection comprises a split retainer ring 100 mounted in complementary grooves 102 in the packoff drive nut 74 and upper metallic seal ring.

Figure 5 shows the packoff 10 driven into the gland 86 and the elastomeric ring 16 compressed due to a reduced radial thickness of the gland thereby forming a pressure seal in the gland. The packoff 10 is driven further down against the friction force of the gland walls, and the upper and lower lips 20 and 22 tend to part to form seals/anti-extrusion barriers after the lower metallic ring 14 abuts a shoulder 104 located on the casing hanger body 74, and also engages a split ring 106 which is held on the casing hanger into a groove 110 in the wellhead in order to lock the casing hanger 74 in the wellhead. At this time the packoff is considered set.

For a more complete explanation of how the tapered lips 20 and 22 react during the movement of the packoff downwardly into the gland, reference is made to the Slyker and Pettit Patent, supra. This patent also sets out the amount of thickness of the elastomeric ring at the center, i.e., wall 32/40 16 as compared to the thickness of the upper and lower metallic seal rings 12 and 14 and how this improved the sealing capabilities of the packoff. The thickness as stated therein this thickness ranged from 8% to 20% greater than the thickness of the metallic seal rings.

However, the reaction of the elastomeric ring 16 during this downward travel or its final set will be discussed in connection with Figures 6 and 7 to which attention is now directed. It will be apparent that utiiizing the technique discussed in these latter Figures, the natural flow of the elastomeric material, i.e., deformation, is not interfered with by the presence of the band 50 and it is to this end that this invention is directed.

Figures 6 and 7 illustrate the packoff 10 of this invention with Figure 6 corresponding to the packoff 10 as shown in Figures 1 and 4, and Figure 7 corresponding to the packoff 10 as shown in Figure 5. To illustrate the deformation of the elastomeric material of the ring 16 when the packoff is both in its unset and set condition, an overlay 112 has been placed over the elastomeric ring 16 and, thus, in Figure 6 the overlay 112 comprises a gridwork of a plurality of rectangles 114 disposed in rows and columns in cross-sectional view with the hand 50 shown only in phantom to illustrate the manner of which the curvature of the band was determined. In this Figure, the rectangles 114 are uniformly distributed and undistorted.

Figure 7 illustrates the packoff 10 in its set condition, the side wall 32/40 near the casing hanger has been compressed so that the side wall 32/40 is now coextensive with the thickness of the upper and lower metallic sealing rings 12 and 14. This compression is represented by the notation delta X. The reaction force of the walls of the gland 86 is represented by the arrows F.

Also, as shown in Figure 7, as the packoff 10 enters the narrower gland 86, the tapered surface 36 engages the tapered inner wall 116 (Figures 4 and 5) with an initial reaction of the elastomeric material to compress, but since the total volume of material remains the same, the packoff 10 elongates slightly. This is represented by the notation delta Y.

The total compression (deformation) of the packoff as shown in Figure 7 is illustrated by a distortion of the rectangles 116. The rectangles 1 14a within the confines 24 and 26 of the lips 20 and 22 are undistorted, but the rectangles 1 14b intmediately above and below the upper lips 20 and 22 undergo a major transformation. This illustrates the area of maximum twist of the elastomeric material by the distortion of the upper and lower portions of the rectangles, especially those rectangles 1 14b nearest the compressed inner wall 32.

The rectangles 1 14c near the height of the bulge 60, on the other hand, have become thinner, horizontally, and closer together.

Utilizing the gridwork illustrating the elastomeric distortion as shown in Figure 7, the curvature of the band was selected. The curvature is such that the band's natural deflection when it is compressed from the casing hanger side and elongated is similar to the material deformation of the elastomeric material when installed in the gland. Within the confines of the lips, i.e. areas 24 and 26, the band 50 was formed straight, i.e. portions 50 and 52, and immediately curving at 56 and 58 toward the bulge, represented at points 1 and 2 until the height of the bulge 60 reaches a point 3, i.e. maximum extent. Utilizing that distortion of the elastomeric material the curvature of the band was driven into the configuration as represented by Figure 6. Thus, the profile of the band 50 in Figure 6 conforms to the profile of the band 50 in Figures 1, 3 and 4.

From the foregoing it can be seen that the concept of providing a band such as 50 which as nearly as possible conforms to the deformation of the elastomeric material when the packoff is set provides the packoff with retrievable capabilities yet does not interfere with the deformation characteristics of the elastomeric material. In other words, the result of the study was that a band 50 was configured to react to the elastomeric material in such a way that latter's deformation characteristics remain the same. That is to say, the elastomeric material was free to deform, as if there was no metallic band in-between. Nonetheless, when it is decided to retrieve the packoff 10, for whatever reason, the same metallic band 50 provided the necessary mechanical connection between the upper and lower metallic seal rings to make retrievability possible.

Claims

1. A packoff for an annular space comprising: an upper metallic seal ring, a lower metallic seal ring, an elastomeric seal ring interposed between said metallic seal rings, and means in the form of a curved, slotted, metallic band in said elastomeric seal ring and connected to said upper and lower metallic seal rings so that force applied to said upper metallic seal ring will apply a similar force on said lower metallic seal ring to provide the packoff with retrievability capabilities.

2. The packoff as claimed in Claim 1 wherein said elastomeric seal ring has a thickness greater than each of said metallic rings.

3. The packoff as claimed in Claim 2 wherein the thickness of said elastomeric seal ring exceeds that of the metallic seal rings by between 8% and 20%.

4. The packoff as claimed in Claim 3 wherein said greater thickness is located on one side of said packoff.

5. The packoff as claimed in Claim 4 wherein each metallic seal ring has a pair of seal/anti-extrusion lips extending in overlapping relationship with said elastomeric ring.

6. The packoff as claimed in Claim 5 wherein said metallic band in cross-section is coaxial with said metallic seal rings in the area of said overlapping relationship and curves toward said greater thickness.

7. The packoff as claimed in Claim 6 wherein said band is provided with elongated vertical apertures which define thin vertical bands of metallic material which are capable of, and driven to, radial flexure in harmony with the elastomeric material upon the deformation of said elastomeric ring.

8. A packoff assembly for sealing an annular space between concentric pressure containing bodies, such as hanger bodies, one of said bodies having a shoulder, comprising: an upper metallic seal ring, a lower metallic seal ring, an elastomeric seal ring interposed between said metallic seal rings and having a thickness greater than each of said metallic rings and said annular space, pairs of seal/anti-extrusion lips on said metallic seal rings extending in overlapping re lationship with said elastomeric seal ring, compression means for compressing said packoff downwardly against the shoulder and causing movement of said upper and lower rings and causing said lips to set, and means in the form of a metallic band within said elastomeric seal ring for mechanically connecting said upper and lower metallic seal rings whereby a pulling force on said upper metallic seal ring will apply a pulling force on said lower metallic seal ring.

9. The packoff assembly as claimed in Claim 8 wherein said metallic band also assists in translating the compression force applied to said upper metallic ring to said lower metallic ring to enhance the capability of the packoff to enter said annular space.

10. The packoff assembly as claimed in Claim 9 wherein said metallic band in crosssection is curved and slotted.

11. The packoff assembly as claimed in Claim 10 wherein said elastomeric seal ring has a thickness greater than each of said metallic rings.

12. The packoff assembly as claimed in Claim 11 wherein the thickness of said elastomeric seal ring exceeds that of the metallic seal rings by between 8% and 20%.

13. The packoff assembly as claimed in Claim 12 wherein said greater thickness is located on one side of said packoff assembly.

14. The packoff assembly as claimed in Claim 13 wherein said metallic band in crosssection is coaxial with said metallic seal rings in the area of said overlapping relationship and curves toward said greater thickness.

15. The packoff assembly as claimed in Claim 14 wherein said band is provided with elongated vertical apertures which define thin vertical bands of metallic material which deflect in harmony with the deformation of said elastomeric ring.

16. A packoff for an annular space comprising: an upper metallic member, a lower metallic member, an elastomeric seal ring interposed between said metallic members, and means in the form of a curved, slotted, metallic band in said elastomeric seal ring and connected to said upper and lower metallic means so that force applied to said upper metallic member will apply a similar force on said lower metallic member to provide the packoff with retrievability capabilities.

17. A packoff assembly for sealing an annular space between ccncentric pressure containing bodies, such as hanger bodies, one of said bodies having a shoulder, comprising: an upper metallic member, a lower metallic member, an elastomeric seal ring interposed between said metallic members and having a thickness greater than each of said members and said annular space, pairs of seal/anti-extrusion lips on metallic member extending in overlapping relationship with said elastomeric elastomeric seal ring, compression means for compressing said packoff downwardly against the shoulder and causing movement of said upper and lower members, and means in the form of a metallic band within said elastomeric seal ring for mechanically connecting said upper and lower metallic member whereby a force applied to said upper metallic members will apply a force on said lower metallic members.

18. A method of preparing a packoff for sealing cylindrical gland in a well system comprising the steps of: providing a pair of circular metallic elements with an elastomeric seal ring therebetween so as to space said metallic elements apart, providing a circular, curved, slotted, flexible metal band within said elastomeric seal ring and connected to said metallic elements.

19. The method of Claim 18 further including the step of making said elastomeric seal ring wider than said gland and compressing said elastomeric material in said gland.

20. The method of Claim 19 further including the step of providing said metal band with curved reaction surfaces which are configured to react in such a manner that the deformation and sealing characteristics of said elastomeric material and lips of circular metallic elements is either unaffected or favorably influenced.

21. The method of Claim 19 further including the step of applying a force on one of said circular metallic elements which is translated to the other circular metallic elements so that said packoff mav finally penetrate the gland.

22. A packoff for an annular space, substantially as hereinbefore described with reference to the accompanying drawings.

23. A packoff assembly for seating an annular space between concentric pressure containing bores, substantially as hereinbefore described with reference to the accompanying drawings.

24. A method according to Claim 18, substantially as hereinbefore described.