GB2196670A - Suspending casing from wellheads - Google Patents

Abstract

A wellhead 13 is secured in position on surface casing 14 and a casing 9 is suspended from a casing hanger assembly 10 within the bore 12 of the wellhead 13. Tension is applied to the casing 9 by lifting the hanger assembly 10 and the casing is kept under tension by supporting it by a seat shoulder 23 extended from the wellhead to engage one of a plurality of landing shoulders 19 on the hanger assembly. The assembly may comprise a plurality of sections 15, 16, with one or more landing shoulders 19 on each section. <IMAGE>

Description

SPECIFICATION Lining shafts Shafts drilled through the ground, for example for the extraction of minerals, require lining by a casing. High temperatures may be encountered, due to the depth underground or presence of hot liquids for example, and the casing may therefore be subject to thermal expansion which may cause buckling of the casing if it is fixed to the shaft wall at two or more points along its length.

This problem can be overcome by applying tension to lengths of casing at the time of installation which causes an increase in length greater than that likely to be encountered due to thermal expansion, so that joints of the casing will not buckle. Some casings have been suspended from a wellhead by a slip type mechanism which lands on an internal load shoulder of the wellhead and wedges directly on the outside of the casing body, the wedge acting on the casing by compression, possibly with the aid of serrated jaws for example which are subject to slip. If slip occurs, several attempts to make the wedge grip have to be made. Furthermore the installation of these slip type suspensions require prior removal of other apparatus such- as blow-out preventers (BOP).

With the development of subsea wellhead, the concept of monobloc wellhead and positive hangers was introduced, i.e. the casing is connected to the bottom of a casing hanger which includes a positive landing shoulder, therefore avoiding the slip type mechanism and its associated high compressive hoop stresses in the body of the pipe at the suspension level. An annular seat is remotely installed between the body of the casing hanger and the internal wellhead housing. The positive hanger system -offers the advantages of avoiding BOP disconnection with the direct consequence of performing the casing hanger cementing and sealing off under BOP protection. Since no casing cutting is necessary, the spools can be permanently connected together into a unitized wellhead body.

Surface wellheads have also adopted similar positive hanger/ unitized wellhead configuration, but when tensioning after cementing is mandatory, the use of existing positive hangers consisting of a single landing shoulder has not been practical since it is difficult to calculate the position required for the positive hanger portion on the casing assembly in advance. The slip type suspension seemed to be the only solution to this problem since it could be applied to various positions along the hanger assembly. The present invention provides an improved positive hanger assembly designed to overcome this problem.

According to the invention there is provided apparatus for suspending a casing within a shaft comprising a casing hanger assembly from which said casing can be suspended and having a plurality of separate landing shoulders located at different elevations, a wellhead and a retractable seat shoulder located for extension into the bore of said wellhead to support said assembly by engaging one of said landing shoulders. With this arrangement, the tensioned casing can be supported by the extended seat shoulder engaging the landing shoulder nearer or nearest to the seat shoulder, so that positioning of the shoulders by calculation in advance is not so critical.

Examples of the invention will now be described with reference to the accompanying drawings, in which: FIGS. 1 to 3 are vertical sections of a casing hanger and seat mechanism in various stages of assembly, FIG. 4 is a horizontal section on line 4-4 of FIG. 3 with the second casing hanger removed, and FIG. 5 is a vertical section on line 5-5 of FIG. 4.

In general terms each form of the apparatus illustrated in the drawings includes a casing hanger assembly 10 with multiple load shoulders 19, a seating mechanism 11 in the longitudinal bore 12 of a wellhead housing 13, for movement between a non-tensioned position (FIG. 1) and a tensioned position (FIG. 2). The wellhead 13, which is secured on top of a surface casing 14, is preferably of the unitized type that permits landing and sealing off more than one casing (the casings being of different diameters and coaxial) in a single wellhead housing.

The multiple load shoulder casing hanger 10 includes a threaded together stack of hanger sections 15 and 16. There may be more than one hanger section 15. The bottom hanger section 16 has the same profile on the top 17 as the hanger section 15 but with a regular casing connection 18 at the bottom. Each of the casing hanger sections 15 and 16 includes at least one landing shoulder 19, and two such shoulders are illustrated in the drawings.

The landing shoulders 19 each has a radial annular upper shoulder surface 1 9a projecting out from the outside diameters 15a or 1 6a of hanger sections 1 5 or 16, a cylindrical outer wall surface- 19b, and a frusto-conical load shoulder surface 19c extending downwardly and inwardly from the outer wall surface 19b.

The landing shoulders 19 are partially slotted to provide flow-by path for fluid during casing lowering and cementing, that is slots are formed in the annular shoulder at spaced positions around the casing hanger. The multiple load shoulder casing hanger assembly 10 is supported below a running tool 20 which features the bottom profile 21 of hanger sections 1 5 with a regular casing connection 22 at the top.

The seating mechanism 11 comprises four arcuate seat segments 23 retracting in respec tive grooves 24 in the wellhead bore 12, each segment being moved by an actuating screw 25 between its retracted and active positions.

The groove 24 has spaced radial annular upper and lower surfaces 24a and 24c projecting out from the wellhead bore 12, between which the seat segment 23 is disposed, with its upper and lower surfaces 23a and 23c in contact with the upper and lower surfaces 24a and 24c. A cylindrical retaining wall 24b extends between the outer edges of the surfaces 24a and 24c through which a radial hole 26 extends to the outside of the housing 13, one hole 26 per 90" around the housing. The head of each segment 23 has a frusto-conical seat surface 23d to co-operate with the frusto-conical load shoulder surface 19c of the casing hanger sections 15 or 16 and a cylindrical wall 23e extending downwardly from the inner edge of segment seat surface 23d.

An actuating screw 25 lies in the hole 26, having a T-shaped head which engages a recess 23f in the rear of the segment to move the head radially of the bore 12 as the screw is rotated in either direction. A threaded portion 25a of the screw 25 is mounted in an internally threaded portion 26a of hole 26.

The outer end 26c of each of the holes 26 is increased in diameter and tapped to receive a packing gland 27 which compresses a sealing element 28 between the actuating screw shaft 25b and the non-threaded portion 26b of the hole 26.

As an alternative to the four separate segments, a compressible split ring 40 can be used as a retractable seat shoulder as shown in FIG. 3. The actuating screws 41 only push in contact with the back of the ring 40a, which then springs back to its rest, retracted position.

Each of the actuating screws 25 is movable between a retracted position (FIG. 1) with walls 23e spaced from the wellhead bore 12 of the wellhead housing 13, and an active position (FIG. 2) with the outer edge of the frusto-conical segment seat surface 23d in line with the wellhead bore 12 of the wellhead housing 13. Once in the retracted position, the actuating screws 25 are inhibited from further retraction by the contact of the rear surface 23b of the segment 23 with the cylindrical retaining wall 24b. When in the active position the screw 25 is limited against further travel into the wellhead by a stop spring 30 that bears against the outer surface 29a of a packing gland cover 29.

When the length of casings has been lowered into the shaft, cement is pumped down the centre of the casings and passes around the bottom of the casings and a distance up the annular passage between the casings and the shaft itself (or any outer casings which may be present). The cement sets, anchoring the lower casings in position, but leaves a number of casings uncemented above the cement level. It is these uncemented casings which are to be tensioned. A lifting force is applied to the running tool 20 and the casing hangers will rise past the seating mechanism on the wellhead. Since the extension of the casing may be uncertain, the plurality of load shoulders is provided on the casing hangers, so that the tension on the casings need only be relaxed a small amount after the seating mechanism has been extended before one load shoulder will engage it.

With reference to FIG. 2, after the first casing is cemented and then after the cement has set, tensioned and landed permanently on the seat segments 23, the running tool 20 and one hanger section 15 is disconnected and retrieved. An annular packoff 31 can then be installed between the hanger 15 and the wellhead housing 13 and locked in place by the set screws 32. Drilling for the second casing string of smaller diameter 43 is begun.

When there is a double shoulder 19 on hanger section 15 or 16, as illustrated, according to the fact that the active hanger 33 is hung from the top 1 9T or bottom shoulder 19B, a short or long packoff is used. FIG. 2 illustrates hanging from the top shoulder 19T and the use of a short packoff 31.

With reference to FIGS. 2 and 3, when drilling for the second casing 43 is completed and prior of running it (ie placing the casings in the drilled shaft), the second casing support 44 is run, orientated and landed.

With reference to FIG. 4, the casing support 44 comprises a casing support body 45 and four arcuate segments 46 sliding inside four windows 47 having a radial axis, cut into the casing support body 45. The casing support body 45 has four vertical by-pass recesses 48 for fluids to by-pass the body in a vertical direction and retains each of the four arcuate segments 46 with two vertical screws 49 with their heads sliding in milled recesses 50.

T-shaped recesses 52 are formed in the rear of the segments 46 and the inner ends 56 of the actuating screws are flanged to fit the recesses. To allow installation of the casing support 44 after the first casing is run, four T-shaped slots 51 extend the full height of the bottom half of the casing support body 45 and line up with the T-shaped recesses 52 cut on the arcuate segments 46 when they are in their retracted position. When casing support 44 is landed on top of the actuation screw ends 56, it is rotated until the slots 51 and recesses 52 are aligned with the screw ends 56, so that it can drop down. The actuation screws 55 are then screwed inwards to move the segments 46 inwards and lock down the casing support 44 in position. The top of the casing support body 45 is formed with a threaded male running profile 53 (FIG.

2) similar to the packoff running preparation 54.

With reference to FIG. 3, after the second casing 43 has been cemented and the cement has set and after the casing 43 has been tensioned and landed permanently the hanger section 37 on the seat segments 46, an annular-packoff 38 can then be installed between the hanger 37 and the wellhead housing 13 and locked in place by the set screws 39.

Claims (7)

1. Apparatus for suspending a casing within a shaft comprising a casing hanger assembly from which said casing can be suspended and having a plurality of separate landing shoulders located at different elevations, a wellhead and a retractable seat shoulder located for extension into the bore of said wellhead to support said assembly by engaging one of said landing shoulders.

2. The apparatus of Claim 1, wherein the assembly comprises a plurality of disconnectable sections, each of them having at least one landing shoulder.

3. The apparatus of Claim 1 or 2, wherein the retractable seat shoulder is a ring compressible to extend into said bore.

4. The apparatus of Claim 1 or 2, wherein the retractable seat shoulder comprises at least two arcuate seat segments.

5. The apparatus of any one preceding claim wherein the retractable seat shoulder is carried by a separate casing support ring.

6. The apparatus of Claim 4 and Claim 5 wherein the seat segments are formed with Tshaped recesses at their rear end, the casing support ring is formed with at least two guides radial of the bore and wherein at least two T-shaped slots extend the full height of the bottom half of the casing support body and line up with the T-shaped recesses when the arcuate segments are in their retracted position.

7. Suspension apparatus substantially as herein described with reference to and as illustrated in the accompanying drawings.