GB2114184A

GB2114184A - Wellhead

Info

Publication number: GB2114184A
Application number: GB08201982A
Authority: GB
Inventors: Jeremy Ian Mockridge
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-01-25
Filing date: 1982-01-25
Publication date: 1983-08-17

Abstract

A wellhead system is disclosed wherein casing hangers 2, 3, 4 are landed independently of one another in a wellhead 1, each forming a metal- metal seal 18 with the wellhead 1. A method and apparatus are disclosed for landing casing hangers 2, 3, 4 at a predetermined pressure. A casing hanger 4 and associated casing string 7 are landed in the well head 1, to contact a bearing surface therein at a predetermined pressure, by a hydraulic jacking tool 30. The casing string is cemented in position whilst the predetermined pressure is maintained and the jacking tool 30 is then removed, allowing the casing hanger 4 to achieve an equilibrium pressure on the bearing surface 8. <IMAGE>

Description

SPECIFICATION Welihead The present invention relates to wellheads, and in particular to subsea wellheads.

Generally, wells for, for example, oil exploration are drilled by drilling concentric bores of decreasing diameter with increasing depth. A casing string, held from above at the wellhead by a casing hanger is located in each bore section to maintain the integrity of the bore wall.

Land based wellheads may consist of a series of spools mounted one on top of the other, each carrying a casing hanger. In subsea wellheads, size is at a premium, as is the number of operations required during any drilling operation.

Accordingly, very compact one-piece wellheads have been developed, combining the spools of the land-based unit into one piece. The first or primary casing hanger to be inserted into the head is held on a taper in the head and subsequent hangers are supported on the preceding hanger, the bearing area of the primary casing hanger taking the full load of all the casing hangers and loads from the associated strings.

Packoffs-a seal and its carrier-- are used to isolate a string from adjacent ones to prevent pressure migration.

In a typical subsea drilling operation, after a borehold of particular diameter has been drilled a casing string is lowered down the borehole, suspended at its top end from the casing hanger.

The hanger is located in the wellhead and the casing cemented to the borehole wall, by pumping a plug of cement down the borehole to return up the outside of the casing string, the packoff is then fixed in position. It can be seen that the hanger will be in tension due to the weight of the casing string, this tension being "cemented in" during the cementing operation. It is probable that some relief of this tension will occur with thermal expansion of the casing string.

The packoff must be located after cementing to allow egress of mud etc. during insertion of the cement. Also it is necessary to provide throughways for the mud through the bearing surfaces. The extreme pressure at the contact between the primary casing hanger and the well head may cause creep over the taper seat, and also will give rise to severe corrosion problems, particularly with sour gas (H2S) wells.

The problem of corrosion has been met in part by the use of expensive steels and surface coatings.

In some prior art devices the packoff body is in compression between casing hangers. If a packoff seal fails then high pressure may build up in the annular cavity around the hanger which may result in the packoff being in a state of "compression and burst", the load bearing capacity of the packoff is reduced.

Casing hangers are subject to compression above the shoulder at which they are located, and tension below from the casing string. If the packoffs are sealing then primarily the hangers would be subject to positive hoop stresses where they are in tension; the most likely mode of failure being tension and collapse due to high annulus pressure from seal leakage.

A first aspect of the present invention provides a wellhead system wherein a casing hanger is hydraulically set in the wellhead to form a metalmetal seal with the wellhead.

In a method of operation of this invention a hanger and casing string will be lowered into contact with the well head, by a hydraulic jacking tool, to a predetermined loading on the headhanger mating surfaces, the string cemented in position, thereby to take weight of the string carried by the jacking tool, and the jacking tool then removed.

To provide for egress of mud etc. during cementing, inspection ports may be provided in the wellhead. These ports will also provide for monitoring of pressure in the annulus outside the casing at the casing hanger, enabling monitoring of gas migration across the seal. It can be seen that the separate packoff seals may be dispensed with.

In a preferred embodiment of the invention "0" ring seals or the like are provided between adjacent hangers or hanger and wellhead, down the wellhead from the metal-metal seal. Also an injection port is provided up from the seal to allow subsequent injection of a sealant where gas migration across the seal is detected.

Another aspect of the invention provides a well head for a subsea wellhead system, the wellhead having a plurality of iocations each for receiving a casing hanger such that casing hangers do not act one upon another.

Yet another aspect of the invention provides a jacking tool for setting a casing hanger in position in a wellhead, comprising means for holding a casing hanger, means for locating the tool at a wellhead, means for adjusting the height of the casing hanger in a wellhead, and means for releasing the casing hanger.

Preferably the tool is hydraulic and there is provided a combination of a wellhead and tool, the tool having stop means for setting a casing hanger at a predetermined height in the wellhead.

When used to set a casing hanger to form a metal-metal seal with the wellhead it is preferred that the stop means allow a predetermined load, say 150,000 Ibs, at the bearing sealing surface.

The present invention further provides a method of assembling a wellhead system, particularly a subsea wellhead system, comprising the steps of lowering a casing hanger, carrying a string, to a wellhead at a position spaced from a bearing surface of the wellhead, lowering the casing hanger on to the bearing surface to a predetermined pressure on the surface, cementing the casing string in position in the borehole, and releasing the tool from the hanger.

The invention will be further described by way of example with reference to the accompanying drawings, in which: Fig. 1 shows a cross-sectional view of a wellhead according to the invention, having three casing hangers and associated strings located therein; Fig. 2 shows a hydraulic jack of the present invention; Fig. 3 shows in part section the hydraulic jack of Fig. 2 in combination with the wellhead of Fig.

1, a casing hanger having been lowered into position.

Fig. 1 shows a sectional view of wellhead having three casing hangers 2, 3, 4 located therein and carrying three casing strings 5, 6, 7, respectively. Primary casing hanger 2 is supported on a taper in the wellhead, forming a primary metal-metal seal with the wellhead at the taper 8. Port 9 enables monitoring of the pressure in the annulus between the wellhead and the lower part 2a of the hanger 9, also port 9 allows egress of mud etc. when the casing string 5 is being cemented to the bore hole wall (not shown). "0" ring seals 10 prevent egress of cement or mud into the taper bearing area 8 during cementing of the string, the mud escaping through monitoring port 9. A secondary seal 11 is provided above the main seal and bearing surface 8.Port 12 is provided for the injection of a sealing compound under pressure if the primary seal at 8 fails, the sealing compound being retained by "O" rings 10 and seal 11. The casing hanger 2 is locked in position by location of circlip 13 in groove 14 in the wellhead wall, annulus 15 being stud bolted to hanger 2 to lock the circlip in position. Casing hangers 3 and 4 are similarly located in the wellhead, on bearing sealing surfaces 1 7, 1 8 respectively, with respective monitoring ports 1 9, 20 sealant injection ports 21,22, and seals 23, 24. "0" ring seals 25 are provided to form a seal between casing hanger 3 and the lower hanger 2, and "0" ring seals 26 from a seal between casing hangers 3 and 4.It can be seen that the casing hangers are held independently of one another in the wellhead, the weight being taken on respective bearing surfaces 8, 1 7, 1 8 of the wellhead, a direct contact seal being formed between each hanger and the wellhead.

Figure 2 shows a jacking tool 30 according to the invention, suitable for locating a casing hanger in a wellhead. The tool comprises two concentric cylinders 31,32; one, 32, slidable axially along the other, 31. Cylinder 32 is located about cylinder 31 slidable in an upper hydraulic chamber 33 and sealed therewith by seals 34.

Fluid is pumped into chamber 33 through pipe 34, causing the cylinder 32 to be lowered relative to cylinder 31; the cylinder being raised by pumping fluid into chamber 39 via pipe 41.

Hydraulic chambers 33, 39 may be annular, or preferably a series of discrete chambers are provided and corresponding piston members, to allow for failure of a seal 34 etc. in a chamber. A stop 35 on cylinder 31 controls the extent of relative downward movement of cylinder 32. The position of stop 35 may be adjustable axially of cylinder 31. Collets 36 are provided on a lower part of cylinder 32 and are adapted to engage with a casing hanger. Annular wedge or wedges 37 is/are movable to force collet 36 radially outwardly to engage with a casing hanger.

Hydraulic fluid is pumped into chamber 38 through pipe 40 to force the collet outwardly, and fluid may be pumped into chamber 39 through 41 to release the wedge from the collet to allow retraction of the collet, fluid being allowed to flow from chamber 38. Wedge 33 seals with walls of chambers 38 and 39 through "0" ring seals 42.

End cap 43, bolted to cylinder 31, slidingly locates the lower end of cylinder 32, seals 44 being provided between the end cap and cylinder 31 and 32.

The landing of a casing hanger at a wellhead will now be described with reference to Fig. 3. In Fig. 3, casing hangers 2 and 3 and associated strings 5 and 6 have already been located in the wellhead 1. Casing string 7 is attached, generally by a screw thread, to casing hanger 4, which in turn is held by the jacking tool 30 via hydraulic collets 36. Collets 36 are shown in engagement with a groove formed between annulus 50 and hanger 4. It is clear that an engagement surface could be provided elsewhere on the hanger. The jacking tool 30 with casing hanger 4 and string 7 is lowered to the wellhead on the end of a pipe string 51. Channels 52 are provided in the tool 30 to allow mud etc. to flow past the tool. The combination may be lowered with a cement plug in the region of the tool to allow for cementing immediately the casing hanger is landed.The jacking tool is lowered with cylinder 32 in its uppermost position, and shoulder 53 engages on a datum level (not shown) provided at the wellhead. In this position the casing hanger is held a small distance above bearing surface 1 8 of the wellhead. Hydraulic fluid is then pumped into chamber 33 through pipe 34 to cylinder 32 to lower the casing hanger 4 onto the bearing surface 1 8. Fluid being allowed to escape from chamber 39 via pipe 41. Stop 35 is set in a predetermined position to allow a maximum bearing force of hanger 4 on bearing surface 18, say 1 50,000 Ibs, sufficient to form a metal-metal seal between hanger 4 and wellhead 1 at the bearing surface. Casing string 7 is then cemented in, mud being allowed to egress through monitoring port 20. After the string 7 has been cemented. Fluid is pumped into chamber 39 to release the wedge 37 from collets 36. The jacking tool 30 is then lifted from the wellhead, the collets 36 being forced back into chamber 39 by virtue of their tapered end faces. It can be seen that a direct contact seal is formed between casing hanger 4 and wellhead 1 at bearing surface 1 8 at a predetermined bearing pressure which may be low enough to avoid stress corrosion in sour gas service. Generally a seal will be a metal-metal seal, though non-metallic surface coatings may interface the bearing surfaces.

The pressure in annulus 54 between casing hangers 3 and 4 is monitored through port 20, when migration of pressure across seal 1 8 is detected sealant may be injected through port 22 to form a fresh seal. At the elevation of the injected sealant the casing hanger is not in tension, hence sealant may be injected at high pressure without risk of a state of tension and collapse.

Each hanger may be set with a jacking tool dimensioned to locate the hanger at the appropriate depth in the wellhead. This will obviate the need for on site adjustments.

Various modifications may be made and it is desired to include all such modifications as properly fall within the scope of the patent granted hereon.

Claims

1. A wellhead system wherein a casing hanger is hydraulically set in the wellhead to form a metal-metal seal with the wellhead.

2. A system as claimed in claim 1, wherein the casing hanger is supported in the welihead on a bearing surface to form the metal-metal seal at said surface, the casing hanger being set on the bearing surface at a predetermined pressure.

3. A method of setting a casing hanger in a wellhead, which comprises setting the casing hanger and an associated casing string to contact the casing hanger with a bearing surface in the wellhead at a predetermined pressure, cementing said casing string in position whilst maintaining said predetermined pressure of said casing hanger on said bearing surface, and thereafter allowing said casing hanger to achieve an equilibrium pressure on the bearing surface.

4. A method as claimed in claim 3, wherein the casing hanger is set in the wellhead by means of an hydraulic jacking tool, the tool being removed to allow the casing hanger to achieve equilibrium pressure on the bearing surface.

5. A method as claimed in claim 3 or 4, wherein a metal-metal seal is formed between the casing hanger and the wellhead.

6. A unified wellhead system comprising a welihead and a plurality of casing hangers, wherein, the casing hangers are landed in the well head independently of one another.

7. A system as claimed in claim 6, wherein a casing hanger is landed on a bearing surface in the wellhead and forms a metal-metal seal with the wellhead at said bearing surface.

8. A system as claimed in claim 7, wherein means is provided for forming a seal to compensate for a breakdown of a said metalmetal seal.

9. A system as claimed in claim 8, wherein the compensating seal is provided above the bearing surface.

10. A system as claimed in any one of claims 6 to 9 wherein means is provided for monitoring pressure in the weilhead radially outwardly of a casing hanger.

11. A jacking tool for setting a casing hanger in a well head, comprising engagement means for engaging a casing hanger, locating means for locating the tool at a wellhead, adjustment means for adjusting the height of an engaged casing hanger relative to a wellhead, and releasing means for disengaging the tool from an engaged casing hanger.

12. A tool as claimed in claim 11, comprising first and second cylinders, said cylinders being substantially coaxial, one slidably located about the other and adjustable relative thereto by hydraulic means, the first cylinder having said locating means for locating the tool at a wellhead and the second cylinder having said engagement means for engaging the casing hanger.

13. A tool as claimed in claim 12, wherein stop means are provided to limit the downward adjustment of a casing hanger relative to the wellhead.

14. A tool as claimed in claims 11,12 or 13, wherein said engagement means comprises an hydraulically movable collet, movable radially outwardly to engage with a casing hanger.

1 5. A wellhead system substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

1 6. A jacking tool for setting a casing hanger in a weilhead, substantially as hereinbefore described with reference to Figs. 2 and 3 of the accompanying drawings.

1 7. A method of setting a casing hanger and associated casing string in a wellhead, substantially as hereinbefore described with reference to Figs.1,2 and 3 of the accompanying drawings.

1 8. The features hereinbefore disclosed, or their equivalents in any novei combination.