Wellhead assembly

Abstract

A wellhead (14') having a tubular wellhead body having a thick main body portion and a first end of which tapers to a reduced thickness slim portion (26), and a wellhead casing (16) a proximal end of

Description

The present invention relates to a wellhead in particular to a subsea wellhead. A wellhead system is a structure which is installed at the top of an oil or gas well. The casing strings which line the well bore are suspended from the wellhead system, and the wellhead system also provides support for a blowout preventer (BOP) stack during drilling, or for a Christmas tree after the wellbore is completed. The wellhead system thus acts as an interface between surface facilities and the casing strings in the wellbore. Typically, the wellhead body is provided with a rigid extension tube known as a wellhead casing which is welded to its lowermost end, and which extends into the upper end of an outer cylindrical casing generally known as the conductor casing. Cement is injected into the annular space between the radially outward facing surface of the wellhead casing and the conductor casing. Where the wellhead is mounted at the top of a subsea well bore, the drilling system often includes a marine riser which extends upwardly from the top of the BOP stack to the drilling rig. Where the drilling rig is floating, the upper end of the marine riser is generally provided with a slip joint and suspended from the drilling rig using a riser tensioner system, both of which accommodate movement of the drilling rig relative to the riser causes by the swell of the ocean. Despite this, during drilling, the wellhead system is exposed to cyclic forces from the drilling rig, the marine riser, as well as from motion of the BOP stack and pressure variations in the well bore. If sufficiently large and experienced over long periods of time, such forces can, over time, cause fatigue damage to the wellhead system, which could ultimately lead to failure of the wellhead system. The effect of these forces is exacerbated by the presence of a rigid column of cement between the wellhead casing and the conductor casing, as the cement reduces the freedom of the extension to flex or move under the influence of the applied forces. The potential for there to be fatigue failure of the wellhead is therefore increased by the cement column, particularly if it sets at an unsuitable or sub-optimal height. Prior patent applications US5029647 and GB2479602 describe a subsea wellhead in which an elastomeric sleeve is mounted around the wellhead extension before the annular space between the wellhead extension and outer casing is filled with cement. The elastomeric sleeve described in GB2479602 is segmented and is claimed to facilitate flexing of the extension in the presence of the cement column, and thus reduce the likelihood of fatigue damage to the wellhead. It is an object of the present invention to provide alternative ways of improving the fatigue performance of a wellhead system. According to the invention we provide a wellhead having a tubular wellhead body having a thick main body portion and a first end of which tapers to a reduced thickness slim portion, and a wellhead casing a proximal end of which is connected to the free end of the slim portion of wellhead body by means of a weld, wherein the slim portion of the wellhead body is at least 0.5m long. The slim portion may have a further reduced thickness region which is adjacent to the free end. The further reduced thickness region may be at least 15cm long. Embodiments of the invention will now be described with reference to the following figures, of which: FIGURE 1 shows an embodiment of subsea wellhead assembly not in accordance with the invention, FIGURE 2 shows a second embodiment of subsea wellhead assembly not in accordance with the invention, FIGURE 3 shows an embodiment of subsea wellhead assembly having a wellhead according to the invention, FIGURE 4 shows a schematic illustration (not to scale) of a longitudinal crosssection through a portion of a wellhead suitable for use with the invention, FIGURE 5 shows a schematic illustration (not to scale) of a longitudinal crosssection through a portion of an alternative embodiment of wellhead suitable for use with the invention, and FIGURE 6 shows a schematic illustration (not to scale) of a longitudinal crosssection through a portion of a wellhead assembly suitable for use with the invention. Referring now to the figures, there is shown a subsea wellhead assembly 10 comprising a wellhead 12 having a wellhead body 14 and a wellhead casing 16, having a proximal end 16a and a distal end 16b relative to the wellhead body 14. A first end of the wellhead body 14 tapers inwardly or transitions, to a slim portion 26 which carries at its free end a weld preparation profile. The weld preparation profile is secured to the proximal end 16a of the wellhead casing 16 by means of a weld 28. The assembly 10 also comprises a tubular conductor housing 18 which has an inner surface which encloses a main passage. The wellhead body 14 is supported in the main passage of the conductor housing 18 through areas of contact between the inner surface of the conductor housing 18 and the outer surface of the wellhead body 14. In this example, the conductor housing 18 has a lower annular weld preparation profile 20, and, by means of a weld 22, supports a casing 24, which in this embodiment is an outer cylindrical conductor casing 24, that extends downwardly from the conductor housing 18 into a bore hole in the seabed (not shown). The lower end of the conductor housing 18 and the conductor casing 24 thus form an outer casing around the first end of the wellhead body 14 and the wellhead casing 16. Typically, the wellhead casing 16 extends down the outer casing 24 at least as far as the seabed, and preferably somewhat further. The outer casing 24 typically extends from 4m to 200m below the sea bed, and the wellhead casing 16 may extend down below the sea bed to the same or almost the same extent. Where the wellhead casing 16 extends for a significant distance, it will be appreciated that it may be made from a plurality of tubular sections which are joined - possibly by means of a screw thread, or by welding - end to end to form a single wellhead casing 16. If this is the case, the wellhead casing 16 is typically made from a plurality of 12m long sections. There is an annular space 30 between the radially outwardly facing surfaces of the first end of the wellhead body 14 and wellhead casing 16 and the radially inwardly facing surfaces of the conductor casing 24 and conductor housing 18. The conductor housing 18 is provided with lateral vent ports 32 which extend laterally through the conductor housing 18 to the annular space 30, thus provided for fluid communication from the exterior of the conductor casing 24 into the annular space 30. In prior art systems, a cement liner is formed in the annular space 30 by pumping cement slurry down the wellbore so that it rises up into the annular space 30. Generally, the cement is pumped up to at least the first end of the wellhead body 14, and possibly as far as the lateral vent ports 32. In this case, the cement liner also occupies a part of the annular space between the conductor housing 18 and the first end of the wellhead body 14. This need not be the case, however, and the cement may not be pumped all the way up to the conductor housing 18, and may simply occupy the space between the conductor casing 24 and the first end of the wellhead body 14, and the wellhead casing 16. Similarly the cement may not be pumped all the way up to the first end of the wellhead body 14, and may simply occupy the space between the conductor casing 24 and the wellhead casing 16. The fluid in the annular space 30 (which is mainly seawater but which may also include and chemicals and debris from the process of drilling the well bore or installing the well head) which is displaced by the cement slurry is pushed out of the annular space 30 through the vent ports 32. In accordance with the present invention, a fluid sleeve 34 is provided, which extends from the outer surface of the wellhead casing 16 and / or the first end of the wellhead body 14 into the annular space 30. By virtue of the provision of the fluid sleeve 34, the cement liner is prevented from directly contacting the wellhead casing 16 and / or wellhead body 14. The fluid sleeve 34 may therefore provide the wellhead 12 with more freedom of movement when the wellhead 12 is externally loaded, and thus may reduce fatigue damage to the wellhead 12 during use. It should be appreciated that the fluid sleeve 34 could surround just the first end of the wellhead body 14, and just the wellhead casing 16 or a portion thereof. Alternatively, it could surround the first end of the wellhead body 14 and either the entire length of the wellhead casing 16 or a proportion of the length of the wellhead casing 16 from the first end of the wellhead body 14 towards distal end 16b. In this embodiment, the fluid sleeve 34 surrounds the entire circumference of outer surface of the first end of the wellhead body and the entire circumference of outer surface of the proximal end 16a of the wellhead casing 16. Advantageously, the fluid sleeve 34 surrounds at least the 20m of the wellhead casing 16 adjacent the wellhead body 14, but could surround up to 50m of the length of the wellhead casing 16. Where the wellhead casing 16 is made up of a plurality of tubular sections, advantageously, the fluid sleeve 34 surrounds the section at the proximal end 16a of the wellhead casing 16, and one or more of the adjacent sections. Various methods of providing the fluid sleeve are proposed. In some embodiments, the wellhead assembly 10 further comprises a retainer which retains the fluid sleeve around the wellhead body 14 / wellhead casing 16 when cement is injected into the annular space 30 to form the cement liner. In the embodiment illustrated in Figure 1, the retainer comprises a tubular retainer sleeve 36. In this embodiment, a first end 36a of the retainer sleeve 36 is secured to and sealed against the slim portion 26 of the wellhead body 14, but it could equally be secured to and sealed against another portion of the outer surface of the first end of the wellhead body 14. The retainer sleeve 36 may extend all the way to the distal end 16b of the wellhead casing 16, but in this embodiment, it does not. In this embodiment, the retainer sleeve 36 extends towards the distal end 16b of the wellhead casing 16 to surround a substantial proportion of the wellhead casing 16. Where the wellhead casing 16 is made from a plurality of tubular sections, the retainer sleeve 36 extends over at least two of the sections at the proximal end 1 6a of the wellhead casing 16. The retainer sleeve 36 is spaced from the outer surface of the first end of the wellhead body 14 and wellhead casing 16 so that the fluid sleeve fills the annular space between the outer surface of the wellhead casing 16 and first end of the wellhead body 14 and the retainer sleeve 36. In this embodiment, the retainer sleeve 36 has an exhaust passage (not shown) which extends through the first end 36a of the retainer sleeve 36 from the space between the outer surface of the wellhead body 14 and the retainer sleeve 36 to the exterior of the retainer sleeve 36, and an exhaust valve (not shown) which is moveable between a closed position in which it substantially prevents flow of fluid along the exhaust passage, and an open position in which flow of fluid along the exhaust passage is permitted. In a preferred embodiment, the exhaust valve is ROV operated. In this case, when the conductor housing 18 is mounted on an underwater wellbore, the space between the retainer sleeve 36 and the wellhead casing 16 and wellhead body 14 fills with water when the wellhead body 14 is mounted in the conductor housing 18. Displacement of the air in the space between the retainer sleeve 36 and the wellhead casing 16 with water is achieved by ensuring that the exhaust valve is in the open position whilst lowering the well head 12 into the conductor housing 18. The exhaust valve can be closed once the well head 12 is mounted on the conductor housing 18, or before the well head 12 is mounted on the conductor housing 18, provided it is closed after the well head 12 is completely submerged and the space between the retainer sleeve 36 and the wellhead casing 16 and wellhead body 14 has filled with water. An ROV is then used to move the exhaust valve to the closed position, before cement slurry is injected into the annular space between the inner surface of the conductor casing 24 and the retainer sleeve 36. As the cement slurry fills the annular space between the retainer sleeve 36 and the conductor casing 24, the water in this space is pushed out of the lateral vent ports 32. Ideally, the injection of cement slurry is stopped before the cement slurry moves past first end 36a of the retainer sleeve 36, to ensure that the cement does not come into direct contact with the first end of the wellhead body 14. The cement need not be pumped all the way up to the conductor housing 18, and may simply occupy the space between the conductor casing 24 and the retainer sleeve 36. The retainer sleeve 36 traps the water in the space between the retainer sleeve 36 and the outer surface of the first end of the wellhead body 14 and wellhead casing 16 and prevents it from being displaced by cement slurry. The cement slurry is then allowed to harden to form a rigid cement liner between the retainer sleeve 36 and the conductor casing 24, whilst the annular space between the retainer sleeve 36 and the wellhead casing 16 remains filled water, and therefore forms the fluid sleeve. It will be appreciated that, depending on how far up the retainer sleeve 36 the cement slurry is pumped, there may be fluid sleeve between the cement liner and the wellhead casing 26 only, or between the cement liner and both the wellhead casing 16 and the first end of the wellhead body 14. In an alternative embodiment, illustrated in Figure 2, the retainer comprises a retainer sleeve 38 having an inner surface, substantially all of which engages with the outer surface of the wellhead casing 16 and the first end of the wellhead body 14. The retainer sleeve 38 engages with the entire circumference of the wellhead casing 16, and may extend all the way to the distal end 16b of the wellhead casing 16 or towards the distal end 16b of the wellhead casing 16 to surround a substantial proportion of the wellhead casing 16. Where the wellhead casing 16 is made up of a plurality of tubular sections, the retainer sleeve 38 advantageously covers the section at the proximal end 16a of the wellhead casing 16, and one or more of the adjacent sections. In this case, the retainer sleeve 38 is made from an absorbent material, such as a gel, an open cell foam, or a hygroscopic material. Where the retainer sleeve 38 has adequate mechanical integrity, for example if it is made from a foam, it may be secured to the wellhead casing 16 using an adhesive. Where the retainer sleeve 38 lacks the mechanical integrity to be secured and remain in position on the wellhead casing using an adhesive, for example where a gel or hygroscopic material are used, the retainer sleeve 38 may be provided by using a porous tube to contain the gel, foam or hygroscopic material. The porous retainer tube may be made from a fabric or fine mesh and may be secured to the wellhead casing 16 using circumferential fabric straps located at the opposite ends of the porous retainer tube. The retainer sleeve 38 is secured to the outer surface of the wellhead casing 16 in its dry form, and therefore absorbs or takes up water as the wellhead 12 is lowered into the sea (during which process it may expand, depending on the material used), and retains the water in the fluid sleeve before cement slurry is injected into the annular space 30 as described above in relation to Figure 1. The retainer sleeve 38 could, for example, be made from a super-absorbent polymer, organic starches, cotton, or a gel. For example, it could be made from sodium polyacrylate, or another super-absorbent polymer that forms a hydrogel when exposed to water or humidity. Other possible materials include Carboxyalkyl cellulose and Cereal-based Polysaccharides. Where the retainer sleeve 38 is formed from a granular material, the granular material may be held around the wellhead casing 16 by means of a flexible, permeable sleeve, which surrounds the outer surface of the wellhead casing 16 and is secured to the wellhead casing 16 and I or first end of the wellhead body 14 at its ends, the granular material being retained in the space between the outer surface of the wellhead casing 16 and the permeable sleeve. In a further alternative embodiment, the fluid sleeve is created by placing a soluble sleeve around the wellhead casing 16 and I or first end of the wellhead body 14 before the wellhead 12 is lowered into the water. The soluble sleeve could be water soluble, but dissolve relatively slowly, so that the soluble sleeve is retained whilst the wellhead is lowered through the water to a subsea wellbore, and for long enough to be present whilst the cement slurry is injected into the annular space 30, but to dissolve away after the cement has completely or partially cured and hardened. An injection line may be provided to facilitate the injection of a solvent to the soluble sleeve after the cement slurry has been injected and allowed to set, either to speed up the dissolution of a water soluble sleeve, or to dissolve a non-water soluble sleeve. The solvent would then form all or part of the fluid in the resulting fluid sleeve. A water soluble sleeve could be made from a water soluble polymer such as polyvinyl acrylate, a carbohydrate such as sugar or water soluble mineral such as salt. A non-water soluble sleeve could be made from polylactic acid, and dissolved using ethylacetate. As a further alternative, rather than being soluble and dissolved by a solvent, the sleeve may be made from a material which reacts with an injected reactant to transform from a solid sleeve to a fluid. The injection of the solvent or reactant may take place once the cement has fully or partially cured and hardened. It would also be possible to choose the melting temperature of the sleeve so that it remains solid at sea temperatures and during injection of the cement slurry, but melts at the normal operating temperature of the wellhead. In a further alternative embodiment, no retainer sleeve is provided. Instead, a unidirectional valve or seal is provided between the wellhead casing 16 and the conductor casing 24 to control flow of fluid into the annular space 30 around the proximal end 16a of the wellhead casing 16. The uni-directional valve I seal is configured to allow flow of cement slurry from the annular space 30 below the valve / seal into the annular space 30 above the seal 40, but to prevent flow of fluid in the opposite direction. During cementing of the wellhead system, cement slurry passes the uni-directional valve I seal and fills the annular space 30 above the uni-directional valve / seal. When the cement starts flowing out of the vent ports 32, and the pumping of cement slurry is stopped. At this point, and before the cement has set, a displacement fluid, which could be water, is pumped into the annular space 30 at the lower end of the wellhead body 14 to displace the cement and push it out of the annular space 30 via the vent ports 32. The displacement of the cement could be assisted by pressurising the displacement fluid, and / or using a fluid with a density which is greater than that of the cement. The displacement fluid could be pumped into the annular space via a port or ports extending through the wellhead body 14 or the conductor housing 18, and could be arranged to direct the fluid against the outer surface of the first end of the wellhead body 14 or wellhead casing 16 to wash the cement slurry away from around the wellhead body 14 / wellhead casing 16 to form the fluid sleeve. A wellhead body 14’ according to the invention is illustrated in Figure 3. In this embodiment, the length of the slim portion 26 of the wellhead body 14’ is increased, thus increasing the separation of the uppermost end of the wellhead body 14’ and the weld 28 between the wellhead body 14’ and the proximal end 16a of the wellhead casing 16. As a result, the weld 28 by means of which the proximal end 16a of the wellhead casing 16 is secured to the wellhead body 14’ is further below the centre of rotation of the wellhead, and this may assist in reducing the forces on the weld 28 due to external loading on the wellhead body 14’. Advantageously, the length of the slim portion 26 is at least 0.5m. The length of the slim portion 26 may be at least one third of the total length of the wellhead body 14’. In one embodiment, the wellhead body 14’ is around 4m in length, and the slim portion 26 is around 1,6m in length. Although not essential, in this embodiment, the thickness of the slim portion 26 is further decreased in a region 26’ adjacent to the very end of the wellhead body 14. This reduced thickness portion 26’ will be more flexible than the remainder of the wellhead body 14’, and also may assist in reducing the forces acting on the weld 28. Advantageously, the reduced thickness portion 26’ is at least 15cm long. The length of the reduced thickness portion 26’ may be at least half of the length of the slim portion 26. In one embodiment, the length of the wellhead is around 4m, the length of the slim portion 26 is around 1,6m, and the length of the reduced thickness portion 26’ is around 0.9m. The fluid sleeve may be applied to this configuration of wellhead body 14’ in any of the ways described above. The fluid sleeve could surround just the first end of the wellhead body 14 - so could extend from the weld 28, or above the weld 28 and around only the slim portion 26 of the wellhead body 14’. In this case, the cement slurry would be pumped up to a level above the weld 28 so that the fluid sleeve separates the cement liner from the first end of the wellhead body 14’. Alternatively, the fluid sleeve could surround the first end of the wellhead body 14’ and the proximal end 16a of the wellhead casing - so extend from above the weld 28 to below the weld 28, as illustrated in Figure 3. In this case, the cement slurry could be pumped to a level below the weld 28 so that the fluid sleeve separates the cement liner from the wellhead casing 16 only, or to a level above the weld 28 so that the fluid sleeve separates the cement liner from both the first end of the wellhead body 14’ and the wellhead casing 16. In all cases it would be possible to provide the fluid sleeve around the wellhead casing 16 only, but then it would be necessary to ensure that the cement slurry was not pumped to a level above the fluid sleeve, in order to prevent the cement liner from filling the space between the outer surface of the first end of the wellhead body 14, 14’ and the conductor casing 24 or conductor housing 18. In a further alternative embodiment, a solid reinforcing sleeve 40 is mounted around the radially outwardly facing surface of the free end of the slim portion 26 of the wellhead body 14, 14’ and the proximal end 16a of the wellhead casing 16, so that the sleeve 40 covers the weld 28, as illustrated in Figure 4. Unlike the elastomeric sleeves described in US 5029847 and GB2479602, the reinforcing sleeve 40 is rigid, and designed to prevent flexing of the slim portion 26 of the wellhead body 14, 14’ and proximal end 16a of the wellhead casing 16 at and in the region of the weld 28. The sleeve 40 therefore supports the welded connection between the wellhead body 14, 14’ and the wellhead casing 16, and when forces are applied to the wellhead body 14, 14’, the sleeve 40 may divert forces away from the weld 28, thus increasing its fatigue life and that of the entire wellhead system 10. Such a reinforcing sleeve could equally be applied to the configuration of wellhead 14’ illustrated in Figure 1 or 3. The reinforcing sleeve 40 could be made from a metal such as steel or from a composite material such as REINFORCEKIT®, which is made from Kevlar bonded using epoxy resin. In the latter case, strips of the composite are wrapped around the weld 28 like a bandage. In the case of the metal sleeve, this could be made as a tubular part which is slid onto the wellhead casing 16 so that it surrounds the radially outwardly facing surface of the proximal end 16a of the wellhead casing 16, before the wellhead casing 16 is welded onto the wellhead body 14, 14’. Once the process of welding the slim portion 26, 26’ of the wellhead body 14, 14’ to the proximal end 16a of the wellhead casing 16 is completed, the sleeve 40 can be slid along the wellhead casing 16 to cover the weld 28. The reinforcing sleeve 40 may then be welded in place, for example by means of a fillet weld 41 at each end of the sleeve 40, or bonded to the wellhead body 14, 14’ and / or the wellhead casing 16. Alternatively, the reinforcing sleeve 40 could equally be made from a tubular part with a longitudinal split, the edges of which are welded together once the reinforcing sleeve 40 is in place over the weld 28, so that the reinforcing sleeve 40 shrinks around the slim portion 26, 26’ of the wellhead body 14, 14’ and proximal end 16a of the wellhead casing 16 after the weld has cooled. As a further alternative, the sleeve 40 could be formed in multiple parts which are mounted around the weld 28, and themselves welded together. In these cases, the sleeve 40 could also be welded or bonded to the wellhead body 14,14 ‘ and / or wellhead casing 16, but this may not be necessary if shrinkage of the sleeve 40 after the welds have cooled is sufficient for the sleeve 40 to be retained in place in an interference fit with the wellhead body 14, 14’ and wellhead casing 16. A back strip 42 may be provided between the sleeve 40 and the wellhead body 14, 14’ / wellhead casing 16 and the sleeve 40 to space the radially inwardly facing surface of the sleeve 40 from the wellhead body 14, 14’ / wellhead casing 16, as illustrated in Figure 5. In a further alternative embodiment, one or more annular inserts 44 made from a hygroscopic material could be mounted around the wellhead body 14, 14’. The hygroscopic material could be nylon, for example. The inserts 44 could be annular and positioned in circumferential grooves around the outer diameter of the wellhead body 14,14’ at the areas of contact between the wellhead body 14, 14’ and the conductor housing 18, as illustrated in Figure 6. Ideally, each insert is shaped such that it does not protrude any further in a radial direction than the surrounding portions of the wellhead body 14, 14’, to allow the wellhead 12 to be mounted in the conductor housing 18 in the conventional manner. The or each hygroscopic insert 44 may have a coating which prevents it from taking on moisture from the environment, for example a grease coating. Due to the tight fit between the radially inwardly facing surface of the conductor housing 18 and the wellhead body 14, 14’, this coating would be scraped off as the wellhead 12 is landed in the conductor housing 18, thus exposing the insert 44 to the surrounding seawater. Each insert 44 then absorbs water, and, as it does so, swells, thus closing any gaps between the wellhead body 14, 14’ and the conductor housing 18, and potentially even preloading these two parts. This could assist in reducing wellhead deflection, and hence the stresses imposed on the weld 28 from forces, in particular bending moments, applied to the wellhead 12. Again, this embodiment of the invention could be applied to the configuration of wellhead 12 illustrated in either Figures 1 or 3, and could be used in conjunction with 5 the fluid sleeve, or solid reinforcing sleeve 44 described above.

Claims

1. A wellhead having a tubular wellhead body having a thick main body portion and a first end of which tapers to a reduced thickness slim portion, and a wellhead casing a proximal end of which is connected to the free end of the5 slim portion of wellhead body by means of a weld, wherein the slim portion ofthe wellhead body is at least 0.5m long.

2. A wellhead according to claim 1 wherein the slim portion has a further reduced thickness region which is adjacent to the free end.

3. A wellhead according to claim 2 wherein the further reduced thickness region io is at least 15cm long.

4. A wellhead according to any preceding claim wherein the slim portion is at least one third of the total length of the wellhead body.

5. A wellhead assembly having a wellhead according to any preceding claim and a tubular conductor housing, the wellhead body being supported by the15 conductor housing by means of areas of contact between the outer surface ofthe thick main body portion of the wellhead body and the inner surface of the conductor housing.

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