Wellhead assembly

Abstract

A wellhead assembly (10) comprising a wellhead (12) having a tubular wellhead body (14, 14') having an outer surface, a wellhead casing (16) extending from a first end of the wellhead body (14, 14'), the wellhead casing (16) having an outer surface and a proximal end (16a) and a distal end (16b) relative to the wellhead body (14, 14'), and the wellhead assembly comprising a tubular outer casing (24) having an inner surface, there being an annular space (30) between the inner surface of the outer casing (24) and the outer surface of the first end of the wellhead body (14, 14') and the outer surface of the wellhead casing (16), wherein the system further comprises a solid cement liner extending from the inner surface of the outer casing (24) into the annular space (30), and a fluid sleeve extending from the outer surface of one or both of the first end of the wellhead body (14, 14') and the wellhead casing (16) into the annular space (30).

Description

Technical Field

[0001] This invention relates to wellheads, particularly subsea wellheads. Background Technology

[0002] A wellhead system is a structure installed at the top of an oil or gas well. The casing string, which is lined within the wellbore, is suspended from the wellhead system, and the wellhead system also provides support for the blowout preventer (BOP) assembly during drilling or the production tree after wellbore completion. Therefore, the wellhead system acts as the interface between the surface facilities and the casing string within the wellbore.

[0003] Typically, the wellhead body is fitted with a rigid extension tube called the wellhead casing, which is welded to the lowest end of the wellhead body and extends into the upper end of the outer cylindrical casing, usually called the guide casing. Cement is injected into the annular space between the radially outward-facing surface of the wellhead casing and the guide casing.

[0004] When the wellhead is installed at the top of the subsea wellbore, the drilling system typically includes a marine riser extending upwards from the top of the BOP assembly to the drilling rig. In the case of a floating rig, the upper end of the marine riser is usually fitted with a sliding joint and suspended from the rig using a riser tensioning system, both of which accommodate the movement of the rig relative to the riser caused by ocean rise. Nevertheless, during drilling, the wellhead system is still subjected to cyclic forces from the movement of the rig, marine riser, and BOP assembly, as well as pressure changes within the wellbore. If these forces are sufficiently large and endured for extended periods, they can cause fatigue damage to the wellhead system over time, potentially leading to wellhead system failure.

[0005] The effects of these forces are exacerbated by the presence of a rigid cement column between the wellhead casing and the pilot casing, as the cement reduces the flexibility or movement of the extension under applied forces. Therefore, the cement column increases the likelihood of wellhead fatigue failure, especially when it is positioned at an unsuitable or suboptimal height.

[0006] Prior patent applications US5029647 and GB2479602 describe a subsea wellhead in which a resilient sleeve is installed around the wellhead extension before cement is filled into the annular space between the wellhead extension and the outer casing. The resilient sleeve described in GB2479602 is segmented and is designed to facilitate the deflection of the extension in the presence of a cement column, thereby reducing the likelihood of fatigue damage to the wellhead. Summary of the Invention

[0007] The purpose of this invention is to provide an alternative method for improving the fatigue performance of wellhead systems.

[0008] According to a first aspect of the invention, a wellhead assembly is provided, the wellhead assembly including a wellhead having: a tubular wellhead body having an outer surface; a wellhead casing extending from a first end of the wellhead body having an outer surface and proximal and distal ends relative to the wellhead body; and the wellhead assembly including a tubular outer sleeve having an inner surface, an annular space existing between the inner surface of the outer sleeve and the outer surface of the first end of the wellhead body and the outer surface of the wellhead casing, wherein the system further includes a solid cement liner extending radially inward from the inner surface of the outer sleeve into the annular space, and a fluid sleeve extending radially outward from the outer surface of one or both of the first end of the wellhead body and the wellhead casing into the annular space, such that the fluid sleeve is surrounded by at least a portion of the cement liner, said portion of the cement liner being located radially outward of the fluid sleeve, and cement in said portion of the cement liner being spaced apart from the wellhead body / wellhead casing.

[0009] Because of the fluid sleeve, direct contact between the cement lining and one or both of the first end of the wellhead body and the wellhead casing is prevented. The fluid sleeve also provides more freedom of movement for the wellhead when it is loaded, and thus reduces fatigue damage to the wellhead after a period of use.

[0010] The cement lining may include a portion that is closer to the distal end of the wellhead casing than the fluid sleeve and has a greater thickness than the portion of the cement lining surrounding the fluid sleeve.

[0011] Preferably, the fluid sleeve surrounds the entire circumference of the outer surface of the first end of the wellhead body. Alternatively or additionally, the fluid sleeve may surround the entire circumference of the outer surface of the wellhead casing and, from the proximal end toward the distal end, surround the entire length of the wellhead casing or a portion of the length of the wellhead casing.

[0012] The wellhead assembly may also include a tubular retaining sleeve that surrounds a portion of one or both of the wellhead body and the wellhead casing, and retains a fluid sleeve around a first end of the wellhead body and one or both of the wellhead casing.

[0013] The tubular retaining sleeve may have a first end fixed to and sealed against the wellhead body or wellhead casing. The tubular retaining sleeve is spaced apart from the outer surface of one or both of the first end of the wellhead body and the wellhead casing, such that a fluid sleeve fills the annular space between the retaining sleeve and the outer surface of one or both of the first end of the wellhead body and the wellhead casing. The retaining sleeve may have: a discharge passage extending from the space between the outer surface of the wellhead casing and / or the outer surface of the wellhead body and the retaining sleeve through the retaining sleeve to the outside of the retaining sleeve; and a discharge valve movable between a closed position and an open position, in which the discharge valve substantially prevents fluid flow along the discharge passage, and in the open position, allows fluid flow along the discharge passage.

[0014] The retaining sleeve may have an inner surface that substantially entirely engages with the outer surface of the wellhead casing and / or the outer surface of the first end of the wellhead body. In this case, the retaining sleeve may be made of an absorbent material, such as a gel, open-cell foam, hygroscopic material, or a soluble or corrosive material, or any combination of two or more such materials.

[0015] According to a second aspect of the invention, a wellhead is provided comprising a tubular wellhead body having an outer surface and a wellhead casing extending from a first end of the wellhead body, the wellhead casing having an outer surface and a proximal end and a distal end relative to the wellhead body, wherein the wellhead further comprises a tubular fluid retention sleeve fixed to the wellhead body such that the fluid retention sleeve extends around the outer surface of one or both of the first end of the wellhead body and the lower wellhead casing, and the fluid retention sleeve retains a fluid layer around one or both of the first end of the wellhead body and the wellhead casing when cement slurry is injected into the space surrounding the wellhead body and / or the wellhead and when the cement solidifies, such that at least a portion of the cement is spaced apart from at least a portion of the wellhead body and / or the wellhead by the fluid sleeve.

[0016] The retaining sleeve may have a first end fixed to and sealed against the outer surface of the wellhead body. The retaining sleeve is spaced apart from the outer surface of the wellhead casing and / or the outer surface of the wellhead body to form an annular space around the outer surface of the wellhead casing and / or the outer surface of the wellhead body, such that a fluid sleeve fills the annular space between the outer surface of the wellhead casing and / or the outer surface of the wellhead body and the retaining sleeve. The retaining sleeve may have: a discharge passage extending from the space between the outer surface of the wellhead casing and / or the outer surface of the wellhead body and the retaining sleeve through the retaining sleeve to the outside of the retaining sleeve; and a discharge valve movable between a closed position and an open position, in which the discharge valve substantially prevents fluid flow along the discharge passage, and in the open position, allows fluid flow along the discharge passage.

[0017] The retaining sleeve may have an inner surface that substantially entirely engages with the outer surface of one or both of the wellhead casing and / or the wellhead body. In this case, the retaining sleeve may be made of an absorbent material, such as a gel, open-cell foam, hygroscopic material, or a soluble or corrosive material, or any combination of two or more such materials.

[0018] According to a third aspect of the invention, a wellhead is provided comprising a tubular wellhead body having an outer surface and a wellhead casing extending from a first end of the wellhead body, the wellhead casing having an outer surface and a proximal end and a distal end relative to the wellhead body, wherein the wellhead further comprises a solid spacer sleeve positioned around one or both of the first end of the wellhead body and the wellhead casing and in contact with the outer surface of one or both of the first end of the wellhead body and the wellhead casing, wherein the spacer sleeve is made of a material that can be dissolved or eroded for replacement by a fluid sleeve.

[0019] The spacer sleeve can be made of water-soluble materials.

[0020] The wellhead can also be equipped with an injection line through which liquid can be injected to contact the spacer sleeve, thereby dissolving or corroding the spacer sleeve.

[0021] According to a fourth aspect of the invention, a method for assembling a subsea wellhead assembly is provided, the subsea wellhead assembly including a wellhead, the wellhead including a tubular wellhead body having an outer surface, and a wellhead casing extending from a first end of the wellhead body, the wellhead casing having an outer surface and a proximal end and a distal end relative to the wellhead body, the method comprising placing the wellhead within a tubular outer sleeve having an inner surface such that an annular space is formed between the inner surface of the outer sleeve and the outer surface of the wellhead casing and the outer surface of the first end of the wellhead body, the annular space being located radially inside the outer sleeve and radially outside the first end of the wellhead body and the wellhead casing, the method further comprising providing a cement lining to the outer sleeve by injecting cement slurry into the annular space, and forming a fluid sleeve between at least a portion of the cement lining and one or both of the first end of the wellhead body and the wellhead casing, such that the fluid sleeve is located radially inside said portion of the cement lining, and such that the cement in said portion of the cement lining is spaced apart from the wellhead body / wellhead casing.

[0022] The method may include injecting cement slurry into an annular space to form a portion of the cement lining that is closer to the distal end of the wellhead casing than the fluid sleeve and has a greater thickness than the portion of the cement lining surrounding the fluid sleeve.

[0023] Preferably, the fluid sleeve surrounds the entire circumference of the outer surface of the first end of the wellhead body. Alternatively or additionally, the fluid sleeve may surround the entire circumference of the outer surface of the wellhead casing and the entire length or a portion of the length of the wellhead casing from the proximal end toward the distal end.

[0024] The wellhead may also include a tubular retaining sleeve that retains fluid around one or both of the first end of the wellhead body and the wellhead casing when cement mortar is injected into the annular space to form a cement lining.

[0025] The retaining sleeve has a first end that can be fixed to or sealed against the outer surface of the wellhead casing or the outer surface of the first end of the wellhead body. The retaining sleeve is spaced apart from the outer surface of the wellhead casing and / or the outer surface of the first end of the wellhead body. When cement slurry is injected into the annular space between the inner surface of the retaining sleeve and the outer casing, the retaining sleeve traps fluid in the space between the outer surface of the retaining sleeve and the outer surface of the wellhead casing and / or the outer surface of the wellhead body.

[0026] The retaining sleeve may have: a discharge channel extending from the space between the outer surface of the wellhead casing and / or the outer surface of the wellhead body and the retaining sleeve through the retaining sleeve to the outside of the retaining sleeve; and a discharge valve movable between a closed position and an open position, wherein in the closed position the discharge valve substantially prevents fluid from flowing along the discharge channel, and in the open position fluid is allowed to flow along the discharge channel, and the method may include the steps of: ensuring the discharge valve is in the open position when the wellhead is submerged for installation on or in the wellhead casing, and then moving the discharge valve to the closed position before injecting cement slurry into the annular space between the inner surface of the outer casing and the retaining sleeve, so as to substantially prevent fluid in the space between the retaining sleeve and the outer surface of the wellhead casing and / or the outer surface of the wellhead body from being displaced by the cement slurry.

[0027] The outer casing can be located in the subsea wellbore so that when the wellhead is placed in the outer casing, the air in the space between the sleeve and the outer surface of the wellhead casing / wellhead body is replaced by water.

[0028] The retaining sleeve may have an inner surface that substantially entirely engages with the outer surface of one or both of the first end of the wellhead body and the wellhead casing. In this case, the retaining sleeve may be made of a material capable of retaining water, and the outer casing is located in the underwater wellbore such that water is drawn in and retained by the retaining sleeve when the wellhead is placed in the outer casing.

[0029] In this case, the retaining sleeve can be made of an absorbent material, such as a gel, open-cell foam, or moisture-absorbing material, or any combination of two or more such materials.

[0030] The retaining sleeve may include a solid spacer sleeve positioned around one or both of the first ends of the wellhead casing and the wellhead body and in contact with the outer surface of the wellhead casing / wellhead body, and the solid spacer sleeve is made of a material that can be dissolved or eroded to be replaced by a fluid sleeve.

[0031] The spacer sleeve can be made of water-soluble materials.

[0032] The wellhead may also be equipped with an injection line through which liquid is injected to contact the spacer sleeve. The method includes the following steps: after injecting and allowing the cement slurry to solidify, liquid is injected into the injection line. The liquid may include a solvent that dissolves the spacer sleeve to replace it with a fluid sleeve. The liquid may also include a chemical reagent that reacts with the spacer sleeve to replace it with a fluid sleeve.

[0033] The wellhead or casing may have at least one injection port extending into the annular space, and the method may include the following steps: after cement slurry is injected into the annular space and before the cement hardens, a portion of the cement slurry is flushed out of the annular space by injecting a non-cementous liquid through the injection port into the annular space.

[0034] According to a fifth aspect of the invention, a wellhead is provided having a tubular wellhead body with an outer surface and a wellhead casing, the proximal end of the wellhead casing being connected to a first end of the wellhead body by means of a weld, the wellhead casing having an outer surface, wherein the wellhead further includes a rigid reinforcing sleeve extending around the outer surface of the proximal end of the wellhead casing and the outer surface of the first end of the wellhead body to cover the weld.

[0035] Preferably, the reinforcing sleeve has an inner surface that engages substantially the entire inner surface with the outer surfaces of both the wellhead casing and the wellhead body.

[0036] The reinforcing sleeve can be welded or bonded to the outer surfaces of both the wellhead casing and the wellhead body.

[0037] The reinforced sleeve can be made of fiber-reinforced composite material.

[0038] The reinforcing sleeve can be made of metal, such as steel.

[0039] The reinforced sleeve can be held in place around the wellhead body and wellhead casing with an interference fit.

[0040] According to a sixth aspect of the invention, a wellhead assembly is provided, the wellhead assembly comprising a tubular wellhead body having an outer surface and a tubular guide casing having an inner surface enclosing a main channel, the wellhead body being positioned in the main channel and supported by a guide housing through a contact area between the inner surface of the guide housing and the outer surface of the wellhead body, wherein at least one insert is provided to form a portion of the outer surface of the wellhead body in a region of the region contacting the inner surface of the guide casing, the insert being made of a hygroscopic material.

[0041] The insert can be made of nylon.

[0042] The insert can be annular and fits into a circumferential groove surrounding the wellhead body.

[0043] The insert may be provided with a protective coating that is removed by contact with the inner surface of the guide housing when the wellhead body is placed in the main channel of the guide housing.

[0044] The wellhead assembly may also include a wellhead casing, the proximal end of which is connected to a first end of the wellhead body by means of a weld.

[0045] According to a seventh embodiment of the present invention, a wellhead is provided, the wellhead comprising: a tubular wellhead body having a thick main body portion and a first end of the tubular wellhead body tapering to an elongated portion with decreasing thickness; and a wellhead casing having a proximal end portion connected to a free end of the elongated portion of the wellhead body via a weld, wherein the elongated portion of the wellhead body is at least 0.5 m long.

[0046] The elongated portion may have a region adjacent to the free end where the thickness is further reduced.

[0047] The area with further reduced thickness can be at least 15cm long. Attached Figure Description

[0048] Embodiments of the present invention will now be described with reference to the following accompanying drawings, in which:

[0049] Figure 1 A first embodiment of a subsea wellhead assembly according to a first aspect of the present invention is shown.

[0050] Figure 2 A second embodiment of the subsea wellhead assembly according to the first aspect of the present invention is shown.

[0051] Figure 3 A third embodiment of a subsea wellhead assembly according to the first aspect of the invention and having a wellhead according to the seventh embodiment of the invention is shown.

[0052] Figure 4 A schematic diagram (not drawn to scale) of a longitudinal cross-section through a portion of the wellhead according to a fifth aspect of the invention is shown.

[0053] Figure 5 A schematic diagram (not drawn to scale) of a longitudinal cross-section through a portion of an alternative embodiment of a wellhead according to a fifth aspect of the invention is shown.

[0054] Figure 6 A schematic diagram (not drawn to scale) is shown of a longitudinal cross-section through a portion of a wellhead assembly according to a sixth aspect of the invention. Detailed Implementation

[0055] Referring now to the accompanying drawings, a subsea wellhead assembly 10 is shown, comprising a wellhead 12 having a wellhead body 14 and a wellhead casing 16, the wellhead casing 16 having a proximal end portion 16a and a distal end portion 16b relative to the wellhead body 14. A first end portion of the wellhead body 14 tapers inward or transitions to an elongated portion 26, the elongated portion 26 having a weld preparation profile at its free end. The weld preparation profile is secured to the proximal end portion 16a of the wellhead casing 16 by a weld portion 28.

[0056] Component 10 also includes a tubular guide housing 18 having an inner surface that encloses the main channel. The wellhead body 14 is supported within the main channel of the guide housing 18 through a contact area between the inner surface and the outer surface of the guide housing 18. In this example, the guide housing 18 has a lower annular weld preparation profile 20 and supports a casing 24 via a weld 22. In this embodiment, the casing 24 is an outer cylindrical guide casing 24 that extends downward from the guide housing 18 into a borehole (not shown) in the seabed. Therefore, the lower end of the guide housing 18 and the guide casing 24 form an outer sleeve surrounding the first end of the wellhead body 14 and the wellhead casing 16.

[0057] Typically, the wellhead casing 16 extends downwards along the outer casing 24 at least to the seabed, and preferably slightly further. The outer casing 24 typically extends 4 m to 200 m below the seabed, and the wellhead casing 16 may extend downwards to the same or nearly the same extent below the seabed. Where the wellhead casing 16 extends a considerable distance, it will be understood that the wellhead casing 16 may be made of multiple tubular sections—which may be connected end-to-end by means of threads or by welding—to form a single wellhead casing 16. If this is the case, the wellhead casing 16 is typically made of multiple 12 m long sections.

[0058] An annular space 30 exists between the first end of the wellhead body 14 and the radially outward-facing surface of the wellhead casing 16 and the radially inward-facing surfaces of the guide casing 24 and the guide housing 18. The guide housing 18 is provided with a lateral ventilation port 32, which extends laterally through the guide housing 18 to reach the annular space 30, thus achieving fluid communication from the outside of the guide casing 24 to the annular space 30.

[0059] In existing systems, a cement lining is formed in the annular space 30 by pumping cement slurry downwards along the wellbore, causing the slurry to rise and enter the annular space 30. Typically, the cement is pumped upwards to at least the first end of the wellhead body 14, and can extend as far as the lateral ventilation port 32. In this case, the cement lining also occupies a portion of the annular space between the guide casing 18 and the first end of the wellhead body 14. However, this is not necessary, and the cement may not be pumped all the way upwards to the guide casing 18; it may simply occupy the space between the guide 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 upwards to the first end of the wellhead body 14; it may simply occupy the space between the guide casing 24 and the wellhead casing 16.

[0060] Fluid displaced by cement slurry (primarily seawater, but may also include chemicals and debris generated during drilling or wellhead installation) is expelled from the annulus 30 through vent port 32.

[0061] According to the invention, a fluid sleeve 34 is provided extending from the outer surface of the wellhead casing 16 and / or the outer surface of the first end of the wellhead body 14 into the annular space 30. The fluid sleeve 34 is located radially inside at least a portion of the cement lining and is therefore surrounded by said portion of the cement lining, with the cement in said portion of the cement lining spaced apart from the wellhead body / wellhead casing. Because of the provision of this fluid sleeve 34, direct contact between the cement lining and the wellhead casing 16 and / or the wellhead body 14 in this area is prevented. The fluid sleeve 34 can therefore provide more degrees of freedom of movement for the wellhead 12 when it is subjected to external loads, and thus can reduce fatigue damage to the wellhead 12 during use.

[0062] It should be understood that the fluid sleeve 34 may surround only the first end of the wellhead body 14 and only the wellhead casing 16 or a portion thereof. Alternatively, the fluid sleeve 34 may surround the first end of the wellhead body 14 and from the first end of the wellhead body 14 toward the distal end 16b around the entire length of the wellhead casing 16 or a portion thereof. In this embodiment, the fluid sleeve 34 surrounds the entire circumference of the outer surface of the first end of the wellhead body and the entire circumference of the outer surface of the proximal end 16a of the wellhead casing 16.

[0063] Advantageously, the fluid sleeve 34 surrounds at least 20 m of the wellhead casing 16 adjacent to the wellhead body 14, but may surround the wellhead casing 16 for a length of up to 50 m. In the case where the wellhead casing 16 consists of multiple tubular segments, the fluid sleeve 34 advantageously surrounds the segment located at the proximal end 16a of the wellhead casing 16 as well as one or more adjacent segments.

[0064] As such, it will be understood that when the cement slurry fills the annular space surrounding the wellhead casing below the fluid sleeve 34, the cement lining includes a portion closer to the distal end 16b of the wellhead casing 16 than the fluid sleeve 34, and the thickness of this portion is greater than the thickness of the portion of the cement lining surrounding the fluid sleeve 34.

[0065] Various methods for setting the fluid sleeve have been proposed. In some embodiments, the wellhead assembly 10 also includes a retainer that holds the fluid sleeve around the wellhead body 14 / wellhead casing 16 when cement is injected into the annular space 30 to form a cement liner.

[0066] exist Figure 1 In the illustrated embodiment, the retainer includes a tubular retaining sleeve 36. In this embodiment, a first end 36a of the retaining sleeve 36 is fixed to and sealed against an elongated portion 26 of the wellhead body 14, but the first end 36a can also be fixed to and sealed against another portion of the outer surface of the first end of the wellhead body 14.

[0067] The retaining sleeve 36 may extend all the way to the distal end 16b of the wellhead casing 16, but in this embodiment, the retaining sleeve 36 does not extend all the way to the distal end 16b of the wellhead casing 16. In this embodiment, the retaining sleeve 36 extends toward the distal end 16b of the wellhead casing 16 to surround a large portion of the wellhead casing 16. When the wellhead casing 16 is made of multiple tubular segments, the retaining sleeve 36 extends at at least two ends of the segments located at the proximal end 16a of the wellhead casing 16.

[0068] The retaining sleeve 36 is spaced apart from the outer surface of the first end of the wellhead body 14 and the outer surface of the wellhead casing 16, such that the fluid sleeve fills the annular space between the outer surface of the wellhead casing 16 and the outer surface of the first end of the wellhead body 14 and the retaining sleeve 36. In this embodiment, the retaining sleeve 36 has a discharge passage (not shown) and a discharge valve (not shown). The discharge passage extends from the space between the outer surface of the wellhead body 14 and the retaining sleeve 36 through the first end 36a of the retaining sleeve 36 to the outside of the retaining sleeve 36. The discharge valve is movable between a closed position and an open position. In the closed position, the discharge valve substantially prevents fluid from flowing along the discharge passage, and in the open position, allows fluid to flow along the discharge passage. In a preferred embodiment, the discharge valve is ROV-operated.

[0069] In this configuration, when the guide housing 18 is installed on the subsea wellbore, the space between the retaining sleeve 36 and the wellhead casing 16 and wellhead body 14 is filled with water when the wellhead body 14 is installed in the guide housing 18. Air is displaced from the space between the retaining sleeve 36 and the wellhead casing 16 by water, and the drain valve is kept open while the wellhead 12 is lowered into the guide housing 18. The drain valve can be closed once the wellhead 12 is installed on the guide housing 18, or even before the wellhead 12 is installed on the guide housing 18, provided that the wellhead 12 is fully submerged and the space between the retaining sleeve 36 and the wellhead casing 16 and wellhead body 14 is filled with water.

[0070] Then, before injecting the cement slurry into the annular space between the inner surface of the guide sleeve 24 and the retaining sleeve 36, the drain valve is moved to the closed position using an ROV. As the cement slurry fills the annular space between the retaining sleeve 36 and the guide sleeve 24, water in this space is expelled from the lateral vent port 32. Ideally, the injection of cement slurry is stopped before it moves past the first end 36a of the retaining sleeve 36 to ensure that the cement does not come into direct contact with the first end of the wellhead body 14. It is not necessary to pump cement all the way up to the guide housing 18, and it can simply occupy the space between the guide sleeve 24 and the retaining sleeve 36.

[0071] The retaining sleeve 36 traps water in the space between the outer surface of the retaining sleeve 36 and the first end of the wellhead body 14 and the outer surface of the wellhead casing 16, preventing water from being displaced by the cement slurry. The cement slurry is then allowed to harden to form a rigid cement lining between the retaining sleeve 36 and the guide casing 24, while the annular space between the retaining sleeve 36 and the wellhead casing 16 remains filled with water, thus forming a fluid sleeve. It will be understood that, depending on how far the cement slurry is pumped above the retaining sleeve 36, a fluid sleeve may exist only between the cement lining and the wellhead casing 26, or both between the cement lining and the first end of the wellhead casing 16 and the wellhead body 14.

[0072] exist Figure 2 In the alternative embodiment of the invention shown, the retainer includes a retaining sleeve 38 having an inner surface that engages substantially the entire inner surface with the outer surface of the wellhead casing 16 and the outer surface of the first end of the wellhead body 14. The retaining sleeve 38 engages the entire circumference of the wellhead casing 16 and may extend to or around the distal end 16b of the wellhead casing 16. Where the wellhead casing 16 comprises multiple tubular segments, the retaining sleeve 38 will advantageously cover the segment located at the proximal end 16a of the wellhead casing 16, as well as one or more adjacent segments.

[0073] In this case, the retaining sleeve 38 is made of an absorbent material, such as gel, open-cell foam, or hygroscopic material. If the retaining sleeve 38 has sufficient mechanical integrity, for example, if it is made of foam, it can be secured to the wellhead casing 16 using an adhesive. If the retaining sleeve 38 lacks the mechanical integrity to be secured to and held in place on the wellhead casing using an adhesive, for example, when using gel or hygroscopic material, a porous tube can be used to accommodate the gel, foam, or hygroscopic material. The porous retainer tube can be made of fabric or fine mesh, and can be secured to the wellhead casing 16 at opposite ends using circumferential fabric strips.

[0074] The retaining sleeve 38 is fixed to the outer surface of the wellhead casing 16 in its dry state, and thus absorbs or draws water when the wellhead 12 is lowered into the sea (depending on the material used, the retaining sleeve 38 may expand during this process), and the retaining sleeve 38 is as described above regarding... Figure 1 The water is held in the fluid sleeve before the cement slurry is injected into the annular space 30.

[0075] The retaining sleeve 38 may be made of, for example, a highly absorbent polymer, organic starch, cotton, or gel. For instance, the retaining sleeve 38 may be made of sodium polyacrylate or another highly absorbent polymer that forms a hydrogel upon exposure to water or moisture. Other possible materials include carboxyalkyl cellulose and cereal-based polysaccharides. In the case where the retaining sleeve 38 is formed of a granular material, the granular material may be retained around the wellhead casing 16 by a flexible, permeable sleeve that surrounds the outer surface of the wellhead casing 16 and is secured at its end to a first end of the wellhead casing 16 and / or the wellhead body 14, the granular material being held in the space between the outer surface of the wellhead casing 16 and the permeable sleeve.

[0076] In another alternative embodiment of the invention, a fluid sleeve can be created by placing a soluble sleeve around the first end of the wellhead casing 16 and / or the wellhead body 14 before the wellhead 12 is lowered into the water. The soluble sleeve can be water-soluble, but dissolves relatively slowly, such that the soluble sleeve is retained as the wellhead is lowered into the subsea borehole by water and exists for a sufficient time while the cement slurry is injected into the annular space 30, but dissolves completely after the cement has fully or partially solidified and hardened. An injection line can be provided to allow the solvent to be injected into the soluble sleeve after the cement slurry has been injected and allowed to solidify, to accelerate the dissolution of water-soluble sleeve or to dissolve non-water-soluble sleeve. The solvent will then form all or part of the fluid in the resulting fluid sleeve.

[0077] Water-soluble sleeves can be made from water-soluble polymers such as polyethylene acrylate, carbohydrates such as sugar, or water-soluble minerals such as salt. Insoluble sleeves can be made from polylactic acid and can be dissolved using ethyl acetate.

[0078] As an alternative, the sleeve is not soluble and is not dissolved by a solvent; the sleeve can be made of a material that reacts with the injected reactants to transform from a solid sleeve into a fluid.

[0079] Once the cement has been fully or partially cured and hardened, solvents or reactants can be injected.

[0080] The melting temperature of the sleeve can also be selected so that the sleeve remains solid at seawater temperature and during cement slurry injection, but melts at the normal operating temperature at the wellhead.

[0081] In another alternative embodiment of the invention, no retaining sleeve is provided. Instead, a one-way seal or one-way valve is provided between the wellhead casing 16 and the guide casing 24 to control the flow of fluid into the annular space 30 surrounding the proximal end 16a of the wellhead casing 16. The one-way valve is configured to allow cement slurry to flow from the annular space 30 below the valve to the annular space 30 above the valve, but is configured to prevent fluid from flowing in the opposite direction.

[0082] During cementing of the wellhead system, cement slurry passes through a check valve and fills the annular space 30 above the check valve. Pumping of the cement slurry is stopped when cement begins to flow out of the vent port 32. At this time, and before the cement solidifies, a displacement fluid—which may be water—is pumped into the annular space 30 at the lower end of the wellhead body 14 to displace the cement and expel it from the annular space 30 via the vent port 32. Cement displacement can be assisted by pressurizing the displacement fluid and / or using a fluid with a density greater than that of the cement. The displacement fluid can be pumped into the annular space via a port or multiple ports extending through the wellhead body 14 or guide housing 18, and can be arranged to guide the fluid against the outer surface of the first end of the wellhead body 14 or the outer surface of the wellhead casing 16 to wash away the cement slurry around the wellhead body 14 / wellhead casing 16, thereby forming a fluid sleeve.

[0083] exist Figure 3 The figure illustrates an alternative configuration of the wellhead body 14'. In this embodiment, the length of the elongated portion 26 of the wellhead body 14' is increased, thus increasing the spacing between the uppermost end of the wellhead body 14' and the weld portion 28 between the wellhead body 14' and the proximal end 16a of the wellhead casing 16. Therefore, the weld portion 28 by which the proximal end 16a of the wellhead casing 16 is fixed to the wellhead body 14' is further below the rotation center of the wellhead, and this can help reduce the force on the weld portion 28 due to external loads on the wellhead body 14'.

[0084] Advantageously, the length of the elongated portion 26 is at least 0.5 m. The length of the elongated portion 26 may be at least one-third of the total length of the wellhead body. In one embodiment, the wellhead body 14' is about 4 m long, and the elongated portion 26 is about 1.6 m long.

[0085] Although not essential, in this embodiment, the thickness of the elongated portion 26 is further reduced in the region 26' adjacent to the very end of the wellhead body 14'. This reduced-thickness portion 26' will be more flexible than other parts of the wellhead body 14 and can also help reduce the force acting on the weld 28.

[0086] Advantageously, the reduced-thickness portion 26' is at least 15 cm long. The length of the reduced-thickness portion 26' can be at least half the length of the elongated portion 26.

[0087] In one embodiment, the wellhead is about 4m long, the elongated portion 26 is about 1.6m long, and the portion 26' with reduced thickness is about 0.9m long.

[0088] The fluid sleeve can be applied to this configuration of the wellhead body 14' in any of the ways described above. The fluid sleeve can simply surround the first end of the wellhead body 14'—therefore, the fluid sleeve can extend from the weld 28, or extend above the weld 28 and only around the elongated portion 26 of the wellhead body 14'. In this case, the cement slurry will be pumped upwards to a height above the weld 28, causing the fluid sleeve to separate the cement lining from the first end of the wellhead body 14'. Alternatively, as... Figure 3 As shown, the fluid sleeve can surround the first end of the wellhead body 14' and the proximal end 16a of the wellhead casing—thus the fluid sleeve can extend from above the weld 28 to below the weld 28. In this case, cement slurry can be pumped to a height below the weld 28 such that the fluid sleeve separates only the cement lining from the wellhead casing 16, or the cement slurry can be pumped to a height above the weld 28 such that the fluid sleeve separates both the cement lining from the first end of the wellhead body 14' and the wellhead casing 16.

[0089] In all cases, a fluid sleeve may be provided only around the wellhead casing 16, but this must be ensured that cement slurry is not pumped to a height above the fluid sleeve to prevent the cement lining from filling the space between the outer surface of the first end of the wellhead body 14, 14' and the guide casing 24 or guide housing 18.

[0090] In another alternative implementation, such as Figure 4 As shown, a solid reinforcing sleeve 40 is mounted on the radially outward surface of the free end of the elongated portion 26 of the wellhead body 14, 14' and the radially outward surface of the proximal end 16a of the wellhead casing 16, such that the sleeve 40 covers the weld 28. Unlike the elastomeric sleeves described in US 5029847 and GB 2479602, the reinforcing sleeve 40 is rigid and designed to prevent bending of the elongated portion 26 of the wellhead body 14, 14' and the proximal end 16a of the wellhead casing 16 at and in the region of the weld 28. Therefore, the sleeve 40 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 can transfer the forces away from the weld 28, thus increasing the fatigue life of the weld 28 and the fatigue life of the entire wellhead system 10.

[0091] This type of reinforcing sleeve can also be applied to... Figure 1 or Figure 3 The configuration of wellhead 14' shown in the diagram.

[0092] The reinforcing sleeve 40 can be made of metal such as steel or composite materials such as... Made, the It is made of Kevlar aramid bonded with epoxy resin. In the latter case, the composite strip is wrapped around the weld 28 like a bandage.

[0093] In the case of a metal sleeve, the metal sleeve can be manufactured as a tubular portion that slides onto the wellhead casing 16 such that the tubular portion surrounds the radially outward surface of the proximal end 16a of the wellhead casing 16 before the wellhead casing 16 is welded to the wellhead body 14, 14'. Once the process of welding the elongated portions 26, 26' of the wellhead body 14, 14' to the proximal end 16a of the wellhead casing 16 is completed, the sleeve 40 can slide along the wellhead casing 16 to cover the weld 28. The reinforcing sleeve 40 can then be welded in place, for example, by corner welds 41 at each end of the sleeve 40, or it can be coupled to the wellhead body 14, 14' and / or the wellhead casing 16.

[0094] Alternatively, the reinforcing sleeve 40 can also be made of a tubular portion with longitudinal cracks, the edges of which are welded together once the reinforcing sleeve 40 is in place on the weld 28, such that the reinforcing sleeve 40 contracts around the elongated portions 26, 26' of the wellhead bodies 14, 14' and the proximal end 16a of the wellhead casing 16 after the weld has cooled. As another alternative, the sleeve 40 can be formed as multiple portions mounted around the weld 28, and said portions are welded together. In these cases, the sleeve 40 can also be welded to or bonded to the wellhead bodies 14, 14' and / or the wellhead casing 16, but this may not be necessary if the contraction of the sleeve 40 after the weld has cooled is sufficient to keep the sleeve 40 in place with the wellhead bodies 14, 14' and the wellhead casing 16 in an interference fit.

[0095] like Figure 5 As shown, a back strip 42 may be provided between the sleeve 40 and the wellhead body 14, 14' / between the wellhead casing 16 and the sleeve 40 to separate the radially inward surface of the sleeve 40 from the wellhead body 14, 14' / wellhead casing 16.

[0096] In another alternative embodiment, one or more annular inserts 44 made of a hygroscopic material may be installed around the wellhead bodies 14, 14'. The hygroscopic material may be, for example, nylon. Figure 6 As shown, the insert 44 may be annular and positioned in a circumferential groove surrounding the outer diameter of the wellhead bodies 14, 14' at the contact area between the wellhead bodies 14, 14' and the guide housing 18. Ideally, each insert is shaped such that it does not protrude further in the radial direction than the surrounding portion of the wellhead bodies 14, 14', allowing the wellhead 12 to be installed in the guide housing 18 in a conventional manner.

[0097] The hygroscopic insert 44, or each hygroscopic insert 44, may have a coating, such as a grease coating, to prevent it from absorbing moisture from the environment. Due to the tight fit between the radially inward-facing surface of the guide housing 18 and the wellhead bodies 14, 14', this coating will be scraped off when the wellhead 12 falls into the guide casing 18, thereby exposing the inserts 44 to the surrounding seawater. Each insert 44 then absorbs water, and in doing so, each insert 44 expands, thus sealing any gaps between the wellhead bodies 14, 14' and the guide housing 18, and possibly even preloading both parts. This can help reduce wellhead deflection and thus reduce the stress on the weld 28 caused by the forces applied to the wellhead 12, particularly bending moments.

[0098] Furthermore, this embodiment of the present invention can be applied to... Figure 1 or Figure 3 The wellhead 12 shown is configured as described above and can be used in conjunction with the fluid sleeve or solid reinforced sleeve 44 described above.

Claims

1. A wellhead assembly comprising a wellhead having: a tubular wellhead body having an outer surface; a wellhead casing extending from a first end of the wellhead body, the wellhead casing having an outer surface and proximal and distal ends relative to the wellhead body, and the wellhead assembly having a tubular outer sleeve having an inner surface, an annular space existing between the inner surface of the outer sleeve and the outer surface of the first end of the wellhead body and the outer surface of the wellhead casing, wherein... The wellhead assembly further includes a solid cement liner extending radially inward from the inner surface of the outer casing into the annular space, and a fluid sleeve extending radially outward from the outer surface of one or both of the first end of the wellhead body and the wellhead casing into the annular space, such that the fluid sleeve is surrounded by at least a portion of the cement liner, the portion of the cement liner being located radially outward of the fluid sleeve, and the cement in the portion of the cement liner being spaced apart from the wellhead body / wellhead casing. The wellhead assembly further includes a tubular retaining sleeve that surrounds a portion of one or both of the wellhead body and the wellhead casing, and retains the fluid sleeve around a first end of the wellhead body and one or both of the wellhead casing.

2. The wellhead assembly according to claim 1, wherein, The cement lining includes a portion that is closer to the distal end of the wellhead casing than the fluid sleeve, and has a greater thickness than the portion of the cement lining surrounding the fluid sleeve.

3. The wellhead assembly according to claim 1 or 2, wherein, The fluid sleeve surrounds the entire circumference of the outer surface of the first end of the wellhead body.

4. The wellhead assembly according to claim 1 or 2, wherein, The fluid sleeve surrounds the entire circumference of the outer surface of the wellhead casing and extends from the proximal end toward the distal end around the entire length of the wellhead casing or a portion of the length of the wellhead casing.

5. The wellhead assembly according to claim 1, wherein, The tubular retaining sleeve has a first end, which is fixed to the wellhead body or the wellhead casing and abuts against the wellhead body or the wellhead casing for sealing. The tubular retaining sleeve is spaced apart from the outer surface of one or both of the first end of the wellhead body and the wellhead casing, such that the fluid sleeve fills the annular space between the retaining sleeve and the outer surface of one or both of the first end of the wellhead body and the wellhead casing.

6. The wellhead assembly according to claim 1, wherein, The retaining sleeve has: a discharge channel extending through the space between the outer surface of the wellhead casing and / or the outer surface of the wellhead body and the retaining sleeve to the outside of the retaining sleeve; and a discharge valve movable between a closed position and an open position, wherein in the closed position the discharge valve prevents fluid from flowing along the discharge channel, and in the open position the discharge valve allows fluid to flow along the discharge channel.

7. The wellhead assembly according to claim 1, wherein, The retaining sleeve has an inner surface, the entire inner surface of which engages with the outer surface of the wellhead casing and / or the outer surface of the first end of the wellhead body.

8. A method of assembling a subsea wellhead assembly, the subsea wellhead assembly comprising a wellhead, the wellhead including a tubular wellhead body having an outer surface, and a wellhead casing extending from a first end of the wellhead body, the wellhead casing having an outer surface and proximal and distal ends relative to the wellhead body, the method comprising placing the wellhead within a tubular outer sleeve having an inner surface such that an annular space is formed between the inner surface of the outer sleeve and the outer surface of the wellhead casing and the outer surface of the first end of the wellhead body, the annular space being located radially inward of the outer sleeve and radially outward of the first end of the wellhead body and the wellhead casing, the method further comprising providing a cement lining to the outer sleeve by injecting cement slurry into the annular space, and forming a fluid sleeve between at least a portion of the cement lining and one or both of the first end of the wellhead body and the wellhead casing, such that the fluid sleeve is located radially inward of said portion of the cement lining, and such that the cement in said portion of the cement lining is spaced apart from the wellhead body / wellhead casing, and in, The wellhead also includes a tubular retaining sleeve that retains the fluid sleeve around one or both of the first end of the wellhead body and the wellhead casing when the cement slurry is injected into the annular space to form the cement lining.

9. The method of claim 8, further comprising injecting cement slurry into the annular space to form a portion of the cement lining that is closer to the distal end of the wellhead casing than the fluid sleeve, and has a thickness greater than the thickness of the portion of the cement lining surrounding the fluid sleeve.

10. The method according to claim 8 or 9, wherein, The fluid sleeve surrounds the entire circumference of the outer surface of the first end of the wellhead body.

11. The method according to claim 8 or 9, wherein, The fluid sleeve surrounds the entire circumference of the outer surface of the wellhead casing and extends from the proximal end toward the distal end around the entire length of the wellhead casing or a portion of the length of the wellhead casing.

12. The method according to claim 8, wherein, The retaining sleeve has a first end, which is fixed to and sealed against the outer surface of the wellhead casing or the outer surface of the first end of the wellhead body. The retaining sleeve is spaced apart from the outer surface of the wellhead casing and / or the outer surface of the first end of the wellhead body. When the cement slurry is injected into the annular space between the retaining sleeve and the inner surface of the outer casing, the retaining sleeve traps fluid in the space between the retaining sleeve and the outer surface of the wellhead casing and / or the outer surface of the wellhead body.

13. The method according to claim 12, wherein, The retaining sleeve has: a discharge channel extending through the retaining sleeve from the space between the outer surface of the wellhead casing and / or the outer surface of the wellhead body and the retaining sleeve to the outside of the retaining sleeve; and a discharge valve movable between a closed position and an open position, wherein in the closed position the discharge valve prevents fluid from flowing along the discharge channel, and in the open position fluid is allowed to flow along the discharge channel, and the method includes the steps of: ensuring the discharge valve is in the open position when the wellhead is submerged for mounting the wellhead on or in the wellhead casing, and then moving the discharge valve to the closed position before injecting cement slurry into the annular space between the inner surface of the outer casing and the retaining sleeve, so as to prevent fluid in the space between the retaining sleeve and the outer surface of the wellhead casing and / or the outer surface of the wellhead body from being displaced by the cement slurry.

14. The method according to claim 13, wherein, The outer sleeve is located in the underwater wellbore, such that when the wellhead is placed in the outer sleeve, the air in the space between the retaining sleeve and the outer surface of the wellhead casing is replaced by water.

15. The method according to claim 8, wherein, The retaining sleeve has an inner surface, the entire inner surface of which engages with the outer surface of one or both of the first end of the wellhead body and the wellhead casing.

16. The method according to claim 15, wherein, The retaining sleeve is made of a material capable of retaining water, and the outer sleeve is located in the underwater wellbore such that when the wellhead is placed in the outer sleeve, water is drawn in and retained by the retaining sleeve.

17. The method according to claim 15, wherein, The retaining sleeve includes a solid spacer sleeve made of a material that can be dissolved or eroded to be replaced by the fluid sleeve.

18. The method according to claim 17, wherein, The spacer sleeve is made of a water-soluble material.

19. The method according to claim 17 or 18, wherein, The wellhead is also provided with an injection pipeline, through which liquid can be injected to contact the spacer sleeve. The method includes the following steps: after injecting cement slurry, injecting liquid into the injection pipeline.

20. The method according to claim 8 or 9, wherein, The wellhead or the outer casing has at least one injection port extending into the annular space, and the method includes the steps of: after injecting cement slurry into the annular space and before the cement hardens, forming the fluid sleeve by injecting a portion of the cement slurry out of the annular space by injecting a non-cementous liquid through the injection port into the annular space.

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