1. Field of the Invention
The present invention relates in general to a method and apparatus to mitigate trapped pressure in a wellhead and in particular to a compressible pressure limiting device for limiting pressure from a void typically located between two crown plugs in a wellhead tree system.
2. Brief Description of Related Art
A horizontal subsea tree has a production outlet extending generally horizontally, in relation to the wellbore, and a bore that is axially aligned with the wellbore. A tubing hanger lands in the horizontal tree and supports a string of tubing extending into the wellbore. The tubing hanger has a vertical passage and a lateral passage extending from the vertical passage and registering with the production outlet of the tree. In some installations an internal tree cap lands in the tree above the tubing hanger, the tree cap normally having a vertical passage that aligns with the vertical passage in the tubing hanger. As a dual safety barrier, a wireline deployed crown plug is installed in the vertical passage of the tubing hanger and another crown plug is installed in the vertical passage of the tree cap. In other installations, the internal tree cap is omitted. In that case, the vertical passage of the tubing hanger is typically plugged with two crown plugs to meet requirements of having dual safety barriers.
Fluid, such as, for example, completion fluid, may be trapped in the vertical passage between the two plugs. The fluid may be relatively cold when it is trapped because the subsea temperature is relatively cold. During production, the well fluid flowing through portions of the wellhead is at a higher temperature and subsequently heats the subsea wellhead. As the fluid trapped between the crown plugs heats up and is restricted from expanding, the trapped fluid pressure can potentially increase above the working pressure of the crown plugs and, thus, damage the integrity of the crown plugs. It is thus desirable to limit the pressure in the void between the crown plugs, without releasing the fluid trapped between the plugs into the environment.
A pressure compensating device can be used to mitigate the pressure increase that can occur when fluid tries to thermally expand in a confined space. In one embodiment, the pressure compensator is located in a wellhead assembly that has a cylindrical bore, a first plug located in and sealingly engaging the cylindrical bore and a second plug located in and sealingly engaging the cylindrical bore. The second plug can be spaced axially apart from the first plug, and thus the cylindrical bore, the first plug, and the second plug define a cavity. Trapped fluid can be retained in the cavity. To mitigate the pressure increase, a pressure compensator having a pair of plates (a first plate and a second plate) can be located within the cavity. The pair of plates can define a void between them and a compressible fluid can be located within the void. When the volume of the wellbore fluid in the cavity increases, it can cause the plates to deflect inward, toward each other.
The inward deflection of the first plate, into the void, can be limited by the second plate such that the first plate does not plastically deform prior to being so limited by the second plate. In one embodiment, the inward deflection of the second plate, into the void, is limited by the first plate such that the second plate does not plastically deform prior to being so limited by the first plate. A cylindrical ring can connect the first plate and second plate and thus define an outer diameter of the void. One or both plates can have a concave surface in relaxed state. Alternatively, one or both plates can have a generally flat surface in its relaxed state. The plates can be made of any of a variety of materials including, for example, metal, polymer, or elastomer. The void between the plates can be filled with a compressible fluid including, for example, a gas such as air, nitrogen, or argon. In one embodiment, the void can be at negative pressure, less than atmospheric pressure, when the plates are in their relaxed state.
The compensator assembly can be located in a frame, or cage, that can be placed in the cavity. The frame can have a sidewall with an aperture so that wellbore fluid can pass through the aperture, into the frame, and thus reach the surface of the compensator plates. More than one compensator can be located in the cavity and, indeed, more than one compensator can be located in a single frame. If more than one compensator is used, a gap can exist between the plates of the two compensators so that wellbore fluid can reach the exterior surfaces of those plates.
So that the manner in which the features, advantages and objects of the invention, as well as others which will become apparent, are attained and can be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiment thereof which is illustrated in the appended drawings, which drawings form a part of this specification. It is to be noted, however, that the drawings illustrate only a preferred embodiment of the invention and is therefore not to be considered limiting of its scope as the invention may admit to other equally effective embodiments.
The present invention will now be described more fully hereinafter with reference to the accompanying drawings which illustrate embodiments of the invention. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and the prime notation, if used, indicates similar elements in alternative embodiments.
Referring to
The tree 100 has an inner wellhead assembly 111 housed within the axial bore 102 of the tree 100. A tubing hanger 112 lands sealingly in bore 102. Tubing hanger 112 is secured to tree 100 by a lock down mechanism 114. A string of production tubing 116 extends through the casing hangers (not shown) into the well for the flow of production fluid. Production tubing 116 is secured to tubing hanger 112 and communicates with a vertical passage 122 that extends through tubing hanger 112. A lateral passage 124 extends from vertical passage 122 and aligns with tree lateral passage 108.
A lower wireline retrievable plug 126, or crown plug, will lock in vertical passage 122 above lateral passage 124, sealing the upper end of vertical passage 122. Seals can form a seal between plug 126 and tubing hanger 112, and dogs, or other types of locking devices, may be used to lock plug 126 in place. In this example, a tree cap 128 inserts sealingly into tree bore 102 above tubing hanger 112. Tree cap 128 has a downward depending isolation sleeve 130 that is coaxial. Sleeve 130 fits within a receptacle 132 formed on the upper end of tubing hanger 112. The interior of sleeve 130 communicates with an axial passage 144 that extends through tree cap 128. Axial passage 144 has approximately the same inner diameter as tubing hanger passage 122.
An upper wireline retrievable crown plug 146 inserts into tree cap passage 144. Various seals can provide sealing between components within tree 100 including, for example, metal seal 148 on crown plug 146, which can engage a surface in passage 144. Dogs, or other types of locking mechanisms, can be used to lock upper crown plug 146 in place. Upper crown plug 146 is a redundant plug for further sealing passage 144, the primary seal being formed by lower plug 126. Upper crown plug 146 and lower plug 126, thus, form dual safety barriers against gas or liquids that may pass up through vertical passage 122. Any type of upper and lower plug can be used to form such safety barriers.
Cavity 150 is a space within tree 100 having a circumference defined by passage 144 and ends defined by lower plug 126 and seal 148 of crown plug 146. Cavity 150 may also include the volume associated with bores or recesses on the top of lower plug 126 or the bottom of crown plug 146. Completion fluids can be trapped in cavity 150 when upper crown plug 146 is sealed in place, which is after tree 100 is installed subsea. Once upper crown plug 146 is installed, the fluid pressure of cavity 150 will not necessarily remain at the hydrostatic pressure of the seawater surrounding tree 100.
The trapped wellbore fluids can thermally expand within cavity 150, causing an increase in pressure. Compensator 152 can be located within cavity 150 to mitigate such pressure increases. Tree 100 is one example of a wellhead assembly. Compensator 152 can be used in any type of wellhead assembly having a cavity which can contain fluids.
Referring to
Void 164 can contain a compressible fluid. The fluid can be, for example, a gas such as air, argon, or nitrogen. Alternatively, the fluid can be a liquid. In one embodiment, the liquid has a high boiling point so that it does not expand significantly when heated. Alternatively, void 164 can contain a mixture of different types of fluids including, for example, multiple gases or combinations of gas and liquid. In another embodiment, void 164 can be evacuated such that the initial pressure is below ambient pressure. Plates 156, 158 can be sufficiently rigid that they generally maintain their shape when void 164 is evacuated. Fill valve 165 can be used to evacuate fluids from void 164 and introduce fluids into void 164.
Plates 156 and 158 can be generally flat and parallel to each other. In one embodiment, plates 156 and 158 can remain generally flat and parallel to each other at a first external pressure within cavity 150. For example, the initial pressure in cavity 150, prior to thermal expansion, may be insufficient to alter the shape of plates 156 and 158, even though such initial pressure is greater than atmospheric pressure. The pressure of the fluid in void 164 or the rigidity of plates 156 and 158 can contribute to the plates remaining generally flat up to a certain external pressure. In one embodiment, the first external pressure can be the hydrostatic pressure of the seawater at the tree 100.
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Crown plug 146 (
Similarly, the lower plate 158 can elastically deform, toward upper plate 156, thereby compressing the fluid in void 164 and allowing the now-expanded wellbore fluid to occupy space previously occupied by lower plate 158. Because the deformation of either or both plates 156, 158 is elastic, the plates can return to their original, relaxed state when the wellbore fluid cools and contracts. Thus, the pressure within cavity 150 does not drop to a pressure significantly lower than its initial pressure.
Referring to
Shell 212 and bell 214 can be joined by any of a variety of techniques. In one embodiment, joint 226 can be a weld, wherein shell 212 and bell 214 are welded together to form a seal. In other embodiments (not shown), joint 226 can include, for example, adhesive seals, threaded connections, and elastomeric seals. In the welded embodiment, port 220 can remain unsealed during the welding process to allow fumes from gap 216 to escape.
Compensator 210 can be introduced into cavity 150 (
In operation of one embodiment of compensator 210, gas can be introduced into gap 216, through plug 224, which can be a check-valve plug, to pressurize gap 216 to a pressure that is greater than atmospheric pressure. The pressure can be selected to support shell 212, such that shell 212 does not deform due to the hydrostatic pressure in cavity 150, but still allow shell 212 to elastically deform when the pressure in cavity 150 increases to a predetermined level above hydrostatic pressure.
Compensator 210 can then be connected to upper crown plug 146 via threads 228, secured against rotation by set screw 230, and lowered into cavity 150 (
While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.