US20050121198A1

US20050121198A1 - Subsea completion system and method of using same

Info

Publication number: US20050121198A1
Application number: US10/982,399
Authority: US
Inventors: Jimmy Andrews
Original assignee: Individual
Current assignee: FMC Technologies Inc
Priority date: 2003-11-05
Filing date: 2004-11-04
Publication date: 2005-06-09
Also published as: WO2005047639A2; WO2005047639A3

Abstract

The present invention is directed to various embodiments of a subsea production system. In one illustrative embodiment, a subsea completion system includes a tubing string, a Christmas tree defining a bottom opening adapted to have the tubing string inserted therein, and means for latching the tubing string to prevent movement relative to the Christmas tree. A method includes coupling a tubing string with a Christmas tree above water and landing the Christmas tree on a subsea wellhead.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 60/517,576, filed Nov. 5, 2003, which is hereby incorporated by reference in its entirety for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to a completion system for oil and gas wells. More specifically, the present invention relates to a subsea completion system in which a tubing string is connected directly to a Christmas tree. In one embodiment, the tubing string and Christmas tree are installed simultaneously.
2. Description of the Related Art
A typical subsea well comprises a high pressure wellhead housing which supports one or more casing hangers located at the upper ends of strings of casing that extend into the well. The system further comprises a tubing hanger that supports a string of production tubing through which the oil and/or gas products will eventually be produced. Such a system further comprises a production tree or Christmas tree, e.g., a horizontal or vertical Christmas tree that contains one or more production bores and a plurality of actuatable valves to control the flow of fluids through the production tree.
Conventionally, wells in oil and gas fields are built up by establishing a wellhead housing and, with a drilling blow out preventer stack installed, drilling down to produce the well hole while successively installing concentric casing strings, which are cemented at the lower ends and sealed with mechanical seal assemblies at their upper ends. In order to convert the cased well for production, a tubing string is run in through the blow-out preventer and a hanger at its upper end landed in the wellhead. Conventionally, the tubing hanger is latched into a wellhead, into a casing hanger inside the wellhead, into an adapter inside the wellhead, or into a tubing head that has been previously latched to the wellhead. Thereafter the drilling blow-out preventer stack is removed and replaced by a Christmas tree having one or more production bores containing actuatable valves and extending vertically to respective lateral production fluid outlet ports in the wall of the Christmas tree.
Such an arrangement introduces many problems that have previously been accepted as inevitable. For example, the fluid connection between the Christmas tree and the tubing string has conventionally been made under water, requiring plugs or barriers to be used to contain well pressure. Such plugs or barriers are conventionally removed via the annulus passageway, requiring wireline access to the annulus. To provide wireline access to the annulus, the annulus valves of the Christmas tree are oriented vertically, resulting in a significantly tall Christmas tree.
Connecting the various components of the completion underwater also presents other problems. For example, it may be difficult to create a proper seal between the Christmas tree and the tubing string and correcting a leak, if one exists, may be difficult. Further, downhole hydraulic and electrical service and control lines are connected subsea, which also may suffer from leaks.
Moreover, each individual component that is installed subsea generally requires a remote installation and retrieval tool. Such tools may be very expensive and must be maintained in proper working order, further increasing cost.
The present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems set forth above.

SUMMARY OF THE INVENTION

The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an exhaustive overview of the invention. It is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later.
In one aspect of the present invention, a subsea completion system includes a tubing string, a Christmas tree defining a bottom opening adapted to have the tubing string inserted therein, and means for latching the tubing string to prevent movement relative to the Christmas tree.
In another aspect of the present invention, a system adapted to be coupled to a subsea wellhead includes a tubing hanger adapted to be positioned in the wellhead, the tubing hanger defining a flow opening extending therethrough; and a Christmas tree operatively coupled to the tubing hanger above water.
In yet another aspect of the present invention, a method includes coupling a tubing string with a Christmas tree above water and landing the Christmas tree on a subsea wellhead.
In another aspect of the present invention, a method includes coupling a tubing string with a tubing hanger above water, landing the tubing hanger on a subsea wellhead, and landing a Christmas tree on the subsea wellhead.
In yet another aspect of the present invention, a subsea Christmas tree defines a lower bore adapted to mate with a tubing string, such that the tubing string is latchable in the Christmas tree.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which the leftmost significant digit(s) in the reference numerals denote(s) the first figure in which the respective reference numerals appear, and in which:
FIG. 1 is a cross-sectional, side view of an illustrative embodiment of a subsea Christmas tree according to the present invention;
FIG. 2 is a cross-sectional, side view of an illustrative embodiment of a tubing hanger according to the present invention;
FIG. 3 is a cross-sectional, side view of the tubing hanger of FIG. 2 and a tubing string suspended from the subsea Christmas tree of FIG. 1;
FIG. 4A is a cross-sectional, side view of a Christmas tree employing a first alternative latching mechanism according to the present invention;
FIG. 4B is an enlarged view of a portion of the Christmas tree of FIG. 4A;
FIG. 5 is a cross-sectional, side view of a Christmas tree employing a second alternative latching mechanism according to the present invention;
FIG. 6 is an enlarged, cross-sectional, side view of a portion of the latching mechanism of FIG. 5;
FIG. 7 is a cross-sectional, side view of the Christmas tree/tubing hanger assembly of FIG. 3 installed on a subsea wellhead;
FIG. 8A-FIG. 8K are cross-sectional, side views depicting an illustrative installation sequence of the subsea Christmas tree of FIG. 1 and the tubing hanger of FIG. 2;
FIG. 9A-FIG. 9D are cross-sectional, side views depicting an alternative illustrative installation sequence of the subsea completions system according to the present invention;
FIG. 10 is a cross-sectional, side view of an alternative embodiment of a completion system according to the present invention;
FIG. 11 is a cross-sectional, side view of an alternative embodiment of a completion system including a tubing head according to the present invention illustrating the tubing hanger of FIG. 2 latched in a tubing head; and
FIG. 12 is a cross-sectional, side view of an alternative embodiment of a completion system according to the present invention including a tubing head comprising a latching means.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer's specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.
The present invention will now be described with reference to the attached figures. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase.
In the specification, reference may be made to the direction of fluid flow between various components as the devices are depicted in the attached drawings. However, as will be recognized by those skilled in the art after a complete reading of the present application, the device and systems described herein may be positioned in any desired orientation. Thus, the reference to the direction of fluid flow should be understood to represent a relative direction of flow and not an absolute direction of flow. Similarly, the use of terms such as “above,” “below,” or other like terms to describe a spatial relationship between various components should be understood to describe a relative relationship between the components as the device described herein may be oriented in any desired direction.
Referring to FIG. 1, one illustrative embodiment of a completion system according to the present invention includes a subsea Christmas tree 100 comprising production valves 102 a-102 c and annulus valves 104 a-104 c. While FIG. 1 illustrates a particular number of production valves 102 and annulus valves 104, the present invention is not so limited. Rather, the number of production valves 102 and annulus valves 104 will be implementation specific. An upper end of the Christmas tree 100 comprises a hub profile, indicated generally at 106, for connected to a subsea riser or other device. A subsea hydraulic connector 108 is disposed on the lower end of the Christmas tree 100 for connecting to a subsea wellhead 402 (shown in FIG. 4). The Christmas tree 100 also defines a bore 112 at a lower end thereof. In the illustrated embodiment, the bore 112 comprises one or more internal sealing surfaces 114 and an internal groove profile 116.
Referring now to FIG. 2, an illustrative embodiment of a tubing hanger assembly 200 according to the present invention comprises an annular tubing hanger body 202 having an upper end 204 and a lower end 206. The lower end 206 of the tubing hanger body 202 is adapted to suspend a tubing string 208 via a threaded connection 210 or by other suitable means. Note that the term “tubing string” is used herein to mean a production tubing string or any other type of tubing string. The upper end 204 of the tubing hanger body 202 is adapted for insertion into the bore 112 (shown in FIG. 1) of the Christmas tree 100. One or more seals or seal assemblies are provided on the upper end of the tubing hanger body 202 for sealing between the tubing hanger body 202 and the sealing surfaces 114 of the Christmas tree bore 112. The seals or seal assemblies may comprise any of several suitable seal configurations that are well known to the art. For example, the seals or seal assemblies may comprise primary metal-to-metal seals 212 a, 212 b (known in the art as “SBMS seals”) and secondary elastomeric “S” seals 214 a, 214 b.
The tubing hanger assembly 200 further comprises an expandable lock ring 216 disposed below a radially extending flange 218 of the tubing hanger body 202. In the illustrated embodiment, a lock mandrel 220 is slidably disposed about the tubing hanger body 202 below the lock ring 216. The lock mandrel 220 rests upon an anti-backoff member 222, such as a threaded nut, which is threadedly engaged with the outer surface of the tubing hanger body 202 below the lock mandrel 220.
Referring also to FIG. 3, when the upper end 204 of the tubing hanger assembly 200 is inserted into the bore 112 of the Christmas tree 100, the seals (e.g., the metal-to-metal seals 212 a, 212 b and the elastomeric seals 214 a, 214 b) engage the sealing surfaces 114 of the bore 112. The Christmas tree 100 is thus sealed to the tubing hanger assembly 200. The lock mandrel 220 is then moved upward to a locked position using a hydraulic jack (not shown) or any other appropriate means. As the lock mandrel 220 moves toward its locked position, a first end 224 thereof engages and expands the lock ring 216 outwardly in a manner known to the art. One or more surfaces 226 a, 226 b of the lock ring 216 engage the profile 116 of the bore 112 of the Christmas tree 100, thus locking the tubing hanger assembly 200 to the Christmas tree 100. The anti-backoff member 222 is then advanced upwardly until it contacts the lock mandrel 220, thus preventing the mandrel from moving away from its locked position.
In the illustrated embodiment, the weight of the tubing string 208 and the tubing hanger assembly 200 is transmitted from the external flange 218 of the tubing hanger body 202, through the lock ring 216 and the Christmas tree profile 116, to the Christmas tree 100. In this way, the tubing hanger assembly 200 can be installed in the Christmas tree 100 and the combined unit can be run into the well.
Note that any other suitable means can be employed within the scope of the present invention for connecting the tubing string 208 to the Christmas tree 100. For example, a hydraulically actuated connector, a threaded connector (e.g., a tubing thread or a low-turn casing connector), a threaded retainer nut, or a shrink-fit coupling can be used to connect the tubing string 208 to the Christmas tree 100. In one particular alternative embodiment, shown in FIGS. 4A-4B, the tubing string 208 is connected to the Christmas tree 100 via a union nut arrangement. A first union nut 400 retains a tubing hanger 402 to an intermediate spoolpiece 404. A second union nut 406 retains the intermediate spoolpiece 404 to the Christmas tree 100. A seal sleeve 408, comprising seal elements 410, 412 on an upper end thereof and seal elements 414, 416 on a lower end thereof, serves as a conduit and seal between the tubing hanger 402 and the Christmas tree 100. Upon make up of the tubing hanger 402 into the intermediate spoolpiece 404, automatic couplers 418 connect the downhole hydraulic and electrical circuits.
In another alternative embodiment, shown in FIG. 5, the tubing string 208 is connected to the Christmas tree via a shrink fit coupling 500. The coupling 500 comprises a pin 502 attached to the tubing string 208 generally at 504 and a box 506 coupled to the Christmas tree 100, generally at 508. The scope of the present invention, however, encompasses an intermediate coupling to attach the box 506 to the Christmas tree 100. Moreover, the pin 502 may be attached to the Christmas tree 100 and the box 506 attached to the tubing string 208.
An enlarged, cross-sectional view of a portion of the coupling 500 is shown in FIG. 6. The pin 502 and the box 506 each define complementary grooves 602, 604 that engage one another. The box 506 also defines an inlet 606, through which a pressurized fluid (e.g., a gas or a liquid) may be introduced via a fluid flow path to the interface between the box 506 and the pin 502. When such pressure is introduced through the inlet 606, the box 506 is expanded and the pin 502 is compressed, such that the grooves 602, 604 are disengaged from one another, allowing the pin 502 and the box 506 to be mated. When pressure is released from the inlet 606, the grooves 602, 604 engage one another to retain the pin 502 and the box 506 together.
Still referring to FIG. 6, the coupling 500 may include, in certain embodiments, one or more sealing elements for inhibiting or preventing fluid from flowing between the pin 502 and the box 506. In the illustrated embodiment, the coupling 500 includes first and second metal-to- metal seals 608, 610. The seal 608 comprises a protrusion 612 defined by the pin 502 that is received into and seals against a recess 614 defined by the box 506. The seal 610 comprises a protrusion 616 defined by the box 506 that is received into and seals against a recess 618 defined by the pin 502. The coupling 500 also comprises a first elastomeric seal 620 disposed in a recess 622 defined by the box 506 that seals against a sealing surface 624 of the pin 502. Moreover, the coupling 500 includes a second elastomeric seal 626 disposed in a recess 628 defined by the pin 502 that seals against a sealing surface 630 of the box 506. Note that the present invention may include one or more of the seals 608, 610, 620, 626 in any chosen combination and location in the coupling 500.
Referring to FIG. 7, when the Christmas tree 100, tubing hanger 200, and tubing string 208 are run as an assembly to the subsea wellhead 702, the subsea hydraulic connector 108 of the Christmas tree 100 connects the lower end of the Christmas tree 100 to the wellhead hub profile 704 in a manner well known to the art. Note that any other suitable means can be employed within the scope of the present invention to connect the Christmas tree 100 to the wellhead 702.
An illustrative installation sequence for the completion system detailed above in relation to FIG. 1-FIG. 7 is described below, with reference to FIG. 8A-FIG. 8K. Note that the present invention is not limited to the installation sequence disclosed below. Referring now to FIG. 8A, a segmented, split stabbing guide 802 is installed over the subsea wellhead 702 using a remotely operated vehicle 804 (shown in FIG. 8I). The stabbing guide 802 helps guide the end of the tubing string 208 into the wellhead 702. As shown in FIG. 8B, the tubing string 208 is suspended from a rig or vessel (not shown) at the water surface. The tubing hanger assembly 200 is installed onto the uppermost joint of the tubing string 208 and is supported using a tubing hanger running tool 806. In the illustrated embodiment, a lower end of the tubing hanger running tool 806 defines a bore 808 that engages the tubing hanger assembly 200 in a manner similar to the engagement of the tubing hanger assembly 200 and the Christmas tree 100.
Referring now to FIG. 8C, the tubing hanger assembly 200 is then lowered to a split support structure 810 at a lower level of the rig or vessel (not shown), such as a moonpool of a rig. Note that the split support structure 810 can be moved between a non-engaged position and an engaged position (shown in FIG. 8C) to support the tubing hanger assembly 200. The tubing hanger running tool 806 is then removed from the tubing hanger assembly 200.
Turning now to FIG. 8D, a set of adjustable or collapsible tree supports 812 (illustrated in a stylized fashion in FIG. 8D) are then installed on the support structure 810. The tree supports 812 can include mechanical or hydraulic jacks or any other suitable device to allow elevational positioning of the Christmas tree 100 and/or the tubing string 208 (with the tubing hanger assembly 200 installed thereon) with respect to one another. A lower riser package 814 is mated with the upper end 106 of the Christmas tree 100 and the Christmas tree 100/lower riser package 814 assembly is landed on the tree supports 812 using a handling tool 816. As illustrated in FIG. 8E, the tree supports 812 are then manipulated to either lower the Christmas tree 100 onto the tubing hanger assembly 200 or to raise the tubing hanger assembly 200 into the Christmas tree 100. The tubing hanger assembly 200 is then locked to the Christmas tree 100 in the manner described above in relation to FIG. 3.
Referring to FIG. 8F, a stress joint 818 and a riser 820 are attached to the lower riser package 814 in place of the handling tool 816. The Christmas tree 100 is then lifted from the tree supports 812 and the tree supports 812 are removed. As shown in FIG. 8G, the lower riser package 814, the Christmas tree 100, the tubing hanger assembly 200, and the tubing string 208 are then run through open water to the subsea wellhead 702 simultaneously (i.e., as a single unit), without the use of a blow-out preventer. In certain configurations, a blow-out preventer or like device may be used in conjunction with the present invention. For example, in certain embodiments, a shut off device or an environmental safety guard that is split may be employed, such that it may be removed after landing the Christmas tree 100.
Referring now to FIG. 8H, a landing joint 822 and a surface flow tree 824 are assembled at the rig or vessel (not shown), and a workover umbilical 826 is prepared for installation of the Christmas tree 100. As shown in FIG. 8I, the segmented stabbing guide 802 is released from the subsea wellhead using the remotely operated vehicle 804. Then, as illustrated in FIG. 8J, the Christmas tree 100 is landed and locked to the wellhead 702, after which the interface between the Christmas tree 100 and the wellhead 702 is tested. Once testing is completed, the stress joint 818, the riser 820, and the lower riser package 814 are retrieved to the surface and a tree cap 828 is installed on the Christmas tree 100, as shown in FIG. 8K.
The present invention embodies a number of distinct advantages and benefits. By making-up the tubing string 208 to the Christmas tree 100 prior to installation in the wellhead 702, the critical connection between the tubing string 208 and the Christmas tree 100 is simplified, thus reducing the cost of the connection and the number of potential leak paths. Furthermore, this connection can be checked at the surface. Similarly, downhole hydraulic and electrical service and control lines can be connected and checked at the surface, thereby eliminating the uncertainty inherent in downhole connections made subsea. The hydraulic and electrical lines may be terminated and connected at the bottom of the Christmas tree 100, or they may be routed through the Christmas tree 100, thus eliminating all connections below and within the Christmas tree 100, which increases the reliability of these lines.
Attaching the tubing string 208 directly to the tree allows the size of the tubing and the production bore to be increased. Heavier tubing strings 208 can be utilized without the use of exotic materials and without increasing the size of the wellhead 702. Thus, higher production flow rates can be achieved. The Christmas tree 100 of the present invention includes all of the valves normally required for a vertical, or conventional, subsea completion system. However, because there are no plugs or barriers that must be pulled via the annulus passageway, wireline access to the annulus is not required. Thus, the annulus valves 104 a-104 c may be moved to the side and/or placed in a separate wing block, as illustrated in FIG. 1. This results in a significant reduction in height of the Christmas tree 100 as compared to a conventional vertical Christmas tree. The need for wireline bore selection devices is also eliminated.
The completion system of the present invention may be installed in either an oriented or an un-oriented configuration. Aspects such of oriented and un-oriented configurations are presented in a commonly-assigned U.S. patent application entitled “Subsea Completion System, and Methods of Using Same” by inventor Christopher D. Bartlett, filed on Oct. 14, 2004 and having attorney docket number 2065.003200, which is hereby incorporated by reference for all purposes. The system may be installed without the use of guide funnels because the tubing string 208 itself guides the Christmas tree 100 into position upon landing on the wellhead 702. Another cost-saving feature of the present invention is the elimination of all remote installation and retrieval tools for the tubing hanger.
Alternatively, as shown in FIG. 9A-FIG. 9D, the tubing hanger assembly 200 and tubing string 208 can be run to the wellhead 702 independently of the Christmas tree 100. As shown in FIG. 9A, the tubing hanger assembly 200 and tubing string 208 is also run through open-water via the tubing hanger running tool 806 and without a blow-out preventer. In such an embodiment, the tubing hanger assembly 200 would be supported in the wellhead 702, as illustrated in FIG. 9B, and, in some configurations, latched thereto. After removing the tubing hanger running tool 806 (shown in FIG. 9C), the Christmas tree 100 is then run subsea and latched to the wellhead 702, the tubing hanger assembly 200, or both (shown in FIG. 9D). It is contemplated that the means for latching the tubing hanger assembly 200 to the Christmas tree 100 could be operable independently of the means for latching the tubing hanger assembly 200 to the wellhead 702. Thus, the tubing hanger assembly 200 could be latched to the Christmas tree 100, the wellhead 702, or both, simultaneously.
It is also contemplated that a single means could be employed that would latch the tubing hanger assembly 200 to either the wellhead 702 or the Christmas tree 100 and simultaneously unlatch the tubing hanger assembly 200 from the other of these two components. In this way, the Christmas tree 100 could be retrieved at a later point in time without necessitating the retrieval of the entire completion string (ie., the entire completion system). Alternatively, if it is desired to also retrieve the tubing string 208, then the entire completion string can be retrieved in combination with the retrieval of the subsea Christmas tree 100.
In another illustrative embodiment, shown in FIG. 10, a tubing hanger 1002 is landed in and latched to the wellhead 702 and the latching means 1000 is housed in the wellhead 702. In such an embodiment, the outside diameter of the tubing hanger 1002 includes only locking grooves 1004 and is reduced in size. The bore 1005 of the Christmas tree 1006 is then of sufficient size to drift the tubing hanger 1002, so that the tubing string 208 and the tubing hanger 1002 can be landed through the Christmas tree 1006. Alternatively, the tubing string 208 and the tubing hanger 1002 can be landed before the Christmas tree 1006.
In yet another alternative embodiment, shown in FIG. 11, the tubing hanger assembly 200 is landed and locked in a separate tubing head 1102. The Christmas tree 100 is then attached to the top of the tubing head 1102 at the surface. The tubing head 1102, tubing hanger assembly 200, and Christmas tree 100 are all run through open-water to the wellhead 702 simultaneously.
An embodiment is also contemplated, as shown in FIG. 12, wherein the tubing hanger 1202 is landed in and latched to a tubing head 1204 and the latching means 1206 is housed in the tubing head 1204. In such an embodiment, the outside diameter of the tubing hanger 1202 includes only locking grooves 1208 and is reduced in size.
In one embodiment, a subsea completion system according to the present invention includes a tubing string, a Christmas tree defining a bottom opening adapted to have the tubing string inserted therein, and means for latching the tubing string to prevent movement relative to the Christmas tree. A method of using the subsea completion system includes coupling a tubing string with a Christmas tree above water and landing the Christmas tree on a subsea wellhead. A subsea Christmas tree defines a lower bore adapted to mate with a tubing string, such that the tubing string is latchable in the Christmas tree.
This concludes the detailed description. The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.

Claims

1. A subsea completion system, comprising:

a tubing string;

a Christmas tree defining a bottom opening adapted to have the tubing string inserted therein; and

means for latching the tubing string to prevent movement relative to the Christmas tree.

2. A subsea completion system, according to claim 1, wherein the means for latching the tubing string in the subsea completion system comprises a tubing hanger assembly adapted to be operatively coupled with the Christmas tree.

3. A subsea completion system, according to claim 2, wherein the Christmas tree defines a bore including a sealing surface and a groove profile and the tubing hanger assembly comprises:

a body including an upper end received in the bore of the Christmas tree and a lower end threadedly connected to the tubing string;

a seal disposed on the upper end engaged with the sealing surface of the Christmas tree bore for sealing between the Christmas tree and the body;

a lock ring disposed about the body;

an anti-backoff member threadedly engaged with the body; and

a lock mandrel slidably disposed about the body and resting on the anti-backoff member, the lock mandrel being engageable with the lock ring to expand the lock ring and engage the groove profile of the Christmas tree.

4. A subsea completion system, according to claim 3, wherein the anti-backoff member comprises an anti-backoff nut.

5. A subsea completion system, according to claim 3, wherein the Christmas tree bore includes a first sealing surface and a second sealing surface and the tubing hanger assembly comprises a metal-to-metal seal engaged with the first sealing surface and an elastomeric seal engaged with the second sealing surface.

6. A subsea completion system, according to claim 1, wherein the Christmas tree comprises an annulus valve disposed on the side of the Christmas tree or in a separate wing block of the Christmas tree.

7. A subsea completion system, according to claim 1, wherein the tubing string is latched above water.

8. A subsea completion system, according to claim 1, further comprising a tubing hanger, wherein the tubing string is latched to the Christmas tree via the tubing hanger.

9. A subsea completion system, according to claim 1, further comprising a tubing hanger and a tubing head, wherein the tubing string is latched to the Christmas tree via the tubing hanger and the tubing head.

10. A subsea completion system, according to claim 9, wherein the tubing hanger comprises a latch.

11. A subsea completion system, according to claim 9, wherein the tubing head comprises a latch.

12. A subsea completion system, according to claim 1, further comprising a tubing hanger, wherein the tubing string is adapted to be latched to a well head via the tubing hanger.

13. A subsea completion system, according to claim 1, wherein the means for latching comprises one of a hydraulically actuated connector, a threaded connector, a threaded retainer nut, and a shrink-fit coupling.

14. A subsea completion system, according to claim 13, wherein the shrink-fit coupling comprises:

a pin attached to one of the tubing string and the Christmas tree; and

a box defining a fluid flow path and attached to the other of the tubing string and the Christmas tree, the box being operatively engageable with the pin,

wherein the box is disengaged from the pin when a pressurized fluid is transmitted through the fluid flow path and the box is engaged with the pin when a pressurized fluid is absent from the fluid flow path.

15. A subsea completion system, according to claim 14, further comprising a sealing element for inhibiting or preventing fluid flow between the box and the pin.

16. A system adapted to be coupled to a subsea wellhead, comprising:

a tubing hanger adapted to be positioned in the wellhead, the tubing hanger defining a flow opening extending therethrough; and

a Christmas tree operatively coupled to the tubing hanger above water.

17. A system, according to claim 16, further comprising a tubing string attached to the tubing hanger and defining a flow opening therethrough, such that the flow opening of the tubing string and the flow opening of the tubing hanger are in fluid communication.

18. A system, according to claim 16, wherein the Christmas tree is operatively coupled to the tubing hanger via a tubing head.

19. A method, comprising:

coupling a tubing string with a Christmas tree above water; and

landing the Christmas tree on a subsea wellhead.

20. A method, according to claim 19, wherein coupling the tubing string with the Christmas tree comprises installing a tubing hanger on an uppermost joint of the tubing string and locking the tubing hanger to the Christmas tree.

21. A method, according to claim 20, wherein locking the tubing hanger to the Christmas tree comprises expanding a lock ring of the tubing hanger.

22. A method, according to claim 21, wherein expanding the lock ring of the tubing hanger comprises engaging a lock mandrel of the tubing hanger with the lock ring.

23. A method, according to claim 20, further comprising lowering the tubing hanger, with the tubing string attached, to a support structure of a rig or vessel.

24. A method, according to claim 23, further comprising landing the Christmas tree on tree supports disposed on the support structure.

25. A method, according to claim 24, further comprising manipulating the tree supports to mate the Christmas tree and the tubing hanger.

26. A method, according to claim 19, further comprising guiding the Christmas tree to the subsea wellhead using the tubing string.

27. A method, according to claim 26, further comprising installing a stabbing guide over the subsea wellhead before guiding the Christmas tree to the subsea wellhead.

28. A method, according to claim 27, wherein installing the stabbing guide comprises installing the stabbing guide with a remotely operated vehicle.

29. A method, according to claim 19, further comprising running the Christmas tree, the tubing hanger, and the tubing string open-water to a well extending from the subsea wellhead.

30. A method, according to claim 29, wherein running the Christmas tree, the tubing hanger, and the tubing string further comprises running the Christmas tree, the tubing hanger, and the tubing string without a blow-out preventer.

31. A method, according to claim 19, further comprising removing a stabbing guide from the subsea wellhead before landing the Christmas tree on the subsea wellhead.

32. A method, according to claim 31, wherein removing the stabbing guide comprises removing the stabbing guide with a remotely operated vehicle.

33. A method, according to claim 19, wherein coupling the tubing string with the Christmas tree comprises installing a tubing hanger on an uppermost joint of the tubing string, locking the tubing hanger in a tubing head, and attaching the tubing head to the Christmas tree.

34. A method, comprising:

coupling a tubing string with a tubing hanger above water;

landing the tubing hanger on a subsea wellhead; and

landing a Christmas tree on the subsea wellhead.

35. A method, according to claim 34, further comprising latching the tubing hanger to the Christmas tree.

36. A method, according to claim 34, further comprising latching the tubing hanger to the wellhead.

37. A method, according to claim 34, wherein landing the tubing hanger on the subsea wellhead further comprises landing the tubing hanger on the subsea wellhead via a tubing head and latching the tubing hanger to the tubing head.

38. A subsea Christmas tree defining a lower bore adapted to mate with a tubing string, such that the tubing string is latchable in the Christmas tree.

39. A subsea Christmas tree, according to claim 38, wherein the lower bore is adapted to mate with the tubing string via one of a tubing hanger, a hydraulically actuated connector, a threaded connector, a threaded retainer nut, and a shrink fit coupling.