Patent Publication Number: US-2005121198-A1

Title: Subsea completion system and method of using same

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&#39;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.