Patent Publication Number: US-9835005-B2

Title: Energized seal system and method

Description:
BACKGROUND 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     As will be appreciated, oil and natural gas have a profound effect on modern economies and societies. In order to meet the demand for such natural resources, numerous companies invest significant amounts of time and money in searching for and extracting oil, natural gas, and other subterranean resources from the earth. Particularly, once a desired resource is discovered below the surface of the earth, drilling and production systems are employed to access and extract the resource. These systems can be located onshore or offshore depending on the location of a desired resource. Further, such systems generally include a wellhead assembly that is used to extract the resource. These wellhead assemblies include a wide variety of components and/or conduits, such as various control lines, casings, valves, and the like, that are conducive to drilling and/or extraction operations. In drilling and extraction operations, in addition to wellheads, various components and tools are employed to provide for drilling, completion, and the production of mineral resources. For instance, during drilling and extraction operations seals and valves are often employed to regulate pressures and/or fluid flow. 
     A wellhead system often includes a tubing hanger and/or casing hanger that is disposed within the wellhead assembly and configured to secure tubing and casing suspended in the well bore. In addition, the hanger generally regulates pressures and provides a path for hydraulic control fluid, chemical injections, or the like to be passed through the wellhead and into the well bore. In such a system, various seals (e.g., annular seals) are often disposed between various components of the wellhead system, such as the tubing spool, casing spool, casing hanger, tubing hanger, pack off assembly, and so forth, to regulate and isolate pressure between such components. For example, such seals may be formed from elastomers, among other suitable materials. Unfortunately, such materials may be susceptible to degradation caused by a wide range of pressures and temperatures to which the materials are exposed within the wellhead system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying figure, wherein: 
         FIG. 1  is a block diagram of a mineral extraction system in accordance with embodiments of the present disclosure; 
         FIG. 2  is a partial schematic of a wellhead system having a sealing system, in accordance with embodiments disclosure; 
         FIG. 3  is a partial schematic of a wellhead system having a sealing system, in accordance with embodiments disclosure; 
         FIG. 4  is a cross-sectional side view of a sealing system, in accordance with embodiments disclosure; 
         FIG. 5  is a cross-sectional side view of a sealing system, in accordance with embodiments disclosure; 
         FIG. 6  is a schematic of a sealing system, in accordance with embodiments disclosure; and 
         FIG. 7  is a schematic of a sealing system, in accordance with embodiments disclosure. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     One or more specific embodiments of the present disclosure will be described below. These described embodiments are only exemplary of the present disclosure. Additionally, in an effort to provide a concise description of these exemplary embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present disclosure, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Moreover, the use of “top,” “bottom,” “above,” “below,” and variations of these terms is made for convenience, but does not require any particular orientation of the components. 
     Embodiments of the present disclosure include a system and method that addresses one or more of the above-mentioned inadequacies of conventional sealing systems and methods. As explained in greater detail below, the disclosed embodiments include sealing systems that can be installed and energized or “pre-charged” to modify the seal contact pressure of one or more seals (e.g., elastomeric seals) of the sealing system. For example, energizing seals of the sealing system may enable an increase of seal contact pressure of the seals at low temperatures. As will be appreciated, at low temperatures (e.g., below 0° C.), elastomeric seals may shrink and/or lose initial contact stress, which may decrease performance of the sealing system. That is, low temperatures may traditionally reduce the sealing capability of elastomeric seals. By energizing or pre-charging the elastomeric seals of the sealing system, the initial contact stress (e.g., seal contact pressure) may be increased initially and thereafter maintained when wellhead temperatures fall. As described in detail below, the seals of the sealing system may be energized or pre-charged via loading (e.g., lateral loading) provided by a gas, a mechanical spring, or other manner of pressurization. 
     Additionally, energizing or pre-charging seals of the sealing system may improve performance of the seals (e.g., elastomeric seals) at high temperatures. Specifically, at high temperatures, the seals of the sealing system may expand, and pre-charged sealing system may absorb forces exerted by the expanding seal. For example, in embodiments of the sealing system having a mechanical spring, the mechanical spring may compress as the seal expands at high temperatures, thereby reducing extrusion of the seal and controlling the seal contact pressure of the seal. Similarly, in embodiments of the sealing system where the sealing system is pre-charged with a gas (e.g., a compressible fluid), the gas may compress as the seal expands at high temperatures, thereby also reducing extrusion of the seal and controlling the seal contact pressure of the seal. Details of the sealing system and various embodiments of energizing and pre-charging the seals of the sealing system to modify seal contact pressures of the seals are described below. 
       FIG. 1  is a block diagram that illustrates a mineral extraction system  10 . The illustrated mineral extraction system  10  can be configured to extract various minerals and natural resources, including hydrocarbons (e.g., oil and/or natural gas), or configured to inject substances into the earth. The mineral extraction system  10  may be land-based (e.g., a surface system) or subsea (e.g., a subsea system). As illustrated, the system  10  includes a wellhead  12  coupled to a mineral deposit  14  via a well  16 , wherein the well  16  includes a wellhead hub  18  and a well-bore  20 . 
     The wellhead hub  18  generally includes a large diameter hub that is disposed at the termination of the well bore  20 . The wellhead hub  18  provides for the connection of the wellhead  12  to the well  16 . For example, the wellhead  12  includes a connector that is coupled to a complementary connector of the wellhead hub  18 . In one embodiment, the wellhead hub includes a complementary collet connector. 
     The wellhead  12  typically includes multiple components that control and regulate activities and conditions associated with the well  16 . For example, the wellhead  12  generally includes bodies, valves and seals that route produced minerals from the mineral deposit  14 , provide for regulating pressure in the well  16 , and provide for the injection of chemicals into the well bore  20  (down-hole). In the illustrated embodiment, the wellhead  12  includes what is colloquially referred to as a christmas tree  22  (hereinafter, a tree), a tubing spool  24  (and/or a casing spool), and a hanger  26  (e.g., a tubing hanger and/or a casing hanger). The system  10  may include other devices that are coupled to the wellhead  12 , and devices that are used to assemble and control various components of the wellhead  12 . For example, in the illustrated embodiment, the system  10  includes a tool  28  suspended from a drill string  30 . In certain embodiments, the tool  28  includes a retrievable running tool that is lowered (e.g., run) from an offshore vessel to the well  16  and/or the wellhead  12 . In other embodiments, such as those for surface systems, the tool  28  may include a device suspended over and/or lowered into the wellhead  12  via a crane or other supporting device. 
     The tree  22  generally includes a variety of flow paths (e.g., bores), valves, fittings, and controls for operating the well  16 . For instance, the tree  22  may include a frame that is disposed about a tree body, a flow-loop, actuators, and valves. Further, the tree  22  may provide fluid communication with the well  16 . For example, the tree  22  includes a tree bore  32 . The tree bore  32  provides for completion and workover procedures, such as the insertion of tools (e.g., the hanger  26 ) into the well  16 , the injection of various chemicals into the well  16  (down-hole), and the like. Further, minerals extracted from the well  16  (e.g., oil and natural gas) may be regulated and routed via the tree  22 . For instance, the tree  12  may be coupled to a jumper or a flowline that is tied back to other components, such as a manifold. Accordingly, produced minerals flow from the well  16  to the manifold via the wellhead  12  and/or the tree  22  before being routed to shipping or storage facilities. 
     The tubing spool  24  provides a base for the wellhead  24  and/or an intermediate connection between the wellhead hub  18  and the tree  22 . Typically, the tubing spool  24  is one of many components in a modular subsea or surface mineral extraction system  10  that is run from an offshore vessel or surface system. The tubing spool may support the tubing hanger and be mounted on a casing spool that supports a casing hanger, or the two spools may be combined into one and support one or more hangers. The tubing spool  24  includes the tubing spool bore  34 . The tubing spool bore  34  connects (e.g., enables fluid communication between) the tree bore  32  and the well  16 . Thus, the tubing spool bore  34  may provide access to the well bore  20  for various completion and workover procedures. For example, components can be run down to the wellhead  12  and disposed in the tubing spool bore  34  to seal-off the well bore  20 , to inject chemicals down-hole, to suspend tools down-hole, to retrieve tools down-hole, and the like. 
     As will be appreciated, the well bore  20  may contain elevated pressures. For example, the well bore  20  may include pressures that exceed 10,000 pounds per square inch (PSI), that exceed 15,000 PSI, and/or that even exceed 20,000 PSI. Accordingly, mineral extraction systems  10  employ various mechanisms, such as seals, plugs and valves, to control and regulate the well  16 . For example, seals and sealing systems are employed to isolate flow and pressures of fluids in various bores and channels throughout the mineral extraction system  10 . For instance, the illustrated hanger  26  (e.g., tubing hanger and/or casing hanger) is typically disposed within the wellhead  12  to secure tubing and casing suspended in the well bore  20 , and to provide a path for hydraulic control fluid, chemical injections, and the like. The hanger  26  includes a hanger bore  36  that extends through the center of the hanger  26 , and that is in fluid communication with the tubing spool bore  34  and the well bore  20 . As will be appreciated, pressures in the bores  20  and  34  may manifest through the wellhead  12  if not regulated and/or isolated. Sealing systems  38  may be disposed between the hanger  36  and the tubing spool  24  to isolate the pressure. In other words, the sealing systems  38  may be disposed in an annular region between the hanger  36  and the tubing spool  24 . Similar sealing systems  38  may be used throughout mineral extraction systems  10  to isolate fluid pressures and flows. For example, sealing systems  38  (e.g., annular sealing systems) may be disposed about other hangers  36 , pack-off assemblies, and other components of the wellhead  12  to isolate various pressures and fluid flows. The sealing systems  38  may include one or more seals, such as elastomer seals. In certain embodiments, the seals included in the sealing systems  38  may be simple elastomer seals or seals having an elastomer component along with other components. For example, a seal in the sealing system  38  may be a metal end cap seal having metal end caps disposed on opposite axial ends of an elastomer seal component. 
     As mentioned above, the sealing systems  38  disclosed herein may be energized and/or charged to modify a seal contact pressure of the one or more seals in the sealing system  38 . In particular, the sealing system  38  (e.g., seals of the sealing system) may be installed within the wellhead  12 , and the sealing system  38  (e.g., seals of the sealing system  38 ) may be initially energized by lateral loading. In certain embodiments, the seals of the sealing system  38  may be initially energized within the wellhead  12  by a pressurized gas. The pressurized gas may establish higher seal contact pressures of the seals against components of the wellhead  12  (e.g., the hanger  26  and the tubing spool  24 ). As a result, the seals of the sealing system  38  may have improved performance (e.g., more effective sealing contact) at high and/or low temperatures. In other embodiments, the one or more seals of the sealing system may be energized by a mechanical spring, such as a Bellville washer. As discussed in detail below, the mechanical spring (e.g., Bellville washer) may place an initial lateral load on the one or more seals to initially increase contact pressure of the one or more seals. At low temperatures, when the seal may shrink, the mechanical spring may expand to maintain seal contact pressure of the one or more seals against components of the wellhead  12  (e.g., the hanger  26  and the tubing spool  24 ). Conversely, at high temperatures, when the one or more seals may expand, the mechanical spring (e.g., Bellville washer) may compress to control the increased seal contact pressure of the one or more seals and reduce extrusion of the one or more seals within the wellhead  12 . 
       FIG. 2  is a partial schematic of an embodiment of the wellhead  12 , illustrating an embodiment of the sealing system  38  disposed between wellhead components. Specifically, the sealing system  38  is disposed between a tubing spool  50  (e.g., tubing spool  24 ) and a tubing hanger  52  (e.g., hanger  36 ). As similarly described above, the tubing hanger  52  is disposed within the tubing spool  50  to secure tubing suspended in the well bore  20 , and to provide a path (e.g., flow path  54 ) for hydraulic control fluid, chemical injections, and so forth. The sealing system  38  in the illustrated embodiment is an annular sealing system disposed about the tubing hanger  52  in an annular region between the tubing hanger  52  and the tubing spool  50 . The sealing system blocks flow and isolates pressure between the tubing hanger  52  and the tubing spool  50 . 
     As mentioned above, the sealing system  38  may be energized or pre-charged via a pre-charging system to modify a seal contact pressure of one or more seals in the sealing system  38 . In certain embodiments, the pre-charging system of the sealing system  38  may include one or more mechanical springs (e.g., a Belleville washer) to energize one or more seals of the sealing system  38 . As described in detail below, the mechanical spring may provide initial lateral loading of the seal of the sealing system  38  and may compress and/or expand to maintain seal contact pressure of the seal during temperature fluctuations. 
     In other embodiments, the pre-charging system of the sealing system  38  may include a pressurized gas (e.g., a compressible fluid) supplied by a gas source and a pump. In such an embodiment, the tubing spool  50  includes an energizing port  56  configured to flow a pressurized gas to the sealing system  38 . The pressurized gas may be supplied by a gas source  58  and a pump  60  that pumps the gas into the sealing system  38 . As discussed in detail below, the pressurized gas within the sealing system  38  may contact the seals of the sealing system  38  and force or bias the seals against surfaces of wellhead  12  components (e.g., the tubing hanger  52  and the tubing spool  50 ) to increase the seal contact pressure of the seals after the sealing system  38  is installed within the wellhead  12 . Additionally, as temperatures fluctuate during operation of the mineral extraction system  10 , the seals may expand and/or contract. At such times, the pressurized gas may compress and/or expand to help maintain the seal contact pressure of the seals against the tubing hanger  52  and the tubing spool  52 . To maintain pressure of the pressurized gas within the sealing system  38  and the energizing port  56 , the tubing spool  50  may include a plug or check valve  62 . As will be appreciated, for embodiments of the sealing system  38  having a mechanical spring to modify seal contact pressure, the tubing spool  50  may not include the energizing port  56 , and the mineral extraction system  10  may not include the gas source  58 , pump  60 , or plug  62 . 
       FIG. 3  is a partial schematic of an embodiment of the wellhead  12 , illustrating two embodiments of the sealing system  38  disposed between wellhead components. Specifically, the illustrated embodiment of the wellhead  12  includes a tubing spool  80 , a casing spool  82 , and a casing hanger  84  disposed within the tubing spool  80  and the casing spool  82 . The wellhead  12  also includes seal pack-off assemblies  86  (e.g., a first seal pack-off assembly  88  and a second seal-pack off assembly  90 ). The wellhead  12  further includes three sealing systems  38 , each of which may be energized or pre-charged to modify seal contact pressures of the one or more seals of each sealing system  38 . A first sealing system  92  (e.g., a cross-over sealing system) is disposed between the tubing spool  80  and the casing hanger  84 . A second sealing system  94  is disposed about the first pack-off assembly  88  between the casing spool  82  and the casing hanger  84 , and a third sealing system  96  is disposed about the second pack-off assembly  90  between the casing spool  82  and the casing hanger  84 . As will be appreciated, each of the first, second, and third sealing systems  92 ,  94 , and  96  may have an annular configuration (e.g., annular seals). In particular, the first sealing system  92  may be annularly disposed about the casing hanger  84 , while the second and third sealing systems  94  and  96  may be annularly disposed about the first and second pack-off assemblies  88  and  90 , respectively. 
     Each of the first, second, and third sealing systems  92 ,  94 , and  96  is energized or pre-charged to modify a seal contact pressure of the respective seals of the first, second, and third sealing systems  92 ,  94 , and  96 . For example, the first sealing system  92  is energized via a pressurized gas. As such, the tubing spool  80  includes an energizing port  98  extending through the tubing spool  80 . Specifically, the energizing port  98  extends from a pump  100  to the first sealing system  92 . As similarly discussed above with respect to  FIG. 2 , gas from a gas source  102  is pressurized by the pump  100  and supplied to the first sealing system  92 . Within the first sealing system  92 , the pressurized gas biases or forces seals of the first sealing system  92  against wellhead  12  components (e.g., the tubing spool  80  and the casing hanger  84 ) to initially increase seal contact pressure of the seals against the wellhead  12  components. The pressurized gas also expands and/or compresses as wellhead  12  temperatures fluctuate to help maintain seal contact pressure of the first sealing system  92 . A plug or check valve  104  disposed at the energizing port  98  may be included to help maintain pressurization of the pressurized gas within the first sealing system  92 . 
     The second sealing system  94  is also energized or pre-charged via a pressurized gas. As such, the casing spool  82  includes an energizing port  106  configured to supply pressurized gas to the second sealing system  94 . Specifically, gas is provided by the gas source  102 , pressurized by a pump  108 , and supplied to the second sealing system  94  via the energizing port  106 . Within the second sealing system  94 , the pressurized gas biases or forces seals of the second sealing system  94  against wellhead  12  components (e.g., the casing spool  82 , the second seal pack-off assembly  86 , and the casing hanger  84 ) to initially increase seal contact pressure of the seals against the wellhead  12  components. The pressurized gas also expands and/or compresses as wellhead  12  temperatures fluctuate to help maintain seal contact pressure of the second sealing system  94 . A plug or check valve  110  disposed at the energizing port  106  may be included to help maintain pressurization of the pressurized gas within the second sealing system  94 . 
     The third sealing system  96  is disposed about the second pack-off assembly  90  between the casing spool  82  and the casing hanger  84 . In the illustrated embodiment, the third sealing system  96  includes a mechanical spring (e.g., a Belleville washer) configured to initially energize one or more seals of the third sealing system  96 . In other words, the mechanical spring (e.g., annular mechanical spring) provides initial lateral loading on the one or more seals of the third sealing system  96  to initially increase seal contact pressure of the one or more seals against wellhead  12  components sealed by the sealing system  38  (e.g., the casing spool  82 , the second seal pack-off assembly  86 , and the casing hanger  84 ). During temperatures fluctuations of the wellhead  12 , the mechanical spring may compress and/or expand to maintain seal contact pressure of the seal against the casing spool  82 , the second seal pack-off assembly  86 , and the casing hanger  84 . As the third sealing system  96  is not energized or pre-charged by a pressurized gas, the casing spool  82  does not include an additional energizing port for the third sealing system  96 . Additionally, a gas source, pump, plug, and check valve are not used to energize the third sealing system  96 . 
     Although the first and second sealing systems  92  and  94  are illustrated as energized by pressurized gas, other embodiments of the first and second sealing systems  92  and  94  may be energized by mechanical springs, and thus may not include the gas source  102 , the pumps  100  and  108 , etc. Similarly, while the illustrated third sealing system  96  is energized by a mechanical spring, in other embodiments the third sealing system  96  may be energized by pressurized gas. As such, other embodiments may include additional energizing ports, pumps, plugs, check valves, etc. Indeed, the first, second, and third sealing systems  92 ,  94 , and  96  may be energized by any of the systems described herein or any combination thereof. For example, in certain embodiments, each of the first, second, and/or third sealing systems  92 ,  94 , and/or  96  may be energized by pressurized gas and one or more mechanical springs. 
       FIG. 4  is a cross-sectional side view, taken within line  4 - 4  of  FIG. 2 , illustrating an embodiment of the sealing system  38 . The sealing system  38  is energized by a pressurized gas supplied by the gas source  58  and pressurized by the pump  60 . The pressurized gas is delivered to the sealing system  38  through the energizing port  56  formed in the tubing spool  50 . 
     The sealing system  38  includes a first seal  120  (e.g., annular seal) and a second seal  122  (e.g., annular seal), each of which are disposed between the tubing spool  50  and the tubing hanger  52 . In the illustrated embodiment, the first and second seals  120  and  122  are metal end cap seals. Specifically, each of the first and second seals  120  and  122  includes an elastomer body  124  (e.g., annular body) with end caps  126  (e.g., metal end caps) disposed on axial ends of the respective elastomer body  124 . In other embodiments, the first and second seals  120  and  122  may be other types of seals (e.g., annular seals), such as elastomer seals, O-rings, and so forth. 
     The first and second seals  120  and  122  are axially offset from one another with a gas region  128  (e.g., annular region) disposed therebetween. The gas region  128  is exposed to the energizing port  56  formed in the tubing spool  50 . As a result, the pressurized gas from the pump  60  may fill the gas region  128  between the first and second seals  120  and  122 . The pressurized gas between the first and second seals  120  and  122  may contact the respective elastomer bodies  124  of each of the first and second seals  120  and  122 . More specifically, the pressurized gas may exert pressure (e.g., force) on the elastomer bodies  124  and thereby exert or apply a lateral load on the elastomer bodies  124 . In this manner, the seal contact pressure of the elastomer bodies  124  against the tubing spool  50  and tubing hanger  52  may be increased, thereby increasing the sealing contact between the first and second seals  120  and  122 , the tubing spool  50  and the tubing hanger  52 . The pressurized gas may be supplied to the gas region  128  and the sealing system  38  prior to operation of the wellhead  12 . As will be appreciated, increasing the initial seal contact stress of the elastomer bodies  124  of the first and second seals  120  and  122  may reduce the impact on the sealing performance of the first and second seals  120  and  122  caused by fluctuating operating temperatures of the wellhead  12 . For example, at lower temperatures when the elastomer bodies  124  may shrink, the pressurized gas within the sealing system  38  may help maintain sealing contact between the first and second seals  120  and  122 , the tubing spool  50  and the tubing hanger  52 . 
     In certain embodiments, the pressure of the pressurized gas within the gas region  128  of the sealing system  38  may be adjusted based on a temperature of the wellhead  12  or other operating parameters to help maintain a desired seal contact pressure between the first and second seals  120  and  122 , the tubing spool  50  and the tubing hanger  52 . For example, the sealing system  38  may include a controller  130  configured to regulate operation of the pump  60  to supply gas (e.g., Nitrogen) to the sealing system  38  at a desired pressure based on feedback from one or more sensors  132 . For example, a first sensor  134  may be configured to measure a temperature within an annular region  136  between the tubing spool  50  and tubing hanger  52 , and a second sensor  138  may be configured to measure a pressure (e.g., gas pressure) within the gas region  128 . In one embodiment, the controller  120  may control operation of the pump  60  to proportionally increase the pressure of the gas supplied to the sealing system  38  as a temperature measured by the first sensor  134  drops. 
       FIG. 5  is a cross-sectional side view, taken within line  5 - 5  of  FIG. 3 , illustrating an embodiment of the sealing system  38  (e.g., the second sealing system  94 ). The sealing system  38  is energized by a pressurized gas supplied by the gas source  58  and pressurized by the pump  60 . The pressurized gas is delivered to the sealing system  38  through the energizing port  106  formed in the casing spool  82 . As discussed above, the second sealing system  94  is a component of the first seal pack-off assembly  88  disposed between the tubing spool  80 , the casing spool  82 , and the casing hanger  84 . In the illustrated embodiment, the pressurized gas within the second sealing system  88  is retained by a plug  150  sealing the energizing port  106 . 
     The illustrated sealing system  38  includes a first seal  152  (e.g., annular seal) and a second seal  154  (e.g., annular seal), each of which are disposed about the first seal pack-off assembly  88 . In particular, the first seal  152  is disposed within a first seal recess  156  (e.g., annular recess) of the first seal pack-off assembly  88 , and the second seal  154  is disposed within a second seal recess  158  (e.g., annular recess) of the first seal pack-off assembly  88 . In the illustrated embodiment, the first and second seals  152  and  154  are elastomer seals, such as O-rings. The first and second seals  152  and  154  are axially offset about a gas region  160  of the sealing system  38 . The gas region  160  is at least partially defined by a recess  162  (e.g., annular recess) formed in the seal-pack off assembly  88 . Additionally, the gas region  160  is exposed to the energizing port  106  formed in the casing spool  82 . As a result, the pressurized gas from the pump  60  may fill the gas region  160  between the first and second seals  152  and  154 . Additionally, the pressurized gas between the first and second seals  152  and  154  may contact the first and second seals  152  and  154  and exert pressure (e.g., force) on the first and second seals  152  and  154 . In this manner, the pressurized gas applies a lateral load on the first and second seals  152  and  154 , thereby increasing the seal contact pressure of the first and second seals  152  and  154  against the casing spool  82 , the casing hanger  84 , and the seal pack-off assembly  88 . In this manner, the sealing contact between the first and second seals  152  and  154 , the casing spool  82 , the casing hanger  84 , and the seal pack-off assembly  88  may be increased. As similarly discussed above, increasing the initial seal contact stress of the first and second seals  152  and  154  may reduce the impact on the sealing performance of the first and second seals  152  and  154  caused by fluctuating operating temperatures of the wellhead  12 . 
       FIGS. 6 and 7  are cross-sectional side views, taken within line  6 - 6  of  FIG. 3 , illustrating an embodiment of the sealing system  38  (e.g., third sealing system  96  disposed about the second seal pack-off assembly  90 . The sealing system  38  is energized by a mechanical spring (e.g., Belleville washer). The embodiments illustrated in  FIGS. 6 and 7  include a seal  200  (e.g., annular elastomer seal) and a mechanical spring  202  (e.g., Belleville washer).  FIG. 6  illustrates the sealing system  38  at a lower temperature, and  FIG. 7  illustrates the sealing system at a higher temperature. 
     As discussed above, the third sealing system  96  is disposed about the second seal pack-off assembly  90  between the casing spool  82  and the casing hanger  84 . The third sealing system  96  includes the seal  200  and the mechanical spring  202  (e.g., Belleville washer), along with a first spacer  204  (e.g., annular spacer), a second spacer  206  (e.g., annular spacer), and a third spacer  208  (e.g., annular spacer). The first and second spacers  204  and  206  are disposed axially between the mechanical spring  202  and the seal  200 . The third spacer  208  is disposed on an axially opposite side of the mechanical spring  208 . In certain embodiments, the third spacer  208  may be axially retained against an axial surface of the casing spool  82 , casing hanger  84 , or other wellhead  12  component beneath the third sealing system  96 . 
     The mechanical spring  202  may expand and/or contract to help decrease, increase, and/or maintain seal contact pressure of the seal  200  against the second seal pack-off assembly  90 , the casing spool  82 , and the casing hanger  84 . For example, the third sealing system  96  may be installed such that the seal  200  experiences an initial lateral loading to increase an initial seal contact pressure of the seal  200  against the second seal pack-off assembly  90 , the casing spool  82 , and the casing hanger  84 . Additionally, in the manner described below, the mechanical spring  202  may reduce seal contact pressure of the seal  200  at higher temperatures, while maintaining seal contact pressure of the seal  200  at lower temperatures. 
     As mentioned above,  FIG. 6  illustrates the third sealing system  96  at lower temperatures. At lower temperatures, the seal  200 , which may be an elastomer seal, may shrink. In such circumstances, a force (e.g., axial force) exerted by the pre-loaded mechanical spring  202  (e.g., Belleville washer) may be transferred to the seal  200  by the first and second spacers  204  and  206 . Specifically, the shrinking of the seal  200  may cause the mechanical spring  202  to expand and exert an axial force on the second spacer  206 , which may transfer the axial force to the first spacer  204 , which may then transfer the axial force to the seal  200 . The axial force on the seal  200  may cause the seal to be further pressed against and between the second seal pack-off assembly  90  and the casing spool  82  or casing hanger  84 . In this manner, seal contact pressure of the seal  200  may be maintained at lower temperatures. 
     At higher temperatures, the third sealing system  96  may appear as shown in  FIG. 7 . As will be appreciated, at higher temperatures, the seal  200  (e.g., elastomer seal) may expand, thereby increasing the seal contact pressure of the seal  200  against and between the second seal pack-off assembly  90  and the casing spool  82  or casing hanger  84 . To reduce and/or control the increasing seal contact pressure, the mechanical spring  202  may be compressed. Specifically, as the seal  200  expands, the seal  200  may exert an axial force on the first spacer  204 , which may transfer the axial force to the second spacer  206 , which may then transfer the axial force to the mechanical spring  202 . As shown in  FIG. 7 , the mechanical spring  202  is compressed from the axial force generated by the seal  200 . The transfer of the axial force from the seal  200  to the mechanical spring  202  and the ability of the mechanical spring  202  to absorb the axial force reduces the potential for extrusion of the seal  200  between the second seal pack-off assembly  90 , the casing spool  82 , and the casing hanger  84 . When the higher temperatures drop, the seal  200  may shrink or contract, and the axial force absorbed by the mechanical spring  202  may be transferred back to the seal  200  by the first and second spacers  204  and  206  to help maintain seal contact pressure of the seal  200 . 
     As discussed in detail above, embodiments of the present disclosure include sealing systems  38  that can be installed and energized or “pre-charged” to modify the seal contact pressure of one or more seals (e.g., elastomeric seals) of the sealing system  38 . Energizing or pre-charging seals of the sealing system  38  may enable an increase of seal contact pressure of the seals at low temperatures. As will be appreciated, at low temperatures (e.g., below 0° C.), elastomeric seals may shrink and/or lose initial contact stress, which may decrease performance of the sealing system  38 . That is, low temperatures may traditionally reduce the sealing capability of elastomeric seals. By energizing or pre-charging the elastomeric seals of the sealing system  38 , the initial contact stress (e.g., seal contact pressure) may be increased initially and thereafter maintained when wellhead  12  temperatures fall. As described above, the seals of the sealing system  38  may be energized or pre-charged via loading (e.g., lateral loading) provided by a pressurized gas, mechanical spring  202 , or other manner of pressurization. 
     Additionally, energizing or pre-charging seals of the sealing system  38  may improve performance of the seals (e.g., elastomeric seals) at high temperatures. Specifically, high temperatures, the seals of the sealing system  38  may expand, and the pre-charged sealing system  38  may absorb forces exerted by the expanding seal. For example, in embodiments of the sealing system  38  having the mechanical spring  202 , the mechanical spring  202  may compress as the seal expands at high temperatures, thereby reducing extrusion of the seal and controlling the seal contact pressure of the seal. Similarly, in embodiments of the sealing system  38  where the sealing system  38  is pre-charged with a pressurized gas (e.g., a compressible fluid), the gas may compress as the seal expands at high temperatures, thereby also reducing extrusion of the seal and controlling the seal contact pressure of the seal. 
     While the present disclosure may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the present disclosure is not intended to be limited to the particular forms disclosed. Rather, the present disclosure is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the following appended claims.