Patent Publication Number: US-2016222761-A1

Title: Subsea Heat Exchangers For Offshore Hydrocarbon Production Operations

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional patent application Ser. No. 62/109,729 filed Jan. 30, 2015, and entitled: “Subsea Heat Exchangers for Offshore Hydrocarbon Production Operations,” which is hereby incorporated herein by reference in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND 
     This disclosure relates generally to subsea hydrocarbon production. More particularly, this disclosure relates to subsea heat exchangers for use with offshore hydrocarbon production systems. 
     The temperatures of hydrocarbon bearing subterranean reservoirs can range from very high (e.g., higher than 400° F.) to very low (e.g., lower than −75° F.). The temperature of any given reservoir is typically dictated by factors such as, for example, the composition of the materials and fluids within the reservoir, the depth of the reservoir, and proximity to other geological features (e.g., hot spots, faults, etc.). During production from a reservoir, produced fluids having extreme temperatures can push the operational limits of the production equipment (e.g., manifolds, risers, piping, etc.), potentially resulting in damage to such equipment. These problems are exacerbated when the production operations are conducted offshore, where the wellhead and much of the production equipment is located on the sea floor, which may be several thousand feet down from the sea surface. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     Some embodiments disclosed herein are directed to a subsea heat exchanger. In an embodiment, the subsea heat exchanger includes a production fluid inlet and a production fluid outlet. In addition, the subsea heat exchanger includes a first heat exchanger unit and a second heat exchanger unit each coupled to the inlet and the outlet, wherein each heat exchanger unit has a central axis, a first end, and a second end opposite the first end. Each heat exchanger unit includes an outer tubular member extending axially from the first end to the second end of the heat exchanger unit, an inner tubular member disposed within the outer tubular member, wherein the inner tubular member extends axially from the first end to the second of the heat exchanger unit, and an annulus radially disposed between the inner tubular member and the outer tubular member. In addition, each heat exchanger unit includes a bridging assembly coupled to the second end of the first heat exchanger unit and the second end of the second heat exchanger unit. The bridging assembly includes a connector having a central connector axis, a first end coupled to the second end of the first heat exchanger unit, a second end coupled to the second end of the second heat exchanger unit, and a throughbore in communication with the inner tubular member of the first heat exchanger unit and the inner tubular member of the second heat exchanger unit. In addition, the bridging assembly includes a tubular stab having a central stab axis oriented parallel to and radially spaced from the connector axis, wherein the tubular stab fluidly couples the annulus of the first heat exchanger unit to the annulus of the second heat exchanger unit. 
     Other embodiments disclosed herein are directed to an offshore production system for producing hydrocarbon fluids from a subterranean well. In an embodiment, the system includes a production tree disposed at the sea floor, wherein the production tree includes a plurality of valves configured to control a flow of hydrocarbon fluids from the subterranean well. In addition, the system includes a riser assembly fluidly coupled to the production tree and configured to flow the hydrocarbon fluids to a vessel disposed at the sea surface. Further, the system includes a heat exchanger disposed on the sea floor. The heat exchanger includes an inlet configured to receive the hydrocarbon fluids from the production tree and an outlet configured to supply the hydrocarbon fluids to the riser assembly. In addition, the heat exchanger includes a plurality of heat exchanger units coupled to the inlet and the outlet. Each heat exchanger unit has a central axis, a first end, and a second end opposite the first end. In addition, each heat exchanger unit includes an outer tubular member extending axially from the first end to the second end of the heat exchanger unit. Further, each heat exchanger unit includes an inner tubular member disposed within the outer tubular member, wherein the inner tubular member extends axially from the first end to the second of the heat exchanger unit, and wherein the inner tubular member of each heat exchanger unit is in fluid communication with the inlet and the outlet. Still further, each heat exchanger unit includes an annulus radially disposed between the inner tubular member and the outer tubular member. Further, the heat exchanger includes a closed thermal processing loop in fluid communication with the annulus of each of the heat exchanger units, wherein the thermal processing loop is configured to circulate a thermal processing fluid through the annuli of the plurality of heat exchanger units. 
     Still other embodiments disclosed herein are directed to a method for cooling hydrocarbon fluids produced from an offshore subterranean well. In an embodiment, the method includes producing hydrocarbon fluids from a production tree disposed at the sea floor to an inlet, and flowing the hydrocarbon fluids from the inlet through an inner tubular member of a first heat exchanger unit. In addition, the method includes flowing the hydrocarbon fluids through a connector of a bridging assembly into an inner tubular member of a second heat exchanger unit, and flowing a thermal transfer fluid through an annulus of the first heat exchanger unit. Further, the method includes flowing the thermal transfer fluid through a tubular stab of the bridging assembly into an annulus of the second heat exchanger unit. 
     Embodiments described herein comprise a combination of features and advantages intended to address various shortcomings associated with certain prior devices, systems, and methods. The foregoing has outlined rather broadly the features and technical advantages of the disclosed exemplary embodiments in order that the detailed description that follows may be better understood. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the embodiments described herein. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed description of the disclosed embodiments, reference will now be made to the accompanying drawings in which: 
         FIG. 1  is a schematic view of an embodiment of an offshore production system including a subsea heat exchanger in accordance with the principles disclosed herein; 
         FIG. 2  is a perspective view of the subsea heat exchanger of  FIG. 1 ; 
         FIG. 3  is a cross-sectional side view of the subsea heat exchanger of  FIG. 1 ; 
         FIG. 4  is an enlarged cross-sectional view of an outer end of a heat exchanger unit within the subsea heat exchanger of  FIG. 1 ; 
         FIG. 5  is a cross-sectional view of a bridging assembly extending between two adjacent heat exchanger units within the subsea heat exchanger of  FIG. 1 ; 
         FIG. 6  is an exploded, perspective view of a heat exchanger unit of the subsea heat exchanger of  FIG. 1 ; 
         FIG. 7  is an enlarged, perspective view of a baffle of the heat exchanger unit of  FIG. 6 ; 
         FIG. 8  is a top schematic view of the subsea heat exchanger of  FIG. 1  illustrating the respective flow paths of the production fluid and thermal transfer fluid; 
         FIG. 9  is an enlarged perspective view of one end of the subsea heat exchanger of  FIG. 1  illustrating the transfer pipes and tubes interconnecting adjacent rows of heat exchanger units; 
         FIG. 10  is a perspective view of an embodiment of a subsea heat exchanger for use within the offshore production system of  FIG. 1 ; 
         FIG. 11  is a perspective view of an embodiment of a subsea heat exchanger for use within the offshore production system of  FIG. 1 ; and 
         FIG. 12  is a top schematic view of an embodiment of a subsea heat exchanger for use within the offshore production system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The following discussion is directed to various exemplary embodiments. However, one skilled in the art will understand that the examples disclosed herein have broad application, and that the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment. 
     Certain terms are used throughout the following description and claims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function. The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness. 
     In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices, components, and connections. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a central axis (e.g., central axis of a body or a port), while the terms “radial” and “radially” generally mean perpendicular to the central axis. For instance, an axial distance refers to a distance measured along or parallel to the central axis, and a radial distance means a distance measured perpendicular to the central axis. 
     As previously described, production fluids having an extremely hot or cold temperature can stress production equipment (e.g., manifolds, risers, piping, etc.), potentially causing damage to such production equipment. This can be particularly problematic in offshore production operations where much of the production hardware is disposed at the sea floor. Embodiments disclosed herein include heat exchangers and related equipment for use within a subsea hydrocarbon production system to either raise the temperature of produced fluids having a very low temperature and lower the temperature of produced fluids having a very high temperature to reduce the thermal stresses experienced by production equipment, thereby offering the potential to reduce the likelihood of damage to such equipment and enhance the operating lifetime of such equipment. In addition, the heat exchangers of at least some of the embodiments disclosed herein reduce the rate of corrosion for subsea production equipment due, at least in part, to the reduction in the temperature of produced fluids. Further, the heat exchangers of at least some of the embodiments disclosed herein reduce (or eliminate) the need for specialized equipment within the subsea production system that are designed and rated to receive and flow fluid of an extreme temperature (e.g., equipment including increased wall thickness, specialized materials, etc.). 
     Referring now to  FIG. 1 , an embodiment of an offshore hydrocarbon production system  10  in accordance with the principles described herein is shown. Production system  10  facilitates the production of fluids from a wellbore  14  extending into a subterranean reservoir. In this embodiment, production fluids comprise hydrocarbons, such as, for example, liquid petroleum, natural gas, hydrocarbon condensate, and combinations thereof. Production system  10  generally includes a subsea production tree  12  mounted to a wellhead  13 , a subsea heat exchanger  100 , a manifold  16 , a pipeline end termination (PLET)  18 , and a riser assembly  20 . Production tree  12 , wellhead  14 , heat exchanger  100 , manifold  16 , and PLET  18  are positioned along the sea floor  7 . In addition, tree  12 , heat exchanger  100 , manifold  16 , PLET  18 , and riser assembly  20  are fluidly connected to one another with a plurality of fluid conduits or jumper lines  17 . 
     Wellhead  13  is disposed at the upper end of a cased wellbore  14 , thereby fluidly coupling production tree  12  to wellbore  14 . Production tree  12  includes a plurality of valves  15  that control the flow of produced fluids from wellbore  14 . Subsea heat exchanger  100  receives production fluids from tree  12  and either raises or lowers the temperature of the produced fluids, as appropriate, such that when the produced fluids exit exchanger  100  they are at an acceptable and/or predetermined temperature within the safe operating temperature range of each piece of downstream equipment (e.g., manifold  16 , PLET  18 , riser assembly  20 , etc.). Manifold  16  receives production fluids from heat exchanger  100 , and, in certain embodiments, also receives production fluids from other wellbores (not shown). Thereafter, the production fluids are passed from manifold  16 , through PLET  18 , and are routed to riser assembly  20 . In this embodiment, riser assembly  20  includes a lower riser assembly  19  disposed at or proximate the sea floor  7  and a marine riser  21  extending vertically from lower riser assembly  19  to a surface vessel  22  disposed at the sea surface  9 . Thus, during production operations, production fluids are received by riser assembly  20  from PLET  18  at the lower riser assembly  19 , and are subsequently routed to vessel  22  at the sea surface  9  through riser  21 . In general, riser  21  can be any suitable riser or conduit for routing production fluids from the sea floor to the sea surface, such as, for example, a free standing riser, a catenary riser, a top tensioned riser, or some combination thereof all while still complying with the principles disclosed herein. 
     Referring now to  FIG. 2 , subsea heat exchanger  100  generally includes a production fluid inlet  101 , a production fluid outlet  102 , a plurality of modular heat exchanger units  120  disposed between and fluidly coupled to the inlet  101  and outlet  102 , and a thermal processing loop  200  fluidly coupled to each of the units  120 . In this embodiment, exchanger  100  is configured to cool (i.e., remove thermal energy from) relatively hot production fluids as they pass from inlet  101  to outlet  102 ; however, in other embodiments, the heat exchanger (e.g., exchanger  100 ) is configured to heat (i.e., transfer thermal energy to) relatively cold production fluids with. As will be described in more detail below, exchanger  100  may be referred to herein as a “modular” heat exchanger, since it is constructed from a plurality of modular heat exchanger units  120  that can be added or removed as needed based on the specific heat transfer requirements and specifications of the associated production system (e.g., system  10 ). 
     Referring now to  FIGS. 2 and 3 , in this embodiment, each heat exchanger unit  120  includes a central or longitudinal axis  125 , a first or outer end  120   a , a second or inner end  120   b  opposite outer end  120   a , an outer tubular member  130  extending axially between ends  120   a ,  120   b , and an inner tubular member  140  concentrically disposed within outer tubular member  130  and extending axially between ends  120   a ,  120   b . Each end  120   a ,  120   b  further includes a stiffening plate  122  that is connected to and supports each outer tubular member  130  and inner tubular member  140  within unit  120 . 
     In this embodiment, heat exchanger units  120  are arranged into three adjacent, parallel rows  110 A,  110 B,  110 C. In particular, two units  120  are disposed within each row  110 A,  110 B,  110 C such that the units  120  within each row  110 A,  110 B,  110 C share a pair of common stiffening plate  122  at each end  120   a ,  120   b  with the corresponding adjacent units within the other rows  110 A,  110 B,  110 C. One or more of the plates  122  may include(s) a pad eye or other similar attachment device (not shown) configured to receive cables and rigging for lowering and/or raising that particular exchanger to and/or from the sea floor  7 , respectively. Moreover, as best shown in  FIG. 3 , the units  120  within each row  110 A,  110 B,  110 C are arranged such that the inner end  120   b  of each unit  120  in a given row  110 A,  110 B,  110 C is positioned axially adjacent the inner end  120   b  of a corresponding unit  120  within that same row  110 A,  110 B,  110 C, and the central axes  125  of the units  120  within each respective row  110 A,  110 B,  110 C are coaxially aligned (note: while only row  110 A is shown in  FIG. 3 , it should be appreciated that each of the other rows  110 B,  110 C are arranged the same). 
     Each outer tubular member  130  is concentrically disposed about axis  125  and includes a first or outer end  130   a , a second or inner end  130   b  opposite outer end  130   a , a radially outer cylindrical surface  130   c  extending axially between ends  130   a ,  130   b , and a radially inner cylindrical surface  130   d  extending axially between ends  130   a ,  130   b . Each outer tubular member  130  extends axially between ends  120   a ,  120   b  of the corresponding heat exchanger unit  120 , and thus, ends  120   a ,  130   a  axially aligned and ends  120   b ,  130   b  are axially aligned. In this embodiment, outer tubular member  130  is comprised of a metallic material (e.g., steel); however, it should be appreciated that a wide range of materials may be used to construct member  130 , such as, for example, carbon-fiber composite. 
     Outer ends  130   a  are secured to the corresponding plate  122  and inner ends  130   b  are secured to the corresponding plate  122 . In this embodiment, ends  130   a ,  130   b  are rigidly secured to the corresponding plate  122  at ends  120   a ,  120   b , respectively, by welding; however, in general, any suitable method for securing two rigid components to one another may be used, such as, for example, bolts, rivets, adhesive, etc. In addition, each end  130   a ,  130   b  is coaxially aligned with an aperture or port  123  extending through plates  122  at ends  120   a ,  120   b , respectively, such that when outer tubular member  130  is secured to plates  122  at ends  130   a ,  130   b , an open passage is defined along axis  125  between plates  122  by outer tubular member  130 . 
     Each inner tubular member  140  is concentrically disposed within a corresponding outer tubular member  130  and includes a first or outer end  140   a , a second or inner end  140   b  opposite outer end  140   a , a radially outer surface  140   c  extending axially between ends  140   a ,  140   b , and a radially inner surface  140   d  also extending axially between ends  140   a ,  140   b . Each inner tubular member  140  extends axially between ends  120   a ,  120   b  of the corresponding heat exchanger unit  120 , and thus, ends  120   a ,  140   a  are axially aligned and ends  120   b ,  140   b  are axially aligned. Radially inner surface  140   d  defines a throughbore  142  extending axially between ends  140   a ,  140   b . As will be described in more detail below, production fluids flow through throughbore  142  during production operations. 
     During assembly, inner tubular member  140  is inserted within the corresponding outer tubular member  130  through port  123  of one of the plates  122  such that (a) inner tubular member  140  is concentrically disposed within outer tubular member  130  as shown in  FIG. 3 ; (b) outer end  140   a  is proximate outer end  130   a ; and (c) inner end  140   b  is proximate inner end  130   b . In addition, once tubular member  140  is fully inserted within tubular member  130  an annulus  132  is formed radially between outer surface  140   c  and inner surface  130   d  that extends axially between ends  120   a ,  120   b . As will be described in more detail below, during production operations, a thermal transfer fluid flows through annulus  132  to facilitate the transfer thermal energy (e.g., heat) away from production fluids flowing within throughbore  142 . Thermal transfer fluid may comprise any suitable fluid for facilitating heat transfer (e.g., convective heat transfer) with another fluid or body. For example, thermal transfer fluid may comprise, for example, seawater, fresh water, corrosion inhibitors, ethylene glycol, propylene glycol, organic acid technology fluid (e.g., DEX-COOL® or ZEREX™), water-soluble oil, mineral oil or combinations thereof. 
     In this embodiment, the axial lengths of outer tubular members  130  (e.g., the length measured axially between ends  130   a ,  130   b ) and inner tubular members  140  (e.g., length measured axially between ends  140   a ,  140   b ) are no larger than the standard length of commercially available pipe, which in some cases is approximately 45 feet. Consequently, the tubulars for constructing tubular members  130 ,  140  may be purchased or otherwise acquired directly from the existing stock of a given supplier without the need to order custom length pipes. In addition, such a length also eliminates the need to weld or otherwise join multiple lengths of pipe to construct tubular members  130 ,  140 , an activity which adds time and costs to the manufacturing of exchanger  100 . However, it should be appreciated that such a length for shells  130  and tubes  140  is not required and each may be disposed at any suitable length while still complying with the principle disclosed herein. 
     Outer end  120   a  of each heat exchanger unit  120  is fluidly connected to either inlet  101 , outlet  102 , or outer end  120   a  of another exchanger unit  120  in the immediately adjacent row (e.g., row  110 A,  110 B,  110 C). Specifically, referring now to  FIG. 4 , in this embodiment, outer end  140   a  of each member  140  includes a flange  150  disposed within a corresponding port  123  on one of the plates  122  that mates with a corresponding flange connector (e.g., flange connector  60 ) on adjacent piping in the manner described below in order to fluidly connect unit  120  within exchanger  100 . While only one end  120   a  of a single exchanger unit  120  is shown in  FIG. 4 , it should be appreciated that each end  120   a  is generally configured to same. Flange  150  generally includes a radially outer annular surface  152  and an end face  154 . Annular surface  152  includes a pair of axially adjacent seal assemblies  151 . Each seal assembly  151  includes an annular recess  153  extending radially inward from annular surface  152  and an annular sealing member  155  (e.g., an S-Seal, lip seal, T-seal, O-ring, etc.) disposed within recess  153 . When inner tubular member  140  is installed within outer tubular member  130 , annular surface  152  slidingly engages both port  123  and radially inner surface  130   d  and each sealing member  155  is radially compressed between surface  130   d  and the corresponding recess  153 . As a result, during production operations, fluid flow between surfaces  130   d ,  153  is restricted and/or prevented by the annular sealing assemblies  151 . Face  154  includes a generally planar engagement surface  154   a  that extends annularly about throughbore  142  and includes an annular recess  156  that extends axially inward from engagement surface  154   a.    
     As is shown in  FIG. 4 , flange  150  at outer end  140   a  mates and engages a corresponding flange  160  disposed on an end of an exterior pipe (e.g., inlet  101 , outlet  102 , etc.). In particular, engagement surface  154   a  on face  154  mates and engages a corresponding engagement surface  164   a  on a face of mating flange  160 . Engagement surface  164   a  on mating flange  160  also includes an annular recess  166  that extends radially inward from surface  164   a  such that when flanges  150 ,  160 , are mated (e.g., bolted) with one another as shown, recesses  156 ,  166  are generally aligned. An annular sealing member  158  (e.g., metallic or elastomeric gasket) is disposed within the aligned recesses  156 ,  166  and is compressed therebetween such that fluid flow between the surfaces  154   a ,  164   a  is restricted and/or prevented by the compressed sealing member  158 . Additional sealing member(s) (e.g., elastomeric sealing ring(s)) may be disposed about the outer periphery of sealing member  158  to prevent seawater from contacting and thus compromising the integrity of member  158  and/or serve as a secondary barrier to prevent fluid flow between surfaces  154   a ,  164   a.    
     Referring still to  FIG. 4 , the radially outer portion of each flange  150  includes a plurality of uniformly circumferentially-spaced apertures or ports  159  extending axially therethrough. Each port  159  receives the end of a transfer tubing member  222  which, as will be described in more detail below, delivers thermal transfer fluid into and out of annulus  132  to facilitate thermal energy transfer with production fluid flowing within throughbore  142  during production operations. Fluid flow between each tubing member  222  and the corresponding flange  150  is restricted and/or prevented by an annular sealing assembly (not shown) disposed radially therebetween. In addition, as is best shown in  FIG. 9 , the diameter of each flange  160  is smaller than the diameter of the corresponding flange  150  such that each tubing member  222  is positioned radially outside connector  160  as it extends axially into the corresponding port  159 . 
     Referring now to  FIGS. 3 and 5 , inner ends  120   b  of the coaxially aligned heat exchanger units  120  within each row  110 A,  110 B,  110 C are connected to one another with a connection or bridging assembly  170 . One bridging assembly  170  will now be described it being understood the other bridging assemblies  170  are the same. As best shown in  FIG. 5 , in this embodiment, bridging assembly  170  includes a central connector  172  extending between opposed inner ends  140   b  and a plurality of uniformly circumferentially-spaced tubular members or stabs  176  disposed about connector  172 . Central connector  172  is an elongate tubular member including a central axis  173  that is aligned with the axes  125  of each of the heat exchanger units  120  within row  110 A, a first end  172   a , a second end  172   b  opposite first end  172   a , a radially outer surface  172   c  extending axially between ends  172   a ,  172   b , and a radially inner surface  172   d  extending axially between ends  172   a ,  172   b . Inner surface  172   d  defines a throughbore  174  extending axially between ends  172   a ,  172   b . Each end  172   a ,  172   b  includes an annular flange  160  as previously described. In addition, inner ends  140   b  of tubes  140  each include a flange  150  that is configured the same as flange  150  on outer ends  140   a . Thus, as is shown in  FIG. 5 , annular surface  152  of flange  150  at inner end  140   b  slidingly and sealing engages with the radially inner surface  130   d  and aperture  123  of the corresponding shell  130  and plate  122 , respectively, in the same manner as described above for outer end  120   a.    
     Each end  172   a ,  172   b  is connected to the inner end  140   b  of one of the tubular members  140  via engagement of mating flanges  150 ,  160 . Specifically, as shown in  FIG. 5 , flange  160  at first end  172   a  of central connector  172  mates with and engages flange  150  of inner end  140   b  of inner tubular member  140  disposed within one of the units  120  within row  110 A (i.e., the unit  120  on the left side of  FIG. 5 ), while flange  160  at second end  172   b  mates with and engages flange  150  on inner end  140   b  of inner tubular member  140  disposed within the other unit  120  within row  110 A (i.e., the unit  120  on the right side of  FIG. 5 ). As a result, throughbore  174  in central connector  172  is coaxially aligned with both throughbores  142  in tubes  140  within row  110 A, thereby forming a continuous production fluid flow path  112 A extending axially between units  120  of row  110 A. As is shown schematically in  FIG. 8 , production fluid flow paths  112 B,  112 C extend axially through rows  110 B,  110 C, respectively, and are configured the same as fluid flow path  112 A described above. 
     Referring again to  FIGS. 3 and 5 , each stab  176  of bridging assembly  170  is an elongate tubular member including a first end  176   a , a second end  176   b  opposite first end  176   a , a radially outer cylindrical surface  176   c  extending axially between ends  176   a ,  176   b , and a radially inner cylindrical surface  176   d  extending axially between ends  176   a ,  176   b . Inner surface  176   d  defines a throughbore  178  extending axially through stab  176 . As best shown in  FIG. 5 , each port  159  of flange  150  on one inner end  140   b  is circumferentially aligned with one port  159  of the opposed flange  150  of the other inner end  140   b . One stab  176  extends axially through each pair of aligned ports  159 —first end  176   a  being received within one port  159  on one of the units  120  within row  110 A and second end  176   b  being received within the aligned port  159  on the other unit  120  within row  110 A. Radially outer surface  176   c  of each stab  176  is allowed to slidingly engage at least one of the corresponding ports  159  such that units  120  within row  110 A may move axially relative to stabs  176 , thereby allowing bridging assembly  170  to accommodate the thermal expansion of units  120  during production operations. In this embodiment, one end (e.g., end  176   a ) is fixed within the corresponding port  159  such that this “fixed” end does not move axially relative to that port  159 , whereas the other end (e.g., end  176   b ) is movably disposed within its corresponding port  159  such that this “free” end can move axially relative to its port  159 . In general, any suitable method for fixing an end  176   a ,  176   b  within the corresponding port  159  may be used while still complying with the principles disclosed herein, such as, for example, welding, a retention sleeve or locking ring disposed within one of the ports  159 , etc. In addition, one or more sealing assemblies (not shown) are included between radially outer surface  176   c  of stabs and ports  159  to restrict fluid flow therebetween during operations. For example, in some embodiments, an annular seal gland extends radially inward from radially outer surface  176   c  and houses an annular sealing member (e.g., an O-ring, sealing ring, etc.) that further engages with port  159 ; however, it should be appreciated that any other suitable sealing assembly may be used while still complying with the principles disclosed herein. Together the annuli  132  and throughbores  178  of stabs  176  define a continuous thermal fluid flow path  113 A through adjacent units  120  within row  110 A. As is shown schematically in  FIG. 8 , thermal fluid flow paths  113 B,  113 C extend axially through rows  110 B,  110 C, respectively, and are configured the same as thermal fluid flow path  113 A described above. 
     Referring again to  FIG. 3 , each heat exchanger unit  120  is supported on sea floor  7  with a plurality of support members  126 . More specifically, each outer end  120   a  is supported by a pair of outer support members  126   a , and each inner end  120   b  is supported by an inner support member  126   b . Each support member  126   a ,  126   b  includes a base or foot  127  disposed along the sea floor  7  and a plurality of columns  128  extending vertically upward from foot  127 . In this embodiment, inner support member  126   b  includes two rows  129 ′,  129 ″ of columns  128  and outer support members  126   a  each include only a single row of columns  128 . Each support column  128  on both inner and outer support members  126   a ,  126   b , respectively, includes a saddle  124  that receives one of the stiffening plates  122  on ends  120   a ,  120   b . Specifically, saddles  124  in columns  128  on outer support members  126   a  receive the stiffening plates  122  disposed at outer ends  120   a  of units  120  within rows  110 A,  110 B,  110 C, while the saddles  124  on columns  128  on inner support member  126   b  receive stiffening plates  122  disposed at inner ends  120   b  of units  120  in rows  110 A,  110 B,  110 C. In addition, as is best shown in  FIGS. 4 and 5 , each saddle  124  has an width W 124  measured axially relative to axis  125  that is greater than the axial thickness T 122  of each plate  122 . Each plate  122  slidingly engages the corresponding saddle  124 , and thus, during production operations, plates  122  are free to slide axially within saddles  124 , such as, for example, to accommodate thermal expansion or contraction of units  120 . In some embodiments, engaged surfaces of plates  122  and saddles  124  are finished or coated with a suitable surface treatment in order to reduce friction therebetween during operations. In still other embodiments, additional friction reducing mechanisms may be employed between plates  122  and saddles  124 , such as, for example, rollers, bearings, etc. 
     Referring now to  FIGS. 3, 6, and 7 , each unit  120  includes a plurality of axially-spaced generally D-shaped baffles  180  disposed within annulus  132  and fixably attached to the corresponding inner tubular member  140  between ends  140   a ,  140   b . As will be described in more detail below, baffles  180  function primarily to direct the flow of heat transfer fluid within annulus  132  during production operations. In addition, in at least some embodiments, baffles  180  also form fin-like appendages along each inner tubular member  140  that effectively increases the surface area of outer surface  140   c , thereby enhancing thermal energy transfer between thermal transfer fluids in annulus  132  and production fluids within throughbore  142  during operations. In this embodiment, each unit  120  includes a total of five uniformly axially spaced baffles  180  mounted to each inner tubular member  140 ; however, it should be appreciated that the number, arrangement, and axial spacing of baffles  180  may be greatly varied while still complying with the principles disclosed herein. 
     Referring now to  FIG. 7 , one baffle  180  will now be described it being understood each baffle  180  is the same. In this embodiment, each baffle  180  is constructed from a pair of rigid plate members that are joined to one another. In particular, each baffle  180  includes a first or main plate member  182  and a second or locking plate member  190  secured to main plate member  182  with a pair of attachment assemblies  188 . Main plate member  182  is generally C-shaped and includes a first planar side  180   a , a second planar side  180   b  facing in the opposite direction as first side  180   a , a planar end surface  181  extending axially between sides  180   a ,  180   b , and a generally cylindrical surface  183  extending axially between sides  180   a ,  180   b  and circumferentially between the ends of planar end surface  181 . A U-shaped recess or notch  184  extends radially inward from end surface  181 . Recess  184  is defined by a pair of radially oriented planar surfaces  185  and a curved surface  186  extending circumferentially about axis  125  between planar surfaces  185 . As will be described in more detail below, inner tubular member  140  is received within notch  184 , and thus, the radius of curvature of surface  186  is the same as the radius of curvature of radially outer surface  140   c  of inner tubular member  140 . Similarly, as will also be described in more detail below, curved outer surface  183  engages the inner surface  130   d  of outer tubular member  130  when baffle  180  is installed within unit  120 , and thus, the radius of curvature of outer surface  130  is the same as the radius of curvature of inner surface  130   d  of outer tubular member  130 . A pair of apertures or through holes  187  extend axially between sides  180   a ,  180   b  on opposite sides of notch  184 . As will be described in more detail below, each aperture  187  receives a bolt  189  to secure main plate member  182  to locking plate member  190 . 
     Referring still to  FIG. 7 , locking plate member  190  includes a first planar side  190   a , a second planar side  190   b  facing away from first side  190   a , a first planar end surface  191  extending axially between sides  190   a ,  190   b , a second planar end surface  192  extending axially between sides  190   a ,  190   b , and a pair of radially outer generally cylindrical surfaces  193  extending circumferentially between planar end surfaces  191 ,  192 . A cylindrical recess or notch  194  extends radially inward from second planar end surface  192  and is defined by a curved surface  195 . As will be described in more detail below, inner tubular member  140  is received within notch  194 , and thus, the radius of curvature of surface  195  is the same as the radius of curvature of radially outer surface  140   c  of inner tubular member  140 . A pair of apertures or throughbore holes  196  extend axially between sides  190   a ,  190   b  on opposite sides of notch  194 . When baffle  180  is installed within the corresponding heat exchanger unit  120 , curved surfaces  193  generally align with the curved surface  183 , and thus, like surface  183 , each of the surfaces  193  also engage with the radially inner surface  130   d  of outer tubular member  130 . Accordingly, like surface  183 , the radius of curvature of surface  193  is the same as the radius of curvature of radially inner surface  130   d.    
     Each attachment assembly  188  includes a threaded rod or bolt  189  and a pair of threaded nuts  197 . To assemble baffle  180 , main plate member  182  is disposed along inner tubular member  140  at a desired location such that inner tubular member  140  is seated within notch  184  and radially outer surface  140   c  is engaged with curved surface  186 . Thereafter, first side  190   a  of locking plate body  190  is engaged with second side  180   b  of main plate member  182  such that the apertures  187  in member  182  are substantially aligned with the apertures  196  in member  190  and radially outer surface  140   c  of inner tubular member  140  is engaged with curved surface  195  within notch  194 . Each of the bolts  189  of assemblies  188  is then inserted within one pair of the aligned apertures  187 ,  196  and nuts  197  are threadably engaged to bolts  189  along each of the first side  180   a  of member  182  and second side  190   b  of member  190 , thereby urging second side  180   b  of member  182  into engagement with first side  190   a  of member  190 , and securing baffle  180  to inner tubular member  140  through a friction fit. It should be appreciated that baffles  180 , once assembled, can be further secured to radially outer surface  140   c  of tubular member  140  by welding, adhesive, or some other suitable method, while still complying with the principles disclosed herein. 
     Referring back now to  FIGS. 3 and 6 , baffles  180  are disposed along inner tubular member  140  in an alternating fashion with each baffle  180  being rotated approximately 180° relative to the each immediately axially adjacent baffle  180  with respect to axis  125 . As a result, when thermal transfer fluid is routed through annulus  132  between ends  120   a ,  120   b  of each unit  120 , it is forced to flow generally sinusoidally around baffles  180 . Without being limited to this or any other theory, such a sinusoidal like flow pattern promotes turbulence within the thermal transfer fluid, which further enhances the transfer of thermal energy between production fluids flowing within throughbore  142  of inner tubular member  140  and the thermal transfer fluid flowing within annulus  132 . To prevent fluid flow between outer curved surfaces  183 ,  193  of members  182 ,  190 , respectively, radially outer curved surfaces  183 ,  193  of baffles  180  (e.g. see  FIG. 7 ) sealingly engage with radially inner surface  130   d . As a result, thermal transfer fluid flowing though annulus  132  is forced to flow around baffles  180  proximate planar end surfaces  181 ,  191 , thereby promoting the sinusoidal like flow pattern described above. In general, surfaces  183  and/or  193  may sealingly engage radially inner surface  130   d  of shell through any suitable method while still complying with the principles disclosed herein. For example, surfaces  183  and/or  193  may engage surface  130   d  with an integral seal, a pre-formed seal, a molded seal, etc. Without being limited to this or any other theory, in embodiments where an elastomeric type seal is utilized between surfaces  183  and/or  193  and  130   d , vibrational energy is at least partially absorbed by the elastomer forming the seal during operation, which thus reduces fatigue wear on inner tubular member  140 . Further, in some embodiments, surfaces  183  and/or  193  are welded or otherwise adhered to radially inner surface  130   d.    
     Referring now to  FIGS. 2, 8, and 9 , during production operations, production fluid flows between inlet  101  and outlet  102  of exchanger  100 . During this process, thermal transfer fluid is also routed along thermal processing loop  200  in order to facilitate the transfer of thermal energy with production fluids. Specifically, in this embodiment, the flowing of thermal transfer fluid along thermal processing loop  200  causes cooling of production fluids such that the temperature of production fluid at outlet  102  is less than the temperature of production fluid at the inlet  101 . Thermal processing loop  200  generally includes a pumping unit  210 , an outlet line  220  extending from pumping unit  210  to heat exchanger units  120 , a recirculation line  230  extending from units  120 , a thermal expansion section  240 , and a cooling unit or radiator  250 . 
     As best shown in  FIG. 2 , pumping unit  210  includes one or more pumps that are disposed within a support frame  212  resting on a mud mat  213 . In this embodiment, two pumps  211   a ,  211   b  are disposed within pumping unit  210 . Pumps  211   a ,  211   b  are variable speed pumps configured such that their speeds may be adjusted by, for example, a controller unit. As the speed of the pumps  211   a ,  211   b  within unit  210  is increased, the flow rate and/or discharge pressure also increases. Similarly, as the speed of the pumps  211   a ,  211   b  within unit  210  is decreased, the flow rate and/or discharge pressure also decreases. During installation, after the other components of exchanger  100  are installed on the sea floor  7 , the pumping unit  210  is lowered, such as, for example, by suspension from a pad eye  214  (e.g., See  FIG. 2 ) attached to unit  210 , until it is seated within support frame  212 . Thereafter, all associated piping (e.g., outlet line  220 ) is fluidly connected to pumping unit  210  by any suitable method, such as, for example, with a remotely operated vehicle (ROV). If at some point, it becomes desirable to replace or repair the pumping unit  210 , the steps for installing the pumping unit  210  are simply performed in reverse order. Specifically, all piping (e.g., outlet line  220 ) is disconnected and removed and unit  210  is lifted out of frame  212  to the surface by, for example, a cable connected to pad eye  214 . Thus, pumping unit  210  may be referred to herein as a “retrievable” pumping unit  210  or a “separately retrievable” pumping unit  210 , as its installation and removal is independent from the installation and removal of the other components of exchanger  100 —meaning, for example, that pumping unit  210  may be removed without requiring the removal of any other components within exchanger  100 . 
     Thermal expansion section  240  is included within loop  200  to accommodate any thermal expansion that might occur within any of the associated lines (e.g., recirculation line  230 ) during operations. As shown in  FIGS. 2 and 8 , expansion section  240  is disposed along recirculation line  230  upstream of radiator  250 . Referring specifically to  FIG. 2 , in this embodiment expansion section  240  includes a U-shaped bend  242  that defines a space  244  sized to accommodate any axial increase in the length of recirculation line  230  due to thermal expansion. 
     Referring again to  FIGS. 2 and 8 , radiator  250  is disposed along loop  200  upstream of pumping unit  210  and downstream of expansion section  240 . In this embodiment, radiator  250  is configured to remove thermal energy (e.g., heat) collected by thermal transfer fluid being circulated through heat exchanger units  120 . Radiator  250  includes a series of pipe curves or bends  252  connected by a plurality of straight sections of pipe  254 . Each of the bends  252  and straight sections  254  are exposed to the surrounding ocean environment. Specifically, without being limited to this or any other theory, each of the bends  252  and pipes  254  adds considerable length to the fluid path that spent thermal transfer fluid must traverse to return to pumping unit  210 . This increased length greatly increases the surface area of pipe that is exposed to the relatively cool ocean environment, and thus promotes a greater transfer of thermal energy between the thermal transfer fluids flowing through radiator  250  and the surrounding sea water during operations. 
     Referring still to  FIGS. 2, 8, and 9 , during production operations, both production and fluid and thermal transfer fluid are routed through exchanger  100  to facilitate the transfer of thermal energy therebetween. For convenience, in  FIG. 8  the flow path of production fluid within exchanger  100  is shown by solid line arrows  105 , while the flow of thermal processing fluid through loop  200  is shown by broken line arrows  205 . Specifically, during operations, production fluid enters exchanger  100  at inlet  101  and is then routed successively through each of the rows  110 A,  110 B,  110 C toward outlet  102 . As production fluid travels through rows  110 A,  110 B,  110 C, it flows successively through the production fluids flow paths  112 A,  112 B,  112 C within each row  110 A,  110 B,  110 C, respectively. Upon exiting one of the production fluid flow paths  112 A,  112 B,  112 C, production fluids are then routed through a transfer pipe to the next successive fluid flow path (e.g., path  112 B,  112 C in rows  110 B,  110 C, respectively). For example, upon exiting fluid flow path  112 A within first row  110 A, production fluids are routed through a first transfer pipe  104  to fluid flow path  112 B within second row  110 B, and upon exiting fluid flow path  112 B, a second transfer pipe  106  routes the production fluids to fluid flow path  112 C within third row  110 C. Each transfer pipe  104 ,  106  includes a pair of flange connectors  160  that mate with the corresponding flange connectors  150  on the respective units  120  within rows  110 A,  110 B,  110 C, in the same manner as previously described above (e.g., see  FIG. 4  and the associated description) (see also  FIG. 9 ). Similarly, both inlet  101  and outlet  102  include flange connectors  160  that mate with the corresponding flange connectors  150  on tubes  140  within rows  110 A,  110 C in the same manner as previously described above (e.g., see  FIG. 4  and the associated description). 
     Referring still to  FIGS. 2, 8, and 9 , as production fluid is routed between inlet  101  and outlet  102  as described above, thermal transfer fluid is routed along thermal processing loop  200  to facilitate cooling of production fluids. Specifically, pressurized thermal transfer fluid is discharged by pumping unit  210  into outlet line  220  where it is routed into a manifold  221  and split into a plurality of transfer tubes  222 , the ends of which are installed within the ports  159  on flange  150  at outer end  140   a  of inner tubular member  140  within one of the heat exchanger units  120  of first row  110 A as previously described (e.g., See  FIG. 4  and the associated description). Thus, tubes  222  provide fluid communication with the annulus  132  in one of the units  120  within first row  110 A. Thereafter, thermal transfer fluid is routed successively through each of the thermal transfer fluid flow paths  113 A,  113 B,  113 C within rows  110 A,  110 B,  110 C, respectively, toward recirculation line  230 . Specifically, upon exiting the fluid flow path  113 A,  113 B,  113 C within a given row  110 A,  110 B,  110 C, respectively, thermal transfer fluid is then routed through a plurality of transfer tubes  222  to the next successive row (e.g., row  110 B,  110 C). In this embodiment, upon exiting fluid flow path  113 A within first row  110 A, a first set of transfer tubes  222 ′ routes the thermal transfer fluid to the flow path  113 B within second row  110 B, and upon exiting flow path  113 B, a second set of transfer tubes  222 ″ routes the thermal transfer fluid to fluid flow path  113 C within third row  110 C. After exiting fluid flow path  113 C, the now spent thermal transfer fluid is routed through a third set of transfer tubes  222 ′″ into a manifold  223  (which is substantially the same as the manifold  221 ), which further directs the fluid into recirculation line  230 . Thus, in this embodiment thermal transfer fluids are routed through heat exchanger units  120  in a parallel flow arrangement with production fluids (i.e., where thermal transfer fluids flow in the same general axial direction along paths  113 A,  113 B,  113 C as production fluid along flow paths,  112 A,  112 B,  112 C, respectively, during operations). However, it should be appreciated that in other embodiments, thermal transfer fluids are routed in a counter flow arrangement with production fluids (i.e., where thermal transfer fluids flow in a generally opposite axial direction along paths  113 A,  113 B,  113 C as production fluid along flow paths  112 A,  112 B,  112 C, respectively, during operations). 
     In this embodiment, thermal transfer fluid within flow paths  113 A,  113 B,  113 C has a temperature less than the temperature of the production fluids within flow paths  112 A,  112 B,  112 C, respectively (i.e., production fluids are relatively hot in this case). Thus, as thermal transfer fluid flows through the annuli  132  of heat exchanger units  120  and the relatively warm production fluids flow through tubes  140 , thermal energy is transferred from the production fluids through tubes  140  to the thermal transfer fluid. Thus, as production fluid flows within throughbores  142  of tubes  140  in rows  110 A,  110 B,  110 C it gradually decreases in temperature such that it is at a minimum temperature when it reaches outlet  102 . Conversely, as thermal transfer fluid flows through the units  120  in rows  110 A,  110 B,  110 C, it gradually increases in temperature such that it is at a maximum temperature when it reaches recirculation line  230 . As a result, upon entering recirculation line  230 , the spent, warm thermal transfer fluid flows through expansion section  240  into radiator  250  where the thermal transfer fluid is cooled in the manner described above. Thereafter, the now cooled thermal transfer fluid is once again routed through pumping unit  210 , thereby repeating the process described above. Thus, the thermal transfer fluid is continuously re-circulated through exchanger  100  and loop  200  throughout heat transfer operations. 
     Referring now to  FIG. 8 , during the above described thermal processing operations, a plurality of parameter measurements (e.g., temperature, pressure, flow rate, viscosity, etc.) can be taken at various points within exchanger  100  and fed (e.g., wirelessly, through cables, etc.) to a controller unit (not shown) that may be disposed subsea, on vessel  20 , or at some other location, such that performance of exchanger  100  can be monitored and adjusted as necessary based on the performance and specifications of the corresponding production system (e.g., system  10 ). In this embodiment, a plurality of measurement assemblies  260  are disposed throughout exchanger  100 , namely at inlet  101 , outlet  102 , recirculation line  230 , and outlet line  220 . Each measurement assembly  260  measures one or more of the temperature, pressure, and flow rate of the fluid flowing through the corresponding tubular (e.g., production fluid, thermal transfer fluid), and then communicates the measurement(s) to the controller unit. In this embodiment, the controller unit is disposed on vessel  20 . Thereafter, either personnel, software applications, or some combination thereof, analyze the measurements from assemblies  260  and determine what, if any, adjustments need to be made by the controller unit to the operating parameters of exchanger  100  (e.g., the pump speed) in order to achieve a predetermined, desired performance. Specifically, in some embodiments, the controller unit adjusts the flow rate of thermal transfer fluid based on the measurements made by assemblies  260  (e.g., temperature) to thereby adjust and control the rate of heat transfer (e.g., convective) between production fluids and thermal transfer fluids. In this embodiment, the flow rate of thermal transfer fluid through exchanger  100  is adjusted by adjusting the speed of pumps  211   a ,  211   b  within pumping unit  210 . In addition, in at least some embodiments, the controller unit may either additionally or alternatively adjust the flow rate of production fluids flowing to inlet  101  by, for example, actuating a choke valve that is disposed upstream of exchanger  100  (e.g., on tree  12 ). Further, in at least some embodiments, one or more heating elements may be installed at certain locations within exchanger  100  (e.g., along line  220  between unit  210  and heat exchanger units  120 ) that can be utilized to alter (e.g., increase) the temperature of the thermal transfer fluid flowing therethrough and thus further adjust the performance of exchanger  100 . Although sensors assemblies  260  are only shown at inlet  101 , outlet  102 , line  230 , and line  220  in this embodiment, in general, sensor assemblies  260  can be disposed at various other locations within exchanger  100  (e.g., within one or more of the heat exchanger units  120 ) either in addition to or in lieu of the locations shown in  FIG. 8 . 
     Referring now to  FIGS. 2, 8, 10, and 11 , as previously described, exchanger  100  may be referred to herein as a “modular” heat exchanger since the construction of exchanger  100  is ultimately based on a plurality of interconnected heat exchanger units  120 . Thus, depending on the needs and specifications of the given production system (e.g., system  10 ), the number and arrangement of heat exchanger units  120  may be modified such that the heat transfer performance delivered by exchanger  100  is optimized in light of those needs and specifications. 
     In particular, to increase the amount of thermal transfer within exchanger  100  (i.e., to increase the amount of temperature change for production fluids between inlet  101  and outlet  102 ), the number of heat exchanger units  120  may simply be increased. This increase in units  120  within exchanger  100  can be accomplished in a number of different ways, all while still complying with the principle disclosed herein, and essentially works to increase the ultimate length of the flow path for production fluids between inlet  101  and outlet  102 . For example, in some embodiments, plates  122  may be enlarged (i.e., extended) in the radial direction relative to the direction of axes  125  and one or more additional rows (e.g.,  110 A,  110 B,  110 C) of units  120  may be added. As another example, in some embodiments, each row  110 A,  110 B,  110 C may include one or more additional units  120  (other than simply two in the embodiment of  FIGS. 2 and 8 ). In these embodiments, it should be appreciated that each of the units  120  of a given row  110 A,  110 B,  110 C will be interconnected with bridging assemblies  170  as previously described above. 
     Conversely, to decrease the amount of thermal transfer within exchanger  100  (i.e., to decrease the amount of temperature change for production fluids between inlet  101  and outlet  102 ), the number of heat exchanger units  120  may simply be decreased. This decrease in units  120  within exchanger  100  can be accomplished in a number of different ways, all while still complying with the principle disclosed herein, and essentially works to decrease the ultimate length of the flow path for production fluids between inlet  101  and outlet  102 . For example, as is shown in  FIG. 10 , in some embodiments, plates  122  may be shortened in the radial direction relative to the direction of axes  125  and one or more rows (e.g., rows  110 A,  110 B,  110 C) of units  120  may be removed. As another example, in some embodiments, one or more heat exchanger units  120  may be removed from each row  110 A,  110 B,  110 C. Specifically, as shown in  FIG. 11 , in some embodiments, each row  110 A,  110 B,  110 C only includes a single unit  120 , and thus, no bridging members  170  are included as no two units  120  are axially aligned along axes  125  in the manner described above. 
     Although subsea heat exchanger  100  is described above for use in transferring thermal energy from production fluids to the thermal transfer fluid to cool the production fluids, embodiments described herein can also be used to transfer thermal energy from the thermal transfer fluid to the production fluids to warm the production fluids. For example, referring now to  FIG. 12 , a subsea heat exchanger  300  for use within production system  10  is shown. In this embodiment, exchanger  300  transfers thermal energy to production fluids as they are routed between inlet  101  and outlet  102 . 
     Exchanger  300  includes many of the same components and features as exchanger  100  previously described, and thus, like numerals are used to describe shared components between exchangers  100 ,  300  and the description below will focus only on the differences of exchanger  300  relative to exchanger  100 . As shown in  FIG. 12 , in addition to features of exchanger  100 , exchanger  300  generally includes a pumping unit  310  in place of pumping unit  210  previously described, and a warming recirculation line  330 . 
     Pumping unit  310  is the same as pumping unit  210  except that pumping unit  310  additionally includes one or more warming devices  311  disposed therein that are configured to warm or heat the thermal transfer fluid that is discharged thereby. In particular, in some embodiments, warming devices  311  within pumping unit  310  include one or more energy elements (e.g., resistive coils) that are electrically powered to generate heat that is transfer to thermal transfer fluid during operations. However, it should be appreciated that any suitable heating elements for increasing the temperature of the thermal transfer fluid flowing through pumping unit  310  may be used while still complying with the principles disclosed herein. It should also be appreciated that in some embodiments, similar heating elements are disposed throughout the exchanger  300  at various locations (either in lieu of or in addition to the pumping unit  310 ). In some of these embodiments, heating elements are disposed within a separate retrievable unit, and in still others of these embodiments, heating elements are permanently installed at various locations within exchanger  300 . 
     Warming recirculation line  330  extends from a valve assembly  320  disposed along recirculation line  230  to pumping assembly  310 . As opposed to line  230 , recirculation line  330  does not include a radiator  250  or similar component configured to cool the fluids flowing therethrough (e.g., along arrows  205 ). Rather, the intent in flowing thermal transfer fluid through line  330  is to maintain a given temperature of the spent fluid after it exits heat exchanger units  120  such that heating operations carried out in pumping unit  310  as previously described (e.g., with one or more energy elements) are enhanced. As a result, in this embodiment, all portions of recirculation line  330  are covered with thermal insulation such that heat loss from the thermal transfer fluid to the ocean environment is minimized. In this embodiment, valve assembly  320  is actuatable between at least three different positions: (1) a first position in which thermal transfer fluids are allowed to flow freely between fluid flow path  113 C in row  110 C and recirculation line  230  but are restricted from flowing into and through line  330 ; (2) a second position in which thermal fluids are allowed to flow freely between fluid flow path  113 C in row  110 C and recirculation line  330  but are restricted from flowing into and through line  230 ; and (3) a third position in which thermal fluids are restricted from flowing into and through both lines  230 ,  330 . In general, valve assembly  320  can be actuated by any suitable method, such as, for example, manual actuation by ROV or other interaction device, automatic actuation by a remote controller unit, etc. In addition, in at least some embodiments, recirculation line  330  may include one or more thermal expansion sections  240  being the same as that included on line  230  and previously described above. 
     During operations, production fluid flows along fluid flow paths  112 A,  112 B,  112 C within rows  110 A,  110 B,  110 C, respectively, of heat exchanger units  120  as previously described. Similarly, thermal transfer fluid flows along fluid flow paths  113 A,  113 B,  113 C within rows  110 A,  110 B,  110 C, respectively, of heat exchanger units  120  as previously described. However, in this embodiment, prior to injection into heat exchanger units  120  and flowing along paths  113 A,  113 B,  113 C, thermal transfer fluids are warmed/heated by the heating devices  311  disposed within pumping unit  310 , preferably to a temperature that is greater than the temperature of the production fluids entering exchanger  100  at inlet  101 . Due to the differences in temperature, as the production fluids and thermal transfer fluids flow along fluid flow paths  112 A,  112 B,  112 C and  113 A,  113 B,  113 C, respectively, thermal energy (e.g., heat) is transferred from thermal transfer fluid across inner tubular members  140  into production fluids. As a result, during operations with exchanger  300 , the temperature of production fluid increases to a maximum at outlet  102  while the temperature of thermal transfer fluid decreases to a minimum when it enters recirculation line  330 . Thus, through use of exchanger  300 , relatively cool production fluids are warmed in order to avoid potential problems associated with such cool production fluids such as, for example hydrate formation. 
     Since exchanger  300  is designed to warm production fluids as previously described, in some embodiments, recirculation line  230  is not included while still complying with the principles disclosed herein. In addition, in at least some embodiments, an additional valve assembly (not shown) is disposed along outlet line  220  which is actuatable to selectively restrict the flow of thermal transfer fluids along line  220  into fluid flow path  113 A in row  110 A. Thus, in these embodiments, both the valve assembly along line  220  and valve assembly  320  may be actuated to prevent fluid flow of thermal transfer fluids both into and out of rows  110 A,  110 B,  110 C of heat exchanger units  120 . Without being limited to this or any other theory, such a flow arrangement may be used to provide a substantially stagnate volume of thermal transfer fluid around tubes  140  within exchanger units  120 , which thus provides an additional insulative barrier to heat transfer between production fluids and the ocean environment. In at least some of these embodiments, additional heating devices, similar to those described above to be included within pumping unit  310  may be disposed throughout units  120  within exchanger  300  to thereby maintain a desired temperature of the stagnant thermal transfer fluids trapped between the closed valve assembly on line  220  (not shown) and valve assembly  320 . 
     In addition, in at least some embodiments, an additional flushing line is included for flushing or removing spent thermal transfer fluid. For example, referring still to  FIG. 12 , exchanger  300  includes a flush line  400  extending from valve assembly  320  to a valve assembly disposed along jumper line  17  downstream of exchanger  300  (see  FIG. 1 ). In this embodiment, valve assembly  320  is additionally actuatable (i.e., additional to the positions described above) to a position where thermal transfer fluid is allowed to flow from flow path  113 C in row  110 C to flush line  400  and is restricted from flowing into either of the lines  230 ,  330 . During normal operations, when thermal transfer fluid is routed through either line  230  or line  330 , valve assembly  320  is actuated to prevent thermal transfer fluid from entering flush line  400 . Additional isolation valves  420  are disposed on each of the recirculation lines  230 ,  330  and flush line  420  to allow for further and finer control of the routing of fluids during operations. Also, in this embodiment flush line  400  includes a one way check valve  430  that is configured to only allow flow along line  400  from valve assembly  320  toward valve assembly  410 . Further, in this embodiment, injection points (or ports)  415  (e.g., ROV injection points) are included along both recirculation lines  230 ,  330 , and each provides an access point for the injection or withdrawal of fluids from the respective line  230 ,  330  by an ROV or other suitable device (e.g., umbilical) during operations. 
     During operations, when it becomes desirable to remove and/or replace the thermal transfer fluid flowing through exchanger  300 , valve assembly  320  is actuated to prevent thermal transfer fluid from entering either of the recirculation lines  230 ,  330 , but allow fluid flow into flush line  400  as previously described. Once thermal transfer fluid flows into line  400  it is routed toward valve assembly  410  and into line  17 , where it can either be sent to the surface (e.g., through riser assembly  20 ) or routed subsea to some other suitable location or collection point (e.g., subsea tank). During these operations, pumping unit  310  continues to run in order to provide the necessary pressure differential to cause positive flow of thermal transfer fluid jumper  17  through line  400 . After all or substantially all of the spent thermal transfer fluid is flushed from exchanger  300  through line  400 , fresh thermal transfer fluid is injected (e.g., by an ROV, pipeline, umbilical, etc.) within line  230  and/or line  330  at injection points  415 , thereby refilling exchanger  300 . In addition, once all spent thermal transfer fluids are flushed from exchanger  300  through line  400 , valve assembly  320  is actuated to once again restrict flow through line  400  while allowing flow through recirculation line  230  and/or line  330  as previously described above. 
     In addition to the flushing operations described above, it should also be appreciated that flush line  400  may also be utilized to flow displaced thermal transfer fluid from exchanger  300  to line  17  when other fluids and/or additives are being injected at injection points  415  in order to prevent an over pressurization of exchanger  300 . Injected additives and/or fluid may include, for example, corrosion inhibitors, biocides, plasticizers, hydrate inhibitors, or some combination thereof. Further, it should also be appreciated that in some embodiments injection points  415  may also be used to induce an initial pressurization of the thermal transfer fluid within exchanger  300  to facilitate future sampling of the thermal transfer fluid for condition monitoring of the fluid properties. 
     In the manner described, embodiments of subsea heat exchangers described herein (e.g., exchangers  100 ,  300 ) can be used to cool production fluids that are produced with temperatures above the operating temperature range of downstream equipment in the production system (e.g., system  10 ) or heat production fluids that are produced with temperatures below the operating temperature range of downstream equipment in the production system (e.g., system  10 ). In addition, the modular design of embodiments of subsea heat exchangers described herein (e.g., exchangers  100 ,  300 ) enables the heat exchangers to be tailored for to the desired thermal transfer performance by adding or removing modular heat exchanger units (e.g., units  120 ) to closely match the specifications and/or needs of the associated production system (e.g., system  10 ). 
     While certain exemplary embodiments have been shown and described, modifications thereof can be made by one of ordinary skill in the art without departing from the scope of teachings herein. For example in some embodiments, thermal insulation may be disposed about each of the heat exchanger units  120  and all associated piping within exchangers  100 ,  300  in order to reduce the thermal influence of the ocean environment during operations. However, it should be appreciated that in at least some of these embodiments the sections and  254  and curves  252  within radiator  250  may be left uncovered to enhance heat transfer between the thermal fluid flowing therethrough and the ocean environment. As another example, while embodiments disclosed herein have shown a heat exchanger (e.g., exchanger  100 ,  300 ) receiving production from a single wellbore (e.g., wellbore  14 ), it should be appreciated that in other embodiments, exchanger  100  and/or  300  may receive production fluids from more than one such wellbores while still complying with the principles disclosed herein. In addition, in at least some embodiments, one or more of the tubes  140  may be replaced with a plurality of tubes extending parallel to one another (e.g., parallel to axis  125 ) rather than a single length of pipe. Further, some embodiments of exchangers  100 ,  300  may also include a hot stab panel or other suitable access point for an ROV or similar device, to enable injection of fluids (e.g., warm fluids, hydrate inhibitors, cleaning solutions, etc.) into any portion of exchanger  100 ,  300 . Still further, in some embodiments, pumping units  210 ,  310  may be utilized to provide pressurized thermal transfer fluid to more than one exchanger  100 ,  300 , respectively, while still complying with the principles disclosed herein. Also, in at least some embodiments, pumping units  210 ,  310  may include one or more filter modules to capture particulate matter that is disposed within thermal transfer fluid, thereby reducing the likelihood of clogging or other failures caused by such suspended particulate matter. Moreover, while the subsea heat exchangers shown and described herein (e.g., exchangers  100 ,  300 ) include a plurality of heat exchanger units  120  that are arranged such that fluids (e.g., production fluid, thermal transfer fluid) flows generally in an S-shaped pattern (e.g., see  FIGS. 8 and 10 ), it should be appreciated that other arrangements may be used. For example, in some embodiments, heat exchanger units  120  are stacked vertically upon one another rather than being spread along sea floor  7  as shown. Thus, the embodiments described herein are exemplary only and are not limiting. While embodiments disclosed herein have included flanges (e.g., flanges  150 ,  160 ) for connecting heat exchangers  120  to one another (e.g., through tubing members  222  and transfer pipes  104 ,  106 , bridging assemblies  170 , etc.) other embodiments may replace one or more of the flanges  150 ,  160  with welded connections in order to eliminate the need for a hydrocarbon containing seal (e.g., seal assemblies  151 ). In addition, other suitable, non-flanged connections may be utilized such as a clamp connector (hydraulic or otherwise) and the like. Also, it should be appreciated that other valve assemblies may be utilized within the exchangers  100 ,  300  in addition to those specifically shown and described above, while still complying with the principles disclosed herein. For example, in some embodiments, additional valves/valve assemblies are attached in and/or around the pumping module  210 , as well as along lines  220 ,  230 ,  330 , etc. 
     In addition, many other variations and modifications of the systems, apparatus, and processes described herein are possible and are within the scope of this disclosure. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims. Unless expressly stated otherwise, the steps in a method claim may be performed in any order. The recitation of identifiers such as (a), (b), (c) or (1), (2), (3) before steps in a method claim are not intended to and do not specify a particular order to the steps, but rather are used to simplify subsequent reference to such steps.