Patent Publication Number: US-2022213764-A1

Title: Subsea Installations Comprising Heated Conduits

Description:
This invention relates to subsea installations comprising conduits that are heated for flow assurance. More specifically, the invention relates to subsea conduits that are fitted with electrical heating systems, such as pipelines and spools as used in the oil and gas industry. 
     Subsea pipelines are used as ‘tie-backs’ to transport crude oil and/or natural gas from a subsea wellhead across the seabed on the way to the surface. Typically, in offshore locations, the oil and/or gas flows up a riser from the seabed to the surface to undergo treatment and temporary storage at a surface installation. 
     Oil and gas are present in subterranean formations at elevated temperature and pressure, which may be increased by the injection of fluids such as steam. On production of the oil or gas, the produced fluid emerges from the wellhead and enters a subsea pipeline in a multi-phase state. 
     During subsequent transportation along the pipeline, the temperature and pressure of the produced fluid have to be kept high enough to ensure a sufficient flow rate across the seabed and up the riser. In particular, various measures are taken to ensure that the internal temperature of the pipeline remains high despite thermal exchange with the surrounding seawater, which is invariably much colder. 
     Low temperature increases the viscosity of the produced fluid and promotes precipitation of solid-phase materials, namely waxes and asphaltenes in crude oil and hydrates in natural gas. Such solid-phase materials tend to deposit on the inner wall of the pipeline and may eventually cause plugs, which will interrupt production. Aside from the high cost of lost production, plugs are difficult and expensive to remove and can even sever the pipeline. 
     In addition, an oil or gas field must occasionally be shut down for maintenance. When production restarts, temperature within the pipeline must be increased quickly so that no plugs will form. 
     The challenges of thermal management increase as subsea pipelines become longer. In this respect, there is a trend toward longer tie-backs as oil and gas reserves are being exploited in increasingly challenging locations. 
     Designers of subsea pipelines have adopted both passive and active approaches to thermal management, either individually or in combination. 
     In passive thermal management systems, the pipeline is thermally insulated to retain heat in the pipeline. Conversely, active thermal management systems add heat to the pipeline. For example, heat may be added by thermal exchange with hot fluids flowing along or around the pipeline. In an alternative approach, heat may be added by electrical heating systems. 
     One example of an electrical heating system is a trace heating system comprising resistive electrical wires or cables running along, and in thermal contact with, the outer surface of a steel flowline pipe. Heat produced by passing an electric current along the cables is conducted firstly from the cables to the pipe wall and secondly through the pipe wall to the produced fluid flowing within the flowline. As an alternative to resistive heating of the cables, heat may be generated by currents induced in the pipeline wall beside the cables. The paper OTC 22631 presented at the 2011 Offshore Technology Conference in Brazil describes these and other approaches to the electrical heating of subsea pipelines. 
     In long resistive trace-heated pipelines powered by a three-phase electrical supply, three heating cables are employed, one for each phase. The cables extend along the pipeline parallel to each other, being arranged either helically around and along the central longitudinal axis of the pipeline or in straight lines parallel to that axis. In the latter case, the cables are suitably equi-angularly spaced around the central longitudinal axis. 
     A neutral point or ‘star end’ is formed by a wye connection at an end of the heating cables. At the star end, the three heating cables are connected together in short-circuit. Such a star end is exemplified in FR 2978006, which discloses a trace-heated pipeline fitted with a triphasic heating system and having redundant heating cables. 
     WO 2007/055592 also shows three heating cables on a pipeline, in this case for induction heating, which are short-circuited together at one end. The sole drawing in WO 2007/055592 is merely schematic and shows the resulting star end spaced from the end of the pipeline, apparently by cantilevered overhanging portions of the heating cables. However, accompanying text in the same drawing refers to short-circuiting at the end of the pipe, which is conventional. 
     Suspending the star end from straight unsupported portions of the cables, as shown schematically in WO 2007/055592, would be structurally impossible. Thus, in the absence of any explanation or further teaching, the arrangement illustrated in WO 2007/055592 can only be interpreted as artistic licence for ease of illustration. 
     WO 2017/182721 shows that heating cables may also be connected electrically by a metallic ring attached to a heated pipeline. The ring serves as a neutral point or star coupling between the heating cables. Hence, a star coupling between heating cables does not necessarily have to be situated at an end of a pipeline. 
     Whilst multiple star-end connections can be made along the length of a trace heating system, the number of such connections has to be minimised because each one creates a short circuit and results in power loss. Another drawback of star ends is that they heat up significantly, which in some situations could cause thermal damage to the cables or to adjoining structures or materials. 
     WO 2018/231562 and U.S. Pat. No. 6,371,693 show further examples of electrically heated subsea pipelines. 
     Against this background, the invention provides a subsea installation for the production of hydrocarbons. The installation comprises: a flowline; a first group of heating cables extending along the flowline; and a termination effecting short-circuit connection between the heating cables of the first group; wherein the termination is located at a subsea location remote from the flowline. 
     The installation may further comprise at least one connecting conduit in fluid communication with the flowline, wherein the termination is connected electrically to the first group of heating cables via at least one second group of heating cables that extends along the or each connecting conduit. 
     The or each connecting conduit may be a jumper or spool in fluid communication with at least one subsea structure that is in fluid communication with the flowline. 
     At least one subsea structure in fluid communication with the flowline, such as a wellhead, a manifold or a pipeline accessory, may be interposed between the termination and the flowline. In that case, the termination may conveniently be mounted on one such subsea structure interposed between the termination and the flowline. Also, electrical connection between the termination and the first group of heating cables may be made via electrical connections in at least one such subsea structure interposed between the termination and the flowline. Conversely, electrical connection between the termination and the first group of heating cables may bypass at least one such subsea structure interposed between the termination and the flowline. 
     The termination may be connected electrically to the first group of heating cables via at least one set of ROV-connectable flying leads. In that case, the installation suitably further comprises connector hubs to which the flying leads are ROV-connectable. 
     The termination suitably comprises: connectors corresponding to the heating cables of the first group; and conductors extending from the connectors to a neutral point. The termination may further comprise heat-exchange formations in thermal communication with the neutral point and/or a housing that supports the connectors and that surrounds the conductors and the neutral point. A mount or foundation may also be provided for supporting the termination. 
     The connectors of the termination may comprise a set of ROS-connectable flying leads or at least one connector hub that is engageable with a set of ROV-connectable flying leads. 
     The inventive concept embraces a termination module for effecting short-circuit connection between a group of subsea heating cables, the termination module comprising: connectors corresponding to the heating cables of the group; and conductors extending from the connectors to a neutral point. 
     The inventive concept also extends to a method of installing a subsea installation for the production of hydrocarbons. That method comprises: placing a termination at a subsea location remote from an electrically-heated subsea flowline; and electrically connecting the termination to a first group of heating cables extending along the flowline, to effect short-circuit connection between those heating cables. 
     A star end located traditionally on a pipeline prevents electrical heating extensions from the pipeline to other structures such as spools. The invention therefore solves the problem of extending electrical heating of a subsea pipeline to other structures or ancillary equipment. 
     Embodiments of the invention provide a termination for an underwater electrical heating system, which system comprises at least three phases, wherein the termination connects the three phases in a short circuit and is located in water separately from the heated equipment. The electrical heating system may for example be a trace-heating system, which may employ resistive heating. 
     The star-end connection may be embedded, encapsulated or enclosed in a sealed package, which may be defined by a tape, a moulded thermoset or thermoplastic polymer, an epoxy, a ceramic or other materials. Such a package may have a radiator shape or formation to optimise heat exchange with surrounding water. Alternatively, the star-end connection may be submerged freely in the surrounding water and so may be exposed directly to that water. 
     Embodiments of the invention also provide an underwater trace-heated system, the system comprising at least one trace-heated flow path for transporting a fluid such as crude oil and trace-heating cables transporting three electrical power phases, arranged along the trace-heated flow path, and at least one star-end connection, located in water, connecting at least one cable for each phase in a short-circuit. The trace-heated system may comprise multiple star-end connections. 
     The flow path may comprise at least one pipeline, jumper or spool, and may include piping in a subsea structure or a wellhead. 
     The trace-heating cables suitably comprise cables arranged along the flow path and electrical connectors between successive sections of cables. 
     Embodiments of the invention also provide a modular star-end connection for an underwater triphasic trace-heating system, the connection comprising an electrical connector hub, cables for the three phases, and a short-circuit connection of at least one cable for each phase. 
     The same star end module can be connected on the same connectors as flying leads, which allows phased development. 
     The invention addresses the problem of heating ancillary components, structures or equipment such as spools and manifolds using electrical energy from a trace-heated flowline. This obviates the alternative of dedicated heating solutions that require associated dedicated power distribution cables. 
     In summary, the invention aims to heat external equipment or structures connected to a trace-heated flowline and to solve the challenges of doing so. The principle of the invention is employed at an end of a heated section of a trace-heated flowline, where the star end is normally located. The star end is instead marinised and located outside the flowline in the surrounding environment. This takes electrical power from the flowline, in effect extending the power-supply capability of the flowline to another subsea location. Heat may thereby be provided to equipment or structures external to the flowline, such as spools or manifolds, using energy conveyed along and beyond the heating section of the flowline. 
     By locating the star end in a cold-water environment, the star end can be cooled because heat generated at the star end due to short-circuiting can be dissipated more readily. The star end can also be repaired or replaced more easily in case of failure. Connections between parts of the system may be based on familiar solutions known for the supply of electrical energy subsea, typically electrical double-barrier connectors. 
     Thus, the invention provides a subsea installation that comprises a flowline and a group of heating cables extending along the flowline. A star end termination effecting short-circuit connection between the heating cables of the group is located at a subsea location remote from the flowline. For example, a subsea structure or fluid conduit in fluid communication with the flowline, such as a PLET or a spool, may be interposed between the termination and the flowline. The termination may be a module that is connectable electrically to the heating cables via flying leads and connector hubs. 
    
    
     
       In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which: 
         FIG. 1  is a schematic side view of a trace-heated subsea installation comprising a star end module of the invention: 
         FIG. 2  is an enlarged schematic view of the star end module shown in  FIG. 1 ; 
         FIG. 3  is a schematic side view of the star end module of the invention connected to a different part of the subsea installation shown in  FIG. 1 ; and 
         FIG. 4  is a schematic side view of a subsea installation in a variant of the arrangement shown in  FIG. 1 . 
     
    
    
     Referring firstly to  FIG. 1  of the drawings, a subsea oil and gas installation  10  is shown resting on the seabed  12 . In this example, the installation  10  comprises a well tree  14  that supplies hydrocarbon production fluid to a subsea pipeline  16  via an intermediate manifold  18  and a pipeline end termination (PLET)  20  at an end of the pipeline  16 . The direction of production fluid flow is therefore from left to right as illustrated. 
     The PLET  20  is an example of a pipeline accessory structure in fluid communication with the pipeline  16 . Other such accessories may be positioned in-line along the length of the pipeline  16  rather than being a termination at an end of the pipeline  16 . 
     Production fluid is conveyed between the well tree  14  and the manifold  18  and between the manifold  18  and the PLET  20  by respective connecting pipes or conduits such as jumpers or spools  22 . The pipeline  16  and the spools  22  are typically wet-insulated with a layer of polymeric thermal insulation that is exposed to the surrounding seawater. However, such insulation is conventional and so has been omitted from the drawings for ease of illustration. 
     The pipeline  16  may extend for several kilometres across the seabed  12  and typically then communicates with a riser that conveys the production fluid to a surface installation such as a platform or an FPSO (floating production, storage and offloading) vessel. The surface installation may also provide power and service fluids to the subsea installation  10 , typically by cables and fluid ducts that extend along the riser to convey the power and fluids in an opposite direction to the flow of production fluid. Such a riser and surface installation are conventional and so have also been omitted from the drawings. 
     The installation  10  exemplified in  FIG. 1  is further simplified to emphasise the main focus of the invention, namely the provision of electrical power to a trace heating system that extends along the pipeline  16  and the spools  22 . For this purpose, a three-phase power supply  24  is shown schematically in  FIG. 1 , connected to respective heating cables  26  that extend in a parallel, mutually-spaced relationship along the pipeline  16  and the spools  22 . In this example, the grouped heating cables  26  are wound helically around the pipeline  16  and the spools  22 . 
     The power supply  24  could, for example, be housed on a surface installation that receives production fluid from the subsea installation  10  as mentioned above. In that case, electrical power for the heating cables  26  of the trace heating system may conveniently be conveyed along a riser extending between the pipeline  16  and the surface installation. 
     The heating cables  26  extend along the pipeline  16  to the FLET  20 , where they terminate in a connector hub  28 A for the connection of a corresponding number of external flying leads  30 , each in series with a respective one of the heating cables  26 . Conveniently, each flying lead  30  terminates at each end in double-barrier connectors that enable connection and disconnection of the flying leads  30  underwater, typically by an ROV. 
     The flying leads  30  effect electrical connection between the connector hub  28 A on the FLET  20  and another connector hub  28 B on the spool  22  that extends between the FLET  20  and the manifold  18 . The connector hub  28 B is connected electrically, in turn, to another group of heating cables  26  that extend along substantially the whole length of the spool  22  to terminate in a further connector hub  28 C positioned on the spool  22  near where the spool  22  joins the manifold  18 . 
     A further set of flying leads  30  extend between the connector hub  28 C and another connector hub  28 D on the other spool  22  that extends between the manifold  18  and the well tree  14 . These flying leads  30  thereby bypass the manifold  18  interposed between the spools  22 . The connector hub  28 D is connected electrically, in turn, to another group of heating cables  26  that extend along substantially the whole length of that spool  22  to terminate in a further connector hub  28 E positioned on the spool  22  near where the spool  22  joins the well tree  14 . 
     Thus, by virtue of the flying leads  30  and the connector hubs  28 A- 28 E, each heating cable  26  on the pipeline  16  is connected in series with a corresponding heating cable  26  on each spool  22 . So, elegantly, the heating cables  26  on the spools  22  are powered via the heating cables  26  on the pipeline  16 , without requiring separate power provisions such as multiple power supplies  24  or multiple connections such as umbilicals from one or more power supplies  24 . 
     In the example shown in  FIG. 1 , a third set of flying leads  30  effects electrical connection between the connector hub  28 E and a further external connector hub  28 F mounted on a star end module  32  of the invention. The star end module  32  defines a neutral point  34  by short-circuiting the heating cables  26  that extend along the pipeline  16  and the spools  22 . The star end module  32  thereby serves as a termination for the electrical circuit that comprises the various heating cables  26 . 
     The star end module  32  is shown here mounted to the well tree  14  but could instead be mounted to a different subsea structure or item of equipment, whether part of the subsea installation  10  or otherwise. In principle, it would also be possible for the star end module  32  to rest on the seabed  12  independently, for example via a dedicated a subsea foundation. 
     The star end module  32  is shown in more detail in  FIG. 2  of the drawings. It comprises a housing  36  that supports the three-phase connector hub  28 F and electrical conductors  38  extending from each respective port or socket  40  of the connector hub  28 F. The three conductors  38  converge and conjoin to define the neutral point  34 . The housing  36  surrounds and encloses, embeds or encapsulates the neutral point  34  and the conductors  38 . 
     In use, heat will be produced in consequence of the short circuit at the neutral point  34 . A heat exchanger  42  in thermal communication with the neutral point  34  comprises external formations such as fins that increase the surface area of the heat exchanger  42 , thereby to promote dissipation of that heat into the surrounding seawater. Similarly, the housing  36  itself may be shaped to define external heat-exchange formations and may be configured to conduct heat away from the neutral point  34 . 
     Where the star end module  32  is to be mounted to a subsea structure or item of equipment such as the well tree  14  shown in  FIG. 1 , the module  32  is conveniently provided with a mount  44  as also shown in  FIG. 2 . The mount  44  may be a temporary coupling or a permanent or integral coupling. 
     The connector hubs  28 A- 28 F, the subsea-connectable flying leads  30  and the star end module  32  facilitate convenient modular subsea expansion or reconfiguration of the subsea installation  10 . In this respect,  FIG. 3  shows the star end module  32  instead mounted on the PLET  20 . The connector hub  28 F of the star end module  32  is connected by a set of flying leads  30  directly to the connector hub  28 A of the PLET  20 , which is connected in turn to the heating cables  26  that extend along the pipeline  16 . 
     Thus, the star end module  32  can conveniently be positioned and connected to define a neutral point  34  by short-circuiting the heating cables  26  of the pipeline  16 . The same star end module  32 , or an additional star end module  32  may be used or re-used to define an alternative neutral point  34  by short-circuiting other heating cables  26  in a larger subsea installation  10  like that shown in  FIG. 1 . 
     Finally,  FIG. 4  of the drawings shows a variant of the arrangement shown in  FIG. 1  in which like numerals are used for like features.  FIG. 4  differs from  FIG. 1  in the arrangement of the electrical connection between the star end module  32  and the heating cables  26  of the spool  22  that adjoins the well tree  14 . Specifically, in  FIG. 1 , the third set of flying leads  30  connects directly between the connector hub  28 E of the spool  22  and the connector hub  28 F of the star end module  32 . Conversely, in  FIG. 4 , electrical connection between the connector hub  28 E of the spool  22  and the connector hub  28 F of the star end module  32  is made via electrical connections in the well tree  14 . 
     Thus,  FIG. 4  shows the third set of flying leads  30  now extending between the connector hub  28 E and a connector hub  28 G mounted on the well tree  14 . The connector hub  28 G is connected electrically to a further connector hub  28 H mounted on the well tree  14 , A fourth set of flying leads  30  effects parallel electrical connections between that connector hub  28 H and the connector hub  28 F of the star end module  32 . 
     Many other variations are possible within the inventive concept. For example, like the connector hub  28 A on the PLET  20 , the connector hub  28 B shown in  FIGS. 1 and 4  could instead be implemented on the PLET  20 . In that case, electrical connections may extend through the PLET  20  between the connector hub  28 B and the associated heating cables  26 . Conversely, the connector hub  28 A could be implemented on the pipeline  16  so that no electrical connections extend through the PLET  20  between the heating cables  26  and the connector hub  28 A. 
     The connector hubs  28 E and  28 H shown in  FIG. 4  could be directly coupled or plugged together without intermediate leads. 
     Similarly, the connector hubs  28 C and/or  28 D shown in  FIGS. 1 and 4  could instead be implemented on the manifold  18 , with corresponding electrical connections being provided through the manifold  18 . Also, the connector hub  28 E shown in  FIGS. 1 and 4  could instead be implemented on the well tree  14 , with corresponding electrical connections being provided through the well tree  14 . 
     Other variations are possible without departing from the inventive concept. For example, the pipeline  16  and/or the spools  22  could by insulated thermally by a dry insulation system based upon a concentric pipe-in-pipe assembly. In that case, the heating cables  26  would typically be disposed within an insulating annulus defined between inner and outer pipes of the assembly. 
     In the examples shown, the parallel heating cables  26  are wound helically around the pipeline  16  and the spools  22  but other arrangements are possible. For example, the heating cables  26  could instead extend substantially straight along the pipeline  16  and/or the spools  22 , substantially parallel to the flow of the production fluid within. 
     The power supply  24  could be connected to the heating cables  26  by a dedicated umbilical hanging from the surface instead of via a riser. Alternatively, the power supply  24  could be located underwater. 
     A set of flying leads  30  could be integrated into or otherwise permanently connected to the star end module  32 , hence potentially obviating the connector hub  28 F shown on the star end module  32  in the drawings.