Patent Description:
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 or gas flows up a riser from the seabed to the surface to undergo treatment and temporary storage at a surface installation.

On production of oil or gas, the produced fluid emerges from a subsea wellhead at elevated temperature and pressure and enters a 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 an uninterrupted flow, at a sufficient flow rate, across the seabed and up the riser. Thus, 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.

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 produced fluid. For example, a pipe-in-pipe (PiP) assembly comprises an inner flowline pipe and an outer pipe in coaxial relation to define an annulus between them, which may be filled with gas and thermally insulating materials. Conversely, active thermal management systems add heat to the pipeline, for example by electrical heating.

One example of electrical heating 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.

Conventionally, PiP structures are used for subsea pipelines that employ trace heating. In that case, the heating cables are disposed within the annulus against the outer surface of the flowline pipe, buried beneath a surrounding layer of thermal insulation material in the annulus. The cables extend along the pipeline parallel to each other, being arranged in straight lines parallel to the central longitudinal axis of the pipeline or, more commonly, spiralling helically around that axis.

Heating cables can be powered by direct current (DC) or alternating current (AC), which can be monophasic current or multiphasic current, typically triphasic current. However, monophasic AC power is less efficient than triphasic AC power and cannot allow effective heating along as great a distance as triphasic AC power. Triphasic AC power is therefore preferred, especially as there is a trend toward longer tie-backs as oil and gas reserves are being exploited in increasingly challenging locations.

In triphasic trace-heated pipeline systems, heating cables are grouped, at least functionally, in triplets being groups of three cables, one cable for each phase. Each group or triplet forms a three-phase star connection circuit. Specifically, a three-phase electrical supply supplies respective phases of alternating current to one end of each of the three cables. At the other end, the three cables are connected together in short-circuit by a wye connection to form a star connection or 'star end'.

Beneficially, where the three heating cables meet at the star end, a neutral point is defined where the sum of the three current phases is nil. This means that no return cable is required.

A star end is exemplified in <CIT> and correspondingly <CIT>, which discloses a trace-heated pipeline fitted with a triphasic heating system and having redundant heating cables. <CIT> teaches the addition of intermediate star connections, rather than having star ends at the end of a pipeline. <CIT>, to the Applicant, discloses relocating the star end to improve its performance. <CIT> discloses a trace heated pipeline with a multiphasic heating system having at least one redundant heating cable.

Typically, there are at least two to four three-phase circuits in a trace heating system, including the option of one spare circuit. In this respect, a trace heating system must operate reliably throughout the long service life of a pipeline, especially as any failure to heat the pipeline, whenever required, could result in a blockage that is difficult or impossible to resolve. As cables of the system are rendered inaccessible by the outer pipe of a PiP assembly, they cannot be maintained or replaced in service.

Tie-backs and therefore heating cables of trace heating systems can have a length in the order of tens of kilometres. To suit fabrication of the pipeline, sections of the cables may be connected in series, thus requiring numerous connections at the junctions between them that each create a potential failure point.

The significant risks and severe consequences of failure of a trace heating system require a high level of redundancy to guarantee availability of the system, for example the provision of two or three independent systems. However, multiplying the number of systems to increase redundancy substantially increases cost and complexity. There is also a problem of lack of space within the annulus of a PiP system, where there is a need to accommodate multiple cables in addition to thermal insulation material within a restricted volume.

Whilst redundancy is relatively easy, if costly, to add to a monophasic trace-heating system, the problems of cost, complexity and lack of space are greatly compounded by the architecture of a conventional triphasic system. As at least three working cables are required for each circuit of a triphasic system to function, failure of any one cable of a triplet group renders all cables of that group inoperative and unavailable for use. Thus, redundancy requires additional cables to be installed in multiples of three.

The present invention takes a different approach to increase redundancy in a multiphasic trace heating system but without necessarily adding heating cables. This approach is to enable the star end architecture to be reconfigured easily so as to modify the role and phase of heating cables in response to faults.

In this respect, the invention aims for broadly the same objective as that outlined at page <NUM> of the paper OTC-<NUM>-MS '<NPL>. That paper discusses a subsea star end coupling system that offers the possibility to recombine heating cables subsea as desired to reinstate operative triplets. For this purpose, each cable terminating at the star end has an individual plug that is co-operable with a subsea switchboard. The plug can be repositioned on the switchboard to enable the cable to be connected to another cable when required to remedy or circumvent a fault.

Inconveniently, reconfiguration must be managed underwater, plug-by-plug and cable-by-cable, using an ROV. This process is slow, intricate, prone to error and vulnerable to any difficulty in removing or inserting any of the plugs. Prolonged ROV operations are also susceptible to interruption caused by adverse weather at the surface.

It is against this background that the present invention has been devised. In one sense, the invention resides in a remedial star connection system for an electrically heated subsea conduit, the system comprising: first and second terminals; first and second connectors that are each complementary to a respective one of the terminals, the first connector comprising at least two contacts arranged to connect with at least two contacts of the first terminal and the second connector comprising at least one contact arranged to connect with at least one contact of the second terminal; and a coupling between the first and second connectors wherein the terminals are exposed to be accessible by the respective connectors underwater to effect short-circuit connection via the coupling between the at least two contacts of the first connector and the at least one contact of the second connector.

The system can be configured in various ways. One way is to modify connections within the first and/or second connectors, for example where the contacts of at least one of said connectors are movable or removable to serve as modifiable connections.

The system may further comprise a junction box to which the first and second connectors are coupled. In that case, the system may be configurable by modifying connections within the junction box, for example movable or removable conductors of the junction box. It is also possible to configure the system by modifying connections within the coupling.

The coupling preferably comprises at least one flexible flying lead. For subsea use, the first and second connectors may be wet-mateable and may comprise ROV handles.

The inventive concept also embraces an electrically heated conduit, comprising: at least one group of heating cables that are powered to carry respective phases of multiphasic AC current and are connected to a first external terminal; at least one additional heating cable that is powered to carry a phase of AC current and is connected to a second external terminal; and a remedial star connection system of the invention, the first connector of the system being engageable with the first terminal to connect with, and to effect short-circuit connection between, at least two of the cables of the group, and the second connector of the system being engageable with the second terminal to connect with the additional cable and, via the coupling to the first connector, to effect short-circuit connection between the additional cable and the short-circuited cables of the group.

The additional cable may be one of a second group of heating cables that are powered to carry respective phases of multiphasic AC current and are connected to the second external terminal.

The inventive concept further embraces a subsea installation comprising at least one conduit of the invention.

The inventive concept extends to a corresponding method of remediating a fault in a heating cable, being one of a group of heating cables powered by respective phases of multiphasic AC current to define a heating circuit of an electrically heated subsea conduit. The method comprises disconnecting a star connection from the conduit, that connection previously joining the cables of the group in short circuit. Then, a remedial star connection is connected instead to the conduit by: connecting a first connector of the remedial star connection to at least two contacts of a first terminal, those contacts being connected to respective intact cables of the group to join those intact cables in short circuit, the cables of the group all being connected to the respective contacts of the first terminal; and connecting a second connector of the remedial star connection, coupled to the first connector, to a second terminal having at least one contact connected to a respective additional intact heating cable of the conduit powered by a phase of AC current, thus bringing that additional cable into short circuit connection with the intact cables of the group to form a new heating circuit.

Conveniently, the first and second connectors may be connected to respective terminals positioned externally to the conduit in such a location as to be accessible by an ROV or a diver when the conduit is installed underwater, those terminals being connected respectively to the group of heating cables and to the additional heating cable.

The additional cable may be selected from among a second group of heating cables that are powered to carry respective phases of multiphasic AC current.

Preliminarily, the method may comprise identifying the heating cable of the first group that has the fault and then configuring the remedial star connection to isolate the heating cable that has the fault. The remedial star connection may also be configured to connect to the additional cable chosen to be brought into short circuit connection with the intact cables of the first group.

When performed on a conduit underwater, the method of the invention may advantageously comprise configuring the remedial star connection before lowering the configured remedial star connection into the water.

Configurable connectors of the invention can also be used in a related method of remediating a fault in a heating cable, being one of a group of heating cables powered by respective phases of multiphasic AC current to define a heating circuit of an electrically heated subsea conduit. That method comprises: identifying the faulty cable; configuring a remedial connection to connect to the faulty cable by configuring contacts of connectors of the remedial connection, for example before lowering the configured remedial connection into water; engaging a first connector of the configured remedial connection with a first terminal of the conduit that is accessible underwater at a longitudinal position on one side of the fault, the cables of the group all being connected to respective contacts of the first terminal; engaging a second connector of the configured remedial connection with a second terminal of the conduit that is accessible underwater at another longitudinal position on an opposite side of the fault, the cables of the group all also being connected to respective contacts of the second terminal; and reinstating the heating circuit by conveying a phase of the AC current along the faulty cable and along a bypass coupling between the first and second connectors, without also conveying at least one other phase of the AC current along the bypass coupling.

In summary, the invention contemplates an externalised star-end termination providing a reconfigurable star-end for a trace heated pipeline, where the pipeline comprises at least three heating cables powered by at least two different phases. For use in a subsea context, the star-end comprises a wet-mate connector to connect cables electrically with corresponding trace heating cables of the pipeline with connections or terminals that are exposed for access by an ROV or a diver. An electrical connection puts at least two cables in short circuit with each other and/or puts at least one other cable directly or indirectly in short circuit with the cables of another similar reconfigurable star-end. An electrical connection can also be made between two or more different trace-heated pipes.

Thus, the invention is concerned with a trace heated pipeline comprising triplets of cables, wherein each triplet comprises distinct cables powered by at least two different phases and is terminated by a connection to a reconfigurable star-end.

Embodiments of the invention provide a reconfigurable star-end connector for a trace heated pipeline comprising at least one triplet of three cables powered by at least two different phases. The reconfigurable star-end connector comprises: a connector to electrically connect internal cables with the corresponding triplet of trace-heating cables of the pipeline; and an electrical connection putting in short circuit together, directly or indirectly, at least one internal cable carrying a first phase with the internal cables carrying another phase of another similar reconfigurable star-end connector.

Where the trace heated pipeline is a subsea pipeline, the connector is suitably a wet-mate connector. Where the pipeline is a PiP structure, the trace-heating cables are suitably inside the annulus with terminals connected to the cables exposed accessibly at a location external to the pipeline, for example on an external surface of an outer pipe of the structure.

The reconfigurable star-end connector may comprise at least one additional plug or connector for directly or indirectly connecting two or more similar star ends together. Thus, embodiments of the invention also provide an assembly comprising at least two reconfigurable star end connectors of the invention, electrically connected together by an electrical connection comprising at least one interconnecting cable. Two interconnected star end connectors may, for example, be mounted on the same pipeline to connect two distinct heating triplets.

Embodiments of the invention also implement a method to mitigate the loss of at least one heating cable of a first cable triplet terminated by a star-end connector, which connector comprises an electrical connector and a star-end connection inside the connector. The method comprises: unplugging the star-end connector terminating the first triplet, and the star-end connector terminating a second triplet; and plugging in, as a replacement, reconfigurable or reconfigured star-end connectors that are electrically connected together directly or indirectly, wherein the internal cable corresponding to the faulty cable of the first triplet is disconnected, and the internal cable corresponding to the same phase of the second triplet is put in short circuit with the other cables of the first triplet carrying at least one another phase via the star-end connectors.

Thus, the invention provides a remedial star connection system for an electrically heated conduit. A group of heating cables powered with respective phases of multiphasic AC current are connected to a first external terminal, and at least one additional heating cable powered with a phase of AC current is connected to a second external terminal.

A first connector has at least two contacts arranged to connect with opposed contacts of the first terminal and a second connector has at least one contact arranged to connect with an opposed contact of the second terminal. The connectors are coupled together so that when they are engaged with the terminals, short-circuit connection is effected between the additional cable and at least two intact cables of the group to reinstate a working heating circuit.

The system can be pre-configured to remediate faults in different cables of a group. The connectors may be wet-mateable for use on subsea conduits.

Referring firstly to <FIG>, a trace heated subsea pipeline <NUM> of PiP configuration comprises an inner pipe <NUM> and an outer pipe <NUM> in coaxial relation defining a thermally-insulating annulus <NUM> between them. Ends of the pipeline <NUM> are closed by conventional terminal modules <NUM> serving as bulkheads that seal the annulus <NUM>.

Heating cables <NUM> extend through the terminal modules <NUM> and along the annulus <NUM> in contact with the external surface of the inner pipe <NUM>. Thermal insulation material that is conventionally also disposed in the annulus <NUM> around the heating cables <NUM> is omitted from the drawings for clarity.

For simplicity, the heating cables <NUM> are shown here as extending longitudinally along the inner pipe <NUM>, parallel to each other and to the central longitudinal axis of the assembly. In practice, the heating cables <NUM> could wind helically around, or extend sinuously along, the inner pipe <NUM> instead.

In this triphasic system, the heating cables <NUM> are grouped in triplets <NUM>. For simplicity, just two triplets <NUM> are shown here. Both triplets <NUM> can be operational simultaneously or the cables <NUM> of one triplet <NUM> can provide back-up redundancy for cables <NUM> of the other triplet <NUM>, only to be activated when required.

The heating cables <NUM> of each triplet <NUM> extend from respective three-phase power supplies <NUM> on the terminal module <NUM> at one end of the pipeline <NUM> to respective star ends <NUM> that are exposed accessibly on the terminal module <NUM> at the other end of the pipeline <NUM>. A wye connection between the cables <NUM> is effected at each star end <NUM>.

Each cable <NUM> of each triplet <NUM> is powered by a different respective phase of alternating current (AC), those phases being designated here as A, B and C.

The grouping of the cables <NUM> in each triplet <NUM> is illustrated here by the proximity of those cables <NUM> to each other, which indeed will usually be the case in practice. However, the main significance of grouping is not the positional relationship between cables <NUM> of a triplet <NUM> but the functional relationship between those cables <NUM>. Specifically, each triplet <NUM> must comprise three cables <NUM> brought together at a neutral point, respectively powered by phases A, B and C, regardless of whether those cables <NUM> are positioned beside each other or otherwise.

Even if one or more of the cables <NUM> is remote from other cables <NUM> of the same heating circuit, the high thermal conductivity of the inner pipe <NUM>, which is typically of steel, helps to avoid significant cold spots within the pipeline <NUM>. The commonly helical or sinuous arrangement of the cables <NUM> also helps to minimise cold spots.

In this example, the star ends <NUM> are defined by removable wet-mateable connectors <NUM>, exemplified here by plugs that are engageable with respective sockets <NUM> that serve as terminals for the triplets <NUM>. The sockets serving as terminals for the triplets <NUM> are positioned to be accessible to an ROV or diver when the pipeline <NUM> is underwater. The sockets <NUM> may, for example, be fixed to an outer surface of the associated terminal module <NUM> as shown. It will be appreciated that the arrangement could be reversed, with fixed plugs serving as terminals instead being co-operable with complementary sockets <NUM> on the removable connectors <NUM>.

Each socket <NUM> is dedicated to a respective triplet <NUM> and contains pins <NUM> that are connected to the respective heating cables <NUM> of that triplet <NUM>. Correspondingly, each connector <NUM> comprises contacts <NUM> that receive the respective pins <NUM> when the connector <NUM> is inserted into a complementary socket <NUM>.

In <FIG>, the connectors <NUM> are configured for normal operation. In particular, the contacts <NUM> within each connector <NUM> are all connected to each other to define the neutral point required to create a star end <NUM> for each triplet <NUM> of heating cables <NUM>. The connectors <NUM> can be left in place in their respective sockets <NUM> for as long as the cables <NUM> of the trace heating system remain intact and the system therefore remains fully operational.

An example of a fault <NUM> is a break in the heating cable <NUM> powered by AC phase C in the first triplet <NUM> as shown in <FIG>. Such a failure of any one cable <NUM> would cause the entire heating circuit defined by that triplet <NUM> to fail. Conventionally, that entire triplet <NUM> and all of its cables <NUM> would be taken out of service and a spare triplet <NUM>, if available, would have to be activated in its entirety instead.

Elegantly, the invention allows the star ends <NUM> to be reconfigured simply by changing the connectors <NUM> in response to failure of a heating cable <NUM>. In accordance with the invention, reconfiguration is achieved simply by removing and replacing the connectors <NUM> of <FIG> with differently-configured connectors <NUM>, as shown in <FIG>, that are arranged to create triplets <NUM> from different combinations of available heating cables <NUM>.

It will be noted that the replacement connectors <NUM> of <FIG> are coupled to each other, for example by a flexible flying lead <NUM>. This coupling between the replacement connectors <NUM> allows intact cables <NUM> of a triplet <NUM> to be connected to another intact cable <NUM>, for example a cable <NUM> of another triplet <NUM>, via another socket <NUM> to create a new functioning three-phase heating circuit. The replacement connectors <NUM> and a coupling exemplified by the flying lead <NUM> together form a remedial star connection system of the invention.

As a result, a single, simple connector replacement operation performed quickly for each triplet <NUM> effects multiple connections of heating cables <NUM> in a desired configuration to circumvent a fault <NUM>. Removal and replacement is conveniently performed by an ROV, for which purpose the connectors <NUM> may have ROV handle formations as shown.

The configuration of the replacement connectors <NUM> of <FIG> is tailored to the nature of the fault <NUM>. In particular, the contacts <NUM> within one connector <NUM> that correspond to the remaining intact heating cables <NUM> of the first triplet <NUM>, namely those powered by AC phases A and B in this example, are connected together. Those contacts <NUM> are connected via the flying lead <NUM> to the contact <NUM> within the other connector <NUM> that corresponds to one of the cables <NUM> of the second triplet <NUM>, in this case the cable <NUM> powered by AC phase C.

Thus, when the replacement connectors <NUM> have been installed in the sockets <NUM>, the heating cables <NUM> of the first triplet <NUM> powered by AC phases A and B are connected to each other and, via the flying lead <NUM>, to the cable <NUM> of the second triplet <NUM> powered by AC phase C. These connections form a star end <NUM> between those three cables <NUM> to create a new functioning three-phase heating circuit. Optionally, the contact <NUM> corresponding to the faulty cable <NUM> can be connected to earth.

Beneficially, therefore, two of the three heating cables <NUM> of the first triplet <NUM> remain operational as part of the new circuit, and only one additional cable <NUM> rather than an entire further triplet <NUM> is required to provide redundancy.

Thus, with reference to <FIG>, a fault is remedied by the following steps. Firstly, at <NUM>, the nature of the fault is ascertained, for example using conventional fibre-optic monitoring cables <NUM> extending along the pipeline <NUM>. Secondly, at <NUM>, when the faulty heating cable <NUM> of a triplet <NUM> has been identified, replacement connectors <NUM> are configured, topside, to connect the surviving cables <NUM> of that triplet <NUM> to each other and to another intact cable <NUM>. In this respect, integrated star-end wiring inside the connectors <NUM> is tailored to the fault but a standard harness may be used in the flying lead <NUM> that joins the paired connectors <NUM>. Thirdly, at <NUM>, the original connectors <NUM> are removed from their sockets <NUM>. Fourthly, at <NUM>, the replacement connectors <NUM> are lowered to the seabed and wet-mated with the sockets <NUM> to make the star end connections that are required to reconstitute a working triplet <NUM> from the chosen cables <NUM>.

Moving on now to <FIG>, this drawing shows a similar pipeline <NUM> with a similar fault <NUM>, namely a break in the heating cable <NUM> powered by AC phase C in the first triplet <NUM>. In this case, the replacement connectors <NUM> are both of a standard configuration, each comprising separate contacts <NUM> that receive the respective pins <NUM> of the complementary socket <NUM>. Each of those connectors <NUM> has a flying lead <NUM> containing conductors that are connected respectively to each contact <NUM>.

In this case, a remedial star connection system of the invention further comprises a junction box <NUM>. The conductors in the flying leads <NUM> connect in turn to respective contacts <NUM> in a junction box <NUM>, in this example via a further plug-and-socket combination <NUM>. This arrangement allows standard connectors <NUM> and a standard harness within the flying leads <NUM> to be coupled to a bespoke, tailored junction box <NUM>.

In this embodiment, therefore, it is the configuration of the junction box <NUM> that is tailored to the nature of the fault <NUM>, not the configuration of the connectors <NUM>. In particular, the contacts <NUM> of the junction box <NUM> that correspond to the remaining intact heating cables <NUM> of the first triplet <NUM>, namely those powered by AC phases A and B in this example, are connected together. Those contacts <NUM> are connected within the junction box <NUM> to the contact <NUM> of the junction box <NUM> that corresponds to one of the cables <NUM> of the second triplet <NUM>, in this case the cable <NUM> powered by AC phase C.

Thus, again, when the replacement connectors <NUM> have been installed in the sockets <NUM>, the flying leads <NUM> and the appropriately configured junction box <NUM> connect the heating cables <NUM> of the first triplet <NUM> powered by AC phases A and B to each other and to the cable <NUM> of the second triplet <NUM> powered by AC phase C. The junction box <NUM> is configured at a topside location and the configured junction box <NUM> is then lowered to the pipeline <NUM>, conveniently with the connectors <NUM> already connected to the junction box <NUM> via the flying leads <NUM> and the plug-and-socket combinations <NUM>.

<FIG> shows how connectors <NUM> of the invention can also be used to energise another trace-heated pipeline <NUM> if those connectors <NUM> are configured with all three contacts <NUM> enabled. Here, connectors <NUM> coupled to respective sockets <NUM> of a first pipeline <NUM> convey three-phase AC current via flying leads <NUM> to corresponding connectors <NUM> engaged with respective sockets <NUM> of a second pipeline <NUM>. The sockets <NUM> terminate respective triplets <NUM> of heating cables <NUM> in each pipeline <NUM>.

Finally, <FIG> shows how connectors <NUM> of the invention can be used to bypass a fault <NUM> between sections of a heating cable <NUM>. Here, longitudinally-spaced sockets <NUM> exposed accessibly to an ROV or diver underwater on the outer pipe <NUM> of a PiP pipeline <NUM> have pins <NUM> that connect to the respective heating cables <NUM> of a triplet <NUM> within the annulus <NUM>. If one of those cables <NUM> develops a fault <NUM> at a location between the connections to the sockets <NUM>, for example due to a failed junction between successive sections of a cable <NUM>, connectors <NUM> at respective ends of a flying lead <NUM> can be configured with contacts <NUM> to connect to the faulty cable <NUM> via the appropriate pins <NUM>. When the connectors <NUM> are inserted into the sockets <NUM>, the phase of current carried by the faulty cable <NUM> can thereby bypass the fault <NUM> via the flying lead <NUM> between the connectors <NUM> serving as a bypass coupling to maintain the integrity of that triplet <NUM>.

Many other variations are possible within the inventive concept. For example, in the arrangements shown in <FIG> and <FIG>, a configurable junction box can be interposed between standard connectors in the manner shown in <FIG>. Similarly, flying leads can connect the connectors to the junction box.

At least one connector may comprise the ends of multiple triplets, with either distinct star ends or connected together to a unique star end, without departing from the invention as defined in the appended claims.

Claim 1:
A remedial star connection system for an electrically heated subsea conduit (<NUM>), the system comprising:
first and second terminals (<NUM>);
first and second connectors (<NUM>) that are each complementary to a respective one of the terminals (<NUM>), the first connector (<NUM>) comprising at least two contacts (<NUM>) arranged to connect with at least two contacts (<NUM>) of the first terminal (<NUM>) and the second connector (<NUM>) comprising at least one contact (<NUM>) arranged to connect with at least one contact (<NUM>) of the second terminal (<NUM>); and
a coupling (<NUM>) between the first and second connectors (<NUM>);
characterised in that the terminals (<NUM>) are exposed to be accessible by the respective connectors (<NUM>) underwater to effect short-circuit connection via the coupling (<NUM>) between the at least two contacts (<NUM>) of the first connector (<NUM>) and the at least one contact (<NUM>) of the second connector (<NUM>).