GB2409473A - Thermoelectric generator in annulus of subsea pipeline - Google Patents

Abstract

A thermoelectric generator is mounted on a subsea structure. In a preferred embodiment, the generator is used to supply power to a valve in the annulus of a double walled oil or gas pipeline. Well fluids at elevated temperature flow though the inner pipe 30 whilst the outer pipe (not shown) is in contact with the cold seawater. The intervening annulus is evacuated to minimise conductive heat loss and the thermoelectric generator 23 is mounted in the annulus, connected to the inner pipe by a heat sink 24 and to the outer pipe by a second heat sink 25. Electricity generated from the thermal gradient is regulated by a voltage regulator 26 and used to power a moisture sensor 28 and a shut off valve 21, 22 that locally closes the annulus should a leak be detected. The electricity may be used to charge a battery 27.

Description

11 Subsea Structure 13 The present invention relates to a subsea structure

having a 14 remote, local power supply. In particular it relates to a remotely actuable waterstop valve.

17 In many subsea structures it is useful to have an electrical 18 power source for actuating and/or controlling various 19 components. Typically electrical power is supplied to subsea structures from the surface via umbilicals or the like. While 21 this is convenient for some applications it requires the use 22 of connectors, commonly known as wet-connectors, which are 23 very expensive.

There are a number of categories of applications where a local 26 electrical power source would be advantageous. For example, 27 continuous lower power subsea applications using a small 28 battery as a buffer, such as, remotely powering subsea 29 monitoring equipment or electronics and remote sensing/instrumentation systems. In the mid power range, 31 power provided using trickle charged storage batteries could 32 be useful for providing power to independent heating of spool 33 pieces at wellheads or intermediate structures, powering of 34 marine equipment located adjacent to pipelines or wellheads and providing charging stations, adjacent to pipelines, for 1 remotely operated vehicles. Further applications include 2 maintaining the charge within a stand-by battery for powering 3 a solenoid, the solenoid being used to trigger mechanical 4 operations such as spring or gravity powered mechanisms or hydraulic systems.

7 A subsea structure where the invention may be used is for 8 example a production line for transporting fluids such as oil 9 and gas to the surface. Such pipelines may use a double walled structure such as a pipe-in-pipe system. A pipe-in 11 pipe system comprises an internal inner pipe within an 12 external outer pipe separated by an annular volume. In such a 13 system the annular space is advantageously used for thermal 14 insulation purposes. Thermal insulation may be improved or enhanced by applying a vacuum to the annular space between the 16 inner and outer pipes.

18 One of the difficulties associated with such pipelines is that 19 of safeguarding the annular space against the ingress of water, for example due to leaks in the external or carrier 21 pipe. Such water ingress into the annular space will reduce 22 the effectiveness of the insulation. To counter the effects 23 of water ingress it is known to compartmentalize 24 longitudinally the annular space using seals or waterstops so as to prevent flooding of the annulus along the length of the 26 pipe-in-pipe system. This facilitates local intervention to 27 repair only the damaged section of pipe.

29 Waterstops may be permanent seals or, preferably, they may comprise valves which have non-sealing and sealing positions.

31 When the annular space is compartmentalized by permanent 32 seals, the vacuum or partial vacuum in the annular space must 33 be created during the manufacture of the double walled pipe.

34 Once manufactured it is then not possible to vary the pressure within the compartments. An ability to vary the pressure I within the annular space would be useful, for example in the 2 case of diffusion of gases into the annulus through the 3 internal or external pipes or a leak which modifies the 4 pressure within the compartment and alters the thermal insulation characteristics of the pipeline.

7 To this end, seal assemblies providing one or more closeable 8 valves facilitating the flow of gas through the waterstop have 9 been devised. Typically in such a case the waterstop comprises one or more tubes or passages through the body of 1I the waterstop, the passages having mechanically actuated 12 valves therein. Such waterstops are useful in that they 13 facilitate the flow of gas within the annular space such that 14 a reduced pressure of vacuum can be created and/or maintained when the pipeline is installed on the seabed.

17 While mechanical means for actuating such waterstop valves are 18 known it would be useful to provide a remotely actuable, 19 electrically operable, waterstop valve useful in controlling the pressure within the annular space during the lifetime of 21 the pipeline and which can be used to separate the annular 22 space into compartments in the case of a leak of water or 23 hydrocarbon fluids into the pipeline (thus preventing flooding 24 of the whole annular space).

26 Accordingly, the present invention provides a subsea structure 27 comprising a thermoelectric device, said thermoelectric device 28 being adapted to produce electricity upon exposure to a 29 thermal gradient between two components of the subsea structure. The thermoelectric device can avail of a 31 temperature difference between two components or parts of the 32 subsea structure to produce electricity remotely when the 33 subsea structure is in place.

1 Preferably the thermoelectric device is adapted to produce 2 electricity upon exposure to a thermal gradient between an 3 external and an internal component of the subsea structure.

The thermoelectric device may act as a direct or an indirect 6 power source. In the latter case, the subsea structure may 7 further comprise a battery for storage of electricity produced 8 by the thermoelectric device.

In one embodiment, the subsea structure is a double-walled 1I pipeline wherein the thermoelectric device is be adapted to 12 produce electricity upon exposure to a thermal gradient 13 between an inner pipe and an outer pipe. The electrical 14 current produced by the thermoelectric device may be used for example to charge a battery or to power an electrically driven 16 operation within the subsea structure.

18 A double-walled pipeline according to the invention may 19 comprise an annular space defined between the inner and outer pipes; a waterstop located in the annular space, having one or 21 more vent passages; and electrically actuable valves for 22 sealing the one or more vent passages situated in a waterstop 23 body. A water sensitive switch is preferably connected to a 24 thermoelectric device and, directly or indirectly, to the valve or valves; the water sensitive switch being adapted to 26 actuate the valve when ingress of water into the annular space 27 is detected.

29 Preferably the waterstop comprises a plurality of vent passages linked to a valve manifold, wherein the 31 thermoelectric device is connected to said valve manifold.

33 The present invention also provides a method of charging a 34 battery, in a subsea structure having a temperature difference between two components, comprising the steps of providing a 1 thermoelectric device; connecting the thermoelectric device to 2 two components between which there is a temperature 3 difference; drawing a current from the thermoelectric device 4 and applying said current to the battery. i

6 The invention shall be described in further detail with 7 reference to the accompanying drawings in which: 9 Figure l is a schematic illustration of the 'Seebeck effect'; 11 Figure 2 shows a thermoelectric device suitable for use in the 12 invention; and 14 Figure 3 shows a waterstop assembly according to the invention.

17 The Seeback effect is a phenomenon whereby an electric current 18 is generated when a circuit containing a thermocouple or 19 junction formed from different materials in the thermoelectric series is subjected to a temperature difference. Figure l is 21 a schematic representation of the Seebeck effect wherein two 22 different metals l and 2 at temperatures Tc and Th respectively 23 generate a potential difference AV when electrically 24 connected.

26 In the following section the invention is described in 27 relation to a pipe-in-pipe system and in particular a pipe-in 28 pipe system comprising a waterstop. However this should not 29 be construed as limiting the generality of the invention - the described aspects of the invention apply equally to any type 31 of subsea structure which exhibits a temperature gradient.

33 Due to the thermally insulating effect of the annular space in 34 a pipe-in-pipe system the inner pipe (and contents thereof) is warmer than the outer pipe (and the surrounding water). The 1 present invention is predicated on the finding that the 2 temperature difference between such elements can be harnessed 3 by way of a thermoelectric device to produce a remote electric 4 power supply. This remote power supply enables components of S a subsea structure to be powered electrically in situ without 6 the need to supply electricity from the surface via an 7 umbilical.

9 Figure 2 a thermoelectric module 10 comprising a series of bimetallic pairs 11. Each bimetallic pair may comprise a pair 11 of materials selected from the group consisting of silicon, 12 bismuth, nickel, cobalt, palladium, platinum, uranium, copper, 13 manganese, titanium, mercury, lead, tin, chromium, molybdenum, 14 rhodium, iridium, gold, silver, aluminium, zinc, tungsten, cadmium, iron, arsenic, tellurium and germanium. Preferred 16 materials for use in the thermoelectric device include bismuth 17 and tellurium. The dimensions of the thermoelectric module 18 are limited only be the particular application. For example, 19 when employed in a pipe-in-pipe system, each module will typically have a width in the range of from about 4mm to about 21 lOOmm, a length in the range of from about 4mm to about lOOmm 22 and a height in the range of about 1.5mm to about 35mm. A 23 series of twelve bismuth/tellurium modules such as that shown 24 in Figure 2 each having a width W of 40mm, a length L of 40mm and a height H of 5mm, operating with a suitable heat sink 26 system, would produce about 14v and a current of approximately 27 0.5A.

29 Typically a waterstop in a pipe-in-pipe structure may comprise up to 20 or more vent passages. The vent passages are 31 preferably grouped together by, and connected to, one or more 32 electrically actuable valve manifolds. The presence of a 33 water sensitive switch in communication with the valve 34 manifolds facilitates closure of the vent passages upon the detection of water or some other fluid in the annular space.

2 Figure 3 shows a particular embodiment of the invention, being 3 a waterstop 20 in a pipe-in-pipe system (for the sake of 4 clarity the outer pipe is not shown; the inner pipe is designated 30). The waterstop comprises a manifold and valve 6 assembly 21 and a valve actuator 22, the valve actuator being 7 connected to a thermoelectric device. The thermoelectric 8 device in turn comprises a thermoelectric module 23 in thermal 9 communication with a flowline heat sink 24 (connected to the flowline or internal pipe) and a carrier heat sink 25 11 (connected to the carrier or pipe). A voltage regulator 26 is 12 provided to regulate the output from the thermoelectric module 13 23.

In operation the waterstop 20 is fitted in a pipe-in-pipe 16 system for transporting oil, gas or other fluids from the 17 seabed to the surface. The temperature difference between the 18 inner and outer pipes is transmitted via heat sinks 24 and 25 19 to the thermoelectric module 23. The electrical current produced can charge a battery 27 (or be used to maintain the 21 charge in the battery 27).

23 A water sensitive switch 28 (with switching relay 29 and 24 attendant circuitry connecting it to the battery 27) may be provided to control the valve. The water sensitive switch 28 26 may, for example, be an electrical resistance circuit or a 27 mechanical float / pressure switch used to trigger a relay to 28 operate the valve actuator 22. The water sensitive switch 29 should be positioned at a suitable distance from the valve assembly 21 in order to allow sufficient time, upon water 31 being detected, for the waterstop to close.

33 The waterstop may be biased towards either closed position 34 with power provided by the thermoelectric device (and any accompanying battery) used to keep the valve open. In the l even that the water sensitive switch 27 is activated by the 2 ingress of water the circuit is broken and the valve closes.

4 In an alternative arrangement, the waterstop may be fitted to the pipein-pipe system in the open position such that 6 activation of the water sensitive switch 27 upon the ingress 7 of water causes the circuit to be closed and the waterstop to 8 be actuated from the open position to the closed position.

As indicated above the present invention is not limited to 11 pipe-in-pipe structures but applies equally to other types of 12 subsea structure, such as for example, well-heads, manifolds, 13 pipeline swivels, hydraulic systems, remote sensing 14 instruments, remotely operated vehicles and the like. The thermoelectric device incorporated in the subsea structures of 16 the invention facilitates any operation which can be driven by 17 electrical power. The thermoelectric energy generated (as a 18 result of a temperature gradient between components of the 19 subsea structure e.g. arising from a temperature difference between oil within a structure and the external water 21 temperature) can be stored in a battery, capacitor or such 22 like for later use.

Claims (1)

1. 2 CLAIMS 4 l. A subsea structure comprising a thermoelectric device,

   wherein the thermoelectric device is adapted to produce 6 electricity upon exposure to a thermal gradient between 7 two components of the subsea structure.

   9 2. A subsea structure comprising a thermoelectric device, wherein the thermoelectric device is adapted to produce 11 electricity remotely when the subsea structure is in 12 place.

   14 3. A subsea structure according to either of Claims l and 2 wherein the thermoelectric device is adapted to produce 16 electricity upon exposure to a thermal gradient between an 17 external and an internal component of the subsea 18 structure.

   4. A subsea structure according to any preceding Claim 21 further comprising a battery for storage of electricity 22 produced by the thermoelectric device.

   24 5. A subsea structure according to any preceding Claim which is a double-walled pipeline defining an annular space 26 between an inner pipe and an outer pipe wherein the 27 thermoelectric device is be adapted to produce electricity 28 upon exposure to a thermal gradient between the inner pipe 29 and the outer pipe.

   31 6. A subsea structure according to Claim 5 further comprising 32 a waterstop located in the annular space and having a vent 33 passage, and an electrically actuable valve for sealing 34 the vent passage.

   1 7. A subsea structure according to Claim 6 comprising a 2 plurality of vent passages connected by a valve manifold.

   4 8. A subsea structure according to either of Claims 6 and 7 S further comprising a water sensitive switch connected to 6 the thermoelectric device and to the valve or valve 7 manifold, the water sensitive switch being adapted to 8 actuate the waterstop when ingress of water into the 9 annular space is detected.

   11 9. A subsea structure according to any preceding Claim 12 wherein the thermoelectric device comprises a pair of 13 materials selected from the group consisting of silicon, 14 bismuth, nickel, cobalt, palladium, platinum, uranium, IS copper, manganese, titanium, mercury, lead, tin, chromium, 16 molybdenum, rhodium, iridium, gold, silver, aluminium, 17 zinc, tungsten, cadmium, iron, arsenic, tellurium and 18 germanium.

   10. A method of charging a battery in a subsea structure 21 comprising connecting a thermoelectric device to two 22 components of the subsea structure between which there is 23 a temperature difference; drawing a current from the 24 thermoelectric device and applying said current to the battery.

   27 11. Use of a thermoelectric device to charge a battery in a 28 subsea structure wherein the thermoelectric device is 29 adapted to produce electricity upon exposure to a thermal gradient between an external and an internal component of 31 the subsea structure.