The present invention relates to thermoelectric generating devices and particularly, but not exclusively, to such devices for use in subsea applications.

In the Offshore Oil Industry new ways are constantly being sought to reduce the high costs of exploration and development. Offshore operators also seek to reduce the use of divers and although remote operating vehicles (ROVs) can take over the function of many divers tasks, they are more expensive to operate and divers are often used for convenience, which carries an inevitable risk of personnel accident.

In order to reduce development costs, wellheads for new subsea wells are completed on the seabed wherever possible rather than on production platforms. Such wellheads require electrical power to operate valve actuators and other remote control systems as well as sensors. This power is generally provided to subsea wells via umbilical cables from existing nearby platforms or from large buoys carrying electrical generating sets. Such apparatus and methods of supplying power is very expensive and _^ although battery packs have been used on some new wells these require to be replaced regularly by divers which is a further inconvenience. Also if the batteries should fail or run down for some unknown reason then operation of the well is impaired or stopped and this adds a further aspect of unpredictability.

It is desirable that such subsea wellheads and other subsea devices should be supplied with electrical power from an autonomous source and not rely on power trans¬ mission from the surface either of the umbilicals or a platform or buoy or from battery packs, for example, as proposed in U.K. Patent Specification No. 1255628.

An object of the present invention is to provide an improved form of thermoelectric generating device for use for example, in subsea environments for supplying power to wellheads and the like.

According to the present invention there is provided a thermoelectric generating device for use with a pipeline carrying a high temperature fluid, said pipeline being surrounded by a cooler medium, said thermoelectric generati device comprising tubular coupling means for being coupled said pipeline and thermoelectric generator means having a predetermined surface area disposed on said tubular couplin means, said thermoelectric generator means comprising a plurality of long narrow strips each extending longitudinal of the tubular coupling means with adjacent strips sub¬ stantially in circumferential contiguity, each strip being bonded to the tubular coupling means and comprising a plurality of thermoelectric elements connected thermally in parallel and electrically in series, said thermoelectric generator means having electrical power output means adapte to be coupled to an electrical load, whereby, in use, the temperature difference between the high temperature fluid

and the surrounding medium is sufficient for said thermo¬ electric generating device to generate sufficient electrica power to supply said electrical load.

In a preferred arrangement, the subsea thermoelectric generating device consists of a short length of pipe (spool piece) flanged at each end for coupling to a wellhead and crude pipeline and which carries an array of thermoelectric generating elements bonded to its outer surface. When thi spoolpiece is inserted into a pipeline carrying crude products from the wellhead, the inner junctions of the thermoelectric generating elements are heated by the crude products via the pipe wall and the outer junctions are cool by the surrounding seawater.

Although the temperature of crude products differs fro well to well a typical value is 100°C. With such a value and a typical temperature drop of 10°C through the pipe wal and a seawater temperature of 10°C then a typical temperatu difference of 80°C is established across the thermoelectric generating elements. With existing thermoelectric element (ThermaTeg, TEG Products, Scotland, UK) in these conditions an electrical output of 0.2 watts per square inch is provided. For a typical wellhead a power requirement is o the order of 200 watts and in order for this to be achieved the physical dimensions of the spoolpiece would require a length of about 32 inches if the diameter was around 10 inc In the thermoelectrical generating device the thermo¬ electric generating elements are in the form of long narrow

strips to conform best to its curved surface and are bonded lengthwise along the pipe wall. The thermoelectric elements cover the entire curved surface between the flanges to minimise seawater making direct contact with, the pipe surface and lowering its temperature. The thermo¬ electric generating elements are connected in a series/ parallel arrangement, the length of each series string being determined by the output voltage required, and the number of parallel elements being selected to provide the power required, with some redundancy against the failure of any one series string.

Preferably said pipeline is a subsea oil/gas pipeline and said high temperature fluid is crude oil/gas and said surrounding medium is separated. Alternatively said pipeline may carry any other hot fluid and the cooler medium can be a liquid or gas or a mixture of both.

Preferably, said pipe means consists of a length of pipe being terminated at each end by a flange, said flange being adapted to be coupled to a subsea wellhead and to said pipeline for permitting the passage of crude product therethrough.

Preferably, said thermoelectric generating elements are bonded directly to the pipeline. Alternatively said thermoelectric generating elements are bonded to a sleeve having a high thermal conductivity, said sleeve having means for being fastened around said pipeline.

Preferably also, said thermoelectric generating elements are disposed to cover substantially the entire surface of said pipe exterior between said flanges and around said periphery. Preferably also, the thermoelectric generating element are connected in series/parallel arrangement, the length of each series string being determined by the output voltage required.

Preferably also, said thermoelectric generating elemen are bonded to said exterior surface by a thermally conducti epoxy material and the outer surface of said thermoelectric generating elements are sealed by an epoxy material.

Conveniently electrical power is taken from said thermoelectric generating device to said electrical load by a bonded-in flying armoured cable of a suitable length.

These and other aspects of the invention will become apparent from the following description when taken in combination with the accompanying drawings in which: Fig. 1 depicts a side view of a thermoelectric generating device in accordance with an embodiment of the invention;

Fig. 2 depicts a sectional view taken on the lines A-A of Fig. 1;

Fig. 3 depicts an ^" enlarged view of a detail σf Fig. 2; Fig. 4 represents a longitudinal section of a thermo¬ electric strip taken on section 4a-4a of Fig. 3;

Fig. 4b shows a similar view taken on the line 4b-4b of Fig. 3 also shown in the P-N junctions;

Fig. 5 is a diagrammatic block diagram circuit of the thermoelectric generating device shown in Fig. 1 j Fig. 6 depicts a cross—section of an alternative embodiment of a thermopile assembly for use with a pipeline , and

Fig. 7 depicts an enlarged detail of Fig. 6. Reference is first made to Fig. 1 of the drawings which depicts a subsea thermoelectric generating device generally indicated by reference numeral 10 which consists of a pipe section 12 generally known as a

"spoolpiece" which has pipe section flanges 14 at each end which are adapted to be connected to like flanges 16 (shown in broken outline) disposed on a wellhead or on an oil/gas pipeline, not shown in the interest of clarity. The pipe section 12 has a thermoelectric generating device generally indicated by reference numeral 18 disposed around its periphery and substantially between the flanges 14. The thermoelectric generating device 18 consists of a plurality of spaced longitudinal thermoelectric strips 20 arranged parallel to the pipeline axis and which are interconnecteα, as will be later αescribed in a series/parallel arrangement to provide electrical power to an electrical load which is typically electrical instrumentation valves, sensors and the like on a wellhead as will be later described in detail.

Reference is now made to Fig. 2 of the drawings which depicts a sectional view taken on the lines A-A of Fig. 1. The pipe section 12 has a pipe wall 21 which defines an interior 22 through which typically oil and gas crude

products flow and there is a temperature difference between the interior 22 and the surrounding seawater indicated by reference numeral 24 across the pipe wall 21. It will be seen that the thermoelectric strips 20 are disposed around the periphery of the pipeline and as best seen in the detail shown in Fig. 3 the surface of the pipeline 12 is covered by an epoxy resin generally indicated by reference numeral 26 to which are bonded thermoelectric strips 20. The outer portion of the thermoelectric strips is covered by an outer layer of epoxy resin 30 and outer contacts 27.

Fig. 4(a) and (b) depict the thermoelectric strip 20, in which Fig. 4 (a) depicts a section through a strip on lines 4a-4a (Fig. 3), and Fig. 4(b) depicts a side view of a strip on 4b-4b. These figures show how the thermoelectric strip 20 is made up from a row of thermoelectric elements 41, and Fig. 4(b) also shows that these elements are sandwiched between an outer substrate 42 and an inner substrate 43, and are connected in series by outer contact pads 27 and inner contact pads 28.

A group of thermoelectric strips would be connected in series by joining the last contact pad on one strip to the same end of the next strip at alternative ends.- Several such series groups may be connected in parallel, to provide redundancy against the failure of any one series string, and this can be tailored to meet specific requirements.

It will be appreciated that thermoelectric elements are connected in series by the contact patches 27, 28, which are etched on the metallised surface of substrates 42, 43, formed of strips of insulating material. These strips are wide enough to carry thermocouplers and their length is determined by the materials supplied. Typically these strips are 4mm wide by 300mm long and they can be cut to fit a spoolpiece shorter than depicted in Fig. 1 or connected in end to end fashion to form longer pieces. Reference is now made to Fig. 5 of the drawings which depicts a block circuit diagram for the thermoelectric generator shown in Fig. 1. It will be appreciated that the generator 18 being composed of alternate 'n' and ^• p ¹ elements electrically connected in series and coupled to contacts 27 and 28 are thermally connected in parallel. The crude oil and gas in the interior of the pipe 22 acts as a heat source and the seawater external to the pipe acts as a heat sink 24. Power is taken across the serial connection from terminals 36 and 38 which are respective positive and negative terminals and are coupled to an electrical load generally indicated by reference number 40. The 'n' and 'p* thermoelectric elements are based on a silicon germanium binary alloy doped with lead and tin as set forth in European Patent Specification No. 185499. Typical electrical and thermal characteristics for these elements are set forth as follows:

- 9 -

^• 'ϊϊ ^1' Type ^" 'p ^1' Type

Seebeck coefficient millivolts C°CI - 0.23 0.16 Electrical resistivity milliohms cm 1.5 1.2 Thermal conductivity watts (cm°C) - 1 0.018 0.018 Figure of merit (°C) - 1 2.10-3 1.2.10-3 Once the ingot is cut into the required size and shape and metal contacts attached to them the metallised elements or dice are soldered to busbars as shown in Figs. 1 through 4. The metal contact pads 27,28 may be produced by a photolith¬ ographic or silkscreen process on a suitable substrate such ° as alumina or aluminium. With this arrangement a high

-density of dice per unit area is realised. In addition, . surface mounting assembly techniques can be used to provide high production rates.

With the arrangement shown in Fig. 5 the temperature 5 difference between the heat source and the heat sink being of the order of 80 C is sufficient with the structure as before descrived to provide electrical power sufficient to drive most components, sensors and the like on an electrical load in the form of a wellhead. ° Reference is now made to Fig.4a of the drawings which depicts a plurality of thermoelectric strips 20 only some of which are shown mounted on a sleeve 50 of high thermal conductivity such as brass, aluminium etc, composed of two generally semicylindrical parts 52,54 which have clamps 56 ⁵ which permit the parts 52,54 to be bolted round a pipeline (not shown) by bolts 58. Referring now to Fig. 7 it will be seen that each strip 20 consists of thermoelectric elements 41

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- 10 - disposed between contacts 27,28 wnicfi, in turn, are connected to inner and outer substrates 42,43, and inner substrates 43 are bonded to the sleeve 50 by adhesive or solder 60. The strips 20 are then set in a potting compound 62. The high thermal conductivity sleeve 50 minimizes heat loss transferred from the hot temperature fluid in the pipeline.

It will be appreciated that various modifications may be made to the embodiments hereinbefore described without departing from the scope of the invention. For example, it will be appreciated that the shape and size of the thermoelectric elements disposed on the pipeline may be varied as required although the arrangement shown is particularly suitable for encompassing a pipeline and maximising the efficiency of the structure. The thermoelectric elements may be of any suitable composition and are not to be construed as being limited to a binary silicon germanium alloy with lead and tin dopants. Further¬ more the power output from such a structure can be readily varied by increasing or decreasing the- length and diameter of the pipe. As a practical matter the power output may be most easily varied by varying the length of the pipe and this of course will vary the length of the thermoelectric elements disposed thereon. It is possible that layers of thermoelectric elements could be disposed in nested rings around the pipe to further increase the power output from a pipe whose length was required to be short. Such arrangements could be connected in series

or in parallel as desired depending on the specific requirements for power and reliability.

Advantages of the invention are that an autonomous supply of electrical power at each well location is provided at a fraction of the cost of umbilical systems and without the need for regular maintenance. The power supply can be varied readily during fabrication and there is no moving parts in that the structure uses a temperature difference between the geothermal heat of the crude products and that of the surrounding seawater to provide power. The structure can be made readily using existing materials and without special manufacturing maintenance or installation techniques.

It will be appreciated that the abovedescribed embodiment is not limited to use in a subsea environment and could, for example, be used in connection with a pipeline carrying flue gases at elevated temperature. Also, for use in the oil or gas industry, the embodiment may be used where the pipeline carries fluids at a substantial temperature difference from that of the surrounding medium, e.g. overland pipelines in very cold climates.