A pipe for a cryogenic fluid and methods of manufacture thereof

Abstract

A method of manufacture is detailed for a pipe for a cryogenic fluid (100). The manufacturing method comprises (100): obtaining an expansion joint (110) and electroforming an inner pipe for a cryogeni

Description

Field of the invention The present disclosure relates to pipes for transporting cryogenic fluids, such as liquid hydrogen, and methods of manufacture thereof. In particular, the present disclosure relates to electroformed pipes for cryogenic fluid. Background Major initiatives are underway to accelerate development of the aircraft fuel lines and ground systems necessary to implement liquid hydrogen technology for aircrafts. While large-scale hydrogen generation and storage systems are under development, the ability to build lightweight systems and fuel aircraft at airports within existing turn-round times remains a potential barrier to introduction of liquid hydrogen (LH2) powered aircraft. On-aircraft systems also need to be lightweight with minimum thermal losses. Refuelling systems are required that could fuel and defuel aircraft with liquid hydrogen. Whetheron or off aircraft, it will be important to minimise hydrogen boil off losses operating in a variety of geographies where ambient temperatures could fluctuate from -40 °C to +50 °C. In addition to ground refuelling, light-weight flexible pipes are needed onboard the aircraft for the onboard fuel system. These also often require complex geometries and pipework. There is currently an absence of lightweight fuel pipes or refuelling equipment to safely deliver pressurised or cryogenic hydrogen to an aircraft. Existing vacuum jacket systems for cryogenic liquids are mostly composed of fixed pipe networks joined by fittings and welds and constructed from stainless steels. Such systems are heavy (~3.5kg I metre length), bulky, and are susceptible to leakage and hydrogen embrittlement when used for liquid hydrogen. Fixing points and flanges have to be welded on, adding to leakage susceptibility and reproducibility difficulties during manufacture. Alternative aluminium structures have to be fabricated by welding preformed parts together with weld lines leak-tested to ensure vacuum tightness. Thermoplastic solutions are new but difficult to produce to vacuum tight specifications. All these issues make existing technology solutions heavy, ill-suited to liquid hydrogen, and expensive with restrictions in design freedom. Systems have been developed for pressurised gaseous hydrogen, e.g. for fuelling hydrogen powered cars in Japan, including leak proof nozzles and valves but these are operating near room temperature and therefore unsuitable for cryogenic operating temperatures. There is therefore a need for lightweight, flexible systems for cryogenic fluids that could be adopted for automated aircraft fuelling with liquid hydrogen. Summary of the invention Aspects of the invention are as set out in the independent claims and optional features are set out in the dependent claims. Aspects of the invention may be provided in conjunction with each other and features of one aspect may be applied to other aspects. An aspect of the invention relates to a method of manufacture of a pipe for a cryogenic fluid. The method comprises obtaining an expansion joint, and electroforming an inner pipe for a cryogenic fluid, wherein electroforming the inner pipe comprises electroforming the expansion joint to form an integral part with the inner pipe. The method also comprises electroforming an outer pipe. The method then comprises assembling the inner pipe within the outer pipe and sealing the outer pipe to form a hermetic seal around the inner pipe. Electroforming is an additive, net-shape forming process that provides a gas-tight barrier as a single piece of metal. During electroforming, metal is deposited by electrolysis onto a preform (such as a mould, model, or mandrel) that may or may not be removed afterwards. Utilising electroforming in the method above may be advantageous to provide a strong pipe structure which can be formed as a complex shape. Electroforming the inner pipe also advantageously co-joins the inner pipe, expansion joints, and optionally integral connector fittings, into a single entity, optionally incorporating other fixing points, feedthroughs and sensors into the structure, whilst also circumventing the need forwelding or joining processes within the pipe. Electroforming hermetically sealed systems may therefore also advantageously minimise welding related failure modes and / or weld defects, enable conformable shapes for optimal thermodynamics, reduce heat losses, and maintain a vacuum without the need for constant pumping. Furthermore, utilising electroforming may provide significant weight savings compared to conventional solutions based on welded steel. As such, the above manufacturing method minimised mass per unit length of the resulting pipe for cryogenic fluid. This can be particularly advantageous for aircraft refuelling systems due to weight and payload implications. The hermetically sealed outer pipe may advantageously provide a secondary containment vessel in the case of a leak. In addition, the hermetically sealed outer pipe may be configured to contain a vacuum. The vacuum may be advantageous to reduce conductive and convective heat transfer. This may be advantageous to provide a pipe that maintains cryogenic fluids with minimum boil-off, preferably less than 1 % cryogenic boil-off. In some examples, electroforming the outer pipe comprises electroforming a first longitudinal section of an outer pipe, and electroforming a second longitudinal section of the outer pipe. The method then comprises assembling the inner pipe between the first longitudinal section and the second longitudinal section of the outer pipe, and joining the first longitudinal section and the second longitudinal section of the outer pipe to form a hermetic seal around the inner pipe. In some examples, joining the first longitudinal section and the second longitudinal section of the outer pipe comprises at least one of welding, brazing, and / or soldering. In some examples, brazing and / or soldering may be preferred. Each of the first longitudinal section and the second longitudinal section of the outer pipe may comprise a flange, wherein the flange of the first longitudinal section and the flange of the second longitudinal section are configured to abut once the outer pipe is assembled. This may be advantageous as the flange may provide a joining surface, wherein joining the first longitudinal section and the second longitudinal section of the outer pipe comprises joining the flange of the first longitudinal section and the second longitudinal section. The first longitudinal section and the second longitudinal section of the outer pipe may each be half sections of the outer pipe. In some examples, the first longitudinal section and the second longitudinal sections may be identical half sections, wherein the first longitudinal section and the second longitudinal section may be electroformed using the same mould or removable mandrel. Alternatively, electroforming the outer pipe may comprise electroforming a complete outer pipe. In such examples, the outer pipe may be electroformed into a removable mandrel, wherein the mandrel is removed once the outer pipe has been electroformed. In some examples, obtaining the expansion joint may comprise electroforming the expansion joint. For example, electroforming the expansion joint may comprise electroforming a multi-layer metallic structure on a removable mandrel. An example multilayering manufacture process is described in patent documents WO 2019 / 016543 and US 11542622. For example, electroforming the multi-layer metallic structure on the removable mandrel may further comprise electroforming a plurality of metallic layers, wherein the thickness and / or composition of the plurality of metallic layers is varied to adjust the elastic modulus and / or yield strength of the expansion joint. The expansion joint may comprise a bellows, however the skilled person will understand that other expansion joint geometries may also be used. In some examples, the outer pipe is electroformed to have a wall thickness of no more than 500 micrometres. This is based on a design boil-off pressure of 5 bar; however the skilled person will understand that the wall thickness may vary for different design requirements. Electroforming the inner pipe may further comprise integrating a cryogenic connector fitting at at least one end of the inner pipe, wherein the cryogenic connector fitting is configured to couple to a cryogenic fluid feed pipe. Electroforming the outer pipe may further comprise integrating vacuum ports and / or fittings, for example to pump, isolate, and monitor the vacuum between the inner and outer Pipe. In some examples, the inner pipe is electroformed using copper. This may be advantageous as copper does not embrittle with hydrogen. Copper also remains ductile at cryogenic temperatures, including to below-253°C for carrying cryogenic liquid hydrogen. The outer pipe may also be electroformed using copper. In some examples, the method may further comprise depositing a silver layer on at least one of (i) an outer surface of the inner pipe, and / or (ii) an inner surface of the outer pipe. This may be advantageous to provide low emissivity for reduced radiative heat transfer to the inner cryogenic pipe. Preferably, the silver layer may be electroformed onto the at least one of (i) an outer surface of the inner pipe, and / or (ii) an inner surface of the outer pipe (e.g. on the inner surface of the first longitudinal section and the second longitudinal section of the outer pipe, prior to assembly). Alternatively or in addition to silver deposition, the outer surface of the inner pipe, and / or the inner surface of the first longitudinal section and the second longitudinal section of the outer pipe may be polished to reduce radiative heat transfer to the inner cryogenic pipe. In some examples, assembling the inner pipe between the first longitudinal section and the second longitudinal section of the outer pipe further comprises incorporating spacer structures between the inner pipe and the outer pipe. The spacer structures may be advantageous to maintain separation between the outer pipe and inner pipe, thereby reducing conductive heat transfer to the inner cryogenic pipe. The spacer structures may comprise polymer, ceramic, or silica micro-sphere insulation. However, the skilled person will understand that any other thermally insulating materials may be used, including for example multi-layer insulation. The electroformed inner pipe and the electroformed outer pipe may comprise at least one bend. Electroforming the inner pipe and outer pipe longitudinal sections to comprise one or more bends may be advantageous to achieve complex shapes without requiring welding between joints or separate pipe sections. As such, via electroforming, the pipes can be formed to be a single entity, including said one or more bends. In some examples, the method may further comprise electroforming an intermediary pipe configured for connection to a cryostat. Assembly then comprises assembling the inner pipe within the intermediary pipe, and the intermediary pipe within the outer pipe. This may be advantageous to form a 3-wall pipe system. In such examples, the inner pipe is configured for cryogenic fluid, the outer pipe is configured for a vacuum, and the intermediary pipe may be configured for a cryogenic coolant fluid. This may advantageously provide a pipe with active cooling to minimise boil-off of cryogenic fluids. Electroforming the intermediary pipe may comprise electroforming a first longitudinal section of the intermediary pipe and a second longitudinal section of the intermediary pipe. The method then further comprises assembling the inner pipe between the first longitudinal section of the intermediary pipe and the second longitudinal section of the intermediary pipe, and joining the first longitudinal section of the intermediary pipe and the second longitudinal section of the intermediary pipe to seal around the inner pipe. A first longitudinal section of the outer pipe and a second longitudinal section of the outer pipe may then also be assembled around the intermediary pipe, and joined as described above. The intermediary pipe may be electroformed using copper. As described above, in some examples, the method may further comprise depositing a silver layer on at least one of (i) an outer surface of the intermediary pipe, and / or (ii) an inner surface of the intermediary pipe. Preferably, silver is deposited on all internal polished surfaces between the inner and middle pipes and the middle and outer pipe, to reduce radiative heat transfer. The method described above is for manufacturing a pipe for cryogenic fluid, however preferably the cryogenic fluid is hydrogen. In such examples, the inner pipe is configured to carry cryogenic liquid hydrogen. It is noted that the resulting pipe of the present invention is configured to carry liquid hydrogen, wherein the hydrogen is liquid due to being maintained at cryogenic temperatures (and not simply high pressure). The assembled pipe may have a mass per unit length equal to or less than 1.6 kg per meter, preferably equal to or less than 1.2 kg per meter, more preferably equal to or less than 1 kg per meter. This is possible due to the electroformed pipes that can achieve a lighter construction than existing steel welded pipes. Equivalent welded steel constructions typically have a mass per unit length ~3.5 kg per meter. In another aspect of the invention, there is provided a pipe for a cryogenic fluid, the pipe being manufactured according to the method of the preceding aspect of the invention. For example, there is provided a pipe for cryogenic fluid, comprising a first electroformed inner pipe configured for cryogenic fluid, and a second electroformed outer pipe, hermetically sealed around the second pipe, configured fora vacuum. In another aspect of the invention, there is provided a pipe for cryogenic fluid, the pipe comprising a first inner pipe configured for cryogenic fluid, a second pipe, surrounding the first inner pipe, configured to receive a cryogenic coolant fluid to cool the cryogenic fluid contained in the first inner pipe, and a third outer pipe, hermetically sealed around the second pipe, configured for a vacuum. Preferably, the first inner pipe, second pipe, and third outer pipe are each electroformed. The first inner pipe, and optionally the second pipe and third outer pipe may be copper. The second electroformed pipe may be configured for connection to a cryostat such that the cryostat is configured to actively cool cryogenic coolant fluid within the second pipe. The cryogenic coolant fluid may be cryogenic liquid or gas, such as a vapour. The cryogenic coolant fluid may be cryogenic helium, for example such as helium vapour. In some examples, the cryogenic helium may be actively cooled at about 10 kelvin (K). The first electroformed inner pipe may comprise an expansion joint, such as a bellows. Optionally, the second electroformed pipe may also comprise an expansion joint, such as a bellows. In some examples, the cryogenic fluid pipe may further comprise a silver layer on at least one of (i) an outer surface of the inner pipe, (ii) an inner surface of the outer pipe, (iii) an outer surface of the intermediary pipe, and / or (iv) an inner surface of the intermediary pipe. Preferably, silver is deposited on all internal polished surfaces between the inner and intermediary pipes and the intermediary and outer pipe, to reduce radiative heat transfer. In some examples, the cryogenic fluid pipe may further comprise spacer structures between the first inner pipe and the second intermediary pipe, and / or between the second intermediary pipe and the third outer pipe. The spacer structures may be advantageous to maintain separation between the respective pipes, thereby reducing conductive heat transfer to the inner cryogenic pipe. The spacer structures may comprise polymer, ceramic, or silica micro-sphere insulation. However, the skilled person will understand that any other thermally insulating materials may be used, including for example multi-layer insulation. The cryogenic fluid pipe may further comprise an insulation layer which surrounds the outer surface of outer pipe. The insulation layer may be, but is not limited to, foam or fibre insulation. The insulation layer may advantageously be provided for impact absorption to protect the outer pipe. The insulation layer may also be thermally insulative. The cryogenic fluid pipe may further comprise a braided metal covering that surrounds the outer surface of the cryogenic pipe. The braided metal covering may advantageously be provided to protect the cryogenic pipe from damage. Drawings Embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1A illustrates a flow diagram of an example method of manufacture for a cryogenic fluid pipe. Fig. 1B illustrates one embodiment of the example method of manufacture of Fig. 1 A. Fig. 1C illustrates another embodiment of the example method of manufacture of Fig. 1A. Fig. 2 illustrates a cross-sectional view of a cryogenic fluid pipe, such as a pipe for liquid hydrogen. The pipe of Fig. 2 may be manufactured according to the method of Fig. 1. Fig. 3 illustrates an example partial cut away view of the cryogenic fluid pipe of Fig. 2. Fig. 4 shows a photograph of an electroformed inner pipe, suitable for use in the cryogenic fluid pipe of the present invention. Fig. 5 shows a photograph of an electroformed longitudinal section of an outer pipe, suitable for use in the cryogenic fluid pipe of the present invention, for example manufactured according to the method of Fig. 1B. Fig. 6 shows a cross-sectional view of a schematic of an example cryogenic fluid pipe, such as a pipe for liquid hydrogen. The pipe of Fig. 6 may be manufactured according to the method of Fig. 1. Fig. 7 illustrates a cross-sectional view of another cryogenic fluid pipe of the present invention, such as a pipe for liquid hydrogen. Fig. 8 illustrates a flow diagram of another example method of manufacture for a cryogenic fluid pipe, such as the cryogenic fluid pipe of Fig. 7. Fig. 9 illustrates a testing set up for testing the vacuum performance of a cryogenic fluid pipe of the present invention. Fig. 10 illustrates a testing set up for testing the heat flux performance of a cryogenic fluid pipe of the present invention carrying cryogenic liquid hydrogen. Specific description Embodiments of the claims relate to cryogenic fluid pipes and methods of manufacture thereof. It will be appreciated from the discussion above that the embodiments shown in the Figures are merely exemplary, and include features which may be generalised, removed or replaced as described herein and as set out in the claims. Fig. 1A illustrates a flow diagram of an example method 100 of manufacture fora cryogenic fluid pipe. Example cryogenic fluid pipes that may be manufactured according to the method of Fig. 1 are shown in Figs. 2, 3, and 6. Example variants of the method 100 of Fig. 1A are also shown in more detail in Figs. 1B and 1C. Firstly, the method 100 comprises obtaining an expansion joint (110). As shown in Figs. 1B and 1C, the expansion joint may be obtained by electroforming the expansion joint, such as a bellows 302, using a multi-layering electroforming process. For example, a mandrel is formed for the expansion joint, for example via CNC machining, 3D printing, injection moulding, or other suitable technique. The mandrel then undergoes electroding for electroforming. Electroforming via a multi-layering process is then undertaken onto the mandrel. Once electroforming is complete, the mandrel is removed. For example, electroforming the multilayer metallic structure on the removable mandrel may further comprise varying the thickness or composition of the multi-layer structure to adjust the elastic modulus and / or yield strength of the expansion joint independently. An example method is described in more detail in patent documents WO 2019 / 016543 and US 11542622. The method 100 then comprises electroforming an inner pipe 204 (120), wherein the expansion joint is integrated into the inner pipe 204 via electroforming. Integrating the expansion joint into the inner pipe 204 via electroforming circumvents the need for welding or other joining methods by instead sealing the expansion joint within the inner pipe with a metallic layer. In this example, the inner pipe 204 is copper. Copper is advantageous as it is one of few electroformable metals, with the added benefit of being easily brazed I soldered to. It also has advantageous properties when used with liquid hydrogen as it resists hydrogen embrittlement and remains ductile at cryogenic temperatures. In more detail, as shown in Fig. 1B, the inner pipe 204 may be formed by pipe bending and soldering fittings, such as the expansion joint, to create an inner assembly. In addition to integrating the expansion joint, cryogenic connectors 304 are also coupled at each end of the inner pipe, for example via soldering. The inner pipe assembly, including the expansion joint and cryogenic connectors 304, is then electroformed. This integrates the cryogenic connectors 304 and expansion joints into the inner pipe, sealing joins, and circumvents the need for welding, or other joining techniques. Alternatively, as shown in Fig. 1C, the inner pipe may be electroformed using an inner pipe mandrel, wherein the inner pipe mandrel is manufactured via CNC machining, 3D printing, injection moulding, or other suitable technique. A photograph of an example electroformed inner pipe 204 is shown in more detail in Fig. 4. The inner pipe 204 integrates a bellows 302 in the pipe 204, and a cryogenic connector 304 at each end of the inner pipe 204. As such, the electroformed inner pipe 204 forms a single piece, without requiring welding. As shown in Figs. 3 and 4, the inner pipe 204 preferably has a “U”-shape. The inner pipe 204 comprises a distal end, a mid portion, and a proximal end, wherein the mid portion couples the distal end and the proximal end. The distal end and the proximal end are substantially parallel, and the mid portion is substantially perpendicular to the distal and proximal ends. The distal end and the proximal end preferably extend in the same direction relative to the mid portion. This configuration may be advantageous for in-service orientation for aircraft refuelling, in particular because the proximal and distal ends may be oriented vertically in use for refuelling. Optionally, the outer surface of the inner pipe is polished, and plated in a silver layer via electroforming. The method 100 also comprises electroforming an outer pipe. As shown in Figs. 1B and 1C, there are two alternative approaches to electroforming the outer pipe. A first approach to electroforming the outer pipe includes electroforming a first longitudinal section of the outer pipe, and a second longitudinal section of the outer pipe. Both sections of the outer pipe are electroformed using copper in this example. The first and second longitudinal sections of the outer pipe are preferably identical halves. In this case, the first and second longitudinal halves of the pipe may be electroformed using a single mould or removeable mandrel. In more detail, a reusable mandrel can be formed by CNC machining, 3D printing, injection moulding, or other suitable technique. The reusable mandrel may then undergo electroding to make the mandrel conductive. The reusable mandrel may then be used to electroform the first longitudinal half section of the outer pipe on the reusable mandrel. Once electroformed, the first longitudinal half section may be removed from the mandrel, and the mandrel may then be reused for electroforming the second longitudinal half section of the outer pipe. The longitudinal sections of the outer pipe further comprise at least one vacuum port 306 along the length of the section, formed by the mandrel. Each vacuum port 306 is configured to be coupled to an external device, such as a vacuum gauge, or vacuum block and blead valve. The sectional edge of each longitudinal section of the outer pipe also comprises a flange 308. A photograph on an example longitudinal section 212a of the outer pipe having been electroformed on a mandrel 500 is shown in Fig. 5. Optionally, the inner surface of each longitudinal section of the outer pipe is also polished and plated with a silver layer via electroforming. Once electroformed, the cryogenic fluid pipe is assembled by arranging the inner pipe between the two longitudinal sections of the outer pipe 140. The flange 308 of the first longitudinal section 212a of the outer pipe 212 and the flange 308 of the second longitudinal section 212b of the outer pipe 212 are configured to abut once the outer pipe 212 is assembled around the inner pipe 204. Optionally, the spacing between the inner pipe and the outer pipe sections is maintained via thermally insulating spacing structures. The cryogenic connectors 304 at each end of the inner pipe 204 are configured to protrude from the outer pipe 212, as shown in Fig. 3. The method then further comprises joining the first and second longitudinal sections of the outer pipe to hermetically seal the outer pipe around the inner pipe. In particular, joining the first longitudinal section and the second longitudinal section of the outer pipe comprises joining the flange of the first longitudinal section and the second longitudinal section. Joining is preferably achieved via brazing and / or soldering the first longitudinal section and the second longitudinal sections. The resulting assembled cryogenic pipe may then be subject to quality assurance and leak checks. A second approach to electroforming the outer pipe includes electroforming the outer pipe as a single entity, for example using copper. In this case, the outer pipe may be electroformed using a single removeable mandrel. The reusable mandrel can be formed by CNC machining, 3D printing, injection moulding, or other suitable technique. The reusable mandrel may then undergo electroding to make the mandrel conductive for electroforming. Electroforming the outer pipe may also include incorporating at least one vacuum fitting into a port 306 along the length of the outer pipe. Each vacuum port 306 is configured to be coupled to a vacuum fitting, such as a vacuum gauge, or vacuum block and blead valve. Optionally, the inner surface of each longitudinal section of the outer pipe is also polished and plated with a silver layer via electroforming. Once electroformed, the cryogenic fluid pipe is assembled by arranging the inner pipe within the outer pipe 140. Optionally, the spacing between the inner pipe and the outer pipe sections is maintained via thermally insulating spacing structures. The cryogenic connectors 304 at each end of the inner pipe 204 are configured to protrude from the outer pipe 212, as shown in Fig. 3. The method then further comprises sealing the outer pipe around the inner pipe to form a hermetic seal. As above, the resulting assembled cryogenic pipe may then be subject to quality assurance and leak checks. In all examples, a vacuum pump may be used to pull vacuum from the vacuum space between the inner pipe and outer pipe. The vacuum is sealed within the vacuum space using a seal off port integrated into the outer pipe, via one of the outer pipe vacuum ports 306. A vacuum gauge, coupled to another vacuum port in the outer pipe may also be provided to measure the current vacuum between the inner and outer pipes. Fig. 2 illustrates a cross-sectional schematic of a cryogenic fluid pipe 200, such as a pipe manufactured according to the method of Fig. 1. The cryogenic fluid pipe 200 comprises an inner pipe 204 for containing cryogenic fluid 202. The inner pipe is preferably electroformed copper. The inner pipe 204 is configured to carry cryogenic fluid 202, such as liquid hydrogen, at or above the boil-off pressure. For example, the inner pipe 204 may be configured to contain liquid hydrogen, or other cryogenic fluid, at a pressure of 5 bar. The outer surface of the inner pipe 204 is polished and comprises a silver layer 206. The polished surface and silver layer 206 are configured to reduce radiant heat transfer to the cryogenic fluid 200 within the inner tube 204. The inner pipe 204 is surrounded by an outer pipe 212. The outer pipe 212 is preferably electroformed copper. The outer pipe 212 is hermetically sealed around the inner pipe 204. The outer pipe 212 is configured to contain a vacuum 210. The vacuum 210 is configured to form a “vacuum jacket” around the inner pipe 204 to reduce heat transfer to the inner Pipe. Insulating spacers are arranged between the outer surface of the inner pipe 204 and the inner surface of the outer pipe 212. The insulating spacers are configured to maintain separation between the inner pipe 204 and the outer pipe 212. The insulating spacers may be 3D printed, laser cut, or moulded. In this example, the insulating spacers are plastic, however the skilled person will understand that other thermally insulating materials may be used. The inner surface of the outer pipe 212 is also polished, and comprises a silver layer 206. As above, the polished surface and silver layer 206 are configured to reduce radiant heat transfer to the cryogenic fluid 200 within the inner tube 204. The cryogenic pipe 200 further comprises an insulation layer 214 which surrounds the outer surface of outer pipe 212. The insulation layer 214 is preferably foam or fibre insulation. The insulation layer 214 is configured for impact absorption, for example to protect the outer pipe 212. Preferably, the insulation Iayer214 is also thermally insulative. The cryogenic pipe 200 also comprises a braided metal covering 216 that surrounds the outer surface of the cryogenic pipe 200. The braided metal covering 216 is configured to protect the cryogenic pipe 200 from damage. Fig. 3 illustrates an example partial cut away view of the cryogenic fluid pipe 200 of Fig. 2. Some features, such as the spacers 208, have been omitted for clarity. In particular, Fig. 3 illustrates a bellows 302 which has been integrated into the inner pipe 204 via electroforming. Preferably, the bellows 302 itself is electroformed, for example according to the method described in patent documents WO 2019 / 016543 and US 11542622. The bellows 302 is configured to deform to accommodate cool-down shrinkage of the inner pipe 204 when exposed to cryogenic fluid. Whilst a bellows 302 is shown in the example shown in Fig. 3, the skilled person will understand that any other expansion joints may be used. The inner pipe 204 also integrates cryogenic connectors 304 via electroforming. The cryogenic connectors 304 are assembled to protrude from the outer pipe 212. The cryogenic connectors 304 are configured to couple to sources and / or sinks of cryogenic fluid, such as cryogenic feed pipes. The outer pipe 212 integrates vacuum ports 306 which are formed during electroforming of the first and second longitudinal outer pipe halves. For example, a first vacuum port may be configured for connection to a vacuum gauge. Another vacuum port may be configured for a vacuum block and blead valve. Fig. 6 shows a specific example of the cryogenic pipe manufactured according to the method of Fig. 1. In this particular example, the inner diameter of the inner pipe was chosen to be 20 mm. During use, the inner pipes will see a drop in temperature of approximately 270°C over a sub-5-second time duration. Copper contracts at a rate of 17 x 10-6 mm / °C, meaning that for a 270°C drop, a contraction of around 5mm per metre would occur. If the pipes were fixed and rigid, this would cause a strain in the material, in turn causing strain in the joints and fittings of the test unit. To accommodate the thermal shrinkage and induced strain, a flexible bellows 302 is incorporated into a straight section of the inner pipe 204. In this example, the bellows 302 is formed of 160 pm wall thickness electroformed copper. To manufacture the inner pipe shown in Fig. 4, readily available 22 mm copper plumbing pipe was used. The pipes, fittings and bellows were all soldered together using leaded plumbing solder, allowing for rapid and efficient manufacture and assembly. The inner assemblies were then electroformed over to ensure a hermetic seal, according to the method of Fig. 1B. The mass per length (excluding fittings) is approximately 1.2 kg / m for a 20mm inner pipe diameter. This is significantly lighter than existing steel welded constructions which typically have an equivalent mass per length of approximately 3.5 kg / m. The outer pipe shown in Fig. 6 has an inner diameter of approximately 50 mm. A reusable tool was precision machined out of tooling block to create the mandrel for electroforming. The tool is symmetrical about the centreline, meaning only one tool is required to make two outer pipe halves, as shown in Fig. 5. KF16 vacuum flange fittings were ‘grown in’ to the outer pipe during electroforming, creating a hermetically sealed, continuous copper piece without joins or seams. Electroformed to 500 pm wall thickness in copper, the two outer pipe halves are mated around the inner pipe and welded, brazed, or soldered together along a flange joint to create a hermetic seal around the inner pipe. The 500 pm wall thickness is based on a design boil-off pressure of 5 bar, however the skilled person will understand that the wall thickness may vary for different design requirements. The “vacuum jacket” between the inner pipe 204 and outer pipe 212 has a cross-sectional width of approximately 14mm. Vacuum testing Preliminary vacuum testing of a cryogenic pipe according to Fig. 6 (unit under test, UUT) was performed using a test set as shown in Fig. 9. This comprises: (i) Vacuum pump (VP) - To pull vacuum from the vacuum space of the unit. (ii) Seal off port and flange bung - To hold vacuum in the vacuum space and disconnect the unit from the vacuum pump. (iii) Vacuum gauge (VG) - To provide vacuum level feedback on the vacuum in the Pipe. (iv) Vacuum switch (VS) - Installed for safety and set at a vacuum level, if it exceeds the set level it will remove power from the test equipment and shut everything down. The vacuum pump was started to remove air from the pipe, with the initial pressure reading recorded from the pressure gauge. Pumping continued until the pressure reached approximately 10“3 mbar, and the system was checked for any significant leaks, which were addressed to ensure maximum airtightness. The vacuum pumping process was maintained for two weeks, during which the vacuum performance was monitored. Regular decay tests were performed hourly, with vacuum results recorded every 15 minutes. After two weeks, the vacuum seal-off connection and pump were removed from the unit under test (UUT). The final pressure reading was recorded upon reaching stability, and the system was observed to maintain this pressure for 24 hours to verify vacuum performance. Further testing demonstrated that said cryogenic pipe could achieve and maintain vacuum better than 10-5 mbar, without requiring continuous pumping. Testing with cryogenic liquid hydrogen Preliminary testing of a cryogenic pipe (unit under test, UUT) according to Fig. 6 for use with liquid hydrogen was performed using a test set as shown in Fig. 10. This comprises: (i) A liquid hydrogen (LH2) supply. (ii) V1 - Isolation valve. (iii) Load cell - A back up to measure the weight delta on introducing LH2. Primarily the RTD is used. (iv) RTD (Resistive temperature device) Thermocouple - To measure the temperature of the LH2, when the temperature flatlines this is an indication that the unit is full of LH2. (v) Vacuum port - To allow vacuum purging. (vi) Pressure transducer (P) &Safety Critical Vent (burst disc) - Safety system that monitors and releases the LH2 If exceeds a set pressure. (vii) Heat exchanger (coil) - Heats the H2 to an ambient temperature. (viii) V2 - Isolation valve. (ix) V3 - Bypass valve. (x) R1 - Back pressure regulator. (xi) Flow meter - To measure the flow and total vented volume of hydrogen, H2. (xii) NRV - Non return valve. In order to quantify how the heat flux through the pipes change as length is varied, two units of different lengths were manufactured. The longer being 1000 mm and shorter being 500 mm. Other than this, the two test units were identical, according to the dimensions set out in Fig. 6. The smaller test unit had a mass of 1.38 kg and a working volume of 0.26 L. The larger unit had a mass of 2.32 kg and a working volume of 0.42 L. The mass per length (excluding fittings) is approximately 1.2 kg / m for a 20mm inner pipe diameter. The LH2 supply line was connected as per procedure, with valve V2 opened and V3 closed. The LH2 supply valve (V1) was gradually opened to initiate the filling process, and regulator R1 was set to achieve a system pressure of 1.5 bar. The fill process was closely monitored through the RTD temperature sensor until stabilisation occurred. R1 was adjusted as necessary to prevent any rapid pressure build-up. Once the system was filled, the LH2 supply valve was closed. The LH2 supply valve was then reopened, and the system pressure was increased to 4 bar by adjusting R1. The pressure rise was continuously monitored, and once the target pressure was reached, V1 was closed, and R1 was set to 0 bar. The hydrogen vent flow was measured using a designated flowmeter, with both the flow rate and total vented volume recorded. The vent pressure was maintained at ambient conditions throughout the measurement process. All data, including flow rate, total volume, vacuum, and temperature, were logged. The boil off flow and temperature were measured to calculate heat flux performance. The cryogenic pipe was monitored throughout for signs of damage caused by thermal shock or loss of vacuum. Test data suggests that heat flux of a cryogenic fluid pipe according to Fig. 6 is estimated to be better than 0.18 W / m. Cryogenic fluid pipe comprising an intermediary pipe for a cryogenic coolant Fig. 7 illustrates a cross-sectional view of another cryogenic fluid pipe 700. The cryogenic fluid pipe 700 comprises an inner pipe 204 for containing cryogenic fluid 202. The inner pipe is electroformed copper. The inner pipe 204 is configured to carry cryogenic fluid 202, such as liquid hydrogen, at or above the boil-off pressure. For example, the inner pipe 204 may be configured to contain liquid hydrogen, or other cryogenic fluid, at a pressure of 5 bar. The outer surface of the inner pipe 204 is polished, and comprises a silver layer 206. The polished surface and silver layer 206 are configured to reduce radiant heat transfer to the cryogenic fluid 200 within the inner tube 204. The cryogenic fluid pipe 700 also comprises a second pipe 402 which surrounds the inner pipe 204. The second pipe 402 is electroformed copper. The second pipe 402 is configured to contain a cryogenic coolant fluid, such as cryogenic helium. In some examples, the second pipe 402 is configured to contain cryogenic coolant fluid at approximately 10 K. At 10 K, cryogenic helium is a vapour. The second pipe 402 is coupled to an external cryostat (not shown). The cryostat is configured to actively cool the cryogenic coolant fluid, such as helium. The inner surface and outer surface of the second pipe 402 are polished, and each comprises a silver layer 206. The second pipe 402 is surrounded by an outer pipe 212. The outer pipe 212 is electroformed copper. The outer pipe 212 is hermetically sealed around the second pipe 402, and the inner pipe 204. The outer pipe 212 is configured to contain a vacuum 210. The vacuum 210 is configured to form a “vacuum jacket” around the second pipe 402 to reduce heat transfer to the inner pipe. The outer surface of the inner pipe 204 is polished, and comprises a silver layer 206. Insulating spacers are arranged between the outer surface of the inner pipe 204 and the inner surface of the second pipe 402, as well as between the outer surface of the second pipe 402 and the inner surface of the outer pipe 212. The insulating spacers are configured to maintain separation between the inner pipe 204, the second pipe 402, and the outer pipe 212. The insulating spacers may be 3D printed, laser cut, ormoulded. In this example, the insulating spacers are plastic, however the skilled person will understand that other thermally insulating materials may be used. An example method 800 of manufacture of a pipe, such as the pipe in Fig. 7, is illustrated in Fig. 8. Firstly, the method 800 comprises obtaining an expansion joint (110). As described above, the expansion joint may be obtained by electroforming the expansion joint, such as a bellows, using a multi-layering electroforming process. The method 800 then comprises electroforming an inner pipe 204 (120), wherein the expansion joint is integrated into the inner pipe 204 via electroforming as described above. Cryogenic connectors, such as cryogenic connectors 304 are preferably also integrated at each end of the inner pipe during electroforming. As shown in Fig. 4, the inner pipe 204 preferably has a “U”-shape. Optionally, the outer surface of the inner pipe is polished, and plated in a silver layer via electroforming. The method 800 also comprises electroforming a second, intermediary pipe. The second pipe has a larger diameter than the first inner pipe, such that the inner pipe is configured to fit within the second pipe. As shown in the method 800, the electroforming the second pipe comprises electroforming a first longitudinal section of the second pipe, and a second longitudinal section of the second pipe (820). Both sections of the second pipe are electroformed using copper. The first and second longitudinal sections of the second pipe are preferably identical halves. In this case, the first and second longitudinal halves of the pipe may be electroformed using a single mould or removeable mandrel. In more detail, a reusable mandrel can be formed by CNC machining, 3D printing, injection moulding, or other suitable technique. The reusable mandrel may then undergo electroding to make the mandrel conductive. The reusable mandrel may then be used to electroform the first longitudinal half section of the second pipe on the reusable mandrel. Once electroformed, the first longitudinal half section may be removed from the mandrel, and the mandrel may then be reused for electroforming the second longitudinal half section of the second pipe. The longitudinal sections of the second pipe further comprise at least one cryostat port along the length of the section, formed by the mandrel. Each cryostat port is configured to be coupled to an external cryostat for active cooling of a coolant fluid, such as cryogenic helium, contained within the second pipe. The sectional edge of each longitudinal section of the outer pipe also comprises a flange 308. Optionally, the inner and outer surfaces of each longitudinal section of the second pipe are also polished and plated with a silver layer via electroforming. In addition, the method 800 also comprises electroforming a third, outer pipe. The outer pipe has a larger diameter than the second pipe, such that the second pipe is configured to fit within the diameter of the outer pipe. As shown in the method 800, electroforming the outer pipe comprises electroforming a first longitudinal section of the outer pipe, and a second longitudinal section of the outer pipe (230), as discussed above. Both sections of the second pipe are electroformed using copper. Once electroformed, the cryogenic fluid pipe is assembled by arranging the inner pipe between the two longitudinal sections of the second pipe (830). Flanges of the first longitudinal section of the second, intermediary pipe and the second longitudinal section are configured to abut once the intermediary pipe is assembled around the inner pipe. Cryogenic connectors at each end of the inner pipe are configured to protrude from the intermediary pipe. Optionally, the spacing between the inner pipe and the second, intermediary pipe is maintained via thermally insulating spacing structures. The method then further comprises joining the first and second longitudinal sections of the second, intermediary pipe to hermetically seal the second pipe around the inner pipe (832). In particular, joining the first longitudinal section and the second longitudinal section of the second pipe comprises joining the flange of the first longitudinal section and the second longitudinal section. Joining is preferably achieved via brazing and / or soldering the first longitudinal section and the second longitudinal sections. The method then comprises a second assembly step comprising arranging the second intermediary pipe (and therefore the inner pipe) between the two longitudinal sections of the third, outer pipe (840). As described above, flanges of the first longitudinal section of the outer pipe and the second longitudinal section are configured to abut once the outer pipe is assembled around the intermediary pipe. Cryogenic connectors at each end of the inner pipe are configured to protrude from the outer pipe. Optionally, the spacing between the intermediary pipe and the outer pipe is maintained via thermally insulating spacing structures. The method then further comprises joining the first and second longitudinal sections of the outer pipe to hermetically seal outer pipe around the second pipe (850), for example as discussed above. The resulting three-walled cryogenic pipe may then be subject to quality assurance and leak checks. In use, a vacuum pump may be used to pull vacuum from the vacuum space between the intermediary pipe and outer pipe. The vacuum is sealed within the vacuum space using a seal off port integrated into the outer pipe, via one of the outer pipe vacuum ports. A vacuum gauge, coupled to another aperture in the outer pipe may also be provided to measure the current vacuum between the intermediary and outer pipes. Advantageously, the vacuum does not require active pumping. In addition, a cryostat may be used to actively cool cryogenic coolant fluid between the 5 inner pipe and second, intermediary pipe. Whilst the method 800 of Fig. 8 discloses electroforming the second, intermediary pipe and outer pipe in longitudinal sections, and subsequently joining said sections, the skilled person will understand that other methods of electroforming each of the second, 10 intermediary pipe and outer pipe may be used. For example, the skilled person will understand that at least one of the second, intermediary pipe and / or outer pipe may be electroformed as a single entity, for example using copper. In this case, said pipe may be electroformed using a single removeable mandrel. 15 In the context of the present disclosure other examples and variations of the apparatus and methods described herein will be apparent to a person of skill in the art.

Claims

1. A method of manufacture of a pipe for a cryogenic fluid the method comprising: obtaining an expansion joint;electroforming an inner pipe for a cryogenic fluid, wherein electroforming the inner pipe comprises electroforming the expansion joint to form an integral part with the inner pipe;electroforming an outer pipe;assembling the inner pipe within the outer pipe; andsealing the outer pipe around the inner pipe to form a hermetic seal around the inner pipe.

2. The method of any preceding claim, wherein obtaining the expansion joint comprises electroforming the expansion joint.

3. The method of claim 2, wherein electroforming the expansion joint comprises electroforming a multi-layer metallic structure on a removable mandrel, for example wherein electroforming the multi-layer metallic structure on the removable mandrel further comprises varying the thickness or composition of the multi-layer structure to adjust the elastic modulus and yield strength of the expansion joint independently.

4. The method of any preceding claim, wherein the expansion joint comprises a bellows.

5. The method of any preceding claim,wherein electroforming the outer pipe comprises:electroforming a first longitudinal section of an outer pipe; and electroforming a second longitudinal section of the outer pipe;wherein assembling the inner pipe within the outer pipe comprises assembling the inner pipe between the first longitudinal section and the second longitudinal section of the outer pipe; andwherein sealing the outer pipe around the inner pipe comprises joining the first longitudinal section and the second longitudinal section of the outer pipe to form a hermeticseal around the inner pipe.

6. The method of claim 5, wherein joining the first longitudinal section and the second longitudinal section of the outer pipe comprises at least one of welding, brazing, and / or soldering.

7. The method of any of claims 5 to 6, wherein each of the first longitudinal section and the second longitudinal section of the outer pipe comprises a flange, wherein the flange of the first longitudinal section and the flange of the second longitudinal section are configured to abut once the outer pipe is assembled; andwherein joining the first longitudinal section and the second longitudinal section of the outer pipe comprises joining the flange of the first longitudinal section and the second longitudinal section.

8. The method of any of claims 5 to 7 wherein the first longitudinal section and the second longitudinal section of the outer pipe are each half sections of the outer pipe.

9. The method of any preceding claim wherein the outer pipe is electroformed to have a wall thickness of no more than 500 micrometres.

10. The method of any preceding claim, wherein electroforming the inner pipe further comprises integrating a cryogenic connector fitting at at least one end of the inner pipe, wherein the cryogenic connector fitting is configured to couple to a cryogenic fluid feed Pipe.

11. The method of any preceding claim wherein the inner pipe is electroformed using copper.

12. The method of any preceding claim wherein the outer pipe is electroformed using copper.

13. The method of any preceding claim further comprising depositing a silver layer on at least one of (i) an outer surface of the inner pipe, and / or (ii) an inner surface of the outerpipe.

14. The method of any preceding claim, wherein assembling the inner pipe within the outer pipe further comprises incorporating spacer structures between the inner pipe and the outer pipe.

15. The method of claim 14 wherein the spacer structures are plastic.

16. The method of any preceding claim wherein the electroformed inner pipe and theelectroformed outer pipe comprise at least one bend.

17. The method of any preceding claim, further comprising:electroforming an intermediary pipe configured for connection to a cryostat; andassembling the inner pipe within the intermediary pipe, and the intermediary pipe within the outer pipe.

18. The method of claim 17 wherein electroforming the intermediary pipe comprises: electroforming a first longitudinal section of the intermediary pipe;electroforming a second longitudinal section of the intermediary pipe;wherein the method further comprises assembling the inner pipe between the first longitudinal section of the intermediary pipe and the second longitudinal section of the intermediary pipe; andjoining the first longitudinal section of the intermediary pipe and the second longitudinal section of the intermediary pipe to seal around the inner pipe.

19. The method of any claims 17 or 18 further comprising depositing a silver layer on at least one of (i) an outer surface of the intermediary pipe, and / or (ii) an inner surface of the intermediary pipe.

20. The method of any preceding claim, wherein the cryogenic fluid is hydrogen.

21. The method of any preceding claim, wherein the assembled pipe has a mass perunit length equal to or less than 1.6 kg per meter, preferably equal to or less than 1 kg per meter.

22. A pipe for a cryogenic fluid, the pipe being manufactured according to the method of any preceding claim.

23. A pipe for cryogenic fluid, comprising:a first electroformed inner pipe configured for cryogenic fluid;a second electroformed pipe, surrounding the first inner pipe, configured to receive a cryogenic coolant fluid to cool the cryogenic fluid contained in the first inner pipe;a third electroformed outer pipe, hermetically sealed around the second pipe, configured for a vacuum.

24. The pipe of claim 23 wherein the second electroformed pipe is configured for connection to a cryostat such that the cryostat is configured to actively cool cryogenic coolant fluid within the second pipe.

25. The pipe of any of claims 23 or 24 wherein the first electroformed inner pipe comprises an expansion joint, and optionally wherein the second electroformed pipe comprises an expansion joint.

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