Lift gas evacuation assembly

Abstract

There is provided a lift gas evacuation assembly (200) comprising an input conduit (210) for receiving a stream of lift gas and gas-lifted material from a riser, an output conduit (220) with an output conduit section (222) for returning the stream to the riser, and an intermediate conduit (230) interconnecting the input and output conduits. The intermediate conduit includes an inner surface section along which lift gas concentrates by gravity during operation. A lift gas evacuation conduit (242, 244) is fluidly connected to the intermediate conduit at the inner surface section for evacuating some of the lift gas. The assembly redirects the stream from an input direction (Din) to a first intermediate direction (D1), then to a second intermediate direction (D2), and finally to an output direction (Dout) coaxially aligned with the input direction (Din).

Description

[0001] LIFT GAS EVACUATION ASSEMBLY

[0002] FIELD OF INVENTION

[0003] The present disclosure relates to a lift gas evacuation assembly, a riser system, use of the riser system, and a method for removing lift gas from a stream of lift gas, liquid and gas-lifted material.

[0004] BACKGROUND

[0005] Gas lift techniques may be used for extracting materials from subsea locations. Gas lift involves injecting gas into a riser pipe to reduce the density of the fluid column, thereby creating a pressure differential that drives the upward flow of materials.

[0006] A challenge faced in effective gas lift operations is managing the behaviour of injected gas bubbles as they rise through the riser pipe. As the bubbles ascend, they expand due to decreasing hydrostatic pressure, potentially leading to flow instabilities and reduced efficiency. This expansion effect becomes more pronounced in deeper water applications, where the pressure differential between the seafloor and surface is greater.

[0007] Another problem in gas lift systems is controlling the gas-to-liquid ratio throughout the riser pipe. As bubbles coalesce and expand during ascent, the gas fraction can become excessive in upper sections of the pipe. This can result in slug flow regimes, reduced lifting capacity, and inefficient use of injected gas. In some cases, the lifting effect of the gas may be entirely lost, meaning that the material being gas-lifted does not reach the top of the riser pipe.

[0008] Lift gas may be removed from the riser pipe along its length to mitigate these challenges and problems. In US Patent 9,719,528, for example, there is described a deaerator, for use at intermediate depths within a riser system. The deaerator creates a rotational flow in the riser pipe using a spiral pipe, thereby separating gas from the slurry / water mixture using centrifugal force. The separated gas gathers along a central axis, where it can be discharged through a ventilation pipe, while the denser mixture swirls around the outside of the gas and can be regathered once the gas has been siphoned off for continued lifting. Another example is shown in Japanese Patent JP 6166308. In the system described in JP 6166308, a riser pipe incorporates slanted sections for releasing gas bubbles into a separate chamber before discharging them.

[0009] While both provide a way of discharging excess lift gas, they require modifications to the riser that present structural challenges. For subsea operations, and especially where a riser is suspended from a vessel or platform at the surface, it is undesirable to introduce instabilities and structural complexities into the riser system.

[0010] SUMMARY

[0011] According to a first aspect, there is provided a lift gas evacuation assembly. The lift gas evacuation assembly is configured to be positioned in-line with a riser between an upstream riser segment and a downstream riser segment, and includes: an input conduit comprising an input conduit section configured to be fluidly connected to the upstream riser segment for receiving a stream of lift gas, liquid and gas-lifted material from the riser; an output conduit comprising an output conduit section configured to be fluidly connected to the downstream riser segment for returning the stream to the riser; an intermediate conduit fluidly interconnecting the input and output conduits and comprising at least one inner surface section along which the lift gas is configured to concentrate by gravity when the lift gas evacuation assembly is in operation; and at least one lift gas evacuation conduit being fluidly connected to the intermediate conduit at the at least one inner surface section for evacuating a portion of the lift gas from the stream, wherein the input conduit is configured for redirecting the stream from an input direction to a first intermediate direction, wherein the intermediate conduit is configured for redirecting the stream from the first intermediate direction to a second intermediate direction, and wherein the output conduit is configured for redirecting the stream from the second intermediate direction to an output direction being coaxially aligned with the input direction.

[0012] Consequently, the input conduit, which may be referred to as the upstream conduit, is configured for receiving the stream from the upstream riser segment along the input direction and redirecting the stream to the first intermediate direction. The intermediate conduit is configured for receiving the stream from the upstream conduit along the first intermediate direction and redirecting the stream to the second intermediate direction, and the output conduit, which may be referred to as the downstream conduit, is configured for receiving the stream from the intermediate conduit along the second intermediate direction and redirecting the stream to the output direction, which is coaxially aligned with the input direction. The input conduit section and the output conduit section may be coaxially aligned along a common axis and, consequently, the input direction and the output direction may be coaxially aligned with the common axis.

[0013] The input conduit section and the output conduit section being coaxially aligned along the common axis allows the lift gas evacuation assembly to be arranged coaxially or substantially coaxially with the riser, thus providing a riser system which is structurally robust. In particular, the arrangement allows the lift gas evacuation assembly to be arranged in plumb or substantially in plumb with the riser, thus reducing stress-buildup in the riser system. Also, the arrangement allows the lift gas evacuation assembly to be carried or supported by the riser, thus making away with having to suspend the lift gas evacuation assembly directly from a resource recovery ship, as is the case in prior art systems (such as JP6166308B2).

[0014] Furthermore, the lift gas evacuation assembly being configured to redirect the flow away from the common axis and then back to the same common axis allows the lift gas to concentrate along the inner surface section such that a predefined portion of the lift gas can be evacuated through the lift gas evacuation conduit in a controlled manner.

[0015] The riser may typically be arranged vertically or substantially vertically in a body of water between a subsea location and a surface location, which may be referred to as a topside location. The gas-lifted material may be subsea mined minerals, e.g. nodules or mineral bearing crust. The liquid in the stream may typically be sea water. The subsea location may typically be a seafloor location or a subsea material collection facility. The surface location may typically be a vessel, such as a resource recovery ship or a floating platform.

[0016] The at least one inner surface section is configured to form an upper or ceiling surface section of the intermediate conduit, thus allowing the lift gas to concentrate at or along the inner surface section by virtue of gravity when the stream is conducted through the intermediate conduit. In other words, the gravitational field of the earth is utilised to allow lift gas, due to its relatively lower density than the other material within the stream, to concentrate or accumulate at or along the inner surface section.

[0017] The first intermediate direction is different to the second intermediate direction. The first intermediate direction and the second intermediate direction are different to the input direction and are different to the output direction. The input and output directions may be vertically upward directions, in use.

[0018] The riser may comprise a plurality of riser segments including the upstream riser segment and the downstream riser segment. By the input conduit section being configured to be fluidly connected to the upstream riser segment "for receiving a stream of lift gas and gas-lifted material from the riser", it may be meant that the upstream riser segment is configured in this way for receiving the stream, comprising a mixture of lift gas, liquid, e.g. sea water, and gas-lifted material, from a portion of the riser upstream of the input conduit section, which may include the upstream riser segment. Similarly, by the output conduit section being configured to be fluidly connected to the downstream riser segment "for returning the stream to the riser", it may be meant that the output conduit section is configured in this way for returning the stream, lacking any lift gas that has been removed via the at least one lift gas evacuation conduit, to a portion of the riser downstream of the output conduit section, including the downstream riser segment.

[0019] References to upstream and downstream herein refer to positions relative to the flow of material along the riser, such that an upstream position is earlier in the riser than a downstream position, and, when used in situ, closer to the seabed.

[0020] The first intermediate direction may form a predefined first angle to the input direction being any one of: within the range of 25-65 degrees; within the range of 40-50 degrees; and 45 degrees. The second intermediate direction may form a predefined second angle to the first intermediate direction being any one of: within the range of 70-110 degrees; within the range of 80-100 degrees; and 90 degrees. These ranges of angles may be particularly useful for redirecting the stream to promote lift gas separation while minimizing flow disruption, as well as to prevent blockages should failure of the system occur. Such angles may also promote concentration of the lift gas along the inner surface section, enabling the removal of the lift gas effectively.

[0021] The output direction may form a predefined third angle to the second intermediate direction being any one of: within the range of 25-65 degrees; within the range of 40-50 degrees; and 45 degrees. This redirection angle allows for smooth reintegration of the stream back into the main riser flow, minimizing turbulence and pressure loss.

[0022] The at least one inner surface section may form a ceiling surface section of the intermediate conduit. Forming a ceiling implies that the at least one inner surface section is to form an upper surface section when the lift gas evacuation assembly is in operation. In other words, the inner surface section in question is the surface section configured to be positioned closest to the topside when the lift gas evacuation assembly is in operation.

[0023] According to one embodiment, the arrangement of the at least one inner surface section allows lift gas and also liquid, e.g. sea water, to be diverted from the stream into the at least one lift gas evacuation conduit. In the least one lift gas evacuation conduit, the liquid, e.g. sea water, surface may be kept at a predefined level by controlled removal of the lift gas from the at least one lift gas evacuation conduit leaving a controlled volume of lift gas in the gas evacuation conduit above the liquid (sea water) surface.

[0024] To prevent liquid (water) droplets from being pulled up through the at least one lift gas evacuation conduit together with the evacuated lift gas, the at least one lift gas evacuation conduit may comprise a first conduit section communicating with the inner surface section and a second, downstream conduit section communicating with the first conduit section, which second conduit section has a larger cross-sectional area than the first conduit section.

[0025] The intermediate conduit may comprise: a first intermediate conduit segment comprising a first inner surface section of said at least one inner surface sections; a first lift gas evacuation conduit of said at least one lift gas evacuation conduits being in fluid communication with the first intermediate conduit segment at the first inner surface section; a second intermediate conduit segment comprising a second inner surface section of said at least one inner surface sections; and a second lift gas evacuation conduit of said at least one lift gas evacuation conduits being in fluid communication with the second intermediate conduit segment at said second inner surface section. Providing multiple lift gas evacuation conduits and multiple intermediate conduit segments may enable more precise and controlled removal of lift gas.

[0026] The first intermediate conduit segment and the second intermediate conduit segment may each be rectilinear. The first intermediate conduit segment and the second intermediate conduit segment may each have a length within the range of 1-50 meters. Providing such shapes and / or lengths of the conduit segment may provide adequate dimensions to allow separation of the lift gas within the intermediate conduit segments. The first lift gas evacuation conduit may be connected to the first intermediate conduit segment closer to a downstream end than to an upstream end of the first intermediate conduit segment. The second lift gas evacuation conduit may be connected to the second intermediate conduit segment closer to a downstream end than to an upstream end of the second intermediate conduit segment. Providing evacuation conduits closer to the downstream ends of the intermediate conduit segments allows the lift gas longer distance to separate from the liquid and mined material within the intermediate conduit segments.

[0027] The first intermediate conduit segment and the first intermediate conduit segment may be mutually perpendicular.

[0028] A load bearing structure may be connected between the input conduit and the output conduit. This will allow the weight of the upstream riser segments, located below the lift gas evacuation assembly, to be transferred to the downstream riser segments, located above the gas lift evacuation assembly, via the load bearing structure. In turn, this may provide structural integrity to the assembly, ensuring reliable operation under the stresses of subsea conditions.

[0029] The load bearing structure may be rectilinear and coaxially aligned with the input conduit and the output conduit. The load bearing structure may be a rectilinear blindpipe connected to the first and second connectors and being coaxially aligned with the first and second connector interfaces.

[0030] The input conduit, the intermediate conduit and the output conduit may be arranged in a common plane. According to a second aspect, a riser system configured for gas-lifting material from a subsea location to a surface location is provided. The riser system includes: a riser configured for gas-lifting the subsea mined material; and at least one lift gas evacuation assembly according to any one of the preceding aspects.

[0031] The riser system may comprise a plurality of said gas lift evacuation assemblies distributed along the riser. Multiple evacuation assemblies along the riser allow for staged lift gas removal, which may improve the gas-lifting efficiency along the riser.

[0032] The riser system may comprise a compressor at the surface location being in fluid communication with the lift gas evacuation conduit(s) of the at least one lift gas evacuation assembly, the compressor being configured to compress evacuated lift gas to be injected in the riser at a location upstream of the lowermost of said at least one lift gas evacuation assembly. This configuration allows for recycling (re-compressing) of the evacuated lift gas, improving the overall energy efficiency of the gas-lifting process.

[0033] A scrubber may be arranged upstream of the compressor.

[0034] The compressor may be a multi-stage compressor and may comprise inter-stage scrubbers (scrubber-compressor-scrubber-compressor- etc.). The removed lift gas can be directed to different inter-stage scrubbers to optimally utilize the remaining compression energy.

[0035] The lift gas may be air.

[0036] The riser, at the surface location, may be subjected to atmospheric pressure. Consequently, at the surface termination the riser is open to the atmosphere. Maintaining atmospheric pressure at the surface simplifies the handling and processing of the lifted materials.

[0037] According to a third aspect, use of a riser system according to any one of the preceding aspects for gas-lifting subsea mined minerals is provided. This application of the riser system enables efficient extraction of valuable minerals from the seabed.

[0038] According to a fourth aspect, a method of evacuating lift gas from a stream of lift gas, liquid and gas-lifted material in a riser system configured for gas-lifting the material from a subsea location to a surface location is provided. The riser system includes a riser comprising an upstream riser segment, a downstream riser segment being coaxially aligned with the upstream riser segment along a common axis, and a lift gas evacuation assembly arranged between the first and the second riser segments. The method comprises the steps of: in an input conduit of the lift gas evacuation assembly connected to the upstream riser segment, redirecting the stream from an input direction being coaxially aligned with the common axis to a first intermediate direction; in an intermediate conduit of the lift gas evacuation assembly connected to the input conduit, redirecting the stream from the first intermediate direction to a second intermediate direction; in an output conduit of the lift gas evacuation assembly connected between the intermediate conduit and the second riser segment, redirecting the stream from the second intermediate direction to an output direction being coaxially aligned with the input direction; and in the intermediate conduit, evacuating a portion of the lift gas from the stream by allowing said portion of the lift gas to exit the intermediate conduit in an upward direction via at least one lift gas evacuation conduit connected to the intermediate conduit. This method provides a systematic approach to separating and evacuating lift gas from the stream, improving the overall efficiency of the gas-lifting process.

[0039] Lift gas and liquid, e.g. sea water, may be diverted in the at least one lift gas evacuation conduit and the method may comprise keeping a liquid, e.g. sea water, surface in the at least one lift gas evacuation conduit, i.e. an interface between the lift gas and the liquid, e.g. sea water, at a predefined level. This may be achieved by controlling removal of the lift gas from the at least one lift gas evacuation conduit.

[0040] The method may comprise: in a first lift gas evacuating step, evacuating a first subportion of the lift gas from the stream by allowing the first sub-portion of the lift gas to exit the intermediate conduit in an upward direction via a first lift gas evacuation conduit connected to a first intermediate conduit segment of the intermediate conduit; and in a second, subsequent lift gas evacuating step, evacuating a second sub-portion of the lift gas from the stream by allowing the second sub-portion of the lift gas to exit the intermediate conduit in an upward direction via a second lift gas evacuation conduit connected to a second intermediate conduit segment of the intermediate conduit. This two-step evacuation process enhances the overall lift gas removal efficiency by providing multiple opportunities for gas separation and evacuation. BRIEF DESCRIPTION OF FIGURES

[0041] Aspects of the disclosure will be described, by way of example, with reference to the following drawings, in which:

[0042] Fig. 1 illustrates a schematic view of a riser system for extracting material from a subsea location to a surface location.

[0043] Fig. 2 illustrates a perspective view of a lift gas evacuation assembly.

[0044] Fig. 3 illustrates a sectional view of the lift gas evacuation assembly shown in Fig. 2.

[0045] Common reference numerals are used throughout the figures to indicate similar features.

[0046] DETAILED DESCRIPTION

[0047] Fig. 1 shows a riser system 100, which is configured for gas-lifting material 120 from a subsea location 10 to a surface location 20. The material may comprise minerals and may be in the form of nodules or mineral-bearing crust. For example, the materials may be polymetallic nodules. The material may be referred to as subsea material, and, when in the process of being lifted within the riser system 100, may be referred to as mined material, subsea-mined material, or gas-lifted material. The riser system 100 transports material 120 from the subsea location 10 to a vessel 30 at the surface location 20, where it can be processed and stored. The vessel 30 is, in this example, a resource recovery ship, but in other examples may be a floating or fixed platform, or another type of vessel or surface location.

[0048] The riser system 100 includes several components arranged vertically from the subsea location 10 to the vessel 30, which are designed to manage the flow within the riser system 100 and ensure efficient lifting of the material 120.

[0049] At the subsea location 10, a collector vehicle 130 is deployed for collecting material 120 from the seafloor. The collector vehicle 130 may be an autonomous underwater vehicle (AUV) or may be a remote-controlled underwater vehicle. The collector vehicle 130 includes various tools and sensors that enable it to identify the material 120, pick up the material 120 and to transfer the material 120 into the rest of the riser system 100. The collector vehicle 130 may also include mining tools to enable dislodging or unearthing of the material 120. The collector vehicle 130 is connected to a jumper 135, which in turn is connected to a lower riser unit 150. The jumper 135 serves as a conduit for transferring the collected material 120 and sea water from the collector vehicle 130 to the lower riser unit 150.

[0050] The lower riser unit 150 is connected to a riser 300, which extends upwards towards the surface location 20. The riser 300 transports the collected material 120 and sea water between the lower riser unit 150 and the vessel 30. To do so, the material 120 is lifted through the riser 300, and this is achieved by introducing lift gas into the riser 300. Introducing lift gas produces a mixture of lift gas, sea water and mined material 120 that is less dense than the mined material 120 by itself, and this allows the mined material to be lifted through the riser 300. The riser 300 extends from the lower riser unit 150 to the surface location 20. A pressure in the riser 300 reduces towards the surface location 20, with the pressure at the surface location 20 and at the end of the riser 300 being atmospheric or near atmospheric pressure.

[0051] Material 120 lifted through the riser 300 is directed through an offtake hose 50 to a separator 48. In the separator 48, the material 120 is separated from water and gases that were also lifted along with the material 120. After separation, the material 120 is deposited in a material store 52 for further processing or storage. The water separated from the material 120 may be treated in a water polisher 54 before being returned to the sea via a water return hose 56.

[0052] The offtake hose 50, separator 48, water polisher 54, material store 52, and return hose 56 form part of a surface system 40. The surface system 40 also includes umbilical cable reels 46, a heave compensator 44, at least one power source 41, and at least one compressor 42. The umbilical cable reels 46 provide a first umbilical cable 140 and a second umbilical cable 160, for providing power and lift gas to the riser system. The first umbilical cable 140 provides an electrical connection to the collector vehicle 130, while the second umbilical cable 160 provides lift gas to the lower riser unit 150. The first umbilical cable 140 and the second umbilical cable 160 are respectively connected to a power source 41 and to a compressor 42 provided on the vessel 30 (the connections are not depicted in Fig. 1). The heave compensator 44 connects to the riser 300 and compensates for motion of the vessel 30 relative to the riser 300, to ensure that the riser 300 is maintained in a stable arrangement throughout mining operations.

[0053] While injecting lift gas enables the material 120 to be lifted from depth, the expansion of lift gas as it rises through the riser 300 could lead to flow instabilities and reduced lifting efficiency without action being taken. As the lift gas bubbles ascend, they expand due to decreasing hydrostatic pressure, and this may cause excessive gas fractions in upper sections of the riser. This can result in unstable flow regimes, reduced lifting capacity, and inefficient use of injected gas. In some cases, the lifting effect may be entirely lost, preventing the material from reaching the top of the riser. These challenges become more pronounced in deeper water applications, where the pressure differential between the seafloor and surface is greater.

[0054] To counteract this, the riser system 100 includes multiple evacuation assemblies 200 positioned along the riser 300 and interspersed between riser segments 305. The evacuation assemblies 200 include a first evacuation assembly 200a, a second evacuation assembly 200b, and a third evacuation assembly 200c. The evacuation assemblies separate and remove lift gas from a stream or flow of lift gas, sea water and mined material 120 travelling along the riser 300. Each evacuation assembly 200 separates some of the lift gas into one or more evacuation conduits 240 and returns the stream, relieved of some of the lift gas, to riser segments 305 downstream, i.e. above, the evacuation assembly 200 for continued travel towards the surface location 20. Accordingly, some lift gas is removed from the stream, while the remaining lift gas, sea water and mined material continues travelling along the riser 300. This ensures lifting efficiency is maintained.

[0055] The evacuation assemblies 200 each include respective evacuation conduits 240. In Fig. 1 , the evacuation assemblies are in fluid communication with the compressor 42, so that lift gas removed from the riser 300 is returned to the compressor 42 for re-use. The compressor 42 is configured to compress evacuated lift gas to be injected in the riser 300 via the second umbilical cable 160. Such re-use of lift gas improves the efficiency of the operation. Alternatively, the lift gas from some or all of the evacuation assemblies 200 may be expelled near the surface via evacuation conduits 240 that extend towards the surface location 20 and that are unconnected to the compressor 42 or other components.

[0056] Turning now to Figs. 2 and 3, a lift gas evacuation assembly 200 is shown. Fig. 2 shows an external, perspective view of the lift gas evacuation assembly 200, while Fig. 3 shows the lift gas evacuation assembly 200 in a schematic, longitudinal section. The lift gas evacuation assembly 200 may be any of the first, second, or third lift gas evacuation assemblies 200a, 200b, 200c shown in Fig. 1.

[0057] The assembly 200 is configured to be positioned in-line with other riser segments 305 within the riser 300. This is shown in Figs. 2 and 3, where the lift gas evacuation assembly 200 is positioned between two riser segments 305, comprising an upstream riser segment 310 and a downstream riser segment 320. The upstream riser segment 310 is shown at the bottom of each figure, while the downstream riser segment 320 is shown at the top of each figure.

[0058] The stream of lift gas, sea water and gas-lifted material being lifted through the assembly 200 passes from upstream riser segment 310 into an input conduit 210 of the assembly 200, the input conduit 210 comprising an input conduit section 212 and an output conduit section 216. The stream then passes from the input conduit 210 to an intermediate conduit 230 of the assembly 200, and from the intermediate conduit 230 to an output conduit 220 of the assembly 200, the output conduit 220 comprising an input conduit section 226 and an output conduit section 222. The output conduit 220 returns the stream to the riser 300 at the downstream riser segment 320. Lift gas is evacuated from the assembly 200 via lift gas evacuation conduits 242, 244 along the intermediate conduit, as will be explained in more detail below.

[0059] Focusing initially on the transfer of the stream into the assembly 200, the input conduit section 212 of the input conduit 210 is fluidly connected to the upstream riser segment 310. This allows fluid, such as the stream of lift gas and material to enter the input conduit 210 from the upstream riser segment 310. Accordingly, the input conduit 210 receives a stream of lift gas and gas-lifted material from the riser 300, and particularly from a portion upstream of the assembly 200 via the upstream riser segment 310 into the input conduit 210. The input conduit section 212 is aligned along a common axis A with the upstream riser segment 310, and therefore receives the stream of lift gas and gas-lifted material from the upstream riser segment 310 along the axis A. The input conduit 210 then redirects the stream from an input direction Din, which is coaxially aligned with the common axis A, to a first intermediate direction D1. The input conduit 210 may therefore be said to be configured to redirect the stream from the input direction Din to the first intermediate direction D1. The input conduit 210 includes a redirection structure 214, which is not visible in Fig. 2 but can be seen in Fig. 3. The redirection structure 214 redirects the stream from the input direction Din to the first intermediate direction D1. The redirection structure 214 therefore achieves an angled surface that is parallel with the direction D1 , and the surface extends across an interior of the input conduit 210 at a position at which an intermediate conduit 230 connects to the input conduit 210.

[0060] The first intermediate direction D1 forms a predefined first angle a to the input direction Din. This angle a is 45 degrees in Figs. 2 and 3, but in other examples may be within the range of 25-65 degrees, and particularly within the range of 40-50 degrees. The specific angle a may be determined based on the specific requirements of the riser system 100 and the characteristics of the material 120 and lift gas.

[0061] As noted above, the input conduit 210 transfers the stream to the intermediate conduit 230. The intermediate conduit 230 fluidly interconnects the input conduit 210 and the output conduit 220. The intermediate conduit 230 includes a first intermediate conduit segment 232 and a second intermediate conduit segment 234. The intermediate conduit 230 connects to the input conduit 210 via the first intermediate conduit segment 232. The first intermediate conduit segment 232 extends from an upstream end 232a to a downstream end 232b. The upstream end 232a of the first intermediate conduit segment 232 is connected with the input conduit 210 and is aligned with the redirection structure 214 to facilitate smooth transition of the stream from the input conduit 210 to the first intermediate conduit segment 232. The first intermediate conduit segment 232 is oriented to extend along the first direction D1.

[0062] The first intermediate conduit segment 232 is connected to the second intermediate conduit segment 234 by a conduit joining segment 236. The downstream end 232b of the first intermediate conduit segment 232 is fluidly connected to an upstream end 234a of the second intermediate conduit segment 234 via the conduit joining segment 236. The second intermediate conduit segment 234 extends from the upstream end 234a to a downstream end 234b, at which the second intermediate conduit segment 234 is fluidly connected to the output conduit 220.

[0063] The second intermediate conduit segment 234 is aligned along a second intermediate direction D2, and the conduit joining segment 236 redirects the stream of material from the direction in which it is flowing along the first intermediate conduit segment 232, which is the first intermediate direction D1 , to the direction in which it is to flow along the second intermediate conduit segment 234, which is the second intermediate direction D2. Accordingly, the intermediate conduit 230 redirects the stream from the first intermediate direction D1 to a second intermediate direction D2.

[0064] The redirection of the stream from the first intermediate direction D1 to the second intermediate direction D2 is facilitated by the design and arrangement of the intermediate conduit 230, and particularly by the arrangement of the first intermediate conduit segment 232 and the second intermediate conduit segment 234. The first intermediate conduit segment 232 and the second intermediate conduit segment 234 are arranged to provide a predefined second angle [3 between the first intermediate direction D1 and the second intermediate direction D2. This angle [3 is approximately 80 degrees in Figs. 2 and 3, but may be in the range of 70-110 degrees, and particularly within the range of 80-100 degrees in other examples. In some examples, the angle may be exactly 90 degrees, and the first intermediate conduit segment 232 and the second intermediate conduit segment 234 may be mutually perpendicular. The specific angle [3 may be determined based on the specific requirements of the riser system 100 and the characteristics of the material 120 and lift gas.

[0065] The arrangement and configuration of the first intermediate conduit segment 232 and the second intermediate conduit segment 234 facilitates separation of the lift gas from the mined material 120 within the stream. The first intermediate conduit segment 232 and second intermediate conduit segment 234 are rectilinear and have a length within a specific range, e.g. within the range of 1-50 meters, to encourage said separation, while the orientation of the segments 232, 234 as described above provides a region in which separation can occur. Particularly, inner surface sections are formed along each of the first intermediate conduit segment 232 and the second intermediate conduit segment 234 along which the lift gas is configured to concentrate by gravity as the stream flows along the segments 232, 234. These inner surface sections include a first inner surface section 233 of the first intermediate conduit segment 232 and a second inner surface section 235 of the second intermediate conduit segment 234. These inner surface sections 233, 235 form ceiling surface sections of the intermediate conduit 230, by which it is meant that, in the orientation within which the assembly 200 arranged, in use, these sections form uppermost surfaces of the first and second intermediate conduit segments 232, 234 and therefore surfaces along which lift gas can concentrate due to gravity and their relatively lower density than the other material within the stream.

[0066] To facilitate removal of the lift gas concentrating by gravity along these surfaces, lift gas evacuation conduits are provided. These lift gas evacuation conduits, which are referred to generally using the numeral 240 in Fig. 1 , include a first lift gas evacuation conduit 242 and a second lift gas evacuation conduit 244. The lift gas evacuation conduits are fluidly connected to the intermediate conduit 230 at the inner surface sections. The first lift gas evacuation conduit 242 is connected to the first intermediate conduit segment 232 at its first inner surface section 233 and the second lift gas evacuation conduit 244 is connected to the second intermediate conduit segment 234 at its second inner surface section 235. The lift gas evacuation conduits 242, 244 are connected to their respective intermediate conduit segments 232, 234 closer to the downstream ends of those segments than to the upstream ends. This means that the first lift gas evacuation conduit 242 is connected to the first intermediate conduit segment 232 closer to the downstream end 232b than to the upstream end 232a, and the second lift gas evacuation conduit 244 is connected to the second intermediate conduit segment 234 closer to the downstream end 234b than to the upstream end 232a. Such positioning enables sufficient separation of the gas from the other material within a particular intermediate conduit segment before reaching an evacuation conduit. The lift gas evacuation conduits 242, 244 evacuate a portion of the lift gas from the stream by allowing the lift gas to exit the intermediate conduit 230 in an upward direction.

[0067] To return the stream, relieved of some of the lift gas, to the riser 300 at the downstream riser segment 320, the intermediate conduit 230 fluidly connects to the output conduit 220, and the stream passes from the intermediate conduit 230 to the output conduit 220. The output conduit 220 fluidly connects to the downstream riser segment 320 at the output conduit section 222 of the output conduit 220.

[0068] The output conduit section 222 is coaxially aligned along the common axis A with the downstream riser segment 320. Accordingly, the output conduit section 222, and the output conduit 220 more generally, are aligned with the input conduit 210 and the input riser segment 310 along the common axis A. This means that the stream is also redirected to flow along the common axis A, and so is redirected from the second intermediate direction D2 to an output direction Dout, which is coaxially aligned with the common axis A and is therefore the same as the input direction Din. The output conduit 220 also includes a redirection structure 224 to facilitate smooth transfer from the second intermediate direction D2 to the output direction Dout.

[0069] The output direction Dout forms a predefined third angle y to the second intermediate direction D2. This angle y is, in this example, the same as the angle a, although in other examples these angles may differ. The angle y may be within the range of 25-65 degrees, and particularly within the range of 40-50 degrees. In some examples, the angle y may be exactly 45 degrees. The specific angle y may be determined based on the specific requirements of the riser system 100 and the characteristics of the material 120 and lift gas.

[0070] In addition to the input conduit 210, the intermediate conduit 230, and the output conduit 220, the assembly 200 also includes a load bearing structure 250, which is connected between the input conduit 210 and the output conduit 220. This structure provides structural support for the assembly 200, ensuring that it can withstand the pressures and forces encountered during operation. The load bearing structure 250 is rectilinear and is coaxially aligned with the input conduit section 212 of the input conduit 210 and the output conduit section 222 of the output conduit 220 along the common axis A. The load bearing structure 250 is a blind pipe in this example, meaning that the load bearing structure 250 has a pipe-like structure and connects the input and output conduits 210, 220, but is arranged so that fluid flow between the input and output conduits 210, 220 via the load bearing structure 250 is prevented. The fluid flow is prevented by the redirection structures 214, 224. In other examples, the load bearing structure 250 may be a different structure to a pipe. The input conduit 210, the intermediate conduit 230, and the output conduit 220 are arranged in a common plane. The load bearing structure 250 is also provided in the common plane. This reduces the footprint of the assembly 200 and provides structural benefits. Generally, when incorporated as part of a riser 300, the assembly 200 enables the riser 300 to have a plurality of segments that are all aligned along a common axis, which provides structural benefits as well as occupying a reduced volume. In other embodiments, however, the input conduit, the intermediate conduit, and / or the output conduit need not necessarily be arranged in a common plane.

[0071] The redirection and removal of lift gas using the assembly can be described as a method involving several steps.

[0072] In the input conduit 210, the stream is redirected from the input direction Din, which is coaxially aligned with the common axis A, to the first intermediate direction D1. In the intermediate conduit 230, the stream is redirected from the first intermediate direction D1 to the second intermediate direction D2. The method then involves evacuating a portion of the lift gas from the flow by allowing the lift gas to exit the intermediate conduit 230 in an upward direction via at least one lift gas evacuation conduit. Using the assembly 200 described above, this may involve two steps, which are a first lift gas evacuating step and a second, subsequent lift gas evacuating step. In the first lift gas evacuating step, a first sub-portion of the lift gas is evacuated from the stream. This is achieved by allowing the first sub-portion of the lift gas to exit the intermediate conduit 230 in an upward direction via the first lift gas evacuation conduit 242. As described above, the first lift gas evacuation conduit 242 is fluidly connected to the first intermediate conduit segment 232 of the intermediate conduit 230. In the second, subsequent lift gas evacuating step, a second sub-portion of the lift gas is evacuated from the stream. This is achieved by allowing the second sub-portion of the lift gas to exit the intermediate conduit 230 in an upward direction via the second lift gas evacuation conduit 244. As described above, the second lift gas evacuation conduit 244 is fluidly connected to the second intermediate conduit segment 234 of the intermediate conduit 230. After said evacuation, in the output conduit 220, the stream is redirected from the second intermediate direction D2 to the output direction Dout, which is coaxially aligned with the input direction Din. Although the examples show an arrangement with two evacuation conduits 242, 244, in other examples a single evacuation conduit may be provided, or more than two evacuation conduits may be provided. Furthermore, in other examples a different number of intermediate conduit segments may be provided. In some examples, directions other than D1 and D2 may be included, such that the intermediate conduit redirects the stream or flow through several different directions between the directions D1 and D2.

[0073] The input conduit section 212 and the output conduit section 216 of the input conduit 210 may each be rectilinear. For example, the input conduit section 212 may be provided as a rectilinear or substantially rectilinear first pipe section, and the output conduit section 216 may be provided as a rectilinear or substantially rectilinear second pipe section that is connected, e.g. welded, to the first pipe section at said first angle a, as is disclosed in Fig. 2. Consequently, said redirection structure 214 may be formed by an internal wall of the output conduit section 216 redirecting the flow from the input direction Din to the first intermediate direction D1.

[0074] For fluidly connecting the input conduit 210 to the upstream riser segment 310, the input conduit section 212 may be provided with a flange 213 being configured to cooperate with a corresponding flange 313 of the upstream riser segment 310. For connecting the input conduit 210 to the intermediate conduit 230, the output conduit section 216 may be provided with a flange 217 being configured to cooperate with a corresponding flange 238a at the upstream end 232a of the first intermediate segment 232.

[0075] The first intermediate segment 232 and the second intermediate segment 234 of the intermediate conduit 230 may be provided as rectilinear or substantially rectilinear pipe sections, and the conduct joining segment 236 may be provided as a curved or bent pipe section, as is disclosed in Fig. 2, redirecting the stream from the first intermediate direction D1 to the second intermediate direction D2.

[0076] For fluidly connecting the first intermediate segment 232 to the conduct joining segment 236, the first intermediate segment 232 may be provided, at the downstream end 232b, with a flange 238b being configured to cooperate with a corresponding flange 237a of the conduct joining segment 236. For connecting the conduct joining segment 236 to the second intermediate segment 234, the conduct joining segment 236 may be provided with a flange 237b being configured to cooperate with a corresponding flange 239a at the upstream end 234a of the second intermediate segment 234.

[0077] The input conduit section 226 and the output conduit section 222 of the output conduit 220 may each be rectilinear. For example, the input conduit section 226 may be provided as a rectilinear or substantially rectilinear first pipe section, and the output conduit section 222 may be provided as a rectilinear or substantially rectilinear second pipe section that is connected, e.g. welded, to the first pipe section at said third angle y, as is disclosed in Fig. 2. Consequently, said redirection structure 224 may be formed by an internal wall of the output conduit section 222 redirecting the flow from the second intermediate direction D2 to the output direction Dout.

[0078] For fluidly connecting the output conduit 220 to the downstream riser segment 320, the output conduit section 222 may be provided with a flange 227 being configured to cooperate with a corresponding flange 323 of the downstream riser segment 320. For connecting the output conduit 220 to the intermediate conduit 230, the input conduit section 226 may be provided with a flange 223 being configured to cooperate with a corresponding flange 239b at the downstream end 234b of the second intermediate segment 234.

[0079] The first and second lift gas evacuation conduits 242, 244 may each be provided with a pipe section 243, 245 that is connected, e.g. welded, to the respective lift gas evacuation conduit 242, 244 to allow lift gas accumulating at the inner surface section 233 and 235, respectively, to be evacuated.

Claims

CLAIMS1. A lift gas evacuation assembly (200) configured to be positioned in-line with a riser (300) between an upstream riser segment (310) and a downstream riser segment (320), comprising:- an input conduit (210) comprising an input conduit section (212) configured to be fluidly connected to the upstream riser segment (310) for receiving a stream of lift gas, liquid and gas-lifted material from the riser (300);- an output conduit (220) comprising an output conduit section (222) configured to be fluidly connected to the downstream riser segment (320) for returning the stream to the riser (300);- an intermediate conduit (230) fluidly interconnecting the input and output conduits (210, 220) and comprising at least one inner surface section along which the lift gas is configured to concentrate by gravity when the lift gas evacuation assembly (200) is in operation; and- at least one lift gas evacuation conduit (242, 244) being fluidly connected to the intermediate conduit (230) at the at least one inner surface section for evacuating a portion of the lift gas from the stream,wherein the input conduit (210) is configured for redirecting the stream from an input direction (Din) to a first intermediate direction (D1),wherein the intermediate conduit (230) is configured for redirecting the stream from the first intermediate direction (D1) to a second intermediate direction (D2), andwherein the output conduit (220) is configured for redirecting the stream from the second intermediate direction (D2) to an output direction (Dout) being coaxially aligned with the input direction (Din).

2. The lift gas evacuation assembly (200) according to claim 1, wherein first intermediate direction (D1) forms a predefined first angle (a) to the input direction (Din)being any one of: within the range of 25-65 degrees; within the range of 40-50 degrees; and 45 degrees.

3. The lift gas evacuation assembly (200) according to any one of claims 1 and 2, wherein the second intermediate direction (D2) forms a predefined second angle ([3) to the first intermediate direction (D1) being any one of: within the range of 70-110 degrees; within the range of 80-100 degrees; and 90 degrees.

4. The lift gas evacuation assembly (200) according to any one of the preceding claims, wherein the output direction (Dout) forms a predefined third angle (y) to the second intermediate direction (D2) being any one of: within the range of 25-65 degrees; within the range of 40-50 degrees; and 45 degrees.

5. The lift gas evacuation assembly (200) according to any one of the preceding claims, wherein said at least one inner surface section (233) forms a ceiling surface section of the intermediate conduit (230).

6. The lift gas evacuation assembly (200) according to any one of the preceding claims, wherein the intermediate conduit (230) comprises:- a first intermediate conduit segment (232) comprising a first inner surface section (233) of said at least one inner surface sections;- a first lift gas evacuation conduit (242) of said at least one lift gas evacuation conduits being in fluid communication with the first intermediate conduit segment (232) at the first inner surface section (233);- a second intermediate conduit segment (234) comprising a second inner surface section (235) of said at least one inner surface sections; and- a second lift gas evacuation conduit (244) of said at least one lift gas evacuation conduits being in fluid communication with the second intermediate conduit segment (232) at said second inner surface section.

7. The lift gas evacuation assembly (200) according to claim 6, wherein the first lift gas evacuation conduit (242) is connected to the first intermediate conduit segment (232) closer to a downstream end (232b) than to an upstream end (232a) of the first intermediate conduit segment (232), and wherein the second lift gas evacuation conduit (244) is connected to the second intermediate conduit segment (234) closer to a downstream end (234b) than to an upstream end (234a) of the second intermediate conduit segment (234).

8. The lift gas evacuation assembly (200) according to any one of claims 6 and 7, wherein the first intermediate conduit segment (232) and the second intermediate conduit segment (234) are rectilinear and mutually perpendicular.

9. The lift gas evacuation assembly (200) according to any one of the preceding claims, wherein a load bearing structure (250) is connected between the input conduit (210) and the output conduit (220).

10. The lift gas evacuation assembly (200) according to claim 9, wherein said load bearing structure (250) is rectilinear and coaxially aligned with the input conduit (210) and the output conduit (220).

11. The lift gas evacuation assembly (200) according to any one of the preceding claims, wherein the input conduit (210), the intermediate conduit (230) and the output conduit (220) are arranged in a common plane.

12. A riser system (100) configured for gas-lifting material (120) from a subsea location (10) to a surface location (20), the riser system (100) comprising:- a riser (300) configured for gas-lifting the subsea mined material (120); and- at least one lift gas evacuation assembly (200a, 200b, 200c) according to any one of the preceding claims.

13. The riser system (100) according to claim 12, wherein the riser system comprises a plurality of said gas lift evacuation assemblies (200a, 200b, 200c) distributed along the riser (300).

14. The riser system (100) according to any one of claims 12 and 13, wherein the riser system comprises a compressor (42) at the surface location being in fluid communication with the lift gas evacuation conduit(s) (242, 244) of the at least one lift gas evacuation assembly (200a, 200b, 200c), the compressor being configured to compress evacuated lift gas to be injected in the riser (300) at a location upstream of the lowermost of said at least one lift gas evacuation assembly (200a, 200b, 200c).

15. The riser system (100) according to any one of claims 12-14, wherein the riser (300), at the surface location (20), is subjected to atmospheric pressure.

16. Use of a riser system (100) according to any one of claims 12-15 for gas-lifting subsea mined minerals.

17. A method of evacuating lift gas from a stream of lift gas, liquid and gas-lifted material (120) in a riser system (100) configured for gas-lifting the material (120) froma subsea location (10) to a surface location (20), the riser system (100) including a riser (300) comprising an upstream riser segment (310), a downstream riser segment (320) being coaxially aligned with the upstream riser segment (310) along a common axis (A), and a lift gas evacuation assembly (200) arranged between the first (310) and the second (320) riser segments, the method comprising the steps of:- in an input conduit (210) of the lift gas evacuation assembly (200) connected to the upstream riser segment (310), redirecting the stream from an input direction (Din) being coaxially aligned with the common axis (A) to a first intermediate direction (D1);- in an intermediate conduit (230) of the lift gas evacuation assembly (200) connected to the input conduit (210), redirecting the stream from the first intermediate direction (D1) to a second intermediate direction (D2);- in an output conduit (220) of the lift gas evacuation assembly (200) connected between the intermediate conduit (230) and the second riser segment (320), redirecting the stream from the second intermediate direction (D2) to an output direction (Dout) being coaxially aligned with the input direction (Din); and- in the intermediate conduit (230), evacuating a portion of the lift gas from the stream by allowing said portion of the lift gas to exit the intermediate conduit (230) in an upward direction via at least one lift gas evacuation conduit (242, 244) connected to the intermediate conduit (230).

18. The method according to claim 17, comprising:- in a first lift gas evacuating step, evacuating a first sub-portion of the lift gas from the stream by allowing the first sub-portion of the lift gas to exit the intermediate conduit (230) in an upward direction via a first lift gas evacuation conduit (242) connected to a first intermediate conduit segment (232) of the intermediate conduit (230); and- in a second, subsequent lift gas evacuating step, evacuating a second sub-portion of the lift gas from the stream by allowing the second sub-portion of the lift gas to exit the intermediate conduit (230) in an upward direction via a second lift gas evacuationconduit (242) connected to a second intermediate conduit segment (234) of the intermediate conduit (230).

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