NO20130715A1 - Water intake risers assembly for an offshore structure, and method for producing a liquid hydrocarbon stream and method for producing a vaporous hydrocarbon stream - Google Patents

Abstract

There is disclosed a water inlet riser assembly (105) suspended from an offshore structure (102) comprising at least one first tubular channel (106) extending longitudinally. A distal portion of the first tubular duct comprising a water inlet section (109). The water inlet section comprises a tubular section having a sidewall circumferentially about the longitudinal direction and defining a longitudinal flow passage with a hole having a first transverse cross-sectional area. The side wall is perforated with several through holes (405) which form multiple water inlet openings. An overall area defined by the multiple through holes is larger than the first transverse cross-sectional area.

Description

The present invention relates to a water intake riser assembly which is suspended from an offshore structure and/or an offshore structure from which the water riser assembly is suspended, as stated in the requirements. In other aspects, the invention relates to a method for producing a liquid hydrocarbon stream using a water intake riser assembly and/or for producing a vaporous hydrocarbon stream using such a water intake riser assembly.

WO 2010/085302 discloses a marine system including a liquefied natural gas (FLNG) plant in/on a sea surface. The FLNG plant can cool and condense natural gas to form LNG, or alternatively heat and gasify LNG. A water riser assembly is suspended in the FLNG plant to take in cold water at depth and convey the cold water upwards to the FLNG plant. The water riser assembly comprises tubular constructions projecting downwards directed into the sea and connected together with several spacers. Cold water is taken in at the distal end at depth, and conveyed further upwards to the FLNG plant. Filters may optionally be provided on each of the bottoms of the tubular structures which help prevent soil, marine life (eg seaweed, algae, fish, etc.) and the like from entering the tubular structures.

A disadvantage of the water riser assembly described in WO 2010/085302 may be that the flow rate of the water in the tubular structures is limited.

In a first aspect, this invention provides a water intake riser assembly suspended from an offshore structure comprising at least a first tubular channel extending along a longitudinal direction and comprising, viewed longitudinally, a forward portion comprising suspension means, followed by a coupling portion, followed by a distal portion comprising a water inlet portion, the coupling portion fluidly connecting the front portion and distal portion, the water inlet portion comprising a tubular portion having a side wall peripherally around the longitudinal direction and defining a longitudinal flow passage with an opening having a first transverse cross-sectional area Ai, said side wall being perforated with a plurality of through holes forming a plurality of heat intake openings, each through hole defining a transverse access path into the flow passage to allow a transverse flow of water to enter the flow passage, a total area defined by the plurality of through holes being greater than d a first transverse cross-sectional area.

Such a water riser assembly may be suspended from an offshore structure to form an offshore structure from which such a water riser assembly is suspended.

In another aspect, this invention provides a method of producing a liquid hydrocarbon stream using a water intake riser assembly and a method of producing a vapor hydrocarbon stream using such a water intake riser assembly.

Process for producing a liquid hydrocarbon stream includes:

- feeding a vaporous hydrocarbon-containing feed stream to an offshore structure; - to form a liquid hydrocarbon stream from at least part of the vaporous hydrocarbon-containing feed stream comprising at least extraction of heat

at least a portion of the vaporous hycocarbon-containing feed stream,

- supplying water to the offshore structure via a water intake riser assembly, - supplying at least part of the removed heat from at least part of the hydrocarbon-containing feed stream to at least part of the water supplied via

the water intake riser assembly;

- then to dispose of at least part of the water.

Process for producing a vaporous hydrocarbon stream,

comprehensive:

- to provide a liquid hydrocarbon stream on an offshore structure; - forming a vaporous hydrocarbon stream from at least part of the liquid hydrocarbon stream comprising

- to add heat to said part of the liquid hydrocarbon stream,

- supplying water to the offshore structure via a water intake riser assembly; - extracting at least a portion of the heat, to supply said portion of liquid hydrocarbon flow stream, from at least a portion of the water supplied via the water intake riser assembly;

- then disposal of at least part of the water.

The invention will now be further illustrated by way of example, and with reference to the accompanying non-limiting drawings, in which: Figure 1 schematically shows a liquid condensed natural gas plant provided with a water intake riser assembly comprising several tubular channels; Figure 2 schematically shows a cross-sectional view of the riser assembly on section plan 2 which is indicated in Figure 1; Figure 3 schematically shows a cross-sectional view of the riser assembly at section plan 3 indicated in Figure 1; Figure 3A schematically shows a cross-sectional view of the riser assembly at sectional plan 3 as indicated in Figure 1, according to another embodiment of this invention; Figure 4 schematically shows an example of a distal part and part of the connecting part of one of the tubular channels; Figure 5 schematically shows a bottom view of the distal part as shown in Figure 4; and Figure 6 schematically shows a perspective view of the distal portion of the water intake riser assembly showing portions of multiple tubular channels when fully suspended.

For the purpose of this description, a single reference number is assigned to a line, in addition to a stream in that line. The same reference number refers to corresponding components, currents or lines.

The present invention describes a water intake riser assembly that can be suspended from an offshore structure, where the water intake part comprises a tubular part having a side wall peripherally around the longitudinal direction and defining a flow passage in the longitudinal direction with an opening having a first transverse cross-sectional area Ai, the side wall is perforated with several through holes each defining a transverse access path in the flow passage to allow a transverse flow of water to enter the flow passage.

This means that the total area defined by the several through holes becomes larger than the first transverse cross-sectional area.

Furthermore, the diameter of the distal part can be kept relatively small because the through holes can be distributed over a relatively large length along the side wall. Hereby, it is arranged to easily pull the tubular channel by pushing it in the longitudinal direction. Preferably, the through holes are distributed over the several circumferences around the side wall. Since the through holes are available in a range of radial directions, the volume of cold water flowing at the highest speed is relatively low compared to taking in water in a longitudinal direction. Furthermore, the risk of complete stoppage of water conveyed to the front part due to clogging of the through holes is reduced if the through holes are distributed not only along the length of the water intake part, but also over the circumference.

Preferably, the first tubular channel is supplemented by at least one second tubular channel which also comprises a front part comprising suspension means, followed by a connecting part, followed by a distal part comprising a water intake part in accordance with the same or similar functions as described above on the first tubular channel. By providing at least two tubular channels it is achieved that water supply from the distal part of the water intake riser assembly to the forward part of the water intake riser assembly is still possible if one of the two tubular channels is blocked from receiving water on the distal part.

The first and second tubular channels can obviously be included in an assembly. The assembly may comprise more than two tubular channels, for example 8 or 9 tubular channels arranged in a rectangular cross-sectional pattern having at least one tubular channel at each of the four corners and a tubular channel between sets of two of the corners. Alternatively, the tubular channels can be arranged in a concentric and/or circular pattern. By increasing the number of tubular channels, the operational risk of blockage can be further reduced.

A pier construction for water intake at sea is described in French Patent Publication No. FR2029224. A tubular pier extends from the sea to the land. Two pillars at the end of the breakwater are submerged in water and extend to the bottom of the sea to support the breakwater. These pillars are hollow and provided with holes in their submerged part. Water is taken in from the sea through the same hollow pillars on which the breakwater rests, with the help of a pump inside them, and conveyed to land through tubular breakwaters. Since the breakwater rests on pillars, the hollow columns are not suspended from the breakwater.

Figure 1 illustrates an example of a marine system 100 in which embodiments of this invention may be implemented. The marine system 100 in this example includes an offshore structure 102 in/on a surface of the sea 104, here represented in the form of a floating structure. The offshore structure 102 may include a liquefied natural gas (FLNG) facility as one example. The FLNG plant can cool and condense natural gas, or alternatively heat and vaporize LNG.

A water intake riser assembly 105 is suspended from the offshore structure 102 in a fully suspended condition. The water intake riser assembly 105 can be used to bring water from the sea to the facility. The water intake riser assembly 105 comprises a bundle 106 of at least one first tubular channel 106A and a second tubular channel 106B. These tubular channels can take in cold water 140 at depth, and convey the cold water upwards to the offshore structure 102. Cold water can be input to heat exchangers to add or remove heat to/from a process carried out on the offshore structure 102. Heated or cooled seawater from the discharge from heat exchangers can be discharged back into the sea at the surface, or alternatively conveyed back to depth with a discharge system.

The first and second tubular channels 106A, 106B generally extend side by side along a longitudinal direction. Viewed longitudinally, each of the tubular tubes has a forward portion 107, then a connecting portion 108, then a distal portion 109. The distal portions of the tubular channels together, when fully suspended, form the distal portion of the water intake riser assembly. Preferably, the distal portion of the water intake riser assembly hangs freely from the ocean floor 103. As an example, the distal portion of the water intake riser assembly hangs at a depth D of between about 130 to 170 meters above the surface of the ocean 104, although the water intake riser assembly may also be at other depths.

The front part 107 comprises suspension means where the tubular channel is suspended from offshore structures 102. Due to the ocean currents, tubular structures 106 can be deflected from the vertical position, up to about 40 degrees or thereabouts (not shown). To accommodate such deflections, the tubular structures 106 can be suspended from the offshore structure through a swivel joint, a ball joint, a riser pendulum or other pivoting hinges or rotatable connections. Reference is made in particular to US 7,318,387 which describes a particularly suitable riser hanger construction involving a flexible load transfer member and a hose to convey the water.

The distal portion 109 comprises the water intake portion, as an example of which will be illustrated below with reference to Figures 4 and 5. The distal portion 109 extends between a first distal end and connecting portion 108. The connected portion fluidly connects the forward portion 107 and the distal part 109. It can be seen in Figure 1 that at least a part of the distal part 109 of the first tubular channel 106A extends further in the longitudinal direction than the second tubular channel 106B.

Only two tubular channels 106A and 106B have been described so far, but bundle 106 may comprise a larger number. Figure 2 shows an exemplary approach or configuration for nine tubular channels (106A through 1061) arranged in a three-by-three rectangular array, according to a particular embodiment. This figure is a cross-sectional view along section plane 2 in figure 1, through several of the tubular channels. The matrix has eight tubular constructions along the periphery and one in the center. The tubular channel 106E in the center can serve as a structural support structure for the spacers. The center tubular channel 106E may or may not convey the water to the surface (ie, may or may not serve as a water intake riser).

In a particular embodiment, the eight tubular channels along the periphery may have outer diameter size d. For the structural tubular channel in the example, the central tubular channel 106E may have an outer diameter less than d. The eight tubular structures along the periphery may be equidistant from each other at a distance of about an outer diameter d. Thus, in this example, the tubular channels (106A to 1061) are placed on a square grid pattern with a grid spacing of about 2d.

Referring again to Figure 1, in order to form the bundle 106, the first and second tubular channels 106A, 106B are laterally connected to each other by means of at least one spacer (HOA; 11 OB, HOC) which cooperates with the respective coupling parts 108 of the tubular channel. By means of such spacers, tubular channels are physically connected or connected. In one embodiment, enough spacers may be provided to keep tubular structures from bumping into each other.

Figure 3 shows an exemplary spacer HOA for nine tubular channels (106A to 1061) arranged in a three-by-three rectangular matrix, according to a particular embodiment. This figure is a cross-sectional view along section plane 3 in figure 1, through spacer 11 OA and several tubular channels. The spacers may each comprise one or more interconnected guide sleeves 306A to 306D and 306F to 3061 through which respective of the tubular channels 106A to 106D and 106F to 1061 are disposed. Posts 307 form the connection. At least one of the posts 307 is fixedly connected to the central tubular channel 106E. In an alternative embodiment, the central tubular channel 106E also passes through a guide sleeve where spacer HOA is supported by alternative means such as a rod, a wire, a chain connected to the offshore structure 102.

The guide sleeves are slidably connected to the tubular channel located therein. Each guide sleeve 306 may define an opening 301, which allows one of the elongate tubular channels to pass freely through and preferably allows limited rotation of the elongate tubular channels about a horizontal axis. The horizontal axis is an axis that lies in a plane symmetrical to spacer 110A, which plane is perpendicular to the longitudinal direction of the passage through opening 301.

Spacer 110A is slidably displaceable in relation to the first and second tubular channels 106A, 106B along the longitudinal direction. In this way, the first and second tubular channels are extracted from a spacer HOA, for example in case one needs to be replaced. Figure 3A shows an alternative embodiment where nine tubular structures are arranged in a concentric matrix, according to one embodiment. In this case, the concentric array is circular. The matrix can alternatively be elliptical, oval, star-shaped, triangular, etc. Furthermore, posts 307 connecting guide sleeves 306 on spacers as shown in Figure 3 have been replaced by a frame or a solid body equipped with holes representing guide sleeves 306 or capable to hold the steering sleeves. This can be used for rectangular arrays or also on other bundle patterns. Figure 4 shows a detailed view of an example of a lower end of one of the tubular channels 106A, including its distal portion 109 and a portion of the coupling portion 108. A guide cylinder 408 may be mounted around a portion of the coupling portion 108 to connect with a of spacers 110. Such a guide cylinder 408 may comprise a different material than the coupling part 108. Preferably, it is a less hard material than the material in the coupling part 108 and/or material on the inside of the guide sleeve to ensure that it wears faster than the coupling part 108 and/or guide sleeves. The coupling part may comprise several of the tubes connected together in a string of connections 409. The inner diameter of the guide cylinder is suitably closely fitted to the outer diameter of the tubular coupling parts. The wall thickness of the guide cylinder is suitable between 1.5 and 3 inches, depending on the outer diameter (larger diameter usually corresponds to larger wall thickness).

The water intake part 403 in the distal part 109 is provided with water intake openings 405 distributed along the water intake part 403.1 embodiments, the coupling part 108 is free from water intake openings. Preferably, the water intake part 403 comprises a tubular part having a side wall 404 peripherally around the longitudinal axis L. In this way, a flow passage is defined in the longitudinal direction L, with an opening 402 having a first transverse cross-sectional area Ai. The water inlet openings 405 are provided as multiple through holes through the side wall 404. Each through hole defines a transverse access port in the flow passage and in operation allows a transverse flow of cold water 140 from the sea into the flow passage.

Appropriately, the total inlet area defined by the flow area through several through holes 405 is larger than the first transverse cross-sectional area Ai. It is thereby achieved that the inlet speed of cold water 140 from the sea just outside the water intake part 403 can still be below a maximum permitted speed (in an example the maximum permitted inlet speed is 0.5 m/s) while the water flow speed inside the tubular channel can exceed it the maximum permissible inlet velocity. In preferred embodiments, the total inlet area is greater than 5 times Ai. Appropriately, the total inlet area is less than 50 times Ai, preferably less than 10 times Ai.

In one particular example, the tubular portion of the water intake portion 403 is made of carbon steel having a steel grade of X70 or equivalent. It can have an outer diameter of about 42 inches and wall thicknesses of about 1.5 cm. The through holes 405 may be drilled through the side wall 404. Preferably, each through hole 405 is less than 10 cm in diameter to prevent large marine units from entering. Preferably, each through hole 405 is larger than 1 cm in diameter to avoid clogging due to build-up of relatively small particles and to avoid large water pressure differentials. In one example, the diameter of the through holes 405 was chosen to be about 5 cm.

Further, the distal portion 109 may include a bottom piece on the distal end 401 to provide a rounded tip. In embodiments, the bottom piece 410 may be mounted to the side wall 404 of the distal portion 109. It may comprise a planar piece 411 projecting downwardly from the water intake portion longitudinally in its plane. The bottom piece 410 can further comprise a guide plate 412 which extends perpendicular to the longitudinal direction L to avoid the inflow of water on the lower tubular end of the water intake part 403. If desired, the guide plate 412 can be provided with one or more smaller through holes 115 to facilitate restricted water access to flow passage 402. These through holes 115 may be of the same or similar size as the through holes 405 in the side wall 404. The planar piece 411 may have a downwardly projecting semicircular or semioval outer contour.

Second and third planar pieces 421 and 431 may also be provided, as illustrated in Figure 5 which has an upward view across the longitudinal distal end 401. Planar piece 411 together with the second and third planar pieces 421 and 431 may form a transverse arrangement of planar pieces projecting radially outward from a central axis CA defined by the intersection where the planar pieces meet. Several planar parts can be provided if desired, which preferably also extend radially from the central axis. Figure 6 schematically shows a perspective view of the distal portion of the water intake riser assembly 105 and shows staggered distal portions 109. The example shows a bundle 106 of eight tubular channels, including the first tubular channel 106A and the second tubular channel 106B. The shown parts of all eight tubular elements are of the same design each with the same components. A spacer 110 is fixedly connected to a central support rod 606. The central support rod 606 projects downwards along the longitudinal direction and also firmly supports an auxiliary part of the spacer 610. The spacer 110 comprises eight guide sleeves 603, but fewer can be installed in other embodiments. The additional spacer comprises four additional guide sleeves 613 of the same design as eight guide sleeves 603, connected together with the arms 607.

In this example, each guide sleeve 603 comprises an upper part 604 directed towards it

front portion of the first and second tubular channels 106A and 106B, and a lower portion 605 facing the distal portion 109 of the first tubular channel 106A. The lower part 605 is cylindrical in shape and surrounds the first tubular member 106A. The tubular element 106A is optionally provided with a guide cylinder 408 as explained above. The upper part 604 is funnel-shaped, and has a larger opening than the cylindrically shaped lower part 605. Additional guide sleeves 613 have a corresponding upper part 614 and lower part 615. This design preferably provides in combination with the base pieces at the distal ends a rounded tip, enables reinsertion of the tubular canal after it has been retracted.

In the example of Figure 6, the distal portion 109 of four of the eight tubular channels includes the first tubular channel 106A, which extends further in the longitudinal direction L than the four remaining tubular channels including the second tubular channel 106B. Thus, if the distal portion 109 of the first tubular channel extends between a first distal end 401 and the coupling portion of the first tubular channel over longer Li, and the distal portion of the second tubular channel extends between the second distal end 601 and the coupling portion to the second tubular channel over a length L2 and then the first distal end 401 extends at least by an amount Li further in the longitudinal direction than the second distal end 601. Of this, the distal parts 109 of the first tubular channel 106A have at least a part which is in the lateral direction (in a plane perpendicular to the longitudinal direction) not overlapping with any part of the second tubular channel 106B.

The total length from the distal end 401 to the bottom string connector 409 can be in the range of 5 to 20 m. In one example this length was approximately 14 m. The length of the water intake portion 403 was in one example 8.5 m and the length of the optional guide cylinder 408 was about 3.4 m.

It should be noted that all tubular channels in the present example are fully suspended for water intake operation, as opposed to being withdrawn from guide sleeves for inspection, replacement or maintenance.

In order to provide sufficient cooling water to the offshore structure 102, in one embodiment, each tubular channel in the assembly need not necessarily be in operation at all times. Thus, one or more of the tubular channels can serve as a redundant water intake riser.

If desired, several filters can optionally be connected to each of the distal parts 109.

If desired, more than one of the described water intake riser assemblies can be suspended from a single offshore structure.

Any or all tubular ducts can be supplied with vortex induced vibration suppressors. Examples are described in, for example, WO 2010/085302.

The water intake riser assembly as described above can be used to supply process water to a process carried out on the offshore structure.

In a specific example, it can be used in a method of producing a liquid hydrocarbon stream comprising: - feeding a vaporous hydrocarbon-containing feed stream to an offshore structure; - forming a liquid hydrocarbon stream from at least part of the vaporous hydrocarbon-containing feed stream comprising at least extraction of heat from

at least a portion of the vaporous hydrocarbon-containing feed stream,

- supplying water to the offshore structure via a water intake riser assembly in accordance with one of claims 1 to 12 suspended from the offshore structure; - supplying at least a portion of the removed heat from at least a portion of the hydrocarbon-containing feed stream to at least a portion of the water supplied via the water intake riser assembly;

- then to dispose of at least part of the water.

A well-known example of a liquid hydrocarbon stream is a stream of condensed natural gas. A variety of suitable installations and arrangements are available in the art for extracting heat from a vaporous hydrocarbon-containing feed stream, particularly a natural gas stream, as well as other processing steps such as removal of unwanted contaminants and components from the feed stream often performed in conjunction with the production of a liquid hydrocarbon stream, and need not be further explained here.

In another example, the water intake riser assembly may be used in a process for producing a vaporous hydrocarbon stream,

comprehensive:

- to provide a liquid hydrocarbon stream on an offshore

construction; - forming a vaporous hydrocarbon stream from at least part of the liquid hydrocarbon stream comprising

- to add heat to said part of the liquid hydrocarbon stream,

- supplying water to the offshore structure via a water intake riser assembly according to one of claims 1 to 12; - extracting at least a portion of the heat, to supply said portion of liquid hydrocarbon flow stream, from at least a portion of the water supplied via the water intake riser assembly;

- then disposal of at least part of the water.

A number of suitable installations and arrangements are available in the field for regasification or vaporization of previously liquid hydrocarbon streams and heat supply to such liquid hydrocarbon streams, and need not be further explained here.

A person skilled in the art will understand that this invention can be carried out in many different ways without deviating from the scope of the appended claims.

Claims (15)

1. Water intake riser assembly suspended from an offshore structure comprising at least a first tubular channel extending along a longitudinal direction and comprising, viewed longitudinally, a forward portion comprising suspension means, followed by a coupling portion, followed by a distal portion comprising a water intake portion, the coupling portion fluidly connects the front part and distal part, where the water intake part comprises a tubular part having a side wall peripherally around the longitudinal direction and defining a flow passage in the longitudinal direction with an opening having a first transverse cross-sectional area Ai, said side wall is perforated with several through holes forming a plurality of water intake openings, each through hole defining a transverse access path into the flow passage to allow a transverse flow of water to enter the flow passage, wherein a total area defined by the plurality of through holes is greater than the first transverse cross-sectional area. 2. Water intake riser assembly according to claim 1, where the through holes are distributed along the water intake part, and where the connecting part is free from the water intake openings. 3. Water intake riser assembly according to claim 1 or claim 2, where the through holes are distributed along the water intake part over a length of at least 5 m in the longitudinal direction. 4. Water intake riser assembly according to claim 3, where the through holes are distributed along the water intake part over a length of less than 20 m in longitudinal direction. 5. Water intake riser assembly according to one of the preceding claims, where the through holes are distributed over most of the circumference around the side wall. 6. Water intake riser assembly according to one of the preceding claims, where the total inlet area is 5 times greater than Ai and/or less than 50 times Ai. 7. Water intake riser assembly according to one of the preceding claims, where each through hole is less than 10 cm in diameter. 8. Water intake riser assembly according to one of the preceding claims, where each through hole is larger than 1 cm in diameter. 9. Water intake riser assembly according to one of the preceding claims, where the first tubular channel comprises an assembly with at least one second tubular channel which generally extends alongside the first tubular channel along the longitudinal direction, said second tube also comprising a front part comprising suspension means, followed by of a connecting part, followed by a distal part, where first and second tubular channels are laterally connected to each other by means of at least one spacer cooperating with the respective connecting parts. 10. Water intake riser assembly according to claim 9, where at least one spacer is slidably displaceable in relation to the first and second tubular channels along the longitudinal direction, where the first and second tubular channels are extractable by at least one spacer. 11. Water intake riser assembly according to claim 10, where the spacer comprises a guide sleeve slidable in contact with the first tubular channel, the guide sleeve comprises an upper part facing the proximal part of the first and second tubular channels and a lower part facing the distal part of the first and other tubular channels, where the lower part is cylindrically shaped surrounding the first tubular element and where the upper part is funnel-shaped and has a larger opening than the cylindrically shaped lower part. 12. A water intake riser assembly according to any one of the preceding claims, wherein the distal portion comprises a bottom piece at the distal end which provides a rounded tip. 13. An offshore structure where a water riser assembly according to one of the preceding claims is suspended. 14. Method for producing a liquid hydrocarbon stream comprising: - feeding a vaporous hydrocarbon-containing feed stream to an offshore structure; - to form a liquid hydrocarbon stream from at least part of the vaporous hydrocarbon-containing feed stream comprising at least extraction of heat from at least part of the vaporous hydrocarbon-containing feed stream, - to supply water to the offshore structure via a water intake riser assembly in accordance with one of claims 1 to 12 suspended from the offshore structure ; - supplying at least a portion of the removed heat from at least a portion of the hydrocarbon-containing feed stream to at least a portion of the water supplied via the water intake riser assembly; - then to dispose of at least part of the water. 15. Process for producing a vaporous hydrocarbon stream, comprising: - providing a liquid hydrocarbon stream on an offshore structure; - forming a vaporous hydrocarbon stream from at least part of the liquid hydrocarbon stream comprising - adding heat to said part of the liquid hydrocarbon stream, - adding water to the offshore structure via a water intake riser assembly according to one of claims 1 to 12; - extracting at least a portion of the heat, to supply said portion of liquid hydrocarbon flow stream, from at least a portion of the water supplied via the water intake riser assembly; - then disposal of at least part of the water.