OVERLOAD PROTECTION SYSTEM FOR LIFT PIPE

Abstract

A system is presented for the protection of a subsea lift pipe from wave action or liquid overload. The pipe has a horizontal portion on the seabed and a buckling bend portion. The system comprises: a flexible pipe having an upper end and a lower end; a plurality of self-contained valves configured to allow liquid to pass through the flexible pipe; the self-contained valves being positioned between the upper and lower ends of the pipe; and further comprising an inlet device coupled with the lower end of the flexible pipe, the inlet device being deflected against a lower wall of the lift pipe.

Description

OVERLOAD PROTECTION SYSTEM FOR LIFT PIPE Technical field The invention relates to the control of hydrocarbon flow in a riser pipe and in particular, relates to the protection of the riser pipe from waves or overload. Background of the invention A hydrocarbon production flowline can be connected to a riser pipe, which transports gas and liquids from a well to the production facility. Flexible riser pipes are used in many subsea systems. This allows the riser pipe to withstand horizontal and vertical movement due to, for example, wave motion. Several different configurations of flexible riser pipes are known in the art (e.g., parallel bend to buckling, parallel bend to S-bend, S-bend, etc.). In each configuration, the riser pipe is typically bent to form at least one buckling bend (i.e., a 'U' shape) and at least one pig bend (i.e., an inverted 'U' shape).As such, the riser pipe will not only extend continuously upward from the flow line in a straight line, but will have several areas with bends or curves, local slopes, and near-horizontal regions. One problem with flexible riser systems is their tendency to accumulate liquid in bends or slopes. At some point, the accumulated liquid could begin flowing unstably toward the base of the riser pipe, causing liquid to accumulate at the base, along the inner walls of the riser pipe, and produce pulsating liquid on the platform. This is known as liquid surge or surge. At some point, the liquid surge or surge could become so severe that it overfills the separators, which in turn causes problems for processing plants and may eventually necessitate abandoning the flow line.Therefore, there is a need for a system that can mitigate the problem of liquid accumulation in a flexible lifting pipe, thereby reducing the risk of liquid overload. Brief description of the invention. According to a first aspect of the invention, a system is provided for the protection against liquid overload of a subsea lift pipe having a horizontal portion on the seabed and a buckling bend portion, the system comprising: a flexible pipe having an upper end and a lower end; a plurality of self-contained valves configured to allow liquid to pass through the flexible pipe; wherein the self-contained valves are positioned between the upper end and the lower end of the pipe; and further comprising an inlet device coupled with the lower end of the flexible pipe, wherein the inlet device is deflected against a lower wall of the lift pipe. The plurality of self-contained valves could be placed in a plurality of locations along the circumferential direction of the flexible pipe. Optionally, the plurality of self-contained valves LZRQnn / zznz / E / YiAi could be placed in a plurality of locations along the longitudinal direction of the flexible pipe. The plurality of self-contained valves could be provided within a wall of the flexible pipe. Alternatively, the plurality of self-contained valves could be provided within a wall of one or more rigid support bodies coupled with the flexible pipe. Each of the rigid support bodies could have a curved shape that matches the inside curvature of the riser pipe. When the riser pipe has a second buckling bend portion, the system could further comprise a second plurality of self-contained valves configured to allow fluid to pass through the flexible pipe, wherein the second plurality of self-contained valves is positioned to be located at the second buckling bend. The system could also include a pressure control system positioned to create a pressure differential between the riser pipe and the flexible pipe. The upper end of the riser pipe could be connected to a first separator and the upper end of the flexible pipe could be connected to a second separator, wherein the second separator has a lower pressure than the first separator. In use, the lower end of the pipe could be in the horizontal portion of the riser pipe, while the self-contained valves are located in the buckling bend portion. Optionally, the system could also include a spacer or weight positioned to push the inlet device against the lower wall of the riser pipe. The inlet device could have a curved shape to match the inside curvature of the riser pipe. The system could also include a reel for unwinding the flexible pipe in order to extend the flexible pipe towards the riser pipe. According to a second aspect of the invention, a method is provided for the protection of a subsea lift pipe having a horizontal portion on the seabed and a buckling bend portion from liquid overload. The method comprises: providing a flexible pipe having an upper end and a lower end; providing a plurality of self-contained valves between the upper and lower ends, each valve positioned to allow fluid to pass through the flexible pipe; extending the flexible pipe into the lift pipe, such that the lower end is in the horizontal portion of the lift pipe and the self-contained valves are located in the buckling bend portion; and extracting fluid from the lift pipe through the lower end of the pipe and the plurality of self-contained valves. Optionally, before extracting liquid from the riser pipe through the lower end of the pipe and the plurality of self-contained valves, the method further comprises creating a pressure differential between the riser pipe and the flexible pipe. Brief description of the drawings Figure 1 illustrates an example underwater flexible riser pipe system; LZRonn / zznz / E / YiAi Figures 2A and 2B illustrate a riser pipe overload protection system according to one embodiment of the inventions; Figure 4A illustrates a radial cross-section through a riser pipe with an inlet device; Figure 4B illustrates a perspective view of an inlet device in a lift pipe; Figures 5A-D illustrate parts of a riser pipe overload protection system; and Figure 6 illustrates a method according to one embodiment of the invention. Detailed description Figure 1 illustrates a typical 'parallel to S-bend' configuration of a subsea riser pipe, where the riser pipe 10 is bent into an 'S' shape through an anchored buoyancy module 14, between the seabed 16 and a buoyancy installation 12. In this configuration, the riser pipe 10 forms a pig's-bend 18 (inverted 'U' shape) through the buoyancy module 14 and a buckling bend 19 (U-shaped) between the buoyancy module 14 and the buoyancy installation 12. An additional buckling bend 13 is formed between the pig's-bend 18 and the horizontal portion 15 of the riser pipe, which rests on the seabed 16. One problem with flexible riser systems in a gas production system is their potential for liquid accumulation. In the horizontal, near-horizontal, or low portions of the riser, fluid velocity is typically low, and fluids may naturally stratify due to gravity. For example, the flowing oil and / or water phases may separate from the gas phase by gravity in the horizontal seabed portion 15 of the riser. Even if this liquid phase is removed from the horizontal portion 15, fluid stratification may still occur downstream, for example, at and around the local minimum of the buckling curve 19. Therefore, liquid accumulation can occur in multiple different parts of the riser 10, increasing the risk of liquid overload. The inventors realized that the aforementioned problem could be solved by extending a flexible pipe into the riser pipe and allowing fluid to enter the pipe in two different ways: first, through an inlet or orifice at the lower end of the flexible pipe; and second, through a plurality of self-contained valves located along the length of the pipe. Generally, these self-contained valves are self-controlled and can selectively open or close depending on whether fluids come into contact with them. Some self-contained valves utilize the Bernoulli effect, which involves a freely movable body located on a valve seat, positioned with a flow path through the valve, to "reverse" the response compared to a conventional valve. LZRQnn / zznz / E / YiAi Thus, these valves can be designed to allow liquid to pass through, but close when the gas content rises above a predetermined level. Typically, self-contained valves are used in drilling to control the inflow of production fluids, although the inventors have realized they can be used in a different context (i.e., mitigating riser tubing overload) than the one described herein. In particular, the self-contained valves described herein could be configured to allow liquid (e.g., water, oil) to pass through the coiled tubing, but close when the fluid flow is primarily gaseous.The design parameters for a particular self-contained valve with these properties will be known, as such, to the expert person, although the use to solve the problem identified by the inventors is not known to the expert person. Figure 2A shows a riser pipe overload protection system according to one embodiment of the invention, illustrating a portion of a riser pipe 201. The upstream end of the riser pipe 201 could be connected to a hydrocarbon production flow line. Alternatively, both the flow line and the riser pipe could be part of a single tubular structure, whereby the portion of the tubular extending upward to the surface is referred to as the riser pipe. In either case, the upstream end of the riser pipe 201 is located on the seabed, forming a horizontal (or nearly horizontal) portion, and the riser pipe 201 carries fluids in a downstream direction 202 toward a production facility. The riser pipe 201 curves upward from the seabed and across to a pig bend 203 and a buckling bend 204.From buckling bend 204, riser pipe 201 curves upwards towards the surface. The curved 'S' shape could be formed by placing riser pipe 201 on a buoyancy aid or underwater arch (not shown). It can be observed that a local minimum is formed at the bend at buckling bend 204, and around this local minimum, riser pipe 201 is nearly horizontal. As such, stratified flow (rather than, for example, mist flow) could occur in this region. The flexible pipe 210, which, for example, is a coiled pipe, terminates in the horizontal portion of the riser pipe 201 located on the seabed. In other words, the pipe 201 is positioned so that the lower end of the pipe 210 is in the horizontal portion. In the embodiment shown in Figure 2A, an inlet device 212 is connected to the lower end of the pipe 210. The inlet device 212 draws fluid from a stratified fluid flow in the horizontal seabed portion of the riser pipe into the flexible pipe 210. The design of the inlet device 212 is configured to capture the fluid flowing in the downstream direction 202, as discussed in more detail later. A plurality of self-contained valves 220 are provided to allow the fluid to be drawn off at locations along the length of the pipe 210. The self-contained valves could be any suitable type of self-contained valves known in the art, which are configured to LZRQnn / zznz / E / YiAi allow liquid (e.g., water, oil) to pass through, although it closes when the fluid flow is primarily gaseous. In the example shown in Figure 2A, the self-contained valves 220 are positioned to allow liquid to pass through the flexible tubing approximately at the local minimum of the buckling bend 204. In this way, any liquid that accumulates or flows toward the buckling bend 204 can be removed. Self-contained valves 220 may be provided within the walls of a rigid support body 222, as shown in the close-up portion of Figure 2A. In this case, the support body 222 has a tubular shape, and the self-contained valves 220 are evenly distributed along the length and around the circumference of the tubular body. In other embodiments, the self-contained valves 220 are instead provided within the wall of the flexible pipe 210 itself. The portion of flexible pipe 210 that connects the inlet device 212 and the support body 222 is of a suitable length so that when the inlet device 212 is in the horizontal seabed portion of the riser pipe 210, the support body 222 is positioned approximately at the lowest point of the buckling curve. During installation, pipe 210 may be uncoiled or otherwise lowered (the first end of the inlet device 212) down into the riser pipe 201. Figure 2B illustrates an alternative embodiment of the invention, wherein a first group of self-contained valves 220a is provided at the buckling bend 204 (in the same manner as in Figure 2A) and a second group of self-contained valves 220b is provided with a first rising region 205 of the riser pipe 201 (i.e., an additional buckling bend). The first and second rigid support bodies 222a, 222b are provided to support the first and second groups 220a, 220b of the self-contained valves, respectively, as illustrated in the approach portions. In this way, liquid extraction is permitted in two additional regions downstream of the inlet device 212. In some configurations, the system also includes a pressure control system, configured to create a pressure differential between the flexible hose and the riser pipe. This allows the fluid to be drawn from the riser pipe into the flexible hose. For example, the pressure control system might include a container that is maintained at a low pressure, for instance, by means of a regulating valve or a pump, with the upper end of the flexible hose connected to the container. Figure 3 shows the results of a simulation of the system as shown, for example, in Figure 2A, where the pressure P along the pipe and the riser pipe is plotted against the distance D along the riser pipe. In the simulation, the pressure P at the upper end of the pipe is maintained at 30 bar, while the pressure in the riser pipe is approximately 55 bar. The position marked 'X' on the graph corresponds to the location of the lowest point on the buckling curve, as illustrated in the inset graph. As the lower end of the pipe is opened by means of the riser pipe inlet device, it can be observed that the pressure in the At the lower end (i.e., D = 0) of the pipe, the pressure is approximately equal to that of the riser pipe, while the upper end of the pipe is approximately 30 bar. This creates a pressure drop along the pipe and thus a pressure differential between the riser pipe and the main pipe. At point X, a pressure differential ΔP of approximately 8 bar is created. In some configurations, the self-contained valves are configured so that the fluid flow rate allowed through each valve varies with the pressure differential ΔP created across the valve. Thus, if the required liquid extraction rate is known (e.g., in m³ / day) and a given pressure differential ΔP is created at the buckling curve, the number of valves required to achieve the liquid extraction rate can be estimated. Referring again to Figure 3, in the simulation, a liquid mass flow rate of 14 kg / s was assumed to be incident on the inlet device at the lower end of the flexible pipe. The liquid capture efficiency of the inlet device was set to 70%, meaning that the remaining liquid flow rate of 4.2 kg / s reaches the buckling curve at point X.Taking the allowable liquid flow rate through each valve as 15 m³ / day when ΔP = 8 bar (and assuming a density of 1000 kg / m³), approximately 24 independent valves are required in the simulation to extract the remaining liquid. Therefore, this example demonstrates that a relatively small number of valves is sufficient to provide adequate liquid extraction. It should be understood that the above simulation parameters are only by way of example, to illustrate the concept that the appropriate type and number of self-contained valves can be calculated or estimated to achieve the required liquid removal capacity. In the simulation described above, at a differential pressure ΔP of 8 bar, each valve is assumed to have a gas flow capacity of 23 m³ / day – meaning that the total gas flow through the self-contained valves is estimated to be 552 m³ / day. It is important to optimize the number of self-contained valves 220. Too few valves 220 will result in a low liquid removal capacity from the buckling curve. If there are too many valves, the gas flow rate being fed from the buckling curve into the coiled pipe 210 will be too high, as will the liquid being drained from the buckling curve. When the gas velocity is too high, the gas will fill the carrying capacity of the coiled pipe 210, thus reducing the liquid removal capacity of the inlet device 212. In some configurations, a plurality of separators is used to create a pressure drop between the riser pipe and the coiled tubing. Typically, a plurality of separators is used in the hydrocarbon separation stage, where the first-stage separator has the highest pressure and the operating pressure is reduced sequentially in each successive separator. The coiled tubing will be able to perform a suction function if the pressure inside the coiled tubing is lower than the pressure inside the riser pipe. This pressure differential can be achieved by connecting the coiled tubing to a separator that has a lower pressure than the next separator in the series. The LZRQnn / zznz / E / YiAi nearby, to which the riser pipe is connected. In other words, the riser pipe is connected to a first separator, and the flexible pipe is connected to a second separator, where the second separator operates at a lower pressure than the first. For example, the riser pipe section is directly connected to a first-stage separator, and the flexible pipe is connected to a second-stage separator. Advantageously, in this way, the pressure differential can be created in the flexible pipe without requiring any additional equipment for the separators already used in the stage separation process. Preferably, the inlet device (e.g., as shown in Figures 2A and 2B) is in contact with the lower wall of the riser pipe. Typically, as described earlier in the horizontal portion of the riser pipe, the fluids naturally stratify under the influence of gravity, creating a liquid-dominant phase that flows as a film along the bottom of the riser pipe. Therefore, it is preferable for the inlet device to be positioned in contact with the lower wall of the riser pipe in order to draw out this liquid film without capturing the gas-dominant phase flowing above the film. Figures 4A and 4B show a specific example of a suitable inlet device 402, where the shape of the inlet device 402 matches a shape of the inner wall of the riser pipe.Figure 4A illustrates a radial cross-section through a riser pipe 401, in which the inlet device 402 is provided. Figure 4B illustrates a perspective view of the inlet device 402. The downstream end 405 of the inlet device 402 is connected to the flexible pipe (not shown). In the longitudinal direction, the inlet device 402 is tapered, with the widest point at the orifice or inlet and the narrowest point at the end 405 that connects to the flexible pipe. The inlet device 402 could be deflected against the inner wall of the well by gravity. Alternatively, the inlet device 402 could be deflected against the upper inner wall by a spacer, springs, or other deflection means. The lower portion 403 of the inlet device 402 at the upstream end has a curve that matches the curve corresponding to the inside diameter D of the riser pipe. As a result, the lower portion 403 of the inlet device 402 is flush with the inner wall of the riser pipe, so that the liquid phase present in the lower portion of the riser pipe will flow into the inlet device 402. In the example shown in Figures 4A and 4B, the upper portion 404 of the inlet device 402 is curved.Alternatively, in some embodiments, the top is flat or concave to reduce the amount of gas flowing into the inlet device 402. Figures 5A-5D illustrate parts of a riser pipe overload protection system according to one embodiment of the invention. As before, the riser pipe 501 is bent into an 'S' shape, as shown in Figure 5A. As in Figure 2B, two sets of self-contained valves are provided in two different regions of the riser pipe: a first set 531 at the minimum of the buckling bend 502 and a second set 541 at the LZRQnn / zznz / E / YiAi first rising section of the riser pipe 501. The first group of self-contained valves 531 is provided within a first rigid support body 533, as shown in Figure 50. Similarly, the second group of self-contained valves 541 is provided within a second rigid support body 543, as shown in Figure 5D. The inlet device 520 is shown in greater detail in Figure 5B. In this example, each of the rigid support bodies 533, 543 and the inlet device 520 has a curved cross-sectional shape, which is coincident with the curvature of the inner wall of the riser pipe. If the pig bend is formed through a buoyancy or submersible arch of relatively small diameter, the curvature of the riser pipe 501 at the pig bend could be large. In this case, when the flexible pipe is passed through the pig bend, the pipe could be permanently deformed. The inventors have realized that the solution to this problem is to use multiple lengths of a smaller-diameter pipe 505 to connect the inlet device 520 to the first support body 531 and the first support body 531 to the second support body 543. In Figures 5A-D, three lengths of a smaller-diameter pipe 505 (e.g., 5.08 cm (2 in.)) connect the inlet device 520 to the second support body 543. Similarly, three lengths of the smaller-diameter pipe 507 connect the second support body 543 to the first support body 533.(Three bundles of pipe have been illustrated as a single line in Figure 5A for clarity.) The downstream end of the first support body 533 is connected to a larger diameter pipe 509 (e.g., 8.89 cm (3.5 in.)). In this way, the total cross-sectional area of ​​the flexible pipe at each point along the length of the riser pipe is kept approximately the same, although the problem of pipe deformation is mitigated. Figure 6 shows a high-level flow diagram describing a method for protecting a riser pipe against surges or overloads of liquid according to the invention. The method comprises providing a flexible pipe having an upper end and a lower end (step 601); providing a plurality of self-contained valves between the upper and lower ends, each valve positioned to allow liquid to pass through the flexible pipe (step 602); extending the flexible pipe into the riser pipe such that the lower end is in the horizontal portion of the riser pipe and the self-contained valves are located in the buckling bend portion (step 603); and drawing liquid from the riser pipe through the lower end of the pipe and the plurality of self-contained valves (step 604). Although the invention has been described in terms of the preferred embodiments noted above, it should be understood that these embodiments are merely illustrative and that the claims are not limited to them. Persons skilled in the art will be able to derive modifications and alternatives in light of disclosure, which are contemplated to fall within the scope of the appended claims. Each feature described or illustrated herein may be incorporated into the invention, either alone or in any combination. LZRQnn / zznz / E / YiAi suitable with any other feature described or illustrated herein.

Claims

1. A system for protecting a subsea lift pipe from liquid overload, having a horizontal portion on the seabed and a buckling bend portion, the system comprising: a flexible pipe having an upper end and a lower end; a plurality of self-contained valves configured to allow liquid to pass through the flexible pipe; wherein the self-contained valves are positioned between the upper and lower ends of the pipe; and further comprising an inlet device coupled to the lower end of the flexible pipe, wherein the inlet device is deflected against a lower wall of the lift pipe.

2. The system according to claim 1, wherein the plurality of self-contained valves is placed in a plurality of locations along the circumferential direction of the flexible pipe.

3. The system according to claim 1 or 2, wherein the plurality of self-contained valves is placed in a plurality of locations along the longitudinal direction of the flexible pipe.

4. The system according to any preceding claim, wherein the plurality of self-contained valves is provided within a wall of the flexible pipe.

5. The system according to any of claims 1-3, wherein the plurality of self-contained valves is provided within a wall of one or more rigid support bodies coupled with the flexible pipe.

6. The system according to claim 5, wherein each of one or more of the rigid support bodies has a curved shape that matches the inner curvature of the riser pipe.

7. The system according to any preceding claim, wherein the riser pipe has a second buckling bend portion and wherein the system further comprises a second plurality of self-contained valves configured to allow fluid to pass through the flexible pipe, wherein the second plurality of self-contained valves is positioned to be located in the second buckling bend.

8. The system according to any preceding claim, further comprising a pressure control system positioned to create a pressure differential between the riser pipe and the flexible pipe.

9. The system according to any preceding claim, wherein an upper end of the riser pipe is connected to a first separator and the upper end of the flexible pipe is connected to a second separator, and wherein the second separator has a lower pressure than the first separator.

10. The system according to any preceding claim, wherein in use the lower end LZRQnn / zznz / E / YiAi 1 1 of the pipe is in the horizontal portion of the riser pipe and the self-contained valves are located in the buckling bend portion.

11. The system according to any preceding claim, further comprising a spacer or weight positioned to push the inlet device against the lower wall of the lift pipe.

12. The system according to any preceding claim, wherein the inlet device has a curved shape that matches the inside curvature of the riser pipe.

13. The system according to any preceding claim, further comprising a reel for unwinding the flexible pipe in order to extend the flexible pipe towards the riser pipe.

14. A method for protecting a subsea riser pipe from liquid overload, the method comprising: providing a flexible pipe having an upper end and a lower end; providing a plurality of self-contained valves between the upper and lower ends, each valve positioned to allow fluid to pass through the flexible pipe; extending the flexible pipe into the riser pipe, such that the lower end is in the horizontal portion of the riser pipe and the self-contained valves are located in the buckling bend portion; and drawing fluid from the riser pipe through the lower end of the pipe and the plurality of self-contained valves.

15. The method according to claim 14, wherein before extracting liquid from the riser pipe through the lower end of the pipe and the plurality of self-contained valves, the method further comprises: creating a pressure differential between the riser pipe and the flexible pipe.

Images