NO341257B1 - Arrangements for flow assurance in a subsea flowline system - Google Patents

Description

Arrangements for flow assurance in a subsea flowline system

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to flow assurance in subsea flowline systems. The invention particularly relates to arrangements for chemicals injection and pigging of flowlines utilized for transport of hydrocarbon production fluids and for fluid injection (water and gas) in subsea wells.

BACKGROUND AND PRIOR ART

In offshore oil or gas production, particularly in deepwater developments, fluid issues need constant attention in order to ensure unhindered transport in flowlines and through other system components such as valves and chokes. To this end, injection of chemicals in the produced fluid is widely used to prevent the formation of hydrates, wax or asphalt that may otherwise deposit and hinder the passage of fluid through the components of the system. Chemicals may also be used for other purpose, and some of the chemicals which can be added to the produced fluid are corrosion inhibitors, hydrate and paraffin inhibitors such as ethanol, methanol, or glycol or diesel, e.g. Injection of chemicals can be made on a continuous basis or on a frequent basis such as in workover operations.

Chemicals for injection are typically supplied from a topside platform via the umbilical. Injection chemicals can this way be supplied to a valve tree installed on the wellhead. The valve tree, also called Christmas tree (XT or X-mas tree) is arranged to direct the produced fluid from the well to the flowline, and comprises the necessary valves and a control module to regulate the flows of produced fluid and injection fluid. For flow assurance in flowlines and for downhole intervention, chemicals may be routed into the flowline or into the annulus that surrounds the production tubing in the well bore.

In production systems, pigging is a method which is frequently used for removing hydrate and wax that has formed deposits on the inter nal wall of the flowline. In systems operating at conditions where pigging is necessary the flowline is connected to a valve via which a mechanical plug, called pig, can be launched into the flowline for further transport with the produced fluid while scraping deposits from the flowline wall. When pigging is applied to the X-mas tree loop the tree must be connected to two flowlines, production and annulus, with similar nominal sizes, to allow the pig round trip.

Chemical injection and pigging may be combined and used simultaneously to provide dissolving of heavy deposits. In a production system, a pig launching valve may be installed in a flowline that connects a production flowline with a chemical injection system on a X-mas tree.

Fluid injection, commonly water or gas, into a field through specific injection wells is a common method used to stimulate the production. A particular technology uses alternating injection of water and gas through the same well which is commonly called WAG (water alternate gas). The control of this process might be, as usual, done by a manifold which alternate the fluid injected. As an alternative, the injection might be controlled directly by the X-mas tree. The tree shall be served by two flowlines, both with permanent supply of fluid, one with gas and the other with water. The tree valves are used, in this case, to control which fluid is injected at a time. Technical concerns might arise with possible hydrate formation caused by minor leakages of gas (or water) which migrate to the water (or gas) flowline environment.

Among the prior art methods and devices for managing hydrate formation in subsea production systems in the patent literature, the following patent publications can be mentioned.

US 2005/217855 Al describes a method and a device for moving a hydrate plug from a pipeline, wherein a pig, which is connected to an umbilical extending to surface, is displaced down into the pipeline until proximity of the hydrate plug, after which a fluid arranged with hydrate-plug- dissolving properties is pumped through the umbilical.

WO 2009/042319 Al describes a method for managing hydrates in a subsea production system. The production system includes a host production facility, a control umbilical, at least one subsea production well, and a single production line. The method generally comprises producing hydrocarbon fluids from the at least one subsea production well and through the production line, and then shutting in the production line. In addition, the method includes the steps of depressurizing the production line to substantially reduce a solution gas concentration in the production hydrocarbon fluids, and then re-pressurizing the production to urge any remaining gas in the free gas phase within the production line back into solution. The method also includes displacing production fluids within the production line by moving displacement fluids from a service line within the umbilical line and into the production line.

WO 2009/042307 Al describes methods for managing hydrate formation in subsea equipment, such as a production line, of a subsea production system. The methods comprise placing a pig in the subsea production system, shutting in production from producers, and injecting a displacement fluid into the system in order to displace a hydrated inhibitor and any remaining production fluids in the production flow line and to further move the pig through the production flow line.

SUMMARY OF THE INVENTION

A general object of the present invention is to achieve flow assurance in subsea hydrocarbon production systems.

It is a special object of the present invention to provide flow assurance via combined chemicals injection and pigging through flowlines and manifolds in a hydrocarbon production system.

It is another object of the present invention to arrange a subsea X-mas tree for chemicals injection and pigging while providing a reliable barrier between an internal chemical injection system and the production fluid route through the X-mas tree.

An additional object of the present invention is to provide, when required, an effective isolation chamber between flowline fluids. This is particularly important in case of flowlines working in WAG injection X-mas tree and WAG manifolds, reducing the hydrate formation risk.

The overall object is met in a subsea flowline system for transport of hydrocarbon production fluid with a chemicals injection and pigging arrangement comprising a crosslink that connects to the flowline system via first and second access points. An isolated crosslink section is defined between first and second pig launching valves that are installed on the crosslink, wherein the isolated crosslink section is connected to a chemicals injection system via an isolation and injection valve.

In one embodiment of the flowline system the chemicals injection system comprises a piston or bladder type accumulator in communication with the isolated crosslink section via the isolation/injection valve.

The accumulator is preferably arranged to be actuated by a remotely operated vehicle (ROV) for discharge of its content of injection chemicals.

The accumulator may be arranged to be filled with injection chemicals by an ROV via a hot stab interface.

The accumulator is in another embodiment arranged to be filled with injection chemicals via an umbilical interface serving the flowline system.

The chemicals injection and pigging arrangement advantageously pro vides a dual barrier in a one way communication from the annulus side to the production side of a X-mas tree.

In one embodiment the flowline system comprises a production bore and an annulus bore that pass vertically through a X-mas tree body, wherein on a crosslink that connects the annulus with a production flowline downstream of a production wing valve on the X-mas tree two pig launching valves are installed in series, each valve defining an end respectively of an isolated crosslink section connected to a chemicals injection system.

In another embodiment the flowline system comprises a production bore that pass horizontally through a X-mas tree body, wherein on a crosslink that connects the annulus with a production flowline downstream of a production wing valve on the X-mas tree two pig launching valves are installed in series, each valve defining an end respectively of an isolated crosslink section connected to a chemicals injection system.

In yet another embodiment of the subsea flowline system, the chemicals injection and pigging arrangement is installed between a manifold export line and a flow assurance flowline section in the flowline system.

SHORT DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be explained in more detail below with reference made to the accompanying drawings, wherein

Fig. 1 is a schematic view showing the invention installed on a subsea vertical X-mas tree, Fig. 2 is a schematic view showing the invention installed on a subsea horizontal X-mas tree, and Fig. 3 is a corresponding schematic view showing the invention installed on an arbitrary subsea flowline system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In Fig. 1 a X-mas tree 1 is shown schematically supported on a wellhead 2 that forms an upper part of a completion to a subsea hydrocarbon well. The X-mas tree 1 comprises a tree body 3 adjoining a tubing hanger 4 which is landed on the wellhead and from which is suspended a production tubing 5. A casing 6 is arranged in concentric relation about the production tubing 5, forming an annulus 7 that provides protection and which serves for downhole interventions. A safety valve 8 may be controlled from topside to cut flow through the production tubing 5. The production tubing 5 and annulus 7 continue vertically through the tree body in a production bore 9 and an annulus bore 10 respectively, the bores 9 and 10 terminated below a tree cap 11. An annulus master valve 12 and an annulus swab valve 13 are stacked vertically on the tree body 2. In similar way the production master valve 14 and production swab valve 15 are vertically stacked on the tree body.

In the vertical X-mas tree 1 production fluid is routed from the production bore 9 via a tree flowline 16 and a production wing valve 17 into the flowline 18 for export. In a similar way, access to the annulus 7 for pressure compensation, bleeding and chemicals injection is arranged via a tree flowline 19 and an annulus workover valve or wing valve 20. The annulus wing valve 20 may be arranged to discharge into the flowline 21 or to other external recipient for bleeding fluid and for pressure compensation of the annulus 7.

In the X-mas tree 1 the annulus and production flowlines are connectable via a crossover line 22 and a crossover valve 23 which provide fluid flow communication in one direction from the production side to the annulus side in the open state of the crossover valve 23. The crossover line 22 connects to the passages 16 and 19 upstream of the production and annulus wing valves 17 and 20 respectively.

In X-mas tree WAG mode for alternated injection of water and gas in the well, the arrangement uses the external flowlines 18 and 21 to supply water and gas.

In one injection step, the fluid of flowline 18 is released to flow via the tree flowline 16 and the production bore 9 maintaining the production wing valve 17 and the production master valve 14 open. The other injection path shall be blocked and the annulus wing valve 20 and the crossover valve 23 shall be maintained closed.

In the other injection step, a different valve arrangement shall be used. The fluid of flowline 21 flows through the tree flowline 19 with annulus wing valve 20 open. The crossover valve 23 maintained open allows the flow though the crossover line 22 into the production bore 9 with the production wing valve 17 closed and the production master valve 14 opened.

A chemicals injection and pigging arrangement 100 is installed on the X-mas tree 1. In the embodiment of Fig. 1 a crosslink 101 is arranged for flow in one direction, from a first access point 102 providing communication with the annulus flowline 21, to a second access point 103 providing communication with the production flowline 18. More precisely the crosslink 101 is arranged for discharge into the flowline 18 downstream of the production wing valve 17. An isolated crosslink section 104 is defined between first and second pig launching valves 105 and 106. The valve 105 and the valve 106 are thus both arranged to receive and to insert into the crosslink 101 a pig (not shown) which can be launched into the flowline 18 at access point 103 and from there driven by production fluid for deposits removal and flow assurance.

The dual pig launching valves 105 and 106 provide a double fluid and pressure barrier in the crosslink 101 between the annulus and production sides of the tree.

The isolated crosslink section 104 is connectable to a chemicals injection system via an isolation and injection valve 107. The crosslink section 104 can this way be charged with a volume of injection chemicals that can be discharged into the flowline 18 or 21, optionally in conjunction with the release of one or two pigs. Additionally, the isolated crosslink section 104 charged with a volume of appropriate injection chemicals will provide an effective barrier to the hydrate formation when the flowlines 18 and 21 are filled with different fluids (water and gas) as occur in X-mas tree WAG mode.

Filling up the isolated crosslink section 104 can be accomplished from topside via the umbilical and a chemicals supply line 108 connecting to an umbilical interface 109. Filling and injection of chemicals can alternatively be accomplished via a (piston or bladder type) accumulator 110 to which the injection chemical can be supplied via the umbilical and a group of check valves 111, 112 and 113 that control the flow directions in the chemicals injection system. The storing capacity of the accumulator is preferably at least equal to the storing capacity of the isolated crosslink section 104.

The accumulator 110 may be arranged for operation by an ROV (Remotely Operated Vehicle) for release of an amount of injection chemicals stored in the accumulator. To that purpose a hot stab interface 114 may be arranged on the X-mas tree for hydraulic activation of the accumulator 110. The hot stab interface 114 may additionally be used for supply of injection chemicals carried on the ROV.

Wiring for monitoring and operation control of the system of Fig. 1, such as high and low pressure hydraulic supply, electricity for power and communication, pressure and temperature transmitters etc, is omitted for clarity reasons, however, operation control is schematically represented in the drawing by the subsea control module 24.

In Fig. 2 a horizontal X-mas tree 1' is shown schematically supported on a wellhead 2' that forms an upper part of a completion to a subsea hydrocarbon well. The X-mas tree 1' comprises a tree body 3' incorporating a tubing hanger 4' from which is suspended a production tubing 5'. A casing 6' is arranged in concentric relation about the production tubing 5', forming an annulus 7' that provides protection and which serves for downhole interventions. A safety valve 8' may be controlled from topside to cut flow through the production tubing 5'. Production fluid is lead to the exterior of the tree body 3' via a horizontal production bore 9'. A plug and a debris cap 11' terminates the upper end of the X-mas tree body 3'.

The annulus 7' communicates to the exterior of tree body 3' via an annulus master valve 12', an annulus workover valve 13' and an annulus isolation valve 20'. The annulus isolation valve 20' may be arranged to discharge into the flowline 21' or to other external recipient for bleeding fluid and for pressure compensation of the annulus 7'.

In the X-mas tree 1' the production bore 9' is connectable to the flowline 18' via production master valve 14' and a production wing valve 17'.

The production side is connectable to the annulus side of the X-mas tree 1' via a crossover line 22' and a crossover valve 23' which provide fluid flow communication in one direction in the open state of the crossover valve 23'. The crossover line 22' is connected to the production and annulus sides respectively upstream of the production wing valve 17' and the annulus isolation valve 20'.

In X-mas tree WAG mode for alternated injection of water and gas in the well, this arrangement uses the external flowlines 18' and 21' to supply water and gas.

In one injection step, the fluid of flowline 18' is released to flow via the tree horizontal production bore 9' maintaining the production wing valve 17' and the production master valve 14' open. The other injection path shall be blocked and the annulus isolation valve 20' and the crossover valve 23' shall be maintained closed. In the other injection step, a different valve arrangement shall be used. The fluid of flowline 21' flows through crossover line 22' with the annulus isolation valve 20' and the crossover valve 23' open. With the production wing valve 17' closed and the production master valve 14' opened, the flow goes though the horizontal production bore 9'.

Similar to the embodiment of Fig. 1 the X-mas tree 1' comprises a chemicals injection and pigging arrangement 100 as disclosed above with reference to Fig. 1. In the embodiment of Fig. 2 a crosslink 101 is arranged for flow in one direction, from a first access point 102 providing communication with the annulus to a second access point 103 providing communication with the production flowline 18'. More precisely the crosslink 101 is arranged for discharge into the flowline 18' downstream of the production wing valve 17'. An isolated crosslink section 104 is defined between first and second pig launching valves 105 and 106. The valve 105 and the valve 106 are thus both arranged to receive and to insert into the crosslink 101a pig (not shown) which can be launched into the flowline 21' at access point 102 and from there driven by production fluid for deposits removal and flow assurance. For more details on the chemicals injection and pigging arrangement reference is made to the description of Fig. 1.

In the embodiment of Fig. 3 the combined chemicals injection and pigging arrangement 100 is shown installed in an arbitrary flowline system 200 applied for transport of hydrocarbon product from a subsea well, or for transport of injection fluids to a subsea well. In the drawing of Fig. 3, the upstream flowline sections 201 and 203 may be the export lines from a subsea manifold arrangement to be connected to risers, PLETs, or other adjoining flowline section. The branch sections 202 and 204 are part of the manifold and will lead directly or indirectly to a connection to the X-mas trees.

The flowline system 200 comprises a chemicals injection and pigging arrangement 100 as disclosed above with reference to Fig. 1. In the embodiment of Fig. 3 the chemicals injection and pigging arrangement 100 is connected to the flowline system 200 in an upstream end of the flow assurance flowline section 202. A crosslink 101 forms a connection between a first access point 102 and a second access point 103 to the flowline system 200. An isolated crosslink section 104 is defined between first and second isolation valves 105 and 106. For more details on the chemicals injection and pigging arrangement reference is made to the description of Fig. 1.

Generally speaking the embodiments of Figs. 1-3 each comprises a chemicals injection and pigging arrangement which can be installed on a subsea flowline system via a crosslink that connects to the flowline system via first and second access points. The access points can be located to provide communication between different sides of the flowline system, such as between annulus and production sides on X-mas trees, or between an upstream location and a downstream location of an arbitrary flowline system. The chemicals injection and pigging system can be installed on vertical X-mas trees, on horizontal X-mas trees, in connection with manifolds or on arbitrary sections of a subsea flowline system applied in hydrocarbon production and transport.

Above three embodiments have been disclosed, all of them incorporating substantially identical layouts of the chemicals injection and pigging arrangement for flow assurance in subsea flowline systems. The claims attached define the presented solution in the general terms that include the embodiments disclosed as well as all other embodiments which can be conceived from the present disclosure.

Claims (9)

1. A subsea flowline system (1, 1', 200) for transport of hydrocarbon production fluid with a chemicals injection and pigging arrangement (100) comprising: - a crosslink (101) that connects to the flowline system via first and second access points (102, 103), - an isolated crosslink section (104) defined between first and second pig launching valves (105, 106) installed on the crosslink (101), wherein - the isolated crosslink section (104) is connected to a chemicals injection system via an isolation/injection valve (107).

2. The flowline system of claim 1, wherein the chemicals injection system comprises an accumulator (110) in communication with the isolated crosslink section (104) via the isolation/injection valve (107).

3. The flowline system of claim 2, wherein the accumulator (110) is arranged to be actuated by a remotely operated vehicle (ROV) for discharge of its content of injection chemicals.

4. The flowline system of claim 2 or 3, wherein the accumulator (110) is arranged to be filled with injection chemicals by an ROV via a hot stab interface (114).

5. The flowline system of any of claims 2-3, wherein the accumulator (110) is arranged to be filled with injection chemicals via an umbilical interface (109) serving the flowline system (1, 1', 200).

6. The flowline system of any previous claim, wherein the chemicals injection and pigging arrangement (100) provides a dual barrier (105, 106) in a one way communication from the annulus side (7, T) to the production side (9, 9^ of a X-mas tree (1, V).

7. The flowline system of claim 6, comprising a production bore (9) and an annulus bore (7) that pass vertically through a X-mas tree body (3), wherein on a crosslink (101) that connects the annulus (7) with a production flowline (18) downstream of a production wing valve (17) on the X-mas tree (1) two pig launching valves (105, 106) are installed in series, each valve (105, 106) defining an end respectively of an isolated crosslink section (104) connected to a chemicals injection system.

8. The flowline system of claim 6, comprising a production bore (9^ that pass horizontally through a X-mas tree body ( 3% wherein on a crosslink (101) that connects the annulus (7^ with a production flowline ( 18r) downstream of a production wing valve ( 17r) on the X-mas tree ( lr) two pig launching valves (105, 106) are installed in series, each valve (105, 106) defining an end respectively of an isolated crosslink section (104) connected to a chemicals injection system.

9. The flowline system of any of claims 1-5, wherein the chemicals injection and pigging arrangement (100) is installed between a manifold export line (201) and a flow assurance flowline section (202) in the flowline system (200).