WO2008056248A2

WO2008056248A2 - A method and a system for hydrocarbon production cooling

Info

Publication number: WO2008056248A2
Application number: PCT/IB2007/003433
Authority: WO
Inventors: John Daniel Friedemann; Eric Smedstad; Kunal Dutta-Roy
Original assignee: Vetco Gray Scandinavia As
Priority date: 2006-11-09
Filing date: 2007-11-09
Publication date: 2008-05-15
Also published as: NO20092219L; GB2456953A; GB2456952A; WO2008056252A2; GB0908290D0; WO2008056252A3; BRPI0718664A2; NO20092218L; RU2009120140A; GB2456954A; GB0908292D0; GB0908291D0; WO2008056250A3; WO2008056250A2; NO20092220L; BRPI0718663A2; RU2009120139A; RU2009120141A; WO2008056248A3

Abstract

A method for cooling a subsea production fluid of hydrocarbon product from above a solids formation temperature for further transport below the said temperature as slurry, comprising the steps of feeding the production fluid through a lumen (8) by which heat is transferred through the lumen wall (9') to an ambient cooling medium for precipitation of material dissolved in the production fluid, as well as the step of dislodging any attaching solid matter from the lumen wall by means of a runner (12) passing with the production fluid flow through the lumen. The production fluid is initially cooled to a temperature still above the solids formation temperature at a location upstream of said lumen (81). The invention also refers to a subsea hydrocarbon production fluid cooling system, comprising a passive unit (2) and an active unit (3) arranged in succession in the production fluid flow, wherein the passive unit (2) comprises a first lumen (8) and the active unit comprises a second lumen (8') arranged to receive the pre-cooled production fluid from the first lumen of the passive unit, and to discharge the same as slurry comprising solids entrained in the production fluid.

Description

A METHOD AND A SYSTEM FOR HYDROCARBON PRODUCTION COOLING

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method and to a system for conversion of a sub- sea hydrocarbon production fluid from above a solids formation temperature to below the said temperature for further transport through a flowline in the form of slurry.

BACKGROUND AND PRIOR ART In long distance subsea production and flowline transport of hydrocarbon products, one of the most challenging issues is related to the problem of cooling of production fluids to within regions of behaviour which are characterized by solids formation and crystallization on the production tubing. The solids may be hydrates which are formed as a mixture of gas and water is cooled under pressure, or wax, asphalte- nes, organic and inorganic salts which are dissolved in the production fluid at production temperature and which precipitate below that temperature or pressure. Obviously, uncontrolled agglomeration and deposition of solids on the tubing interior successively result in reduced flow.

Several technologies have been developed to either heat the flow system or to insulate the flow system, and by this way keeping the combination of temperature and pressure in the production fluid to a region at which said solids formation is either avoided or kept to a minimal level.

A third technology is to accept the heat and pressure loss and to control the process. This solution can generally be referred to as "cold or sub-cooled flow technology". In cold flow solutions, method and apparatus are provided by which the production fluid is cooled to a solids formation temperature at a location from where the production fluid is further transported as slurry at the lower temperatures.

Prior art contains several examples on this approach to the problem. Relevant background art may be found in US 6,070,417, e.g., which discloses a method that is carried out by means of an apparatus defining a continuous lumen. The lumen has a thermally conductive wall from which heat is extracted from outside, as the pro- duction fluid flows through the lumen, resulting in formation of solid material attaching on the inner surface of the lumen wall. A flexible runner circulates with the production fluid in the lumen and dislodges material deposited on the inner surface of the lumen wall. The material is then carried as solids in slurry out from the lumen, together with the production fluid. The lumen is contained in a heat ex- changer containment through which a coolant medium is circulated in order to lower the temperature of the production fluid to a solids formation temperature. Drawbacks in prior art solutions are complex installation and unstable or unsatisfactory operation. None of the prior art solutions have paid significant attention to installation ability and operability of a sub-sea system. The cold flow technology essentially protects a flowline downstream of a cold flow device. Flow upstream of the cold flow device is still susceptible of flow assurance issues such as wax and hydrates. For this reason, the flowlines upstream of the cold flow device must be insulated and/ or treated, as is usual practise today.

SUMMARY OF THE INVENTION An object of the invention is therefore to provide a method and a system that improve the cold-flow technology through simplified installation, reduced installation costs, and enhanced and steady operation of a slurry formation system and method.

The object is achieved by a method for cooling a subsea production fluid of hydrocarbon product from above a solids formation temperature for further transport below the said temperature as slurry, comprising the steps of feeding the production fluid through a lumen by which heat is transferred through the lumen wall to an ambient cooling medium, such as sea water, for precipitation of material dissolved in the production fluid, as well as the step of dislodging any attaching solid matter from the lumen wall by means of a runner passing with the production fluid flow through said lumen. According to the invention, the production fluid is initially cooled to a temperature still above the solids formation temperature at a location upstream of said lumen.

The initial cooling step provides more efficient control of hydrate and /or wax formation in a temperature transition zone. The initial cooling also permits reduced dimensions and a more compact design of structures operating in the transition zone, as the amount of energy to be transferred from the production fluid to a cooling medium in the transition zone is likewise reduced in result of the preceding cooling step.

The method further comprises the step of feeding the production fluid through a passive unit comprising a first lumen, as viewed in the flow direction of production fluid, the first lumen preferably having no runner passing there through.

Advantageously, the method further comprises the step of feeding the production fluid through the first lumen in the form of a helix configuration arranged in the passive unit.

In a preferred embodiment, the method includes the step of feeding the pre-cooled production fluid from the passive unit through a second lumen in the form of a he- lix configuration arranged in an active unit located downstream of the passive unit, and running a flexible runner through the second lumen of the active unit.

The object is also achieved by a subsea hydrocarbon production fluid cooling sys- tern comprising a passive unit and an active unit arranged in succession in the production fluid flow, wherein the passive unit comprises a first lumen having an inlet in flow communication with a hydrocarbon product flowline, said first lumen arranged to receive the production fluid at a temperature above a solids formation temperature and to discharge the same at a lower temperature above the solids formation temperature via an outlet, said outlet from the first lumen being in flow communication with an inlet to a second lumen arranged in the active unit, from which second lumen the production fluid is discharged as slurry via an outlet in flow communication with the flowline downstream of the active unit.

In the preferred embodiment, at least one of the lumens of the passive and active unit has a helix configuration. A helical lumen of the active unit is preferably arranged for the passage of a flexible runner there through.

In advantageous embodiments, the helical lumen of the passive and/ or the active unit is arranged vertically on a substructure. The helical lumen of the passive and active units may be stacked vertically above each other in a substructure. The substructure, supporting the passive and/ or active units, can be arranged on the sea bed.

Because solids formation may also be enhanced by pressure loss, the flow related pressure loss in the device can be controlled in such a manner that the combined pressure and temperature losses are controlled to minimize the rate of formation of the formed solids.

One or both of the passive and active units may be encased in a housing, containing a cooling medium.

Permanently mounted devices may further be arranged to provide external circulation of cooling medium surrounding the lumen/ lumens. The system may further be configured to allow access such as for cleaning purposes.

The piping comprised in the system may further advantageously be coated in view of increasing heat transfer to the ambient and to reduce accumulation of fouling material from the surrounding environment. SHORT DESCRIPTION OF THE DRAWINGS

In the following, the invention will be more fully described in connection with the accompanying drawings, schematically illustrating preferred embodiments of the invention. In the drawings,

Fig. 1 shows the set up of cooperating passive and active units in a system according to the invention;

Fig. 2 shows in a side view a preferred embodiment of a cooling loop comprised in the passive and/ or active units, and

Fig. 3 illustrates schematically the cooling process including passive and active steps according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the drawings, reference number 1 indicates a cut out portion of a flowline 1 through which a produced hydrocarbon fluid is transported in a flow direction F from a production site located upstream, i.e. at the left hand side of the drawing, towards a sea-based or land-based host plant located downstream, at the right hand side of the drawing. According to the invention, the production fluid is cooled in steps from a production temperature at which hydrates and/ or wax are dissolved in the fluid, to a lower temperature at which the hydrates and/ or wax precipitates into solid matter entrained in the fluid. The stepped cooling process relies on a passive unit 2 in cooperation with an active unit 3 located in series therewith and downstream. Each unit 2, 3 comprises a cooling loop including a continuous lumen defined by a tubular wall. The successive passive and active units 2 and 3 are preferably located as close as will be practical to a production site or a manifold connecting to a number of hydrocarbon product wells. Albeit the passive and active units are shown as separated in the drawing, a distance between the units may be reduced to a minimum. Each of the passive and active units may be individually enclosed in a separate housing 4, or they may be together enclosed in a common housing containing a cooling medium. Alternatively, the passive and active units 2 and 3 may be exposed to the ambient sea-water.

At the passive unit 2, the production fluid having a temperature above a solids formation temperature enters via an inlet 5, passes a cooling loop 6 and exits via an outlet 7 at a reduced temperature which is still above the solids formation temperature. The production fluid exiting the outlet 7 of the passive unit is thus still characterized by hydrates and/ or wax dissolved in the fluid. The cooling loop 6 com- prises a lumen 8 which is defined by a tube-shaped lumen wall 9, through which heat is transferred from the production fluid to an ambient coolant, such as sea- water, illustrated in the drawing through the arrow H. Valves 10 are controllable for bypass of production fluid and for isolation of the cooling loop 6. Controls and monitoring devices 11 may be enclosed in a separate capsule and connected to a remote operator's control via power and signal wires (not shown).

At the active unit 3, the production fluid which exits from the passive unit at a temperature above a solids formation temperature enters via an inlet 5', passes a cooling loop 6' and exits as slurry via an outlet 7' at a temperature which is below the solids formation temperature. The production fluid which enters the inlet 5' of the active unit thus contain hydrates and/ or wax dissolved in the fluid, which is suc- cessively cooled to below the solids formation temperature in the active unit to exit as slurry via the outlet 7' of the active unit. Corresponding to the passive unit 2, the active unit 3 comprises a lumen 8' which is defined by a tube-shaped lumen wall 9', through which heat is transferred from the production fluid to an ambient coolant, such as sea- water, illustrated in the drawing through the arrow H. Valves 10' are controllable for bypass of production fluid and for isolation of the cooling loop 6'. Controls and monitoring devices 11 ' may be enclosed in a separate capsule and connected to a remote operator's control via power and signal wires (not shown).

In contrast to the passive unit 2, the active unit 3 has a movable pig or runner 12 circulating with the production fluid inside lumen 8'. Thus in the active unit, the temperature of the production fluid is reduced below a hydrate and/ or wax formation temperature, eventually resulting in solids attaching to the lumen wall. As is known in the art, the runner is effective for dislodging any attaching solid matter from the lumen wall. To this purpose, a flexible runner 12 is preferred. A flexible runner may be formed with radial flanges supported from a flexible stem, the flanges being operative for scraping the inside of the lumen wall upon passage of the runner through the lumen. Gate valves through the lumen wall for entrance and exit of the runner are known "in the art, and indicated in the drawing at 15.

In a production fluid cooling system, one or more passive and/ or active units may be arranged in the flow of production fluid, if appropriate. Each of the passive and active units 2, 3 may comprise a valve and control block and the associated cooling loop/loops mounted on a substructure. The cooling loops may be placed directly on the seabed, and equipped with appropriate means and controls meeting with shut- in inhibition requirements.

With reference to fig. 2, an advantageous embodiment of a cooling loop and lumen in a stepped hydrocarbon product cooling system according to an embodiment of the invention will be explained. A cooling loop 6, 6' of helix configuration is sup- ported from a sub-structure 13. The sub-structure 13 and the cooling loop 6, 6' may be seated on the sea bed and open to the ambient sea-water, or may be enclosed in a housing (not shown) containing a cooling medium into which heat is transferred from the production fluid through the helical wall 9, 9', said wall defining a helical lumen for passage of the production fluid. The helical loop 6, 6' may be supported in vertical orientation as illustrated. The vertical orientation will provide the advantage of drainage control and simplified inhibition in case of shut-ins. Al- ternatively, the sub-structure may be arranged for a horizontal support of the helical cooling loop. In both cases, columns 14 may be comprised in the sub-structure for fixation of the cooling loop.

The helical loop 6, 6' is connected to the flowline 1 via inlet and outlet piping, equipped with corresponding bypass and isolation valves 10" substantially as discussed above. Control and monitoring equipment 11" would typically be correspondingly supported on the sub-structure 13. The necessary wiring and connectors for power supply and control can be composed of proven sub-sea equipment known to the skilled person.

The cooling loop of helical configuration may advantageously be applied to one or both of the passive and active units 2 and/ or 3, respectively. The set up includes the units arranged in succession, horizontally side by side, or vertically above each other.

In the helical cooling loop 6' comprised in the active unit 3, a flexible runner 12 is insertable through adequate gates for passage through the helical lumen together with the production fluid. Suitable structure of such gate is already known in the art and need no further explanation. Gates are advantageously duplicated to make possible the use of backup runners, if appropriate.

The passive and active units 2, 3 are thus arranged in succession, and operative for a stepped cooling of the produced hydrocarbon fluid as is schematically illustrated in fig. 3. Apparent from the drawing, the passive unit 2 is located on the upstream side of a temperature window representing a hydrate and/ or wax transition zone, wherein the fluid is cooled in the active unit 3 to below a hydrate and/ or wax formation temperature.

Dimensioning of the system and cooling loops 6, 6' of the passive and active units is a matter of design with respect to production parameters, such as composition and temperature of the production fluid, depth of sea, piping dimensions, etc. The calculations that would be required for the necessary transfer of heat energy from the production fluid to the ambient cooling medium are familiar to the skilled person. The first step of the cooling system will thus mainly depend on the length and di- ameter of the cooling loop 6, defining the first lumen 8 of the passive unit 2. For an estimate of the building space required and the size of structures suggested herein, the piping of cooling loops 6, 6' may have a diameter of 6, 8 or 10 inches, e.g. The length of the cooling loop 6', defining the second lumen 8' of the active unit 3, may in this example be in the order of about 250-500 m. The stack height in a vertical orientation of the helical loop may be estimated to, e.g., 15 times the inner diameter of the helical piping. Obviously, these figures are presented merely as an example.

Obviously, in a subsea cooling system the cooling loops should be designed according to methods appropriate to marine applications. Surface coatings when applied should be chosen to ensure optimal heat transfer and simultaneous control of surface accumulations and corrosion by-products. A preferred embodiment would typi- cally include use of corrosion resistant materials in the construction of recycle/cooling loops as an alternative to the use of corrosion resistant coatings. Another preferred embodiment foresees the use of thin coatings applied externally to protect the piping from marine growth.

The helical designs illustrated herein allows for access to the tubing for removal from accumulations to the piping surface. Such removal can be performed using permanently mounted pumping devices designed to provide sufficient circulation of external water to remove marine growth or sediments from the piping. When used continuously, these circulation devices can also increase the cooling capacity of the cooling loops. Such a system can be installed in several modified embodiments of the present invention. The illustrated embodiment also allows for cleaning using remotely or robotically operated devices or by direct access via divers or other human operated submarine devices.

The cooling system is preferably designed to be self-draining, and may preferably also include features for chemical injection and inhibition of the system during shut-in periods of the field operation.

Solids formation may be further enhanced by pressure loss. The flow related pres- sure loss in the device can be controlled in such a manner that the combined pressure and temperature losses are controlled to minimize the rate of formation of the formed solids. To this purpose, tubing and flow-paths may advantageously be designed to control both the pressure and temperature loss in order to control the formation of solids formed by pressure /temperature reduction.

The invention is of course not in any way restricted to the embodiments described above. On the contrary, many possibilities to modifications thereof will be apparent to a person skilled in the art without departing from the basic idea of the invention such as defined in the appended claims.

Claims

1. A method for cooling a subsea production fluid of hydrocarbon product from above a solids formation temperature for further transport below the said temperature as slurry, comprising the steps of feeding the production fluid through a lumen (8') by which heat is transferred through the lumen wall (9') to an ambient cooling medium, such as sea water, for precipitation of material dissolved in the production fluid, as well as the step of dislodging any attaching solid matter from the lumen wall by means of a runner (12) passing with the production fluid flow through said lumen (6'), characterized by the step of initially cooling the production fluid to a temperature still above the solids formation temperature at a location upstream of said lumen (8').

2. The method of claim 1, characterized by the step of feeding the production fluid through a passive unit (2) comprising a first lumen (8) having no runner.

3. The method of claim 2, characterized by the step of feeding the production fluid through the first lumen in the form of a helix configuration arranged in the passive unit (2).

4. The method of any of claims 2 or 3, characterized by the step of feeding the pre- cooled production fluid from the passive unit (2) through a second lumen (8') in the form of a helix configuration arranged in an active unit (3) located downstream of the passive unit, and running a flexible runner (12) through the second lumen (8') of the active unit (3).

5. The method of any previous claim, characterized by the step of controlling the formation of solids through a corresponding control of both pressure and temperature.

6. A subsea hydrocarbon production fluid cooling system, characterized by a pas- sive unit (2) and an active unit (3) arranged in succession in the production fluid flow, wherein the passive unit (2) comprises a first lumen (8) having an inlet (5) in flow communication with a hydrocarbon product flowline (1), said first lumen (8) being arranged to receive the production fluid at a temperature above a solids formation temperature and to discharge the same at a lower temperature above the solids formation temperature via an outlet (7), said outlet (7) from the first lumen (8) being in flow communication with an inlet (5') to a second lumen (8^}) arranged in the active unit (3), from which second lumen (8') the production fluid is discharged as slurry via an outlet (7') in flow communication with the flowline (1) downstream of the active unit (3).

7. The system of claim 6, characterized in that at least one of the lumens (8; 8') of the passive and active unit has a helix configuration.

8. The system of claim 7, characterized in that a helical lumen (8') of the active unit (3) is arranged for the passage of at least one flexible runner (12) there through.

9. The system of any of claims 7 or 8, characterized in that the helical lumen of the passive and/ or the active unit is arranged vertically on a substructure (13).

10. The system of claim 9, characterized in that the helical lumens of the passive and active units are stacked vertically above each other on the substructure.

11. The system of any of claims 6 to 10, characterized in that the substructure, supporting the passive and/or active units, is arranged on the sea bed.

12. The system of claim 11, characterized in that one or both of the passive (2) and active (3) units are encased in a housing (4) containing a cooling medium.

13. The system of claim 12, characterized in that devices are permanently mounted and arranged to provide external circulation of cooling medium surrounding the lumen/ lumens .

14. The system of any previous claim, characterized in that it is configured to allow access, such as for cleaning purposes.

15. The system of any previous claim, characterized in that piping comprised in the system is coated to increase heat transfer and to reduce accumulations of fouling materials from the surrounding environment.