SUB-COOLED HYDROCARBON PRODUCTION METHOD AND SYSTEM INCLUDING MACERATION OF PRECIPITATES

TECHNICAL FIELD OF THE INVENTION The present invention relates to a method and to a system for conversion of a subsea 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, asphaltenes, 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 an upstream location, from where the production fluid is further transported as slurry at the lower temperature.

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 disclose 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 production fluid flows through the lumen, resulting in formation of solid material 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 exchanger containment through which a coolant medium is circulated in order to lower the temperature of the production fluid to a solids formation temperature. US 6,774,276 disclose method and apparatus by which the flow of fluid hydrocarbons is sub-cooled in an upstream heat exchanger from where it is

introduced into a reactor where it is mixed with particles of gas hydrates which are also introduced into the reactor. The effluent flow of hydrocarbons from the reactor is cooled in a downstream heat exchanger to ensure that all water present is in the form of gas hydrates. The flow is then treated in a separator to be separated into a first flow and a second flow. The first flow has a content of gas hydrate and is recycled to the reactor via pumps to provide the particles of gas hydrates mentioned before. The second flow is conveyed to a pipeline to be further transported.

A problem in this design is the heating effect caused in pumps. The recycled fluid has to be significantly sub-cooled to ensure that the fluid which is recycled to the reactor is within the hydrate or wax formation temperature window. As the fluid is sub-cooled in the upstream heat exchanger, solids deposition in tubing may occur ahead of the reactor. Drawbacks in prior art solutions are thus 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 the 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 device and method.

The object is achieved by a method for conversion of a subsea hydrocarbon production fluid from above a solids formation temperature to below the said temperature for further transport through a flowline in the form of a slurry, comprising the steps of recycling a partial volume of the production fluid through at least one cooling loop connecting a downstream discharge from the flowline with an upstream inlet to the flowline, and macerating the solids, which are formed in the production fluid by mixing with the recycled flow, by means of a motor driven impeller introduced in the fluid flow between the flowline inlet and the flowline discharge.

In a preferred embodiment, the recycled fluid is injected into the production fluid. Advantageously, the recycled fluid is injected into a low pressure area induced in the production fluid by means of a Venturi nozzle incorporated in the flowline.

Optionally, a runner may be arranged for passing through the cooling loop and operative for dislodging solid matter that has precipitated on the inner wall of the recycling loop.

The object is also achieved in a system for conversion of a subsea 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. The system comprises at least one production fluid recycling loop connecting a downstream discharge from the flowline with an upstream inlet to the flowline, the recycling loop having a lumen defined by a lumen wall through which heat is transferred from the production fluid to an ambient cooling medium, such as sea water, and a motor driven impeller incorporated in the flowline between the flowline inlet and the flowline discharge.

A Venturi nozzle is preferably arranged in the flowline adjacent to the inlet from the recycling loop, and operative for injection of the recycled fluid into the flowline.

In a preferred embodiment, the impeller is incorporated in the flowline through a flowline section having an upstream inlet to the impeller, which inlet is non- coaxially arranged with respect to a downstream outlet from the impeller. The impeller, which may be formed as a worm screw, is advantageously driven in rotation by a submersible electric motor. Optionally, entrance and exit ports can be are arranged in the recycling loop for the passage of a runner there through.

Multiple valves and flow paths may be arranged proximate to the junction of the recycling loop to the device inlets, and allowing for optimization of the flow behaviour at the junction.

A preferred embodiment includes the implementation of the system as integrated part of an in-line subsea pumping station. The system may use pressure-loss followed by boosting to stabilize or avoid the dissolution process of solids from a production fluid.

The system may 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.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The invention will be more fully described below with reference to the accompanying diagrammatic drawing.

In the drawing, reference number 1 indicates a cut out portion of a flowline 1 through which a subsea hydrocarbon production 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. Incorporated in the flowline 1 is a flowline section 2, having an upstream inlet 3 in flow connection with a downstream outlet 4. Inlet 3 and outlet 4 are non-coaxially aligned, connected through an angular flowline section 5. The flowline section 5 may be curved in alternative embodiments. From the inlet 3, flowline section 5 directs the production fluid to an impeller 6 from which the production fluid is discharged via the outlet 4, into the downstream length of flowline 1.

The impeller 6 is driven in rotation by means of a motor 7, via the motor shaft 8 which connects to the impeller 6. Preferably, the motor 7 is an electrical motor for subsea use. The impeller may be formed with a number of blades 9 arranged in succession, and shaped for maceration of solid matter entrained in the production fluid. To this purpose, the impeller blades may also be associated with stationary formations supported adjacent the impeller blades, providing in cooperation a grinding effect. The impeller 6 may alternatively have the shape of a worm screw associated with a cylindrical envelope which tightly surrounds the impeller. However, and since macerator designs are known from the literature, it is within reach of the skilled person to find a suitable design for maceration of solid matter entrained in the production fluid.

Downstream of the impeller 6, a partial flow of the production fluid is discharged into a conduit 9 via a discharge outlet 10. The conduit 9 directs the partial flow back to an inlet 11 into the flowline 1, located upstream of the impeller 6. The conduit 9 thus forms a loop for a portion of the production fluid to be recycled into the main flow. The recycling loop has a lumen defined by a lumen wall through which heat is transferred from the production fluid to an ambient medium, such as sea water, the heat transfer illustrated in the drawing through the arrows 12. The recycling loop 9 thus also operates as a cooling loop for the recycled flow of production fluid.

The partial flow through recycling/ cooling loop 9 is controlled through an adjustable throttling device 13, such as a remotely controlled throttle valve 13. One or more check valves 14 near the inlet 11 into flowline 1 ensures that the flow direction through the loop 9 cannot be reversed in result of possible pressure peaks in the production fluid.

The partial flow through recycling/ cooling loop 9 is driven by means of an injection device. In the illustrated embodiment, the injection device is realized in the form of a Venturi nozzle 15 arranged in flowline 1 in the region of the inlet 11. The Venturi nozzle creates in the production fluid a region of reduced pressure by which the partial flow is driven through loop 9 and injected into the production fluid. In result of the mixing in of cooled production fluid, the temperature of the production fluid is reduced. The recycled flow through the at least one cooling loop enhances the cooling of the main flow, and provides nucleation sites for solids formation in the main flow.

Valves placed at the outlet of each recycle loop enable the control of the recycle flow into the flowline inlet. This allows the adjustment of turbulence that may occur in the flow at the connection point. By manipulating these valves, the operator can achieve additional control over solid particles formed immediately after the cooled recycled fluid's entry into the main flow through the flowline.

The necessary valve components, wiring and connectors for power supply and control can be composed of proven sub-sea equipment known to the skilled person. By a proper dimensioning, temperature monitoring and adequate throttling, the temperature of the production fluid can in this way be reduced to below a solids formation temperature upstream of the impeller 6, for successive maceration of precipitated solid matter by the impeller. Dimensioning of the system 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 fluid to the ambient cooling medium are familiar to the skilled person.

Optionally, the system may comprise first and secondary recycling/ cooling loops 9 and 9', or even more if desired. The recycling/ cooling loop 9, 9' may additionally comprise helically formed sections of its length in order to extend the exposure to ambient cooling medium. The recycling/ cooling loop 9, 9' may also optionally comprise entrance and exit ports, known per se, for a runner to pass through the loop for dislodging any solid matter that may accumulate on the inner wall of the recycling/ cooling loop.

The disclosed system can be used both to boost fluid flow and to grind produced solids for further transport to a surface receiving facility, and can be based on a subsea booster pump. An integrated booster pump implementation provides several benefits, such as

• through the use of the pump, cooling process and speed can be controlled;

• footprint on the seafloor is minimized, as are the number of system components;

• the cold flow process can mechanically impact solidified hydrate and/ or wax to ensure it is sufficiently pulverized as to flow effectively;

• an ESP style pump can also be used making retrieval of components easier while also mechanically macerating the flowstream:

• the booster pump implementation improves economics (abandonment pressure) with a further improvement of economics over conventional cold flow technology.

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 typically 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 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.

If appropriate, the cold flow device discussed above 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 lumens /cooling loops.

The system may further be configured to allow access such as for cleaning purposes and 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 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.

Solids formation may be further 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. 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. Because the rate of appearance of formation for solids in a depressurization process is typically much higher than the resolvation process as the pressure is increased, the combination of this system with a boosting mechanism is beneficial because the formation potential is significantly reduced and the driving force is in fact towards resolvation. Solids formed in recycling loop 9, 9' will then be stabilized and further dissolution halted. Thus the pressure losses at throttling devices 13, 15 provide a driving force for dissolution which is stabilized by the pump devices. The pressure loss in the effective vena contracta of inflow device 15 is followed by pressure recovery which provides the same stabilizing effect. 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.