HYDROCARBON FLUID

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a system arranged to ensure stable transport of wax and hydrate prone hydrocarbon crude fluid in subsea pipelines at pressure and/or temperature formation conditions for wax and hydrates commonly referred to as cold flow. More precisely, the present invention relates to a system designed to enable cold flow of wax and hydrate prone hydrocarbon fluid by solids precipitation and disintegration.

BACKGROUND AND PRIOR ART

Feeding hydrocarbon well fluid through long distance subsea pipelines in which the temperature in the fluid falls below a temperature at which solidification of wax or hydrates can be expected entails special problems in terms of precipitation and deposition of solid matter in the bulk fluid or on pipeline walls which may finally lead to clogging and choking of the pipeline.

A well-known and widely used method to avoid choking of a pipeline at cold flow conditions involves dislodging of deposited solid matter from the pipeline wall by means of a plug that is ejected into the pipeline for transport with the well fluid, a method which is commonly referred to as "pigging". Plugs or "pigs" are frequently launched into the pipeline at an upstream end of a cold flow section and removed from the pipeline at a downstream position, such as at the surface platform e.g. Pigging requires subsea installations in the form of pig storage, manifolds and valves to manage launching and removal of pigs from the pipeline.

It is also known in the art, see e.g. US 8,256,519B2, to enable cold flow conditions by maceration of solids which are caused or allowed to precipitate at an upstream position, creating this way a slurry which is released into the pipeline for transport through the cold flow section.

SUMMARY OF THE INVENTION An object of the present invention is to provide an alternative system for precipitation of solids in a hydrocarbon well fluid, and for successive disintegration or maceration of the so formed solids for further transport with well fluid in a pipeline at cold flow conditions.

Another object of the present invention is to provide a hydrocarbon solids precipitation and disintegration system that enables cold flow through a structurally non-complex design that is reliable in operation.

Still another object of the present invention is to provide a hydrocarbon solids precipitation and disintegration system of modular design which can readily be up- sized or downsized.

Yet another object of the present invention is to provide a hydrocarbon solids precipitation and disintegration system of compact design and high controllability.

One or more of these objects will be met in a system for enabling cold flow of wax and hydrate prone hydrocarbon fluid, the system comprising

• a cooling cylinder of sufficient length to permit solids to precipitate and deposit on the inside of the cooling cylinder as hydrocarbon fluid passes through the cooling cylinder,

• a rotatable scraper extending through the cooling cylinder, the scraper in rotation capable of dislodging pieces of solid matter from the deposits on the inside of the cooling cylinder,

• wherein the scraper is driven in rotation by an electric motor installed in the fluid flow, the motor having an open rotor providing through-flow for fluid. Implementation of a motor with a through-flow rotor provides freedom of choice with respect to installation of the motor and the scraper. The motor can thus be drivingly connected to either the upstream end or to the downstream end of the scraper. The motor can also alternatively be inserted between two successive scraper lengths. Placing the motor at the inlet to the cooling cylinder may be advantageous since in this embodiment the rotor will avoid being hit by pieces of solid matter in the bulk flow or dislodged from the cylinder wall by the rotating scraper. The motor can be realized as a permanent magnet (PM) motor wherein permanent magnets are carried in the periphery of a radially bladed and open rotor, whereas electromagnets and stator windings are supported on a casing that surrounds the open rotor. The permanent magnets can be arranged on a circular ring member connected the outer ends of the rotor vanes of a PM motor stage, e.g., or the permanent magnets can be carried in the ends of the rotor vanes. In either case the rotor shaft is drivingly coupled to the scraper shaft, directly or indirectly via gear box or other transmission.

Embodiments of the invention comprise a PM motor with a radially bladed rotor wherein the rotor vanes are provided a pitch angle, the rotor vanes thus transferring motor power to the fluid in a pump or booster implementation, or in a mixer implementation, e.g., wherein the through-flow rotor operates as impeller in an axial flow pump. Embodiments of the invention further comprise two or more PM motors with through- flow rotor coupled in an axially stacked motor assembly.

The scraper is realized in the form of a screw having a radial blade that runs helically about a central shaft. A non-driven end of the scraper may be journalled for rotation in a bearing support with an annular passage for through-flow of fluid and solid matter.

The cylinder wherein precipitation takes place can be cooled through natural convection. To this purpose the cylinder may be formed externally with cooling flanges that increase the heat transferring contact surface with the ambient sea.

The cylinder may alternatively be provided a forced convection system wherein seawater is set in motion about the cooling cylinder, or wherein seawater or other cooling media is forced through a cooling jacket that surrounds the cylinder. Alternatively or in addition to the above, cooling may be assisted through injection of compressed gas preferably from the production into the fluid in the cooling cylinder, in order this way to enhance precipitation by expansion induced cooling, utilizing the Joule-Thompson effect. The cooling cylinder can be arranged downstream of a passive or active mixer wherein the fluid is homogenized by mixing the different phases of a multiphase fluid from a subsea hydrocarbon well. A mixer for this purpose can be based on the same motor design as the PM motor that drives the scraper.

The cooling cylinder can be arranged upstream of a grinder wherein solids in the fluid leaving the cooling cylinder are macerated into finer matter or into a slurry for further transport under cold flow conditions.

A grinder for this purpose can be based on the same motor design as the PM motor that drives the scraper. The mixer and/or the grinder can be provided as a stacked motor assembly wherein two or more successive rotors are operated in contra rotating or co-rotating directions.

The capacity of the present solids precipitation and disintegration system can be adapted to requirements in different ways: by arranging cooling cylinders in serial or parallel expansion of the system, and/or by upsizing the length and diameter of the cooling cylinder.

The system of the present invention can be applied in a pigging system, if appropriate. In such application of the system, a chamber for forming hydrocarbon based plugs is connected to the outlet end of the cooling cylinder, such that solid matter dislodged from the cylinder wall and in the bulk fluid can be pressed into the plug forming chamber by the rotatable scraper. The plug can be launched from the plug forming chamber via a plug launching valve through which the plug can be released to travel with well fluid through the cold flow section of the pipeline. The hydrocarbon plug may in this way function as an artificial pig.

SHORT DESCRIPTION OF THE DRAWINGS

The above discussed aspects of the invention will now be explained with reference made to the accompanying, schematic drawings. In the drawings, Fig. 1 illustrates a precipitation chamber in a system for enabling cold flow through a subsea pipeline according to the present invention,

Fig. 2 illustrates in longitudinal sectional view the precipitation chamber of Fig. 1 and associated units that may form part of the system,

Fig. 3 illustrates an expanded system comprising several precipitation chambers arranged in series, Fig. 4 illustrates an expanded system comprising several precipitation chambers arranged in parallel,

Fig. 5 shows the system arranged for expansion induced cooling, and Fig. 6 shows the system adapted for production of hydrocarbon plugs for pigging purposes.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to Fig. 1, a system 1 for precipitation and disintegration of solids in a hydrocarbon well fluid stream F comprises a precipitation chamber defined between inlet end and downstream end of a through flow cylinder 2, a cooling arrangement 3 on the outside of the cylinder, a rotatable scraper 4 extending through the cylinder, the scraper 4 having a diameter slightly less than the inner periphery of the cylinder, a motor 5 drivingly connected to the scraper, and a bearing support 6 supporting the scraper in a non-driven end thereof.

As shown on larger scale in Fig. 2, the assembly forms a precipitation chamber wherein wax and hydrate prone crude fluid is subjected to pressure and/or temperature formation conditions for wax and hydrates during passage through the cooling cylinder 2. The solidified products, see reference S, which deposit on the inner periphery of the cylinder, are successively dislodged mechanically from the cylinder wall 7 through the rotating action of the scraper, the scraper successively driving the pieces of solid matter towards the downstream end of the cooling cylinder. In order to ensure precipitation the cooling cylinder 2 needs flow length and cooling capacity sufficient to reduce the fluid temperature to the required level. Several process parameters may need to be taken into account such as the initial pressure or temperature in the fluid, cooling method in terms of active or passive cooling or both, ambient seawater temperature and underwater currents, e.g. Since the operational conditions change from one application to another the disclosure contains no recommendation with respect to length and diameter of the cooling cylinder. However, as will be understood from the following, the disclosure gives examples on designs which can be applied to enable sufficient length of cooled flow through the cooling cylinder. For example, cooling of the cylinder can be effected in different ways. In the embodiment of Fig. 2 cooling is accomplished by circulation of cooling medium in a pipe length 8 that runs in a spiral about the cylinder. Pipes for circulation of cooling medium can alternatively be arranged to run longitudinally or in parallel on the outside of the cylinder 2. Pipes for cooling medium can be installed in a cooling jacket 9. Alternatively, seawater can be fed through such a cooling jacket to effect cooling by forced heat convection. Forced heat convection can alternatively be achieved by creating motion in the seawater that surrounds the cylinder. Passive heat convection can be accomplished by arranging flanges or fins on the outer periphery of the cylinder in order to increase the contact surface with the ambient sea. If appropriate, measures for passive and active cooling in combination can be applied to the cooling cylinder 2.

Precipitation can be further enhanced by injection of produced gas G which may preferably be compressed in a gas compressor 10 to be introduced in the fluid inside the cylinder 2, as indicated by the arrows in Fig. 5.

With reference again to Fig. 2, the scraper 4 comprises a radial blade 11 which is turned in a spiral about a central shaft 12, forming a helical screw. The downstream end of the scraper is supported for rotation in a bearing 13. The upstream end of the scraper is drivingly connected to the motor 5 which will be further explained below.

In the embodiment of Fig. 2 the motor 5 is a permanent magnet motor comprising permanent magnets 14 carried on a rotor 15. The electromagnets 16 and associated stator coils are arranged on a casing that surrounds the rotor. More precisely, the permanent magnets 14 are carried in the outer ends of a set of rotor blades or rotor vanes 17 which extend radially from a central shaft/rotor shaft 18. The rotor shaft 18 and rotor vanes 17 form an impeller that has an annular opening through the motor 5. The rotor shaft 18 is drivingly coupled to the scraper shaft 12.

The flow of well fluid through this part of the system follows an in-line, annular flow passage through the motor, the cooling cylinder and the bearing support. It will be understood that transition between annular flow and pipeline flow obviously requires corresponding manifolds or other flow transition devices to connect the system to import and export pipe lines connecting to the system in the upstream and downstream ends thereof.

The motor 5, the cooling cylinder 2 with the scraper 4 and the bearing 13 form together a central unit in the cold flow enabling system. Embodiments of the system may however include supplementary devices to assist in enabling cold flow conditions.

In the upstream end of the system a mixer 19 may be arranged to effect even distribution of wax or hydrate prone constituents in a multiphase well fluid. The mixer 19 may be a passive, stationary installation, having flow guides 20 which create turbulence in the well fluid, e.g. Alternatively, an active mixer may be arranged and based on mixing elements that are driven in rotation.

An active mixer 21 is advantageously based on the PM motor that is used as the motor 5 which drives the scraper. More precisely, two or more motors 5 can be stacked in axial relation and arranged with rotor vanes 17 designed to accelerate the flow through the respective rotors. Enhanced mixing and homogenization of a multiphase flow can be accomplished if successive rotors are propelled in contra rotating directions. A combined booster/mixer can in this way be provided to prepare the fluid for the cold flow enabling process which takes place in the cooling cylinder. Improved disintegration and maceration of solid matter that is discharged with the well fluid from the cooling cylinder can be accomplished through the arrangement of a grinder unit 22 in the downstream end of the system. A grinder unit may likewise be based on the PM motor that is used as the motor 5 which drives the scraper. Enhanced maceration and homogenization of the flow through the cold flow system can be accomplished if successive rotors of the grinder unit are propelled in contra rotating directions. The cold flow enabling system of the present invention is readily expandable to meet various demands and requirements related to conditions prevailing at different production sites. This is illustrated in the embodiments of Fig. 3 and Fig. 4.

For example, in a case where the length of a single cooling cylinder is deemed not to be sufficient for precipitation of all wax and hydrate prone constituents in the fluid, the serial layout of multiple in-line cooling cylinders is an available option for implementation of the present invention.

Fig. 3 shows a set of cooling cylinders arranged in a serial layout. Each cooling cylinder may be equipped with a separate motor 5 to drive the respective scrapers 4 in rotation. Alternatively, a single motor 5 or a multi-staged/stacked PM motor 5 can be arranged to drive two or more interconnected scraper screws in their respective cylinders. A boosting/mixer unit 19, 21 and a grinder unit 22 may be arranged upstream and downstream, respectively, of the in-line cooling cylinders in serial layout.

An alternative expansion scheme is shown in Fig. 4 illustrating a set of cooling cylinders arranged in parallel layout. Although not illustrated in Fig. 4 it will be understood that each cooling cylinder is equipped as previously discussed. In the upstream ends the cooling cylinders communicate with an import pipeline via a manifold pipe 23, and if appropriate via a booster/mixer unit 20, 21. In the downstream end the cooling cylinders communicate with a cold flow pipeline C via a manifold pipe 24, and if appropriate also via a grinder unit 22.

In the parallel layout of Fig. 4 the flow F is split and distributed on multiple cooling cylinders, each of which receives a part of the total flow. The cooling cylinders can thus be designed to have reduced diameter for increased cooling and precipitation efficiency.

A system according to the present invention can further be extended to include functionality for pigging. To this purpose the cooling cylinder is operatively connected to a hydrocarbon plug forming chamber 25, wherein artificial hydrate pigs can be formed to be successively launched downstream via a plug launching valve 26. The operation includes precipitation and build-up of solid matter on the wall of the cooling cylinder while the valve is kept open. Valve is then closed where upon the scraper screw is rotated to dislodge and transport solid material for compaction in the hydrocarbon plug forming chamber. The valve is then opened to release the hydrocarbon plug which is successively launched into the cold flow pipeline.

It will be appreciated from the above that the system as shown provides great flexibility in operation, installation and capacity to ensure cold flow of wax/hydrate prone hydrocarbon crudes over long distances subsea. Modifications of the disclosed embodiments are possible without leaving the scope of the invention as disclosed above and defined in appended claims.