MOTOR COOLING FAN AND/OR AUXILIARY BEARINGS

DESCRIPTION

The present invention relates to a subsea motor-compressor unit for processing a working fluid . The subsea motor-compressor unit according to the present invention comprises, integrated in a housing, a motor and a compressor.

In extraction plants for extracting natural gas from a subsea field, submersible integrated motor-compressor units are usually placed directly on the seabed . Generally a subsea motor-compression unit comprises a centrifugal compressor pushing the extracted natural gas to the mainland, the compressor being arranged in a housing together with a motor, usually consisting of an electric motor.

The compressor of the motor-compressor unit could be fluidly connected with an external separator machine placed between the well and the inlet of the unit.

The subsea motor-compressor unit of the kind of the present invention usually has a vertical configuration having a vertical shaft on which are arranged both the rotor of the electric motor and the centrifugal impellers of the compressor, the shaft is supported by a plurality of magnetic bearings, comprising radial bearings and axial thrust bearings. To each bearing is further associated an auxiliary bearing .

With the vertical configuration the drainage is due to the gravity and the footprint is minimized . A main drawback of the motor-compressor units of the known type usually used in subsea installations, is represented by the fact that due to the wet droplets contained in the gas processed by the unit it is possible to have fouling formation both at start and during operation of the motor compressor. Fouling formations are dangerous because may cause performance degradation of the motor-compressor unit and or failure of the motor. A week point of the motor-compressor unit is represented by the bearings, both the radial and thrust bearings and the auxil iary bearings, particularly interested by fouling formation as it will be explained more in details in the following .

As it has been said, motor-compressor units used in subsea environments in the production or transport of hydrocarbons are provided with a shared rotating shaft supported by a rotor-bearing system . The motor drives the compressor in order to generate a flow of compressed process gas. As the motor drives the compressor, heat is generated . In case of electric motor, heat is also generated by the electrical systems that are characteristic of electric motor drivers. Heat is also generated through the windage friction resulting from the rotating components operating in pressurized gas. If this heat is not properly dissipated, it negatively affects the performance of the motor and can damage the insulation of the stator. Increased temperatures can also adversely affect the rotor-bearing systems of both the compressor and motor, thus leading to bearing damage and/or failure. For cooling the motor and bearings in a subsea motor-compressor unit, is provided a cooling circuit which may be an open loop cooling circuit or a quasi-closed-loop cool ing circuit where gas is drawn from the process stream at some point in the compression process.

Only a small amount of process gas is fed into the cooling circuit from the process stream. The quasi-closed-loop cool ing circuit often uses a small blower to circulate the cooling gas through the cooling circuit. In subsea applications, the cool ing gas is typically cooled in a sea water- cooled heat exchanger.

This process gas is then passed through the motor and bearing areas to absorb heat.

Unfortunately, notwithstanding the small amount of process gas used, there is a significant drawback with the cool ing circuit approach to subsea motor-compressor cooling : the presence of wet droplets and/or of heavy hydrocarbons even in the gaseous state in the process gas leads to fouling formation at start up and during the operation of the motor-compressor unit, especially but not only at the areas where the gas flow is slow and/or at stationary parts.

The fouling formation, in contact with hot parts of the unit, became sol id or semi-solid, or very viscous, and particles stick to both static and rotating parts of the compressor flow path, adversely affecting the aerodynamic form leading to a decrease in mass flow, efficiency, pressure ratio and surge margin . This impl ies an increase in the required electrical power in order to maintain a constant production/delivery rate.

Additionally, the bearings, and especially auxiliary bearings of the motor-compressor shaft, are affected by fouling formation . In fact, each auxil iary bearing usually comprises a rolling bearing or a plain bearing which works in case that the magnetic bearing stops working .

One example of known cleaning apparatus for subsea compressor units is disclosed in EP1 907705B1 , wherein a system for cleaning compressors that are situated at a difficultly accessible location, e.g ., on or near the seabed or downhole in a well bore, comprises a cleaning liquid line extending between a read ily accessible liquid source and the compressor. The prior art document fails to teach how to clean the motor of a motor- compressor unit.

In EP1 907705B1 the liquid source may be a line for supplying hydrate inhibitor, anti-foam chemicals, barrier liquid, demulsifier or other types of chemicals to a subsea production or processing activity.

Alternatively, the liquid source can be an accumulator tank situated in the vicinity of the compressor. In this case, the accumulator tank is in communication with a high pressure l ine diverting high pressurized gas from the compressor to boost the pressure of the cleaning liquid in the accumulator tank and evacuate the cleaning l iquid .

The compressor often comprises more than one compressor stage. The l iquid is preferably injected in the intake flange of the compressor. The washing liquid will flow through the compressor and knock loose particles that have adhered internally in the flow path . The washing liquid leaves the compressor via the compressed gas l ine and can be carried with the gas to a subsequent station for separating the washing liquid from the gas.

The injected inhibitor l iquid must be injected in the intake flange of the compressor as well as the washing liquid . SUMMARY The present invention relates to a subsea motor-compressor unit for processing a working fluid comprising a washing apparatus.

According to a preferred embodiment of the present invention, a motor- compressor unit for processing working fluid comprises, integrated in a single unit housed in a case, a motor and a compressor, and a washing apparatus.

According to a first aspect of the invention, the integrated motor- compressor unit comprises a washing apparatus for selectively washing during operation both the motor and the compressor. Additionally, according to a further aspect of the present invention, the integrated motor-compressor unit is provided with a washing apparatus for selectively washing thrust bearings and auxil iary bearings of both the motor and the compressor.

Preferably, the washing of the motor-compressor unit is performed by means of a washing liquid, preferably MEG (monoethylene-glycol).

Further details and specific embodiments will refer to the attached drawing, in which :

- Figure 1 is a sectioned side schematic view of a typical integrated motor-compressor unit for subsea installation; - Figure 2 is a section side schematic view of a detail of the washing apparatus according to the present invention, the detail is referred to the motor area;

- Figures from 3 to 5 are enlarged section side schematic views of the washing apparatus of Figure 2 according to the present invention .

DETAILED DESCRIPTION

The following description of an exemplary embodiment refers to the accompanying drawings. The following detailed description does not limit the invention . Instead, the scope of the invention is defined by the appended claims.

Reference throughout the specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed . Thus, the appearance of the phrases "in one embodiment" or "in an embodiment" in various point of the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. With reference to Figure 1 , it is shown an integrated motor-compressor unit 1 0 comprising a compressor 20 and a motor 30, preferably an electric motor, directly connected to said compressor 20, which are integrated in a single unit.

The motor-compressor unit 1 0 comprises a box or casing 50 in which said compressor 20 and said electric motor 30 are housed . The casing 50 may be realized in a single piece or, alternatively, it may comprise multiple parts.

Said compressor 20 and said electric motor 30 are preferably separated by an intermediate diaphragm 40 thus avoiding that process gas comprising solid and/or liquid particles could pass from the compressor into the motor area.

Said motor 30 and said compressor 20 are both coupled to the same axial shaft 60. Alternatively, said compressor 20 could be coupled to a first shaft portion and said motor 30, particularly the rotor of said motor, could be coupled to a second shaft portion, the two shaft portions being connected by means of a joint.

Due to the fact that said compressor 20 and said motor 30 are coupled to the same shaft 60, or to a plurality of shaft portions joined together, the motor 30 unit and the compressor 20 unit are not separated, and the process gas processed by the compressor passes through both . The process gas is also used for cooling the motor in the cooling system: for cooling the motor and bearings in the subsea motor- compressor unit, is provided a closed-loop cooling circuit where gas is drawn from the process stream .

Due to the presence of wet droplets in the process gas, both the compressor 20 and the motor 30 are subject to fouling formations, both at start up and during operation of the motor-compressor unit.

Additionally, the motor-compressor unit 1 0 preferably comprises three magnetic radial bearings and a magnetic axial thrust bearing, each one of said bearings having an auxiliary bearing . More in details, with reference to figure 1 , an example of motor-compressor unit 1 0 may comprise three auxiliary bearings 61 , 62, 63 for supporting both a rotor of the electric motor 30 and a rotor of the compressor 20.

Each magnetic radial bearing can also support axial thrusts, depending on the way they are mounted . Therefore, for example, one or more of said three auxiliary bearing can work also as an axial bearing contrasting axial thrusts.

Preferably said auxil iary bearings 61 , 62, 63 are rolling bearing or plain bearing which works in case that the magnetic bearing to which the auxiliary bearing is associated stops working .

A first bearing 61 and a second bearing 62 of said three auxiliary bearings are preferably positioned and support said shaft 60 of said electric motor 30 close to its ends. A third bearing 63 of said three auxil iary bearings preferably supports and is positioned on a free end of said rotor of said compressor 20.

In this preferred configuration, the number of auxiliary bearings for supporting the electric motor 30 and compressor 20 is reduced to the min imum possible, as three supporting bearings are used .

Said motor-compressor unit 1 0 preferably comprises a further thrust bearing 64, preferably an active magnetic thrust bearing suitable for supporting axial thrusts, preferably situated on said rotor of said electric motor 30. The cool ing of the motor-compressor unit is preferably effected by means of a quasi-closed circuit, not shown in the figures, equipped with an external exchanger and a fan 70 situated inside the unit at the end of the rotor of said electric motor 30, to circulate the cooling gas through the stator and rotor parts. The auxiliary bearings 61 , 62 of the motor side and the thrust bearing 64 are cooled by the same gas which is circulating in the motor, by means of the system and the fan 70.

For the third auxiliary bearing 63 of the compressor side, the cool ing is guaranteed by means of a gas discharge from the compressor sent to the third auxiliary bearing 63 through a dedicated channel . Due to the fact that process gas is used for cooling the bearings, the same are particularly subject to fouling formations.

Each auxil iary bearing 61 , 62, 63 usually comprises a bearing preferably rolling bearing or plain bearing which works in case that the rolling bearing stops working .

The auxil iary bearings are stationary during the normal operation of the motor-compressor unit, and therefore are particularly subject to foul ing formations.

A collection sump 1 00 is further provided at the end of the compressor rotor of the compressor 30, within the casing 50. The collection sump 1 00 is suitable to collect completely the liquid possibly entered inside the motor-compressor unit 10 during the subsea installation and the liquid still present in the unit during the operation.

A drainage system is provided inside the motor-compressor unit 1 0 in order to drain liquids from both the motor and the compressor to the collection sump 1 00.

The configuration of said motor-compressor unit 1 0 can be either horizontal or vertical depending on the particular installation demands.

When the configuration is vertical, with the compressor at the bottom, the collection sump 1 00 is preferably provided inside the casing 50 and under the compressor 30. According to a preferred embodiment of the present invention shown in Figures from 2 to 5, said motor-compressor unit 1 0 further comprises, provided inside the motor-compressor unit 1 0, a washing apparatus, generally indicated with the reference number 80 in the attached figures, said washing apparatus comprising one or more washing devices for selectively washing specific areas of said unit 1 0.

More in details, each of said washing devices for selectively washing specific areas of said unit 1 0, in turn comprise at least a dedicated delivery duct 81 , 82, 83, 84, 85, 86, 87, 88, 89, 90 for delivering a washing agent to a target area of said unit 1 0 to be washed .

Advantageously, each delivery duct 81 , 82, 83, 84, 85, 86, 87, 88, 89, 90 comprises a spray nozzle hydraulically connected to the end of the delivery duct facing the target area to be washed . The spray nozzles are not shown in the drawings. The washing apparatus 80 is connectable to an external source of a washing agent.

Said washing agent is preferably a pressurized washing fluid, more preferably the washing fluid is MEG (monoethylene glycol).

Advantageously, said washing apparatus 80 comprises at least one of the following dedicated washing devices: a first dedicated washing device for washing the fan 70 comprising a first dedicated delivery duct 81 for delivering a washing agent to said fan 70; a second dedicated washing device for washing the first auxiliary bearing 61 comprising a second dedicated delivery duct 82 for delivering a washing agent to said first auxiliary bearing 61 ; a third dedicated washing device for washing the second auxiliary bearing 62 comprising a third dedicated delivery duct 85 for delivering a washing agent to said second auxil iary bearing 62; - a fourth dedicated washing device for washing the third auxiliary bearing 63 comprising a fourth dedicated delivery duct 86 for delivering a washing agent to said third auxiliary bearing 63; a fifth dedicated washing device for washing the thrust bearing 64 comprising a fifth dedicated delivery duct 83 for delivering a washing agent to said thrust bearing 64; a sixth dedicated washing device for washing the motor comprising a sixth dedicated delivery duct 84 for delivering a washing agent to the motor 30, preferably to the upper motor surface 31 ; a seventh dedicated washing device for washing the intermediate diaphragm 40, comprising a seventh dedicated delivery duct 87 for delivering a washing agent to said intermediate diaphragm 40; an eight dedicated wash ing device for washing the compressor 20 the compressor 20 and the collection sump 1 00, comprising an eight dedicated delivery duct 88 for delivering a washing agent to said compressor 20 and to said collection sump 1 00; said eight dedicated washing device preferably further comprising additional dedicated ducts 89, 90 for delivering a washing agent to specific areas of said compressor 20.

According to one embodiment of the present invention shown in the attached Figures, the washing apparatus 80 is contained inside the motor-compressor unit 1 0. More in details, the washing devices comprises a duct and a spray nozzle facing the target area to be washed which are completely contained inside the casing 50.

The casing 50 has preferably a substantially cyl indrical shape.

When the casing has a substantially cylindrical shape, each of the ducts for delivering a washing agent to the auxil iary bearings 61 , 62, 63 preferably further comprises a circumferential channel provided in the casing 50 all around the circumference. Thanks to the circumferential channel the washing agent could be delivered to the corresponding auxil iary bearing 61 , 62, 63 along their whole external circumference through a plurality of spray nozzles, spaced along said circumferential channel .

The washing agent is preferably a washing l iquid . More preferably the washing agent comprises MEG (monoethylene glycol), which is usually available in subsea boosting stations for other uses.

Otherwise, the washing agent could be any other washing agent suitable to remove the foul ing formations.

The washing agent is injected into the washing apparatus 80 through inlet points 91 , 92, 93 specially provided on the casing 50. Alternatively, the washing agent is injected into the washing apparatus 80 though inlet points which coincide with flanged apertures already provided on the casing 50.

The delivery ducts of said washing apparatus 80 are hydraulically connected to said inlet points 91 , 92, 93 provided on the casing 50.

The washing agent sprayed by the spray nozzles of said ducts is then drained through the drainage system already provided in the motor- compressor unit.

Depending on the injection point of the washing agent, part of said washing liquid may be processed by the compressor and therefore evacuated through the compressor outlet.

When the motor-compressor unit has a vertical configuration, the drainage is facilitated by gravity. Therefore, according to a preferred embodiment of the present invention dedicated spray nozzles are provided for each critical component of the motor-compressor unit.

According to a preferred embodiment of the present invention, the washing apparatus as described above is provided with controlled valves for controll ing the flow of the washing agent through the delivery ducts, so that the washing agent can be delivered to a predetermined area to be washed in a selective manner.