SE1400602A1 - Detachable pressure compensator for subsea applications and a subsea system comprising the same - Google Patents

Description

15 20 25 30 needs to be pressure compensated for is so large that the pressure compensators required become a substantial part of both the total weight of the subsea unit and cost. These large pressure compensators also make the handling and transportation more difficult, and the required support structures also complicate the product design.

Most of the required compensating volume required is due to temperature changes during storage and handling, and the pressurisation when the equipment is lowered to the seabed and installed subsea. Only very small changes in oil volume are actually caused by the operation of the equipment.

Complex pressure compensators are thus utilised to accommodate oil volume changes before the equipment is actually brought into use.

SUMMARY A general object of the present disclosure is to solve or at least mitigate problems of the prior art.

Hence, according to a first aspect of the present disclosure there is provided a detachable pressure compensator connectable to a subsea unit, wherein the detachable pressure compensator comprises a first connector arranged to connect the detachable pressure compensator to a subsea unit and to provide a route for liquid to flow between the detachable pressure compensator and the subsea unit, a chamber, a dielectric liquid contained in the chamber, an active pressure compensating device, and a control system arranged to control the active pressure compensating device to transport dielectric liquid from the chamber to the subsea unit.

By means of a detachable pressure compensator the oil volume change caused during transportation and/ or installation can be compensated for as required. The pressure compensator of the subsea unit may thereby be kept small compared to existing pressure compensators; the pressure compensator of the subsea unit may in particular be designed to handle any pressure fluctuations that a subsea unit may be subjected to when installed at the seabed. 10 15 20 25 The equipment may be made both smaller and more efficient than a permanently installed pressure compensator. The cost would also be very much reduced compared to existing designs of pressure compensators. Also the required support structures would be signiñcantly less, reducing both weight and cost of the equipment.

The detachable pressure compensator is only temporarily attached or connected to a subsea unit. It is retrieved either prior to deployment in the sea, i.e. after storage/ transportation, or after installation on the seabed.

According to one embodiment the active pressure compensating device comprises a pump.

According to one embodiment the active pressure compensating device comprises a motor arranged to operate the pump.

According to one embodiment the control system is arranged to control the IIIOtOT.

According to one embodiment the control system comprises a pressure sensor arranged to sense a pressure in the detachable pressure compensator.

According to one embodiment the first connector is remotely controllable to disconnect from the subsea unit.

According to a second aspect of the present disclosure there is provided a subsea system comprising a subsea unit comprising: an enclosure, an electrical device arranged inside the enclosure, a pressure compensator, and a dielectric liquid arranged inside the enclosure, a second connector; and a detachable pressure compensator according to the first aspect, wherein the first connector is connectable to the second connector to thereby allow dielectric liquid to be transported from the detachable pressure compensator to the subsea unit.

According to one embodiment the first connector comprises a first valve and wherein the second connector comprises a second valve. 10 15 20 25 According to a third aspect of the present disclosure there is provided a method of providing pressure compensation for a subsea unit, by means of a detachable pressure compensator connected to the subsea unit, wherein the detachable pressure compensator comprises: a first connector arranged to connect the detachable pressure compensator to the subsea unit and to provide a route for liquid to flow between the detachable pressure compensator and the subsea unit, a dielectric liquid chamber, an active pressure compensating device, and a control system arranged to control the active pressure compensating device to transport dielectric liquid from the chamber to the subsea unit, wherein the method comprises: a) connecting the first connector of the detachable pressure compensator to a second connector of the subsea unit to provide a flow path for the dielectric liquid from the chamber to the subsea unit, which subsea unit comprises an enclosure, electrical equipment arranged inside the enclosure, a pressure compensator, and a dielectric liquid arranged inside the enclosure, b) providing pressure compensation in the subsea unit by controlling the active pressure compensating device to transport dielectric liquid from the chamber to the subsea unit, c) sealing the flow path, and d) disconnecting the detachable pressure compensator from the subsea unit.

According to one embodiment step c) is performed prior to deployment of the subsea unit into sea water.

According to one embodiment step c) is performed upon installation of the subsea unit at the seabed.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, etc. are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, etc., unless explicitly stated otherwise. 10 15 20 25 BRIEF DESCRIPTION OF THE DRAWINGS The specific embodiments of the inventive concept will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 schematically shows a detachable pressure compensator; Fig. 2 schematically shows a subsea system comprising the detachable pressure compensator in Fig. 1 and a subsea unit; and Fig. 3 is a flowchart of a method of providing pressure compensation of a subsea unit by means of the detachable pressure compensator in Fig. 1.

DETAILED DESCRIPTION The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplifying embodiments are shown. The inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art.

Like numbers refer to like elements throughout the description.

Fig. 1 shows a block diagram of a detachable pressure compensator 1. The pressure compensator 1 is connectable to and disconnectable/detachable from a subsea unit, i.e. a subsea module comprising electrical equipment such as power electronic building blocks forming a power converter, and/ or a transformer.

The detachable pressure compensator 1 comprises a first connector 3 arranged to connect the detachable pressure compensator 1 to a subsea unit and to provide a route or flow path for dielectric liquid to flow from the detachable pressure compensator 1 to the subsea unit.

According to one variation, the first connector 3 may be remotely controlled.

Disconnection of the detachable pressure compensator 1 from a subsea unit 10 15 20 25 may for example be controlled remotely by means of a remotely operated underwater vehicle (ROV), or it may be disconnected by a diver.

The detachable pressure compensator further comprises a chamber 5 which contains a dielectric liquid, an active pressure compensating device 7, and a control system 9. The control system 9 is arranged to control the active pressure compensating device 7 to transport dielectric liquid from the chamber 5 to the subsea unit via the first connector 3.

The active pressure compensating device 7 may for example comprise a pump. According to one variation the active pressure compensating device 7 also comprises a motor arranged to operate the pump. In this case, the control system 9 is arranged to control the motor. The active pressure compensator 7 may according to one variation also be able to transport a dielectric liquid from the subsea unit to the chamber 5; dielectric liquid flow may hence be bi-directional.

The control system 9 may comprise a pressure sensor or a pressure gauge 11 arranged to sense a pressure in the detachable pressure compensator 1, and send the measured pressure va1ue(s) to the control system 9. The pressure sensor 11 may for example be arranged to measure a pressure at the first connector 3. In general the pressure sensor 11 is arranged to measure a pressure which provides a basis for the control system 9 to determine whether it is necessary to provide any dielectric liquid from the chamber 5, and also to determine the amount of dielectric liquid to be transported to the subsea unit in order to obtain a pressure inside the subsea unit close to the ambient pressure surrounding the subsea unit.

Fig. 2 shows a subsea system 13 comprising the detachable pressure compensator 1 and a subsea unit 15. The detachable pressure compensator 1 is in the example in Fig. 2 connected to the subsea unit 15. For this purpose, the subsea unit 15 comprises a second connector 21 arranged to be connected to the first connector 3 of the detachable pressure compensator 1. 10 15 20 25 30 The subsea unit 15 has an external housing or enclosure, and comprises a passive pressure compensator 17, a dielectric liquid and an electronic or electrical device 19. The external housing may for example be made of a material which has high thermal conductivity, preferably metal such as steel e.g. Stainless steel. The dielectric liquid fills the entire interior space of the subsea unit 15 so as to prevent the occurrence of any air gaps between the internal surface of the external housing and any internal component, such as electronic device 19, contained in the subsea unit 15. The dielectric liquid counteracts deformation of the external housing when the subsea unit 15 is subjected to an ambient subsea pressure higher than a pressure that the external housing can withstand without deformation. The dielectric liquid may for example be oil or an ester, and prevents short circuit of any electronic or electric device contained in the subsea unit 15.

The passive pressure compensator 17 is arranged to transmit ambient subsea pressure to the inside of the external housing. Thereby a pressure difference between ambient subsea pressure and pressure inside the external housing may be reduced. The passive pressure compensator 17 may for example be defined by a mechanically flexible portion of the external housing, or a membrane such as an impermeable membrane. The interior volume of the subsea unit 15 and thus the pressure inside the external housing is hence dependent of the ambient subsea pressure.

The electronic device 19 is adapted to withstand the high pressures present at depths at which the subsea unit 15 is to be installed. Such devices are know in the art, and will therefore not be elaborated any further herein. Examples of electronic devices are capacitors, for example capacitors based on metalized film technology or on oil-soaked film-foil technology, and power electronic devices such as insulated gate bipolar transistor (IGBT) modules, integrated gate-commutated thyristors (IGCT), diodes, and thyristors.

According to one variation of the subsea system 13, the first connector 3 comprises a first valve 23a and the second connector 21 comprises a second valve 23b. Alternatively, one of the detachable pressure compensator 1 and 10 15 20 25 the subsea unit 15 may comprise both the first valve 23a and the second valve 23b; this would typically be the subsea unit 15. In may also be said that the subsea system 13 comprises both the first valve 23a and the second valve 23b.

Alternatively, according to one variation, the subsea system 13 comprises only one valve for controlling liquid flow from the detachable pressure compensator 1 to the subsea unit 15. The subsea system 13 may thus comprise single seal or a double seal valves.

The operation of the subsea system 13 will now be described in more detail with reference to Fig. 3. In particular, a method of providing pressure compensation for the subsea unit 15 by means of the detachable pressure compensator 1 will be described.

In a step a) the first connector 3 of the detachable pressure compensator 1 is connected to a second connector 21 of the subsea unit 15, to thereby provide a flow path for the dielectric liquid from the chamber 5 to the subsea unit 15.

This connection is typically provided onshore, prior to transportation of the subsea unit 15 to the site of installation.

In a step b) pressure compensation in the subsea unit 15 is provided by controlling the active pressure compensating device 7 to transport dielectric liquid from the chamber 5 to the subsea unit 15.

Pressure compensation is obtained by means of the control system 9, which obtains a pressure in the detachable pressure compensator 1 measured by the pressure sensor 11, whereby the control system 9 is able to determine the amount of dielectric liquid to be transported to the subsea unit 15, and thus how to control the active pressure compensating device 7.

The pressure compensation may be performed until storage and/ or transportation is completed, or until installation on the seabed has been completed. The size of the chamber 5 and the amount of dielectric liquid, e.g. oil, in the chamber 5 upon commencement of the pressure compensation is designed based on what type of pressure compensation is to be provided by 10 15 20 25 30 the detachable pressure compensator, i.e. only for onshore transport/ storage or also for installation on the seabed.

After the subsea unit 15 has been installed on the seabed, or is to be deployed in the sea, the single or double seal valves are closed, i.e. the first valve and/ or the second valve is/ are closed. Thus, in a step c) the flow path is sealed.

In a step d) the detachable pressure compensator 1 is disconnected from the subsea unit 15. As previously mentioned, step d) may be performed prior to deployment of the subsea unit 15 into sea water or upon installation of the subsea unit 15 at the seabed. In the former case, the subsea unit 15 itself would have to be able to compensate for the compression during installation, but temperature fluctuations during storage and transportation would be compensated for by means of the detachable pressure compensator. In the latter case, the detachable pressure compensator 1 is retrieved from the sea after it has been disconnected. Only the very small pressure compensator 17 provides pressure compensation for the subsea unit 15 at the seabed, to compensate for any small oil changes during operation.

If it is needed to retrieve the subsea unit 15 from the seabed, according to one variation the subsea unit comprises a system for compensating for the increase in oil, during decompression. This could for example be realised by a system connected to the seal valves before retrieval, or by a bladder already connected to the subsea unit, but isolated by a valve that needs to be opened.

It is envisaged that the detachable pressure compensator and subsea system presented herein finds applications within the oil and gas industry for example for subsea HVDC/HVAC power provision systems, i.e. power transmission and power distribution systems, as well as offshore power generation such as wind energy, tidal energy, wave energy, and ocean current energy.

The inventive concept has mainly been described above with reference to a few examples. However, as is readily appreciated by a person skilled in the 10 art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.

Claims (11)

10 15 20 25 11 CLAIMS

1. A detachable pressure compensator (1) connectable to a subsea unit (15), wherein the detachable pressure compensator (1) comprises: a first connector (3) arranged to connect the detachable pressure compensator (1) to a subsea unit (15) and to provide a route for liquid to flow between the detachable pressure compensator (1) and the subsea unit (15), a chamber (5), a dielectric liquid contained in the chamber (5), an active pressure compensating device (7), and a control system (9) arranged to control the active pressure compensating device (7) to transport dielectric liquid from the chamber (5) to the subsea unit (15).

2. The detachable pressure compensator (1) as claimed in claim 1, wherein the active pressure compensating device (7) comprises a pump.

3. The detachable pressure compensator (1) as claimed in claim 2, wherein the active pressure compensating device (7) comprises a motor arranged to operate the pump.

4. The detachable pressure compensator (1) as claimed in claim 3, wherein the control system (9) is arranged to control the motor.

5. The detachable pressure compensator (1) as claimed in any of the preceding claims, wherein the control system (9) comprises a pressure sensor (11) arranged to sense a pressure in the detachable pressure compensator (1).

6. The detachable pressure compensator (1) as claimed in any of the preceding claims, wherein the first connector (3) is remotely controllable to disconnect from the subsea unit (15).

7. A subsea system (13) comprising: 10 15 20 25 12 a subsea unit (15) comprising: an enclosure, an electrical device (19) arranged inside the enclosure, a pressure compensator (17), and a dielectric liquid arranged inside the enclosure, a second connector (21); and a detachable pressure compensator (1) as claimed in any of claims 1-6, wherein the first connector (3) is connectable to the second connector (21) to thereby allow dielectric liquid to be transported from the detachable pressure compensator (1) to the subsea unit (15).

8. The subsea system (13) as claimed in claim 7, wherein the first connector (3) comprises a first valve (23a) and wherein the second connector (21) comprises a second valve (23b).

9. A method of providing pressure compensation for a subsea unit (15), by means of a detachable pressure compensator (1) connected to the subsea unit (15), wherein the detachable pressure compensator (1) comprises: a first connector (3) arranged to connect the detachable pressure compensator (1) to the subsea unit (15) and to provide a route for liquid to flow between the detachable pressure compensator (1) and the subsea unit (15), a chamber (5), an active pressure compensating device (7), and a control system (9) arranged to control the active pressure compensating device (7) to transport dielectric liquid from the chamber (5) to the subsea unit (15), wherein the method comprises: a) connecting the first connector (3) of the detachable pressure compensator (1) to a second connector (21) of the subsea unit (15) to provide a flow path for the dielectric liquid from the chamber to the subsea unit, which subsea unit (15) comprises an enclosure, electrical equipment (19) 10 13 arranged inside the enclosure, a pressure compensator (17), and a dielectric liquid arranged inside the enclosure, b) providing pressure compensation in the subsea unit (15) by controlling the active pressure compensating device (7) to transport dielectric liquid from the chamber (5) to the subsea unit (15), c) sealing the flow path, and d) disconnecting the detachable pressure compensator (1) from the subsea unit (15).

10. The method as claimed in claim 9, wherein step c) is performed prior to deployment of the subsea unit into sea water.

11. The method as claimed in claim 9, wherein step c) is performed upon installation of the subsea unit at the seabed.