WO2013072682A2

WO2013072682A2 - Improvements in or relating to subsea power distribution

Info

Publication number: WO2013072682A2
Application number: PCT/GB2012/052823
Authority: WO
Inventors: Brendan Hyland
Original assignee: Wfs Technologies Limited
Priority date: 2011-11-15
Filing date: 2012-11-14
Publication date: 2013-05-23
Also published as: GB201119652D0; WO2013072682A3

Abstract

A method of fault monitoring in a subsea power line used for transporting electrical power from a surface location to a remote subsea installation having a plurality of cable sections with each pair of adjacent sections being connected via a transformer, the method comprising the steps: providing a fault monitoring system on the remote side of each transformer; monitoring each cable section using the fault monitoring system and recording monitoring data at the fault monitoring system; guiding a mobile underwater vehicle to a fault monitoring system; wirelessly transmitting the monitoring data from the fault monitoring system to the mobile underwater vehicle; and reviewing the monitoring data to determine a fault condition on the respective cable section.

Description

IMPROVEMENTS IN OR RELATING TO SUBSEA POWER DISTRIBUTION

The present invention relates to subsea power distribution and in particular, though not exclusively, to a wireless system for monitoring earth leakage on extended tie- backs.

As energy requirements increase, technology is being developed to find and exploit new energy sources in our oceans. Offshore wind farms are being constructed and tidal power generators are seeing a resurgence in interest. Oil and gas exploration and production is also venturing further into deeper waters and more remote locations. As a result large numbers of subsea installations are being constructed. In many instances control modules are located upon the installations together with process sensors, to control and operate the facilities. Electrical power needs to be delivered to the control modules and sensors. Power lines are typically run from surface to the subsea location. These power lines can extend to thousands of kilometres in length. When installing power lines of extended length, transformers are located at intervals in the line to regulate power and voltage levels between the sections. As with any subsea deployed equipment the power lines are subject to harsh environmental conditions. This can result in damage to the line and the loss of power transfer. It is therefore common to perform earth leakage monitoring on the line. This is achieved by using surface equipment which 'peers down' the line and detects any faults on the line and may also give an indication of the location of the fault. Unfortunately, where a transformer or other type of Galvanic isolator is used inline, the surface equipment can only 'peer down' to the location of the transformer. Thus the line on the remote side of the transformer cannot be monitored from the surface. GB 2 476 1 52 to Viper Subsea Limited describes a subsea line monitoring device comprising a diagnostic unit, first connector, the first connector being directly connected to the diagnostic unit such that the integrity of a line or device connected to the first connector may be indicated by an output signal. The line monitoring device may be temporarily implemented while the cable or device is deployed. Degradation of the integrity of the line may be the result of earth leakage, a short circuit or line insulation resistance. Should the insulation fail then the line monitoring device may be used to detect an over current condition. The connectors may be electrically interconnected so as to allow the transmission of power there between. The diagnostic unit may have an internal power supply which may be a rechargeable battery. The diagnostic device may include a display to display the output signal. The device may comprise transmission means to transmit the output signal to a remote location, the transmission means may be a hydro-acoustic transmission means. The device may comprise a memory for storing the output signal. One or more electrical parameters may be sensed by the diagnostic unit.

This monitoring device is for use when a fault in the form of a low line insulation resistance is measured from the surface. The monitoring device is then deployed on a temporary basis to the sea bed in close proximity to the connectors and cables to be tested. For example, it may be installed by a diver or using a remotely operated vehicle (ROV). One of the connectors associated with the cable to be checked is disconnected from the electrical distribution unit and connected instead to the device. The connector provided on the cable of the device is connected to the vacant connector provided on the subsea distribution unit. Once the connections have been made in this manner, the monitoring device can check for any earth leakage faults.

This prior art device requires there to be a fault monitoring system to act from the surface to indicate that there is a problem around the distribution unit. Monitoring on the remote side of a transformer cannot be achieved from surface and thus this device cannot be used for permanent monitoring over an extended tie-back with multiple transformers.

It is an object of the present invention to provide a subsea power line including fault monitoring which obviates or mitigates at least some of the disadvantages in the prior art.

According to a first aspect of the present invention there is provided a subsea power line for transporting electrical power between a surface location and a remote subsea installation comprising a plurality of cable sections with each pair of adjacent sections being connected via a transformer and wherein a fault monitoring system is located on the remote side of each transformer.

In this way, each section of cable can be monitored independently to determine if a fault exists on that section of cable. The subsea power line can deliver power to the subsea installation or carry generated power from a subsea installation to surface.

Preferably, the fault monitoring systems are permanent in-line fault monitoring systems. In this way, any fault measured is the result of a fault in the cable section and not in the connections to and from the monitoring system. Alternatively, the fault monitoring systems may be retrofitted to existing power lines. In this way, ageing power distribution systems can be monitored.

Preferably, the fault monitoring systems include a memory to store monitored data and a transmitter to wirelessly transmit monitored data. In this way, there does not need to be a control line to transfer the data back to the surface.

Preferably each transmitter has an electrically insulated magnetic coupled antenna.

Alternatively, each transmitter has an electric field coupled antenna. The antenna may be a wire loop, coil or similar arrangement. Such antenna create both magnetic and electromagnetic fields. The magnetic or magneto-inductive field is generally considered to comprise two components of different magnitude that, along with other factors, attenuate with distance (r), at rates proportional to 1 /r² and 1 /r³ respectively.

Together they are often termed the near field components. The electromagnetic field has a still different magnitude and, along with other factors, attenuates with distance at a rate proportional to l/r. It is often termed the far field or propagating component.

In this way the data is transmitted as an electromagnetic or magneto-inductive signal. Preferably also, the fault monitoring systems include a rechargeable battery. In this way, power from the power line is not required to operate the fault monitoring system. Preferably, the power to recharge the battery is transmitted by magnetic coupling between first and second transceivers, the first transceiver being located in the fault monitoring system and the second transceiver being located on a mobile underwater vehicle brought to the fault monitoring system. In this way, there is no need for direct electrical conductive contact. More preferably, each transceiver includes a circular coil structure surrounded by a flux guiding enclosure that inductively couples energy from a primary coil in the second transceiver to a secondary coil in the first transceiver.

Preferably each fault monitoring system monitors for earth leakage in the respective cable section. In this way deterioration and/or reduction in power transmission is monitored. Alternatively, each fault monitoring system may measure an electrical parameter selected from a group comprising: resistance, impedance, voltage, phase, current and power, in order to monitor a fault. Advantageously each fault monitoring system includes a time domain reflectometer to assist in determining the location of the fault on the cable section. Each cable section may be from around 1 km in length to hundreds of kilometres on length. Preferably each cable section is at least 50km in length. More preferably, each cable section is at least 100km in length. In this way, a power line to a very remote installation can be monitored. Preferably, there is a plurality of transformers and a plurality of fault monitoring systems. In this way, a power line to a very remote installation can be monitored.

Preferably the fault monitoring system includes a microprocessor for storing and undertaking commands based on the fault monitoring data. In this way, power can be shut-down and/or regulated in the cable section at the transformer.

According to a second aspect of the present invention there is provided a method of fault monitoring in a subsea power line used for transporting electrical power from a surface location to a remote subsea installation having a plurality of cable sections with each pair of adjacent sections being connected via a transformer, the method comprising the steps:

(a) providing a fault monitoring system on the remote side of each transformer;

(b) monitoring each cable section using the fault monitoring system and recording monitoring data at the fault monitoring system; (c) guiding a mobile underwater vehicle to a fault monitoring system;

(d) wirelessly transmitting the monitoring data from the fault monitoring system to the mobile underwater vehicle; and

(e) reviewing the monitoring data to determine a fault condition on the respective cable section.

In this way, fault monitoring can be performed at the remote side of a transformer without the requirement for a control line from the surface or across the transformer. Preferably, there are a plurality of transformers and a plurality of fault monitoring systems and wherein the method includes the step of systematically guiding the mobile underwater vehicle to each fault monitoring system and harvesting the monitoring data. Preferably, the method includes the step of storing the monitoring data in a memory at the fault monitoring system. In this way, data can be stored until the mobile underwater vehicle is deployed to collect it.

Preferably, the method includes the step of transmitting the data as an electromagnetic and/or magneto-inductive signal. Signals based on electrical and electromagnetic fields are rapidly attenuated in water due to its partially electrically conductive nature. Propagating radio or electromagnetic waves are a result of an interaction between the electric and magnetic fields. The high conductivity of seawater attenuates the electric field. Water has a magnetic permeability close to that of free space so that a purely magnetic field is relatively unaffected by this medium. However, for propagating electromagnetic waves the energy is continually cycling between magnetic and electric field and this results in attenuation of propagating waves due to conduction losses. The seawater provides attenuation losses in a workable bandwidth which still provide for data transmission over practical distances.

Preferably, the fault monitoring systems include a rechargeable battery which is recharged by transmitting power by magnetic coupling between first and second transceivers located in the fault monitoring system and the mobile underwater vehicle respectively. In this way, there is no need for direct electrical conductive contact. As the power transfer is achieved by magnetic coupling while the data transfer is by electromagnetic or magneto-inductive signals, the data can be transferred with the mobile vehicle at a greater distance from the fault monitoring system than that required for power transfer. Alternatively, data and power transfer can be simultaneous.

Preferably, the method includes the step of compressing the data prior to transmission. In this way the occupied transmission bandwidth can be reduced. This allows use of a lower carrier frequency which leads to lower attenuation. This in turn allows data transfer through fluids over greater transmission distances so that the mobile underwater vehicle does not require to be close to the fault monitoring system to collect data. Preferably, the data transmission is bi-directional. In this way, command and control signals can be transferred to a microprocessor in the fault monitoring system. The method may then include the step of regulating power on the cable section.

Preferably, the method includes the step of monitoring for earth leakage in the respective cable section. Alternatively, the method may include the step of monitoring by measuring an electrical parameter selected from a group comprising: resistance, impedance, voltage, current, phase and power. Advantageously, the method may include the step of performing time domain reflectometry to assist in determining the location of the fault on the cable section.

Preferably the mobile underwater vehicle is an autonomous underwater vehicle (AUV). More preferably, the mobile underwater vehicle is a remotely operated vehicle (ROV). Such vehicles are already used in a subsea environment. An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings of which:

Figure 1 is a schematic illustration of a subsea environment including a subsea power line according to an embodiment of the present invention; Figure 2 is a block diagram of a transmitter for use in a system of the present invention; Figure 3 is a block diagram of an antenna for use in the transmitter of Figure 2.

Reference is initially made to Figure 1 of the drawings which illustrates a subsea power line, generally indicated by reference numeral 1 0, for transporting electrical power from a surface location 12 to a remote subsea installation 14 according to an embodiment of the present invention. The power line 1 0 is formed from a number of cable sections 16a-g. At the junction 18 between each pair of cable sections, 16a,b for example, there is located a transformer 20. Such an arrangement of cable sections 16 and connecting transformers 20 is as known in the art for transporting power over large distances. For the present invention the cable sections 16 are of the order of at least 50km in length and may typically be greater than 10Okms. These distances are found at offshore wind farms 22, offshore power generation facilities, deep ocean observatories, deep ocean sensor networks and subsea installations for oil and gas production. The power line 10 formed of multiple cable sections 1 6 can be referred to as an extended tie-back. The cable sections 16 are laid across the sea-bed 28, with a first cable section 1 6a crossing the sea level 30 to meet the first transformer 20 on the sea-bed 28. The final cable section 16g connects to the subsea installation 14 which, in the example embodiment, is the support structure 32 of a wind turbine 34. While only a single wind turbine 34 is shown, it will be appreciated that there may be a distribution unit at the subsea installation 14 to distribute power over an array of wind turbines as would typically be arranged for an offshore wind farm 22.

In the present invention a fault monitoring system 24 is located on the remote side 26 of each transformer 20. The fault monitoring system 24 may be an in-line monitoring system installed when the power line 1 0 is laid or may be retrofitted to the power line 1 0. There is also a fault monitoring system 24a at the surface location 12. Each fault monitoring system 24 is as known in the art for monitoring earth leakage in a cable to which it is attached. In this case, the fault monitoring system 24a monitors the first cable section 16a. As is known, this technique of 'peering down' the cable 16a can only operate to the junction 18 because of the presence of the transformer 20. Any galvanic isolator, such as a transformer, prevents monitoring at points passed the junction 18 on the remote side 26 of the transformer 20. While each fault monitoring system 24 primarily monitors for earth leakage, the system 24 may also measure resistance, impedance, voltage, current and power to assist in monitoring. Additionally, a time domain reflectrometry measurement can be taken along the cable section, the analysis of which can be used to indicate a location of a fault on the cable section 16. By locating a fault monitoring system 24 at the remote side 26 of each transformer 20 on the power line 1 0, every section of cable 16a-g can be monitored.

Each fault monitoring system 24 includes a memory to store the monitored data. The monitoring system may be programmable so that measurements are only taken at intervals to conserve the power at the fault monitoring system 24. Each fault monitoring system 24 has a power supply 36. The power supply 36 is a rechargeable battery so that the fault monitoring system 24 can operate even when power fails in the power line 10.

Monitoring data stored in the memory at the fault monitoring system 24 is collected via an ROV 40. The download of data between the fault monitoring system 24 and the ROV 40 is achieved by wireless transmission through the seawater 38. Fault monitoring system 24 includes a first transceiver 42 and there is a second transceiver 44 mounted on the ROV 40. Reference is now made to Figure 2 of the drawings which illustrates parts of each transceiver 42,44. In transceivers 42,44 the sensor interface 56 receives monitoring data from the measurements made in the fault monitoring system 24 on the cable section 16, which is forwarded to data processor 58. Data is then passed to signal processor 60 which generates a modulated signal which is modulated onto a carrier signal by modulator 62. Transmit amplifier 64 then generates the desired signal amplitude required by transmit transducer 66. In the transceiver 44, there is a control interface 68 which sends command signals to the data processor 58 which are transmitted by the above described path. These command signals can be used to identify the wireless transceivers 42,44 to each other and determine if the transceivers 42,44 are within proximity or range to transmit data and/or power. The transceivers 42,44 also have a receive transducer 70 which receives a modulated signal which is amplified by receive amplifier 72. De-modulator 74 mixes the received signal to base band and detects symbol transitions. The signal is then passed to signal processor 76 which processes the received signal to extract data. Data is then passed to data processor 58 which in turn forwards the data to control interface 68. For the transceiver 44, there is also a memory 78 which can store data for onward transfer. In this way, data is transmitted from transceiver 42 and collected at transceiver 44. The data is stored in memory 78 located on the ROV 40. While transceivers 42,44 contain both a receiver and a transmitter, it will be apparent that for data transfer, the fault monitoring system could have a transmitter and the ROV could have a receiver. By including transceivers 42,44 this provides the opportunity to download commands to the fault monitoring system 24 from the ROV 40. Such commands may be procedures to carry out in the event of determination of a fault, such as shutting down the transformer and preventing electrical flow in the power line 10.

Figure 3 shows an example of an antenna that can be used in the transmitter and receiver of Figure 2. This has a high permeability ferrite core 80. Wound round the core are multiple loops 82 of an insulated wire. The number of turns of the wire and length to diameter ratio of the core 80 can be selected depending on the application. However, for operation at 1 25 kHz, one thousand turns and a 10:1 length to diameter ratio is suitable. The antenna is connected to the relevant transmitter/receiver assembly parts described in Figure 2 and is included in a sealed housing 84. Within the housing the antenna may be surrounded by air or some other suitable insulator 86, for example, low conductivity medium such as distilled water that is impedance matched to the propagating medium 38. The antenna can also be used to magnetically couple energy between the transceivers 42,44. In this regard the housing acts as a magnetic flux guide and the multiple loops 82 with the ferrite core 80 provide a transformer when a pair of transceivers are brought together. In order for successful energy transfer the two transceivers must be arranged close together, there being an acceptable gap of only 1 -2cm. Thus the range for power transfer is much smaller than the range for data communication. Coupling efficiency reduces as frequency increases because of leakage inductance effects. Eddy current losses increase with frequency so also act to reduce the bandwidth available for data transmission. Data and power transmission can be separated in frequency to allow simultaneous operation of the two functions. Transfer efficiency is more critical for power transfer than for data communication applications so a higher frequency will usually be assigned to the data communication signals. While a transceiver 42,44 is described with a common antenna for transmit and receive, separate antennas may be used. Additionally, a separate transmitter coil arrangement can be provided solely for power transfer. In this way, power can be transmitted from the ROV 40 to the battery of the power supply 36 in the fault monitoring system 24. This achieves recharging of the battery.

In use, fault monitoring systems 24 are installed on the remote side 26 of each transformer 20 in a subsea power line 1 0 which is connected between a surface station 12 and a subsea installation 14. They may be fitted during the construction phase or alternatively they may be retrofitted to take measurements when required. The fault monitoring systems 24 can be programmed to make measurements at predetermined intervals and save the data in an on board memory. When the data requires to be retrieved an ROV 40 including a transceiver 44 travels underwater to the each junction 1 8. At or near the junction 18, transceivers 42,44 will identify themselves to each other when in range. Data can then be transferred by the process described with reference to Figure 2. The ROV 40 can then be repositioned with the transceiver 44 close to a transceiver 42 on fault monitoring system 24. At this much smaller range, magnetic coupling for power transfer can occur and batteries within the fault monitoring system 24 can be recharged. In this way the fault monitoring systems 24 can effectively be sealed for life as the data is harvested wirelessly and recharging is also achieved wirelessly. This arrangement not only removes the need for wet mate connectors at the subsea transformers 20, but also removes the requirement to have a data line running back to the surface station 12. The collected data can then be downloaded from the ROV 40. Advantageously, an ROV 40 can harvest all the data from all the fault monitoring systems 24 located on a power line in a single trip. On the same trip each fault monitoring systems 24 can also be recharged to ensure sufficient power is available to record measurements until the next trip is due. Alternatively, the ROV can be selective in which fault monitoring systems 24 it collects data from and/or which fault monitoring systems 24 it transfers power for recharging. If power transfer is taking place, then the transceivers can be positioned close together, and in this position data can also be transferred. However, as the positioning for power transfer is more critical than for data transfer, the ROV can be moved off and away from the junction 1 8 for data transfer.

The principle advantage of the present invention is that it provides a fault monitoring system for monitoring all the sections in a subsea power line of an extended tie- back.

A further advantage of at least one embodiment of the present invention is that it provides a fault monitoring system for monitoring all the sections in a subsea power line that does not require a data line back to the surface.

A yet further advantage of at least one embodiment of the present invention is that it provides a fault monitoring system for monitoring all the sections in a subsea power line which uses rechargeable batteries so that it can remain operational when a fault occurs in the subsea power line.

It will be appreciated by those skilled in the art that various modifications may be made to the invention herein described without departing from the scope thereof. For example, the mobile underwater vehicle may be a manned submarine. The mobile underwater vehicle may be used to transfer data and control signals between the fault monitoring systems other subsea installations which could provide a central processing unit. While the description is for power distribution from surface to a subsea installation, the system can be reversed and equally be used from a subsea power distribution facility to surface, with the remote side now being closer to the surface. In addition, whilst an ROV has been described as collecting data, it may alternatively be any suitable underwater vehicle, or a diver which actions the collection of data.

Claims

1 . A subsea power line for transporting electrical power from between a surface location to and a remote subsea installation comprising a plurality of cable sections with each pair of adjacent sections being connected via a transformer and wherein a fault monitoring system is located on the remote side of each transformer.

2. A subsea power line as claimed in claim 1 wherein the fault monitoring systems are permanent in-line fault monitoring systems.

3. A subsea power line as claimed in claim 1 wherein the fault monitoring systems are retrofitted to existing power lines.

4. A subsea power line as claimed in any preceding claim wherein the fault monitoring systems include a memory to store monitored data and a transmitter to wirelessly transmit monitored data.

5. A subsea power line as claimed in claim 4 wherein each transmitter has an electrically insulated magnetic coupled antenna.

6. A subsea power line as claimed in claim 4 wherein each transmitter has an electric field coupled antenna.

7. A subsea power line as claimed in one of claim 5 and claim 6 wherein the antenna may be a wire loop, coil or similar arrangement.

8. A subsea power line as claimed in any preceding claim wherein the fault monitoring systems include a rechargeable battery.

9. A subsea power line as claimed in claim 8 wherein the power to recharge the battery is transmitted by magnetic coupling between first and second transceivers, the first transceiver being located in the fault monitoring system and the second transceiver being located on a mobile underwater vehicle brought to the fault monitoring system.

10. A subsea power line as claimed in any preceding claim wherein each fault monitoring system monitors for earth leakage in the respective cable section.

1 1 . A subsea power line as claimed in any preceding claim wherein each fault monitoring system includes a time domain reflectometer to assist in determining the location of the fault on the cable section.

12. A subsea power line as claimed in any preceding claim further comprising a plurality of transformers and a plurality of fault monitoring systems.

13. A subsea power line as claimed in any preceding claim wherein the fault monitoring system includes a microprocessor for storing and undertaking commands based on the fault monitoring data.

14. A method of fault monitoring in a subsea power line used for transporting electrical power from a surface location to a remote subsea installation having a plurality of cable sections with each pair of adjacent sections being connected via a transformer, the method comprising the steps:

(a) providing a fault monitoring system on the remote side of each transformer;

(b) monitoring each cable section using the fault monitoring system and recording monitoring data at the fault monitoring system;

(c) guiding a mobile underwater vehicle to a fault monitoring system;

15. A method of fault monitoring wherein there are provided a plurality of transformers and a plurality of fault monitoring systems and wherein the method includes the step of systematically guiding the mobile underwater vehicle to each fault monitoring system and harvesting the monitoring data.

16. A method of fault monitoring as claimed in any one of claim 14 and claim 15 wherein the method includes the step of storing the monitoring data in a memory at the fault monitoring system.

17. A method of fault monitoring as claimed in any one of claim 14 to claim 16 wherein the method includes the step of transmitting the data as an electromagnetic and/or magneto-inductive signal.

18. A method of fault monitoring as claimed in any one of claim 14 to claim 17 wherein the fault monitoring systems include a rechargeable battery wherein the method includes the step of recharging the battery by transmitting power by magnetic coupling between first and second transceivers located in the fault monitoring system and the mobile underwater vehicle respectively.

19. A method of fault monitoring as claimed in claim 1 8 wherein power transfer is achieved by magnetic coupling while the data transfer is by electromagnetic or magneto-inductive signals wherein the method includes the step of data transfer with the mobile vehicle at a greater distance from the fault monitoring system than that required for power transfer.

20. A method of fault monitoring as claimed in any one of claim 14 to claim 20 wherein the method includes the step of compressing the data prior to transmission.

21 . A method of fault monitoring as claimed in any one of claim 14 to claim 21 wherein data transmission is bi-directional.

22. A method of fault monitoring as claimed in any one of claim 14 to claim 22 wherein the method includes the step of monitoring for earth leakage in the respective cable section.

23. A method of fault monitoring as claimed in any one of claim 14 to claim 22 wherein the method includes the step of monitoring by measuring an electrical parameter selected from a group comprising: resistance, impedance, voltage, current , phase and power.

24. A method of fault monitoring as claimed in any one of claim 15 to claim 25 wherein the method includes the step of performing time domain reflectometry to assist in determining the location of the fault on the cable section.