NO20160231A1

NO20160231A1 - Method, modem and system for communication of digital data on subsea power line

Info

Publication number: NO20160231A1
Application number: NO20160231A
Authority: NO
Inventors: Heyn Halfdan Magnus
Original assignee: Fmc Kongsberg Subsea As
Priority date: 2016-02-11
Filing date: 2016-02-11
Publication date: 2016-12-21

Description

TECHNICAL FIELD

The invention relates to communication of digital data on a subsea power line cable.

More specifically, the invention relates to a method for communication of digital data on a subsea power line cable, wherein the digital data is modulated onto a power signal transferred on the subsea power line cable by a particular modulation scheme.

The invention also relates to an associated power line modem and a system for communication of digital data between a topside location and a subsea location.

BACKGROUND

There is a general need for providing efficient and reliable communication between a topside location and a subsea location, in particular when operating subsea installations for exploiting hydrocarbon from a well located at the seabed.

A subsea power line cable may be arranged between at topside location and a subsea location, with the main purpose of transferring electric power from a power supply at the topside location to power consuming devices or arrangements at the subsea location.

Various approaches exist for communicating digital data between a topside location and a subsea location.

In some approaches, the data are transferred through a wired communication line, for instance, an electrical or optical communication line, which is separate from the subsea power line cable.

In other approaches, the subsea power line cable is utilized not only for transferring electrical power, but also for providing communication between the topside and subsea locations. In such cases, power line modems may be arranged at the topside and subsea locations in order to modulate a data signal onto the power line and to demodulate the power line signal in order to extract a data signal.

US-8 199 798 describes a method for communicating binary data on a subsea power line, which establishes a point-to-point connection between a subsea modem and a topside modem. The communication utilizes orthogonal frequency division multiplexing (OFDM) for modulating the binary data onto the electric power signal.

ODFM modulation used as a modulation approach in subsea power line communication, as in US-8 199 798, may however have certain shortcomings: OFDM requires the use of a cyclic prefix, which may be a copy of part of a transmitted symbol that is appended at the beginning of the next. This results in communication overhead and hence reduced transmission rate.

The spectral localisation of the OFDM subcarriers may be quite weak. This may result in interference between data symbols under frequency offsets, and a need for large separation between neighbouring systems.

Hence, there is a need for providing improved communication of digital data on a subsea power line cable.

SUMMARY

The invention has been defined in the independent claims. Advantageous embodiments have been defined in the dependent claims.

The invention provides a method for communication of digital data on a subsea power line cable, wherein the digital data is modulated onto a power signal transferred on the subsea power line cable by filter bank multi carrier modulation, FBMC modulation.

The invention also provides a power line modem, which is configured to communicate digital data by performing a method as set forth in the present specification.

The invention also provides a system for communication of digital data between a topside location and a subsea location, the system comprising a first power line modem arranged at the topside location,

a second power line modem arranged at the subsea location, each power line modem being of a type set forth in the present specification, and

a subsea power line cable communicatively interconnecting the first and second modems.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic block diagram illustrating a system for communication of digital data on a subsea power line cable. Figure 2 is a schematic flow chart illustrating a method for communication of digital data on a subsea power line cable. Figure 3 is a schematic block diagram illustrating further aspects of a system for communication of digital data on a subsea power line cable. Figure 4 is a schematic block diagram illustrating a power line modem and an associated coupler. Figure 5 is a schematic block diagram illustrating further possible features of a power line modem and an associated coupler.

DETAILED DESCRIPTION

Figure 1 is a schematic block diagram illustrating a system for communication of digital data on a subsea power line cable.

The system 100 includes a topside subsystem 110 and a subsea subsystem 160,

which are interconnected by an umbilical 150. The umbilical 150 may have a length in the range of about 100 m up to tens or several hundred kilometers. The umbilical 150 includes an electric power line 152 which transfers, i.a., electric power from the topside subsystem 110 to the subsea subsystem 160. The electric power line 152 included in the umbilical 150 is also used to communicate data from the topside subsystem 110 to the subsea subsystem 160, and vice versa.

The umbilical 150 may also include additional lines, e.g. hydraulic control or power lines, and/or electric or optic communication or control lines. It may consist of a single section, or include several interconnected sections, connections, step-out connectors, and/or branching arrangements.

The topside subsystem 110 may e.g. be installed on a topside hydrocarbon production control facility, which may be located on an onshore site, onboard a vessel, such as a drilling ship, or on a platform, such as a drilling platform.

The subsea subsystem 160 may e.g. be installed on the seabed, for instance in association with at least one wellhead of a subsea hydrocarbon production well.

The topside subsystem 110 includes a topside power supply device 120, which is arranged for the purpose of supplying electrical power to power-consuming subsea equipment and devices that are included in or connected to the subsea subsystem 160. The topside power supply device 120 provides electric power at its output line 122, which is further electrically connected to the power line 152 in the umbilical 150. The topside power supply device 120 may be a power distribution device or power conversion device, which distributes or converts electric power provided by a remote electric power source, e.g. on land. Alternatively, the topside power supply device 120 may be an independent source of electric power. In either case, the topside power supply device 120 may advantageously be an AC power supply device.

The topside subsystem 110 further includes a topside control device 130. The topside control device 130 is arranged with, i.a., the purpose of establishing and performing digital communication, advantageously two-way communication, with various subsea equipment that are included in or connected to the subsea subsystem 160.

For instance, the topside control device 130 may be configured to provide control data that are transferred via the power line 152 in the umbilical 150 to subsea equipment such as actuators, valves, etc, for the purpose of controlling the actuators, valves, etc. The topside control device 130 may also be configured to receive data provided by various subsea equipment and transferred via the power line 152 in the umbilical 150, for instance data provided by subsea sensors, local subsea control devices, etc.

The topside subsystem 110 further includes a topside modem 140, which is configured to modulate a data signal received on a data signal line 132, provided by the topside control device 130, onto the power signal in the power line 152. To this end, the modem 140 includes or is connected to a coupling device 126, which may be an inductive coupler arranged to be inductively connected to the power line 152. The modulated power signal is transferred through the power line 152 in the umbilical 150 to the subsea subsystem 160.

The topside modem 140 is also configured to demodulate the signal that is decoupled by the coupling device 126 from the power signal provided on the power line 152 in the umbilical 150. Thus, the topside modem 140 will provide a data signal on the data signal line 132.

The topside modem 140 is configured to perform the above-mentioned modulating by means of a predetermined modulation scheme, namely, filter bank multi carrier modulation, FBMC modulation.

Likewise, the topside modem 140 is configured to perform the above-mentioned demodulating by means of filter bank multi carrier demodulation, FBMC demodulation.

The use of FBMC modulation and demodulation may, i.a., provide the following effects and/or advantages, compared with background art modulation approaches, including the background art OFDM communication:

- Increased bandwidth efficiency or increased data rate for a given bandwidth

- In FBMC, neighbor subcarriers are separated by combination of a tailored frequency offset together with a tailored timing offset, while subcarriers further apart are separated by effective filtering. - Efficient filtering of a plurality of channels, which leads to improvements, e.g., better cross talk suppression, when several systems are present, e.g., in redundant cables which are closely arranged carrying FBMC signals in neighbor frequency bands. - Improved tolerance to noise outside the utilized frequency band, such as colored noise, narrow band noise, power noise.

A FBMC modem frequency coordination mechanism may ensure crosstalk suppression in that the lines never utilize same frequency bands (in opposite directions, i.e. that one line have communication from topside to subsea and the other from subsea to topside). FBMC is particularly well suited to filter out any noisy frequency bands from (near end) neighbor power line systems or any other frequency selective noise from any other known or unknown neighbor system.

B. D. Tensubam: "A Review on FBMC: An Efficient Multicarrier Modulation System", Int. Journal of Computer Applications (0975-8887), Volume 98- No. 17, July 2014, discloses further elements and features of FBMC modulation scheme and FBMC demodulation scheme, which may be utilized in embodiments of the present method for communication of digital data, powerline modem and system for communication of digital data.

The communication may further be time multiplexed. This has the effect of enabling multidrop communication, i.e. communication between a plurality of subsea control devices and the topside control device 130. In the case of time multiplexed communication, two or more subsea modems may be connected to a power line 152 in the umbilical and addressed one at the time by the topside modem.

The method may further comprise a near end crosstalk avoidance process. The crosstalk avoidance process may introduce synchronization in the time domain. This may be useful in a situation wherein two or more power and communication lines are located close to each other in the umbilical, e.g. two separate pairs in one quad. In such a case, a synchronization mechanism ensures that the lines never utilize simultaneous communication in opposite directions, i.e. that one line have communication from topside to subsea and the other from subsea to topside. If the near end crosstalk process is not employed, a receiving modem may in some cases pick up signals from a transmitting modem that is located in the same end of the umbilical.

An advantage with the arrangement of near end crosstalk avoidance is that all lines in the umbilical can use the whole available frequency spectrum without limiting the communication of the subsea modem that has the most data to send. In a subsea system the vast majority of the communication may usually be from subsea to topside. In most normal operations the communication from topside is only short commands.

Advantageously, the FBMC modulation and demodulation includes staggered filtering. Advantageously, the staggered filtering may be in the form of Offset Quadrature Amplitude Modulation, O-QAM. This facilitates maximum spectral efficiency.

Alternatively, in some cases, the FBMC modulation and demodulation may include one of the following modulation and demodulation features:

- Offset Quadrature Phase Shift Keying, O-QPSK, and

- Cosine Multi Tone, CMT.

The FBMC modulation and demodulation advantageously includes use of a plurality of subcarriers, and each subcarrier is advantageously assigned a certain power level.

The power level and the signaling alphabet, i.e., number of bits per subcarrier symbol, may advantageously be selected to adjust a characteristic of the communication on the subsea power line cable.

This characteristic may, i.a., include a data rate of the communication, or an error rate of the communication, or both.

Further with reference back to figure 1, the subsea subsystem 160 includes a subsea modem 170, which may advantageously be functionally identical to the topside modem 140 described above.

The subsea modem 170 includes or is connected to a coupling device 176, similar to the coupling device 126 of the topside modem 140, e.g. an inductive coupler arranged to be inductively connected to the power line 152. The modulated power signal is transferred through the power line 152 in the umbilical 150 to the topside subsystem 110.

The power line 152 proceeds through the coupler 176 to a subsea power terminal 172 which is connected to power-consuming subsea equipment 190.

The subsea subsystem 160 further includes a subsea control device 180, which is communicatively connected to the subsea modem 170. The subsea control device 180 may be configured to receive control data from the subsea modem 170 and transfer such control data to subsea equipment such as actuators, valves, etc, for the purpose of controlling the actuators, valves, etc.

The subsea control device 180 may also be configured to receive data provided by subsea equipment, for instance data provided by subsea sensors, additional, local subsea control devices, etc, and to transfer these data to the subsea modem 170.

The subsea subsystem 160 further includes or is electrically connected to at least one power consuming subsea equipment 190. This may include a variety of equipment disposed subsea, e.g. on the seabed, for instance a wellhead, or elements of a wellhead, valves, pumps, compressors, control devices such as the subsea control device 180, etc.

Figure 2 is a schematic flow chart illustrating a method for communication of digital data on a subsea power line cable.

The method initiates at the initiating step 210.

First, in step 220, a power signal is provided at a topside location.

Next, in step 230, a digital data signal is modulated onto the power signal by FBMC modulation. This step may, e.g. be performed in the topside modem 140. Possible details of the communication, including the FBMC modulation, has been disclosed above with reference to figure 1.

Next, in step 240, the modulated power signal is transferred to a subsea location.

Next, in step 250, the modulated power signal is demodulated at the subsea location, resulting in a digital data signal to be provided at the subsea location. This step may, e.g. be performed in the subsea modem 170. The modulated power signal is demodulated by FBMC demodulation. Details of the communication, including the FBMC demodulation, has been disclosed above with reference to figure 1. Also, in this step 250, power is transferred to the power-consuming devices located subsea.

In addition to the illustrated steps, a digital signal provided at the subsea location may be modulated onto the power signal in the subsea modem. Also, a step of demodulating the power signal, resulting in a digital data signal at the topside location, may be performed in the topside modem 140. Such modulation and demodulation includes FBMC modulation and demodulation, respectively.

The above process may be repeated or terminated at the terminating step 260. Figure 3 is a schematic block diagram illustrating further aspects of a system for communication of digital data on a subsea power line cable. Figure 3 includes the entire system illustrated in figure 1. Hence, the above description associated with figure 1 is incorporated in the present description of figure 3. Only the additional elements have been specifically described here.

In figure 3, the topside subsystem 110 is identical to the topside subsystem 110 shown in figure 1. However, the topside modem 140 in figure 3 is particularly adapted to provide time multiplexed communication. Moreover, in figure 3, the subsea subsystem 160 includes two subsea modems 170, 171, with respective couplers 176, 177 to the power line 152. The two subsea modems 170, 171 are connected to respective subsea control devices 180, 181.

In the configuration shown in figure 3, the subsea modems 170, 171 are configured to isolate data submitted in separate time slots or time intervals on the power line 152. Since the topside modem 140 is adapted to provide time multiplexed communication, it may be enabled to address data to be responded to by the modems 170, 171 in respective time slots or periods.

It should be understood that not only two, but virtually any plurality of subsea modems, such as three, four, five, six or more, could be arranged in a similar configuration as the one illustrated in figure 3. In this case, the topside modem should be configured to provide time multiplexed communication with a corresponding number of individual time slots or periods.

As will be understood from the above explanation, multidrop communication may be obtained by configuring the modems and their associated operating methods with time multiplexed communication.

Figure 4 is a schematic block diagram illustrating a FBMC power line modem and an associated coupler.

In figure 4, the power line modem has for simplicity been identified with the reference numeral 140, which also identifies the topside power line modem 140 shown in figures 1 and 3. It should however be appreciated that any one of the exemplary power line modems disclosed in the present specification, e.g. the subsea power line modem 170 shown in figure 1 and/or the subsea power line modems 170 and 171 shown in figure 3, may also have further features and elements as those disclosed herein with reference to the power line modem exemplified in figure 4.

The power line modem 140 includes a digital processing unit 142. A data signal line is connected to the digital processing unit. For readability and simplicity, the data signal line has been identified with the reference numeral 132, which also identifies a data signal line connected to the topside control device 130 as illustrated in figure 1. However, if the power line modem is a subsea power line modem, it should be appreciated that the data signal line may likewise be the signal line 182 connected between the subsea power line modem 170 and the subsea control device 180. As a further alternative, the data signal line may be the data signal line 133 connected between the subsea power line modem 171 and the subsea control device 181, as illustrated in figure 3.

The digital processing unit 142 is communicatively connected to a digital to analog sampler unit 144, which is further communicatively connected, e.g. by a transmit communication line, to a combiner unit 127 in the coupling device 126. The digital processing unit 142 is also communicatively connected to an analog to digital sampler unit 146, which is further communicatively connected, e.g. by a receive communication line, to the combiner unit 127 in the coupling device 126. The combiner 127 is arranged to combine the transmitter signal (TX) on the transmit communication line and the receiver signal (RX) on the receive communication line. The combiner unit is further communicatively connected to a high pass filter 128 in the coupling device 126. The high pass filter 152 is connected to the power line 152. The high pass filter 128 is operable to separate the high voltage and low frequency power system from the lower voltage and higher frequency communication signal. The connection to the power line can be done in parallel with high voltage capacitors or in series with the power line with an inductive current transformer.

Figure 5 is a schematic block diagram illustrating further possible features of a power line modem and an associated coupler.

In figure 5, the digital processing unit 142 is communicatively a first digital signal processor 390 and a second digital signal processor 300. The second digital signal processor is configured to perform FBMC modulation of a signal provided by the digital processing unit 142. The FMBC modulated signal is further provided to an interpolation filter 310 which is communicatively connected to the second digital signal processor 300. The interpolation filter 310 may e.g. be implemented as a programmable circuit, such as an FPGA circuit. The interpolation filter 310 is further communicatively connected to a digital to analog converter 320, which is further communicatively connected to a filter 330, whose output is further connected and provided to a power amplifier 340. The power amplifier 340 is connected to the coupler 126 which finally connects the digital communication signal to the power line 152.

The coupler 126 is also connected to the low noise amplifier 350 which picks up a communication signal derived from the power line 152 by the coupler 126. The low noise amplifier 350 is further communicatively connected to a filter 360, whose output is communicatively connected to an analog-to-digital converter 370. The output of the analog-to-digital converter 370 is communicatively connected to a decimation filter 380, which may be implemented as a programmable circuit, such as an FPGA circuit. The output of the decimation filter 380 is connected to the first digital signal processor 390, which is configured to perform FBMC demodulation of its input signal and provide a FBMC demodulated signal to the digital processing unit 142.

Claims

1. Method for communication of digital data on a subsea power line cable, wherein the digital data is modulated onto a power signal transferred on the subsea power line cable by filter bank multi carrier modulation, FBMC modulation.

2. Method according to claim 1, wherein the communication further is time multiplexed.

3. Method according to claim 2, further comprising a near end crosstalk avoidance process.

4. Method according to one of the claims 1-3, wherein the subsea power line cable extends between a topside location and a subsea location.

5. Method according to claim 4, wherein the subsea power line cable also transfers electric power from the topside location to the subsea location.

6. Method according to one of the claims 1-5, wherein the FBMC modulation includes staggered filtering.

7. Method according to claim 6, wherein the staggered filtering includes offset quadrature amplitude modulation, O-QAM.

8. Method according to one of the claims 1-7, wherein the FBMC modulation includes use of a plurality of subcarriers, each subcarrier being assigned a certain power level.

9. Method according to claim 8, wherein each subcarrier is associated with a certain signaling alphabet.

10. Method according to claim 9, wherein said power level and said signaling alphabet are selected to adjust a characteristic of the communication on the subsea power line cable.

11. Method according to claim 10, wherein said characteristic includes a data rate of the communication, or an error rate of the communication, or both.

12. Method according to one of the claims 1-11, further comprising FBMC demodulation of a signal transferred on the subsea power line cable

13. Power line modem, configured to communicate digital data by performing a method as set forth in one of the claims 1-12.

14. System for communication of digital data between a topside location and a subsea location, comprising a first modem as set forth in claim 13, arranged at the topside location, a second modem as set forth claim 13, arranged at the subsea location, and a subsea power line cable communicatively interconnecting the first and second modems.