GB2451079A

GB2451079A - Subsea electronic modules with variable rate modems controlled by adjustable clocks

Info

Publication number: GB2451079A
Application number: GB0713809A
Authority: GB
Inventors: Gerald Ian Robinson
Original assignee: Vetco Gray Controls Ltd
Current assignee: Baker Hughes International Treasury Services Ltd
Priority date: 2007-07-17
Filing date: 2007-07-17
Publication date: 2009-01-21
Also published as: GB0713809D0

Abstract

Underwater electronic modules in oil drilling and other applications are interconnected via networks. In order to optimise the transmission speed over the network the surface module negotiates a data rate with each undersea module. This may involve sending messages and then measuring the amount of errors in the received version. The data rate is set by adjusting the modem clock. This can be done by changing the frequency of the oscillator 8 or by selecting 14 one of a number of frequency dividers 9-13 fed by the oscillator. A crystal or voltage controlled oscillator (VCO) may be used.

Description

* 2451079 Variable Speed Modem This invention relates to a communications system, an underwater well installation and a method of selecting the data rate of each of a plurality of modems within a network.

Communications systems incorporating a network using a plurality of modems are well-known in many different fields of industry. Fig. I shows one such system as may be used in an underwater well installation, in this case a subsea hydrocarbon fluid production facility. Control and monitoring of the facility is typically achieved by the sending and receiving of data via modems between a land-or sea-based control platform I and subsea electronics modules (SEMs) 2, 3, 4 and 5. Each SEM is mounted on a so-called "Christmas tree" located on the sea bed, and is connected, via a common subsea termination unit 7, to the platform 1 via an umbilical 6, which may be many kilometres long.

The rate at which data can be transmitted through the umbilical 6 between the surface and subsea via the modems varies considerably, being dependent on both the characteristics of the umbilical, e.g. its length, and the number of nodes present. The nodes result from the splitting of the transmitted and received data between individual trees (and thus SEMs) on the seabed. Currently, a choice has to be made between existing designs of either a low-speed or high-speed modem. A low-speed modem suffers from a relatively poor data transfer rate, while use of a high-speed modem may cause a severe loss of data rate when the modem is too fast for the network. The speed of the modem is dependent upon its clock frequency, which in a conventional modem is constant and defined by a crystal-controlled oscillator. The process of calculating the optimum operating speed of a modem, when operating in a multiple modem network, when the communication lines between the modems are not precisely defined is difficult and time intensive.

It is an aim of the present invention to provide a modem of variable speed which can thus be set to provide a maximum data rate for a particular network configuration. The speed of the modem, and hence data rate may then be adjusted by assessment of the network performance.

The data rate between modems is typically defined by the speed of the internal clock within the modem. The aim of the invention is achieved by making the clock frequency variable, so that it can be adjusted to achieve the fastest data rate for the network for an acceptable error rate.

According to a first aspect of the present invention, there is provided a communications system as set out in the accompanying claims.

According to a second aspect of the present invention, there is provided an underwater well installation as set out in the accompanying claims.

According to a third aspect of the present invention, there is provided a method of selecting the data rate of each of a plurality of modems within a network as set out in the accompanying claims.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:-Fig. 1 schematically shows a typical communications system network for an underwater well installation; Fig. 2 schematically shows a first embodiment of a variable speed modem in accordance with the present invention, which permits either manual or automatic selection of data rate; Fig. 3 schematically shows a second embodiment of a variable speed modem in accordance with the present invention in an arrangement which permits manual selection of the data rate; and Fig. 4 schematically shows a third embodiment of a variable speed modem in accordance with the present invention in an arrangement which permits automatic selection of the data rate.

Fig. 2 shows a modem arrangement in accordance with a first embodiment of the present invention which permits either manual or automatic selection of data rate. A crystal oscillator 8 feeds a chain of digital divider circuits 9-13, the outputs of which are connected to a digital selector circuit 14. This is operable to select one of the divider outputs and feed it to modem 15. Modem 15 is linked far communication with a communications network. The modem 15 is controlled, via an independent control port, by processor 16. In a particular embodiment, the processor 16 is a SEM microcontroller, i.e. the processor which typically controls operation of a respective well, however a separate processor may be alternatively be employed. The processor 16 also controls the selection of divider outputs by the digital selector circuit 14. is

The crystal oscillator 8 produces a fundamental clock frequency of 8.6018 MHz. The first digital divider 9 divides this frequency by 32, producing an output carrier frequency A of 268.8 kI-Iz, providing a data rate of 115.2 kbps. The second digital divider 10 divides frequency A by 2, producing an output B of 134.4 kl-Iz, providing a data rate of 57.6 kbps. The third digital divider 11 divides frequency A by 3, producing an output C of 89.6 kHz, providing a data rate of 38.4 kbps. The fourth digital divider 12 divides frequency C by 2, producing an output D of 44. 8 kHz, providing a data rate of 19.2 kbps.

The fifth digital divider 13 divides frequency D by 2, producing an output E of 22.4 kFIz, providing a data rate of 9.6 kbps.

All of these frequencies A-E are available for selection simultaneously. The digital selector circuit selects the desired frequency using digital AND selector circuits.

For manual selection of the data rate, the operating modem clock frequency can be selected by a command from the control platform I (see Fig. 1), via the modem network.

To set up the network optimally using a manual method, a test message is passed between the modems of the network at a first, e.g. low clock frequency, and the data error rate is determined by comparing received test messages with the original. The test message can be resent at progressively higher clock frequencies, manually selected by a command from the control platform I, until a specified data error rate threshold is reached. At this point, the clock frequency of the modem is locked. This enables the performance of the network to be optimized.

Alternatively, the data rate of each modem in the system may be selected automatically.

Here, the processor 16 would contain a test routine, for example a software package, to perform the following functions: a) Generate test messages which are sent to the modem 15 to transmit over the network; b) Receive returned test messages from the network via modem 15; c) Compare the returned test messages with the originally-generated test message and detect any errors; d) Determine whether the error rate is below a stored acceptance threshold level; and e) Change the selected modem clock frequency.

For a communications network such as the example illustrated in Fig. 1, the mode of operation for set-up could be: a) Set each of the modems at the control platform 1 and at SEMs 2, 3, 4 and 5 at their lowest clock frequencies; b) The processor 16 of the control platform modem (hereinafter designated "modem A") is programmed (e.g. pre-programmed) to accept a request to generate test messages; c) Modem A transmits a number of test messages, say 50, addressed to modem B (i.e. the modem at SEM 2) over the network; d) Modem B recognizes the messages as test messages, stores them and returns them to the network addressed to modem A; e) Modem A receives the returned test messages, compares them with the original test messages, detects errors and calculates the error rate. If the error rate is below the stored acceptable error threshold rate, it sends a command message to modem B to increase its clock frequency to the next available frequency and increases its own clock frequency to match; f) Steps c), d) and e) are repeated as many times as necessary until the acceptable error rate is reached. At this point, modem A sends a message to modem B to reduce its clock frequency to the previous available frequency and also reduces its own clock frequency to match. The D/A input value of modem A is stored; g) Modem A reverts back to the lowest clock frequency and repeats the steps c) to f) for each of the other modems; h) Having determined a set of the highest clock frequencies that achieves the specified data error rate between modem A and each of the other modems, the clock frequency of modem A is locked to the lowest frequency of the set, to ensure communication between all modems is below the specified error data rate.

Such a process enables the automatic optimization of modem data rates, without the need for detailed analysis of the network performance prior to installation.

In certain situations it may be feasible for modem A to cormnunicate with each other modem at that modem's maximum frequency (which may be higher than the lowest frequency of the set). However, the time saving gained by using this higher frequency will often be more than offset by the additional time spent in changing the data rates for each modem.

Fig. 3 shows a modem arrangement in accordance with a second embodiment of the present invention which enables the data rate of the modem to be adjusted manually.

A digital to analogue convertor (DIA) 17 is used to provide DC voltage output. The DC output voltage is derived by the D/A 17 from a voltage source Va from a stable voltage supply unit 18. D/A 17 is an 8- bit device with digital inputs connected to switches 19 which provide a voltage supply Vd, chosen as suitable for the logic I input requirements of the digital inputs of ID/A 17. The DC output voltage can therefore be incrementally set from 0 volts to Va by closing the switches 19 in a binary order. Switches 19 could be mechanical switches for example.

A voltage-controlled oscillator (VCO) 20 is connected to D/A 17, VCO 20 having a resistor I capacitor network determining its oscillating frequency and thus the clock frequency of the modem. A varactor diode is used as the capacitive element. The varactor diode of VCO 20 is fed with the output DC voltage from D/A 17, so that the capacitance of the varactor diode changes with variation of this voltage. Since the capacitance of a varactor diode is small, typically between 50 and 500 picofarads, the oscillator runs at several megahertz. This frequency is reduced (if necessary) to suit the modem by a frequency divider 21 with a counter circuit and Schmidt input which converts the typically sine wave output of VCO 20 into a fast-edged pulse train at a reduced frequency suitable for the modem clock.

To set up the network optimally, a test message is passed between the modems of the network at a first, eg. low clock frequency, and the data error rate is determined by comparing received test messages with the original. The test message can be resent at progressively higher clock frequencies, manually adjusted by means of switches 19, until a specified data error rate threshold is reached. At this point, the clock frequency of the modem is locked. This enables the performance of the network to be optimized.

Alternatively, the maximum clock frequency of each modem may be determined in turn.

The frequency for the system would then be set to the lowest of these maximum frequency values, in a similar manner as for the automatic selection method described above in relation to the first embodiment.

Fig. 4 shows a third embodiment of the present invention which enables the data rate of the modem to be adjusted automatically.

This embodiment shares several of the same components as the second embodiment, namely D/A 17, stable voltage supply unit 18, VCO 20 and frequency divider 21. In this embodiment, the digital inputs of D/A 17 are connected to analogue switches 22 which are controlled by a processor 16.

Modem 15 receives clock signals from the frequency divider 21 and is operable to transmit and receive data to and from processor 16 and the communications network.

The processor 16 contains a test routine, for example a software package, to perform the following functions: a) Generate test messages which are sent to the modem 15 to transmit over the network; b) Receive returned test messages from the network via modem 15; c) Compare the returned test messages with the originally-generated test message and detect any errors; d) Determine whether the error rate is below a stored acceptance threshold level; and e) Operate analogue switches 22 in binary steps to increase the modem clock frequency.

For a communications network such as the example illustrated in Fig. 1, the mode of operation for set-up could be: a) Set each of the modems at the control platform 1 and at SEMs 2, 3, 4 and S at their lowest clock frequencies; b) The processor 16 of the control platform modem (hereinafter designated "modem A") is programmed (e.g. pre-programmed) to accept a request to generate test messages; c) Modem A transmits a number of test messages, say 50, addressed to modem B (i.e. the modem at SEM 2) over the network; d) Modem B recognizes the messages as test messages, stores them and returns them to the network addressed to modem A; e) Modem A receives the returned test messages, compares them with the original test messages, detects errors and calculates the error rate. If the error rate is below the stored acceptable error threshold rate, it sends a command message to modem B to increase its clock frequency by one increment (using switches 22) and increases its own clock frequency by the same increment; f) Steps c), d) and e) are repeated as many times as necessary until the acceptable error rate is reached. At this point, modem A sends a message to modem B to reduce its clock frequency by one increment and also reduces its own clock frequency to match.

The D/A input value of modem A is stored; g) Modem A reverts back to the lowest clock frequency and repeats the steps c) to for each of the other modems; h) Having determined a set of the highest clock frequencies that achieves the specified data error rate between modem A and each of the other modems, the clock frequency of modem A is locked to the lowest frequency of the set, to ensure communication between all modems is below the specified error data rate.

The above described embodiments are exemplary only, and many other variations are possible within the scope of the claims. For example, although the communications system has been described with reference to an underwater well installation, it may be equally applicable in many other technical fields. Similarly, the layout of the networks may vary between applications.

There are various other methods for changing the clock frequency of a modem, which may be selected as appropriate. While the embodiments described above limit the selected clock frequency to one of a set of possible frequencies (i.e. determined by the digital dividers or analogue switches), it is conceivable that a continuously variable clock frequency could be used, for example by appropriate driving of a VCO.

The clock may be internal or external to the modem.

The processing means used may comprise the modem's internal processor, rather than a separate processor 16.

With the third embodiment described above, it is possible to dispense with the analogue S switches 22. In this case, the digital output voltages from the processor should match the digital input voltages required by the D/A. The binary digital output from the processor may then be connected directly to the binary digital input of the D/A.

The data rate selection methods described may also be used to identify any faults in the network, for example in the event of degradation of network performance. -10-

Claims

1. A communications system comprising a modem for transmitting data, the rate of data transmission by the modem being dependent on a clock frequency of the modem, wherein the clock frequency is variable to permit variable data rate transmission by the modem.

2. A communications system according to claim 1, wherein the clock frequency is selectable from a set of frequencies.

3. A communications system according to either of claims 1 and 2, comprising means for selecting the clock frequency.

4. A communications system according to claim 3, wherein the selecting means comprises means for adjusting a fundamental clock frequency.

5. A communications system according to claim4, wherein the fundamental clock frequency is determined by crystal oscillator.

6. A communications system according to either of claims 4 and 5, wherein the adjusting means comprises at least one digital divider for producing a frequency output different to the fundamental frequency.

7. A communications system according to claim 3, wherein the selecting means comprises a voltage controlled oscillator and means for controlling the input to the voltage controlled oscillator.

8. A communications system according to claim 7, comprising an array of switches to control the input to the voltage controlled oscillator.

9. A communications system according to claim 8, wherein the switches of the array are electronically controlled.

10. A communications system according to claim 8, wherein the switches of the array are operable manually.

Ii. A communications system according to any of claims 3 to 9, comprising processing means for controlling the switches.

12. A communications system according to claim 11, wherein in use the modem transmits data to a network, and the processing means receives data from the network and controls the selecting means in dependence on characteristics of the data received.

13. A communications system according to any preceding claim, comprising a second modem with a respective second variable clock frequency and data rate.

14. A communications system according to claim 13, wherein the second clock frequency of the second modem may be varied in response to a command sent by the modem to the second modem.

15. A communications system according to any preceding claim, wherein the system is employed in an underwater well installation.

16. An underwater well installation comprising a communications network including first and second inter-communicable modems, comprising means for adjusting the respective data rates of said first and second modems.

17. A method of selecting the data rate of each of a plurality of modems within a network, the data rate of each modem being determined by an associated clock frequency of each said modem, comprising the steps of: a) providing each of the plurality of modems with means for varying the clock frequency of the respective modem; b) setting a first modem of said plurality at an initial clock frequency and a second modem of said plurality at a respective second clock frequency; c) sending a test message from the first modem to the second modem; d) sending a return message from the second modem to the first modem in response to the test message; e) comparing the accuracy of the return message with respect to the test message; and, if the accuracy lies below a predetermined threshold: f) changing the clock frequency of the first and second modems; and g) repeating steps c) to e) until the accuracy lies above the threshold.

18. A method according to claim 17, further comprising, in a network with at least one further modem, the steps of: h) sending a test message from the first modem to a further modem; i) sending a return message from the further modem to the first modem in response to the test message; j) comparing the accuracy of the return message with respect to the test message; and, if the accuracy lies below a threshold: k) changing the clock frequency of the first and second modems; 1) repeating steps i) to j) until the accuracy lies above the threshold; and m) repeating steps h) to I) until the clock frequency of all such further modems has been adjusted if required.

19. A method according to claim 18, including the step of: n) setting the clock frequency of the first modem to the lowest clock frequency of any modem within the network.

20. A method according to claim 19, wherein steps a) to n) are carried out automatically.

21. A method according to either of claims 19 and 20, wherein steps a) to n) are controlled by a processing means associated with the first modem.

22. A method according to any of claims 17 to 21, wherein the network forms part of an underwater well installation.

23. A communications system substantially as herein described with reference to the accompanying drawings.

24. A method of selecting the data rate of each of a plurality of modems within a network as herein described with reference to the accompanying drawings.