Pipeline communication system

The invention relates to a pipeline Communication system for providing monitoring and/or control of pipeline based plant and equipment.

SUMMARY OF THE INVENTION

In patent documents WO-A-9326115 and FR-A-2097265 undersea pipeline systems cooperating with an offshore drilling platform are disclosed having a communications link.

The present invention is concerned with a land based system.

According to the invention there is provided a pipeline communication system for an electrically conductive pipeline insulated from the adjacent ground and including; transmitter means for sending information from a remote station using the pipeline and the adjacent ground as conductors; receiver means for receiving information at a remote station using the pipeline and the adjacent ground; and means employing recovery techniques to enhance the degraded received signals due to their passage along the pipeline and the adjacent ground.

Further according to the invention a communication method employing an electrically conductive pipeline insulated from the adjacent ground the method including transmitting information from a remote station by using the pipeline and the adjacent ground as conductors, employing recovery techniques to enhance the degraded received signals to recover the information therein.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference to the accompanying drawings in which:

FIG. 1

shows an example of a pipeline network utilising the invention;

FIG. 2

shows a transmitter for the pipeline communication employing frequency shift keying techniques;

FIG. 3

shows an alternative transmitter arrangement where the signal is amplified and applied directly to the pipe;

FIG. 4

shows a receiver suitable for receiving transmitted signals taken from the pipeline;

FIG. 5

shows a clock recovery arrangement for use with the output from the receiver in

FIG. 4

;

FIG. 6

shows an alternative analog non-coherent receiver to that of

FIG. 4

with some constructional advantages;

FIG. 7

shows correlation using a digital version of the receiver of

FIG. 4

; and

FIG. 8

shows the digital arrangement for clock recovery from the receiver of FIG.

6

.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1

shows a typical pipeline arrangement employing the invention. The pipeline

10

includes junctions

11

and

12

. Equipment or plant such as governor modules

14

and transformer/rectifiers (T/R)

15

form part of the network. A number of monitors including flow monitors

16

, governor monitor

17

and hazard monitor

18

periodically provide information to the network controller

20

. This information can pass to the system control

22

via outstation

21

using a telemetry link operating over one of the normal means of data communications, with access by radio link, the PSTN or other suitable equipment. To communicate to the controller

20

, the monitors use the pipeline as a conductor. In order to use the earth as the return conductor, the metal pipe is insulated therefrom. Although transmission signals may be directly applied to the pipe, typically they will be superimposed on the existing cathodic pipe protection system.

In addition to monitoring signals, control signals can be sent from controller

20

to adjust governors, transformer/rectifiers or other plant provided. Hence two way communication is available.

The communication system employs low frequency (typically 5-20 HZ but could be as high as 200 HZ) low bandwidth modulation techniques. Modulation is typically in the form of fast frequency shift keying (FFSK). Because of the length of pipeline that can be involved, typically up to 50KM, the attenuation of the signals can be high as well as associated noise levels giving rise to low signal to noise-ratios. Hence it can be that noise is so high that the signal to noise ratio can be low, typically 5dB. To overcome this, signal recovery techniques are employed typically using matched filters. Repeaters

23

can also be employed to increase signal levels and will be sufficiently intelligent to monitor embedded address destinations and hence the need for selective regeneration. The transmitter uses FFSK applying a tone of one frequency to represent an input bit with a data sense of ‘1’ and a second frequency tone to represent an input bit with a data sense of ‘0’. These data bits are clocked at a data rate of typically 10 bits per second. The system is not limited to two tones. Additional tones can be used to send multiple data bits in parallel.

FIG. 2

shows the structure of a transmitter suitable for transformer/rectifier sites. The transmitter

30

comprises waveform generators

31

-

34

which transmitter receives the data at input

35

which selects 10 Hz tone as an indication of data ‘1’ and a 15 Hz tone as a data ‘0’ via selector

36

. Using Fast Frequency Shift Keying (FFSK) at 10 baud, the modulated data signal is applied by modulating part of the current applied to the pipe (current switch

37

) which provides cathodic protection (CP) as a result of the pipeline resistance. The signals are in digital form with multistage representations of the two modulating tones. Each state of these tones represents the switching levels for the transformer/rectifier pipeline devices. The bandwidth of the pipe is such that the switching harmonics are considerably reduced through their passage in the pipe so are of no great concern at the receiver.

Where stand-alone sites occur the data signal is amplified and applied directly to the pipe using a transmitter

40

as shown in FIG.

3

. The transmitter includes waveform generators

41

-

44

which transmitter receives the data at input

45

and selects 10 Hz or 15 Hz as before. The signals are applied to the pipeline via low pass filters

46

,

47

and amplifier

48

. The signals are all digital representations but imposed on the pipe CP as an analog.

The receiver needs to cope with signal to noise ratio of only 5dB for a typical pipe distance of 40 KM. A suitable receiver is shown in FIG.

4

. The receiver

50

is a non-coherent FFSK matched filter receiver. The receiver

50

includes amplifier

51

, low pass filter

52

and automatic gain control

53

and through the band pass filter

54

. The filter output is passed via amplifier

55

to detector

56

. This allows the analog signal from the pipe to be received and the digital data recovered into streams F

1

and F

2

. Matched filters

57

and

58

and summer

59

provide the recovered (uncloacked) data at its output. Separate matched filter paths are used for each tone and data decisions are made by comparison of the resultant energy output from each filter. Symbol recovery is derived from the data decision stream. Each matched filter is implemented as an integration of the incoming data stream multiplied by a sine wave at the respective sub-carrier frequency (over each successive study symbol period). This is an analog autocorrelation of each tone with the received incoming data stream. In order to recover the transmitted clocks accompanying the transmitted data, the circuit of

FIG. 5

is employed. The clock recovery circuit

60

includes a sample delay

61

and data slicer

62

. A digital voltage controlled oscillator

63

and phase locked loop

64

and doubler

65

. Clock recovery is effected by tracking both the peaks of the autocorrelation streams to form a new clock sampling stream derived from the instantaneous value from either of the correlated streams. This clock sampling stream is recovered at the clock rate.

In practice the analog non-coherent receiver can be modified to that shown in FIG.

6

. The components include low pas filters

70

and sample delay

72

on one path and low pass filter

71

and amplifier

73

on the other path passing to summer

74

and hence to delay

61

. Other paths include delay

75

and mixer

76

. The output from mixer

76

goes to band pass filter

77

and then to amplifier

78

before input to comparator

79

.

When digital implementation is required a receiver can be constructed as shown in FIG.

7

. This uses matched filters and employs a series of correlators. The analog first stage output is received by summing amplifier

81

and converted into digital form via analog to digital converter

82

. The dual path filtering

57

and

58

is provided as before now in digital form. As shown in the upper (expanded) path a sampler

83

with delays

84

and adders

85

, dividers

86

and summer

87

to path sampler

88

and comparator

89

. Hence, the received analog signal is amplified, filtered and fed to an automatic gain control loop. Typically the LPF has a −3 dB cut off at 20 HZ. The AGC uses a bandpass filter with −3 dB points at 7.5 HZ and 17.5 HZ to select the band of interest for detection. The level detector input is used to adjust the gain of the loop so that the output AC peak to peak signal most closely fits the peak to peak full scale (FS) limit of the digital convertor (ADC). The ADC gives typically

10

bit resolution. F

1

and F

2

paths are concerned respectively with each matched filter.

Clock recovery associated with the transmitted information has to be extracted from transmitted information when received using correlation techniques employing data registers as is shown in FIG.

8

.

The devices

61

-

65

are provided as before and now digital data in the registers

90

with associated summer

91

and comparator

92

utilise samples available from the F

1

and F

2

sample streams. The nature of the communication channel used by this system is such that equalisation is usually required for acceptable data performance. If the same hardware is to be used with many different pipe types, sizes and coatings then the pipe frequency and phase characteristics may be extremely varied. Automatic Equalisation is therefore included to compensate for gross amplitude non-linearities with frequency at design level of around +/−15 dB across the modulation band for the currently preferred modulation scheme. Other forms of equalisation could be implemented if another modulation scheme was used. The new equalisation scheme may then require a different amount of fixed or automatic equalisation, accordingly.

The communications bearer system described here forms a data network for remote monitoring between the controller on each section of electrically isolated pipe and the monitors. The remote monitoring data network (RMDN) detailed here uses a multilevel priority access technique using Carrier Sense Multiple Access (CSMA) with N-Persistent retries. Two priority levels are used at present. High Priority (HP) and Low Priority (LP), however more levels may be used in the future according to user demands. At present, all LP monitoring messages are allowed M-access_retries and all HP alarm and control messages are allowed P-access_retries where M, N and P are statistically chosen to ensure that all HP messages are successful and a percentage Q (acceptable to the user) of LP message get through.

The system has been designed to operate over a channel at ranges where the SNR is better than 5 dB and the signal level is greater than 100 mV at the receiver. These parameters have been chosen as a compromise of current technology versus perceived cost of the marketable product. Performance at lower threshold and SNR values could be considered in the future, according to market trends.

Repeater functionally is provided to route messages via repeaters between initiating and destination units that are physically separated by a distance greater than the maximum Inter-Unit Distance (IUD). The system described here, can support up to 512 repeater units, allowing a network of between 1 and 10, 220 m in length, per electrically isolated section. The maximum serviceable length of pipeline chosen here has been identified from typical long-haul pipelines in the world to date. The protocol may be changed to accommodate longer pipelines in the future, but this is seen as not likely to be cost effective, today.

A proprietary protocol is used for data operation over the remote monitoring network between the monitor and the system control. The protocol is designed to realise the optimum data performance over the system by matching the data techniques used to the communications channel formed by the pipe.

This system uses up to 5 pre-defined data packet sizes and up to 16 user defined data message types within each packet. Each packet has Forward Error Connection (FEC) and bit interleaving applied to it on transmission. Error checking is performed using a standard CRC. The system uses standard run in and byte synchronisation techniques. The byte synchronisation word used is 32 _bits long and there are 5_different words used, one to identify each of the different packet types.

The number of packets and user defined messages has been chosen to suit the present Gas Market. However, these parameters could be adjusted for another market in the future.

Within each data packet, two way routing fields are used to route packets via units capable of repeating messages.