Abstract: In order to ensure the normal operation of pipeline transportation, it is of great practical significance to conduct early warning of pipeline damage. It is an important part of early warning monitoring to determine the location of the incident quickly and accurately. The system scheme of pipeline early warning monitoring and positioning is designed. The principle of monitoring and positioning is analyzed. It is pointed out that the time difference between the damage signal transmitted to the adjacent monitoring stations on the upstream and downstream is the main factor affecting the precise positioning. The GPS timing is used as the benchmark. The clock synchronization subdivision technology can more accurately capture the moment when the acoustic signal reaches the adjacent monitoring stations, and the experiment results are good. Provide a new and effective technology for quickly and accurately locating the location of the damage.
Keywords: GPS; clock subdivision; pipeline; destruction; positioning
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1 Introduction The pipeline transportation industry has the advantages of low cost, energy saving, high safety, long transmission distance and stable supply. It has unique advantages in transporting liquids, gases and slurries, especially in industries such as petroleum, chemical, natural gas and urban water supply. China's pipeline construction has developed rapidly in the past two decades, and it is clearly stated in the 11th Five-Year Development Plan that four major oil and gas pipelines will be built.
As the age of the pipe grows, the leakage caused by natural damage becomes more and more serious. Especially in the major oil production areas and the wide oil and gas pipelines, man-made destruction of theft has occurred. Pipeline leakage will not only affect the normal operation of pipeline transportation, but also cause major economic losses. When transporting toxic, corrosive, flammable and explosive media, it will also pollute the environment and cause fire and explosion accidents. Therefore, the detection of pipeline leakage and the location of the damage point have extremely important practical significance .
Among the various causes of pipeline leakage accidents, man-made vandalism (punching, cutting, drilling) has surpassed the natural factors and ranked first . Therefore, it is possible to detect the location when the pipeline safety is threatened or just damaged. The early warning system can prevent problems before they happen. Quickly and accurately determining the location of the incident is an important part of early warning monitoring. This design uses GPS reference clock subdivision technology to accurately obtain the time when the characteristic signal is transmitted to each monitoring station, and calculates and monitors the difference between the time stamps. The distance is obtained by separately obtaining the velocity of the characteristic signal to the upstream and downstream, thereby determining the exact location of the pipeline damage.
2 system design and positioning principle
2.1 System overall design The whole system is shown in Figure 1. It consists of several monitoring stations, GPRS network, Internet network and a data service center . The monitoring station is distributed along the pipeline and is responsible for signal collection, information processing and transmission. The data service center computer has an IP address, and uses the data transmission terminal (DTU) and the pipeline intelligent detection terminal unit (RTU) to transparently transmit data through the Internet network and the GPRS wireless channel, realizing real-time monitoring of the working status of the transmission pipeline, fault alarm, Integrated management functions such as breakpoint location.
Figure 1 Pipeline damage prevention monitoring and its positioning system
System working process: Install a sound pressure sensor on each of the four adjacent stations of the pipeline. The probe protrudes into the interior of the pipeline and contacts the fluid in the pipeline. When somewhere in the middle of the pipeline is damaged or leaks, the characteristics of the damage point The sound waves propagate to the sound pressure sensor along the pipeline to the two ends. After the sound wave signal is preprocessed, the characteristic parameters that can represent the sound wave signal are extracted. When the characteristic parameters match the pattern characteristics obtained by the mode training, the synchronous clock interrupt program is started immediately. And note the moment when the characteristic sound wave arrives, so as to prepare for the calculation of the damage point position compared with the time when the characteristic sound wave signal reaches the other station; the propagation speed of the characteristic sound wave in the fluid and the clock synchronization accuracy of each station determine the damage point. Positioning accuracy, using the standard UTC time of the Global Positioning System (GPS) satellite, can provide TTL level seconds pulse, through the clock subdivision processing can obtain more accurate time information in the second; time-labeled characteristic sound wave signal is encoded In the future, the GPRS wireless data device will be used as the data terminal unit (DTU) to move the packet data industry.
2.2 Positioning principle When the sound pressure sensor receives the characteristic sound wave signal, the position of the damage point can be calculated according to the time difference of the characteristic signal propagating to the upper and lower monitoring stations and the propagation speed of the sound wave in the tube. Factors such as the pressure distribution and density distribution of the propagation medium and the physical form of the medium will affect the speed of sound wave propagation. In addition, the influence of the flow velocity of the fluid itself should be considered, because the characteristic sound wave propagates upstream against the flow direction of the fluid, and downstream is the flow direction of the fluid, so the positioning principle shown in Fig. 2 is adopted. Fast response speed and high positioning accuracy.
Figure 2 Schematic diagram of positioning principle
D â€“ the distance between two monitoring stations 1, 2 in the pipeline, m;
s â€” the distance from the destruction point to the upstream site 1, m;
Î”s â€” monitoring the known distance between stations 1 and 3, 2 and 4, m;
T0 â€” the time at which the acoustic signal is generated, s;
T1, t2, t3, t4â€”the time at which the acoustic signal propagates to stations 1, 2, 3, 4, s;
Î”t13, Î”t24, Î”t12, Î”t01, Î”t02â€”the time difference between the two stations 1 and 3, 2 and 4, 1 and 2, station 1, station 2 received the acoustic signal, s;
The propagation speeds of sound waves along the pipeline up and down are:
vä¸Š=â–³s/â–³t13 v==s/â–³t24 (1)
The positioning equations are listed according to known conditions:
s = vä¸ŠÂ·â–³t01 D- s = vä¸‹Â·â–³t02 â–³t12=â–³t01 - â–³t02 (2)
Substituting equation (1) into equation (2) to get the positioning formula:
s = (DÂ·â–³t24+â–³sÂ·â–³t12) / (â–³t13+â–³t24) (3)
It is known from the formula (3) that the position of the damage point can be accurately determined as long as the monitoring stations at the upstream and downstream of the pipeline accurately receive the characteristic sound wave signal with the time stamp attached thereto.
3 Based on GPS reference clock synchronization and segmentation
3.1 Field Processing Unit The field processing unit is a remote terminal unit (RTU) that collects, processes, and transmits field data, including a core processing unit, a sound pressure sensor, a voice recognition unit, a GPS receiving device, and a GPRS data wireless communication device. Figure 3 shows the hardware connection of each part of the field processing unit.
Figure 3 RTU system hardware components block diagram
The on-site processing unit is controlled by the C8051F020 microcontroller, without the need to extend the serial port, ROM and RAM. The MCU has two hardware serial ports. The baud rate generation of these two serial ports is independent, and it does not occupy the timer. It is quite flexible to use, and communicates with the GPS timing module and the GPRS DTU respectively. The GPS module sends the time information from the serial port in a fixed format. The MCU receives the data sent by the GPS module, parses out the useful time data for encapsulation, and then delivers the second serial port to the GPRS DTU in a prescribed format, using the GPRS network. Send the data to the Internet.
3.2 GPS reference clock Synchronization failure point location requires the time when the known characteristic sound wave signal propagates to the upstream and downstream sites. Obviously, if you want to get an accurate time difference, you must ensure that the data used for analysis has the same start time. Otherwise, even if the arrival time of the sound wave can be accurately captured, it will be biased because the start time of the signal is inconsistent. The sampling time of the station is not synchronized, so the calculated time difference is meaningless, let alone precise positioning. Therefore, it is important to choose a higher precision timing system to provide uniform time for each site.
Conventional clock frequency generation methods have certain problems, and the positioning accuracy cannot meet the requirements. If the crystal ages, it is susceptible to changes in the external environment; the atomic clock will also produce deviation after long-term use, and it needs timing calibration. In this design, the way to solve this problem is to use the Global Positioning System (GPS) to uniformly time the stations, ensure the clock synchronization of each station, and add the time information collected in the collected data, thereby reducing the error. .
GPS provides timing and positioning functions globally, and GPS users anywhere in the world receive signals from satellites through low-cost GPS receivers to obtain accurate spatial position information, synchronization time stamps, and standard time . In the design of the system, the remote terminal device RTU is embedded in the GPS signal receiving device, and the sampling clock is periodically calibrated. Each remote terminal device uses the calibrated sampling clock as a sampling basis to completely solve the problem that the system clock of the distributed system is not synchronized.
Use the Trimble Lassen SQ/IQ GPS receiver module to use GPZDA data information in NEMA0183 format. When the GPS receiver is synchronized with the GPS clock, it will receive the message. The format is $ GPZDA, hhmmss.ss, xx, xx, xxxx,, *hh. The information includes hours, minutes, seconds, days, months, years, and sum checksums.
3.3 clock subdivision
The GPS time information is synchronized with the second pulse, and one pulse is sent every second. If the receiving time is between two adjacent second pulses, only reading the GPS UTC time will cause a large error, so that the positioning is more accurate and the clock is fine. Minute. C8051F020 microcontroller 16-bit timer has two registers of high byte and low byte, the maximum count can reach 65536, programmed to make each machine cycle 20Î¼s, the timing time can be up to 1.31s, enough to complete the time in the interrupt per second timing. Assign high byte data to variable timerH, low byte data to variable timerL, convert to decimal value after processing, which is the number of counts, multiply by 20Î¼s to get accurate seconds time, assign value to a stored time data The structure timerTime can achieve an accuracy of 20Î¼s. Figure 4 is the clock synchronization and subdivision timing diagram, C8051F020 program flow chart shown in Figure 5.
Figure 4 Clock synchronization and subdivision timing diagram
Figure 5 core processing unit program flow chart
The working process of clock subdivision is as follows: C8051F020 uses the interrupt mode to receive the UTC time information of GPS. The PPS of the GPS receiver generates INT0 interrupt every second. After responding to the interrupt, in the INT0 interrupt service routine, the timer is cleared after starting. In order to perform clock subdivision, GPS time frame information is received through UART0. When the next PPS triggers INT0, the same timer is cleared after starting, and the time data structure is updated. In this way, the INT0 interrupt is repeated and interrupted once every second. The external INT1 is triggered by the P11 port of the dedicated DSP chip and is set to the highest priority. If the external INT1 interrupt is triggered, it indicates that the voice recognition unit has successfully destroyed the sound wave recognition, and the main program immediately Turning to the INT1 interrupt service routine, reading the high byte and low byte registers of the timer, the CPU is processed to combine the time information with the GPS time frame to create a time stamp.
3.4 Experimental results The destruction information message contains the year, month, day, hour, minute, second, precision 20 s microsecond time information and the destruction mode feature code, for example, as shown in Figure 6, 085026.82366 06, indicated in April 2006. At 8:50:26 and 823,660 microseconds on the 23rd, the DSC received the corrupted mode code from the station 13573100603 as 06 information message.
Figure 6 Received message information
If the DSC receives the message information of each monitoring station upstream and downstream of the pipeline, and decodes and extracts the time when the acoustic signal is transmitted to each station, it is simply processed by the program to identify the damage mode, and the damage point can be accurately determined by formula (6). Positioning, the theoretical error is less than 1m.
4 Conclusions Using GPS timing as the benchmark clock synchronization subdivision technology, the time at which the damaged characteristic acoustic wave signal propagates to each monitoring station can be accurately obtained. The time label can be used to determine the velocity of the breakpoint characteristic signal to the upstream and downstream respectively, and then The exact location of the pipeline damage can be determined by using the difference between the time stamp and the distance between each monitoring station. Good results have been achieved in the experiment. Before the pipeline safety is threatened or damaged, an early warning is issued, and the location of the incident is quickly and accurately determined, so that the passive inspection line is turned into an active and targeted attack, which can effectively curb the drilling and theft phenomenon, saving a lot of manpower and material resources. And financial resources, providing a new and effective technical guarantee for national property security and environmental protection.
The main innovations of this paper are as follows: (1) Determining the speed measurement method of sound wave propagation along different directions of the pipeline, and deriving the calculation formula of the actual damage point location; (2) Using GPS timing and clock subdivision technology for each monitoring station of the pipeline The clock synchronization, improve the positioning accuracy of the damage point, and realize the early warning monitoring of the precise positioning of the transmission pipeline damage.
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