Towards an Enhanced Telluric Compensation Methodology

Transcription

1 Towards an Enhanced Telluric Compensation Methodology Chijioke Ukiwe & Shamus M c Donnell Hunter M c Donnell Pipelines Services Inc. Presented at AUCSC 2011 by Gord Parker, C.E.T., CP2 Edmonton, Alberta, Canada

2 This Work This paper is a progress report of an ongoing R&D project to develop an enhanced Telluric and AC interference compensation methodology. Research commissioned by Spectrum XLI Partial funding from the Government of Canada This presentation was previously delivered at the NACE Northern Area Western Conference.

3 Telluric Current A telluric current (from Latin tellūs, "earth") is an electric current which moves underground or through the sea. The currents are extremely low frequency and travel over large areas at or near the surface of Earth. Telluric currents are also observed in the Earth's crust and mantle.

4 Telluric currents are primarily induced by changes in the outer part of the Earth's magnetic field, usually caused by interactions between the solar wind and the magnetosphere or solar radiation effects on the ionosphere.

5 Some Telluric Current Facts The magnitude varies, but can be in the order of 100's to 1000's of Amperes. Local variations in the conductivity of the Earth's crust affect the density of these currents as they follow the path of least resistance. This effect can be used to detect the presence of low-conductivity ore bodies beneath the crust in geological surveys. Along with the currents induced in the Earth's crust, any man-made conducting objects on or beneath the Earth's surface will also have large currents induced in them. This can cause power grid systems to trip from the overload. High sun-spot activities can buffet the Earth's magnetic field, causing fluctuations which in turn induces telluric currents on earth conductors, increasing the likelihood of such interruptions.

6 How do telluric currents affect pipelines? The sun emits energy & Particles Earth s geomagnetic field Magnetic field Induced electric field Voltage swings in CP systems during close interval survey CIS Induced electric currents on pipelines Source: space weather Canada

7 Telluric currents: economic importance V Voltage Test Post CSE Time Pipeline Pipe-to-soil potential measurement Difficulty to pipeline integrity assessment if uncorrected could result in incorrect interpretation and improper adjustment of CP system = Possibility of Corrosion

8 Telluric on pipe-to to-soil potentials: practical example 5 Seconds grid 2 Seconds CP Interruption Cycle Low frequency Temporal Voltage swing = V at t for any pipeline location 40 Second Telluric Current Cycle Telluric current frequency varies; cycle times of a few seconds to minutes.

9 AC Interference on pipe-to to-soil potentials 4 second grid 2 second CP Interruption Cycle Multiple Voltage swings superimposed on the pipe-to-soil Potential: High frequency AC 50/60 Hz Low Frequency telluric 1/40 th Hz

10 Telluric characteristics: Temporal AND Regional effects Telluric Magnitude At the same moment in time 300mV A B 100mV PIPELINE DISTANCE C 500mV D 400mV Regional Effects on pipeline structures on telluric variation: Valves, bends, rectifiers, anodes, insulating flanges Telluric Potential Variation up to ~100 mv/km at same moment

11 Telluric characteristics: Temporal effects 2. Space weather effects At a given location along the pipeline, PSP changes with time 20+ mv/sec > 100mV / cycle telluric effect has been recorded!

12 What is Telluric Compensation? Voltage Time Voltage Time Exclusion of Noise on PSP Data during CIS

13 Basis for Telluric Compensation Stationary PSP data (from stationary data logger (SDL)) summed over time yields stable average PSP PSP Comparable regional effects of telluric on PSP within several kms SDL PSP data can be used to correct for any PSP variation in close proximity PSP

14 Average PSP? Use statistics carefully, considering duration of temporal effects, and size of data pool. Using data from a short time period, other than the actual survey period can result in error. There are three kinds of lies: lies, damned lies, and statistics. Benjamin Disraeli ( )

15 Current Field Practices for Telluric Compensation 1. Real-time telluric compensation; based on stationary reference electrode placed at start of survey. SRE Data Logger + V - V Telluric Pipe-to-soil potential variations are corrected real-time

16 Current Field Practices for Telluric Compensation 1. Real-Time Compensation: pit-falls 1. - Unreliable representative average PSP - Difference in regional telluric effects on pipe can render correction useless High Telluric magnitude at A SRE A B Error in compensation increases away from Stationary electrode Suppressed Telluric magnitude at B

17 Current Field Practices for Telluric Compensation 2. Single Stationary Data Logger Compensation Post Processing For Compensation - Reliable representative average PSP during survey - Difference in regional telluric effects on pipe can render correction useless SRE + SDL Differences in regional telluric currents at A & B affect correction method Suppression of Telluric magnitude at B A B

18 Current Field Practices for Telluric Compensation 3. Multiple SDL Telluric Correction A C Approximately linear telluric profile at any Given time, t within pipe section, AB B

19 Multiple SDL Telluric Correction: Underlying Assumptions Based on Distributed Source Transmission Line (DSTL) Theory SDL placement spacing ~ 6 8 km Electrical continuity of telluric currents = uniformity of soil and pipeline characteristics Statistical average PSP is calculated from pool of readings that span entire survey period.

20 Multiple SDL Telluric Correction SDL-A SDL-B

21 Multiple SDL Telluric Correction Enhancements Field work Enhancements = Easier and faster Data Post-processing 3-4 SDLs Waveform Log Strategic SDL Positioning And more reliable results!

22 Enhanced Telluric Compensation: unique advantage ~ capturing FULL telluric variation trend along pipeline Telluric Magnitude Pipeline A B C D

23 4 SDL Telluric Compensation Method: example 300 Telluric PSP Variation MP: MP: MP: MP: Telluric Magnitude (mv) Time (Sec.)

24 4 SDL Telluric Compensation Method: Practical safeguard against error! PSP Variation (mv) SDL Placement (miles)

25 Enhanced Telluric Compensation: Fast Interruption cycle Telluric Variation can exceed 50 mv per second times cycle length = significant! Use of Fast Interruption Cycle of about ~1 second = ON:OFF (~3:1) = <50mV per cycle (simple 1 potential / cycle correction) Longer cycle makes it necessary for independent Telluric correction for ON and Instant Off PSP; different magnitude at start of cycle and end of cycle!

26 Multiple SDL Telluric Correction Enhancements: (2) Post-processing (a) Continuous waveform logging of the PSP Need to ensure the same readings are selected from both survey data and SDL if different technologies used, errors can result

27 Multiple SDL Telluric Correction Enhancements: (2) Post-processing (b) Simple moving average system on SDL data before telluric compensation Careful and thorough Choice of each ON and OFF PSP data before Telluric correction using Simple moving averages Of telluric and high Frequency AC interferences

28 Multiple SDL Telluric Correction Enhancements: Final Post-processed data CI Survey Compensation Survey Pol Critical low Comp Pol Critical High -1.3 Volts (CuCuSO4) Station (ft)

29 Even further telluric enhancements Recent advances in research holds promise for the use of telluric trends to forecast telluric magnitude over longer sections of pipelines = more strategic SDL placements and time of survey. Ability to compensate for high frequency interference; 50/60 Hz AC using high frequency stationary data loggers.

30 Conclusions The use of 3 ~ 4 Stationary data loggers to track telluric pattern and ensure accurate compensation Fast cycles are preferred to avoid under/over-compensation to either ON or OFF pipe-to-soil potential data Continuous waveform logging can enhance data accuracy Advanced statistical approaches can enhance post-processing and hence, overall pipeline integrity

31 Acknowledgement The Authors are grateful to the National Research Council of Canada for financial support through an Industrial Research Assistantship fund

32 Questions?