Survey Methods
OICCG have a wide working knowledge of survey methods including direct current voltage gradient (DCVG), cathodic protection survey (CPS) Close Interval Potential Survey (CIPS) and pipeline current mapping (PCM). PCM, CPS and CIPS are methods used to locate and assess the external coating and level of CP along the pipelines. When used in conjunction with GPS and GIS, these techniques compliment one another and provide a comprehensive assessment of the pipelines.
Pipeline current mapper (PCM)
Level of protection survey (LPS)
Cathodic protection survey (CPS)
Close interval potential survey (CIPS)
Close interval polarized potential survey (CIPPS)
Pipeline integrated gauging (PIG)
Global positioning system (GPS)
PIPELINE CURRENT MAPPER (PCM)
The PCM instrument provides an accurate means of locating a buried pipe and assessing the coating. A low “near DC” frequency (4Hz) is used to simulate the DC current generated by cathodic protection. The low frequency avoids the losses caused by capacitance so nearly all the signal loss is due to coating faults or shorts to other structures. Conventional pipe locators cannot practically detect such a low frequency, so the PCM receiver is fitted with a precision, high performance magnometer. This detects and measures the 4Hz magnetic field. Advanced signal processing techniques filter and amplify the signal, so that push button current measurements (magnitude and direction) of the “near DC” signal can be made. Integral data logging functions enable the storage of this data so that current loss versus distance can be plotted.
Careful use of the PCM gives the location of coating faults to within
a few meters. The addition of the A-frame attachment enables the operator
to pinpoint the fault to within a few centimeters.
CATHODIC PROTECTION SURVEY (CPS)
The CPS instrument locates and measures rectified Cathodic Protection (CP) impressed signals without the use of an ancillary transmitter. Current readings are made with a single keystroke. In this way the ripple content of CP currents are measured and used for pipeline analysis.
Pipeline coating condition is responsible for current changes. Current
change is quantified as the ratio of current before and after a coating
problem. If a pipeline is electrically consistent along its length
then the rate of change will be uniform, and the graph is a straight
line showing the change/unit length. Once the faulty section is defined,
regular close interval measurements will pinpoint the fault location.
This technique exposes those points that differ from normal.
CLOSE INTERVAL POTENTIAL SURVEY (CIPS)
CP systems are routinely monitored at test points at intervals of approximately 1-2 km apart. The limitation of this routine monitoring is that the status of a defect located remote from a test point remains undetected. To obtain an accurate assessment it is necessary, therefore, to record potential measurements at a greater number of locations than simply at the test point installed as part of the pipeline system.
The CIPS technique involves the measurement of the pipe potential
at intervals of less than one pipe depth, typically 1-2 meters. A
trailing cable provides contact with the pipe and a static potential
recorder was employed to discern between stray current influences
and coating defects. CIPS yields a comprehensive pipe to soil potential
profile, which highlights areas of under protection at points remote
from the test points. An integral data logger enables the structure
potential versus time and distance to be plotted.
GLOBAL POSITIONING SYSTEM (GPS)
The GPS surveyor fixed prior to, and during the course of the PCM survey, reference points, (known points), roughly dividing the pipes length into manageable zones. These established base lines were utilized in the post GPS processing procedure. The GPS survey is undertaken using two L1 GPS receivers observing the same satellite constellation simultaneously. By placing the “reference receiver” at a known point, time theoretical measurements expected at this point are compared with those actually observed. The second mobile receiver is used to occupy all nominated features such as PCM pegs, pipe markers and CP posts, closing on another known point of reference. This method is known as “Stop and Go” and combines the advantages of Static and Kinematic survey techniques.
Typically the position data accuracy, captured during this method of survey would be in the order of sub meter, with the expected case being centimeter accuracy in good survey conditions. These conditions are expected where unrestricted skies prevail. Areas inhibited by overhead obstacles such as tree cover and tall side obstructions, not conducive to ideal GPS conditions, are treated using traditional survey techniques.