There are several above ground techniques with which it is suffucient to walk and measure coating defects over the entire lenght of the pipeline. A great advantage of these techniques is that you don’t need to perform destructive measurements or to excavate the pipeline. The 5 most common techniques are:
- Pearson technique
- Close Interval Potential Survey (CIPS technique)
- Pearson en CIPS combined
- Direct Current Voltage Gradient Technique (DCVG technique)
- External Coating Direct Assessment technique (ECDA technique)
Coating inspection should be integrated in a variety of activities to monitor the condition of the asset.
The Pearson technique is an AC current technique, that is probably the oldest inspection technique for above the surface pipeline inspection. With the aid of a transmitter, an AC signal is put on the pipeline, for example using a C.P. test post or any other location where metallic contact with the pipeline is available. Furthermore, a temporary grounding or earthing is necessary.
At sections where no coating defects are present, a regular small signal loss over the pipeline length will take place. At places where the external coating shows defects, the ac signal will leave the pipeline into the soil towards the earthing. Due to this AC current transport in the soil, a voltage gradient will appear. This voltage gradient can be detected above ground: Two surveyors walk behind each other over the pipeline with a distance between them of 8 – 10 meters. They have metallic contact with the soil (heal contacts or sticks) and are linked together by means of a metallic cable. The voltage difference between the 2 surveyors due to the AC current in the soil can be received by an AC receiver carried by one of the surveyors. The amplified signal can be heard using a head phone.
In case the 2 surveyors walk behind each other, the maximum signal indicating a coating defect will appear 2 times, when the 2 surveyors pass the coating defect. At the moment that the coating defect is precisely located in between the 2 surveyors, the SC signal will be minimal.
The Pearson technique is difficult to apply in areas where stray currents are present or where many other metallic objects are present in the direct vicinity of the pipeline. A further difficulty is thatthe Pearson technique will face problems in relatively dry soils: no good contact with the soil.
An AC signal of 1000Hz is commonly used with thick layer coatings such as asphalt bitumen, coal tar, polyethylene. For thinner layer coatings such as liquid epoxy or fusion bonded epoxy, a frequency of 175 – 200 Hz is used.
In more modern versions of the Pearson technique, it can be integrated with automatic recording of the survey data and automatically intepretated in conjunction GPS locations from the surveyer.
2. Close Interval Potential Survey (CIPS technique)
A pipeline protected against external corrosion by the application of cathodic protection, will be surrounded by potential gradients due to the voltage drop (IR drop) in the soil.
A defect in the external coating will locally change the potential gradients in the soil because the CP current is locally increased.
Compared with locations where no coating defects are present, higher potential differences will occur in the vicinity of a coating defect. These potential differences can be detected at the soil surface. Besides that, the degree of polarisation (protection potential) at the location of the coating defect can be measured with the so called on / off measurements. The C.P. system is periodically switched on and off. If on / off potentials are measured every 1-5 meters, a continual potential profile in the longitudinal direction of the pipe can be derived. This data can be very instrumental to get detailed information about the actual performance of the cathodic protection system, the technical status of the external coating system and the influences of possible interference. In practice, on / off potential measurements are normally done with the following intervals:
- Every meter above the pipeline
- Every 5 meters above the pipeline
- 4 “on” potential measurements and 1 “off” potential on every 5 meters above the pipeline
An important consideration at choosing an on / off cycle of the C.P. system is the condition that during longer periods, the actual polarisation potentials do not change due to the off periods. Too long off periods will cause the effect that depolarisation will occur and from that, no reliable potentials are measured. It is therefore that in practice, a minimum on/off cycle of 4 to 1 is being applied.
Before a CIPS survey is started, the exact routing of the pipeline must be clear. This is necessary because the CIPS surveyor must walk exactly above the pipeline in order to get reliable measurements of the C.P. potentials. As an alternative, a second surveyor with a pipe locator could walk in front of the CIPS surveyor in order to be sure walking exactly above the pipe.
In order to measure adequate off potentials, it is necessary that all rectifiers and other CP current sources sacrificial anodes) are switched off synchronically. Furthermore, all metallic contacts with other pipelines and third party constructions, which also are cathodically protected, must be disconnected from the pipeline to be measured. The surveyor with data logger and a Cu/CuSO4 – reference cell, walks over the pipeline while a metallic wire, connected between the pipe connection and the surveyor, is rolled off. During the survey, a constant contact between the Cu/CuSO4 – reference cell and the soil is necessary. It is therefore wise to use 2 reference cells providing a continues contact.
The distance from the starting point is measured by the wire, that is rolled off. During he CIPS survey, additional data such as road crossings, water crossings, fences, etc. can be put into he data logger. These objects are very relevant for good interpretation of data at a later stage.
In stray current areas, where potentials can fluctuate considerably, a static data logger can be installed in order to discriminate between potential fluctuations due to coating defects and potential fluctuations due to the stray currents in the soil.
3. Pearson en CIPS combined
It is feasible to perform a combined CIPS / Pearson survey. Besides switching the C.P. system, which is required for the CIPS measurements, an AC signal, required for the Pearson measurements, is put onto the pipe.
4. Direct Current Voltage Gradient Technique (DCVG technique)
The DCVG – inspection technique is being developed to be the most common technique to localize and characterize external coating defects.
To explain this method in a nutshell, it a method with which a pulsing DC current signal is put onto the pipeline. As a current source, the rectifier of the CP system can be used or a temporary current source like battery or other DC current source. If the rectifier is used as a current source, the anode bed of the CP system can be used as the positive pole. Using an external current source, a temporary anode bed as an earthing is used
The pulsing current signal is created by periodically switching the current consumption to the pipeline by means of an interrupter. The frequency of the switching signal is mostly 1/3 sec. ‘on’ en 2/3 sec. ‘off’.
At the location of a defect in the external coating, current from the CP system will flow from the anode bed in the direction of the coating defect. This current will cause a potential field around the defect. In figure 22 are also drawn the lines with equal potential, the so called equal potential lines. Due to he pulsing CP current, the potential field will also pulse. This pulsing potential field can be measured at soil level with a mV meter and 2 reference cells).
Every time when the current source is switched on, a potential difference is being measured. When the current source is “off”, no potential difference is measured (no current no potential field). The indicator on the millivolt meter will fluctuate with the same frequency as the electric potential field, the frequency as the interrupter.
The most negative reference cell will point, when the current source is on, at the direction of the coating defect. Moving on towards the coating defect, a situation will be reached in which both reference cells are located at the same equal potential line. In that situation no fluctuation on the millivolt meter will be seen: in the ‘on’ situation as in the ‘off’ situation, no potential difference exists between the 2 reference cells. Normally, the coating defect will be located exact in the middle of the 2 reference cells in the longitudinal direction of the pipe.
At the located coating defect, the same measurement is repeated in the rectangular direction. In that way the exact epicentre of the coating defect is located. A wooden peg can be placed to indicate the exact place of the defect.
- No synchronisation of rectifiers needed
- The survey can be done by one person
- Gives information on the performance of the cathodic protection
- Can be applied in stray current areas
- Exact location of external coating defects
- Gives information on the relative size of defects
- Gives information on anodic / cathodic behaviour
Information from the last 3 points can be very important for prioritising repair and rehabilitation activities.
- The survey progress per day is dependent on the quality of the external coating
- It is necessary to walk the whole length of the pipeline
- DCVG application is not possible under tarmac roads
- No ‘hard copy’ of the results
5. External Coating Direct Assessment technique (ECDA technique)
The ECDA technique is developed for pipelines that cannot be inspected on external corrosion by Intelligent Pigging. The data gathered with this technique will provide pipeline owners insight in the actual technical condition of the pipeline. This information can be used to decide on actions regarding a possible rehabilitation of pipeline (section)s.
The ECDA technique is a combination of existing techniques such as for example DCVG and CIPS. Necessary also is the measurement of the soil resistivity on regular distances. An absolute condition, using this technique, is that all coating defects, even the smallest, must be localized and characterized.
The ECDA technique is not very often used up to now, due to the fact that it is labour intensive and consequently relative cost expensive.