Corrosion Under Insulation is a major threat to oil and gas, petrochemical and fertilizer plants in general. It occurs mainly due to ingress of water through the insulation which then comes in contact with the metallic pipes underneath. Corrosion is expedited by elevated temperatures; poor coating on the pipes and chemical environment especially chlorine as in the case of marine structures, etc. There are various types of CUI like galvanic, chloride, acidic or alkaline. In effect it causes wall thinning of the metallic pipes fabricated using carbon or low alloy steel and stress corrosion cracking (SCC) or pitting corrosion in case of stainless steel pipes. Since it occurs underneath the insulation nothing will be visible on the surface until the leakage is observed outside the insulation but by then it would have led to serious safety issues which could probably end up in shut down of plant.
Therefore it is very critical to have a periodic monitoring mechanism in place to detect CUI.
Conventional NDT methods to monitor CUI and their limitations
Visual inspection: This is considered the simplest and cheapest mode of inspection to look for corrosion on the surface of pipes. From experience the plant inspectors will pick areas where the probability of corrosion attack is more. After the insulation are removed the pipe surfaces are visually checked by the inspectors to look for corrosion, deteriorated coatings, pitting, rusty or loose scale formation on the external areas of pipes. But this has so many limitations as the wall thinning normally starts from inside cannot be seen from outside. Furthermore the starting of stress corrosion cracking is incredibly small that naked eye cannot see, therefore this method of inspection is not recommended except for detection of gross corrosion.
Manual Ultrasonic thickness gauging (UTG): This is one of the widely used techniques to monitor corrosion. It uses pulse-echo technique, wherein the operator uses normal/0 degree, single crystal or preferably twin crystal transducer in contact with the external surface on the pipe being tested after application of couplant, the longitudinal ultrasonic waves now pass through the wall thickness and bounces/reflects from the inner walls of the pipe and return to the probe, by setting the actual velocity of the material under test the operator can record the available wall thickness of the pipe. This is later compared with the drawing to calculate the actual wall thickness remained after corrosion. But major limitation is mainly removal of insulation to allow direct contact with external metal surface. Though with latest thickness gauges removal of paint or coating is no longer required but the probability of detection of cracks are not possible and the inspection results are dependent on the skill of the operator and time consuming. If wrong velocity is chosen by operator then the whole data obtained will be erroneous.
Conventional Eddy current: This is very effective and widely used method based on electromagnetic induction for detection of CUI especially Stress Corrosion cracking in stainless steel pipes as most grades of stainless steel pipes are not ferromagnetic in nature thereby magnetic flux leakage testing is not feasible. The effect of wall thinning is detectable but eddycurrent is a comparison method meaning it will not give specific value of thickness, it will only show a variation in impedance graph. This testing is effective even if there will be little insulation but with increasing insulation thickness the probability of detection (POD) is mitigated due to lift-off factor. Also for general carbon and low alloy steel due to high permeability again the eddycurrent will not give proper results due to poor electrical conductivity. Moreover eddycurrent generated are mostly limited to surface and subsurface, so with increased wall thickness of pipe, this technique may not be adequate.
Advanced NDT techniques to monitor CUI and their limitations
Phased Array: This technique is expensive and uses complex equipment, scanners and software but still is widely being used in the field of corrosion mapping. The basic concept of phased array is no different from conventional UT as both works on pulse – echo technique. Instead of single crystal in conventional UT, phased array uses an array of elements or crystals which can be electronically excited to achieve constructive interference at desired focus depth hence eliminating the problems associated with near field effect in conventional UT. Moreover beam steering and focussing is possible and automation of inspection is possible which helps to reduce time drastically using line scanning. Also permanent record of corrosion mapping is available which facilitates reanalysis at later stages. Using modern techniques like HydroFORM, Olympus TM, is specifically designed for detection of wall thickness reductions and also distinguish corrosion from other anomalies like hydrogen induced blistering and fabrication induced lamination. But major limitation, equipment is very expensive and being a contact method the removal of insulation and surface preparation will be needed. Furthermore operator needs to be highly trained for operation and data interpretation of results.
Long Range Ultrasonic Testing (LRUT)
LRUT is a becoming more popular these days for detection of CUI especially in pipelines onshore/offshore, risers, refineries, etc because it eliminates the need for removal of insulation except the portion where the probes are placed around the pipe. LRUT uses ultrasound with very low frequency which enables them to travel long distances through pipes wall thickness under the insulation and when it comes across sections with reduced wall thickness, the sound energy is reflected back to the probe which is recorded and interpreted. To further investigate other NDT techniques like manual UT or RT can be carried out to pin point the defect location and characterization. The advantage of LRUT is that around 300 meters of pipe can be scanned by wrapping the probes around one location no matter the pipeline is buried underground or covered by thick concrete blocks this technology still remains effective. Disadvantages being very expensive equipment and requires highly trained operator for scanning and interpretation.
Pulsed Eddy current (PEC): This is an advanced electromagnetic inspection technique that has revolutionized detection of CUI on ferrous pipes. The advantage over other techniques being no need for removal of insulation, even presence of aluminium, concrete, or galvanized steel weather jacket, rebars on the outer surface does not hinder the PEC inspection. The technique uses a larger dimension probe placed on top of the insulation. As current passes through the probe coil a strong magnetic field is generated which penetrates through the insulation and reaches the buried pipe and stabilizes inside the material thickness. The current is then turned off immediately and this in turn set up eddycurrents inside the component due to electromagnetic induction that diffuses inwards and loses strength. This reduction in eddycurrent is monitored by the PEC probe and used to determine wall thickness. The limitations are PEC will only detect gross corrosion; it cannot distinguish between near surface and far surface defects, cannot detect small pitting corrosion, difficult to use on smaller diameter pipe elbows. Also equipment is expensive and requires trained operator for scanning and interpretation.
The effect of CUI can be severe leading to sudden and catastrophic failure, especially in systems that operate at high pressure. Localised pitting corrosion can cause leaks, potentially releasing hazardous chemicals into the environment. Therefore it is of paramount importance to establish a proven monitoring method in place which is reliable, economical and has higher probability of detection. It is not advisable to choose contact NDT techniques being time consuming and costly as they require removal of insulation to give reliable results.
To conclude advanced techniques like LRUT , Pulsed eddy current are the best available techniques to identify areas of gross corrosion without removing the insulation and the affected areas can be further investigated using conventional NDT methods or phased array to derive possible conclusion on the exact nature of corrosion under insulation.
Prepared by, Siji Sidharthan
Mudiame International Limited
ASNT Level III UT, RT, MT, PT, VT
PCN Level 3 UT, RT, MT, PT
PCN Level 2 TOFD, PAUT
NDE Level II ECT