Magnetic Eddy Current (mectm) as a Novel Technique for the Internal Inspection of Cra-lined Pipe

CRA-lined pipelines, i.e. pipes with an inner pipe of a corrosion resistant alloy inside of a ferritic steel pipe carrying the hoop stress, pose a challenge to inspect with conventional in-line inspection technologies. Existing UT technology is not able to inspect through the interface of a CRA and a ferritic steel component. Additionally, current MFL technology is not able to sufficiently magnetise the ferritic steel pipe in order to inspect this type of pipe. The current contribution investigates the possibility of Magnetic Eddy Current (MECTM) to carry out such an inspection. A prototype internal inspection tool has been devised. Several types of sample defects have been produced into the ferritic steel carrier pipe as well as into the CRA-inner pipe. The defect types anticipate the integrity issues that an internal inspection tool would need to address. Several pull tests under controlled speeds have been carried out. The results are presented. Influences of tool speed, magnetisation level and eddy current parameter are investigated. Special attention is drawn to the distinction of different defect types. The MECTM technology was found to be a suitable inspection technique for CRA-lines pipes. This is achieved through several adaptations for this particular task. The technology may also fill other existing inspection improvement opportunities, like the detection of small volumetric features or the inspection of heavy walled small diameter gas pipelines. Introduction Cladded and CRA-lined pipe has been in use in the Oil and Gas industry for a long time now. The main purpose is to transport hydrocarbon with high contents of corrosive components. Hence they are mainly found in upstream sour service. The idea is to use an inner pipe or layer of a CRA (Corrosion Resistant Alloy) to avoid internal corrosion and to use an outer layer of ferritic steel (carrier pipe) to carry the hoop stress. The alternative would be a pipe of full Duplex or CRA material. However, such a pipe is more expensive as the required volume of the more expensive CRA material is much higher. Two principally different methods of manufacturing are common today. In the first pipe type the CRA-layer is metallurgically clad to the ferritic layer. This is achieved with different processes, one being the cladding of the CRA layer onto the ferritic steel slab before producing the final pipe. The other method is mechanical bonding. As there is no metallurgical bond between the two metals in the final pipe it is called lined pipe. This is of course not to be confused with the lining of pipes with an inner polymer coating. The process often consists of a CRA pipe to be inserted into a ferritic steel and then to expand it into the inner diameter of the carrier pipe. The CRA pipe is shorter than the carrier pipe. At the ends the inner CRA layer is produced with a weld overlay. At the ends the layer is thus metallurgically bonded even for this type. The main reason for this production method is to ensure that the annulus is not open to the atmosphere and no water ingress is possible. Amongst others this allows for proper welding of the pipe. The in-line inspection of a pipeline of CRA lined or clad pipe has remained a challenge for existing ILI technologies. The limitations in applying existing ILI inspection technologies strongly depend on the configuration of the CRA layer. For ultrasonic wall thickness measurement the metallurgically clad pipe does not present a problem. As the difference in acoustic impedance between the two steel layers is small a full transmission of the UT pulse is present and a normal pulse-echo method can be applied [1].