Design of an EMAT Guided Wave Collar for Coated Riser Inspection

The area close to the submerging point of off-shore steel catenary risers is difficult to access for inspections, since neither divers or climbers can work safely under the severe splash conditions. Hence, there is a need for technologies that allow remote or automated inspection of the so-called splash zone. Systems based on long range ultrasonic guided waves (GW) are an interesting option, especially if permanently-attached sensors are used to monitor the region continually. Operation with conventional piezoelectric sensors usually requires removal of the external coatings which are usually present, and this can lead to additional corrosion problems. In this work a GW system based on electromagnetic acoustic transducers was developed and tested on pipes covered with a 3 mm thick polyethylene coating. Initial studies of the transducer configuration were performed using finite-element analysis and then validated experimentally. A prototype sensor array was manufactured and then used for inspection of machined defects, initially in dry conditions in the laboratory and later in a full-scale test in an 15m depth water tank. In these given lift-off conditions, the system was capable of detecting defects corresponding to a 2.5% cross section area loss at a distance of 8m.