Corrosion Erosion Detection A new Corrosion-Erosion monitor (CEM) has been developed by the Bergen-based ultrasonic instrumentation company ClampOn, to measure wall thickness loss in pipes, plates or other metal structures. The device was designed to address demands by operators, for reliable systems able to monitor changes in wall thickness on subsea pipes and other structures on a permanent and continuous basis. “In subsea applications, the hostile environment can present a challenge to detecting and measuring corrosion and erosion,” said Olav Brakstad, sales manager at ClampOn. “The industry has developed a number of techniques, many of which have drawbacks.” “In larger pipelines, intelligent pigs are used routinely to carry out inspection at regular intervals. Alternatively, in smaller un-pigable pipelines, operators can use cableoperated tools. “These not only represent a costly operation to carry out, but also may require the greater expense of shutting down well production.
“By virtue of the way they work, a majority of integrity monitoring systems are often of limited value to the operators. This is because any erosion/ corrosion found in a pipe is rarely distributed evenly,” said Brakstad. “It is far more common that some areas are more susceptible than others. It is these ‘hot-spots’ that require greater attention.” Subsea items such as manifolds, templates and jumpers are traditionally inspected by pre-installing corrosion/ erosion sensors or by using remotely operated vehicle (ROV)-operated sensors. “These ROV-operated sensors do not always provide reliable readings,” said Brakstad. “While there are various such devices on the market that scan these hotspots, they are generally single point measurements. By their very nature, they do not provide constant readings and may not give any indication of critically thin wall thicknesses.” What is ideally required, is a constant monitoring of the pipe’s condition over a large area by a non-intrusive system. This was the driver that prompted ClampOn to develop its CEM system. While normal ultrasonic testing is conventionally used for measuring wall thicknesses, CEM is based on
dispersing of guided lamb waves in order to track changes in the thickness of a pipe wall. The wavelengths used are equal or greater than the thickness of the structure. This results in much lower attenuation. It affords guided wave inspection the potential to extend ultrasonic corrosion measurements in pipes over greater distances.
The ClampOn CEM uses active ultrasound, with up to 16 pairs of transducers working in a pitch-catch mode. Most commonly, transducers are placed on two rings around the pipe at pre-determined points. A typical arrangement would place these transducers rings 1m apart. “Factors such as spurious arrivals and mode separation place restrictions on the maximum and minimum distance between transducers,” explained Brakstad. “These transducer rings would be normally used to monitor average wall thickness loss along this entire section. By the careful positioning of the transducers, however, it is possible to monitor infrastructure that is normally inaccessible, for example, sections of pipeline buried in the seabed.” Limitations imposed by transducer sensitivity makes it effective on 5mm–35mm wall thicknesses. It has, however, a sensitivity between 0.1% and 1%, depending on the temperature. Long term monitoring programmes may reveal local thickness variations representing a significant percentage of the average wall thickness. Integral to the CEM system is an algorithm that incorporates a patented use of constant group velocity (CGV) modes. These provide maximum sensitivity to changes in wall thickness within the constraints imposed by the necessary robustness that the technique requires. “As the instrument is based on ultrasound,” explained Brakstad, “one advantage of using the technique
ROV-installable version of the CEM
UT2 August 2012
The August edition of UT3, the magazine of the SUT