The Efficiency of Phased Array UT for Corrosion Mapping
Published on 23rd November 2020
Several inspection standards which cover the maintenance of large assets frequently state that a relatively small collection of sparsely separated spot measurements is needed to estimate the interval until the next inspection or the asset’s remaining life.
Application Note By: Prof. Neil Pearson - Senior Scientist - Advanced Technologies
Corrosion mapping is being adopted more and more for non-destructive testing of structures with large areas. Through regular thickness measurements, mapping aims to improve the quality and quantity of information to describe the condition of the asset. Ultrasonic Testing (UT) is commonly employed on the large surface areas with in-service assets such as pipelines and storage tank shells. With such large areas to cover, manual inspection can be time consuming, tedious and with sparse spot checks, have a low probability of detection. Automated corrosion mapping can improve on all these aspects and is becoming more common to assess the integrity and better estimate the remaining life of these assets.
Several inspection standards which cover the maintenance of large assets frequently state that a relatively small collection of sparsely separated spot measurements is needed to estimate the interval until the next inspection or the asset’s remaining life. Inspecting these large assets can be very time consuming leading to the inspector and asset owner striking a balance between the time available to conduct the inspection and the level of measurements required to determine its condition. By simple approximation, some guidelines recommended a number of 10mm2 spot measurements on a 15 meter (49 feet) high shell wall of a storage tank 50 meters (164 feet) in diameter would cover approximately 0.0012% of its surface; as the saying goes, like finding a needle in a haystack. Coupled with hazards from rope access or increased costs with scaffolding access to elevated areas, the inspections are challenging to say the least.
Introducing automated corrosion mapping to the inspection can reduce the overall cost to access elevated positions in a safe manner. Automation can also dramatically increase the probability of detecting wall loss by rasterizing a UT probe across the surface and collecting spot UT measurements at a consistent interval. The Rapid Motion Scanner (RMS) from Eddyfi Technologies is a remote crawler for corrosion mapping that can perform this task by creating a 2D colour coded map of measurements. The RMS uses magnetic wheels for adhesion and can climb surfaces orientated in any direction. Positional accuracy of the corrosion map is achieved with a fixed scanning rail arm with its weight evenly distributed onto two separate tractor modules that run in tandem to reduce the risk of the scanner veering off course. With a range of scanning rails from 300 to 600 millimeters (12 to 24 inches), corrosion maps created with the RMS can then be stitched together —even at different resolutions— using our CMAP tool to create a holistic view of the asset condition.
The RMS can conduct high-resolution spot checks at 0.5mm x 0.5mm (0.02in x 0.02in) intervals through to something as low as 50mm x 50mm (2in x 2in). The resolution chosen is a compromise between the time allocated to conduct the inspection and the largest discontinuity the asset owner is willing to miss. Alternatively, a fast-coarse scan on a low-resolution could be conducted first to find the areas of interest followed by high-resolution scans. Of course, the inspection time can be reduced by only targeting areas of the asset that are normally associated with corrosion, e.g. the product interface level in a storage tank.
Phased array UT (PAUT) extends on the regular spot measurement of the single crystal method by using an array of N ultrasonic transducers (or elements). Through electronic scanning of these elements, a wider scan area can be acquired as opposed to the standard single element. When the probe is swept, a strip of measurements is created. With minimal overlap, far less raster sweeps of the probe is required to cover the same area to that of a single crystal probe. Using the RMS with a PAUT probe and one of the self-contained M2M MantisTM or Gekko® units incorporating the ultrasonic pulser, data storage and multi-touch display with workflow user interface can drastically improve the efficiency of each scan. For example, a 1800mm (71in) long, 300mm (12in) wide scan with a high measurement resolution of 1mm x 1mm (0.04in x 0.04in), a raster scan with a single crystal probe would take around 55 minutes to complete. With a wider probe, less sweeps and at the same measurement resolution, the RMS coupled with a PAUT probe would take around 8.5 minutes; a 6-fold improvement in corrosion mapping efficiency.
In addition to standard PAUT, the Mantis and Gekko can perform Total Focusing Method (TFM) when corrosion mapping to further elevate the quality of the reported indications by synthetically focusing all points under the array of the probe. For each probe position, an element in the array is fired once and all elements listen for reflections. This is repeated for each element resulting in a collection of N B-Scans. Through the TFM method, an image is constructed by summing specific points from each of the B-Scans to give a focused, “cross-sectional like” representation of the component under the array. This leads to better defect representations, more intuitive analysis, and faster inspection. This can be conducted in real-time on the Mantis and Gekko to deliver reportable results directly to the inspector with the same mapping efficiency to standard PAUT. One example benefit of TFM for corrosion mapping is the presentation of the defect shape. For example, a dome-end like corrosion area on the far side of an inspection sample can have reflectors at an incident angle greater than 45°. For the single crystal approach, the sound would not reflect to the probe location and so the roll-off of the defect would show severely reduced amplitude. With an array probe and with the TFM approach, the width of the probe would result in the further elements detecting the 45° reflections resulting in a holistic representation of the corrosion profile.
For over a decade, the corrosion mapping instruments offered by Eddyfi Technologies has evolved to offer enhanced efficiency and imaging. to learn more about our corrosion mapping solutions on offer to meet your needs.