Pipelines start to deteriorate the moment they commissioned and placed in-service. Pipeline corrosion and metal loss from the inside (ID) or outside (OD) diameter are almost inevitable. From the moment a pipeline commissioned, it begins to deteriorate, with the specific pipeline corrosion mechanism depends on the product transported, operating and ambient temperature, metal alloy used, insulation, operational environment and other factors. This article focuses the rapid screening of pipelines with guided wave ultrasonic testing (GWUT) followed up by high- resolution scanning of internal corrosion detected using a phased array ultrasonic testing (PAUT) dual element probe technology.
From the moment line pipe is manufactured and placed in service, it is subject to flaws and suffers deterioration. Corrosion is one of the primary deterioration mechanisms and depending on the corrosion rate may impact the pipeline performance in the short or long term. Pipeline corrosion may be initiated through a variety of electrochemical mechanisms at any time in a pipeline’s lifecycle requiring a pipeline non-destructive testing (NDT) plan for screening and metal loss quantification. Guided wave ultrasonic testing (GWUT) of pipelines is an effective method to detect thinning or erosion, stress corrosion, pitting corrosion and general corrosion. Phased array ultrasonic testing (PAUT) dual element probe technology (DLA) is an excellent follow-up NDT method for accurate assessment of the corrosion length, width and depth.
Pipeline Corrosion Assessment with Guided Wave Ultrasonic Testing
Pipeline guided waves are used to inspect long lengths of pipe, ranging from 20 to 300 feet, for reductions in percent cross-sectional area loss (%CSA). There are a few different instrumentation platforms from which pipeline GWUT is performed including the MSS, GUL and Teletest systems based on piezoelectric and magnetostrictive sensor technologies. While the different GWUT instruments reference comparable advantages, they all converge to using the torsional T(0,1) mode  for primary inspections. GWUT torsional modes are characterized by particle displacement in the angular direction compared to the radial/axial directions as shown below. The main advantage of the guided wave T(0,1) mode is less attenuation due to product, coatings, etc.
Figure 1: GWUT torsional displacement compared to radial/axial displacement.
Example data from a 9-inch OD pipe with 0.1875” nominal wall thickness is shown below. The GWUT sensor is installed at 0’. GWUT is focused in the backward direction with the first 90 degree elbow weld reflection observed at approximately 7’. A separate data set is created by focusing GWUT in the forward direction with the first 90 degree elbow weld observed at approximately 15 and 20 feet. A 2% CSA detection threshold was set and all indications above this level are tagged across the GWUT +/- directions. A potential corrosion event was detected at 12’ and was followed up with phased array ultrasonic testing (PAUT) dual element probe technology. This area was investigated further with PAUT DLA at +/- 12” from estimated location of the pipeline corrosion.
Figure 2: GWUT pipe data on section with 90 degree elbow in + and – directions. Area was investigated further with PAUT at +/- 12” from estimated location of the pipeline corrosion.
The dual element linear phased array probe is used for the same reason that a standard ultrasonic dual element probe is used. By separating the transmitter and receiver element(s), the ultrasonic pulser is decoupled from the receiver. As a result, there is no main bang or dead zone allowing for ultrasonic thickness measurement of thinner materials and/or detection of near surface discontinuities. A top and side view of the Olympus 7.50L32 REX1-IHC dual element linear phased array probe is shown below. In the top view, the individual element configurations of the pulsing and receiving elements are shown. While the transducer is 32 elements long, there are a total of