Introduction
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.
Pipeline Corrosion
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 [1] 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 64 elements. Odd number elements 1-63 are on the bottom side of the PAUT probe as shown. Even number elements 2-64 are on the top side of the PAUT probe as show. A Rexolite block is used to prevent wear to the bottom of the PAUT transducer. The delay due to the Rexolite block must be subtracted out using the wedge delay calibration similarly to how a zero or probe delay function is adjusted on standard ultrasonic testing instrument.
Figure 3: Top and side view of PAUT DLA probe.
Example dual element linear phased array probe data is shown below for a standard 0.250” to 1.000” calibration block. The DLA probe is long enough to acquired data across three different steps in a single linear PAUT scan. As the virtual probe aperture is swept from left to right, the 0.250, 0.500, and 0.750” steps are observable on a single linear scan.
Figure 4: Example dual element linear phased array probe wedge delay calibration data.
Example PAUT data from the GWUT area identified during the pipeline screening process is shown below. The video shows the standard analyses that is used for the DLA PAUT probe using the Olympus X3 platform. Clockwise from bottom right are the following presentations of phased array data: 1) PAUT A-scan generated by the current virtual probe aperture (VPA). VPA is selected using the data cursor in the B-scan, C-scan, or S-scan. 2) Encoded PAUT B-scan is generated along the 1-D encoder direction or scan axis. This view shows the cross-section of the pipeline along the scan axis. 3) The C-scan shows the top view of the area scanned by the DLA probe width and scan axis length. PAUT C-scan pixels, or data points, are color coded to Gate A reflection amplitude or depth measured. 4) PAUT S-scan measures the part thickness across the index axis of the DLA probe.
The ID metal loss length may be measured using the PAUT B-scan or C-scan using the light blue scan axis and scan axis measurement cursors. In this case the ID corrosion starts at approximately 0.40” and ends at 1.00” for a total length of 0.60”. The width of the metal loss is measured using the PAUT S-scan green index axis, using index axis measurement cursors, and is approximately 1.00”. Most importantly, the depth is measured with either the B-scan or S-scan. The metal loss is flat and occurs at a depth of 0.120”.
Figure 5: Example dual element linear phased array probe ID corrosion data.
Summary
Guided wave ultrasonic testing screens long lengths of pipe quickly but requires follow-up NDT for metal loss quantification. High element – wide pitch PAUT dual element linear phased array probes provide high resolution encoded data that support accurate metal loss sizing in length, width, and depth directions.
[1] Rose, J.L., 1999, Ultrasonic Waves in Solid Media, Cambridge University Press.