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  • Guided Wave Testing - Maximizing Buried Pipe Corrosion Knowledge from each Excavation

    Andy Crompton

    10731 E. Easter Ave., Ste. 100 Centennial, CO 80112

    Structural Integrity Associates, Inc. Email: [email protected]

    Roger Royer

    301 Science Park Road, Ste. 121 State College, PA 16803

    Structural Integrity Associates, Inc. Email: [email protected]

    Mark Tallon

    Progress Energy Brunswick Nuclear Plant

    8470 River Road SE Southport, North Carolina 28461

    Stephen F. Biagiotti, Jr. P.E.

    10731 E. Easter Ave., Ste. 100 Centennial, CO 80112

    Structural Integrity Associates, Inc. Email: [email protected]

    ABSTRACT Excavation and Direct Examination of buried piping using conventional non-destructive examination (NDE) has been the traditional inspection approach for decades and remains the only quantitative method for piping evaluations in plants when internal in-line inspection tools cannot be used due to geometry or other constraints. This difficult to assess piping presents many challenges, including limited effectiveness of traditional indirect inspection tools, high cost and operational concerns associated with excavations, and the ability to evaluate only a small sampling of a piping system. Many inspection technologies exist for buried pipe assessments; however, no indirect techniques provide the ability to yield quantitative wall loss values suitable for ASME fitness for service calculations beyond whats exposed in the excavation. An evolving technology, guided wave testing (GWT), has many applications including the ability to provide assessment information beyond the excavation.

    In this paper, the application of GWT for buried piping inspection will be discussed. We will review: principles behind its operation; the competitive technologies on the market; challenges for the technology; management of data within the Electric Power Research Institute (EPRI) industry standard buried pipe database (BPWorks 2.0); trending; case histories showing how GWT can be used to extend the knowledge gained during an excavation by screening adjacent areas for more significant corrosion than observed in the excavated and exposed area; coupling GWT results with other inspection technologies to gain an enhanced interpretation of the overall condition of the line; and how to incorporate this data into an effective structural and/or leakage integrity program as part of the reasonable assurance process.

    PRINCIPLES OF GWT

    Guided waves are acoustic, mechanical, or electromagnetic

    waves with propagation characteristics dictated by the properties and boundaries of the medium in which they travel. Ultrasonic

    Proceedings of the ASME 2012 Pressure Vessels & Piping Conference PVP2012

    July 15-19, 2012, Toronto, Ontario, CANADA

    PVP2012-78561

    1 Copyright 2012 by ASME

  • guided waves, used for screening to detect changes in cross-section, are elastic mechanical stress waves introduced into the pipe wall and constrained to propagate along the axis of the pipe segment within the pipe wall. The waves are of relatively low frequency (15-85 kHz) and are scattered and/or reflected by changes in the pipe cross section or acoustic impedance. Reflections can originate from corrosion, welds, supports, coating interfaces or transitions, earth entrances, etc. One main difference between using guided waves for screening and traditional ultrasonic testing or bulk waves is their ability to size and locate defects. Further comparison of these techniques is provided in Table 1.

    Table 1. Differences between Bulk and Guided Waves

    Bulk Wave Guided Wave Time consuming for large areas Rapid screening

    Point by point scan directly beneath sensor

    Single sensor position propagates waves on either side of the sensor

    Partial coverage Volumetric coverage Defect must be directly accessible to be detected

    Detected defects may not be directly accessible

    Accurate defect sizing Approximate circumferential and axial sizing For piping applications, the types of guided waves used for excitation are either longitudinal or torsional. Longitudinal waves are characterized by the compression motion they create along the axis of the pipe. A drawback to longitudinal waves is they are not ideal when considering the inspection of liquid filled pipes as they can propagate into that medium causing distortion of the data. A torsional wave is most commonly used for pipe inspection and results in the twisting of the pipe preventing the energy from transferring into the pipe media. Upon impinging on a reflector, each of these excitation methods result in the reflection of symmetric and mode converted flexural waves. Flexural waves are generated from reflectors which are not symmetric about the pipes circumference such as an isolated corrosion patch. Therefore, the symmetric and flexural responses can be used to characterize a given reflector. As an example, a uniform weld will produce a large symmetric reflection with little flexural response. In contrast, isolated corrosion or pitting will produce a large flexural reflection with a lower symmetric component. Torsional modes may be more or less sensitive to certain types of defects than longitudinal modes due to the different in/out of plane displacement components each mode is comprised. Penetration power of both types of waves can vary significantly with frequency and boundary conditions. Therefore, depending on the application and requirements, one mode may be better suited than the other Guided wave transducer arrays are typically comprised of a comb configuration which can be modified either during construction or in the field to alter the frequency band used for excitation and analysis. One main factor that influences the frequency is the transducer spacing whereas a smaller spacing results in higher analysis frequencies and greater resolution (due

    to the shorter wave lengths) which can result in shorter screening distances. The frequencies available are also governed by the boundary conditions, or more specifically the pipe size and thickness, which influence the guided wave dispersion curves. The guided wave dispersion curves for a given structure represent all wave modes available for excitation [1]. Each reflector along a pipe segment is characterized by its amplitude, flexural, and frequency responses. A symmetric amplitude response is present for all reflectors and is proportional to positive or negative cross-sectional change (CSC) when compared to the total surface area exposed by a transverse cut of the pipe. A non-symmetric amplitude response is created if the feature is non-symmetric and is used to estimate the features length around the circumference of the pipe, also known as circumferential extent (CE). For example, a weld has 100% CE; however, a defect with a given cross-sectional wall loss is more critical for a smaller CE as shown in Figure 1.

    Figure 1. Wall loss as a function of CSC and CE

    The attenuation or decay of the signal is logarithmic over the inspection distance down the pipe in either direction from the sensor location. The attenuation can be caused by pipe appurtenances, wrappings, linings, contents, soil loading, and/or general corrosion. Reflections from a series of defects and the associated energy loss are shown in Figure 2. Screening is generally conducted by comparing the known CSC of girth welds to unknown features such as corrosion. Welds are generally assumed as 22.5% CSC (positive) and vary from weld to weld; however, weld measurements can be taken to improve the accuracy of the screening. A flange or threaded coupling represents 100% CSC and therefore no or little energy is transmitted beyond this feature, marking the end of the diagnostic range for all examinations. A unique feature such as a 45 degree bend in most cases distorts the wave such that data

    2 Copyright 2012 by ASME

  • may become unreliable for interpretation beyond the feature and generally marks the end of the test.

    Figure 2. Reflections from Guided Waves

    Due to logarithmic decay, a distance amplitude correction (DAC) curve is applied to the data as a method to compensate for loss of amplitude over distance and assign a constant amplitude (%CSC) over a distance. Some of the curves used during analysis are: the weld DAC, used as the first means of proper amplitude correction and reference; call DAC, used for a visual threshold for feature detection; and a noise DAC, which can be used to determine the signal-to-noise ratio (SNR). Some common types of DAC curves placed over guided wave data are shown in Figure 3.

    Figure 3. DAC Curves

    The sensitivity achieved for a test represents the percent CSC that can be detected at a given SNR. One element contributing towards the confidence of identifying and classifying features is the SNR. It should be noted that CSC is one measure for reflections and many other factors influence the reflection of guided waves at a defect. For this reason, during the analysis the technician may also characterize indications in the testing range based on the reflectors flexural, frequency, and focused response. The inspection range is typically a function of the required sensitivity. For example, if the sensitivity is increased for GWT from 5% to 10% at a constant SNR the distance for the inspection will also increase. However, smaller features less than the specified 10% CSC sensitivity may not be discernible from the noise in the signal. Further illustration of sensitivity over distance is shown in Figure 4 using the same DAC Curves

    as Figure 3. This figure displays two responses of similar amplitude prior to applying the DAC curves. The first peak occurs in an area where the decay is rela