Laser-Based Ultrasound for the Inspection of Gas Pipelines

Ultrasonics has proven to be an effective method for detecting a variety of defects in gas transmission pipes including cracks, wall thinning and corrosion pits. The use of Lamb waves for the detection of defects and in situ process monitoring applications has been successfully pursued for many years [1–6]. The use of a laser-based ultrasound (LBU) inspection technique to detecti defects is attractive because of the potential for rapid inspection of large areas and because it is noncontact with large standoff distances. Owing to its noncontacting and remote nature, the LBU technique is being investigated as an alternative technology to piezoelectric transducers or electromagnetic acoustic transducers (EMATs) for the rapid nondestructive inspection of pipelines. Currently, the preferred methods for introducing ultrasonic waves into the pipe are by using a piezoelectric transducer in a liquid-filled wheel or an EMAT. In field use, the wheel or the EMAT is attached to a moveable platform (known as a pig), which travels along the length of the transmission line. The wheel must maintain contact with the pipe wall during the inspection. Although the EMAT is a noncontact sensor, it must be operated close to the pipe’s surface. The contact and near-contact requirements can result in a loss of data when pipe irregularities such as dents or joints between sections cause the wheel or the EMAT to lift off from the surface of the pipe. The liquid-filled wheel uses longitudinal waves that propagate into the wall of the pipe. For a complete inspection of the pipe’s circumference, many wheels must be used. The EMAT generates a Lamb wave in the wall of the pipe that can be directed either circumferentially or axially along the pipe. Although the LBU technique also uses Lamb waves, unlike EMAT systems, the detection sensitivity of the LBU system does not decrease with increased separation from the part. However, a potential difficulty for LBU techniques is that Lamb waves are a family of guided waves that exist in plate-like structures, and a large number of modes of vibration may coexist in a given plate thickness. A laser that has been focused to a spot or line represents a broadband Lamb wave source in both the temporal and spatial frequency domains, which leads to the simultaneous excitation of many modes. Consequently, LBU techniques for generating Lamb waves have generally been pursued only when the lowest order symmetric or asymmetric mode was needed, probably because these modes are generated and detected with the greatest efficiency and thus offer a de facto mode selection mechanism since these modes dominate the others that may be present. We previously demonstrated [7] a mechanism for efficiently generating and selecting a single Lamb wave mode using simulated arrays. In this paper, we describe the implementation of a laser array for the generation of Lamb waves. We also present some preliminary results of a study of the characteristics of Lamb wave modes to identify suitable modes for detecting defects in pipelines. The features that are important include the generation and detection efficiency of the Lamb wave modes, the mode’s energy distribution, and the velocity dispersion of the waves.