Residual Stresses Measurement and Inner Geometry Inspection of Pipelines by Optical Methods

Abstract

This paper describes three different optical systems designed to be used outside the lab for pipeline inspections. The first one is a robust and portable ESPI based hole-drilling unit with radial sensitivity used for residual stresses measurement. The device has a special diffractive optical element that produces an achromatic interferometer. The displacement component around the hole drilled is measured by ESPI with radial in-plane sensitivity and is fitted by least square methods to evaluate residual stresses. An infield application for analyzing the integrity of a gas pipeline is presented as an application example. The second system is a conical laser triangulation device to measure the geometry of the inner surface of pipes. A laser beam is deflected by the tip of a 45° conical mirror and produces a radial light plane that intercepts the inner surface of the pipe producing a bright ring all way around 360°. The image of the light ring is used to compute the radius of about 1400 points in each section while the device is moved along the pipe axis. Finally, the third system uses active photogrammetry to measure in cylindrical coordinates the details of the inner geometry of pipe junctions and welding seams. It was designed to inspect welded joints, to check weld seam quality as well as to identify transverse and angular misalignment between adjacent sections.