Ultrasonic Lamb wave technique for measurement of pipe wall thickness at pipe supports

• US 5,965,818 A
• Filed: 01/15/1998
• Issued: 10/12/1999
• Est. Priority Date: 01/15/1998
• Status: Expired due to Fees

• Current Assignee: Shell Oil Company (Shell Plc)
• Original Assignee: Shell Oil Company (Shell Plc)
• Inventors: Wang, Weicheng David
• Primary Examiner(s): CHAPMAN JR, JOHN E
• Application Number:
  US09/007,391
• Time in Patent Office: 635 Days
• Field of Search: 73/597, 73/598, 73/643, 73/637, 73/638
• US Class Current: 73/598
• CPC Class Codes: G01B 17/02 for measuring thickness
• CPC Class Codes: G01N 2291/02854 Length, thickness
• CPC Class Codes: G01N 2291/0423 Surface waves, e.g. Rayleigh waves, Love waves
• CPC Class Codes: G01N 2291/2634 cylindrical from outside
• CPC Class Codes: G01N 29/07 by measuring propagation velocity or propagation time of acoustic waves

Abstract

This invention is a new inspection technique using ultrasonic Lamb waves to measure reduction of pipe wall thickness due to localized corrosion at pipe supports. The technique uses two transducers in a pitch-catch mode to send and receive a selected Lamb wave. The wave travels along the pipe wall in the circumferential direction. The reduction of the pipe wall thickness at the pipe support changes the time-of-flight between the two transducers. By comparing time-of-flight data measured from the pipe support area and from areas adjacent to the pipe support, one can quantify the change of time-of-flight due to the pipe corrosion wall loss at the support. A mathematical model, a sizing algorithm and a PC program have been developed to quantitatively relate in time-of-flight to the minimum remaining wall thickness. With them, one can quantify the minimum remaining pipe wall thickness at a pipe support by simply measuring the change in the time-of-flight.

Claims

1. A method for quantifying the remaining wall thickness of a pipe at a pipe support without lifting the pipe from the support and without any need for calibration samples, comprising the steps of:

a) installing a transmitting transducer and a receiving transducer on said pipe on opposite sides of said pipe support;

b) transmitting a highly dispersive and monotonic part of an S_o mode Lamb wave, whose frequency multiplied by thickness values are equal to or less than the value corresponding to the group velocity minimum, into said pipe with said transmitting transducer;

c) receiving said Lamb wave with said receiving transducer;

d) measuring the time required for said transmitted Lamb wave to be received by said receiving transducer, said time being identified as the time-of-flight;

e) without changing the instrument settings, but with said transducers repositioned to points adjacent said pipe support, repeating steps a) through d) to obtain a reference time-of-flight;

f) determining the change in time-of-flight due to localized corrosion wall loss by subtracting the time-of-flight measured at the pipe support from the reference time-of-flight; and

g) determining the pipe thickness at said pipe support by using said change in time-of-flight.

2. The method of claim 1 wherein a model, assuming that corrosion at the pipe support progresses in a tangential cut manner, is used to relate said time shift to the minimum remaining wall thickness.