Abstract
The purpose is to determine the suitability of ultrasonic thickness testing of corroded tubular members for the offshore industry. The research can be divided into two parts: 1) determining the error in an ultrasonic thickness measurement associated with measuring the wall thickness of a corroded tubular member and 2) determining how the strength calculations are affected by an error in a wall thickness measurement. This thesis is based on the first phase of a research project funded by Mineral Management Service of the Department of the Interior, Shell Deepwater Development and Mobil Technology Company. The second phase of that project will be to identify any common patterns of corrosion, which may be useful in developing a protocol for in-situ testing. Thickness measurements have been taken on corroded tubular members under a variety of surface conditions with both an ultrasonic thickness device and a micrometer. The error of the measurements has been determined by taking the standard deviation of a series of two measurement groups taken at the some point under the same surface conditions. The ultrasonic thickness measurements have also been compared to the micrometer measurements to determine any systematic errors in measurement. For the typical situation in which the wall thickness is small compared to the diameter of the member, a series of strength calculations have been performed taking into account the following possible conditions: 1) a wall loss that is constant around the circumferences and 2) a wall loss that is greater on one side than the other. It has been found that the second case is sometimes the worst situation because the eccentricity resulting from the nonuniform wall loss causes a bending moment in the member, resulting in failure earlier than would be predicted by a simple axial load model. Depending on the amount of corrosion and the length to diameter ratio of the member, the limiting mode of failure may be symmetric yielding, elastic buckling, or yielding due to eccentric corrosion. The lower bound on member strength has been found by consideration of all three failure modes.
Konen, Keith Forman (1999). Ultrasonic thickness measurements on corroded steel members: a statistical analysis of error. Master's thesis, Texas A&M University. Available electronically from
https : / /hdl .handle .net /1969 .1 /ETD -TAMU -1999 -THESIS -K65.