Development of Non-invasive Ultrasound Inspection Techniques through Steel

Abstract

Using normal incident, pulse-echo ultrasound is a well-established approach in the non destructive testing and evaluation community for measuring wall thickness and detecting defects. If the sensors are permanently installed, this opens for a high sensitivity to relatively small changes in the wall thickness. At present, intrusive probe technology is commonly used for rapidly determining the corrosion rate of steel structures. However, non-intrusive methods are often desired. The first part of this thesis focuses on developing a new ultrasound inspection technique in relation to high resolution corrosion monitoring of internal steel surfaces in the presence of thermal variations. The method extracts both the change in steel thickness and temperature from a set of ultrasonic signals in a monitoring setup, eliminating the need for an independent thermometer. The analysis illustrates the importance of temperature compensation and how even small thermal variations can give an erroneous corrosion rate if not corrected for.

In many applications it is required to extract information from materials through steel. Two examples taken from the oil and gas industry are deposits monitoring and cement evaluation through a tubing wall. A challenge when using pulse-echo ultrasound for such applications is the high impedance ratio between the steel and the material under observation causing most of the energy to reverberate back and forth in the steel structure. Signal interference together with a large contrast ratio between the steel reverberations and the desired echo may complicate the detection scheme. It is therefore a need for further development of robust techniques addressing such issues. The second part of this thesis hence deals with feature extraction from materials through steel. It is entirely related to a new bi-layer transducer design or to a dual frequency technique which can be applied with the new transducer. The dual frequency technique is based on transmitting two pulses with a frequency separation of about 1:10 simultaneously and coaxially. The low frequency (LF) pulse manipulates the acoustic properties of the medium nonlinearly, while the high frequency (HF) imaging pulse propagates under the influence of the LF pulse. In order to transmit such pulse complexes, the transducer consists of two piezoceramic elements which have a common aperture. In the second paper the HF part of such a transducer is characterized, and a methodology for characterizing paraffin wax deposited on steel is presented. The method is based on modeling the electro-acoustic channel of the transducer when only the HF element is active. The model is then _tted to measured data which in turn facilitates inversion of wax parameters. The third paper investigates the same transducer when both elements are active. A method for detecting water films through steel by applying radiation pressure is then presented as well as the modeling and suppression of acoustic crosstalk from the LF to he HF element. The next paper analyzes the two-way dual frequency wave propagation in a plane layer when assuming normal incident, plane wave propagation. In particular, it is discussed how a nonlinear delay between two HF pulses propagating on opposite polarity of the LF pulse accumulates for different boundary conditions and layer properties. This is important for the last paper, where a refined design and prototype testing of the dual frequency transducer is discussed. Experimentally, nonlinear delays are obtained from a water layer both in transmission mode and in pulse-echo mode. It is also illustrated how these delays can be used for suppressing the reverberations in steel relative to the desired echo signal.