TWI Industrial member Report Summary 896/2008
By C Nageswaran, C R A Schneider and N Decourcelle
Background
Pipes clad on the internal or external surface with another material are used in various applications. The inspection of welds between clad pipes using ultrasonic testing presents a significant challenge because different acoustic velocities in the cladding, the parent pipe material and the weld metal can adversely affect the transmission of ultrasound. The inspection of clad pipes used as risers for transporting oil and gas in deep-water fields is particularly challenging where severe fatigue loading and an acidic environment make it necessary to be able to detect relatively small flaws.
Differences in acoustic velocities between materials cause partial reflection of the incoming ultrasonic beam at the cladding interface and refraction and attenuation of the transmitted beam. The amplitude and position of flaw signals may be affected, particularly in the region of weld caps. The nature of the bond between the cladding and weld metal can affect the inspection of welds, particularly of the root.
Theoretical models of ultrasonic inspection systems can, in principle, be used to design the inspection of clad pipes (as well as ultrasonic inspections more generally) by:
- Allowing inspection procedures to be optimised;
- Allowing inspections to be qualified more quickly and cost-effectively by avoiding the need for large numbers of test specimens;
- Providing physical insight into inspection data.
It is, however, necessary to validate such models and establish any limits to their validity, especially if the models are to be used to underpin the inspections of safety-critical items.
CIVA is an advanced ultrasonic beam modelling package for the NDT industry. The aim of this project is directed towards the validation of the CIVA software and its underlying theoretical formulation against experimental data, with a view to its eventual application to modelling ultrasonic inspection of clad pipes. The work also has more generic relevance to the ultrasonic testing (UT) of layered metallic components in general. For instance, it is relevant to the modelling of UT of clad and unclad pressure vessels, as used in the nuclear industry, and for the detection of flaws in weld zones, parent plate or the cladding, and to components with external cladding as well as those with internal cladding.
In addition to CIVA, another theoretical model, SimulUS, was available to provide verification. SimulUS was developed by PeakNDT (Derby, UK) solely for the generation of sound fields by single element and phased array probes. It is designed with a fast algorithm with the aim to allow the NDT technician to quickly visualise sound fields generated by common NDT probes as an aid to making better inspection choices. SimulUS is less versatile than the more general purpose CIVA, and is unable to predict interactions between the sound field and features in the component being inspected. In 2006 Whittle (2006) presented the validation of SimulUS in modelling the sound fields created by phased array probes, and this validation is extended to different configurations in this work.
Initial validation of the CIVA model as applied to unfocused beams in unclad material was presented in an interim report (Schneider et al, 2005). This is the final report for the project.
Objective
Validate recent theoretical models of ultrasonic inspections (using both focused and unfocused beams) against experimental data and established models.