Guided Wave Tomography for Accurate Corrosion Mapping in Inaccessible Areas

Accurate corrosion depth mapping in inaccessible areas is a problem of major importance across a wide spectrum of industries. While several thickness gauging techniques are available, they are only applicable when the area to be inspected is directly accessible. In fact, standard gauging methods require a probing sensor to be scanned over the area where corrosion damage is expected. However, this is not always possible as access can be limited due to the presence of other structural members. As an example, determination of the depth of corrosion at supports of pipelines is a major issue in the petrochemical industry. At present the only reliable way to determine the corrosion depth accurately is to lift the pipe from the support and to use standard methods, thus resulting in a very costly and potentially hazardous inspection procedure. Here, we propose a tomographic approach similar to X-ray CT. However, instead of using ionizing radiation we employ guided ultrasonic waves that can be transmitted across the inspection area from a remote and accessible transducer location. While the interaction of photons with matter can be described by simple ray models in X-ray CT, scattering, diffraction and refraction phenomena characterise the encoding of mechanical property information in guided wave signals. These phenomena add much complexity to the problem of retrieving thickness maps in GWT and represent the main challenge of this proposal. Therefore, at a fundamental level this programme aims at developing a general approach to GWT that can address this complexity borrowing ideas developed in geophysical exploration and medical diagnostics. From a more applied perspective, we propose to develop a field deployable prototype for mapping corrosion at supports which will serve the twofold purpose of maintaining the research focussed on practical problems and of facilitating the translation of the proposed technology to industry. Moreover, the prototype will have a flexible design that will allow its application to corrosion mapping problems in inaccessible areas other than pipe supports to ensure that the proposed technology will have an impact across a wide spectrum of industries. This proposal is being submitted within the UK Research Centre in NDE to the targeted research programme, the funding for which is earmarked by EPSRC for industrially driven NDE research. It is supported by Shell and Petrobras who are contributing 90k cash as well as in-kind contributions to the project.

Accurate thickness mapping of large engineering structures is critical to assess the integrity and residual life of mechanical components subject to erosion or corrosion damage. Corrosion is a major issue in a wide range of industries, aerospace, petrochemical, and civil infrastructure being typical examples. It is estimated that the direct cost of corrosion to industrialized countries represents a significant proportion of the gross domestic product (GDP), with some analysts suggesting that the cost of corrosion to the US alone is as high as 3.1% of its GDP. Although various thickness gauging devices are commercially available they are not applicable when the area to be inspected is not directly accessible. Notably, water initiated corrosion tends to occur where the presence of multiple structural features causes water accumulation. As an example, accurate determination of the depth of corrosion in pipelines at supports is a major issue in the petrochemical industry. At present the only reliable approach is to lift the pipe from the support, if the pipe is not welded to it, and to use standard thickness gauging methods. However, this is very costly and can be hazardous for the inspectors. Corrosion mapping in inaccessible areas is therefore an open challenge and the aim of this proposal is to develop a general methodology that can lead to improved detection and accurate depth characterization of corrosion damage from a remote and safe location. Improved detection is central to avoid catastrophic failure while better characterization leads to a more accurate assessment of the state of damage which can then be used to predict the residual service life of a structure. Therefore, the impact of this proposal is far reaching and encompasses economic benefit to industry and improved safety conditions for people. The former is primarily defined by the direct costs associated with structural failure and indirect costs due to production outages that in many circumstances can exceed the direct costs. Moreover, the proposed technique would reduce the current hazardous conditions under which inspectors have to operate as well as reducing the likelihood of catastrophic failures which can result in the loss of lives. In addition to the collaborating companies, Shell and Petrobras, the proposal has been supported by all the industrial members of the UK Research Centre in NDE (RCNDE) that currently includes Rolls-Royce, BNFL, Airbus, dstl, RWE npower, Shell, BP, Alstom, SERCO Assurance, Petrobras, British Energy, E.ON, Tenaris, Network Rail and HSE. The industrial members have recognised the economic benefit resulting from the lower operational costs that can be achieved by limiting production outages for maintenance. As a result, the proposal has been approved by the academic and industrial board of the RCNDE to proceed as a targeted research programme, the funding for which is earmarked by EPSRC for industrially driven NDE research. The research project has been designed to facilitate the translation of fundamental science into exploitable technology within the three year timeframe of the proposal. Indeed a significant part of the research effort is devoted to the development of a field deployable prototype. The prototype will embody the main technological components developed within this programme i.e. new software and hardware, thus providing a template for the next generation system to be used for the commercial exploitation of the technology. This final stage will be managed either directly via the Imperial technology transfer company, Imperial Innovations, or through Innovations licensing a third party supplier to distribute it to industry and to provide the requisite support.