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Continuous monitoring of corrosion damage in pipelines requires sensor systems that can achieve high precision (repeatability) to detect and size subtle wall thickness (WT) losses. In fact, corrosion typically progresses at a rate of 1 mm per year or less which poses very demanding performance requirements if the state of the pipe has to be assessed on a weekly or even monthly basis. Guided ultrasonic wave tomography (GUWT) has emerged as an attractive approach for continuous monitoring owing to its ability to map WT losses over an extended pipe section. This is possible because GUWT employs ultrasonic waves that are guided by the pipe wall to travel a large distance from the source transducer. Defects along the path of the guided wave cause a perturbation of the signal which is then interpreted by model-based inversion schemes to determine the WT loss. In addition to damage, there is a vast number of time-dependent operational and environmental (O & E) factors that can perturb the signal also when damage is not present. To achieve high precision, it is therefore essential that GUWT can detect the changes in the signal due to damage and suppress those caused by other benign factors such as temperature variations. Although numerous studies have been conducted to estimate the accuracy of various implementations of GUWT, precision has received less attention due to the challenges associated with reproducing realistic O & E conditions in the lab. This work focuses on the effect of temperature and presents the results of a continuous monitoring experiment conducted on a pipe bend kept outdoors and exposed to weather conditions for a period 21 months. It is shown that the pipe undergoes a relatively severe temperature cycling with typical daily peak-to-trough variations of 20 ∘ C that cover the range from - 15 ∘ C to + 51 ∘ C as the seasons alternate. The effect of temperature is suppressed by exploiting the spatial diversity of array measurements and combining multiple datasets measured during the monitoring period. The maximum WT loss is estimated with high precision exhibiting a standard deviation not exceeding 0.4% of the nominal WT. [ABSTRACT FROM AUTHOR]

Abstract: The transmission of guided ultrasonic waves across corrosion or erosion damage encodes information about the defect depth. Tomography maps the depth profile from multiple transmission experiments performed under different insonification angles by solving the so-called inverse problem; the accuracy of the depth estimation being dependent on the range of angles available for the inversion. Practical application of tomography to tubular structures, such as pipes and bends, requires the use of two ring arrays of ultrasonic transducers at the two ends of the pipe section to be inspected. However, such a configuration leads to an insufficient angular coverage when considering the signals that travel along the shortest temporal path between a pair of transducers. This paper introduces a general inversion method that extends the range of insonification angles by exploiting the information carried by the signals that wrap around the pipe multiple times before reaching the receive array, thus resulting in superior image resolution and increased depth estimation accuracy. In addition, to address typical thermal fluctuations encountered during continuous monitoring, a strategy that combines a temperature compensation scheme with the intrinsic thermal stability of electromagnetic acoustic transducers (EMATs) is developed and tested with full-scale experiments performed on a schedule of 40, 8″ diameter steel pipe instrumented with two ring arrays of EMAT transducers. It is shown that for an irregularly shaped defect the proposed inversion method yields maximum depth estimations that are as accurate as single point ultrasonic thickness gaging measurements and over a wide temperature range up to 175°C. The results indicate that advanced inversion schemes in combination with EMAT transduction offer great potential for continuously monitoring the progression of corrosion or erosion damage in the oil and gas industry. [Copyright &y& Elsevier]

Ultrasonic guided waves offer the possibility of inspecting large areas of structures from a small number of sensor positions. However, inspection of complex structures is difficult as the reflections from different features overlap. Estimating the number and amplitude of the wave packets in ultrasonic time traces is crucial for the development of a guided wave inspection system, in order to detect and locate damage. Deconvolution has been extensively used in geophysical applications to resolve overlapping echoes in the recorded signals. The main objective of this work was to evaluate the applicability of the deconvolution approach for enhancing the resolution of ultrasonic time traces in structural health monitoring (SHM). Numerical simulations on strongly overlapping signals were carried out to evaluate the performance of the two techniques that have been considered: (i) Wiener filter, (ii) single most likely replacement. It was shown that the relatively narrow bandwidth of the input signals and phase shifts between different reflections limit the benefits obtained from deconvolution and it was concluded that deconvolution is unlikely to be useful for guided wave SHM applications. [ABSTRACT FROM AUTHOR]