This paper discusses the latest developments in non-invasive corrosion/erosion monitoring with guided wave tomography for subsea and topside asset management. There are many technologies available for corrosion/erosion monitoring topside, but much fewer subsea. The subsea environment is difficult to access for instrument maintenance and verification of monitoring data, and many pre-installed systems have proven to be of little use. A technology for non-intrusive wall thickness monitoring based on ultrasonic guided waves has been available for permanent installation topside and subsea for some years. A subsea retrofit solution has also been delivered. The system utilizes ultrasonic guided Lamb waves to monitor wall thickness over extended areas; much larger than any spot measurement system with a comparable number of transducers. The ClampOn CEM® received a Spotlight on Technology award at the 2012 OTC. Monitoring data from this instrumentation platform are well suited as basis for tomographic processing to produce two-dimensional wall thickness maps of pipe sections. A technique has been established that holds promise of unprecedented capability for wall thickness monitoring, offering previously unavailable real-time imaging of eroding wall surfaces. Points of minimum wall thickness can be located, their depths can be measured, and their developments over time can be tracked.
Tomographic methods are used with ultrasonic guided wave wall thickness monitors to provide two-dimensional maps of the wall thickness throughout a monitored area. Tomographic monitoring systems are typically configured with 16 to 32 non-intrusive ultrasound transducers positioned in predetermined patterns for optimal coverage and resolution. Other monitoring systems have much fewer transducers, positioned according to the pipe geometry and wall area of main concern. Although these systems are not optimized for wall thickness mapping, the same or similar tomographic methods can improve analysis and interpretation of the datasets they produce. Algorithms are sought for implementation in responsive, interactive software for browsing and analysis of recorded wall thickness monitoring data. Simple tomographic algorithms are also desirable for use in embedded software. A "light weight" tomographic algorithm is outlined, and a prototype implementation is tested against a proven full-scale tomography software package. The two approaches produce similar results on small datasets corresponding to wall thickness monitoring systems with few transducers installed. For the larger datasets generated for high resolution wall thickness mapping, the reference software is superior in terms of both computational load and accuracy.
Throughout the oil and gas industry corrosion and erosion damage monitoring plays a central role in managing asset integrity. This paper describes a technology for monitoring of wall-loss rates across continuous areas of pipeline wall. Permanently installed ring arrays of non-invasive ultrasonic transducers delimit a monitored section. The arrays transmit and receive guided ultrasonic waves that insonify the entire pipe section. The received signals are processed by a numerical algorithm to produce a two-dimensional map of wall thickness loss. The algorithm measures wall thickness loss differentially, detecting change between an initial and a current state of the pipe. Wall loss can be estimated with accuracy beyond the pipe manufacturing tolerances. Moreover, a strategy that combines a robust temperature compensation scheme with the intrinsic stability of electromagnetic acoustic transducers (EMATs) is used to address the intrinsic thermal dependency of all ultrasonic technologies, always experienced in field applications. Full-scale experiments are presented demonstrating maximum-depth estimation uncertainty below 0.5 % of initial wall thickness with excellent thermal stability up to 175 °C.
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