Whilst corroding subsea pipes remains a significant concern, detailed corrosion mapping of these pipes has required computer imaging capability which, until now, subsea permanent monitoring systems have been unable to handle, due to the computing capability and energy required in performing such mapping. This paper addresses the challenges and possibilities of online permanent tomography imaging/mapping of pipes, adopting new technologies and algorithms using low power instruments and providing real time processing. Standard tomographic wall thickness mapping became an option mid-2010s, with the integration of a software package.
Systems with full coverage of a pipe section provide three-dimensional maps of wall thickness loss, with key results like minimum wall thickness and defect positions transmitted to the control system. Many wall thickness monitors are equipped with fewer transducers than what is required for standard tomographic wall thickness mapping. The intention and challenge of the "Tomography Light" system is use these simpler and more cost-effective systems and still be able to achieve tomography mapping of a local area where corrosion or erosion is expected to be most severe.
While corrosion and erosion in subsea pipes remains a significant concern, instrumentation has been developed to provide detailed mapping of wall thickness loss. In the mid-2000s the Corrosion-Erosion Monitor was introduced, which utilizes guided ultrasonic waves (GUW) to monitor mean wall thickness loss between non-intrusive ultrasound transducers installed permanently on pipe sections with typical length 0.7 m–1 m [1, 2, 3, 4]. The tomography software package was developed in the 2010s [5, 6, 7, 8] to produce two-dimensional wall thickness maps from the GUW monitoring data.
Tomographic methods can be computationally intensive and typically require a computer or high-capacity automation controller to run. This is sometimes disadvantageous in self-contained subsea monitoring systems, ideally outputting fully processed measurement results in engineering units. Since long-term reliability is critical and low power consumption is often desirable, a solution is sought for three-dimensional wall thickness mapping with rugged electronic hardware such as digital signal processors (DSP) and field-programmable gate arrays (FPGA). Current Subsea Corrosion Erosion Monitor installations use from 6 to 16 ultrasound transducers. The number of GUW propagation paths across a monitored pipe section ranges from 7 to potentially more than 400. Typical systems currently in field operation utilize fewer than 50 wave paths in periodic measurements for automated wall thickness monitoring. The tomography softwares algorithm can be downscaled to operate with reduced computational load for measurement datasets of such small size. An alternative approach is studied in parallel, as a possible complementary supplement, referred to herein as "light-weight" tomographic wall thickness mapping [9].
