Abstract
Guided wave ultrasonic testing is fast developing into an efficient screening tool to rapidly inspect long sections (50+ m) of offshore pipelines and risers for corrosion. The technique enables rapid inspection and screening of sections of lines from a single inspection location, like an offshore platform. Over the last year, the authors organization has applied ultrasonic guided waves to the inspection of a variety of offshore and onshore assets through the excitation of a particular wave propagation mode known as the torsional mode. Torsional guided waves were chosen due to its anticipated advantages in terms of an extended choice of parameter selection and also because of its insensitivity to the type of liquid hydrocarbon/crude oil filling the lines.
This paper details our experience in using ultrasonic guided waves with special emphasis on offshore installations like export and import risers. As part of the endeavor to increase the confidence in the technique, extensive verification was carried out on risers that could be removed from service. Additional verification was carried out on the in-house 75-meter test loop. From these studies it was possible to determine the optimum inspection parameters for the types of defects found in the lines. A novel method was developed in-house to approximate the local corrosion depth from the parameters reported from guided ultrasound. Case studies from two crude oil import risers are presented to illustrate the experiences. The unique challenges of inspecting offshore risers and the testing considerations are discussed. The benefits and limitations of the technique are discussed, and typical guidelines for riser inspection using guided wave are suggested.
