Corrosion and Crack Detection in an Offshore Riser and Flowline

Inspection of an unpiggable flowline without a subsea launcher/receiver

A 1.3 km (0.8 mi), 10” flowline transporting oil from a platform to a subsea manifold was to be inspected for corrosion and cracks for the first time. Experts had calculated that movement of the flowline in the water could have caused cracks in the girth weld area. As a typical offshore riser and flowline, it was not designed for pigging. No traps existed on either end of the line, and no pumps were available for moving a pig. Theoretically, a temporary subsea trap could have been installed underwater to conduct a conventional ILI. However, the support vessel and the ROV for the subsea launch, among other things, would have incurred enormous costs and risks for the operator.

Graphic showing sketch of a flowline and riser inspection.

Our solution

After thorough consideration of all operational and environmental factors, ROSEN offered a much more efficient and cost-effective solution: The experts from our Tethered Solutions division would launch a tethered bidi self-propelled ultrasonic inspection tool from a temporary trap on the platform. This would require neither the installation of a subsea launcher/receiver nor that of a pump with connecting lines. Additionally, it would obviate the need for expensive equipment, such as a diving support vessel and a ROV.

Due to space problems on the platform, our experts recommended a vertical ILI tool launch. The required winch with the cable was installed on the upper deck, and the cable was guided with several guiding wheels from the winch to the launcher trap. The customized inspection tool was equipped with the following:

  • Strong crawler in the front
  • Ultrasonic wall thickness measurement unit
  • That is at the same time a geometry unit to detect pipe deformations
  • TOFD (time-of-flight diffraction) unit for crack detection and sizing
  • Electronic and storage modules

Once everything was ready to go, the trap was filled air-free with oil and the tool launched vertically into the pipe. It switched into inspection mode, and the crawler moved forward, pulling the other modules into the line, which immediately started measurements and data collection. The ROSEN inspection team controlled the movement and tension of the cable from the control room while viewing the inspection data in real time. The pipe tally was prepared even while the pig travelled down. Thus, the team always knew the tool’s exact location and stopped it right at the manifold. The data was downloaded and the tool switched into return mode. On the return inspection run, the tool stopped at every girth weld and circumferentially scanned the area with the TOFD sensors. Only a few hours after the inspection was finished, our experts handed over a first report to the operator. 

Your benefit

The customized solution yielded substantial savings of cost and effort for the operator. Furthermore, it significantly reduced the risk of the operation by rendering any subsea work obsolete.

All requested inspection data was delivered in one run with outstanding reliability and accuracy. Based on these high-quality results, the operator was able to execute a corrosion growth assessment and a pipeline lifetime calculation. All this was valuable input for a detailed repair plan and an integrity management plan, including required inspection cycles.

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