Underwater radiography

TWI is leading the FlexiRiserTest consortium in developing a prototype system for the inspection of flexible risers used to transport oil from the sea bed.

The oil is typically moved into offshore platforms or Floating Production Storage and Offloading (FPSO) units but there is currently no method of examining, in situ, the cross section of underwater risers and flow lines to ensure that they continue to work reliably.

The prototype uses an external gamma radiography technique comprising an oppositely positioned gamma source and digital flat-panel detector to image the internal walls of the risers.

Existing radiography-based inspection of underwater structure uses film or phosphor plates that have to be returned to the surface for development and processing.

It is believed to be the first time that a digital flat-panel detector has been used for underwater radiography inspection. The application of the digital detector allows for rapid production of radiographic images of the riser, which can be instantly relayed via an umbilical to a host computer at the FPSO.

A robot has been developed to deploy the radiography-based inspection system that is capable of crawling along the external surface of the flexible riser. It is also able to rotate 360^o about the riser axis so that the riser can be completely imaged.

Since the new inspection technique acquires a significant amount of images, Automated Defect Recognition algorithms have been developed to help reduce the number of images presented to the operator.

The inspection technology has been tested to a depth of 20m so far, and has been shown to detect broken tensile wires in flexible risers, but the partners in the project are confident that much greater depths would be possible with further development.

The work is as a result of a two-year European Framework six-part funded project, shared between nine organisations. The  detector technology is deployable now and can be used in underwater inspection solutions by TWI Member companies.