A challenging inspection operation on a 13’ Oil Export Flexible Riser having a tight 55° wire angle structure in the outer and inner armour layer was successfully performed in Q4 of 2015.
This was a challenging inspection not only because of the 55° wire angle structure in the outer and inner armour layer but also a the presence of a thick 15.3mm outer sheath with an anti-wear tape 2.5mm between the two armour layers.
Due to the absence of the pressure armour layer, this flexible riser is susceptible to Stress Corrosion Cracking (SCC) and Hydrogen Induced Cracking (HIC) in both armour layers. The aim of the inspection operation was to scan the flexible riser from the installation down through the sag bend for cracking defects in both the tensile armour layers.
Prior to this inspection campaign, the MEC-FITTM technique which was originally capable of inspecting flexible pipes with wire angle structures of up to 37° was verified to determine its suitability for inspecting flexible pipe with a 55° wire angle structure and a thick outer sheath. A simulated test sample with EDM cracks in 45° and 90° angle and various flaw types was established by placing wires at different locations and wire layer positions.
The MEC-FITTM technique was successfully verified in the blind tests with results showing that all through-cracks in all the layers representing the simulated tight wire structure and thick sheath were detected while part-depth cracks were detected in the outer layer.
To perform the inspection operation, the MEC-FITTM technique was adapted onto a rebuilt MEC-Hug Crawler tool to enable the circumferential scanning to accommodate the 55° flexible wire angle structure. The tool was also customised for the deployment by an inspection-class ROV from the installation which represented a substantial cost saving in comparison to the deployment using a work-class ROV from the vessel.
The Client was not only satisfied with the successful inspection operation and the cost saving from the deployment by the inspection-class ROV, but more importantly that the riser operation could be continued safely.