BIDI riser inspection

Pipeline Description:

High pressure gas pipeline, 6” WT 12.7mm, operating pressure 131 bar. Length approx. 10km. The pipeline connects a 20” subsea natural gas transmission line with the Thistle platform in the northern North Sea. It consists of (1) the subsea tee piece with valve station, (2) the sea line and (3) the riser. 

Why is it challenging?

There are no possibilities to install a pig trap at the subsea end of the line. Further, for operational reasons it is not possible to depressurize the line. In view of UT-inspections, flooding the riser with water was no option. 

Target of the inspection:

To determine external corrosion on the riser. The riser is located inside a casing that had an unknown history.