Remote operated ultrasonic inspection system examined subsurface brace welds on lattice tower legs by using a scanner moving in circumferential tracks 48 meters down inside the leg.

The offshore installation South Arne is located in the Danish sector of the North Sea where the water depth is about 60 meters. The platform, operated by Hess, sits on a concrete oil storage tank. The topside is supported on the concrete storage tank by one concrete column leg and a steel lattice frame for well head conductors.

Brace welds on two of the lattice tower legs should be inspected for defects in the heat affected zones at the leg side of the brace welds. As the target welds were located about 25 meters below surface the feasibility and costs of different inspection methods involving divers and a DSV were evaluated by the platform operator and based on this the possibility of internal inspection with ultrasound was chosen.

The selected solution, provided by The Department of Advanced NDT Global at FORCE Technology, was to introduce a scanner system into the hollow and dry lattice leg through a small opening cut in the upper leg section for that purpose. The maximum allowable size of the opening was set to be 250 mm wide and 500 mm high. Light, video camera end a magnetic wheel scanner (modified version of AWS-5D) equipped with 7 UT probes was manually lowered 48 meters inside the slightly tilted leg to the target depth.

Arrived there, the scanner’s guide wheels were released allowing magnetic wheel to get contact with the leg wall. From that moment scanner movements were controlled by operator’s topside via the P-scan system 4 instrumentation.

The scanner with its 4 magnetic wheels and two motors could be moved in circumferential tracks and up or down as well. The approximate depth of the scanner was verified by visual identification of girth welds with known position. The exact position was then found and verified by the use of the ultrasonic signals from the welds. 

Due to the complex configuration of probes and their view angles it was possible to examine for defects, independent of defect direction based on rather few track runs. The scan data could be viewed and evaluated top-side simultaneously with progress of the scan operation. The inspection revealed satisfactory integrity of the examined brace welds.