Typically inline inspection is carried out for geometry-, corrosion- and cracking
defects. Regarding cracking, most of the time axial cracking (SCC, seam weld
cracks....) is observed. Ultrasonic crack inspection tools are available for the reliable
detection of such cracks using conventional ultrasonic shear wave technique suitable
for detection but with limited depth sizing capabilities.
Under special conditions (bending forces, poor welding) cracking can also appear in
circumferential direction. For this type of cracks, the same type of tool can be used
with a modified sensor carrier providing similar detection and sizing performance as
for axial cracking. In offshore applications, however, accurate sizing in particular of
crack depth is an important requirement as offshore verification and repair work is
usually very cost-intensive. In order to provide a high POD together with good sizing
performance a tethered 10" tool was developed for inspection of circumferential
cracks. It contains a pulse - echo (PE) unit for reliable detection and a TOFD (time-offlight-
diffraction) unit for accurate sizing of circumferential cracks. The PE technique
is used for fast screening during the forward inspection. The TOFD measurement
which is basically stationary is performed on the backward inspection by stopping at
every location where a crack indication was detected by the PE inspection. Extensive
blind testing of the new tool was performed. The comparison of the results from ILI
inspection and destructive testing showed that the ILI tethered tool fully met the
specification with regard to detection and sizing. After successful validation the tool
was used for a tethered inspection of a 10" offshore pipeline (length 8 km).
In the paper the inspection concept and the setup of the new tool are described.
Results from the qualification tests as well as from the first inspection run performed
with the new tool will be presented.