Page 367 - Pipelines and Risers

338 Chapter 18

Detection and sizing performance are dependent on pipeline diameter, wall thickness,
magnetic permeability and tool speed. Tool speed is limited to less than 0.5 m/s due to the
low frequency applied to generate the eddy currents. The maximum wall thickness that can be
inspected with a RFEC tool depends on test frequency in combination with pipe magnetic
permeability. For carbon steel pipes the maximum inspectable thickness is approximately 10 -
12 mm.

18.2.3 Intelligent Pigs for Purposes other than Metal Loss Detection
General
If one excludes metal loss detection then, broadly speaking, pipeline inspection by intelligent
pigging can be categorized into the following five groups of inspection capability:-

0 Crack detection
Calipering
Routesurveying
Freespan detection
0 Leakdetection

The purpose of this section is to briefly describe the tools and techniques that are currently
available with respect to the above inspection requirements.

Crack Detection
British Gas have developed a crack detection pig based on ultrasonic wheel probes. This pig
is called the Elastic Wave Inspection Vehicle and can be operated in both gas and liquid
pipelines. The first prototype was a 36-inch pig which contained 32 wheel probes. In addition
a 30-inch pig has been built. Main difficulties with this technology has been on data
interpretation with regards to minimizing the rate of false calls. However, in recent years
much work has been carried out by British Gas on data analysis algorithms to discriminate
real cracks from spurious indications. British Gas claim that the number of false calls has
decreased significantly by their recent improvements on data analysis.

PTX have developed an ultrasonic crack detection pig that aims to detect both internal and
external longitudinal cracks in clean liquid pipelines. The tool can also detect potential fatigue
cracks in the longitudinal weld seam. Note that this pig cannot be run in gas pipelines unless
this is done in a liquid slug. The key in the concept is the complete coverage of the pipe by a
large number of ultrasonic piezoelectric transducers (512 for a 24-inch pig).

Calipering
Caliper pigs measure internal profile variations like dents, ovality and internal diameter
transitions with the primary objective being to detect mechanical damage andlor ensure that a
less flexible metal loss inspection pig can pass through the pipeline. Caliper pigs are normally
designed to be flexible and can pass 25% ID reductions.