Page 368 - Pipelines and Risers

Pipehe Inspection, Maintenance and Repair 339

Most of the Caliper pigs are equipped with mechanical sensors (fingers) that follow the inner
profile of the pipe wall. Typically, these pigs can detect dents and ID reductions of between
1% and 2% of the pipe diameter. A drawback of the mechanical caliper pig is that false
readings can be obtained from debris or solid wax. Established contractors that offer services
with mechanical caliper pigs are Pipetronix, Enduro Pipeline Services and TD Williamson
(TDW). Some tools have the additional capability to measure the bend radii.

H Rosen Engineering (HRE) offers a service with a caliper pig that uses eddy current
proximity probes and which is called the Electronic Gauging Pig (EGP). The 8 probes are
mounted in a conical nose at the front or rear of the pig. This pig has the advantage that the
pig is very rugged and insensitive to debris or wax. When required the EGP can be mounted
with a larger cone by which the sensitivity can be increased from about 1.5% ID reduction to
about 0.5% ID reduction, at the expense of the pig’s flexibility.

Route Survey
The Geopig of BJ Pipeline Services (formally Nowsco) is the market leader for route
surveying. The Geopig was developed by Pulsearch, Canada in the mid eighties with the aim
to measure subsidence in the “Norman Wells” pipelines in Canada which lie in an active
permafrost region. The Geopig is capable of determining the latitude, longitude, height, bend
location and curvature and center point of a complete pipeline in a single run. The heart of the
Geopig is a strapdown inertial measurement unit giving an accuracy on location of 0.5 m/km
and a curvature with a radius up to 100m. Two fixed rings with ultrasonic probes are mounted
to measure the internal profile of the pipeline. In liquid pipelines undamped and unfocused
2.5 MHZ transducers are used. The sensors for gas service operate at 250 KHz and require a
minimum internal pressure of 10 bar. A footprint of the sonar on the wall has a diameter of
1Omm. The accuracy of the sonar to measure dent depths is +I- 2.5 mm.

Some pipeline operators have found good use of the Geopig to assess the pipeline profile for
upheaval buckling and the necessity for rock dumping.

An alternative to the Geopig is offered by Pipetronix in the form of their Scout pig, which
uses inertial navigation by means of built-in gyroscopes.

Freespan Detection
British Gas have developed the Burial and Coating Assessment (BCA) pig based on neutron
backscattering, that aims to detect freespans. However, the BCA pig has not become a
commercial success because of its limited competitiveness with respect to remotely operated
vehicle (ROV) inspection.

HRE have recently developed a freespan detection pig based on gamma ray technology.

BJ Pipeline Services claim that their Geopig (see previous section) can detect freespans by
measuring vibrations of the pipeline when the pig passes an unsupported section however, this
capability has not yet been field proven.