Page 365 - Pipelines and Risers

336 Chapter I8

becomes high at surface lengths of about 20 mm. Depth sizing accuracy of the remaining wall
thickness is in the order of +/- 1 mm for pits and +/- 0.5 mm for general corrosion at a
confidence level of about 80%. Small pits can be missed. This performance is achieved by the
state of the art tools.

The depth sizing error is absolute and independent of nominal pipe wall thickness. The
relative error however, will increase significantly for smaller wall thickness. Most pipeline
operators conclude that ultrasonics is more suited for thick wall pipe than for thin wall pipe. A
threshold wall thickness of 7 mm is generally chosen below which ultrasonic pigs are not
recommended for use.

The amplitudes of the inner and outer wall echoes must exceed pre-set threshold values to be
detected. The echo signal can be attenuated by fouling, roughness of surfaces, tilting of probe
and curvature of surface profile. Dirt at the bottom of the line during a survey may mask the
most critical defects.

A rough internal pipe surface, e.g. due to corrosion, may result in a double inner wall
reflection causing the tool to ignore the second reflection coming from the outer wall. When
this shortcoming is not realized the metal loss is reported to be external with a completely
wrong depth.

Applicability
Ultrasonic pigs can be applied under the following conditions :-

Diameter range from 6-inch to 60-inch;
Velocities from 1 mfs through to 3 ds;
For pipe wall thickness above 7 m;
Only for liquid products unless the tool is run in a batch of liquid.

High Frequency Eddy Current
Principle
High Frequency Eddy Current (HFEC) technology has been developed for monitoring
internal corrosion in heavy wall, small diameter pipelines.

HFEC proximity sensors are mounted on a polyurethane sensor carrier and applied for two
different types of measurement so called global and local. The local sensors measure the
distance from the sensor to the pipe wall. The global sensor is used to measure the distance
from the center of the carrier to the local sensors. The combination of the measurements from
local and global sensors provides the internal profile of the pipeline by which both internal
pitting and general corrosion can be determined.