Page 360 - Pipelines and Risers

Pipeline Inspection, Maintenance and Repair 331

sensor technology and on data processing, storage and analysis. Despite all the developments
on the mechanical design of pigs and on the inspection technology, intelligent pigs should not
be seen as being infallible. Each different tool has inherent limitations on inspection
capabilities that should be realized. Various experiences within the industry whereby
unsatisfactory inspection results were obtained emphasize this point. The main causes for
unsatisfactory results have been; no appreciation of the limitations of the inspection tool,
selection of the wrong technique and/or contractor, poor performance of the contractor and
lack of expertise to interpret and analyze the inspection results. With regards to the frequency
for intelligent pig inspection, there is no norm in the industry and the requirement for
intelligent pigging depends on the operators inspection philosophy, and the nature and
operationaI risks (Chapter 18.1.2) of the pipeline. Indeed there are many pipelines that are not
designed to be or have never been intelligently pigged.

18.2.2 Metal Loss Inspection Techniques
General
Several techniques are available for the inspection of pipelines using pigging technology
however, each different technique and tool has inherent limitations on inspection capabilities
that should be realized. The type of pig chosen will depend on the purpose of the inspection
and the nature of the inspection data required.

Although on occasions the objectives of pipeline inspection using an intelligent pigging tool
may vary, in general it is the requirement to detect metal loss that concerns most operators of
oil and gas pipelines.

Several techniques are applied in metal loss intelligent pigs, these are:

Magnetic flux leakage
Ultrasonics
High frequency eddy current
Remote field eddy current

Magnetic Flux Leakage
Principle
About 90% of all metal loss inspections are performed with magnetic flux leakage (MFL.)
pigs hence, the MFL technology can be regarded as the most important technique for metal
loss inspections of pipelines.

The magnetic flux leakage technique is based on magnetizing the pipe wall and sensing the
MFL of metal loss defects and other features. From the MFL signal patterns it is possible to
identify and recognize metal loss corrosion defects, but also other features such as girth
welds, seam welds, valves, fittings, adjacent metal objects, gouges, dents, mill defects, girth
weld cracks and large non-metallic inclusions.