Page 163 - Pipeline Pigging Technology

Pipeline Pigging Technology

economies, and there is a growing awareness that maintaining the integrity
of such a strategic asset during its operational life has significant benefits. This
realization is reinforced by considering both the financial and the environ-
mental consequences of failures.
British Gas first formulated a policy for the condition monitoring and
periodic revalidation of its 17,000km of high-pressure gas transmission
pipelines in the 1970s, the corner-stone of which was to replace the tradi-
tional hydrostatic pressure test with a more quantitative and cost-effective
means of assessing pipeline integrity. Detailed technical and investment
appraisals confirmed that, for defined categories of pipeline defect, on-line
inspection would have major performance and financial benefits over the
pressure test. The investment study assumed that in the absence of a suitable
commercial inspection service, it would be necessary to develop a system
capable of the required performance standard. The technical study acknowl-
edged the fact that a pressure test, whilst being a valuable aid to the
commissioning of new pipelines, was both costly and disruptive as a revalidation
method and further, could not fulfil the requirement for a quantitative
measure of pipeline condition.
A pipeline must be designed to withstand the operational stresses associ-
ated with transportation of the product, and must also be protected as far as
possible from damage and degradation during its operational life. In this latter
respect, even the product, which is usually under pressure and occasionally
at high temperatures, may be chemically-aggressive by its nature and because
of contaminants. Thus, the pipeline may suffer damage to the internal as well
as the external surface, a fact which must be accommodated by the inspection
system. This requirement must also be combined with the facility for unam-
biguously responding to 'defined class(es) of defect in a potentially-aggressive
product, and a pipeline environment in which the conditions are unknown
in terms of debris and internal surface deposits. It is this combination of
requirements which imposes the need for careful selection of the inspection
technique and a highly-robust engineering solution.
British Gas undertook a detailed study of all available inspection tech-
niques, which revealed that magnetic-flux leakage (MFL) was the preferred
method for metal-loss inspection in a pipeline environment. Since that time,
the technique has been the subject of major innovations and refinements by
British Gas, particularly in respect of physical design, which have set it apart
from other competitive systems.
British Gas began production of magnetic-flux leakage based inspection
systems in the size ranges appropriate to its own pipelines, and since the late
1970s regular inspection operations have taken place in the high-pressure
pipeline network to continuously monitor its condition and thus ensure its
integrity.

144