Page 322 - Pipeline Pigging Technology

Assessment of pipeline defects

THE ASSESSMENT OF PIPELINE

DEFECTS DETECTED DURING

PIGGING OPERATIONS

THE ADVENT of high -resolution magnetic-based on-line inspection and
monitoring equipment now allows operators to thoroughly assess the integ-
rity of a pipeline. This equipment can findall significant defects in the line, and
it is then the operators' responsibility to determine whether these defects
require repair.
The significance of many pipeline defects can be assessed using proven,
simple analytical methods. These methods can be applied to assess defects
detected in-service, or to plan inspection schedules for corroding pipelines.
This paper describes the variety of pipe-wall defects that can be detected
during pigging, and summarizes their assessment methods. The incorpora-
tion of these methods into condition-monitoring plans is discussed, and finally
an overall defect assessment methodology is presented.

INTRODUCTION

Periodic inspection of oil and gas transmission pipelines often reveals
corrosion defects. Some 'intelligent' on-line inspection tools can accurately
detect, size and locate pipe-body corrosion (Fig.l). Following detection, the
significance of these corrosion defects can be assessed using either estab-
lished analytical methods[l-3], company[4] or national codes[51. Where
corrosion is still active, a further on-line inspection can re-size corroded areas
and a corrosion rate can be estimated from the two inspection reports. This
rate, combined with further assessment of the significance of the corrosion,
can be used to give a long-term assessment of the integrity of a corroding
pipeline or, alternatively, allow an operator to instigate improved or alterna-
tive methods of controlling corrosion.
Mechanical damage is the major cause of service failures in onshore and
offshore pipelines handling petroleum or gas[3]. However, as pipelines age

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