Page 91 - Pipeline Pigging Technology

Pipeline Pigging Technology

In order better to understand how to inspect or make a condition
assessment for flexible pipe, one must first make a comparison between the
general properties and characteristics of flexible pipe with that of steel pipe.
Some of these differences are illustrated in Table 1 [Neffgen,1988].
As can be seen from Table 1, considerable differences exist between rigid
and flexible pipe. Flexible pipe's complex behaviour in practice means:

bending moments and strains cannot be easily calculated;
some component materials exhibit non-linear behaviour;
differences exist between component elastic moduli which must be
analytically explained;
strain distribution around the pipe is axi-symmetrical.

DEFECTS AND MODES OF FAILURE

To understand the structure of flexible a pipe is to appreciate the
complexities of its behaviour and then to relate those to the presence and
significance of defects. The purpose of any inspection programme is princi-
pally directed at [Bea et al ,OTC,1988]:

detection and documentation of defects which can lead to a significant
reduction in serviceability characteristics;
defining what should be inspected, when, and how;
establishing a long-term database and feedback loop;
establishing the significance of a defect and/or the need for remedial
action.

Such an inspection programme initially must focus on the identification
and determination of "...significant defects which can affect structural capa-
bility, i.e. the ability of the structure to remain serviceable (not to fail) during
its projected operational life" [Bea etaL, 1988]. The importance of establish-
ing a database for pipe defects and understanding how such defects can
propagate are important in relating significance with regard to failure modes.
Two modes of failure have been identified as having principal impacts on
structural integrity, those being wear and fatigue. Veritec [Veritec joint
industry report, 1987] has defined wear as "...the damage to a solid surface
caused by the removal or displacement of material by the mechanical action
of a contacting liquid, solid, or gas. Wear is mostly mechanical, but may
combine with chemical corrosion".

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