Page 382 - Pipeline Pigging Technology

High-accuracy calliper surveys

construction bend tangent points. Piecewise continuous isoparametric finite
element shape functions are then automatically fitted to measured centreline
coordinates bounded in each element length, using least squares adjustment
procedures.
Similarly, cylindrical shape functions are fitted to the sonar data in each
pipe joint, thus giving diameter and wall-thickness data that can be statisti-
cally compared with the pipe specifications, and with random deviations
from internal diameter expectations due to dents, wrinkles or internal
corrosion effects. The finite element centrelines are then consistently mapped
to a reference plane initial geometry, representing a datum strain and stress-
free geometry of straight pipe and construction bends for structural simula-
tion and reliability analysis work. The cylindrical fitted data are amended at
this stage for internal pressure and thermal effects.
The centreline tangent vector misalignment at welds is computed and
used to correct the displacement vector used in the kinematic and structural
simulation work, so that normal construction "doglegs" at girth welds are not
included in the structural demand computations of structural deformations
and curvatures.
All data-acquisition statistics are propagated through to the functional
structural model for damage search, kinematic analysis, simulation, multi-run
rectification and correction for temperature and pressure differences, and
static reliability-analysis work including stability. The kinematic analysis is the
minimum required analysis effort necessary for an objective location of any
existing damage. Additional analysis is done in accordance with client
requirements.

Applications

The uses of the Geopig for pipeline geometry surveying have been
expanded beyond the original curvature monitoring, strain measurement and
precise location, to include high-accuracy calliper. The ability to interactively
analyze all features and make determinations of the structural significance of
those features has enhanced the ability of operators to maintain operating
conditions of pipelines. In addition, the Geopig can be used to evaluate
corrosion problems and, based on considerations of the total pipeline
condition, determine which dents, wrinkles, or wall thinning need to be
replaced to maintain system integrity.

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