Page 87 - Pipeline Pigging Technology

Pipeline Pigging Technology

As a result of the high consequential inertial loads imposed largely by
differential motions between the vessel and the seabed and, as a result,
environment forces, flexible risers have been used to effectively provide a
motion-compensation system.
The increased availability of various flexible pipe designs has increased the
industry's need for greater awareness concerning pipe properties, ageing
effects, fatigue lifetime, and inspectability. What is clear is that flexible pipe
is not a product of a "black-box technology", and can be technically assessed
and verified with regard to its overall integrity. However, in order to formulate
both a methodology and a programme for the inspection of flexibles, it is
essential to have a clear appreciation of their construction aspects and
correspondingly complex behaviour. In this way the presence and signifi-
cance of defects can be related to any impact on structural reliability.

UNDERSTANDING PIPE CONSTRUCTION

Flexible steel reinforced pipe is a generic term defined by the American
Petroleum Institute [API, RP 17b 1987] as being "... a composite of layered
materials which form a pressure containing conduit. The pipe structure
allows large deflections without a significant increase in bending stresses".
Pipes are reinforced axially and radially by the incorporation of steel chords,
flat tendons, helixes and/or cylindrical carcasses; construction will either be
of the bonded or non-bonded types.

Bonded pipe construction

Bonded pipes are those where the component materials are applied as
alternating layers (polymer, steel, fabric) using chemical bonding agents to
achieve initial adhesion strength. Elastomeric materials and textile-reinforced
fabric plies are laid over and between several layers of cross-wound, pre-
tensioned steel reinforcing elements preventing steel-to-steel contact. To
achieve a homogeneity as a single structure, the pipe is vulcanized in a
carefully-controlled heating oven (applying temperature in a stepwise man-
ner together with pressure to the structure) permitting cross-linking of the
polymer structure and curing of the matrices involved.
In a bonded pipe, flexibility is provided by axial and shear deformations,
and there are virtually no relative movements between interfacing surfaces.
This is especially important when considering wear rates and, ultimately,

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