Page 90 - Pipeline Pigging Technology

Pigging for flexible pipes

Steel pipe Flexible pipe

homogeneous material inhomogeneous construction
construction
non-layered construction layered construction
near-round shape slightly oval shape
monolithic material composite of materials
low dynamic fatigue high dynamic fatigue resistance
resistance
simple structural behaviour complex structural behaviour
low flexibility (up to 500 x i.d.) high flexibility (8-10 x i.d.)
smooth bore smooth or rough bore

Table 1. Comparison of properties and characteristics for rigid and
flexible pipes.

outside diameter to form a durable yet resistant covering capable of taking
abrasion forces while also resisting hydrocarbon fire (typically to Lloyds
Bulletin at 700°C for 30mins without loss of content).
The composite construction also serves to reinforce the individual pipe
components and enhance their individual strengths. By embedding steel
chords used for axial reinforcement in elastomer matrices, Pag-O-Flex of West
Germany has found [Joint Industry Report, 1987] that the breaking load in
long-term axial pull tests for embedded steel chord is considerably greater
than that for bare steel chord. This is particularly important when considering
riser applications, where a catenary configuration is used and combined
loadings occur in the steel reinforcement due to internal pressure, tension,
and bending effects.
Other composites, such as epoxies, graphites, and glass fibres, also offer
significant technical benefits by combining high fibre strength with good
material resistance to corrosion or chemical degradation. However, compos-
ites [Lefloc'h,1986] are often difficult to assess with regard to structural
strength and changes in mechanical properties due to the influences of ageing
and material degradation over time. Certain properties in material construc-
tion can lead to a degree of variability in product qualities and a lack of precise
knowledge as to which property principally governs at any one point in an
operational lifetime. Furthermore, distribution of stresses within individual
layers is not always linear or simple to assess. It can be said that such
composites exhibit a complex rather than simple structural behaviour, i.e.
the material behaves anisotropically (forces do not act in a single direction);
the construction is inhomogeneous; and the failure modes can be compound.

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