Page 92 - Pipeline Pigging Technology

Pigging for flexible pipes

Wear or fretting of steel components, not fatigue, has been found by Pag-
OFlex after 2V£ years of dynamic testing of 6-in x 6000psi riser pipes to be the
most probable mode of failure. Wear is of particular concern for dynamic
flexible riser systems because pipes are bent towards their minimum radius
of curvatures, and may also be subjected to high crushing loads both during
installation and operation (especially at touch-down points and over steel
arches). O'Brien and others [OTC 4739,1984] have stated that "a deepwater
catenary system is prone to wear because of the overall system elasticity and
surge motions". These wear concerns increase with system motions, water
depth, imposed loads, and the overall excursions of the riser configuration.
Fatigue, i.e. the development of weaknesses in the polymeric or steel
components due to repeated cycles of stresses, has proven difficult to
quantify. To relate stress levels in individual pipe layers to cycles to failure it
has been necessary to perform long-term (more than 1 year) component and
pipe dynamic tests at simulated operational and environmental conditions. As
stated above, Pag-O-Flex's joint industry programme subjected pipes to
dynamic bending and tension exposed to 100-year storm conditions for more
than 20million cycles without pipe failure, i .e. no loss of pressure or fluid [Pag-
OFlex, JITP Report, 1987]. Through the development of S-N curves for both
component and pipe structure, as well as improvements in ultimate capacity
models, a better understanding of fatigue lifetime can be gained. The other
modes of failure for flexible pipe can be summarized as being [Veritec JEP/
GF2,1987]:

disbondment of bonded components;
fretting or internal wear;
corrosion of steel components;
fatigue failure of component part(s) or the structure itself.

Inspection of flexible pipes is complicated not only because of the
composite, layered construction but also because of a pipe's complex
behaviour. Because of the high design safety factors and surplus strength
elements used in its construction, the pipe can compensate for the presence
of defects. Favourable aspects concerning such a matrix-type construction to
be noted are: that a high degree of structural redundancy exists; and gradual
leakage rather than sudden rupture is the most probable effect of a failure.
This factor should be reassuring to operators, particularly when transporting
live crude or gas in flexible pipe.
Efforts in the inspection of flexible pipe can therefore be focussed
primarily around two categories of defects [Neffgen,Subtech,1989] which
can have an impact on the structure because of leakage:

73