Rocker pipe solution to alleviate settlement induced distress in flexible pipes

A number of earlier researchers investigated the soil structure interaction parameters
which affect the structural behaviour of buried flexible pipes. However, it was not until
the mid-1990s that the importance of rocker pipe design to accommodate differential
settlement raised awareness of the consequent absence in current design procedure [BS
EN 1295-3; 1998].
This study widens the understanding of the effect of differential ground movements on
the behaviour of flexible pipes to address concerns raised to the Committee European de
Normalisation. Many pipeline failures result from the excessive strains developed in the
vicinity of the junction between a pipe and a settling structure. Case studies of such
failures are presented in this thesis, which demonstrates that it can occur not only in
large diameter pipeline but also in small diameter domestic pipeline systems. A method
of analysis and the use of developed appropriate rocker pipe length is an industrially
useful outcome of this research.
Analytical solutions for flexible pipes have been developed based on the concept of
beams on elastic foundation approach. Non-dimensional relationships have been
developed and are presented in the form of charts. These charts permit hand calculations
and rapid verification of structural design of the pipeline and, thus, assess the integrity
of the existing pipelines located in areas with ground instability. Knowledge of the soil
strength and sub-grade modulus is required, along with pipeline geometry and pipe
stiffness, to apply the non-dimensional relationships. The soil parameters can be
measured in situ or estimated using empirical correlations. The cause for failure has
been investigated with the proposed analytical soil structure interaction approach which
identifies the zone of distress in the pipeline. The analysis has been extended to
demonstrate how a rocker pipe system can be incorporated to alleviate such distress.
The concept of rocker pipes has also been investigated in the study through laboratory
scale testing. Soil box tests, with ground conditions defined through plate/pipe load
testing, have been developed. Full-scale testing on flexible pipes instrumented with
FlexiForce pressure sensors at the pipe invert, strain gauges measuring strain due to
induced differential settlement on the pipe crown and deflection transducers measuring
deflection profiles along the pipe length due to differential settlement of the end
structure, have been used to validate the mathematical modelling.
Ile combination of rocker pipes with flexible joints and elastic effects removes the
distress imposed by the differential settlement, and provides the possibility for a better
comparison with the data obtained from both theoretical analysis and experimental tests.
Results from each of these approaches are compared with those from experiments. It is
concluded that there is need for a rational design procedure analysis for rocker pipes to
be incorporated into codes of practice, such as EN 1295.

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