Mitigation against lateral buckling and axial walking of subsea pipelines

There is a growing concern amongst offshore pipeline operators over the instability
problem of lateral buckling and axial walking of offshore pipelines which is linked with
elevated operating temperature and pressure. While some mitigating options are
available to tackle this phenomenon, in most cases these are expensive and
impracticable in deep waters, and none ofthem involves the modification of the ambient
soil properties typically characterised by very low undrained shear strength (Cu) and
high water content (w). In recent times, the use of engineered buckle solutions has
become generally accepted as a cost effective and elegant solution. This option
involves laying the pipeline in a snake configuration where some specific sections are
designed to move during operation while others relatively stable. This option depends
on accurate understanding of pipe-soil interactions which presently poses the greatest
uncertainty in pipeline design. Furthermore, in order to ensure that the buckles are
formed as predetermined, the ambient soil strength must be sufficient to resist the pipe
motion at locations designed to be relatively stable or the entire design approach would
be undermined.
This dissertation presents laboratory investigations at both small and pilot scale directed
at using the electro-kinetic phenomenon (EK) to treat the soil around a partially buried
pipeline with the aim of increasing the pipeline stability to lateral buckling and axial
walking. The influence of the EK treatment was assessed by evaluating the changes in
the soil Cu, wand the soil resistance to vertical, lateral and axial displacements of pipe
sections. Additionally, large-scale pipe-soil interaction studies were conducted to study
the soil deformations, especially the real time study of the berm of soil formed and the
development of the soil resistance during pipe motion.
Preliminary results of the application of EK in geotechnical engineering to offshore
pipelines show up to 600% increase in Cu, 14% decrease in w and 190% increase in the
pull out force thus implying promising outcomes which could form the basis for
subsequent research in this area.