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.