Flowlines and pipelines in offshore areas are generally laid on the surface of the seabed under their own weight without being buried in a trench, these pipelines may experience lateral buckling and axial walking due to expansion caused by their operating temperatures and pressures. Analysis of pipe-soil interaction is required to assess, control and limit buckling. This analysis should include geotechnical data collection and interpretation, and evaluation of the lateral and axial resistance that a pipeline develops to resist the expansion forces.
This paper consists of three parts. The first part discusses the importance of site-specific geotechnical characterisation in order to identify strength and deformation parameters of the very shallow layers. Reference is made to recent project in deepwater offshore West Africa.
The second part is a summary of results of pipe-soil interaction experiments conducted on reconsolidated soft clay recovered from deepwater offshore West Africa. These laboratory experiments included measurements of axial and lateral resistances of pipe sections partially embedded in clay. Extensive instrumentation was used to measure forces and displacements of the tested pipe sections.
In the third part, simplified methods and practical calculations are proposed for the evaluation of the axial and lateral pipeline resistance mobilization curves. These calculations are based on general soil mechanics rules and calibrated using the results of the pipe-soil interaction experiments presented in the second part. Parameters such as pipe over-penetration and the development of soil berms during lateral movement are briefly presented, as they may greatly influence the lateral soil resistance.
Flowlines and pipelines are commonly used in deepwater oil and gas developments. They may operate at high temperatures and may be subjected to lateral buckling and excessive walking.
Pipeline buckling occurs when the compressive effective axial force becomes large enough due to thermal expansion, for example. For deepwater flowlines and pipelines laid on the seabed, buckling is generally produced in the horizontal plane (horizontal buckling). Pipeline ‘walking’ us a ratcheting response consisting of axial displacements of the pipe-line and is caused by start-up/shut down cycles.
General pipeline models (finite element or other models) are used to check pipe integrity. Realistic rules for interaction laws between the soil and the pipeline should be integrated into these models. The objective of this paper is to propose simplified methods and practical calculations to evaluate the axial and lateral pipeline resistance curves to be integrated into the general pipeline models.
Reliable information concerning soil layering and properties should be collected. The soil investigation programme should include high quality geophysical and geotechnical surveys. A geophysical survey allows detection of ?geohazards? and provides regional geological overview. The geotechnical survey should allow collection of high quality soil samples. In situ geotechnical tests should be also performed, such as cone penetrometer tests (CPT), T-bar test, etc. Particular attention should be paid to investigate the very shallow strata)the first metre).
Characterizing the top metre of deepwater sediments for geotechnical engineering purposes can be achieved by two complementary methods.
Performing in situ testing using lightweight seaved modules