Alasdair Maconochie

Pipeline Unburial Behaviour in Loose Sand

Small diameter pipelines are routinely used to transport oil and gas between offshore production plants and the mainland, or between remote subsea well-heads and a centralised production facility. The pipelines may be placed on the soil surface but it is more usual that they are placed into trenches, which are subsequently backfilled. For the buried pipelines a well established problem has been that of upheaval buckling. This occurs because the fluid is usually pumped through the pipes at elevated temperatures causing the pipeline to experience thermal expansion which, if restrained, leads to an increase in the axial stress in the pipeline possibly resulting in a buckling failure. A secondary phenomenon that has also been identified, particularly in loose silty sands and silts, involves floatation of pipelines through the backfill material, usually shortly after burial. At the University of Oxford a project sponsored by EPSRC and Technip Offshore UK Ltd has commenced to investigate in detail the buckling and floatation problems. The main aim of the research programme is to investigate three-dimensional effects on the buckling behaviour. The initial experiments involve the more typical plane strain pipeline unburial tests to explore the relationship between depth of cover, uplift rate, pipeline diameter and pullout resistance under drained and undrained conditions. The second and main phase of experiments involves inducing a buckle in a model pipeline under laboratory conditions and making observations of the pipe/soil response. This paper will describe the initial findings from the research including a) plane strain pipe unburial tests in loose dry sand, and, b) initial small scale three-dimensional buckling tests. The paper will then describe the proposed large scale three-dimensional testing programme that will be taking place during 2006 and 2007.Copyright

CAISSON: A Suction Pile Design Tool

The two main suction pile design methods that are generally applied and accepted within the industry are 3D Finite Element analysis and limit equilibrium. The limit equilibrium method involves assuming a number of failure mechanisms with the mechanism offering the least resistance adopted for design. The limit equilibrium suction pile design software CAISSON has been developed and validated by Cathie Associates for Technip. It is currently in use for rapidly and reliably determining the critical failure mechanism and ultimate holding capacity of initiation, mooring and hold back suction piles in clay. CAISSON has been developed as a stand-alone program written in Visual Basic with a user-friendly program interface implemented to allow for efficient computations. The failure mechanisms employed in CAISSON were identified initially using 2D FE results from PLAXIS. The failure mechanisms identified were further calibrated using 3D FE modelling in ABAQUS and FLAC to account for the influence of side shear within the limit equilibrium equations adopted in CAISSON. The current version of CAISSON can analyse suction piles with L/D aspect ratios from 0.5 to 5 installed in clay of uniform or linearly increasing undrained shear strength. Additional program features include computation of inverse catenary shapes for anchor chains, anisotropic undrained shear strength profiles, pile tilt and pile misalignment. The development and validation of CAISSON is presented in this paper along with a case study and a short parametric study to identify the significance of the CAISSON input parameters that govern the ultimate holding capacity of suction piles. Planned upgrades to CAISSON will also be presented.Copyright