Pipe–soil Interaction – definition of lateral and axial coefficients of friction including consideration of pipe embedment during installation and subsequent operation through detailed soils modelling based on gathered/surveyed soil properties. Also determination of static and dynamic soil stiffness. This feeds other conventional and specialist design activities.

A key issue for on-bottom stability is the influence of pipe embedment assumptions on the level of axial expansion and the extent of lateral restraint. Recent survey data showing deepwater pipeline/flowline embedment post-installation (but prior to hydrotest) indicates installation dynamics produce a mean embedment several times that typically predicted by embedment analysis for an empty line.

For axial expansion the lower bound value of embedment governs however for lateral buckling it may be the upper bound that requires consideration for the governing case. Accurate prediction of embedment relies on near-surface geotechnical knowledge, rather than the traditional approaches in this area, for pipe-soil interaction and DEEPSEA has developed this through a number of projects, particularly the investigation of SCR pipe-seabed interaction in the touchdown zone. This knowledge and the available toolset are applied to the embedment, expansion and lateral buckling design aspects to provide enhanced design of the system.

Lateral buckling – including preliminary assessment with analytical tools, detailed analysis with finite element modelling of sufficient/entire line length. Lateral buckling mitigation design in terms of concept (snaking, sleepers, buoyancy, intermittent restraint etc.) selection and detailed design to define reliable solution.

The need for the line to axially extend is the driving factor for the lateral buckling phenomenon, therefore, accommodation and controlled promotion, rather than restraint, of this axial expansion results in economic relief of the axial force by letting the pipeline do the work.

Whether lateral buckling is an issue for a flowline depends on a combination of parameters:

• Operating temperature (and to a lesser extent pressure, see above)
• End restraints/expansion accommodation – alleviation of expansion forces for short lines can lead to reduction/elimination of lateral buckling as a design consideration
• Length of line – for long lines an anchor length will develop (length dependent on soil conditions and embedment) resulting in the full thermal expansion force being locked into the system. For short lines, typically 2-4 miles in length, the full buckling force is unlikely to develop
• Submerged weight of line – this is affected by the quantity of insulation applied to the pipe for steady state/cooldown (see above) and this reduces the critical temperature for lateral stability (under environmental loading as well as lateral buckling)