Flotation to trigger lateral buckles in pipelines on a flat seabed

Triggering lateral buckles in subsea pipelines can be an effective way to ensure that the thermal expansion is spread over several buckles at a controlled spacing, rather than concentrating in a single buckle that may become overstressed, or causing excessive displacements at the ends of a pipeline where spool-piece connections or jumpers may become overstressed. For a given location where a buckle is to be triggered one calculates that axial load that would develop if the buckle fails to form, and ensures the buckle would be triggered at or below this axial load. This paper gives simple analytical expressions for triggering of such lateral buckles by applying buoyancy to the pipeline, e.g., by inflatable parachute-like bags, which can later be removed. The expressions apply for a single buoyancy load, two equal buoyancy loads, or distributed buoyancy over a specified length. They explicitly solve the design problem of establishing the required amount of buoyancy for a given axial load at which the buckle must be triggered. It is shown quite generally that bifurcation buckling into a lateral buckling mode occurs before vertical instability, and at moderate uplift displacements. The analytical results are based on a flat seabed, the theory of moderately large deflections, and elastic pipe behavior. An example involving a single point buoyancy load is also solved by finite-element analysis based on large deflection theory to verify the accuracy of the approach.