Axial resistance of smooth polymer pipelines on sand

The axial resistance of pipelines is an important design input, influencing a variety of analyses such as buckling and axial walking. As such, accurate assessment of the frictional behaviour of the soil-pipeline interface is necessary to properly model axial behaviour. Smooth polymer coated pipelines are commonly used subsea, yet despite their common application, limited guidance exists in the main governing standards concerning the expected level of axial friction to be used in design. Related guidance that does exist (e.g. BSI, 2016) suggests a minimum friction coefficient of 0.55 for sand-pipeline interfaces. This paper reviews various aspects of sand-polymer direct shear interface testing that must be considered and presents the results of some experimental research TechnipFMC have undertaken in collaboration with the University of Bristol. These results indicate that a sand-pipeline friction coefficient of 0.55 is often unrealistic for smooth polymer coated pipelines and in many design scenarios a lower frictional coefficient is more appropriate. The experimental test program considered the main factors believed to influence axial friction of smooth polymers on sand including D50 grain size, sand density and a range of stress levels (including the low stresses expected for subsea pipelines). All tests were conducted fully saturated to mimic subsea conditions and the roughness of the pipe coating samples was thoroughly characterised. TechnipFMC project experience has found that use of lower axial friction is sometimes beneficial (e.g. axial feed-in to trigger buckle initiation). In other cases, a higher axial friction may be better for design (e.g. limiting axial walking). Being able to better characterise the friction range is therefore important to ensure a robust design and to assist in avoiding more costly mitigation measures where they may not actually be needed.