Shearing Resistance of Rough Polymer-Sand Interfaces-Paper

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The study investigates axial friction between rough polymer pipeline coatings and seabed sands, an important factor in pipeline expansion, walking, and buckling design. Earlier research focused largely on smooth coatings and fine-grained soils, while guidance for pipelines on sand was limited.<br /><br />Researchers tested four sands spanning fine to coarse grain sizes, at loose and dense states and submerged stress levels of 2, 5, and 15 kPa. Using an improved direct shear apparatus, they compared polypropylene surfaces with two manufactured roughness levels: laser-engraved (average roughness, Ra, of 13 μm) and laser-engraved then sand-blasted (Ra of 35 μm). Results were compared with prior tests on smooth polymer and sand alone.<br /><br />Friction generally increased as normal stress decreased, particularly below 5 kPa. Finer grains and denser sand also tended to raise resistance. The 13 μm surface increased friction over smooth polymer but showed no dilation. The rougher 35 μm surface produced peak strength and clear dilation in dense sand. Across the tests, resistance rose consistently from smooth polymer to engraved polymer, sand-blasted polymer, and finally sand alone. However, even the roughest surface did not always reach sand-alone strength, indicating that commonly used roughness thresholds may not reliably predict fully rough behaviour for polymer coatings.<br /><br />At stresses below 5 kPa, peak friction coefficients were approximately 0.4–0.6 for the 13 μm surface and 0.65–1.0 for the 35 μm surface; corresponding residual values were 0.3–0.55 and 0.45–0.75. The results broadly align with prior research, but some field pull tests report higher resistance than laboratory tests can explain. Further investigation of very low stress effects and field conditions is needed to improve pipeline design estimates.

Keywords

axial friction

pipeline coating

rough polymer

seabed sand

direct shear test

sand grain size

normal stress

pipeline design

buckling

friction coefficient