Relationship between texture of polypropylene coatings and interface friction for sand at low stress levels

Pipelines in the offshore sector make common use of polypropylene (and other polymer) coating systems to protect the infrastructure and provide thermal insulation. High pressure high temperature pipelines are subject to large axial loads due to restraint by seafloor soils of thermal strains in the pipe wall. Friction between pipe coating material and seafloor soil plays a defining role in the build-up of axial stress and the formation of lateral buckling. Accurate quantification of pipe-soil interface friction is key to robust pipeline stability design and possibilities to enhance or manipulate the friction coefficient may be attractive to designers. An extensive campaign of soil and interface tests using a range of granular materials and polypropylene surface specimens engineered to achieve varying surface textures was carried out. The results show that interface friction primarily depends on stress level and the magnitude of the surface texture in relation to the particle size. Herein, a new relative texture parameter, T-a, is developed that, unlike alternative relative roughness parameters, can be obtained using conventional profilometry measurement techniques. An expression for estimating the friction coefficient in relation to texture and stress level is proposed that can serve as a useful predictor of pipe-soil interface friction.