Interface shear strength of polypropylene pipeline coatings and granular materials at low stress level

Lawrence W De Leeuw,Andrea Diambra,Matt S Dietz,Henry Milewski,George Mylonakis,A Tarantino,E Ibraim

doi:10.1051/e3sconf/20199213010

Lawrence W De Leeuw, Andrea Diambra + Show 5 more

Open Access

https://doi.org/10.1051/e3sconf/20199213010

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Abstract

A range of sands were tested in direct and interface shear at very low stresses to determine the interface shear strength of polypropylene pipeline coating counterfaces and to evaluate interface efficiency. Polypropylene has a wide range of applications as a coating material in the offshore environment, so quantification of interfacial strength is an important component for geotechnical design. Direct shear tests show classic peak-postpeak stress-displacement and stress-dilatancy behaviour whereas interface tests show an elastic, perfectly plastic type behaviour for both loose and dense samples with no appreciable volumetric response. Interface efficiencies generally range between 0.3 and 0.7 dependent on both grain size and stress level. Normalised roughness is used to relate the surface roughness to the grain size and shows that the greater interface strength with smaller grained sands can be explained by their greater effective roughness. The relationship between stress ratio and normalised roughness for sand-polypropylene resembles established relationships for sand-steel interfaces.

Highlights

Subsea hydrocarbon pipelines and electrical cables are laid either directly on the seafloor or buried in shallow trenches to protect them from adverse hydrodynamic loading or damage from fishing gear
Direct and interface shear testing was carried out using the Winged Direct Shear Apparatus (WDSA) developed by Lings and Dietz [7]
The results have shown that the hardness of the surface is not the only parameter governing the interface friction

Summary

Introduction

Subsea hydrocarbon pipelines and electrical cables are laid either directly on the seafloor or buried in shallow trenches to protect them from adverse hydrodynamic loading or damage from fishing gear. Polypropylene coatings are applied to electrical cables in the form of a dense yarn sometimes mixed with bitumen. A common technique for pipeline coating is extrusion of polypropylene into a strip to wrap around the pipe to form a relatively smooth coherent surface and this type of application is the focus of this research. On dynamic seabeds comprising loose sands, loads transferred from pipe to soil in response to thermal expansion and contraction causing buckling or pipeline walking are restrained principally by the strength of the polypropylene-soil interface. Numerous researchers have explored the interface strength of polymers and sands including M- and HDPE, PVC, Epoxy, plexiglass [1-4 among others] and it appears customary to link the interface strength to the hardness of plastic polymers [4]. The effect of soil gradation on interface strength of polypropylene has previously been explored [5]

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