Interface shear response of polypropylene pipe coatings and influencing pipe buckling using enhanced textures

Abstract

Pipelines are an integral part of offshore infrastructure supporting the oil and gas industry and the consequences of their failure have severe economic and environmental ramifications. Changes in pipe internal pressures and temperatures from the as-laid condition to their operational state cause large thermal expansions. When axial strain from thermal expansion is resisted by the pipe-soil friction, the effective axial force in an unburied pipeline is relieved by lateral friction-sliding-buckling. The phenomenon of pipeline buckling is a significant challenge in managing the global stability of high pressure-high temperature offshore on-bottom pipelines. Pipelines are commonly given a protecting coating to aid in protection from damage and to provide thermal insulation. The use of polypropylene in this application is prevalent but relatively recent so correct quantification of the interface shear strength between marine sand soils and polypropylene is key to robust global stability design.
Herein, an extensive campaign of soil and interface shearbox testing has been undertaken to determine and evaluate the shear response of polypropylene surfaces. Parameters such as soil grading, density, surface texture, stress level, and cyclic behaviour have been investigated. The results show that the efficiency of the interface is strongly dependant on the soil grading and the surface texture at the interface. The shear response of soils at the interface with smooth surfaces is bilinear, characterised by an initially linearly elastic response at very small horizontal displacements, that transitions rapidly to a near constant shear stress plateau. Surfaces with greater roughness provoke a dilatant soil shear response more typical of a soil-only behaviour. Greater magnitude of surface texture engenders greater dilation leading to greater peak shear strengths. A relationship has been developed which can aid designers in predicting interface friction for polypropylene surfaces and sandy soils given surface texture, soil grain size, and stress level input parameters.
The application of the experimental results to real-world problems was investigated through numerical modelling in Abaqus of an approximately 5 km long pipe on a rigid seafloor using friction penalty and non-linear springs to model pipe-soil interaction and force-displacement response. The impact on global stability and buckling parameters of changes in pipe-soil friction and of applying a differential friction regime along the pipe was investigated. Numerical analysis results showed that such techniques are able to significantly change the distribution and magnitude of buckles.

Date of Award	21 Jan 2021
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Andrea Diambra (Supervisor) & George Mylonakis (Supervisor)