Polymer Inner Liner Strain Boundaries – From Factory to Service

Abstract The polymer based inner liner (fluid barrier layer) is a critical component in flexible pipes, designed to effectively contain the fluids transported within the pipe. High performance polymers, i.e., polyvinylidene fluoride (PVDF), cross-linked polyethylene (XLPE), high-density polyethylene (HDPE), are known for their excellent chemical resistance and durability, and are often used as inner liner material in flexible pipes. The selection of inner liner material depends on many factors like service temperature, exposure fluid, and dynamic bending requirements. Also, in certain cases, the ductility of polymers is significantly reduced which becomes a critical consideration, especially when the material is subjected to localized and abrupt strains. This study investigates the correlation between the yield point and the mechanical capacity of inner liner material to validate the strain limits in relation to static and dynamic applications. PVDF liner material is analyzed in this work due to its known inherent strain limited material properties, under certain conditions, and thus requires critical consideration during manufacturing, installation, and service. The research analysis covers static tensile testing investigating strain capacity, dye penetrant inspection (DPI) and fatigue performance (full-scale and small-scale testing) to establish acceptable design requirements for PVDF inner liners used in unbonded flexible pipes. This work indicates that one of the most complicated and challenging stages during a flexible pipe’s lifetime is during manufacturing/installation where extreme care needs to be taken to limit the strain in the inner liner.