Abstract

Pressure armour in unbonded flexible pipes is constructed using helically-wound profiled wires specifically designed to provide hoop strength; in high pressure applications, several layers are wrapped to provide the necessary hoop strength. During service, the wires in individual layers are in contact and move relative to each other due to dynamic flexing. Analysis of such interfaces is complex, as the contact between the layers is non-uniform, discrete and nonconforming. Discrete contact geometry often leads to high stress hot spots that can lead to regions of fretting/wear in the wire under certain conditions. Knowledge of the local interactions between individual layers and the ability of the pressure armour wires to withstand such local contact under applied dynamic sliding conditions is essential to guarantee the necessary integrity and durability of the pipe to withstand service pressure loading.

This paper discusses a pipe with two pressure armour layers. A finite element model has been developed to predict both the dynamic interaction between layers and the contact stick-slip behaviour between the wires under dynamic loading. The numerical results of this work, together with the fretting/wear damage behaviour of the wire material developed in a previous study, have been used to assess the fretting/wear damage of the pressure armour wires of the pipe. An evaluation method has been proposed to predict the safe loading envelope that would not lead to wear/fretting of the pressure armour wires.

The method involves the evaluation of subsurface stress distribution at the wire contacts in the pipe, which is used with material yield properties to predict the size of microscopic plastic zones. Loading cycles to crack nucleation are predicted using experimental data generated under similar loading conditions. A brief discussion is provided on the fretting damage behaviour of flexible pipes under service conditions.

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