A Simplified Model of the Carcass Axial Capacity for Unbonded Flexible Pipes Under Differential Pressure Loading

Abstract

Abstract The formation of hydrate plugs can lead to a significant operational risk to a flexible pipe when uncontrolled differential pressure builds up across a blockage. After a hydrate plug is completely formed, it can fill all the voids within the carcass profile and the gap between the carcass and pressure sheath layers. This bonding mechanism constrains the relative movement of the carcass and pressure sheath layers at the hydrate plug section. The axial end cap force from the differential pressure acts on the formed hydrate plug and the load is directly transferred to the carcass and pressure sheath layers. If large enough, this load could tear the carcass layer apart, and damage the pressure sheath layer which leads to catastrophic failure of the pipe structure. It is important to assess the limiting capacity of such failure mode to assure safe operation. In this paper, a carcass axial capacity prediction model is presented for flexible pipes under differential pressure loading based on the characteristics of the deformations of the carcass and pressure sheath layers near the hydrate plug. The total axial capacity is predicted by the combination of a 2D finite element model for the carcass layer and a simplified analytical model for the pressure sheath layer. Detailed quantitative analysis results of a given flexible pipe are presented and compared with the test results. The design parameters impacting the model prediction are explored.