Deep Water Carcass Development: Effects of Carcass Profile on Collapse Resistance

The increasing application of flexible pipes in deep water requires extension of the product’s capability through higher design pressures with large diameters, while one of the most important structural capacities is collapse resistance. In addition to the introduction of new materials and manufacturing capability, the carcass structure is also expected to be optimized for maximum purpose. The carcass is an interlocked metallic construction that is used as the innermost layer to prevent for example, either totally or partially, the collapse of the internal pressure sheath due to decompression, or the pipe, due to external hydrostatic pressure, tensile armour pressure, and mechanical crushing loads. During manufacture of the carcass, its profile may differ from the nominal designed profiles and this may affect the collapse resistance. In this paper the effects of the carcass profile on the wet collapse strength of flexible pipes via numerical simulation are investigated. The wet collapse strength is referred to as the hydrostatic collapse strength of the pipe when the annulus is assumed to be flooded with seawater. In this condition, the external hydrostatic pressure acts directly on the barrier layer above the carcass. Using ANSYS, a 3D finite element model with the consideration of helical effects and cyclic boundary condition, which was developed to check the effect of each part of the carcass profile on the collapse strength by varying the design parameters, e.g. inclination angles, etc, and manufacturing ovality. As both verification and comparison, the principle of the effects is explored and explained analytically. The results can be used as an acceptance criterion on the as-built profiles in the manufacturing process, and as a guideline for the innovation and optimization of the carcass for maximum performance.