Development of an Integrated Analytical Model to Predict the Wet Collapse Pressure of Flexible Risers

Abstract

A flexible riser is a multi-layered pipe device which enables deep-water production by connecting seabed facilities to floating vessels. To withstand huge hydro-static pressure, it is required to have strong collapse capacities. At present, the collapse capacity of a flexible riser is designed based on a "wet collapse'' concept, in which the outer sheath is damaged and the seawater has flooded the annulus. For a given water depth, the hydro-static collapse design of a flexible riser needs to be confirmed by a wet collapse calculation. Calculating the wet collapse pressure of flexible risers is always challenging since the layers within risers are different in geometries and materials. Such a complex cross-sectional configuration makes the numerical simulation become the main approach for collapse analysis, which is quite time-consuming for the design stage. As the production is moving towards ultra-deep water fields, the design of new riser product is being required to achieve a balance between collapse resistance and self-weight. Therefore, there is a demand to develop an efficient tool to facilitate the collapse design. In view of this, this thesis presents an integrated analytical model, which addresses three challenges in the wet collapse analysis of flexible risers: the interlocked layer profiles, the geometric imperfections and the pipe curvature. The integrated analytical model contributes to the hydro-static collapse design of flexible risers, which can provide the designers a rapid feedback for their designed cross-section configuration. In our research work, the whole collapse analysis conducted by the proposed analytical model takes less than one hour to finish the prediction. Most of the time is spent on modeling the metallic layers for obtaining their equivalent properties. The actually wet collapse calculation given by this analytical model takes only a few seconds. By contrast, the numerical simulation requires 8-12 hours for modeling and consumes 2-3 days on average to complete one job. For companies that are developing new riser product to enable the ultra-deep water production, this proposed integrated analytical model can effectively facilitate the collapse design of new riser products.