Abstract

Periodic shutdowns, which decompress service pressure down to atmospheric pressure, are common in offshore flexible pipe systems. In high pressure and high temperature operations, the use of multi-layer barrier flexible pipes is also common. A multi-layer barrier usually combines inner sacrificial and pressure sheaths and, under certain circumstances, the annular gap between these sheaths is filled with fluid. When this fluid is trapped in this gap, which may occur due to a rapid decompression on shutdown operations or an accidental opening of an upstream valve, the flexible pipe system may fail. The trapped fluid tends to be released slower than the fluid that transiently flows under the bore thus generating a differential pressure, which is function of the rate of decompression, and may induce the collapse of the inner carcass. Therefore, the assessment of the magnitude of this differential pressure is crucial to prevent safe decompression rates. This article describes a numerical model, based on finite differences, to predict the differential pressure magnitude for risers in catenary configurations. The model is developed considering fundamental fluid dynamics principles, such as the conservation equations, for a one-dimensional geometry. The results are compared to previously published literature data for a given 4” flexible pipes under two different geometric configurations (horizontal and catenary). The results from the proposed model agreed quite well with those from a previous proposed approach for horizontal configurations, while the catenary configuration analyses evidenced the impact of the gravity parcel on the differential pressure between the pressure sheath and the bore of the pipe.

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