Session: 04-01-02 Flexible Pipes and Umbilicals II

Paper Number: 78325

78325 - Gas Diffusion in Flexible Pipes: A Comparison Between Two- and Three-Dimensional FE Models to Predict Annulus Composition 

The Brazilian oil and gas industry vastly uses flexible pipes. These pipes are responsible for water, oil, and/or gas transport. Currently, the high concentration of carbon dioxide and hydrogen sulfide presents in Brazilian pre-salt wells has been introducing new challenges to the use of flexible pipes. Due to the pressure sheath permeability, the internal fluid permeates into their annulus. The water present in the annulus combined with the permeation of acid gases (CO₂ and H₂S) shall lead to the formation of a corrosive solution for the steel wires in the annulus. This corrosive environment may induce a failure mechanism, SCC (Stress Corrosion Cracking) and HIC (Hydrogen Induced Cracking) responsible for drastically reducing structure lifetime. A downside is that the flexible pipes permeation phenomenon may be very complex. The permeate rate depends on temperature, gases partial pressures, and the distribution of the free volume. Therefore, new tools are required to understand better the fluid permeation between the flexible pipes’ layers.

Therefore, this paper shows and compares two models based on the finite element method to predict the annulus composition of flexible pipes. Both models consider the temperature gradient effects on the material properties, and a flash calculation combined with a stability analysis provides vapor and liquid phases composition. On the one hand, the first approach deals with a two-dimensional model that does not consider the three-dimensional shielding effects of the armor wires geometry. On the other, the second develops a complete three-dimensional model of the cross-section geometry.

The results indicate that, while being faster, the two-dimensional approach shows higher concentration results than the three-dimensional approach. Furthermore, the difference between the two approaches indicates that the shielding effects of the helicoidal wires are relevant.

Presenting Author: João Marcos Bastos Vieira Federal University of Rio de Janeiro

Authors:

João Marcos Bastos Vieira Federal University of Rio de Janeiro
José Renato Mendes De Sousa Federal University of Rio de Janeiro