Numerical investigation of gas permeation and condensation behavior of flexible risers

Flexible risers are widely employed in deep-sea oil and gas development. Methane, carbon dioxide, water vapor, and other small molecular gases can permeate into the sheaths and accumulate with water condensation in the annulus, which may cause overpressure and corrosion accidents. A mechanistic model was proposed to describe the permeation of gas phase in the multiphase flow though the flexible riser inner surface and its condensation in the internal annular space. The governing equations were solved efficiently by a combination of the control volume method and the finite difference method. After validation of the models, the thermo-hydraulic characteristics, concentration fields, annulus conditions, flow patterns, and some key parameters were comprehensively investigated. The results showed that with increasing the inlet pressure and temperature, the initiation time of venting and its duration decreased exponentially. The venting frequency can be adjusted by selecting an appropriate shielding factor and venting pressure. When the shielding factor is larger than 0.75 or the venting pressure is greater than 500 kPa, the venting cycle will be significantly prolonged. The gas permeation rates varied in a wide range of 2 times with flow patterns.