Cool-Down Temperature Abnormity Caused by Subsea Pipeline Topography

An abnormity of temperature trend during cool down has been observed. During shut-in of insulated pipeline and riser systems that carry multiphase wellstream, the temperature at pipeline segments with large geometrical variations may decrease below the local ambient temperature. The low temperatures at these locations increase risk of hydrate formation during system cool-down. The cause of such abnormity may be explained by the mass transfer between gas-liquid interface, the phase re-allocation, and inner energy re-distribution due to the combined effects of gravitational and buoyancy forces. Pressure, mass flowrate, fluid compositions, flow orientation, and external and internal heat transfer during system cool-down are the key factors for the phase-redistribution between gas and liquid, and along the system. Susceptibility of a system for cool-down temperature abnormity should be assessed during design stage and operation procedures should be developed to manage it. An explanation of the governing mechanisms of this phenomenon is given in this paper, for a wide range of system parameters. The discussions are based on a series of theoretical and numerical studies of gas/condensate and oil/gas flows in subsea flowline and riser system after system shutdown.