The mechanisms of phase re-distribution of gas/condensate flow in a deepwater steel lazy-wave riser after system shutdown have been studied numerically. The investigated system consists of a 15-mile long subsea pipeline tieback to a floating vessel, via a 9,800-ft long lazy-wave production riser. The subsea well is located at 6,350 ft of water. The system is insulated, and transports a gas-condensate mixture with liquid loading of 10 stb/mmscfd. This study reveals that besides pressure, the internal heat transfer during system cool-down is a key factor for the phase re-distribution between gas and liquid, and along the system. The liquid holdup variations are caused by the interfacial mass transfer between gas-liquid interface and phase re-distribution due to the combined effects of gravitational and buoyancy forces. Fluid cool down temperature “overshoot” in the lazy wave riser valley during system cool down has been observed. The pressure effect on the cool down temperature overshoot has been studied. The phenomenon is discussed based on fundamental heat transfer, phase equilibrium, and multiphase flow principles. The lazy wave riser configuration is a promising option for deepwater development, and gas/condensate flow is a multiphase flow phenomenon commonly encountered in raw gas transportation. The results of this study improve the understanding of multiphase flow transient behavior in deepwater pipeline/riser systems, and benefits gas/condensate production system design.

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