Analytical Transient Thermal Model for Predicting Cooldown Temperature in a Subsea Pipe-in-Pipe Flowline System

Background: For the reliable operation of deep-water production systems transporting hydrocarbons, the critical flow assurance problems to manage are wax and hydrate formation and their deposition due to the heat transfer between the production system and surroundings. Wax and hydrate deposition can restrict the flow with significant production loss. Methods: The adequate thermal insulation of such systems can allow the retention of thermal energy in the production system to ensure that the fluid cooldown temperature remains above the wax and hydrate formation temperatures. This may provide sufficient time for taking the preventive measures during an emergency shut-in operation. The purpose of this paper is to predict the cooldown temperature of a subsea Pipe-in-Pipe flowline system transporting crude oil from the reservoir to the host facility using a recently developed transient thermal model. The model incorporates the energy redistribution inside the system through the internal temperature gradient and allows to determine the requirement of thermal insulation of the Pipe-in-Pipe system to retain sufficient heat before the fluid temperature falls below the hydrate and wax temperatures. Results: The model results are compared with an industry accepted commercial simulator results to illustrate the accuracy of the model for emergency shut-in operations. Conclusion: With limited pipeline configurations and fluid properties, the model can successfully predict the requirements of thermal insulation and cooldown temperature efficiently and economically.