Systematic experimental and modeling approaches to designing a safe operating strategy for a 5-km deepwater-subsea-flowline case study are presented to address unplanned shutdown and restart events for waxy-crude production. The measurements confirmed that the fluid behaves like Bingham plastic when it is allowed to become gel at the seabed temperature of 4°C. The cool-down period was modeled using the transient simulator validated by measurements and was predicted to take 21 hours. The restart pressure was then modeled for both stock-tank and at-line pressure conditions. These restart pressure requirements were found to be 2,500 and 2,100 psi, respectively, for stock-tank and at-line pressure conditions. Also, the use of pour-point depressants demonstrated that the fluid would not form gel at the seabed temperature of 4°C. However, the current shut-in wellhead pressure of 2,500 psi is deemed adequate to restart the lines in the event of unplanned shutdown without the use of chemicals. The presence of a subsea pig-launching pump provides a safety factor for restart in case the line pressure is released to atmospheric conditions. Hence, the operating strategy does not require injection of pour-point depressants at the current state. However, in future when the shut-in wellhead pressure falls below 2,500 psi, the operating strategy is expected to be modified accordingly.
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