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 deg.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 psi and 2,100 psi, respectively for stock-tank and at line pressure conditions. Also, the use of inhibitor treatments demonstrated that the fluid would not form gel at the sea bed temperature of 4 deg. 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 wax inhibitors 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.

Summary 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.