Umbilical Flushing Optimization During an Extended Shutdown

In this Case Study, AFS developed an umbilical flushing strategy for a client to reduce corrosion risk of the subsea flying leads during a planned, extended shut-in. Prior to restart the original production chemical would be pumped back through the umbilical until reaching the terminus. The client planned to use EGMBE as a compatible chemical solvent to displace corrosion inhibitor, hydrate inhibitor, and asphaltene inhibitor, respectively, from the umbilical lines.

Issue

The Deepwater production platform was equipped with 8 umbilical lines dedicated to production chemicals, each exceeding 30,000 ft in length equating to a total tubing volume greater than 3,300 gallons. Given displacement operations often require multiple tubing volumes to achieve full displacement, the calculated flushing volumes and total pumping times were exceptionally large. Additionally, the increased risk of introducing various flow assurance failures if inhibitors were not injected during on-line production times required a detailed operations procedure to minimize production downtime and efficient chemical use while minimizing flow assurance risks.

Process

AFS developed an in-house model for umbilical flushing using primary mass transfer principals: advection and diffusion. Here, advection occurs mainly in the axial direction, while diffusion occurs primarily in the radial direction. Based on these principals, the properties of the fluids, and displacement operating conditions, the model output includes the concentration of the fluids at the flying lead terminal as a function of time.

The umbilical displacement process was found to operate in a laminar flow regime, which impacted flushing rate efficiencies.  Lower flushing rates led to reduced EGMBE volumes while increasing pumping times.

The umbilical flushing models showed EGMBE volumes could be reduced by reinjecting the production chemicals prior to full displacement with EGMBE. Models predicted a window in which the flying leads were fully displaced with EGMBE, while the umbilical was backfilled with 1/4 to 1/3 volume of production chemicals. Table 1 summarizes the optimized EGMBE injection volumes and EGMBE pumping times prior to shut-in. Figure 1 shows the concentration of CI and EGMBE at the subsea flying lead over time and injected volumes.

Table 1. Summary of optimized EGMBE flushing times and volumes

Figure 1. CI and EGMBE concentration at the steel subsea flying lead terminals

Outcome

The flushing guidelines developed by AFS were successfully used in field operations for extended shut-ins, allowing for protection of the subsea steel flying leads. The client was able to reduce EGMBE volumes by over 4,400 gallons and pumping times ranging 60-145 hours through the optimized flushing protocols.

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