In this Case Study, AFS developed an umbilical flushing laboratory test designed to examine the sensitivity of product and displacement solvent viscosity on the effect of flushing volume required to displace production chemicals.
Issue
Throughout field life, the chemicals used to treat flow assurance problems associated with subsea tiebacks change with varying field conditions. To better address production issues and provide the best overall asset performance, chemicals often need to be changed. The selection of displacing solvents and buffer fluids, as well as associated volumes required, are critical to a successful chemical swap as incompatibilities can be detrimental to subsea delivery systems by creating blockages.
An industry “rule of thumb” typically uses one to two tubing volumes to fully displace the incumbent fluid from the system. However, this can often lead to an inefficient displacement by either increasing time and cost or leaving the system vulnerable to potential blockages.
Process
AFS developed a dynamic lab test to determine the flushing volume needed to displace different production chemicals with varying compatible solvents by flowing fluid through a lab-scale umbilical. The flushing volumes for chemical displacement were assessed by examining the effect of solvent choice, based on the product and solvent viscosity.
A constant flow rate was used to ensure the Reynolds number of each solvent remained consistent in the laminar regime with varying product viscosity. EGMBE (6cP) and water (1.5cP) required two to three tubing volumes to reach 5vol% concentration of the incumbent chemicals. Meanwhile, MEG (45cP) required 1.5 tubing volumes to reach 5vol% concentration of aqueous scale inhibitor without any prominent tail.
The film-forming corrosion inhibitors exhibited a notable long tail of lower concentrations with testing vials never reaching 100% transition after three tubing volumes.
Table 1 summarizes the tubing volumes required to reach 5vol% product concentration. Figure 1 depicts the tubing volumes needed to reach various concentrations of production chemical throughout the chemical transition.
Table 1. Umbilical flushing volume lab testing summary
| Incumbent Product | Incumbent Viscosity | Displacement Solvent | Flow Rate (ml/min) | Solvent Viscosity | Tubing Volumes (5vol% Incumbent) | Desired Maximum Incumbent Concentration |
| Film-Forming CI 1 | 71 cP | EGMBE | 10 | 6 cP | 2.9 | 5% |
| Film-Forming CI 2 | 12 cP | 10 | 2.1 | 5% | ||
| Aqueous SI 1 | 15 cP | MEG | 10 | 45 cP | 1.4 | 5% |
| Aqueous SI 2 | 23 cP | 10 | 1.5 | 5% | ||
| Aqueous SI 1 | 15 cP | Water | 10 | 1.5 cP | 1.9 | 5% |
| Aqueous SI 2 | 23 cP | 10 | 1.9 | 5% |
Figure 1. Lab measured required tubing volumes during chemical transitions
Outcome
Based on the lab testing, the industry “rule of thumb” for umbilical flushing volume requirements is relatively accurate for most production chemicals.
Corrosion inhibitors and other film-forming chemicals are the outliers, taking more tubing volumes to displace the incumbent product. Additionally, the displacing solvent with the higher viscosity reached target concentrations the quickest.
Umbilical flushing optimization is complex and understanding its sensitivity is imperative to avoid unwarranted time and cost loss during displacement operations. Additional lab testing variables and chemical characteristics will be assessed in future studies to provide further test-based field recommendations.
For more information about AFS laboratory services, please visit our laboratory page or contact us to discuss your specific project challenges in more detail.