The Role of Wax Inhibitors for Flow Assurance Below WAT in Long Tiebacks

The low temperature of the seabed can lead to wax precipitation of the oil in offshore production, clogging the flowlines, particularly in long tieback systems. To prevent this, production systems are designed to keep the oil always above the WAT throughout its trajectory, which limits the length of the flow lines. An oil from a small field located 60 km from a FPSO vessel, where the seabed temperature is 12 °C, was investigated. The WAT and gelation temperature (GT) of the oil were  ~ 17 and ~ 9 °C by microcalorimetry and rheology, respectively. The net present value is US$ 589 million for an insulated pipeline and US$ 689 million if no insulation is used. The latter case is economically more attractive, if the wax is removed with frequent pigging operations. However, for a similar oil in very deep waters where the seabed temperature is 5 °C, restarting oil flow is energy-intensive due to the yield stress at this temperature, necessitating inhibitors. Surfactant wax inhibitors, while not affecting GT, the yield stress temperature, and WAT, reduced the yield stress value by 70–90%, softening the gel. A commercial polymeric wax inhibitor lowered GT and WAT to approximately −21 and 10 °C, respectively, and eliminated yield stress. All inhibitors reduced oil viscosity by about 60% at 5 °C, facilitating wax removal with pigs.