Subsea deepwater flowlines transporting crudes are subjected to shutdowns for operational or emergency reasons. For waxy crudes, the low temperature of the surrounding seawater may lead the oil to cool below its pour point. In this situation, a gelled structure composed of paraffin crystals distributed through a viscous matrix is formed, blocking the flowline. The formation of this gel, as well as its aging, present serious complications for the flow restart process, because the required pumping pressure becomes greater than the normal operating pressure. The restart of waxy crude flowlines has close connection with the complex rheological properties of gelled crudes and startup pressure propagation. To better understand the restart process, rheological tests and restart pressure measurements in a pipe were performed. Using a rheometer, essential parameters that describe the behavior of the waxy crude oils, such as WAT (Wax Appearance Temperature), flow curves, yield stress and thixotropy were characterized. Yield stress measurements can be used to estimate the minimum pressure required to restart the flow. However, as the yielding process is time-dependent, phenomena such as thixotropy, shrinkage, compressibility, thermal effects and aging are enormously relevant for the gel breakup. Furthermore, the time required to restart a line blocked with gelled waxy oil depends mainly on the applied pressure and viscosity that decreases with time. Thus, start-up tests were also performed in a horizontal pipeline blocked with gelled waxy crude oil, once when these phenomenas are not considered, the use of conservative pressure values would lead to unnecessary costs for the restart process.
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