Factors affecting gel strength include thermal history, shear history, stress loading rate (SLR), aging, and composition of the waxy crude. The most controversial factor is the cooling rate, which some have argued increases the gel strength at higher values (Ronningsen et al. 1992) while others advocate the reverse effect (Henaut et al. 1999). Lee et al. (2007) showed that higher cooling rate increases the gel strength if the failure mechanism is adhesive and vice versa if the failure mechanism is cohesive.
It is important to note that no standard test for determining the yield stress of waxy crude oils has been adopted by the petroleum industry. This is because of the very poor repeatability in any given instrument and the poor reproducibility between the different tests. Chang et al. (1998) attributed poor repeatability and reproducibil-ity of the yield values to their strong dependence upon not only what the sample is experiencing (i.e., temperature and shear rate) but also what the sample has experienced (i.e., thermal and shear history). Ronningsen et al. (1992) reported a reproducibility of ±20% of yield point results obtained from their model pipeline setup, while Lee et al. (2007) reported a better success on the order of ±10%.
Gelation Pressure Effects: Reality or Academic Concept?
The effect of hydrostatic pressure on gelation of waxy crudes is an issue of great practical importance in subsea pipelines connected to long vertical risers as depicted schematically in Fig. 1. However, most restart studies have been carried out at ambient conditions and attempts have been made to apply the results obtained to conditions involving deepwater systems. During shut-down of such a pipe-line transport system for maintenance purposes or emergency, the crude cools under quiescent conditions to the seabed temperature. In today’s deepwater systems, running in waters up to 10,000 ft deep, a temperature gradient that could differ considerably from the bottom of the sea to the surface is a possibility. This is more so during hot seasons when the water at the surface could be at temperatures as high as 20°C, while the seafloor temperature could remain as low as 5°C. Thus, under such conditions, the waxy crude in the pipe at the sea floor can be gelled (assuming its pour point is higher than the seafloor temperature), while at some distance from the seabed, liquid waxy crude in the riser lies above the gelled seg-ment. This will exert some hydrostatic head on the segment under-going gelation. Assuming a head of 0.3psi/ft (typical of crudes in the Gulf of Mexico), at depths of say 7,000 ft, pressures as high as 2,100 psi could be exerted on the gelling segment. The questions then are, “Does this pressure affect the strength of the gel formed, and how does this affect the restart conditions of the pipeline?” Investigation of this interesting issue is the primary focus of the experimental studies conducted in the present work.
不通啊。。。