(355g) Study on the Pig Motion in Waxy Crude Oil Pipeline While Wax Removing Operation

Wax
precipitates when waxy oil temperature decreases in pipeline transportation. A section
of slurry (mixture of wax deposits and oil flow) may come into being when
pigging is carried out for removing wax deposit, which can even lead to the
getting stuck of a pig. So, it is necessary to find reliable tools to predict
many variables related to the motion of a pig, such as the pig velocity, the
pressure profile along the pipeline and the bypass flow rate, which are
important to pigging efficiency and safety.

Reliable
predictions of above variables demand appropriate models to handle (i) basic
problems like the friction force between pipeline wall and pig, the resistant
force of deposits and parameters of slurry formed in front of pig as well as
(ii) the solution to combine the motion of pig, the motion of fluids and the
motion of slurry.

To tackle these
two problems, a mathematical model was developed to simulate the motion of a pig
in waxy crude oil pipeline. In contrast with prior studies in which pigs moved
in liquid and gas pipelines, wax deposits attached to pipeline wall was taken
into consideration for the first time. So in addition with the fluid flow
equations and dynamic equations of pig where key parameters like wax resistant
force and friction force were obtained through experiments, a model to simulate
the increasing slurry in front of pig was also established. All of them were
solved together using the method of characteristics. Note that interaction
between pig and fluid flow (or slurry) was described by mass conservation
equations at boundaries. Combining pig motion with the pump operating
characteristics, pig velocity and pipeline inlet pressure under any conditions of
wax thickness can be obtained through the model (figure 1). Wax resistant force
turns out to be two orders of magnitude smaller than pressure drop of the slurry
section even when the thickness of wax deposits is 0.1m. Bigger diameter of bypass
hole in the pig can, on one hand, reduce the viscosity of slurry which benefits
the pig motion, while decrease pig velocity on the other hand, so an optimal
diameter of bypass hole is recommended for the sake of pigging time consumed. According
to the calculation results, when bypass diameter increases from 0.01m to 0.03m,
the pressure drop of slurry decreases by nearly 15% after the pig moved for the
same time.

Figure 1 Time evolution of pig
velocity and pipeline inlet pressure under different thickness of wax deposits
(h)

Pigs
would get stuck once the pipeline inlet pressure exceeds the available pumped
pressure, so pipeline
inlet pressure was taken as a criterion to judge whether a pig would get stuck
(figure 2). Finally, sensitivity analysis in view of the effect of related
parameters on the increasing rate of pipeline inlet pressure was performed and
the results indicated an increasing sequence: thickness of wax deposits,
friction force between pipeline wall and pig, viscosity of slurry.

Figure 2 Illustration of pressure
drop along the pipeline during pigging at different times (the red sign
represents the pig position)