We have found that pressure drop is proportional to viscosity and diameter to the fourth power for laminar flows. Thus as the viscosity of a liquid gets much larger than water, it will require either very high pressure drop (high operating costs) or large pipes (high capital costs) to pump. Obviously, if the liquid is valuable enough then it will be worth it and pipeline built. However for a less valuable material, pumping as a single phase liquid may not be economically viable. Such is the case for very heavy crudes. Pumping costs can be so large that the oil could never be sold for profit.
However, what if your company or country has access to only this kind of heavy crude oil. It is in your interests to solve this problem and produce a new technology that transport the crude efficiently enough so that it can be sold for profit.
The idea that could be used is "lubricated" transport where the viscous oil is confined to a "core" region in the center of the pipe and a much less viscous liquid (e.g. cheap also, water) flows in the annular region. The pressure drop is reduced because while the water is sheared highly, the viscous oil core is not subject to much deformation thus does not contribute to pressure drop.
For more about the real problem you will need to read elsewhere. Real core-annular flows have waves on the interface which usually act to stabilize the core flow, although could sometimes break up the core.
A general reference for this topic is Fundamentals of two-fluid dynamics by D. D. Joseph and Y. Y. Renardy, Springer Verlag, 1993.
There is also a CNN science/technology video on this topic "staring" Dan Joseph and his lab.