Pour point depressant
Pour point depressants (PPD) are chemical additives that influence the shape of wax crystals and therefore the viscosity and yield stress of a crude oil as it is cooled. They can significantly reduce both normal pumping pressures and yield stress after an extended shut down.
Typically, pour point depressants are polymers at concentrations of 20-50 percent in a hydrocarbon solvents (e.g. toluene). Injection rates of as high as 100-1000ppm are commonly required. Therefore this is not a cheap option, and chemical costs alone can run into thousands of dollars a day. However pour point depressants have been used economically in the Indian Bombay High, Caspian Sea Chirag and North Sea Beatrice fields.
They must be injected while the oil is still hot, before wax crystals are first formed. Their performance depends very much on crude type and the additive itself, and there is no guarantee that a suitable pour point depressant can be found for every waxy crude. However, viscosity or yield stress can be significantly improved.
It should be noted that additive doped waxy crude is even more sensitive to mechanical shear. Some additives perform excellently in the absence of shear and not at all with shear. Evaluation of additives is therefore a complex process.