The deposition of paraffins was studied in a laboratory simulation of the subsea operational conditions, where the oil pipelines laying on the seabed are subjected to low temperatures. An experimental apparatus was built for a pipe-flow model submerged in a chilling bath of near frozen water. Streamwise pressure and temperature gradients were measured to evaluate the differences in the behavior of paraffin deposition between one-phase oil flow and two-phase oil-water flow. Most works on the behavior of paraffin deposition consider one liquid phase only, while two phases - oil and water - are considered here. Measurements in two-phase oil-water flows show a reduction of the head loss compared to one-phase oil flows, when both are subjected to paraffin precipitation, and photographs show that the layer of paraffin deposits on the pipe wall was significantly reduced in the presence of water.
Introduction
Petroleum is a mixture of hydrocarbon molecules, amongst which we find the alkanes, whose general formula is CnH2n+2, including linear, ramified and cyclical chains with simple covalent bonds, where the number o carbon atoms may range from a few to hundreds. Above a certain molecular weight the molecules will present a solid phase under standard conditions, known as paraffin, but while in their natural underground petroleum reservoir the paraffins may be in a stable liquid phase solution of oil. However, when the temperature of the oil falls below a certain value, determined from the thermodynamics of phase equilibrium, called the temperature of paraffin precipitation (TPP), the solubility of the paraffins in the oil decreases and the precipitation of solid particles occurs. The solid particles of paraffin may agglutinate, and also, through physical-chemical processes, they could deposit in the neighboring solid surfaces.
The deposition of paraffins in submarine pipelines poses a serious problem for the offshore petroleum production. The warm oil from the petroleum reservoir, emerging from the well-head, is cooled down by the seawater, surrounding the pipeline at temperatures as low as 4 °C (around 40 °F), to the point where paraffin precipitation occurs somewhere along the pipe. After some time, the continuous precipitation leads to deposits in the internal walls of the pipe, clogging the lines and promoting an increase in the head loss of the flow. Consequently, there is an increase in the pressure gradient required to maintain the flow, and the flow rate is reduced. In time, a complete obstruction of the pipeline may occur. Significant financial losses result from the reduction of revenue and from the increase of maintenance costs. Figure 1 shows a section of a pipeline with paraffin incrustation (from an offshore field in the Campos basin in Brazil).
