Abstract
Pigging has been recognized as the most cost-effective method for preventing flow restriction by wax deposits in subsea flowlines. However, the piping mechanics for wax removal in pipelines is still very poorly understood.
A unique test facility was designed and constructed for experimental studies on the mechanics of wax removal in pipelines. The test facility consisted of a test section, a support structure, an apparatus to pull the pig through the test pipe, and a computer-based data acquisition system. The test section was 21-ft long and was made from 3-in. inner diameter schedule-40 steel pipe. The mixure of commercial wax and mineral oil was casted inside the test section at different wax thicknesses and oil contents. A series of experiments were performed to investigate the wax removal mechanics with three different types of conventional pigs, i.e., disc, cup and polly pigs.
The experiments showed that a typical wax removal process using a pig followed four distinct phases, namely, wax breaking, plug formation, accumulation and production phases. Wax accumulating can be very significant, and is expected to be the dominating factor for the force required for moving a pig in long pipelines. As wax thickness and hardness increases, the required force to move the pig increases. The shape and material of the pig have a profound effect on the wax removal performance. While the disc pig provides the most efficient wax removal, the force requirement is excessive, especially for thicker and harder wax deposits. The wax removal performance of a cup pig is very similar to that of a disc pig. However, the cup pig can withstand higher load without mechanical damages than the disc pig. The polly pig offers the poorest wax removal performance.
