Pipeline Pigging Part 1: Cleaning Pigs & Pigging Strategy
Build-up of deposits can create conditions for accelerated localized corrosion, which may be caused by under-deposit corrosion, localized acidic conditions and/or bacterial attack (microbiologically induced corrosion - MIC). This may result in sections of the pipeline needing repair or replacement before the end of their anticipated design life. Deposits will also impact throughput due to the reduction in the effective pipe diameter, which may require up to 140% increase in pressure in the line to maintain flow (Titratsoo 1).
This paper is the first part of a two part series and presents an overview of pig selection criteria for cleaning and maintenance of the pipelines, and covers high level guidance on establishing a progressive pigging strategy.
Pigs are tools traveling through the pipe, which are driven by the pressure differential across the pig. Several factors can affect the seal between the pig and pipewall; in addition, fluids accumulated in the flow will also be effective in pig selection. Some of these factors are presented in Table 1. There are many different pig designs available which can be grouped under two main categories- utility and in-line inspection (ILI) pigs. Cleaning and sealing pigs fall under utility pigs. Cleaning pigs are mainly used to remove accumulated solids and debris, whereas the sealing pigs are utilized mostly to remove accumulated liquids, separate dissimilar fluids, dewatering, etc. Selection of the correct pig type for a given application is also dependent on features such as pig material, length, sealing discs, driving cups, guiding discs and additional tools attached such as blades and brushes.
