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.
To get a very dirty pipeline clean, it is necessary to use a sequence of pigs called a “pig train” or progressive pigging. The selection of the pigs, the sequence and the number of pigs is often determined from experience, but this can change in the field, as the debris comes in from each run. Also, there is a class of pigging that does not use mechanical pigs but such things as gels, liquid batches and even ice.
Comparison of debris on pipeline bottom vs. pipe wall deposits. The actual type of fouling is dependent upon several factors including: the production fluid characteristics such hydrocarbon product, water, and gas composition; the presence and composition of common oilfield contaminants, such as hydrogen sulfide, carbon dioxide, mercury, arsenic, zinc, lead, iron, and manganese and operating conditions (e.g., temperature, pressure, pH, flow regimes, etc.).