Pigging Submarine Outfalls

Abstract

As an early component of a comprehensive program to reduce pollution in Boston Harbor, two 80-yr-old cast-iron outfalls, each 1.5 m (60 in.) in diameter and about 1.6 km (1 mile) long, were cleaned in a pigging operation. The operation restored the hydraulic capacity of these pipelines to about 175% of the prepigging capacity. Of particular concern were preserving the integrity of very old and possibly fragile pipe, and minimizing the environmental impact of the cleaning operation, with its discharge of scourings to the marine environment. The planning, monitoring, and execution of the project are described in detail. On each of the two lines, a dye test was conducted to confirm that there were no ruptures in the line. Then the diffuser dome at the downstream end was removed, and the sewage effluent was diverted to the outer outfall. A series of flexible pigs was inserted at the upstream end and driven through the pipe one at a time by a pump-supplied pressure. The progressive improvement in hydraulic capacity is plotted against the progressive number of pig passes made. The planning, monitoring, and execution of the project are described in detail.