THE DIRECTIONALLY-DRILLED GAS PIPELINE CROSSING OF THE RIVER RIBBLE: THE 1050MM DIA TREALES TO BURSCOUGH GAS PIPELINE

This article describes the difficult but successfully completed project of constructing a gas pipeline under the River Ribble at Preston, England, near its mouth. Large training walls had already been built at the location of the crossing. It was thus understandable why the Environment Agency and National Rivers Authority placed strict restrictions on its construction. Transco, the pipeline construction branch of British Gas, is aware of the need for environment protection. In 1996, it commissioned a feasibility study from Smit Land & Marine to study the possibilities of drilling a 1050m diameter pipeline by directional drilling. It was found that this method was feasible, but would require a comprehensive geological, geophysical, and geotechnical investigation. Some very interesting geological features and ground conditions were found, which could not have been predicted from simple borehole studies. The geology was found to be various layers of sand, gravel, and sandy clay, lying over sandstone. The article lists the five specific environmental constraints, and describes: (1) the grouting works; (2) drilling technique; and (3) health, safety, and environmental aspects of the project. Experience showed that there should have been a larger drilled radius, and a 1525mm diameter hole for the pipe.