Prestressing Buried Pipelines by Heating With Air

Abstract

Buried pipelines operating at elevated temperatures experience high longitudinal compressive stresses because the surrounding soil prevents thermal expansion. At high operating temperatures, buried pipelines can push through the soil at bends and buckle catastrophically. In soft soils they can lose lateral stability, and they can develop plastic failures. Thermally induced problems can be prevented with varying degrees of success by using thicker wall pipe, higher strength steel, longer radius bends, deeper burial, better backfill compaction, and/or prestressing during construction. Prestressing is most appropriate for pipelines operating at temperatures more than 80°C above ambient. One technique for prestressing a buried pipeline, that has been found to be both easy and economical for a liquid sulphur pipeline in Alberta, is to heat it with hot air and bury it while it is still hot. Pipe diameter and prestressing temperature both have a significant impact on the kind of heating equipment that is required.