Scour Studies for a Gas Pipeline Crossing in Negro River, Brazil

Each pipeline river crossing is a unique project and depends on the local environmental conditions. Sediment movement or erosion of the river bed can, for example, originate free spans superior to those for which the pipeline was designed to resist. A thorough understanding of the sediment dynamics is necessary in order to guarantee the integrity of the pipeline from damages caused by variation of the river bed. PETROBRAS designed and installed two gas pipelines in Negro River, state of Amazonia, Brazil. The new pipelines will have a total length of 383 km through the rain forest, 10 km of which was laid along the river bed and a second branch crossing the river over 5 km. In order to guarantee the structural safety of the pipeline, characterization of river hydraulics and bed dynamics was carried out with acquisition of environmental data, numerical modeling and surveying. Current and flow measurements with ADCPs, hydrodynamic and morphological modeling were carried out. Multibeam, side scan sonar and SBP surveys at different times were compared in order to assess large scale changes in the river bed and presence of bed forms. Hydraulic characterization of the Negro River indicated relatively low bed mobility in comparison to the Amazon River, for example. Finally, parametric calculations for onset of scour, length and depth of scour holes and related time scale, as well as bottom velocities over the hydrological cycle were used to obtain parameters for engineering design (route stability, free span analysis, etc.), protection requirements and survey periodicity for monitoring of free span development along the pipeline.