Ultrasonic measurement techniques as applied in inspection devices such as pigs are frequently used for the assessment of oil pipeline integrity. Since a coupling medium (e.g. water) between the detection device and the pipe wall is required, this technique is however not directly applicable to a gas pipeline. This leaves two options: a short water batch propelled by pressurised air and the use of a half batch, which means the pipeline is completely filled with water during inspection and must then be emptied again during a second pig run.
In the case study presented here, a short batch run was not feasible since this approach was found to lead to unacceptably high pig velocities due to occasional steep declines of the pipeline in question. The half batch approach chosen for this reason, however, required detailed preparatory modelling of the resulting hydrostatic pressures to avoid both overpressures and water hammer effects during the pig run due to the large height differences of several hundred meters. It also presented new practical challenges such as the management of large amounts of water and the requirement for high pressures during both pig runs. An overview of the inspection carried out is given, with an emphasis on the solutions developed and implemented to address these aspects.