During standard pre-commissioning of new pipelines, a hydrotest is normally carried out to verify the integrity of the linepipe and welds. Then water must be removed from the system, and this is usually performed by propelling a train of mechanical pigs through compressed air. The residual water after pigging shall be eliminated in order to avoid contamination of the process fluid during first filling and early operation; this is normally obtained by dry air purging at low pressure (close to atmospheric). Based on past experience, the amount of free water left in the pipeline after a correct dewatering operation is quite small, i.e. equivalent to a thickness order of pipe surface roughness.
Standard drying requirements specify that the WDP (Water Dew Point) of dry air exiting from the pipeline at the end of purging operation shall not be above -20°C at atmospheric pressure; however, in recent projects more stringent specifications have been established, due to higher purity requirements of the process fluid.
In this paper, possible options relevant to the drying operation have been analyzed, as referred to the pre-commissioning of a new long distance large diameter gas pipeline to be operated with a process gas having a WDP below -100°C. Calculations of the residual water removal time have been carried out by considering purging with ultra-dry air and vacuum; furthermore, an additional contingency operation time due to the presence of a finite liquid pool residual from a poor dewatering has been evaluated also. A comparison among the different options has been carried out, based on timing and cost impact. Air drying models developed in the past have been applied and fitted to the current study conditions.