Several techniques can be applied when drying a pipeline, however drying by using super dry air have been found to be the most economically compared to for example vacuum drying or drying by using methanol or nitrogen.
When a pipeline is to be dried be using super dry air, the individual test section are linked together to form one or more drying sections. Where permanent pig- launcher and receivers are not available temporary traps must be connected to the line and the drying unit is then connected to the pig launcher by flexible hoses.
Super dry air with a dew point of approximately -40ºC or lower are injected in to the pipeline and as the air passes through the section it will absorb moisture which have been left after the hydrostatic test.
During the drying operation, special foam pigs having a large absorptive capacity and high abrasion resistance are launched and propelled through the pipeline, these foam pigs distribute the residual water left in the line onto the pipe wall, thus increasing the areas of contact between residual water and air.
By using a new technology it is today possible to manufacture foam pigs which are extremely abrasion resistance. By using these long run foam pigs, drying have successfully been carried out in sections of 200 to 230 km, even in uncoated pipelines.
Therefore drying by using super dry air and long run foam pigs are also advantageously in offshore pipelines.
When a dry foam pig is received, all mainline- and bypass valves are set in half open position to dry out the valve cavities. Purging of the mainline, block valve stations and all low point drains will continue until the discharge air at all locations are below the requested dew point.
As an option high density polly brush pigs can be launched as the mainline is getting dry, this combined with a high velocity of dry air will bring out dust particles from the pipeline.
Moisture g/m³ air