The first operation during pre-commissioning is to clean and fill the pipeline. This is usually, though not always, performed as a single operation. The purpose of these activities is to prepare the line for hydrotesting and to ensure that the line is clean for the use of inspection tools, or that at least that debris left over from the construction phase (potentially damaging to the pipeline or associated process equipment) is cleared out. Progressive Pigging is generally run in a controlled manner with a number of pigs with increasing aggressive characteristics through a pipeline. The volume of material received/collected with each pig is carefully monitored at the receiving pig trap, pit (usually onshore), or tanks, and is used to guide the selection of the following pig types that are subsequently run.