Pig signaling is a method of indicating when the pig has reached a certain point in the pipeline. This is usually achieved by attaching a triggering device or “signaler”. This may be activated by the pig physically moving a lever or a plunger, which protrudes, into the line (referred to as intrusive), or by remotely sensing the pig’s presence from outside the pipe wall by, for example, a change in the magnetic field. This is usually referred to as a non-intrusive signaler.
Pig signalers are required at the very least at the exit of the pig traps. Traditionally these have been mechanical devices, which flip up a flag when the pig actuates a pressure or mechanical sensor during its passage. Then have the disadvantage that they need to be well maintained and need rest prior to the passage of the next pig. Non intrusive signaler- i.e. those which do not require any interference with the pipe integrity are now the preferred method and indeed are the only feasible method for subsea use.