For the determination of the design basis of the inlet separator in the offshore oil and gas plant, one of the most crucial factors is the liquid surge volume generated mainly by the transient phenomena such as riser slugging and pig-induced slugging. In particular, pig-induced slugging frequently brings about the most serious slug problems. In the study, a combination of a by-pass pig and an inlet- separator dual control valve scheme is introduced for the application, of which pigging operation is not possible by a conventional method because it causes inlet separator flooding. First, for the purpose of the estimation of the simulation accuracy, the stability map generated from an S-shaped riser experiment is compared with the prediction of OLGA simulation. And then, it is demonstrated that a combination of a by-pass pig and and inlet separator dual control scheme makes it possible to operate stable pigging, even though a conventional pigging is not available due to the excessive pig-induced slugging.