Abstract: Pigging is a challenging operation for submarine pipelines with diameter less than 102 mm. Such small-diameter pipelines will expose to higher risks of pig sticking than large-diameter pipelines when traditional pigging operation is adopted. Through analysis on the impurity types and sizes in the small-diameter pipeline composed by 40.6 mm steel pipes and flexible pipes and study on the starting of impurities in the pipeline, a hydraulic pigging process with proper flow velocity of cleaning fluid more than three times of set velocity of impurities is chosen through calculation and field verification. It proves that the hydraulic pigging process can effectively remove the impurities in small-diameter pipelines. Thus, without pigs, the pigging process with pigging fluid at proper flow velocity is applicable to small-diameter pipelines of single material or mixed materials, which can avoid the risk of pig sticking and improve operation efficiency and lower operation cost while achieving the same pigging effects.