During the subsea pipeline trench dredging for the newly-built offshore oil terminal at a port in East Africa，the hard rock layer is encountered.To solve this problem，this paper studies and contrasts the conventional processes such as rock hammer breaking，backhoe excavating，and underwater reef blasting.We apply a rotary drilling rig to assist the grab dredger in hard rock layer dredging，which does not need to deploy professional ships such as the cutter-suction dredger.The results show that the grab dredger can smoothly dredge the hard rock layer after the operation of the rotary drilling rig，and the working efficiency and cost are well controlled.The research can serve as a reference for similar projects.