Abstract: To investigate the formation process and blockage of hydrates in deepwater oil-gas multiphase pipelines, the formation process of hydrates in subsea risers and subsea horizontal transportation pipelines was numerically simulated by OLGA, based on the phase equilibrium curve of hydrate formation from condensate gas and the hydrate kinetic model(CSMHyK v2.0). It is shown that under the process parameters of one certain subsea pipeline, a large amount of hydrates are formed in both risers and horizontal transportation pipelines, and the viscosity of hydrate slurry increases 10 times in risers and 18 times in horizontal transportation pipelines. The formation rate of hydrate in risers decreases from the sea floor to the sea level and finally approaches to zero, while that in horizontal transportation pipelines remains unchanged. The area of hydrate formation and the viscosity of hydrate slurry decrease with the decline of pipeline outlet pressure. When the outlet pressure of risers and horizontal transportation pipelines are controlled at 3 MPa and 2 MPa, respectively, the formation of hydrates can be avoided and the smooth flow of pipelines can be ensured.