Local scour under two pipelines of different diameters in steady currents is investigated numerically. The two pipelines are arranged in the so-called piggyback configuration where the small pipeline is located directly above the large pipeline (as shown in Fig. 1). The Reynolds-averaged Navier-Stokes equations and the transport equation of the suspended sediment concentration are solved using a finite element method. The sediment mass conservation equation is solved for predicting the bed scour profile. The numerical model is firstly validated against the scour below a single pipeline where the experimental data are available. Then the model is employed to simulate the scour below two pipelines in steady currents. Computations are carried out for the diameter ratio (the small pipe diameter (d) to the larger one (D)) of 0.2 and the gap (G, between the two pipelines) to the large diameter ratio G/D ranging from 0.0 to 0.5. It is found that the flow and the scour profiles are influenced significantly by the gap ratio.