Abstract: Model experiments were conducted in a large wave flume to investigate the hydrodynamic loads and frequency characteristics of piggyback pipelines. The effects of the current parameters, the wave parameters, the relative angle θ between the main and the secondary pipes, and the gap-to-diameter ratio e/D between the main pipe and the seabed were considered. The results show that under current-only conditions, when θ ranges between 60° and 150°, the drag coefficient C_D of the main pipe increases significantly compared to that of a single pipe. Depending on θ, the wall-free main pipe may experience an average vertical force either toward or away from the seabed. Under wave-only conditions, the amplitude of the lift force for a fully buried pipeline is significantly amplified compared to the case of e/D = 0, and showing a trend opposite to the DNV recommended values for single pipes. Under combined wave-current conditions, the frequencies of the lift and drag forces on the main pipe exhibit subharmonic and harmonic frequencies of the wave frequency, with the dominant frequency of the lift force being the subharmonic frequency.