The invention relates to a computer implemented method and tool for determining pressure-drop in multiphase pipeline flow where the effective surface roughness, keff, of liquid film coated sections of the inner pipeline wall is assumed to be equal to the maximum hydraulic roughness, ksmax. The maximum hydraulic roughness is further assumed to be proportional to a maximum stable droplet size, ddropletmax, i.e.: keff=ksmax=K·ddropletmax, where K is a correlation coefficient. The invention further relates to applying the computer implemented method for designing a pipeline-based fluid transport system for transport of multiphase fluids.