A computer-implemented method for improving flow of a multiphase mixture in a fluid transport system by determining pressure drop of low-liquid loading flows, the method comprising: obtaining one or more physical dimensions of a vertically oriented pipe configured to transport a multiphase flow; obtaining one or more physical parameters of the multiphase flow in the pipe from sensors; determining an effective roughness of a liquid film of the multiphase flow on an interior wall of the pipe using the one or more physical dimensions of the pipe, and using the one or more physical parameters of the multiphase flow; determining a pressure drop in the pipe using the effective roughness of the liquid film, wherein the pressure drop is given by the following equation: − dp dx = 4 τ i D g + ρ g g wherein: τ i represents shear stress, D g represents the diameter of a gas included in the multiphase mixture, g represents acceleration due to gravity, and ρ g represents the density of the gas and a liquid in the multiphase mixture characterized by: determining the effective roughness comprises determining effective roughness due to viscosity of the liquid film and surface tension of the liquid film, determining one or more operating parameters of the fluid transport system based on the pressure drop in the pipe, comprising: determining an operating rate of a pump of the fluid transport system that provides a target amount of pressure, adjusted for the determined pressure drop and operating the pump using the operating rate; or determining a diameter for the pipe that provides a target amount of pressure, adjusted for the determined pressure drop and operating the fluid transport system using a second pipe corresponding to the diameter. obtaining one or more physical dimensions of a vertically oriented pipe configured to transport a multiphase flow; obtaining one or more physical parameters of the multiphase flow in the pipe from sensors; determining an effective roughness of a liquid film of the multiphase flow on an interior wall of the pipe using the one or more physical dimensions of the pipe, and using the one or more physical parameters of the multiphase flow; determining a pressure drop in the pipe using the effective roughness of the liquid film, wherein the pressure drop is given by the following equation: − dp dx = 4 τ i D g + ρ g g wherein: τ i represents shear stress, D g represents the diameter of a gas included in the multiphase mixture, g represents acceleration due to gravity, and ρ g represents the density of the gas and a liquid in the multiphase mixture characterized by: determining the effective roughness comprises determining effective roughness due to viscosity of the liquid film and surface tension of the liquid film, determining one or more operating parameters of the fluid transport system based on the pressure drop in the pipe, comprising: determining an operating rate of a pump of the fluid transport system that provides a target amount of pressure, adjusted for the determined pressure drop and operating the pump using the operating rate; or determining a diameter for the pipe that provides a target amount of pressure, adjusted for the determined pressure drop and operating the fluid transport system using a second pipe corresponding to the diameter. determining the effective roughness comprises determining effective roughness due to viscosity of the liquid film and surface tension of the liquid film, determining one or more operating parameters of the fluid transport system based on the pressure drop in the pipe, comprising: determining an operating rate of a pump of the fluid transport system that provides a target amount of pressure, adjusted for the determined pressure drop and operating the pump using the operating rate; or determining a diameter for the pipe that provides a target amount of pressure, adjusted for the determined pressure drop and operating the fluid transport system using a second pipe corresponding to the diameter. determining an operating rate of a pump of the fluid transport system that provides a target amount of pressure, adjusted for the determined pressure drop and operating the pump using the operating rate; or determining a diameter for the pipe that provides a target amount of pressure, adjusted for the determined pressure drop and operating the fluid transport system using a second pipe corresponding to the diameter.