Continuous water-assisted flow (CWAF), where a water layer surrounds a viscous oil core, provides low energy, long distance transport of heavy oil and bitumen without requiring heating or solvent addition. In industrial applications of CWAF, the pipe wall is fouled by a thin coating of oil, an effect not considered in many studies of water-lubricated pipe flows. In the present study, a new method to model pressure loss in the water-assisted pipeline flow of heavy oil is introduced. The hydrodynamic effects produced by the wall-fouling layer are incorporated in the model as input parameters for CFD simulations. The most important of these parameters are the thickness of the wall-fouling layer and the equivalent hydrodynamic roughness it produces. The CFD methodology described here was developed on the ANSYS-CFX platform and is able to capture the effects of the wall-fouling layer, the hydrodynamic roughness produced by this layer, and the water hold-up. The new CFD model was validated using previously collected data from tests conducted in two separate pipeline loops (100 and 260 mm in diameter), using a range of oil viscosities, water fractions, and mixture velocities. Compared to existing models, the one presented here provides more accurate predictions and requires significantly fewer computing resources. Because the model was developed using a physics-based approach, it is a useful tool in evaluating the effects of pipe diameter, oil viscosity (or temperature), water cut, and mixture velocity on pressure losses in water-assisted heavy oil pipelines.