Summary Flow pattern, pressure drop, and water holdup were measured for oil/water flow in horizontal, hilly terrain (±0.5 and ±3°), and vertical pipelines at a temperature of approximately 35 (± 5) °C and a pressure of approximately 245 kPa using the large-scale multiphase-flow test facility of Japan Oil, Gas and Metals Natl. Corp. (JOGMEC). Test lines of 4.19-in. (106.4-mm) inner diameter (ID) and 120-m total length were used, which included a 40-m horizontal or hilly terrain (near-horizontal) and a 10-m vertical test section sequentially connected. The flow pattern was determined by visual observation with video recordings, and a flow-pattern map was made for each condition. New flow patterns were identified for horizontal and hilly terrain flow, such as oil flow in a snake-like shape at top of pipe at high rate of water flow, and water flow at bottom of pipe at high rate of oil flow. New holdup and pressure-drop data are presented for each flow condition. Flow rate and inclination angle influence holdup and pressure-drop behaviors. In vertical flow, when the oil superficial velocity exceeds a certain value, the pressure drop decreases exponentially as the superficial oil velocity, vSO, increases. Slippage between the phases was analyzed using the measured water holdup plotted against the input water cut with inlet-oil flow rate as a parameter and slip velocity vs. measured water holdup. It was found that the slippage changed significantly with slight changes in inclination angle. This paper provides new experimental data of flow pattern, water holdup, and pressure drop measured particularly at horizontal, hilly terrain, and vertical conditions, with large-diameter pipes. This is indispensable information for developing reliable prediction models for oil/water two-phase and gas/oil/water three-phase flow in pipelines. Introduction In the petroleum industry, the joint flow of two immiscible liquids such as oil and water in pipes commonly occurs at facilities for production and transportation of oil (i.e., horizontal, inclined, or vertical pipes) in wellbores and flowlines. In offshore fields, these pipelines can be of considerable length before reaching the separator facilities. The pressure required to transport the fluid over long distances is highly influenced by the pressure drop that can be significantly affected by the mixture properties of the oil and free water. As the amount of free water increases as the field matures, a reliable prediction of pressure drop and water holdup is extremely important for the optimum design of pipeline systems in the industry. For a two-phase mixture of oil and water flowing together in a pipe, different internal flow geometries or structures can occur, depending on the flow rates of the two phases and the geometrical variables of the pipes, as well as the flow conditions and physical properties of the phases. The different interfacial structures are called flow patterns. Knowledge of the flow patterns that could occur under a given set of conditions leads to better prediction of oil-/water-flow behavior. In addition, accurate interpretation of experimental data requires reliable prediction of flow pattern, water holdup, and pressure drop. The flow characteristics of oil/water mixtures are generally different from gas/liquid systems. In oil/water flow, the different flow structure is mainly caused by the small buoyancy effect and lower free energy at the interface, allowing the formation of shorter interfacial waves and small dispersed-phase droplet size. Therefore, the results of gas/liquid flow cannot be applied directly to oil/water flow in most cases.