Wax deposition in subsea pipelines transporting waxy crude oil under oil-water stratified flow remains a critical flow assurance challenge. This study systematically investigates wax deposition characteristics using an experimental flow loop that simulates deep-sea conditions. Experiments were conducted with waxy simulated oil and deionized water under varying superficial velocities of both phases. Results show a crescent-shaped deposition layer exclusively on the upper oil-wetted wall, with no deposition on the water-contacted lower wall. Notably, the deposit near the oil-water interface exhibits higher wax content and enriched heavy components compared to the top wall, indicating localized aging behavior. A key finding is that increasing oil or water superficial velocity reduces deposition thickness but enhances wax content and promotes aging. The deposit mass per unit area correlates negatively with oil-phase actual velocity, while wax content and heavy component concentration show strong positive correlations. This work provides novel insights into the circumferential heterogeneity and phase-specific aging of wax deposition in stratified oil-water flow, offering a foundation for improved predictive models.

Oil-water stratified flow, a fundamental pattern in multiphase pipe flow, is commonly encountered in offshore petroleum production and transportation. Although hydraulic characteristics of this flow regime have been extensively studied, accurate prediction of its heat transfer behavior under non-isothermal conditions remains a challenge. In this study, we develop a three-dimensional heat transfer model for oil-water stratified flow by integrating the energy conservation equation with established flow models and coupling it with momentum conservation. Turbulence is resolved using a low-Reynolds-number k-ε model. The phase interface is captured via a minimum energy model, and the irregular physical domain is transformed into a regular rectangular region using bipolar coordinates to simplify grid generation and numerical solution. The model was validated against experimental measurements of average outlet temperatures for both phases, showing relative errors within 5%. Results further reveal how water cut influences the axial temperature distribution and highlight three-dimensional temperature profiles during non-isothermal flow. This model provides theoretical insights and practical tools for optimizing thermal management and ensuring safety in offshore petroleum pipeline operations.

Wax deposition in oil-water stratified flow is commonly encountered onshore and offshore oil production pipe systems, and typically reduces transportation capacity of oil. The accurate predicted model of wax deposition has becomes an indispensable approach to design effective remediation strategies. However, a reliable mechanistic model for wax deposition prediction in oil-water two-phase stratified pipe flow is lacking to validate the deposition process. In this work, a three-dimensional (axial, radial, and angular) robust wax deposit model for oil-water stratified circular pipe flow was developed. The model of formation of a gel deposit based on the first principles of rheology was developed, associated with the results obtained from hydrodynamics and heat/mass transfer simulations. The predictions for wax deposition are found to compare satisfactorily with experimental data with two different oils for single phase and four different water cuts for oil-water stratified pipe flow. It can be seen from the wax gelation mechanism that an increase in water cut can help to reduce the wall/oil-deposit interface shear stress, thereby leading to an increase in the degree of gelation as well as the deposit rate. Furthermore, a local deposit analysis in the circumferential direction was conducted, for water cut 75% and total flow rate 5 m³/h, which provided insights to understand that the thickness on pipe wall was roughly uniformly distributed locates near the top of the pipe and the nearer the position gets close to two points, where the oil-water interface contacts the inner wall, the deposition thickness quickly dropped to 0. It was attributed to the fact that a roughly uniformly thickness far away from the oil-water interface contact the inner wall resulted in the slowly changes temperature along the circumferential pipe wall wetted by oil.