Empirical Heat Transfer Model for Slug Flow and Bubble Flow in Vertical Subsea Pipes

Abstract

Abstract The prediction of flow assurance related problems for subsea pipelines, such as paraffin deposition, alphaltene deposition, corrosion, hydrate formation, cold oil line startup etc, all partially rely on valid heat transfer models. Compared with single-phase heat transfer and two-phase hydraulics, two-phase heat transfer is much more complicated because the mechanisms of heat transfer for specific flow patterns are different. A variety of empirical models have been developed for specific flow pattern of two-phase liquid-gas heat transfer. Some of them have been proved fairly accurate through research and literature review, such as Aggour model for bubble flow and Rezkallah model for slug flow both in vertical pipes. However, when these models are applied to subsea cooling conditions, the accuracy is not satisfactory. In this paper, the basic knowledge of two-phase flow is introduced. A new empirical heat transfer model is developed for both slug flow and bubble flow in vertical subsea pipelines. Experimental data proves this model works better than the existing models. Literature review indicates that this is the first model that can be applied to two flow patterns. From the results seen, the model can predict good accuracy. The paper therefore recommends the application of this model. Introduction Liquid-gas two-phase flow widely exists in oil production, transportation and nuclear industry. Compared with single-phase heat transfer and two-phase flow hydraulics, two-phase heat transfer has not been fully understood. This on one hand is because the complication of the two-phase flow performance, but also because this topic has not been deeply investigated. In fact, two-phase heat transfer is a very crucial issue. For example, paraffin deposition, hydrate formation, asphalt deposition, scale precipitation and cold oil line startup have become very challenging issues for offshore petroleum industry. The prediction, prevention and remediation of the above troublesome and costly problems all partially rely on valid heat transfer model. For example, heat transfer method is the most widely used method for paraffin deposit thickness calculation. That is, with accurate heat transfer model, the heat transfer coefficient can be obtained. With heat transfer coefficient, the temperature at the wall can be obtained. Thus the temperature across boundary layer can be calculated, which is the driving force for paraffin deposition and aging phenomena. Thus, deposition rate can be obtained for remediation operation, like pigging. As we can see, a valid heat transfer model is crucial. On the other hand, no model for cooling condition has been developed from former literature review, even though a variety of models for heating condition have been developed. To simplify the calculation, it is helpful to have a unified model that can be applied to two flow patterns.