(261f) Thermal Options for the Retrofit of Pipelines to Enable Transporting Heavier Oils

Depletion of conventional oil reservoirs is leading to extraction of heavier oils, characterized by very high viscosity. Some reservoirs are in cold locations and pipelines are exposed to extreme cold conditions. Oil viscosity, which is the key physical property for pipeline design, greatly increases due to gradual cooling along the line, and application of drag reduction methods may be required. However, changes in oil quality are difficult to foresee for the entire lifetime of a pipeline (typically decades). As a result, pipelines that originally were optimally designed (even overdesigned) to transport less viscous oils may face problems if the feedstock varies significantly. A possible scenario, here considered, is the transition in crude oil feedstock towards heavier quality. The question addressed is whether it is possible to use a pipeline as originally designed with heavier oil, or whether drag reduction techniques have to be applied. The high capital cost of pipeline infrastructure makes the construction of new systems expensive and the re-utilization of existing infrastructures a desirable, although challenging task.

The main current drag reduction techniques involve viscosity reduction, and are mainly classified into¹: thermal (heating), dilution and water emulsion. Emulsion and dilution reduce viscosity by adding up to 30_vol% of water or other diluent, leading to decreased oil throughput, and require additional auxiliary facilities. Thermal methods, on the other hand, permit to fully use the pipe capacity to transport the oil itself, but require local or continuous heating.

In this paper, a thermo-hydraulic model for a buried, insulated pipeline is used to study a realistic pipeline section. The physical properties of the oil, including viscosity, vary over the length of the pipeline as a function of local temperature. The modular formulation of the pipeline model permits building multiple section structures by connecting a number of pipeline sections in series, and introducing intermediate processing, such as heating, pumping, flow diversion, etc. as required. In this work, thermal management policies (heating) are considered. The impact of changes in oil feedstock are analysed and several thermal retrofit options, including initial heating and various intermediate heating options are evaluated, with consideration of a variety of design and operational constraints.

The results show that suitable heating strategies can potentially be used to retrofit existing pipelines which were initially designed to transport light oil, to transport heavier feedstock. Higher viscosity of heavy oils increases the pressure drop, and initial heating and reduced throughput may be required to enable transportation over long distances. Through intermediate heating, viscosity is reduced, pressure drop requirements are met, and significant reduction in throughput loss can be achieved, leading to very significant economic savings. The trade-off between throughput losses and the required heating is quantified, and optimum locations for the heating stations are identified which minimise heating requirements.

The optimal heating retrofit option depends on the oil type and future transportation requirements, which are often difficult to predict over the long term. In a retrofit, it is important to consider an optimal economic/energy design for the case on hand, but also to avoid introducing bottlenecks that might limit later future adaptation to more viscous feedstock. It is shown that this leads to a complex combinatorial problem that requires a detailed techno-economic analysis and optimization.

This analysis heavily depends on the heating technology used. Current heating technologies are restricted to fired heaters, heat exchangers or electrical heating elements. These require building small processing stations and/or power generation stations along the pipeline, have a significant impact on the existing infrastructure, often lead to additional pressure drops and are typically rather inflexible.

Finally, some technology requirements are identified and discussed that would enable the design of thermal heating stations with higher flexibility in retrofit design and operation, including  low impact on existing infrastructure, easy set-up, dismantling and retrofit, modular installation, flexible capacity addition and low capital cost.

1. Martínez-Palou R, Mosqueira MDL, Zapata-Rendón B, et al. Transportation of heavy and extra-heavy crude oil by pipeline: A review. J Pet Sci Eng. 2011;75(3-4):274–282. 

Acknowledgments

This research was performed under the UNIHEAT project. The authors wish to acknowledge the Skolkovo Foundation and BP for financial support, and Hexcell Ltd. for providing part of the modelling framework used in this work.