Abstract

The development of computational fluid simulation software for the desktop computer when combined with simple calculations and model testing for confirmation provide process equipment designers with exceptional tools to advance the performance of oil production equipment. Specifically, this article detail the CFD models, calculations and testing methods employed to identify and improve the vertical fluid hydraulics in horizontal vessels. CFD simulations provided insight into the mechanisms responsible for both the uniform distribution and collection of oil/water production fluids. Simple calculations provided confirmation of uniform longitudinal distribution through perforated spreaders and collectors. Simple hydraulic simulations using a basic "watertable" provided visual confirmation of the CFD results as well as allowing "fine-tuning" of the resulting design changes. These new developments have permitted significant increases in vessel throughput without sacrificing performance.

Introduction

As the world's oil production increasingly consists of heavier, viscous oil, the difficulty of dehydrating it also increases. Not only must the equipment designers rely on more aggressive electrostatic treatment methods; they must also examine the fundamentals of fluid hydraulics within the process vessel. It has been assumed that uniform fluid hydraulics in large horizontal vessels operating with vertical fluid movement, such as oilfield electrostatic treaters, was routinely achieved. However, with CFD simulations and lab experiments it has been determined that flow uniformity is not easily achieved.

An electrostatic dehydrator works by establishing an oil/water interface at approximately 30% of the vessel diameter. A set of longitudinal distributors, either pipes or open bottom boxes, are installed slightly above the interface. These distributors, perforated along their length on both sides with uniformly spaced orifices, introduce the oil/water mixture into the vessel. The flow exits horizontally from these orifices and gradually turns to flow vertically upward toward the collector located in the top of the vessel. As dispersed water droplets within the mixture coalesce to larger sizes, they settle against the rising oil flow. The oil is dehydrated as the entrained water is first coalesced by the electrostatic field and then separated by gravity. The overhead collector consists of a pipe arranged longitudinally along the top of the vessel. Orifices are either placed on both sides or the top of the collector pipe and arranged to achieve uniform collection of the rising oil.

After CFD software became readily available it was easy to mathematically simulate the fluid hydraulics under the variety of operating conditions experienced during operation. These simulations could also include conditions encountered in dehydrators installed on FPSO facilities. While CFD simulations can confirm the movement of production fluids through a typical electrostatic dehydrator, it also permits the designer to quickly investigate alternative fluid control methods. It is prudent to confirm the CFD results by visualconfirmation whenever possible until the designer is comfortable with the CFD simulations. Surprisingly, the first spreader and collector designs thought to produce uniform fluid distribution were, in fact, responsible for the poor distribution observed.

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