Reducing Separation Train Sizes and Increasing Capacity by Application of Emerging Technologies
Authors: Frankiewicz Ted , Browne Michael M. , Lee Chang-Ming 
Published: October 2008

Recent developments in the understanding of separator hydrodynamics and the development of high G-force separation technologies allows offshore operators to significantly increase separator capacity/size ratios. By viewing the gas/oil/water separation train as an integral system, performance demands on upstream, higher pressure components can be relaxed, allowing for further reductions in the size of high-pressure vessels. This paper will review design options for reducing the size and weight of separation trains while maintaining or improving their performance.

Recently designed separation trains are used to illustrate the compact separations concepts as compared to more conventional systems. Separator performance is illustrated by computational fluid dynamic (CFD) analysis.

Introduction

The state of the art in gas-oil-water separations has been advancing rapidly in response to the demands for lighter, less expensive separation equipment. As a result of the introduction of various cyclonic technologies, and the development of mathematical modeling tools such as computational fluid dynamics, the performance of separators can be improved even as vessel size and fluid residence times decrease. In addition, the philosophy of separations is changing as operators recognize the benefits of trading reduced size and performance of expensive high pressure separators for more efficient, lower cost second stage equipment which is capable of providing on-spec products (gas, oil, and water) from variable and highly contaminated feed streams.

To minimize size, the primary separator in a production train generally effects only gas/liquid separation. The liquid residence time in a 2-phase separator varies typically from 1 " 3 minutes, depending upon fluid viscosity and density, while the liquid residence time in a corresponding 3-phase separator will be 3 " 10 minutes, depending both on the relevant fluid properties and the operator's tolerance for risk relative to separator performance.

The downside of high-pressure, 2-phase separations is the formation of tight emulsions and reverse emulsions that are more difficult to break in downstream equipment. Consequently, the size and/or complexity of second stage separators, FWKO's, oil dehydrators, and water treatment equipment is often increased.

Despite all efforts to properly engineer a separator, process upsets will happen. Unexpected severe slug flow, control system failure, and pipe or flow line pigging are common operational causes of such upsets. It thus becomes important to design oil dehydration and water treatment equipment to accept the resulting separator upsets with minimal impact on product quality.

In this paper, the technological concepts for increasing separator efficiencies and improving their ability to mitigate process upsets is described. Also, concepts for improving the performance of oil dehydration and water treatment systems are discussed. By utilizing and integrating the spectrum of separation systems, a reduction in equipment size and weight can be realized without compromising process performance or robustness.

Gas/Liquid Separation

Conventional Separation.

Traditionally, bulk gas/liquid separation is accomplished in short liquid residence time horizontal or vertical separators. Often, fluid flow into the separator is gas-dominant, forcing fluid level in a horizontal separator to be minimized. To compensate, the vessel size is increased to reduce gas velocity and minimize gas carry-under to lower pressure separators.