Restraints Influence Topside Facility Design
Authors: Slocum D.J. 
Published: October 2008

The design of topside facilities for offshore platforms is subject to many restraints, such as space and weight limitations, the need for self sufficiency, drilling support, gas utilization, secondary recovery, and environmental regulations, which make the design of these facilities different than that of onshore facilities.

While this paper does not provide solutions directly applicable to all platform, it does document previous solutions and provides conceptual ideas and process alternatives that warrant consideration for future platforms.

INTRODUCTION

The process facilities on a production platform must both separate the crude from associated gas and produced water and provide a means of shipping the crude, utilizing the gas, and disposing of the water. In addition to the process facilities, support and utility facilities are also required, the size of which is dependent on the extent of processing. Restraints, or, conversely, opportunities, influence the selection of process facilities for each individual platform. As each item of equipment requires deck space, the extent of processing has a major impact on platform size and project cost. But in the final analysis, each platform must accomplish its primary function - production.

This paper examines how different restraints have given rise to significantly different concepts for installation of processing facilities on platforms. Consideration is given to crude processing, gas handling, produced water treatment, secondary recovery, and drilling, as well as their impact on utility and support facilities.

CRUDE PROCESSING

Two of the factors which greatly affect the crude processing facilities installed on a platform are:The extent of treatmentDewatering characteristics of the crude

Extent of Treatment

Under a simple treatment scheme, the crude is only degassed to the point where vaporization can be suppressed in a pipeline to shore. Onshore, the crude is stabilized and dehydrated. Produced water remains with the crude and is transported from the platform with the oil. Thus, produced water treatment and disposal are not required on the platform.

Crude processing is essential when the crude must be stabilized and dehydrated. This is the situation when the crude is loaded onto ships, the amount of produced water is excessive, or there are no onshore facilities. Short of fractionation, the oil can be stabilized by flashing at elevated temperatures and low pressures. A flash at 140°F and slightly positive pressure will produce a crude with a Reid vapor pressure of less than 10 psia. Free phase water can be removed in three-phase production separators. Produced water treatment facilities are thus required on the platform.

Dewatering Characteristics

Crude oil dehydration is enhanced by elevated temperatures, residence time, and chemical injection. As defined by Stoke's law, liquid-liquid separation is facilitated by:Greater difference in gravities occurring at elevated temperaturesReduced viscosity - occurring at elevated temperaturesMore time - provided by longer retention in the separatorsLarger droplet size - aided by addition of chemicals which coalesce fine water particles into large droplets