Challenges Associated With The Design Of Oil-Gas Separation Systems For North Sea Platforms

Abstract

ABSTRACT The conditions encountered in North Sea oil-gas separation facilities design and operations require that the system be both highly selective and extremely flexible within the space and weight limitations of a production platform. To better understand the approaches used by process engineers in attempting to solve these difficult and sometimes conflicting challenges, this paper discusses the objectives, some of the more significant factors which influence the potential solutions to these objectives, and some of the approaches utilized to meet these challenges. OBJECTIVES 1. Produce Specification Crude Oil Crude oil specifications may be either for offshore tanker loading or for delivery to an oil pipeline at the platform. Crude oil specifications are usually defined relatively simply, through limitations on vapor pressure and on BS&W (basic sediment and water) content. BS&W is normally limited to a nominal percentage, such as 0.5%, and meeting this specification is outside the scope of this paper. The oil-gas separation system in most cases? does not significantly affect whether or not the oil will meet the BS&W specification, since for those oils where this is a problem special emulsion treating is required independent of the oil-gas separation system. Crude oil vapor pressure is the specification which most influences the design of oil-gas separation systems. For offshore tanker loading, the oil may be limited to a vapor pressure in the range of 8 to 14 pounds RVP (Reid Vapor Pressure). RVP refers to a standard method of vapor pressure testing utilizing a specific test cylinder assembly and determined at a temperature of 100°F. RVP is not identical with TVP (true vapor pressure), which is the actual vapor pressure exerted by a liquid in equilibrium with a vapor at any given temperature. Oil leaving an oil-gas separator operating at 15 psia and 1000F will have a TVP of 15 psi at 100°F. Because of the nature of the RVP test cylinder, TVP of a liquid at 100°F will always be slightly higher than RVP. The amount of this difference is a function of the liquid composition and can be estimated. Crude oil vapor pressure limitations for pipeline delivery may be comparable to those for tanker loading. However, a pipeline can be specifically designed as a high vapor pressure system to handle gas liquids components mixed with the crude oil, and it is quite possible that North Sea oil pipeline systems will be developed in this manner .It is not expected to be feasible to recover NGL components separate from oil and gas on the offshore platforms and construct separate NGL pipeline systems. It also may not be possible in every field to design a separation system which meets both gas specifications and low crude oil vapor pressure specifications unless an intermediate product is also made. This will be more fully discussed later in this paper.