Design of Offshore Gas Pipelines Accounting For Two-Phase Flow (includes associated papers 6401 and 6402 )
Authors: Gould T.L. , Ramsey E.L. 
Published: December 2007

In the past, offshore gas pipelines with simultaneous flow of liquids havebeen undersized. Many offshore designs have been performed using thePanhandle flow equation with an efficiency factor that is adjusted with gasflow rate. Correlations are used to show that efficiency does not changewith flow rate for horizontal lines, but does change with liquid loadingand underwater terrain.

Introduction

This paper describes new techniques and conceptsrequired to obtain the most efficient design of offshoregas pipelines with simultaneous flow of liquids. Recentdevelopments in multiphase flow methods can besuccessfully applied to "wet gas" design problems.In the past, offshore gas pipelines were designedwithout cognizance of several important problems.The first problem is the effect of liquid loading ofthe gas stream. The liquid may be introduced into theflow stream from several sources, but in almost all cases, offshore gas pipelines are transporting a wetgas. Even though operators may consider the line dry, there is usually some condensation of either water orhydrocarbon components as the gas is moved towardshore. The most prevailing source of liquids is fromthe formation. The formation will tend to graduallydry itself out around the wellbore over a period oftime by producing small amounts of connate fluidfrom even so-called dry gas pools. In addition, waterconing and retrograde condensation of hydrocarbonsin the reservoir may also account for liquidproduction along with the gas.Substantial liquid production is usually separatedfrom the gas stream at the platform. However, lowrates of liquid production are usually passed directlyinto the pipeline to shore. Indeed, many pipeline contracts are written with the assumption that a certainliquid volume will be moved along with the gas. Thesimultaneous flow of gas with small liquid loadingsis an accepted practice. However, a problem arisesin the initial design of these pipelines becausesingle-phase gas flow equations are used. Even the bestdesigns merely adjust the "flow efficiently" to accountfor the presence of liquid in a gas line.The second problem often neglected in offshorepipeline design is that of inclination. When gaspipelines are viewed as dry systems, it is perfectlyreasonable to neglect the change of elevation since it usuallyis no more than several hundred feet. However, theeffects of inclination, and specifically terrain, arecrucial in determining the performance of a wetsystem; i.e., gas and liquid flowing simultaneously. Ina wet system that is inclined upward in the directionof flow, the liquid will tend to accumulate and let the gas "slip" by. This concept of slip implies that gas istraveling at a different velocity than the liquid in thepipe. The degree of slip or "liquid holdup" is afunction of diameter, flow rates, flow regimes, and other variables. The magnitude of this liquid holdup hasbeen shown by Beggs and Brill and others to be afunction of the inclination of the system.

Objectives

The first objective is to show the relationship betweenmultiphase flow models and the single-phase gasequations used in current practice. Although there aremany single-phase gas-flow formulas available in the literature this paper will concentrate on thePanhandle A and modified Panhandle equations.

JPT

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