Design of Offshore Gas Pipelines Accounting For Two-Phase Flow (includes associated papers 6401 and 6402 )
- T.L. Gould (INTERCOMP Research Development and Engineering Inc.) | E.L. Ramsey (Pipe Line Technologists, Inc.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- March 1975
- Document Type
- Journal Paper
- 366 - 374
- 1975. Society of Petroleum Engineers
- 1 in the last 30 days
- 438 since 2007
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In the past, offshore gas pipelines with simultaneous flow of liquids have been undersized. Many offshore designs have been performed using the Panhandle flow equation with an efficiency factor that is adjusted with gas flow rate. Correlations are used to show that efficiency does not change with flow rate for horizontal lines, but does change with liquid loading and underwater terrain.
This paper describes new techniques and concepts required to obtain the most efficient design of offshore gas pipelines with simultaneous flow of liquids. Recent developments in multiphase flow methods can be successfully applied to "wet gas" design problems. In the past, offshore gas pipelines were designed without cognizance of several important problems. The first problem is the effect of liquid loading of the gas stream. The liquid may be introduced into the flow stream from several sources, but in almost all cases, offshore gas pipelines are transporting a wet gas. Even though operators may consider the line dry, there is usually some condensation of either water or hydrocarbon components as the gas is moved toward shore. The most prevailing source of liquids is from the formation. The formation will tend to gradually dry itself out around the wellbore over a period of time by producing small amounts of connate fluid from even so-called dry gas pools. In addition, water coning and retrograde condensation of hydrocarbons in the reservoir may also account for liquid production along with the gas. Substantial liquid production is usually separated from the gas stream at the platform. However, low rates of liquid production are usually passed directly into the pipeline to shore. Indeed, many pipeline contracts are written with the assumption that a certain liquid volume will be moved along with the gas. The simultaneous flow of gas with small liquid loadings is an accepted practice. However, a problem arises in the initial design of these pipelines because single-phase gas flow equations are used. Even the best designs merely adjust the "flow efficiently" to account for the presence of liquid in a gas line. The second problem often neglected in offshore pipeline design is that of inclination. When gas pipelines are viewed as dry systems, it is perfectly reasonable to neglect the change of elevation since it usually is no more than several hundred feet. However, the effects of inclination, and specifically terrain, are crucial in determining the performance of a wet system; i.e., gas and liquid flowing simultaneously. In a wet system that is inclined upward in the direction of flow, the liquid will tend to accumulate and let the gas "slip" by. This concept of slip implies that gas is traveling at a different velocity than the liquid in the pipe. The degree of slip or "liquid holdup" is a function of diameter, flow rates, flow regimes, and other variables. The magnitude of this liquid holdup has been shown by Beggs and Brill and others to be a function of the inclination of the system.
The first objective is to show the relationship between multiphase flow models and the single-phase gas equations used in current practice. Although there are many single-phase gas-flow formulas available in the literature this paper will concentrate on the Panhandle A and modified Panhandle equations.
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