Design calculations for oil and gas pipelines - Common aspects and specific topics -

The fundamental equations which describe the flow through gas and liquid pipelines are formally identical. However, the wide gap between the compressibility ranges of gases and liquids leads to quite different transport characteristics for both kinds of media. While for oil pipelines and pump stations specific topics like batch operation, surges, and possible slack line scenarios have to be considered, a detailed design calculation for gas pipelines and compressor stations require substantial knowledge of real gas thermodynamics. The general relationship between pressure difference along a pipeline section and volume flow is given through the combination of mass conservation and the force balance. An extension of Bernoulli ́s law applies to steady-state liquid flow with non-constant density profile (e.g. batch operation), whereas Ferguson ́s formula is valid for gas flow through pipelines. The dynamic behaviour of a pipeline system is calculated by real time simulation of a complete model including all existing station elements like pumps, valves, etc. in terms of a one by one mapping of the real world. The energy equation is the link between flow mechanics and thermodynamics and is the basis for the temperature model which again is formally identical for liquid and gas pipelines. Especially for gas pipelines a realistic temperature model turns out to be essential. Heat exchange with the environment shows considerable differences for onshore and offshore pipelines. This article briefly outlines some layout and design considerations for transport capacity and power consumption of oil and gas pipelines. The impact of uncertainties of assumptions on the results is shown with examples. Pipeline Technology Conference 2007 Rev. 2 2007/03/22 © 2007 3S Consult München GmbH 2/12 Basic data for a design study From the technical point of view a design study for a pipeline requires at first the designated route with an elevation profile along the line and the desired throughput. Further the flow related physical properties of the fluid (liquid or gas) and an adequate thermodynamic equation of state have to be determined. The design code (e.g. ASME etc.) is the framework for layout mainly under safety aspects. In addition to these general information demands all the specific boundary conditions of the individual project have to be respected. Constraints due to environmental protection are just one of such conditions. Results of a design study Diameter and wall thickness along the pipeline and the optimal placement of stations are the main results that should be determined by the hydraulic study. The layout of pump and compressor stations includes the determination of the expected power consumption for operation later on. Most of these questions can be answered by steady-state calculations. The operation philosophy, the control concept and the indispensable instructions in case of emergency are elaborated with a detailed model of the entire pipeline including its stations. A leak detection and leak location system is mandatory for oil and gas pipelines. The choice of the instrumentation and the SCADA system should be part of the considerations from the beginning. Common aspects for oil and gas flow Basic equations There are four equations which describe (1) the continuity of mass, the conservation of (2) momentum and (3) energy, and finally (4) the equation of state. All four equations have to be solved simultaneously to include all the hydraulic and thermodynamic phenomena being relevant for flow through pipelines. The four variables to be simultaneously calculated are flow velocity v, pressure p, temperature T, and the density ρ = ρ (p,T). The set of equations is identical for liquids and gases except for the equations of state. Especially for natural gases there are a variety of approaches toward a precise thermodynamic description of their properties, which in general can be derived from the individual gas composition. General approach A detailed pipeline model is the fundament for the reliable calculation of all steadystate scenarios as well as the dynamic behaviour of the pipeline during operational intervention or disturbances like accidents, breakdown of power supply and other emergency cases. The model is hierarchically structured like the pipeline itself. There are stations connected by pipeline sections. The internal construction of the stations has to be mapped into the model element by element. Each element then has to be Pipeline Technology Conference 2007 Rev. 2 2007/03/22 © 2007 3S Consult München GmbH 3/12 configurated according to its actual parameters – characteristics of pumps and valves, closing times of valves, etc. Also the control sequences are fully included in the model simulation. Figure 1 Overview of a pipeline model comprising detailed stations and pipeline sections. Visualisation of the simulation results via dynamic hydraulic profile. Steady-state flow Steady-state flow hydraulics yields the relation between pressure and throughput, finally the transport capacity of the pipeline. The alliance of continuity of mass and momentum (reduced to steady-state conditions) leads directly to the pressurethroughput-relation. Gases and liquids, however, are described differently. For liquids the pressure head is used instead of the pressure itself – which leads to the well known Bernoulli equation, here being extended for a non-constant density profile. The description of gas flow at typical pipeline pressure and throughput requires a complete real gas thermodynamic treatment. The large compressibility of gases leads to spatially varying density due to the pressure drop along the line and due to the elevation profile. The latter effect was first introduces by Ferguson. The basic equation of the pressure(head)-throughput-relation is given by (cf. the “Glossary” for the notation): 43 42 1 4 4 4 3 4 4 4 2 1 43 42 1 force friction force driving on accelerati state steady v D x z g x p x v v 2 2 ⋅ ⋅ − ∂ ∂ ⋅ ⋅ − ∂ ∂ − = ∂ ∂ ⋅ ⋅