Network Analysis Introduction

1
Network Analysis - Introduction

• Transmission Planning Code Workshop 2
• 1st May 2008

2
Introduction

• Network Analysis is used extensively by National
  Grid within the planning process
• Aim of this presentation to introduce the
  fundamental concepts behind the network analysis
  tool used by National Grid for planning

3
Network Analysis why do it?
Network Analysis why do it?

• NTS is a complex network of pipes, compressors,
  regulators (control valves), multi-junctions and
  many other plant items required to transport high
  pressure gas safely
• Gas leaving the NTS must be at adequate pressure
• NTS must be able to meet the 1 in 20 peak day
  demand security standard
• Mathematical models required to understand gas
  flow and pressure behaviour under different
  supply/demand conditions
• Natural gas behaviour described by complex
  thermodynamic and hydraulic equations

4
What models are required?

• Models are derived from fundamental physical and
  thermodynamic laws
• Conservation laws (mass, momentum, energy)
  describe motion of a fluid under pressure and
  temperature changes in a fluid
• Equation of state describes relationship between
  pressure, temperature and volume for gases or
  mixtures of gases
• Major assets are modelled these induce the
  largest pressure drops, control the flow of gas
  and control pressure
• Pipes
• Compressors
• Regulators (control valves)
• Network models are built up by joining these key
  components together
• Represents the topology of the network

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What input data is required?

• Network model topology describes only the major
  physical assets of the system
• Additional data required to describe e.g.
• Volumes of gas flowing into and out of network,
• Design limits of components and other connected
  facilities
• Operating behaviour, gas properties, altitudes...
• Data may be time dependent and vary with ambient
  conditions e.g. air temperatures
• Models are usually visualised and manipulated
  through a graphical interface due to the amount
  of data required, and the complexity of the
  network topology
• National Grid currently use the FALCON network
  analysis tool for planning

6
What output data is produced?

• The key outputs from the software are
• Pressures at each modelled node (e.g. supply
  nodes, demand nodes, pipe ends, compressor
  inlets/outlets)
• Volumetric flows through components
• Gas properties and temperatures at each node
• All other outputs are derived from these results
  e.g.
• Linepack in each pipe
• Compressor power used
• Models are deterministic i.e. they are not
  statistical models which simulate random or
  uncertain events

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How is capacity modelled?

• Entry and exit capacity not modelled directly -
  commercial rather than physical concepts
• Derived by analysing the network under different
  supply/demand conditions
• Transmission capability usually analysed under
  steady flow assumptions (for planning needs)
• Flexibility analysis is analysed assuming offtake
  flows are profiled across a gas day (transient
  analysis)
• Fixed flow conditions input to the model and
  analysed
• All pipeline and plant must operate within design
  limits
• Analyst changes the operating setpoints and
  configuration of the network to find a model
  solution that supports the flow patterns
• Each fixed flow condition must currently be
  analysed separately
• Requires skilled analysts to find a solution
  quickly to a steady flow problem
• A transient analysis problem may take an analyst
  several hours to solve

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How are capacity constraints identified?

• Capacity constraints are identified for planning
  needs through
• Breach of minimum pressures required at an
  offtake and/or
• Over-pressurisation at an entry point or within
  the network
• Constrained volume at an entry point is
  determined by the maximum flow that can be
  achieved at that entry point without breaching
  pressure limits
• Constrained volumes will change with supply and
  demand patterns
• Assumption of full plant availability used for
  longer term view

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Summary

• Network analysis is needed to understand the
  behaviour of the NTS under different supply and
  demand conditions, due to the underlying complex
  dynamics of natural gas flowing through the
  system
• Outputs are the hydraulic variables of the system
  (pressures, flows)
• Nodal capacity values are derived from different
  studies using the network models however these
  depend heavily on...
• Supply and demand patterns
• Minimum and maximum operating pressure
  requirements
• Plant availability and capability

10
Questions?