Neste Jacobs Oy / Hans Aalto1 ”Transfer Functions for Natural Gas Pipeline Systems” Dr. Hans Aalto

Neste Jacobs Oy / Hans Aalto2

3 Main pipeline system components: Compressor stations, pipeline segments and offtakes km Compressor station discharge pressures are usually used to operate the pipeline

Neste Jacobs Oy / Hans Aalto4 The dynamics, i.e. response to discharge pressures (and offtake gas flow changes) is slow or very slow How would we obtain the transfer function between 2 variables of a given pipeline? - Identify from true pipeline system data - Identify from dynamic simulator data “Direct method”: from design data to transfer functions!

Neste Jacobs Oy / Hans Aalto5 Start with the PDE for a (=each!) pipeline segment This is the simplest isothermal PDE model for pipelines in the horizontal plane only and with small gas velocities!

Neste Jacobs Oy / Hans Aalto6 Discretize w.r.t to the space co-ordinate z, using N elements (nodes) for each segment (!) i=1,2,….N Compressor station between node “k-1” and “k” : PI- controller of discharge pressure manipulating gas flow: PI

Neste Jacobs Oy / Hans Aalto7 … Linearize this large ODE model in a given steady state operating point where x ^ [ΔP 1 Δq 1 ΔP 2 Δq 2 … ΔP N Δq N ] T or:

Neste Jacobs Oy / Hans Aalto8 Matrices A (2Nx2N) and B (2Nxm) depend on the geometry, physical parameters, node partition and steady state data = design (engineering) information C (1x2N) is needed just to select which state variable is of interest The rest is easy, obtain the transfer function from (A,B,C) using standard methods ??? eg. ss2tf of Matlab

Neste Jacobs Oy / Hans Aalto9 NO! Transfer function from large system is difficult, even if dominating time constants may be obtained. In our case, numerator dynamics has relevance!

Neste Jacobs Oy / Hans Aalto10 => Use Linear Model Reduction techniques! Truncation: Solve P and Q from Compute Hankel singular values Arrange eigenvectors of PQ into: The upper N r <<2N submatrices of: provide a greately reduced linear state space system

Neste Jacobs Oy / Hans Aalto11 Balanced truncation: P and Q required to be diagonal... Transfer function from reduced model with N r = 3…4 is easily obtained with standard methods!

Neste Jacobs Oy / Hans Aalto12 Pipeline system w. 6 segments, 8 offtakes, 4 compressor stations and 70 nodes => 2N=140 CS1 CS2 CS Pb Pa

Neste Jacobs Oy / Hans Aalto13 Transfer function from CS2 discharge pressure to “Pa”, far downstream CS2 [time constant]=minutes!: ~ Dito for “Pb”, close to CS2:

Neste Jacobs Oy / Hans Aalto14 Compare nonlinear ODE model and linear reduced order model (step response of Pa to CS2) Time, minutes Pa [bar] deviation from steady state

Neste Jacobs Oy / Hans Aalto15 Further development - Non-isothermal pipeline - High gas speed - Vertical direction - Reduce nonlinear ODE model first, then linearize = Proper Orthogonal Decomposition LOPPU!