  We examine the problem of optimal transport capacity expansion planning for a
gas pipeline network to service the growing demand of gas-fired power plants
that are increasingly used to provide base load, flexibility, and reserve
generation for bulk electric system. The aim is to determine the minimal cost
set of additional pipes and gas compressors that can be added to the network to
provide the additional capacity to service future loads. This combinatorial
optimization problem is initially formulated as a mixed-integer nonlinear
program, which we then extend to account for the variability that is inherent
to the demands of gas-fired electricity production and uncertainty in expected
future loads. We consider here steady-state flow modeling while ensuring that
the solution is feasible for all possible values of interval uncertainty in
loads, which results in a challenging semi-infinite problem. We apply
previously derived monotonicity properties that enable simplification of the
problem to require constraint satisfaction in the two extremal scenarios only,
and then formulate the robust gas pipeline network expansion planning problem
using a mixed-integer second order cone formulation. We consider case studies
on the Belgian network test case to examine the performance of the proposed
approach.
