For the large scale contractors, check the Engineering News Record (ENR) listing of the Top 500 Design Firms. Accompanying the Top 500 list are rankings in different sectors including the Top 20 Transportation firms.
Better off to target Houston. There are no full time pipeline engineering companies in Austin that I know of although you might find the odd consulting engineer that does some gas flow and gathering lines from time to time. I knew some pipeliners that lived near Austin and drove to Houston every day, but that was a long time ago when gasoline was durt cheap. You might want to glance at the price on the pump before you consider that. Better to look in the internet yellow pages under engineers/pipeline. Look in the Austin listings, if any, then compare against the Houston listings.