by Clifton Linton, Editor, Energy Transport Insider

One is an occurrence, two is a coincidence and three is a trend.

The midstream energy sector is on the cusp of seeing a trend in mergers. In the first half of September, Enbridge Inc. and Spectra Energy Corp., announced plans to merge.

Meanwhile Enterprise Products Partners LP and Williams Companies Inc. announced that their merger talks had fallen apart.

The sector has long grown through smaller c...