TransCanada Corp. announced a US$848-million offer Monday to complete its takeover of Columbia Pipeline Group by buying up all the corporate ownership units of an investment affiliate.

The Calgary firm committed to pay US$15.75 each for 53.8 million units of Columbia Pipeline Partners LP, an 11.3% premium over the previous month average trading price.

TransCanada already has effective control over the investment vehicle’s operations as a result of acquiring its managing partner, Columbia Pipeline Group, in a US$13-billion takeover announced in March and closed in July (see Daily GPI, July 5).

Columbia Pipeline Partners holds interests in Columbia Gas Transmission, Columbia Gulf, Millennium Pipeline, Hardy Storage, Columbia Midstream and CEVCO.

The operating companies run a web of transportation, processing and storage facilities reaching from the Gulf of Mexico to New York State. Through CEVCO, the network also includes ownership rights to Appalachian shale gas and liquids deposits.

Columbia Pipeline Partners said in a statement that a committee of independent directors will review TransCanada’s offer and a vote by unit owners will be held. No target dates were set. Both sides of the transaction described it as a financial follow-up to the larger group takeover.