Williams Cos. agreed to pay nearly $6 billion to expand its ownership of Access Midstream Partners LP, a move aimed at increasing the natural-gas pipeline company’s presence in areas with growing energy output from shale formations. Under the deal, Williams would acquire from Global Infrastructure Partners II the remaining 50% of the privately held general-partner interests in Access Midstream that the Tulsa, Okla., company didn’t already own. Williams also would acquire 55.1 million of Access’s limited-partner units, bringing William’s stake in the limited partnership to 50%. At the close of trading Friday, the limited-partner units had a market value of $3.6 billion. Access gathers natural gas and natural-gas liquids pumped from wells in Texas, Pennsylvania and Oklahoma, processes them and delivers them to larger pipelines, a business that Williams said it expected would grow rapidly. Access, which is based in Oklahoma City, was formed in 2009 as Chesapeake Midstream […]