U.S. natural gas pipeline giant Williams could be close to one of its biggest acquisitions ever as it is in late-stage talks to acquire gas pipeline operator Momentum Midstream in a deal estimated to be worth about $5.5 billion, sources with knowledge of the development have told Bloomberg. Tulsa-based Williams could announce within weeks the deal to buy Momentum Midstream from private equity firm EnCap Flatrock Midstream, according to Bloomberg’s anonymous sources. The agreement is not a done deal yet, as no final decision on the transaction…