The business, which includes pipelines and processing stations that remove valuable crude-like liquids from gas, owns or has rights to about 4,000 kilometres of pipeline systems and 21 million barrels of storage capacity.
It has 232,000 barrels per day of fractionation capacity and natural gas liquids produced from 8.3 billion cubic feet per day of gas processing capacity.
Natural-gas liquids like ethane, propane and condensates are stripped out of natural gas by fractionation plants. Because they are priced in relation to oil, producing NGLs is now more profitable than producing gas, whose price has stagnated on weak demand and high supplies.
The transaction is expected to be completed by the end of the first half of 2012. Shares in BP were down 0.3 percent to 459.15 pence at 1:10 p.m.
BP was advised on the sale by Credit Suisse.
(Reporting by Matt Scuffham; editing by James Davey)