Free-falling oil prices are the latest headache for the global mining giant, which is already grappling with a dam burst at a Brazilian mining venture and a collapse in prices for iron ore , the focus of its biggest division. The company’s shares have plunged 16% in the early days of 2016. The write-down against the energy unit surpasses US$5.5 billion of charges reported in 2013 across several assets, previously its biggest impairment on record. It also adds to a series of write-downs against that business, including a pretax charge of US$2.8 billion last year , and another of similar size in 2012. BHP isn’t alone. Resources giants around the world have written down the value of assets amid the sharp downturn in markets. BHP plowed about $20 billion into U.S. shale-gas businesses in 2011, and has since spent billions a year exploring and developing assets to become one […]