Plunging global energy prices forced BHP Billiton on Friday to book a $7.2 billion pre-tax write-down against the value of its struggling onshore United States assets, as the mining giant works to rein in costs and reduce risk. The decision came as miners globally struggle to cope with collapsing commodity prices and China's once insatiable appetite waning. Sharp falls in oil prices have ravaged the bottom line of miners across the world, pushing smaller players to the brink while tearing billions in revenue out of the budgets of resource-dependent economies such as Australia. BHP spent $20 billion in 2011 on shale oil and gas assets in the U.S., but the move increasingly backfired with a dramatic fall in prices.