SYDNEY: 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 US assets, as the mining giant works to reign in costs and reduce risk. The decision came as miners globally struggle to cope with collapsing commodity prices and China’s once insatiable appetite — boosted by an unprecedented investment boom in the world’s second-largest economy — waning. Sharp falls in oil prices have ravaged bottom line of miners across world, pushing smaller players to the brink while tearing billions in revenue out of the budgets of resources-dependent economies such as Australia.