BHP Billiton on Tuesday announced its financial results for the December 2013 half year, with net income surging 83 percent to 8.11 billion U.S. dollars on revenues that rose 5.9 percent to 33.95 billion U.S. dollars. BHP Billiton Chief Executive Officer Andrew Mackenzie said the strong performance was driven by a substantial improvement in productivity and additional volume from the company's low-risk, largely brownfield investment program. "The commitment we made 18 months ago to deliver more tons and more barrels from our existing infrastructure at a lower unit cost is delivering tangible results," he said. "Annualized productivity led volume and cost efficiencies totaling 4.9 billion U.S. dollars are now embedded and this is expected to increase to 5.5 billion U.S. dollars by the end of the 2014 financial year." A 65 percent increase in net operating cash flow and a 25 percent reduction in cash outflows from investing activities have driven a 7.8-billion-U.S.-dollar increase in free cash flow during the period. Mackenzie said the group believes an average rate of return of greater than 20 percent is achievable for its portfolio of major development options. The interim dividend was raised by just 3.5 percent to 59 U.S. cents, up 2 cents from that of last year but lagging well behind major rival Rio Tinto's 15 percent increase in its final dividend last week. Mackenzie said the payout is in line with the company's established practice. BHP Billiton also predicted that with strong free cash flow projected, it's net debt of 27.1 billion U.S. dollars is expected to approach 25 billion U.S. dollars by the end of the 2014 financial year. Investors marked BHP shares higher. The stock jumped 2.3 percent to 38.89 AU dollars, its highest close for a year, outperforming the 0.2 percent rise in the broader market index.