SEOUL - As Shin Hyun Song takes the helm of the Bank of Korea this week, the seasoned economist is taking on an ambitious task: making the won a global currency without jolting capital flows, just as Asia's fourth-largest economy enters a turbulent phase.Shin, 66, is best known internationally for warning of the 2008 global financial crisis well before it hit. Now he is proposing to 'rebuild a framework' that could unwind ​a protective fortress around the won — a system he helped design himself when he last served in Seoul as a presidential adviser in 2010.'Even China is struggling to ‌internationalise the ‌yuan. So realistically it would be a challenging goal,' said Chang Jaechul, an economist at Pinnacle Economic Research Institute.