Bermuda-based Frontline Ltd., the world’s biggest operator of supertankers, plunged more than 10 percent to its lowest stock price since May 2003 yesterday.The company closed at 47.02 kroner in Oslo trading, down 5.38 kroner, a fall of 10.25 percent.Frontline has suffered from falling ship hire rates as a result of a glut in the global supply of tankers. Its shares are down by more than two thirds so far this year.Rival Bermuda-based tanker operator Nordic America Tankers Ltd is also down by nearly 30 percent this year and fell $1.08 to close on $18.28 in New York trading yesterday.