NEW YORK (AP) ¿ Greek shipping company Tsakos Energy Navigation Ltd. said Tuesday it will generate at least $15.5 million from a two-year charter extension on a tanker. The shipping company didn't identify the oil company that extended its use of the Pentathlon, a seven-year-old double hull suezmax tanker. Tsakos said the gross-revenue figure assumed only a minimum payment rate, but suggested it would get a higher rate when the extension begins in early November. Tsakos said it was the third extension in a row with the same major oil company since 2003. Tsakos operates 47 double-hull vessels with five more ships expected to be delivered through 2011. U.S.-traded Tsakos shares rose 12 cents to $16.12 in midday trading.