MEXICO CITY, Dec 8 (Reuters) - Plans set out by Petroleos Mexicanos (Pemex) to carry out a series of liability management transactions will reduce the Mexican state oil company"s debt and refinancing risk, according to Moody"s Investors Service. read more "Pemex"s liability management transactions are credit positive because they reduce debt and refinancing risk," Moody"s said in a statement. Reporting by Adriana Barrera