Two empty barges being towed down the Ohio River near Pittsburgh, Pennsylvania broke free from its tug and struck a moored third barge causing it to go adrift.  The moored barge was loaded with a cargo lubricating oil floated down the river until it went aground on a sandbar just before the West End Bridge.   It is listing to starboard, but not taking on water.  The two empty barges were picked up by a tug.   Another barge may have been struck which had in its hold a cargo of anhydrous ammonia, a chemical used in agriculture and refrigeration.  Reports state there was no visible damage and no signs of oil has leaked out.  The Coast Guard is conducting an investigation into how the barges broke free and checking for any releases of pollution.   No on was injured in the incident.