WORKERS AND SUPERVISOR WERE PULLING I BEAMS FROM THE PREHEATER TOWERS ON THE OUTSIDE OF VESSEL. ONE OF THE BEAMS BECAE STUCK AND THE SUPERVISOR INSTRUCTED WORKERS NOT TO GO INSIDE VESSEL WHILE HE LEFT TO RETRIEVED FALL PROTECTION. AGAINST SUPERVISOR ORDERS 1 WORKER ENTERED THE VESSEL WHILE THE 2 WORKERS BACKS HAD TURNED. THE WORKER SLIPPED AND FELL APPROX. 57 FT.