BP's new fleet of oil tankers, already dogged by cracked rudders and missing anchors, now has a new glitch. Fleet managers have been forced to replace deck fixtures called mooring bitts on three of four ships after tests showed they were defective and one violently broke down. Mooring bitts are stout metal posts around which ropes are lashed for tugging on ships or securing them to a dock. On Sept. 12, the tanker Alaskan Navigator was approaching the dock in Valdez when a bitt on the starboard bow broke off as a tug boat pulled on a mooring line, according to people with the U.S. Coast Guard, the ship's operator and a Valdez-based oil-industry watchdog group. When it broke, the heavy iron bitt shot over the side of the ship and plunked into the water.