[aaronmdjones]

It saved them time on switching the fuel they were using. Within US waters they were required to either burn cleaner fuel or scrub the dirty (high-sulfur) diesel fuel they would use in open waters.

They didn't have a fuel scrubber and they didn't want to spend the time flushing the dirty fuel out of the fuel lines to switch to the clean fuel, so they bypassed the fuel lines and fuel pumps for generators 3 and 4 and used the fuel line flushing pump as a fuel pump to feed generators 3 and 4 with clean fuel (marine gas oil) instead.

They would then presumably start generator 1 and/or generator 2 once in open waters, feeding them with the regular, cheaper, dirtier diesel fuel, and shut down generators 3 and 4.

Bypassing the fuel lines and fuel pumps for generators 3 and 4 made them prone to the very failure they experienced.

The ship would not have been built this way; it wasn't up to code.