Well, as I know, Siemens S7 3xx and 4xx is a good example. You can Google its training materials. Also, Siemens has some training courses from programming to maintenance.
1. Conformally coated I/O and Controller to handle corrosive marine environment.
2. capability of interface to a myriad of other PLCS since there will be a lot of vendor skids that make up an oil rigs ( gas compressors, generators, fire&gas systems, pipeline pumps.)
3. Handle a lot of I/O in one cabinet since space is premium. Believe me, it is more expensive/sq foot than a Manhattan millionaire pad.