Per Goetz Liedtke on the SCADAlisterve, “Siemens is the market leader in oil drilling control systems. The Deepwater Horizon drilling rig had both Siemens and GE Fanuc equipment, but I believe the latter was in the Blow Out Preventer (yes, off-shore oil drilling rigs use PLCs or controllers in the underwater equipment - even 5000 feet down).”
Both Siemens PLCs and GE Fanuc PLCs have known control system cyber vulnerabilities. I googled Siemens PLC and oil drilling to get the following: