Automation Systems

Function and operating principle

A PLC reads inputs, runs its program in a fixed scan cycle and writes outputs. On a drilling rig, PLCs run mud pump sequences, drawworks and top drive interlocks, pipe handling and power management. Remote I/O racks sit near the machines and talk to the CPU over a fieldbus. On a vessel, an integrated automation system handles machinery alarms, ballast, tank gauging and power management through distributed controllers and operator stations.

Configurations

• Single CPU with local and remote I/O, common for single machines.
• Redundant or hot-standby CPUs, where a second controller takes over without stopping the process.
• Distributed control systems, where many controllers share one database and engineering tool.
• Safety PLCs, certified to IEC 61508, which run emergency shutdown and fire and gas logic. These belong with alarm and shutdown systems, not standard PLC spares.

Key specifications

Obsolescence and lifecycle

Most used automation purchases are driven by OEM lifecycle status. Siemens moved the SIMATIC S7-300 into phase-out in October 2023 and to type discontinuation in October 2025, after which modules are sold only in spare-part quantities. The S7-1500 is the successor. Rockwell has discontinued the PLC-5 and SLC 500 controllers, and points users to ControlLogix and CompactLogix. Check the OEM lifecycle database for each catalogue number, because one module can sit in a different phase from the rest of its family.

Standards and certification

• IEC 61131-2 sets the equipment and test requirements for PLCs, and IEC 61131-3 defines the programming languages.
• Class type approval for marine automation follows IACS UR E10 environmental tests and UR E22 for computer-based systems. DNV-OS-D202 applies to automation on DNV-classed offshore units.
• IACS UR E27 sets cyber resilience requirements for computer-based systems on new construction contracted from 1 July 2024.
• Remote I/O mounted in a zone needs ATEX or IECEx certification, and intrinsically safe I/O needs its loop documentation under IEC 60079-14.

Failure modes and inspection

• Electrolytic capacitors dry out with age, in service or on the shelf.
• Memory backup batteries go flat and can take the program with them.
• Connector pins corrode in humid storage.
• Electrostatic damage from unprotected handling may not show until the module is under load.
• Fans and power supplies fail first in hot cabinets.

A bench test in a rack with the right firmware, a check of each channel and a soak test under power catch most faults. Inspect boards for heat marks and contamination.

Documentation

Ask for catalogue numbers, series letters and firmware versions, whether each module is unused, tested used, removed working or repaired, the repair house's test report and warranty where relevant, and the storage conditions. For a complete system, ask for the program backup and engineering software version.

Integration and compatibility

A newer firmware release may not accept an older program or rack, and some CPUs cannot be downgraded. Engineering software licences and runtime licences usually stay with the original buyer. On a classed system, swapping a module for a different version can need class acceptance.

Refurbishment and when used makes sense

Unused surplus in sealed anti-static packaging is the strongest choice for a discontinued platform. Tested used modules suit a spares stock, and repaired modules make sense with a test report and a warranty from the repair house. If the rig has a long contract ahead, price a lifetime spares buy against a planned migration to the current family.

Related equipment

For the cabin side, see drilling control systems. For motor and switchboard controls, see electrical control systems. Naventr publishes the catalogue numbers and revisions the Seller supplies so you can match them to your rig.