PLC information note issued by IMCA

Published on 15 October 2012

Programmable Logic Controllers (PLCs) are widely used in a variety of small and large pieces of equipment on marine construction vessels, for example, cranes, winches, handling systems, dive systems and power management systems. The International Marine Contractors Association (IMCA) has issued a PLC information note to help ensure that operation and failure modes are understood, and highlighting the importance of carrying out an FMEA (failure modes and effects analysis) and testing the components and complete systems.

“When a PLC controlled piece of equipment is in use and a main power failure occurs, the equipment should revert to a fail-safe position, for example, brakes will be applied, valves may close or open and pressure may be ‘locked in’ or prevent movement.” Explains IMCA’s Technical Director, Jane Bugler.”The equipment should be in a safe state. The PLC Central Processing Unit (CPU) has a battery back-up power supply that should still allow the software programme to run and detect that a main power failure has occurred and take the necessary action.

“However, given the same piece of equipment, what may happen if the PLC CPU is manually reset without main power failure during equipment operation? The control programme may revert back to a start-up position which could, for example, release brakes and vent pressure. This may put personnel and equipment at serious risk of injury or damage.

“The behaviour of such PLC controlled equipment during restarting or software resetting of the PLC CPU unit should be fully understood. The consequences of restarting or manually resetting the software of such control systems during specific operations should be fully risk assessed. This should apply to any PLC controlled equipment where (high-) potential risk of injury or damage exists.”

The information note (IMCA M 15/12) highlights steps to be taken if PLC controlled equipment is new, or if an existing PLC is replaced with a new CPU, or the software programming of a PLC CPU has been modified or accessed in any way; and also provides useful references to PLC controlled equipment.