WAYPOINT : Taking back control
George Shaw from the Royal Institute of Navigation looks at feedback control systems in DP and asks what safety lessons can be learned from the marine and aviation sectors when it comes to their use
Feedback control systems occur widely in the natural world. A sensor detects what’s happening in a given situation and feeds back the information to the system it is linked to. This feedback is then used to inform and make adjustments to correct an error or achieve a desired result. For decades, autopilot technology has made use of feedback control systems to support safe navigation of aircraft and ships. However, despite long experience of using them, outcomes can still go spectacularly awry.
When it comes to marine vessels, DP usually controls two or more of the ship’s dynamical parameters: position displacements, rotations or rates of change. While skilled mariners can readily disengage the autopilot setting on their ship whenever they need to take over manual navigation, taking back control during a DP operation can be problematic due to the tighter tolerances involved. The limited speed of human reactions and the precision required to make incredibly accurate corrections may prevent a DP operator from continuing operations manually in certain circumstances – exiting the operation safely may be the best option. Safety must come first – all interventions must primarily safeguard the vessel, personnel, equipment and infrastructure.
The feedback control systems used in DP have their own inherent limitations, even when they are fault-free. Sensor measurements may not consistently meet tight position tolerances, which could be less than a metre. In challenging environments such as strong gusts of wind or freak waves, a ship’s thrusters may have insufficient power to meet the demands made of them by the system. The dynamics can become uncontrollable. In adverse circumstances, the control loop itself can become unstable, causing the dynamic responses to oscillate or move the vessel away from its intended position.
SAFETY MUST COME FIRST - ALL INTERVENTIONS MUST PRIMARILY SAFEGUARD THE VESSEL, PERSONNEL, EQUIPMENT AND INFRASTRUCTURE
Lessons to be learned
The maritime sector can learn from the experiences of the aviation industry in terms of human factors interacting with dynamic control systems, and the effect this can have on safety. In 2018/19, problems with the Boeing 747 MAX stall-prevention MCAS (manoeuvring characteristics augmentation system) led to two tragic aircraft crashes that killed all 346 passengers and crew members onboard. In both situations, a single attitude sensor was set up to force the nose of the aircraft down if a stall condition was detected. The sole sensor failed and the aircraft’s pilots struggled to intervene to correct (or even disengage) the misplaced demands of the MCAS. Irrecoverable flight instability ensued. The pilots’ handbooks provided inadequate training to handle the situation and the cockpit system did not give sufficient warnings.
When propulsion control is used for DP and maritime navigation, it can improve manoeuvrability for larger ships, but it also has its own inherent control limitations. In November 2023, the Spirit of Discovery cruise vessel, fitted with innovative propulsion capable of vectoring thrust over a full 360°, encountered problems during a storm in the Bay of Biscay. As part of its safety response, the vessel veered sharply and stopped for a prolonged period. This action severely traumatised the passengers and injured around 100 people. DP operators need to understand and closely monitor system behaviour and warnings, and continuously assess when and how it is necessary to intervene. Good seamanship is essential, as is enhanced training and experience. As autonomous systems become more commonplace, control systems related to DP will become an increasingly significant part of navigation. Mariners actively seeking to improve their DP skills should be well placed to meet future challenges at sea and onshore.
Contact RIN at: www.rin.org.uk | 1 Kensington Gore, London, SW7 2AT | Tel: +44 (0)20 7591 3134