Development of functional safety requirements for DP-driven servicing of wind turbines

The adage “prevention is better than cure” is at the heart of safety principles. However, effective accident prevention is challenging in complex, highly automated systems such as modern DP-driven vessels, which are supposed to safely transfer technicians in often unfavourable environmental conditions. FMEA analysis, which is required for DP-driven vessels, is helpful to build-in a necessary level of redundancy and thereby mitigate consequences of failures, but not particularly helpful to inform preventive measures, not least against functional glitches in controlling software. In this paper we develop a set of functional safety requirements which are aimed at prevention of causal factors behind drift-off, drive-off and other hazardous scenarios. For this purpose, we use a systemic hazard analysis by STPA, which delivers both failure and interaction-based (reliable-but-unsafe) scenarios. The functional requirements cover both design and operational (human element related) requirements, which are then ranked based on our proposed heuristic. The ranking is not predicated on statistics or expert option but instead it is proportional to the number of hazardous scenarios a requirement protects against, hence indicating the relative importance of the requirement. The paper also summarises the suggested areas of safety improvement for DP-driven vessels.