A comparative safety assessment for Direct Current and Direct Current with hybrid supply power systems in a windfarm Service Operation Vessel using System- Theoretic Process Analysis

Abstract

As windfarms are moving further offshore, their maintenance has to be supported by the new generation Service Operation Vessels (SOV) with Dynamic Positioning capabilities. For the SOV safe operations it is crucial that any hazardous scenario is properly controlled. Whilst international regulations require the implementation of Failure Modes and Effects Analysis (FMEA) for SOV power systems, FMEA has been criticised for not addressing properly failures in control systems. In this study, System-Theoretic Process Analysis (STPA) is employed for identifying the hazardous scenarios in terms of Unsafe Control Actions (UCAs) in Direct Current (DC) and DC with batteries power systems. Then the identified UCAs are ranked based on their risk. The results demonstrate that the number of hazardous scenarios derived by the STPA increases in a power system with batteries in comparison to a conventional DC power system, thus depicting higher complexity of this system. However, the increase in overall risk is small and within acceptable limits, whilst the risk reduces for a number of UCAs leading to Diesel Generator overload sub-hazard.

A Novel Method for Safety Analysis of Cyber-Physical Systems—Application to a Ship Exhaust Gas Scrubber System

“…The CASA method incorporates a wider system context, considers the software failures, thus addressing the CPSs software-intensive character of CPSs, and incorporates the system temporal behaviour in the Fault Tree thanks to the inclusion of the ESI approach. The incorporation of the system temporal aspects is an advantage compared to other studies using FMEA [29], FTA [30,31], Bayesian Networks [32], and STPA [34][35][36][37][38].…”

“…The importance metrics estimation is an advantage compared to Petri Nets based approaches [20,27,28]. As it was demonstrated, in the CASA method, a more detailed system safety model was developed than STPA based ranking approaches [34][35][36][37], which supports more accurate criticality/importance analysis.…”

“…STPA has been also used to derive test requirements for CPSs [33]. A number of studies applied approximate ranking to scenarios derived using STPA [34][35][36][37][38].…”

“…Whilst the temporal relations of the investigated system were incorporated in [20], no importance analysis was implemented due to computational limitations. The STPA results ranking was applied based on an approximate estimation of the considered safety metrics [34,[36][37][38], whilst the study in [35] did not consider the system interactions in detail.…”

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

“…The control diagram in Figure 3 also shows other controllers such as the power management system (PMS). The interactions between these systems were included in the presented analysis, however PMS hierarchy and other systems were analysed in a separate study also presented in this conference (V. Bolbot et al, 2019).…”