Abstract
Ensuring safety and reliability constitutes one of the primary concerns associated with the approval of autonomous ships. Currently, multiple risk-based studies on autonomous ships are ongoing but still lack adequate safety assurance to get approval. This paper presents a comparative risk analysis that specifically focuses on identifying hazardous scenarios related to operating in or switching to different system control modes during different phases of autonomous operation. The objective is to gain insight into how the choice of control mode corresponding to different autonomy levels affects safety in different phases of the operation and under various circumstances. The analysis is based on a use case autonomous ship under the SEAMLESS project that operate method is applied to identify the unsafe control actions (UCA)s and related hazardous scenarios during unmooring, unberthing, and port departure phases. Initially, a concept of operations (CONOPS) is formulated for the use case vessel to describe the functional design conditions and scenarios. The application of STPA and the categorization of safe modes of operations will help to improve the CONOPS in further studies. This paper aims to analyze hazards associated with the operational phases mentioned earlier across various potential control modes of the ship. It compares UCAs and loss scenarios to ascertain whether certain control modes are safer or more hazardous than others during different operational phases. Finally, based on these findings, the paper proposes recommendations. The research presented in this paper will help to enable the safe and reliable operation of autonomous ships for short sea service (SSS) and lead to its approval.