Review and comparison of the modeling approaches and risk analysis methods for complex ship system.

Abstract

Marine industry is leaning towards autonomous vessels with companies such as Rolls-Royce and Kongsberg leading the development. However, this rapid technological change invites greater risks and responsibilities for marine professionals. Ship systems are getting more complex with time as the interactions between components are increasing and software are getting embedded. As a result, the nature of risks in modern systems can be different than in the traditional systems, where the risks were mostly limited to human errors and component failures. However, for identifying risks in modern complex systems, it is first important to understand the structural composition of the system, and the component’s behavior, functions and interactions. Although, modern systems are quite different than traditional systems, traditional system-safety engineering techniques developed are still widely used.

This thesis aims to review a modern modeling approach known as Systems Modeling Language (SysML) and a risk analysis method known as Systems-Theoretical Process Analysis (STPA); and compare them against widely used traditional methods known as the Tree structure method and Fault Tree Analysis (FTA). SysML, developed in 2006, is a graphical modeling language which presents structural composition, component functions, behavior, constraints and requirements of a system. SysML aims to support the analysis, specification, design, verification and validation of complex systems. STPA, developed in 2011, is a risk analysis method which aims to identify and mitigate risks in a complex system. Unlike traditional methods such as Fault Tree analysis (FTA), STPA focuses on risks due to the unsafe control actions and component interactions. Furthermore, STPA can be also used during the early phases of the system development process to generate safety constraints and requirements for a safer design of the system.

This thesis also includes a workshop with Rolls-Royce where FTA, STPA, SysML and the Tree structure method were applied to a sample complex ship system. The results and feedback received from the workshop are presented and analyzed.

The results suggest that the modern methods such as SysML and STPA are more suitable than traditional methods for modeling and identifying risks in a complex ship system if the results of the method’s implementation are considered. SysML presents several aspects of systems in a model which are missing in the Tree structure method, such as the requirements of a system, and behavior and interaction of components. Furthermore, it also provides a model that can be used as a tool for conducting an analysis of a system. Similarly, STPA succeeds on identifying higher number of risks related to component interactions and human errors in comparison to FTA, as STPA analyzes all possible control actions in a system, whereas FTA only analyzes the risks that are known to the analysts. However, some drawbacks of SysML and STPA have also been identified. Although the methods are suitable for complex ship systems, the methods have higher degree of complexity and require more time for an analysis in comparison to traditional methods. Furthermore, some solutions to improve the identified drawbacks of SysML and STPA are proposed in this thesis. Finally, some viable future research topics to improve the research results are presented.