Nowadays an increasing popularity of alternative designs can be observed challenging the IMO Regulations of SOLAS (International Convention for the Safety Of Life At Sea). Examples are passenger ships with larger main vertical zones, novel types of survival crafts and new materials. This desire for innovative solutions combined with the society’s need for increasingly safer transport is expected to be satisfied by risk-based ship design and approval. The process of alternative ship design and arrangements, as described in MSC/Circ.1002 and MSC.1/Circ.1212, requires a risk analysis to demonstrate that the risk contribution of the novel design is less or equal to the present design. Thus the application of this process can also be regarded as risk-based design. The application of risk-based design is driven by the need for continuous improvement of the efficiency leading to lower costs for design, manufacturing or operation, because it offers the required frame for the development of new innovative solutions. IACS defines safety as absence of unacceptable levels of risk to life, limb and health. Risk is defined as a measure of likelihood that an undesirable event will occur together with a measure of the resulting consequence within a specified time, i.e., the combination of the frequency or probability and the severity of the consequence. Risk-based design involves risk assessment and risk evaluation criteria that can be defined, for instance, on basis of historical data or the ALARP process (As Low As Reasonably Practicable) combined with cost-benefit analysis. In this paper the definition of a risk evaluation criterion for systems by means of ALARP and cost-benefit analysis is presented and illustrated by a sample design of a ship fuel oil system. The risk contribution tree used for the analysis is composed of fault trees and event trees. A cost-benefit analysis is applied to establish a target system risk criterion in form of a target system failure probability. Problems related to the discrete structure of systems are discussed. The work shows that different risk analysis methods are required to describe the escalation chain from a component failure to a potential accident and its consequences.

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