A new approach is proposed for safety investigations of complex multistate systems. These systems have dependent components, called critical infrastructures, with variable operating conditions. The safety function of the critical infrastructure system is defined and determined for an exemplary “m out of l” critical infrastructure. In the fully-developed model, it is assumed that system components have multistate exponential safety functions with interdependent departure rates from subsets of safety states. A critical infrastructure safety model is adopted for an oil pipeline transportation system operating in a maritime port.

Basic concepts of the safety analysis of ageing multistate systems are introduced. The system components and the system multistate safety functions are defined. The mean values and variances of the multistate systems’ lifespans in the safety state subsets and the mean values of their lifespans in the particular safety states are defined. The multistate system risk function and the moment of exceeding the critical safety state are introduced. A series safety structure and a parallel-series safety structure of the multistate systems with ageing components are defined and their safety functions are determined. The multistate system safety models are applied to the prediction of safety characteristics of a maritime ferry.