Reliability Theory of Structures

The marine industry can improve its performance in designing systems, subsystems and components on which its operations depend on by using risk-based methods and tools. In an environment of increasingly complex engineering systems and structures, the concern about the operational safety of the latter continues to play an important role in their design, construction and operation procedures. A rational and quantitative approach for assessing the failure probabilities and the impact of failure of the structure on its environment (human beings, natural environment) is needed.

The risk-based methods allow the engineer to evaluate the safety and risk of complex engineering systems under various conditions. This approach contributes to the reduction of the cost of unnecessary and expensive re-engineering, repair or replacement of the system.

The risk-based methods may be classified into risk management, which includes risk assessment and risk control, or risk communication. Risk itself can be defined as the potential for loss as a result of a system’s failure and can be measured as a pair of factors, one being the probability of occurrence of an event and the other being the consequence associated with the event’s occurrence.

Risk assessment is a scientific process by which the risk of given situations for a system is modelled and quantified. Risk assessment includes risk analysis and risk evaluation. Risk control is achieved through monitoring and decision analysis. Finally, risk communication is classified according to its target audience, either the media and the public or the engineering community, and can be defined as an exchange of information among individuals, groups and institutions.

On the other hand, reliability of a system can be defined as the system’s ability to fulfil its design purposes for a specified period of time. Reliability is, therefore, the probability that the complementary event will occur to failure. Based on the above it can be easily deduced that reliability is one of the major components of risk and therefore, plays a major role in the process of the risk-based methods.

The reliability theory deals with the scientific confrontation of uncertainty in structures, whether they are naval or mechanical, and has as target the estimation of safety and the ability of well functioning. The knowledge of this scientific region has been evolved rapidly the last years, and became a tool for planning methodologies with wide use in many practical applications.

The appropriate way of using the reliability theory of structures, includes the deep comprehension of a variety of natural problems and that cannot be done automatically. The use of this theory ¶in the field of shipbuilding has as target:

1. The achievement of economy in cost-weight for some predetermined acceptable safety index for the whole structure,
2. The insurance of a united level of reliability.

The uncertainties considered till now were the load acting on a structural element and its resistance. More generally a range of uncertainties may need to be considered. These might include various environmental conditions, workmanship and human error, and prediction of future events. Identification of uncertainties for complex systems may be difficult. Usually it is advantageous to use a systematic scheme to help to enumerate all operational and environmental loading states and, for each, consider possible combinations of error or malfunction. This is essentially ‘event-tree analysis’. Rather similarly, the systematic development of all possible forms of hazard to which a structure might be subjected has been termed ‘hazard scenario analysis’ even for structural systems. More generally, techniques such as ‘brain storming’ may be of use. All techniques rely on having available expert opinion and up-to-date information on which to base assessments. There are various ways in which the types of uncertainty might be classified. One is distinguishing between ‘aleatory’ (or intrinsic) uncertainty and ‘epistemic’ uncertainty. The first refers to underlying, inherent uncertainties and the latter refers to uncertainties which might be reduced with additional data or information, better modelling and better parameter estimation.

So each engineer that deals with the planning and construction of structures should know and comprehend the reliability theory of structures. This theory is used either indirectly in constructional codes, which have been developed according to the theory of reliability, either directly in cases of structures that have high uncertainties or probability of failure. The objective in both situations is to be achieved an economic structure with a minimal, but sufficient, safety index and well functioning.