The aim of this paper is to introduce the notion of operational safety, i.e, the influence of the action of man on the safety of a system. Explicitly or not, this action must be taken into account when designing a structure. For a given operation, man has :

  • to decide whether or not to attempt carrying out, as a result of his judgment of the environmental conditions, and

  • if his decision is to go ahead, to perform it.

The decision and the eventual performance of the operation constitute the action of man, modifying the environmental loads. Both the decision and the performance of the operation involve errors, with respect to the conditions taken for design. These errors combine with the error inherent in any design analysis, due to calculation of environmental forces and modelization of the structure, to form the error in demand. Quality of materials, building, and maintenance also deviate from the-design standards, resulting in an error in capability. The design safety factor, giving an excess of design capability over design demand; must be such that safety is achieved, i.e. that, in operation, capability remains larger than demand, in spite of the influence of all the above mentioned errors, to an acceptably high probability. To carry out illustrative calculations, on a number of systems for marine work, Gaussian distributions are taken for the errors in the factors affecting capability and demand. These calculations show the influence of the various safety parameters on the number of safe operations allowed by the design.


The problem of safety of units for work at sea, and more particularly of offshore units, is given more and more attention. Oil companies, operators, governments, and regulatory bodies strive to obtain a high degree of safety of these units, but, at the same time, owners (oil companies or operators) must make sure that this search for safety does not result in an uneconomical over-design of the units. To achieve both goals, an assessment of safety must be made, i.e the probabilistic distributions of capability and demand must be found, for each constitutive element of the structure, the failure of which is considered unacceptable.

At design, both demand and capability are evaluated and the design is based on a safety factor, or a set of safety factors, leaving some margin of design capability over design demand. In operation, demand and capability deviate from their design value. Safety in operations is achieved when the influence of these deviations does not absorb the margin of safety at design. The assessment of safety consists in finding out whether or not the design safety factor is insufficient or more than sufficient to ensure safety for an acceptably large number of operations.

Design demand results from an analysis of the motional and/or structural behaviour of the unit, in a given set of environmental conditions.

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