Platforms in the North Sea have been traditionally designed using working stress linear elastic methods. These methods are perfectly adequate to simulate normal operational conditions experienced on offshore installations. Use of these methods in design for accidental events, however, leads to over-design which can be costly to fabricate and operate. The criteria for accidental loads are focused on prevention of loss of lives and property, and protection of the environment. As such, they are related to loss of containment or collapse of supporting structures. With the relatively recent technology progress in development of non-linear methods, an elasto-plastic approach is more frequently adopted which enables a platform to be designed for accidental loads with improved safety at a reasonable cost as a result.

This paper describes how use of non-linear methods leads to enhanced knowledge on resistance of structures to accidental loads and improved, safer design. It outlines selection of topside framing arrangement for improved ductility, non-linear verification of jacket imperfections and plastic collapse due to ship impact. The method can also be used for assessment of structural resistance to fire.

It would be an impossible task for a regulatory body to specify all possible accidental scenarios in designing an offshore platform. It is therefore rather the implication of statements in various Rules, Codes of Practice or recommendations which concern us, e.g. risks to the integrity of TSR (temporary safe refuge), escape routes, embarkation points, etc. within a specified period of endurance.

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