The present paper addresses the problem of optimal design of portfolios of fixed offshore structures. A new framework for design is developed where the effect of dependency in the performance of structures subject to common extreme load events is taken into account in the design by inclusion of the follow-up consequences resulting from the simultaneous failure of several structures in the portfolio. First the special aspects of optimal design subject to follow-up consequences are addressed from the perspective of structures portfolio risk management. Thereafter the problem of optimal design of groups of structures is defined with special considerations to the assessment of the relation between the design, the probability density function of the life cycle benefits and the number of structures considered (in a group). Using this model basis the optimum design of fixed steel offshore platforms where the capacity of the structures against extreme wave loads can be expressed as function of the Reserve Strength Ratio (RSR) is considered. Thereafter parametric studies are conducted to illustrate the significance of the number of structures considered in a group, the correlation between the extreme loads acting on the different structures and the significance of including the follow-up consequences into the design optimization problem.

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