Optimality and Acceptance Criteria in Offshore Design

Abstract

The optimal design of offshore structures is formulated as a decision theoretical problem. The objective is to maximize the expected net present value of the life cycle benefit. The general optimization problem is simplified by taking into account the cost impacts of a possible reconstruction of the structure. The analytical solution to this problem has been derived for the case, where failure events follow a stationary Poisson process. The life cycle benefit is formulated in terms of the production profile, the design and construction costs, failure costs and reconstruction costs. In order to assess the effect of potential loss of lives, the costs of fatalities are included applying the concept of the Implied Costs of Averting a Fatality (ICAF). The suggested approach to optimal design, which can be applied for any type of offshore structure, is exemplified considering the special case of steel structures. Here, it is standard to represent the ultimate structural capacity in terms of the Reserve Strength Ratio (RSR). For the purpose of illustration, the relation between material usage and RSR, which is valid for monopod structures, is applied. Optimal RSR’s and corresponding annual failure rates are assessed for both manned and unmanned structures covering a wide range of different realistic ratios between the potential revenues and costs for construction, failure and reconstruction.