A probabilistic approach to lifetime design of offshore platforms

Abstract

Offshore platforms are considered critical infrastructure as any disruption in their lifetime service can rapidly result in a great loss to arise. While these structures are often designed for their initial construction cost, it is worth considering a lifetime-based design so that both direct and indirect costs are involved in the design process. Here, a probabilistic-based approach to life-cycle-cost (LCC) analysis of offshore platforms is proposed. A fixed offshore platform is designed first based on the current design regulations and for a 100-year return period. For the effect of LCC on design optimization, the simultaneous effect of the wave, current, and wind merging are probabilistically considered. The structural elements are designed for five different models; one model based on the current design requirements and the rest for more than the requirements. The LCC of each model is accordingly determined. The results show that the code-based model is not optimal when is compared with a lifetime cost period; it is necessary to increase the size of structural elements by up to 10% to meet an optimum point. Results show that with a 5% increase in the initial cost, a decrease in the LCC up to about 46% is observed. The work presented here is to stimulate stakeholders to promote the LCC-based design of important structures to reduce lifetime costs.

A Corrosion- and Repair-Based Reliability Framework for Offshore Platforms

“…µ N U means the equivalent normal distribution; σ N U is the standard deviation of the equivalent normal distribution. The loop in Figure 2 is iterated to determine a reliable β until it converges to a desired value (∆β ≤ 0.005) [20,40]. The reliability index of the i-th member, β, which is uniquely related to the probability of failure, can be expressed as Equation (7) [32,39].…”

“…It is worth noting that the accumulation of damage reduces the remaining strength of the structural members or its overall capacity which can increase the failure risk. The cumulative PoF over t years can be expressed using Equation (12) [20].…”

“…If Z is the number of mutual damage modes considered in the dynamic analysis, then each damage mode can be defined by the level of corrosion damage in a set of elements. Hence, the P f a in year t can be expressed via Equation (13) [20].…”

“…Finally, the annual reliability index (β a ) for design lifetime is calculated using Equation ( 15) [20].…”

“…Their study focused on the impact of wave and wind loads on the dynamic response characteristics of the offshore structure under various environmental load cases. Hajinezhadian and Behnam [20] addressed the design optimization of offshore platforms and demonstrated that code-based designs are not always optimal from a lifetime perspective.…”