Correlation Between In-Place Analysis and Reserve Strength Ratio for Fixed Offshore Platforms: A Parametric Assessment

Aging fixed offshore platforms increasingly require structural reassessment to support life-extension and modification decisions. Conventional linear in-place analysis provides member-level demand–capacity checks under prescribed load combinations, while nonlinear collapse analysis yields the Reserve Strength Ratio (RSR) as a global indicator of structural redundancy and ultimate capacity. Although both approaches are widely used in offshore engineering practice, their results are often interpreted independently, leading to conservative decisions and potentially unnecessary strengthening measures. This study investigates the correlation between in-place analysis results and RSR values for fixed offshore platforms using a simulated yet engineering-realistic dataset developed in accordance with industry practice and offshore design standards. Fifteen representative jacket platforms with varying configurations, water depths, and structural conditions were assessed using linear in-place analysis and nonlinear pushover collapse analysis. Statistical relationships between RSR and key in-place performance indicators, including maximum member utilization and the percentage of overstressed members, were examined. The results demonstrate a clear inverse trend between RSR and in-place utilization ratios, with platforms exhibiting RSR values exceeding approximately 1.6 generally showing acceptable global and local performance. However, noticeable scatter and isolated exceptions highlight that RSR cannot replace detailed in-place checks. The findings support the use of RSR as a preliminary screening indicator to inform engineering judgment and optimize reassessment strategies for brownfield offshore assets.