STRUCTURAL INTEGRITY ASSESSMENT FOR LIFE EXTENSION AND RETROFITTING OF AGING OFFSHORE PLATFORMS

Abstract

Hydrocarbons continue to play an important role in meeting global energy demand. In India, offshore oil and gas facilities, especially in the western offshore region, contribute significantly to domestic production and energy security. However, many fixed jacket platforms in fields such as Mumbai High are now operating beyond their actual design life and are exposed to long-term deterioration due to wave loading, corrosion, fatigue, accidental damage, and changing operational requirements. These factors make periodic reassessment and re-certification essential to ensure the continued structural integrity and safe operation of aging offshore platforms. The present study focuses on the structural reassessment of an existing fixed jacket type offshore platform based on the recommendations of the American Petroleum Institute (API) for life extension assessment. Site-specific metocean data for the Gulf of Kutch, including wave, wind, and current conditions corresponding to both operating and extreme storm environments, are considered during the analysis. A global linear in-place structural analysis is performed using SACS v24 to evaluate the adequacy of jacket members and tubular joints under combined operational and environmental loading conditions. Member and joint utilization checks are performed in accordance with API RP 2A, 22nd Edition, Working Stress Design, 2014 provisions to identify overstressed structural components. For loading conditions where the platform does not meet the prescribed acceptance criteria, a nonlinear ultimate strength assessment is performed using static pushover analysis to capture the actual structural response under extreme environmental loading. Incremental wave loading is applied until structural collapse to investigate the failure sequence, redistribution of internal forces, and ultimate collapse behaviour of the jacket system. The Reserve Strength Ratio (RSR), also referred to as the Collapse Load Factor (CLF), is evaluated to determine the residual strength and redundancy of the platform. The study also examines strengthening and retrofitting techniques for overstressed subsea tubular joints, which remain one of the major challenges in rehabilitation of aging offshore structures. Retrofit schemes using ring stiffeners and friction-grip clamp systems are evaluated for strengthening deficient joints and facilitating iv member replacement without extensive modification to the existing structure. To investigate local joint behaviour in detail, Component-Based Finite Element Analysis (CBFEA) is carried out using IDEA StatiCa 26.0. Detailed finite element models of tubular joints incorporating ring stiffeners and clamp arrangements are developed to evaluate stress distribution, load transfer mechanisms, and structural performance under applied loading conditions. The results of the study indicate that the adopted reassessment and strengthening approach improves both global and local structural performance of the aging jacket platform. From the in-place analysis, 20 tubular joints are found overstressed under extreme storm loading. After grout filling, most of the joints meet the unity check requirement and for remaining critical joints retrofitting procedures are performed. The pushover analysis shows adequate global reserve strength, with the minimum RSR obtained as 1.80 against the required value of 1.60, and the maximum RSR obtained as 3.20. For the proposed mechanical friction-grip clamp, all IDEA StatiCa checks remain within allowable limits, with maximum bolt utilization of 87.2%, weld utilization of 76.8%, and first buckling factor of 43.85. The ring stiffener model also shows improvement in local stress distribution, with the equivalent stress in members reducing from 355.3 MPa to 286.9 MPa. These results confirm that targeted retrofitting using ring stiffeners and friction-grip clamp systems can improve the residual capacity and support the life extension of the existing offshore jacket platform.