Incorporation of Axial Aging Effects of Pile Foundations into Pushover Analysis of Offshore Jacket Platforms

Abstract

The integrity assessment of jacket platforms using non-linear pushover analysis in the past had shown that most of the platform failure occur due to the lack of strength in the pile foundation. When the failure of jacket platforms in extreme weather conditions were studied, it was observed that the foundation was intact while the platform failed. This disagreement between the simulation and the actual condition can be explained by the phenomenon of Aging of pile foundations. Experimentally, the axial capacity of pile foundations have been found to be improving with time due to aging. The rate of improvement of the capacity can be empirically predicted using the properties of the soil in which the pile is installed. An empirical equation namely Skov and Denver equation was utilised to determine the improvement in capacity of offshore jacket piles in this study. This improvement was incorporated into the pile-soil modelling of jacket platforms using a new, yet very simple technique of stepping up the axial soil structure interaction curves. Pushover analysis of two offshore jacket platforms with the modified pile-soil model was done using the software SACS. The RSR (Reserve Strength Ratio) obtained from the pushover analysis showed significant difference due to the incorporation of the axial aging effects of the piles. Jacket A has showed a maximum improvement in RSR of 11% and a maximum reduction of RSR of 11% whereas Jacket B has showed a maximum improvement in RSR of 27% and a maximum reduction of RSR of 17%. The study has given a good insight into the changes in behaviour of a jacket platform due to aging of its pile foundations and is expected to improve the structural integrity assessment techniques of aged offshore jacket platforms.