ULTIMATE STRENGTH ASSESSMENT FOR FIXED STEEL OFFSHORE PLATFORM

Currently, more than 80% of Malaysia’s offshore platforms are aged 30-40 years which is beyond the design life of 25 years. Structural assessments are needed to gauge the platforms for the extended use. The two common methods widely used are the simplified ultimate strength analysis and static pushover analysis. Simplified ultimate strength is attained when any of member, joint, pile steel strength and pile soil bearing capacity reaches its ultimate capacity. This is the platform’s ultimate strength. Static pushover analysis generally concentrates on RSR (Reserve Strength Ratio) and RRF (Reserve Resistance Factor) for the ultimate strength. This report summarizes a study of the ultimate strength of jacket platforms designed using API RP2A-WSD 21st Edition (2000) using SACS software with the module for Full Plastic Collapse Analysis. Two types of analyses have been carried out. First the ultimate strength of jacket platform with different number of legs is determined and in the second part the collapse load of platforms for different bracing configuration is studied. The non-linear pushover analysis is done by programming the software to analyze the structure with a set of incremental load until the structure collapses. The non-linear analysis module will distribute the load to alternative load paths available within the jacket framework until the structure collapse or have excessive deformation. In order to cater to uncertainties and distribution of data, several criteria of platform site location, age of service, type of platform, number of legs and other critical-related characteristics were considered. From the first phase of study, it is seen that \ a platform with more legs has higher ultimate strength compared to less number of legs. Hence a bigger jacket platform with eight legs is stiffer than smaller platform. The bracing configuration study shows that the X-bracing contributes highest rigidity to the whole platform by retaining the platform until the highest load compared to other configurations.