Strain Based Failure Assessment for Offshore Structures Under Seismic Loading

Under seismic loading, structural hot spots can experience very high levels of stress and many random stress reversals. Conventional stress based methods cannot assess the failure state in detail when stress is beyond the elastic limit and nominal stress reversals are more than double the yield stress. A method has been created to fully reproduce the true stress/ strain history by using 1) generalised Masing’s rule with equivalent cyclic energy dissipation to model cyclic stress/strain relation, 2) Neuber’s method to calculate inelastic strain concentration factor, and 3) relative effective notch factor determined from comparing S-N curves of different joint classes. From this reproduced strain history, strain cycles can be counted and low cycle fatigue analysis can be conducted by using Miner’s rule and by estimating damage from the strain based failure criteria such as Coffin-Mason method. This method has been implemented in a numeric procedure and coded in a FORTRAN program called CYSTRA (as for CYclic STRain Analysis). It takes input of “nominal” random stress history directly from general structural software, linear or non-linear, local or global, and calculates extreme strain and strain cycles at multiple hot spots for the whole structure efficiently. Thus it greatly facilitates failure assessment for offshore structures which can have a large number of hot spots within the structure, unlike mechanical devices commonly assessed in strain based analysis where detailed FE based methods can be used.