This paper presents formulations and procedure of an efficient calculation of stress spectra and fatigue damage of offshore structures with deteriorated members in the uncertainty space. Calculation modeling of member deteriorations is represented by equivalent spring systems, which can be determined on basis of damage detection and stiffness degradation, with a deterioration uncertainty parameter. Redistributions of the member and system stiffness matrices and the load vectors are expressed in incremental (decremental) forms. The updated system stiffness-matrix is sated in terms of stiffness- and deterioration-uncertainties and the updated system load-vector is stated in terms of deterioration- and loading-uncertainties. Using the Neumann expansion solution technique, the inversion of the updated system stiffness matrix is expressed in terms of uncertainty parameters so that the reliability iteration can be performed without requiring repetitive inversion of the stiffness matrix. The deterioration- and uncertainty-update of the stiffness matrix requires resolution of the eigenvalue problem. This problem is reformulated in terms of uncertainty variables and an efficient solution algorithm is presented. An extra uncertainty parameter is used in structural transfer functions to represent damping uncertainties. Having expressed wave forces as functions of uncertainty variables, formulations of transfer functions of displacements and member internal forces are presented in the uncertainty space, which enable the reliability calculation to be efficient and fast. Apart from uncertainties of structural and loading origins, uncertainties arising from environmental origin, which appear in the spectral-analysis, are summarized. These are related to the modeling of random waves and wave-current interactions as well as to the long-term probability-distribution model of the significant wave height. Uncertainties in SCF, damage model (S-N line), non-narrowness of the stress process, long-term probability distribution of sea states and in the damage at which failure occurs (reference damage) are considered in fatigue-related uncertainties. An example is presented to demonstrate the application of the approximate analysis procedure to the mean value response analysis of deteriorated structures.

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