The paper deals with the research results of authors aiming to establish the methodology of assessing the safety of offshore structures, based on traditional strength condition in extreme sea state (corresponding to the current design standards, ULS) but taking into account of the fatigue damage accumulated in thepast, called as "the expanded strengtlt condition", and based at the same time on traditional fatigue accumulative damage condition (corresponding to the current design standards, FLS) but taking into account of the fatigue damage in extreme sea state, called as "the expanded fatigue condition". This methodology was performed by the probability model and structural reliability theory. This paper is taken from the National Research Project KC09.16/06-10, in which the application research was realized by Pham Hien Hau and used in her doctoral thesis defended at Liege University - Belgium. This methodology was registered and accepted at the Intelligent Property Department of the Science and Technology Ministry.