The authors developed the algorithm which applies the quantitative risk analysis (QRA) to the rational safety assessment for a very large floating structure, considering a mooring system as an example. In the beginning they describe the rational or modern safety assessment based on QRA, and discuss about the general safety for a very large floating structure which is required to obtain the public acceptance. Then they consider the mooring system for the very large floating structure as an example to show the possibility of the application of QRA, and developed the time domain simulator for the estimation of mooring line forces which is used to collect the data base of the accident while the method to estimate accidental costs, that is to say, consequences is shown, and finally they discuss the algorithm of the safety assessment based on QRA.