This paper deals with a progressive collapse simulation of multiple mooring dolphin system in a huge box shaped, shallow drafted floating structure which is now being proposed as one of the candidates of an offshore airport in Japan. The progressive collapse simulation is recently being well-used to check the robustness of the mooring system after an initial failure of one mooring dolphin. This recognizes the possibility of failure even though a design satisfies the requirements of the ultimate limit state. Model experiments and time domain simulations have been carried out in order to investigate how the collapse of multiple mooring dolphins progresses after a first passage failure of one mooring dolphin and how the dynamic model of collapse process can be represented. The probability distribution of mooring system collapse time after the initial failure is studied.