Rising needs for heavy transport operations are intensified by the expanding offshore industry worldwide. Whenever very large and heavy objects have to be transported, only semi-submersible heavy transport vessels are capable of carrying this special cargo. Accidents in the past during operations of these vessels highlight the requirement of analyzing the operation procedures in more detail. Especially the submerging process of the main working deck is very critical regarding the hydrostatic stability. A new numerical progressive flooding simulation method will be presented for applications like accident investigations or damage stability assessments. This method is modified to fit the special requirements of simulating the operational behavior of semi-submersible vessels in the time-domain. A direct approach is chosen, which computes the flux between the compartments based on the Bernoulli equation and the current pressure heads at each intermediate step. Losses due to viscous effects are taken into account by empirical discharge coefficients. This method will be used to simulate the submerging operation in the time-domain to point out critical situations regarding the stability of the vessels and the cargo. This will be compared to accidents which occurred in the past. Furthermore, recommendations for operational procedures are proposed.

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