Estimation of extreme response from operations involving transients

All marine operations have steps where the dynamic behavior changes in a transient way. In fact – it is the purpose of every operation to make changes. A process dominated by transient or highly non–linear response must be analyzed differently from a stationary case. The applicability and limitations of well–known statistical approaches are discussed through simple examples. In all the examples results from time–domain simulations in irregular waves are basis for the estimation.

The limiting sea state for an operation is regularly determined from estimated extreme loads to be expected during the critical phases of the operation. Stationary simulations can be performed, with the system in one or a few selected situations that are considered to give the largest dynamic response. Alternatively, simulations can be performed in which the system is brought through the transient phase, comparable to the situation in the real operation. In order to obtain statistically confident response, repeated deployments in irregular seastates should be simulated, with different realizations of the wave field. It is demonstrated that the last method gives more realistic results, since an unrealistic build–up of oscillations that can be observed in stationary cases, is avoided.

Keywords: Hydrodynamics, Response, Transients, Statistics.