In this paper an analysis of necessary and sufficient conditions for persistency of excitation for a class of moored semisubmersibles with single-harmonic incoming wave is presented. The semiphysical model structure in 6 degrees of freedom is shown to contain a large number of parameters and states. The nonlinear system dynamics is able to produce chaotic behaviours and bifurcations. The analysis had concluded that the regressors for parameter estimation must expand a space of few amount of dimensions in order to ensure exponential convergence of all estimates. This condition is plausible to be fulfilled under chaotic responses. A real moored crane-platform is taken as case study in order to evaluate the theoretical results.