Offshore platforms and windmills are constructed by assembling huge mechanical structures transported by heavy lift vessels. The construction process comprises two interconnected operations, the dynamic positioning (DP) of the vessel and the lifting of heavy loads. The DP system is commonly designed and tuned for the case that there is no load or for the case that the heavy load is free-hanging (mode 1). During the transition from the free-hanging to the case that the vessel is connected to a heavy load which is mounted to the platform (mode 2), the DP system may not be able to preserve the position stability of the vessel, jeopardizing human and system safety. The goal of this work is to design an intelligent monitoring system for the early detection of the transition between the two construction modes by adopting a nonlinear state estimation approach. Simulation results are used for illustrating the effectiveness of the proposed construction mode detection system.