Dynamic responses of the floatover system with nonstationary mating processes: An experimental study

Floatover method has been widely applied in the installation of both fixed and floating platforms, which has the advantages of high efficiency, economy, and large lifting capacity compared with other installation methods. Few studies could numerically and experimentally simulate the continuous load transfer process which shows significant nonstationary characteristics. This paper conducts experimental research on the nonstationary mating process of the floatover system. The ballasting system based on the control logic of feedback adjustment is developed to adjust the draft and inertia properties of the floatover vessel, and the validation tests are conducted to verify the feasibility and reliability of the system. The load buffer devices are designed carefully and the piecewise linear spring simulation method is applied to simulate the nonlinear stiffnesses of the elastic cushion. The stationary model for 0% and 100% load transfer stages and the nonstationary model for continuous load transfer process are established and tested in the model tests, and the dynamic responses are analyzed by using statistical and spectral methods respectively. Stationary model tests indicate that the spectral characteristics and distribution of LMU (Leg Mating Unit) dynamic loads in transverse symmetric positions are similar, and the statistical values are also relatively close under different wave heads. The nonstationary dynamic responses are segmented according to the physical process, time-frequency spectrum and piecewise statistics diagram to find the critical stage. The greatest dynamic load occurs in the intermediate stage of the mating process when the larger stiffness of the LMU acts and the steel-to-steel contact will also occur.