Dynamic analysis of offshore structures with non-zero initial conditions in the frequency domain

• The approach was applied to offshore structures with non-zero initial conditions.
• New studies on loads reconstruction and response comparisons with the ANSYS were made.
• A larger model order for environmental loads is suggested.
• A steel cantilever beam is used to demonstrate the proposed method.

The state of non-zero conditions is typically treated as fact when considering the dynamic analysis of offshore structures. This article extends a newly proposed method [1] to manage the non-zero initial conditions of offshore structures in the frequency domain, including new studies on original environmental loads reconstruction, response comparisons with the commercial software ANSYS, and a demonstration using an experimental cantilever beam. The original environmental loads, such as waves, currents, and winds, that act on a structure are decomposed into multiple complex exponential components are represented by a series of poles and corresponding residues. Counter to the traditional frequency-domain method, the non-zero initial conditions of offshore structures could be solved in the frequency domain. Compared with reference [1], an improvement reported in this article is that practical issues, including the choice of model order and central-processing-unit (CPU) time consumption, are further studied when applying this new method to offshore structures. To investigate the feasibility of the representation of initial environmental loads by their poles and corresponding residues, a measured random wave force collected from a column experiment at the Lab of Ocean University of China is used, decomposed, reconstructed and then compared with the original wave force; then, a numerical offshore platform is used to study the performance of the proposed method in detail. The numerical results of this study indicate that (1) a short duration of environmental loads are required to obtain their constitutive poles and residues, which implies good computational efficiency; and (2) the proposed method has a similar computational efficiency to traditional methods due to the use of the inverse Fourier transform technique. To better understand the performance, of time consumption and accuracy of the proposed method, the commercial software ANSYS is used to determine responses from the time history analysis. Comparisons of the results between the proposed method and the ANSYS results demonstrate that the proposed method produces high accuracies that are similar to the time-domain method and good computational efficiencies, which are similar to the traditional frequency-domain method. Finally, a steel cantilever beam is used to demonstrate the proposed method.