Simplified algorithm for evaluating the hydrodynamic performance of very large modular semi-submersible structures

This paper develops a simplified algorithm for evaluating the hydrodynamic performances of mobile offshore bases (MOBs) in the preliminary design stage based on the following reasonable assumptions: no navigational velocity exists, and marine environment loads only consider the random and irregular wave forces at high sea states. MOBs are a representative structural style for very large modular semi-submersible structures in deep-sea areas. Simplified formulas for different parameters in the dynamic equations of MOBs are derived, including added mass, restoring force, and the constraint forces of connectors. The revised Morison equation is used to estimate the wave forces on MOBs because the components of MOBs can be treated as small-scale components by comparing them to the wavelengths at high sea states. During this process, the shielding effects of the interactions between multiple components of MOBs are considered to estimate the effects of wave diffraction and other fluid-structure interactions on the results of wave forces. A 3-module numerical MOB model at high sea states is considered as a case study. The hydrodynamic responses of the MOB modules are calculated under random and irregular wave conditions with different wave angles. The statistical results obtained using this simplified algorithm in the time domain exhibit close agreement with the results of experiments reported in the literature and obtained using the potential flow theory (PFT) method. Additionally, the relative errors among the 3 methods fall within the permitted range for practical engineering, which adequately validates the accuracy, feasibility and reliability of the methodologies developed in this paper. The statistical results of the hydrodynamic responses among 3 adjacent MOB modules under the same sea state are also compared. Finally, this paper discusses the response spectra associated with the time-domain results of MOB module. This study provides significant support for the evaluation of the hydrodynamic performances of very large modular semi-submersible structures.