Modelling the Stability of a Semi-Submersible Offshore Platform Under Tsunami Waves

Abstract

This study investigates the dynamic stability of a GVA 4000-type semi-submersible offshore platform (1:200 scale model) under tsunami wave loads using the Smoothed Particle Hydrodynamics (SPH) numerical approach coupled with MoorDyn to model the catenary mooring system. Simulations were conducted using DualSPHysics under three tsunami intensity scenarios represented by wave-tank water heights of 1,800 mm (T1), 1,900 mm (T2), and 2,000 mm (T3), corresponding to the wave conditions used in physical modeling at the TDMRC tsunami flume facility. The simulation results show that an increase in tsunami intensity from T1 to T3 increases the maximum surge response by 15.3% (from 0.131 m to 0.151 m) and the maximum mooring stress by 13.3% (from 283.09 N to 320.72 N), with a positive correlation between these two parameters. Pitch was recorded as the dominant rotational motion mode (7.25°–.55°), while roll remained below 0.21° across all scenarios, indicating good transverse stiffness of the twin-pontoon configuration. Multiple linear regression analysis of wave propagation data from seven wave gauges showed that wave arrival time (t50) is well explained by the spatial position and intensity of the scenario (R 2 = 0.964), while the final water surface elevation (η∞) is more strongly influenced by the specific characteristics of each scenario (R 2 = 0.532). The distribution of mooring stresses shows that the highest loads are concentrated on the mooring lines on the wave-incident side (L0 and L1), with no indication of mooring failure or capsizing in any of the tested scenarios. Overall, the results indicate that the twin-pontoon semi-submersible platform configuration with eight catenary mooring lines possesses adequate dynamic stability to withstand the range of tsunami load intensities analyzed in this study.