Experimental and numerical investigation on seakeeping performance of a semi-submersible platform navigating in head waves

The new semi-submersible platform features a safe navigation capability in complex sea conditions, making it well-suited for scientific exploration and conducting marine full-scale experiments. This study focuses on the nonlinear heave and pitch motions of the platform, using a combination of experimental and numerical methods. Numerical calculations adopt the time-domain Rankine panel method (RPM) and computational fluid dynamics (CFD) method. The seakeeping performance of the platform was investigated under a range of conditions including speed, wavelength, and wave steepness. The Froude number effect and its impact on heave and pitch motions were discussed, with detailed analyses conducted on variations in total heave force and pitch moment, Froude-Krylov, and radiation forces. The nonlinear effects of wave steepness on motions and forces were investigated under various conditions. The results show that the platform has strong nonlinearities in the heave and pitch motions at Ak >= 0.08, and the current study shows that the pitch and heave motions can be approximately linear at Ak <= 0.04. Under the condition of 7 / H = 40, the reduction relative to the linear condition near the peak of the heave motion is up to 36.7% and 43.8% for pitch. The heave amplitude decreases by 69% and the pitch amplitude by 69.5% at 7 / H = 20.