Experimental investigation of nonlinear springing of ultra-large container ship in regular waves

The issue of ship structural fatigue strength caused by springing is receiving increasing attention from designers. To investigate nonlinear springing more precisely on ultralarge container ships that have large-opening structural characteristics, a new segmented ship model with a U-shaped backbone was designed and fabricated in this paper. The novel U-shaped backbone can simultaneously simulate the vertical, horizontal, and torsional stiffnesses as well as the shear center of a full-scale container ship. Measurements of the vertical and horizontal bending moments of the ship model were achieved by assessing the corresponding bending strains of the U-shaped backbone. Integration of the shear flow method was developed for direct measurements of torsional moments. This method was validated through the calibration of the U-shaped backbone. On the basis of the new segmented ship model, vertical, horizontal, and torsional nonlinear springing tests were conducted successfully for the 20000 TEU container ship in the seakeeping basin of the CSSRC, and their characteristics and mechanisms were investigated. The test results reveal that nonlinear springing is a special case of high-frequency vibration that is induced by the harmonic excitation force. The resonance frequency component of the vertical bending moment of the midship is almost 2.5–4.5 times the corresponding wave frequency component when the ship experiences double-frequency vertical nonlinear springing at high speed under different wave headings. The severity of torsional nonlinear springing is not as pronounced as that of vertical nonlinear springing.