Prediction of vertical responses of a container ship in abnormal waves

Abstract

A body-nonlinear time domain code based on strip theory is used to calculate the vertical ship responses of a containership in extreme sea conditions. The numerical method calculates the radiation forces based on Cummins formulation. A practical engineering approach is followed for calculation of the radiation/diffraction forces for instantaneous wetted surface of the hull. The Froude–Krylov and hydro static forces are also calculated for instantaneous wetted surface area of the hull. The numerical method calculates the vertical responses of a container ship in abnormal waves embedded in a real deterministic sea and the results are compared with the model test data. Results obtained from the numerical code that uses body-nonlinear hydrodynamic and hydrostatic forces are compared with results from another code that uses linear radiation/diffraction force and body-nonlinear Froude–Krylov and hydrostatic force. Time series of the vertical ship motion, vertical bending moment at the midship, and the relative motion at the bow are calculated and compared. Additionally, short term distribution of peaks of the vertical motion and bending moment, and the largest vertical bending moment peaks are analyzed. It is observed that the body nonlinear radiation/diffraction forces significantly improve prediction of the ship responses in extreme waves.