A fast time domain method for predicting of motion and excessive acceleration of a shallow draft ship in beam waves

The excessive acceleration has been incorporated into the framework of the second generation intact stability criteria (SGISC) at IMO 53rd meeting. The vulnerability criteria for excessive acceleration, including Level 1 and Level 2 and the direct stability assessment (DSA) are used to evaluate the sample ship. In DSA, the comparative study of five theoretical approaches is performed. One modified approach is to adopt a 5-DOF nonlinear mathematical model. Based on ordinary strip theory, the modified diffraction force is proposed to obtain more accurate motion of shallow draft ships in beam waves. The nonlinear restoring force is calculated through pressure integration on instantaneous wetted surfaces. Consider the three-dimensional effects of hydrodynamics force around the bow and stern of a shallow-draft ship, the added mass, damping coefficient and diffraction force in the direction of heave is modified respectively. Considering the nonlinear roll diffraction force caused by the non wall-sided hull, the instantaneous value of the roll diffraction force is obtained by adopting the method of invoking the steady-state solution in frequency domain corresponding to the corresponding draft of each section at each instant. In this paper, the sway, heave, roll response and lateral acceleration as well as vertical acceleration at measuring point are compared and analyzed by five approaches. The influence of modified diffraction force on time domain prediction of ship motion and excessive acceleration is studied. The model test has been carried out to verify the prediction method of ship nonlinear motion and acceleration response. The results show that the roll RAO and lateral acceleration obtained by the modified calculation method are the closest to the test values compared with other four methods. Compared with linear theory, the method proposed in this paper can obtain more accurate results of ship motion in a relatively short time, which is effective for direct prediction and evaluation of the ship’s lateral acceleration response in waves. •The numerical model is established for shallow draft ship acceleration calculation.•The modification of diffraction force for ships in beam waves is proposed.•The comparative study of five theoretical approaches is performed.