## 1 Introduction  
Accurately predictions of ship hydrodynamics and wave-induced loads are the fundamental work for hull structural strength assessment. In the past decades, considerable efforts have been made to investigate the seakeeping performance and wave load response of ships, which includes two-dimensional (2D) strip theory [1; 2], three-dimensional (3D) panel theory [3] and 2.5D high speed slender body theory [4]. For the cases of rigid ships the structural deformation is negligible during the determination of hydrodynamic responses. However, with the increasing demand for large-dimension high-speed ships, the structural deformation of flexible ships can no longer be ignored and the hydroelastic effects should be considered during the hydrodynamic and wave load calculations [5]. The hydroelasticity theory, which was first proposed in the field of aerodynamics [6], considers the effects of mutual interaction among inertial force, hydrodynamic force and elastic force [7]. Ship hydroelasticity theory has been widely developed in the community of Naval Architecture and Ocean Engineering (NAOE) since the 1970s [8]. The 2D hydroelasticity theory combines the strip theory with linear beam theory [9]. The 3D hydroelasticity theory combines the 3D potential flow theory with 1D beam theory or 3D structural finite element method (FEM) [10]. A comprehensive review of ship hydroelasticity theory, which includes 2D (or 3D) linear (or nonlinear) frequency-domain (or time-domain) methods can be found in [11]. The influence of different levels of nonlinearities,which includes linear, Froude-Krylov nonlinear, body nonlinear, body exact and smooth waves methods, on symmetric hydrodynamic responses for a ship operating in harsh waves with large amplitude motions can be found in References [12; 13]. On the other hand, although the computational fluid dynamics (CFD) tool, which is on the basis of solving Reynolds Averaged Navier-Stokes (RANS) equations has been incorporated into the methodology of hydroelasticity, the immensity of time cost still makes it stay at early stage and far from practical engineering application [14]. Therefore, the potential flow theory constitutes an invaluable tool in the prediction of ships hydroelastic responses. Large vessels with pronounced flare bow sailing in harsh weather are more susceptible to frequent and harsh wave impacts, which will result in large increase of transient hull girder vibration loads called whipping. The slamming induced whipping loads are primarily an ultimate strength issue and it decays rapidly due to the structural damping effects [15]. Moreover, the springing responses of hull girder occur even in low wave states and it may last for a considerable long time once excited. The magnitude of springing loads is usually low; however, the number of cycles can be very large, which highly contribute to the fatigue damage of the structure [16]. The time-domain hydroelasticity theory sufficiently considers the effects of structural responses on the hydrodynamic forces and therefore provides opportunities for the investigation of whipping and springing loads. [PERSON] and [PERSON] [17] investigated the springing and whipping characteristics of a large container ship by segmented flexible model tank experiments. [PERSON] et al. [18] developed a fully coupled hydroelastic model which combine a 3D Rankine panel method, a 1D or 3D FEM, and a 2D Generalized Wagner Model (GWM). [PERSON] et al. [19] conducted a comparative and benchmark study on the impact loads predicted by using CFD tools OpenFOAM and STAR-CCM+ with experimental data. To date, majority of the literature work are focusing on the investigation of ship hydroelastic responses in regular waves [20; 21; 22]. However, the realistic irregular waves are much more complex compared with the regular waves and they are associated with strongly nonlinearity and randomicity. [PERSON] et al.