## 1 Introduction
[23] studied the effects of bow flare variation on the vertical responses of bulk carrier, containership and passenger ship in abnormal waves and extreme irregular seas. [PERSON] and [PERSON] [24] studied the slamming load characteristics of a chemical tanker in irregular waves experimentally and numerically. [PERSON] and [PERSON] [25] predicted the extreme loads on ultra-large containership with structural hydroelasticity. In the authors' previous work [26], a 3D time-domain nonlinear hydroelasticity theory was developed to predict ship motion and load responses in regular waves; and the proposed hydroelasticity theory is extended to irregular wave case in this paper. In fact, ships are subjected to a wide variety of different sea states during their whole lifetime, and the cycle times of alternate loads may be very large. Therefore, long-term prediction of wave-induced hull girder loads considering the effect of various operational circumstances is of great importance for the determination of the extreme design loads and the subsequent ultimate strength assessment. [PERSON] et al. [27] assessed the maximum wave-induced loads occurred on a fast monohull during its lifetime by long-term distribution calculations of nonlinear vertical bending moment (VBM) amidship. [PERSON] and [PERSON] [28] developed a simplified algorithm for the estimation of long-term extreme value by identifying the most important sea states with limited number. [PERSON] and [PERSON] [29] presented a nonlinear time-domain formulation for ship motions and loads estimation and also extended with a nonlinear long-term statistics method. The wave load response of ship in harsh or extreme irregular waves and the structural safety is a special reason of concern. Since ships operate in unknown severe and high-risk environments, marine accidents due to structure failure may occur at seas. For example, on 18 January 2007, the 275-m-long containership MSC NAPOLI was caught in stormy weather and suffered severe structural damage and the crew were forced to abandon the ship and escaped with the help of helicopter (see Figure 1a). Another example may be the five-year-old 8100-TEU containership MOL COMFORT who broke in two in rough weather off Yemen on 17 June 2013 (see Figure 1b). In order to properly evaluate the hull structural strength and safety, in the recent years the International Association of Classification Societies (IACS) stressed the contribution of whipping loads to extreme loads and its influence on hull girder ultimate strength by revising the chapter URS11A [30]. [PERSON] et al. [31] presented a direct calculation method for the assessment of the ultimate strength of a 10,000 TEU container ship and the margin of safety between the ultimate capacity and the maximum expected moment is established. The China Classification Society (CCS) issued calculation guidance notes that evaluate the effects of whipping and springing loads on hull structural fatigue strength [32]. The CCS also plans to issue calculation guidance notes for assessing the effects of whipping and springing loads on hull structural ultimate strength. The work involved in this study is conducted in collaboration with CCS for such purpose of investigation. In this paper, a 3D time-domain nonlinear hydroelasticity theory is presented to study the motion and load responses of a large flexible ship with pronounced bow flare in irregular waves. The hydrodynamic part is simulated by 3D potential flow theory and the structural part is addressed by 1D Timoshenko beam theory, and they are fully coupled by modal superposition method to consider the hydroelastic effects. The slamming loads calculated by momentum impact method are included into the hydroelastic motion equation. To validate the numerical results, experiments of a segmented ship model advancing in irregular head waves were carried out in a towing tank.