Research Of Effect Of Nonlinear Wave Load On Ship Structural Strength

Over the past decade, with the development of shipping industry and the stimulation of scale economies effect under the background of global economic, ship design, especially the cargo ship, is progressing toward bigger, faster, and more professional. These ships regularly have large dimensions. Besides, in structural design stage, high-strength steel is widely used to reduce the structure weight. Hence, the stiffness of a large scale ship is much smaller than that of a traditional ship and the hull girder natural frequencies decrease relatively. At the same time, ships like large naval vessels or container ships are usually with complicated moulded lines and large flares, so the problem of slamming cannot be ignored. For these ships in harsh environment, the nonlinear impact loads, such as flare and bottom slamming, are more prone to provoke instantaneous and intense overall vibration, which is called whipping.Whipping results in severe structural damage, puts forward higher requirements for hull girder ultimate bending moment capacity and will reduce the fatigue life of structure. In this case, for structural strength assessment, mainly fatigue damage calculation and ultimate strength checking, the influence of hull girder vibration should be included in certain conditions, as well as the influence of nonlinear impulsive wave loads. This paper established a simplified nonlinear wave load calculating approach, taking slamming into account and the effect of nonlinear slamming loads on fatigue damage and ultimate strength assessment was studied on this basis. The main contents are as follows:1) Ship motions and linear section loads were calculated with 3-D linear frequency domain theory, and were transformed to time domain. Hull girder vibration characteristics were analyzed with transfer matrix method. Nonlinear time domain vibration equation including nonlinear slamming force was established to compute nonlinear section loads.2) The section modulus was used as the conversion factor to calculate stress time history.The total accumulated fatigue damage and wave induced fatigue damage were calculated separately. The fatigue damage contribution from whipping and the influence factor on vertical bending moment of slamming were analyzed.3) The hull girder ultimate bending capacity was computed with NFEM software Abaqus.Bending moment of transverse sections was analyzed for short-term and long-term. Partial safety factor of whipping in ultimate strength checking criteria was studied on. Existing hull girder ultimate strength checking approaches including slamming were summarized and compared for their applicability.4) On the basis of taking into account the nonlinear vibration of slamming, a complete set of methods, including nonlinear wave load calculating, cumulative fatigue damage analyzing, and hull girder ultimate strength checking, were establishment, taking a large container ship for example.