【作者】 施兴华；

【导师】 王善；

【作者基本信息】 哈尔滨工程大学 ， 固体力学， 2008， 博士

【摘要】 船体结构的自然损伤包括腐蚀和疲劳,舰船的环境载荷也是引起船体结构强度问题的主要因素。在分析船体结构可靠性时,不但要考虑载荷的时变性,而且还要考虑船体强度随时间的退化,即与传统的可靠性分析采用极值理论相比,加入了时间因素,相应的可靠性分析即为基于随机过程的船体结构时变可靠性分析。近年来,舰船结构的时变可靠性的研究开始引起了很多学者的关注。在通常情况下,船舶结构由于周边环境及波浪载荷的反复作用,将不可避免的产生疲劳和腐蚀,这将贯穿整个船舶的使用过程。结构强度由于其所处的环境及所受的载荷,将会出现腐蚀和疲劳的情况,而导致其强度随时间而变化,故必须处理为随机过程。疲劳裂纹的扩展以及舰船构件厚度的降低将导致船体梁有效截面模量减小,最终使其总纵抗弯强度减小,使舰船的可靠性随时间而降低。针对疲劳和腐蚀对舰船结构强度的影响,本文提出一种舰船结构时变可靠性的分析方法。根据舰船在服役期间的载荷环境特点,采用断裂力学及一般腐蚀原理,研究了在疲劳和腐蚀作用下总纵强度随时间的变化规律,基于随机过程理论,建立了船体梁截面模量的随机时变模型。然后应用上穿率分析,结合并联系统的可靠性分析方法对舰船结构的时变可靠性进行了分析计算。这种分析方法既能反映疲劳和腐蚀因素导致的船体梁结构可靠性随时间的连续变化情况,又避免了繁琐的数值积分,简单可行。通过算例与年度瞬时可靠性进行比较,表明该方法能更精确地预测在役舰船的时变可靠度,从而为服役舰船的可靠性、维修性、保障性决策提供参考。当船舶在航行期间遭遇恶劣海况时,将产生严重的砰击现象。砰击诱导的动态载荷效应具有同低频波浪载荷效应相当的数值,它会导致船舶整体颤振和局部损伤。基于随机过程理论,建立了船体梁在砰击、波浪及静水载荷联合作用下功能函数的随机过程模型,考虑波浪载荷和砰击载荷幅值的相关性,采用上穿率及并联系统可靠性分析方法求解此随机过程的可靠性。通过算例分析了舰船结构的阻尼率、砰击率及相关系数对可靠性的影响,并计算了舰船结构各种可能的失效模式及相应的可靠性。分析表明,结构的阻尼率对砰击载荷作用下的结构可靠性起着决定性的的影响,即阻尼率较小时,砰击载荷对结构的可靠性影响很大,且舰船结构的受压屈曲破坏是结构的主要失效模式。舰船甲板结构的动力屈曲是在砰击、波浪等载荷联合作用下引起的,其舯部一旦发生屈曲破坏,随后的载荷将引起屈曲变形在船舯部位的增长进而引起整个船体梁失效。因此,研究舰船甲板的可靠性具有重要意义。考虑舰船甲板受压屈曲破坏的三种主要失效模式,采用随机过程理论和并联系统可靠性分析方法对其进行分析计算,发现甲板结构的侧屈失效是最易发生的。虽然这只是局部的破损,但也会降低舰船整体结构的强度,可能导致在下次恶劣海浪中发生整体破坏。因此,适当加强舰船舯部的加筋或对已屈曲加筋及时加固,可有效地提高甲板整体结构的可靠性,为舰船的可靠性、维修性、保障性提供参考。舰船整体失稳即舰船倾覆是造成人员及财产重大损失的严重事件,历来受到造船界与航运界的极大重视。舰船倾覆因涉及到舰船外载荷的随机性和大幅度横摇的强非线性而使问题十分复杂。波浪载荷和风载荷都具有随机性,这就导致舰船倾覆是具有一定概率的随机事件。确定该随机事件的发生概率,对工程实际具有一定的参考价值。以路径积分法为基础,采用Gauss-Legendre公式探讨了随机风浪作用下舰船的运动及其倾覆概率计算。考虑阻尼力矩、复原力矩的非线性及风浪的随机性,建立了随机风浪中舰船运动的非线性微分方程,应用路径积分法给出白噪声随机扰动和定常风倾力矩作用时横摇角概率密度函数随时间的演变,按照现有的倾覆准则给出舰船整体失稳即倾覆概率的表达式。通过算例,验证了路径积分法的准确性,分析了各个参数对横摇角概率密度的影响,计算得出了不同风速、不同航速下的倾覆概率,以及倾覆概率随时间的变化。研究表明,此方法简便可行,并能在数量上预报舰船在随机风浪下的倾覆概率。更多还原

【Abstract】 The environmental loads and the damage of ship structure included corrosion and fatigue fractures were the primary factors of the ship strength problem. In this case, the time variability of the loads was considered, so was the degradation of the ship strength. Namely, compared with the traditional method which adopted the extremum theory, the method presented in this paper considered the factor of time, so the corresponding reliability analysis was the time-dependant reliability analysis for ship structure based on random theory. Recently, many scholars have paid much attention to the study of time-dependant reliability of ship.Generally, the fatigue and corrosion would inevitably happen to the ship due to the repeat effect of environment and wave loads, which would be all over the ship life. Based on above instance, the ship strength would change with time, so it must be treated as a random process. The expanding of fatigue crack and the reducing of component thickness would induced the decreasing of effective section module, finally the vertical bending strength reduced, which would have impact on the time-dependant reliability.A method for time-dependent reliability assessment of the strength of a degraded ship hull girder in the presence of fatigue and corrosion was developed. According to the characteristics of loading environment for existing ship structures, the variation of strength with time due to fatigue and corrosion was studied by applying fracture mechanics and general corrosion theory, then the time-variant model of ship hull modulus was formulated using the random process theory. The time-dependent reliability of ship was analyzed with up-crossing analysis and the reliability method of parallel system. This analysis can not only exhibit continuous decrease of reliability of the ship under fatigue and corrosion but also avoid numerical integration, which was easy to compute. Furthermore, the time-dependent reliability and annual instantaneous reliability was compared through an example, which indicates the time-dependent reliability of ship structures could be predicted using the proposed method in a more reliable way, and it could provide some references to the decision of the reliability, maintenance and safeguard.When the atrocious sea happened in the sailing period of ship, the serious slamming would happen. And the slamming induced dynamic loads effect was equal to the effect of low wave loads, which would induced the whole flutter failure and local damage of ship. Based on the random process theory, the random process model of performance function of the ship hull girder subjected to slamming, wave-induced and stillwater loads was set up. According to the real condition, the correlation of the peak value of slamming and wave-induced load was considered. The reliability analysis was done by means of up-crossing analysis and parallel system reliability method. Finally, the influences of damping ratio, slamming rate and coefficient of correlation on the reliability were analyzed, and the reliability of all failure modes were also calculated. It was indicated that the damping ratio dominated the influence of slamming load on the reliability. Namely, when the damping ratio was little, the influence was great. The results also show the buckling failure was the primary one.The buckling of ship deck structure was induced by the combined of slamming and wave loads, once the middle section was destroy, then the ship hull girder would be failure when it subjected to the loads. So the reliability analysis of the ship deck subjected to slamming load was significant to the design of ship. Considering the three primary buckling failure modes of ship deck, the reliability was studied by the random process theory and parallel system reliability method. The result showed that the stiffener tripping failure was the most dangerous, although it was partial destroy, which would reduced the ship strength and happened the whole failure at the next serious sea. Thereby, reinforcing the stiffeners of middle ship or strengthening the buckled stiffeners properly would be enhancing the reliability of whole ship deck, which can provide some references to the ship design.The unstable of the whole ship, i.e., the capsizing of ship was the serious incident which made greatly losing of personnel and wealth, so more attention was paid on by shipbuilding circle and sailing circle. The ship capsizing study included the random of the loads and the nonlinear of great range rolling, which was complicated. Because of the random of the wave and wind load, the capsizing was a random event. The determination of this event probability could provide references to the engineering.The path integration method, based on Gauss-Legendre integration scheme, was applied to the motion of ship subjected to random wind and beam seas. Considering the nonlinear damping moment, restoring moment and random wind and seas, the nonlinear differential equation for ship’s rolling motion was established. The rolling angle probability density was analyzed using the path integration method, when the ship was subjected to white noise disturb and a stationary wind moment, then capsizing probability was also calculated according to the presented rules. The presented method was capable of producing accurate results, which could be proved through the example. At the same time, the influence of each parameter on the probability density was analyzed. Furthermore, the capsizing probability at different velocity of wind and sailing was calculated, and the probability with time was also computed. It could be seen that this method was easy to achieve. Thus the rolling capsizing probability of warship in random wind and beam seas could be predicted quantitatively by the proposed method.更多还原