【作者】 吴晓源；

【导师】 崔维成；

【作者基本信息】 上海交通大学 ， 船舶与海洋结构物设计制造， 2008， 博士

【摘要】 浮式生产储油卸油轮(FPSO)是具有油船外形的近海结构物,与普通的油船相比,FPSO将承受更为严峻的载荷条件,因而其船体结构要具有特殊的疲劳耐久度和断裂强度,以确保在作业位置按照生产要求持续工作40年以上直至油田开采结束。根据对受损FPSO损伤情况的调查,开孔及纵骨穿越横框架处的连接节点等结构都是易于产生疲劳损伤的部位,因而有必要在有限元数值计算的基础上对结构形式进行符合实际建造条件的改进。FPSO的装卸载比油船更为频繁,因而装卸载对船体造成的低周疲劳损伤也不容忽视。由于结构节点在设计上的不合理或生产建造、监督管理等环节的漏洞,或是遭遇台风等恶劣海况,在FPSO船体结构上将会产生裂纹。在焊趾处裂纹萌生的初期,通过对裂纹进行打磨的方法进行修理,可以达到延长结构疲劳寿命的目的。目前国际国内对FPSO疲劳强度的研究工作主要集中在如何预防裂纹的产生上,对于产生裂纹后的结构进行的研究则较少。本文作为“大型船舶结构的超规范研究”的后续研究,主要研究工作有如下几个方面:(1)对国内外船舶结构疲劳强度的研究方法、低周疲劳强度的研究进展以及FPSO疲劳强度的研究情况进行了综述。(2)对某散货船和某入坞维修FPSO部分舱段的损伤情况进行了实船调查,对采集的数据进行了分析。(3)基于对FPSO开孔疲劳损伤调查的结果,采用考虑焊缝的体单元模型,对开孔处的应力集中系数进行了三维有限元数值计算,参照船厂的实际建造情况,提出了较目前规范更便于建造的结构形式并给出了相应的应力集中系数计算参考值。(4)基于对老龄船舶纵骨穿越横框架处疲劳损伤情况的调查,参考JTP规范中建议的节点连接形式,采用考虑焊缝的体单元模型,对FPSO舷侧水线位置纵骨穿越横框架节点的应力集中系数进行了三维有限元数值计算,给出当计算疲劳寿命不满足设计要求时对结构可能采取的改进形式。(5)分析了低周疲劳对FPSO船体结构疲劳寿命的影响,以某FPSO船体结构中纵骨穿越横框架连接节点为研究对象,采用考虑低周疲劳修正的疲劳校核方法进行了疲劳寿命的计算。(6)提出了对FPSO船体结构中具有一定板厚的含裂纹T型焊接连接节点进行打磨消除裂纹的修理方法。在拉弯载荷工况下对不同的T型节点裂纹修理切口形式进行了简化的二维有限元数值计算,得到相应的应力集中系数,并将应力集中系数较小的切口形式应用到采用体单元的三维有限元数值计算中,分析了板厚和修理切口长度对T型节点应力集中系数的影响。(7)考虑在含修理切口的T型节点切口底部再次出现表面裂纹的情况,建立含表面裂纹的有限元模型,采用线弹性断裂力学的方法,对拉弯载荷工况下不同尺度的表面裂纹最深处及端部的应力强度因子进行了系列有限元数值计算,并根据计算结果回归得出适用于含修理切口T型节点切口底部萌生的表面裂纹的应力强度因子经验计算公式。(8)对T型节点焊趾处的裂纹修理进行了试验研究,验证了采取打磨消除裂纹延长结构疲劳寿命的可行性,将试件断口上预制裂纹及其扩展的观察结果与计算值进行了比较,检验了经验公式的工程适用性。通过本文的研究得出的主要结论如下:(1)采用考虑焊缝的体单元模型,对开孔及纵骨穿越横框架连接节点处的应力集中系数进行有限元数值计算,能够更好的模拟实际结构。在船厂目前所具备的建造条件下,将环绕开孔的加强形式改为Type G型,既便于施工,又能有效降低开孔处的应力集中;扶强材、防倾肘板和背肘板的趾端设置成软趾的形式有利于降低应力集中,而采用背肘板的结构由于横框架间纵骨跨距的减小,应力集中的降低较为明显。(2)装卸载产生的低周疲劳损伤在FPSO的疲劳强度计算中占有较为重要的作用,综合考虑高、低周疲劳损伤进行计算的结果显示,考虑低周损伤后,结构节点的疲劳寿命由原来的92年下降到60年,这能够对不满足设计寿命的FPSO的损伤情况进行解释。(3)对FPSO船体结构中具有一定板厚的含焊趾处表面裂纹T型焊接节点进行裂纹打磨消除后,焊趾处将产生一定形状的表面缺陷。对拉弯载荷工况下含修理切口(表面缺陷)T型焊接节点的应力集中系数进行简化的二维有限元数值计算以及采用体单元的三维有限元数值计算表明:在修理切口表面半宽大于深度时,采用椭圆型的切口形式产生的应力集中系数将小于U型切口形式;不考虑焊趾的影响、减小母板板厚以及增加修理切口的长度都将增大焊趾处表面缺陷的应力集中系数;采用平板表面缺陷来代替焊趾处表面缺陷的简化方法偏于安全。(4)采用线弹性断裂力学的方法,对含修理切口的T型节点切口底部的表面裂纹最深处及端部,在拉伸、弯曲载荷工况下的应力强度因子进行了有限元数值计算,并根据计算结果回归得出适用于含修理切口T型节点切口底部萌生的表面裂纹的应力强度因子经验计算公式。(5)对FPSO典型焊接节点进行的短期修理试验表明,即便在焊趾处的萌生裂纹已扩展到一定程度,采取沿焊趾方向贯穿板厚的切口对裂纹进行打磨消除,仍可以有效地延长结构的疲劳寿命;并通过对断面的观察,检验了基于线弹性方法得出的含修理切口T型节点切口底部表面裂纹的应力强度因子经验公式的工程适用性。更多还原

【Abstract】 Floating Production Storage and Offloading vessels (FPSO) are the ship-shaped offshore installations, which will subject to much harsher environmental loads compared to trading oil tankers. Therefore, particular fatigue endurance and fracture strength should be satisfied for the hull structures of FPSO to ensure the continuous operation for more than 40 years during the entire production period in a supposed field.According to the investigation on damaged FPSOs, fatigue damages are prone to arise in the structural sites or units as cutouts and connections of longitudinal/frame transition, which bears out the necessity to improve the practical structural form based on fine finite element numerical calculations. Since the loading and unloading are quite frequent for FPSO compared to oil tankers, the low cycle fatigue damage must be certainly considered. Fatigue cracks may be initiated by many factors as structural misalignments, the design and operational flaws, and harsh environmental conditions. During the crack initiation stage at the weld toe, cracks can be repaired by grinding treatment to elongate the fatigue life of the structure. To date, the research work related to the fatigue strength of FPSO is mainly on how to prevent crack initiation but neglects the treatment of damaged structures. As the post-research of“beyond rule research on large ship structures”, the following efforts have been made in this thesis:(1) A literature review of the related research works, including the current fatigue strength study methods of ship structures, the research progress of low-cycle fatigue strength and the existed conclusions for the fatigue strength of FPSO, is summarized, in which a description of the existing approaches, development procedure and applied conditions is introduced.(2) Practically investigated the damage conditions and distributions of the hull of a FPSO repaired in dry dock and a certain bulk carrier, the analyses on the obtained data are carried out.(3) On the basis of the investigation results for the fatigue damages of FPSO cutouts, three dimensional finite element numerical calculations are carried out to calculate the cutout stress concentration factors. And a new structural form which is more convenient for construction compared to current rules is put forward accordingly with the reference values of stress concentration factors.(4) On the basis of the investigation results for the fatigue damages at connections of longitudinal/frame transition of aged ships and according to the advised connection form in JTP rule, three dimensional finite element calculations are carried out to obtain the stress concentration factors at the connection of longitudinal/frame transition in the water line of FPSO, using the solid element model with the consideration of weld. And the possible improved structural forms were analyzed when the demanded fatigue life is not satisfied.(5) The effects of the low cycle fatigue damage on the hull structures of FPSO are analyzed and the fatigue life of a connection of longitudinal/frame transition for a certain FPSO hull structure is calculated using the fatigue assessment method considering low cycle fatigue correction.(6) The weld repair method is proposed for the cracked T-joints of FPSO hull structures, and the simplified two dimensional finite element calculations are carried out on the repaired T-joints under the tension and bending load conditions. Accordingly, the stress concentration factors are solved and the optimum repair profiles with the relative lower stress concentration factor are adopted in three dimensional finite element analyses with solid elements, which are used for analyzing the effects of plate thickness and repair profile length on the stress concentration factors of the joints.(7) Considering the condition that surface cracks may arise at the bottom of the repair profile of T-joints, a series of finite element models are established combined with the theory of linear elastic fracture mechanics for the calculations of the stress intensity factors at the bottom and the end point of surface cracks with different dimensions under tension and bending load conditions. Some empirical equations for the stress intensity factors of surface cracks initiated at the bottom of the repair profile of T-joints are obtained by analyzing the calculation results.(8) An experimental study on the weld repair by crack grinding treatment at the weld toe of the T-joint is made to validate the feasibility of the repair method for elongating the fatigue life of the structure. And the engineering applicability is proved to be satisfactory by comparing the results from the pre-cracked test and the finite element analyses. Through the studies,the following main conclusions can be drawn:(1) The stress concentration factors at the sites of cutouts and the connections of longitudinal/frame transition are analyzed by finite element method using solid elements with the consideration of weld for better modeling the practical structures. Under the current construction conditions of the ship yards, substituting the reinforcement of cutout with Type G will be more convenient for construction and effectively decrease the cutout stress concentrations. Soft toe setting at the weld toe of stiffener, tripping bracket and backing bracket is proved to be favorable for lowering the stress concentration factor while the structure with a backing bracket most obviously decreases the stress concentration factor as it reduces the space of longitudinals.(2) The low cycle fatigue damage caused by frequent loading and unloading affects a lot on the FPSO fatigue strength calculations. The reasons of the facts with unsatisfied demanded fatigue life could be illustrated by considering the low cycle fatigue together with high cycle fatigue.(3) Surface cracks may arise at the bottom of the weld repairs when repair by crack grinding treatment is implemented for the cracked T-joints of FPSO hull structures. A series of simplified two dimensional finite element models and more rational three dimensional models are established for the calculations of the stress concentration factors of T-joints with repair profiles (surface flaw) under tension and bending load conditions. It can be shown that the stress concentration factor of elliptical weld repair profile would be lower than that of U-shaped weld repair profile when the surface half width of the repair profile is larger than its deepness. The stress concentration of the surface flaw at the weld toe would be increased when neglecting the effect of weld toe, decreasing the thickness of the base plate and increasing the length of the repair profile. And the simplified method of substituting the surface flaw at weld toe by that in a plate would bring more conservative results.(4) Based on the theory of linear elastic fracture mechanics, a series of finite element analyses are carried out to calculate the stress intensity factors at the bottom and the end point of surface cracks with different dimensions under tension and bending load conditions. And empirical equations for the stress intensity factors of surface cracks initiated at the bottom of the repair profile of T-joints were obtained by analyzing the calculation results.(5) An experimental study on the short repair at the weld toe of the typical T-joints of FPSO is made to show that the crack grinding repair method is feasible for elongating the fatigue life of the structure even when the weld toe cracks had propagated to a certain extent. And the engineering applicability is proved to be satisfactory by comparing the results from the pre-cracked test and the finite element analyses.更多还原