Cyclic behavior and strain energy-based fatigue damage analysis of mooring chains high strength steel

This study investigates the low-cycle fatigue behavior of mooring chains high-strength steel grade R4 under different strain amplitudes and strain ratios at room temperature. A fatigue test program has been carried out on small low cycle fatigue specimens cut from large mooring chains. The experimental results characterize the cyclic stress-strain relationship, the mean stress relaxation behavior, and the cyclic plasticity parameters of the material. Strain energy density is correlated with fatigue life through a simple power-law expression and very well represented by Basquin-Coffin-Mansion relationship. Further, a non-linear elastic-plastic material model is calibrated to the experimental stress-strain curves and used for the estimation of energy dissipation in the specimens under applied cyclic loads. The predicted fatigue life using the calibrated material parameters demonstrates a close agreement with the experimental fatigue life. Numerical simulations are carried out to analyze local plastic straining and assess crack initiation at the pit site of corroded mooring chains considering the multiaxial stress state. An energy-based approach is employed to estimate the number of cycles needed for a crack to initiate from an existing corrosion pit.     

Remaining fatigue life assessment of corroded mooring chains using crack growth modelling

Failures caused by the combined actions of fatigue, corrosion and wear are important safety concerns for mooring chains used on floating structures in the oil and gas industry. Prediction of remaining corrosion fatigue life based on surface condition could therefore be a useful tool for the continued safe operation of corroded chains. This paper investigates the use of crack growth modelling for estimating the remaining corrosion fatigue life of mooring chains that exhibit significant pitting corrosion damage. A crack growth modelling approach is used to produce remaining fatigue life estimates for a selection of severely pitted mooring chains. Using fatigue crack growth rate test results for grade R4 high strength mooring chain steel, empirical crack growth laws are presented for free corrosion and cathodic protection conditions at load ratio R = 0.1. Two different methods for establishing equivalent cracks from surface scans of corrosion damage are presented. The mooring chains are proof loaded as part of their manufacturing process. Residual stresses introduced during this process have therefore been determined by finite element analysis and accounted for in the fatigue crack growth predictions. One of the equivalent crack models, accounting for the single dominant corrosion pit, provided quite accurate fatigue life predictions when compared with full scale test results.     

一种基于冰载荷引起的船体结构疲劳损伤分析方法

目前对基于冰载荷引起的船体结构强度问题已经发展的较为成熟,但是基于冰载荷引起的结构疲劳强度的研究比较少。针对该问题,如果有一种简化方法来指导早期的结构设计,对船舶设计者来说是很有现实意义的。本文对英国劳氏船级社发布的ShipRight FDA ICE_^[1]作概括性的介绍,旨在向读者介绍这一简单而高效的评估方法以及技术背景和注意要点等。最后文章根据这一方法给出数条油船和LNG船的算例,并验证通过该方法改善疲劳节点的可行性。     

深水悬链复合锚泊线疲劳损伤计算

以某座Spar平台的锚泊系统为研究对象,首先利用三维绕射理论计算Spar平台主体波浪力,得到平台的总体运动响应时程。再建立复合锚泊线的二维非线性有限元动力分析模型,基于DelVecchio(1992)提出的经验公式,采用迭代的方法计算复合锚泊线的刚度。锚泊线和海床之间的接触作用基于刚性海床假定,基于Morrison公式计算锚泊线的惯性力和拖曳力荷载,根据计算得到的平台主体运动响应时程作为锚泊线顶端输入条件,在时域范围内进行复合锚泊线的动力分析。计算得到中国南海某海域各短期海况条件下复合锚泊线应力的时间历程曲线,采用雨流法对其计数得到对应于各短期海况条件下的疲劳载荷谱。最后根据Miner线性累积损伤模型,对复合锚泊线在长期海况条件下的疲劳损伤进行比较计算。     

Structural behavior and design of high-strength steel welded tubular connections under extreme loading

The present work is motivated by the increasing need for cost-efficient solutions in offshore structural systems for wind energy production and for improvement of their structural performance. The structural behavior and design of high-strength steel welded tubular connections (yield strength higher than 700 MPa) subjected to monotonic and strong cyclic loading is investigated. In the first part of the paper, an experimental investigation is presented on high-strength steel tubular X-joints subjected to monotonic and cyclic loading far beyond the elastic limit of the material, leading to weld fracture. Two grades of weld metal material are employed in the welding process of the specimens. The experimental results indicate that the weld material grade has a significant influence on the deformation capacity of the welded connection under monotonic loading conditions, and its low-cycle fatigue life. The experimental procedure is simulated using advanced finite element models, elucidating several features of joint behavior and complementing the experimental results. Overall, a good agreement is found between numerical simulations and experimental results, in terms of both global response and local strains at the vicinity of the welds. Furthermore, the structural performance of the welded tubular joints under consideration is assessed using available design methodologies in terms of both ultimate strength and low-cycle fatigue resistance, in an attempt to validate an efficient design methodology for low-cycle fatigue. The results from this research effort are aimed at developing the necessary background for the possible use of high-strength steel in tubular steel lattice structures, particularly in offshore platforms for renewable energy production. They can also be used as a basis for the possible amendment of relevant design specifications and recommendations for including special provisions for high-strength steel structural systems.     

Selection of appropriate numerical models for modelling the stresses in mooring chains

Mooring chains are key components for floating platforms. The failure of these components can be catastrophic in terms of the economic and environmental impacts, especially when dealing with the potential failure of FPSOs. However, mooring failures have been regularly occurring much earlier in their service lives than expected, with almost 50% of the reported failures happening in the first 3 years of 20-year design lives. Although the operating stresses play a major role in determining the failure mechanisms of mooring chains, the methods of predicting the operating stresses in mooring chains vary in the openly available literature, and the accuracy of these different numerical methods for predicting types of mooring failures is unknown. There is currently little evidence provided for when one model is appropriate for a particular scenario. Therefore, this paper benchmarks the different available methods for modelling mooring chains under tension, including FE models found in the literature. These models are calibrated and verified against previous studies and compared with experiments and a developed FE explicit model. There is a significant difference in the way that the numerical models behave, which are discussed in terms of their applicability and limitations in modelling mooring chains. The results of this study show that the explicit modelling approach should be utilised for accurate assessment of mooring lines, as it provides the most realistic response, with a substantial reduction in the computational cost and without any convergence problems.     

Study of fatigue crack nucleation processes by crystalline FE analysis

 In order to accumulate knowledge about how material compositions and manufacturing methods affect fatigue strength, this paper investigates the relationship between the swiftness of the changes in the macroscopic indices of the crack initiation process and the hardening properties of a material. This is done by calculating the cyclic deformation behavior of a f.c.c. single crystal. The relationship between the swiftness of the changes and the crystal geometries is also examined by calculating the deformation behavior of a f.c.c. crystal with a high Schmid factor buried in a large crystal with a low Schmid factor. In addition, a multiscale hardening rule based on forest theory is developed in order to examine the microscopic mechanisms of fatigue slip band (persistent slip bands, PSBs) formation. The validity of the theories presented is examined by comparing the changing nature of the measured and calculated hysteresis loop shapes of f.c.c. single crystals. Inhomogenous slip deformation through the crystal is also investigated, and inferences are then drawn about the microscopic mechanisms of cyclic hardening and PSB formation. Received: August 5, 2002 / Accepted: December 18, 2002 Address correspondence to: N. Osawa (osawa@naoe.eng.osaka-u.ac.jp) Updated from the Japanese original, which won the 2002 SNAJ prize (J Soc Nav Archit Jpn 1998;184:351–363, 1999;185:283–292 and 186:535–544, 2001;190:539–551)     

双轴加载下船底纵骨疲劳寿命试验与数值模拟研究

以实尺度制作了疲劳试验的试件,对45000DWT江海直达散货船船底纵骨与水密及非水密实肋板连接处进行了双轴恒幅循环载荷下的疲劳试验,记录了热点处的疲劳裂纹扩展长度和对应循环次数,获得船底纵骨疲劳特性;根据不同节点处裂纹萌生和扩展情况,得出非水密节点处是疲劳危险处;用动态应变仪记录热点处的应变,根据新版《船体结构疲劳强度指南》中的热点应力法计算热点应力,用试验数据拟合了一条S-N曲线,与指南提供的D曲线进行了对比分析,验证了D曲线的合理性。同时用MSC.Patran软件建立有限元模型,选取2007版《船体结构疲劳强度指南》中名义应力法对应结构形式的F曲线,用MSC.fatigue模块计算了相应节点的总疲劳寿命,将有限元结果和试验结果进行对比,验证了热点应力法的准确性。     

A regression and beam theory based approach for fatigue assessment of containership structures including bending and torsion contributions

Container shipping has been expanding dramatically during the last decade. Due to their special structural characteristics, such as the wide breadth and large hatch openings, horizontal bending and torsion play an important role to the fatigue safety of containerships. In this study the fatigue contributions from vertical bending, horizontal bending and torsion are investigated using full-scale measurements of strain records on two containerships. Further, these contributions are compared to results from direct calculations where a nonlinear 3D panel method is used to compute wave loads in time domain. It is concluded that both bending and torsion have significant impacts on the fatigue assessment of containerships. The stresses caused by these loads could be correctly computed by full-ship finite element analysis. However, this requires large computational effort, since for fatigue assessment purposes the FE analysis needs to be carried out for all encountered sea states and operational conditions with sufficient time steps for each condition. In this paper, a new procedure is proposed to run the structure finite element analysis under only one sea condition for only a few time steps. Then, these results are used to obtain a relationship between wave loads and structural stresses through a linear regression analysis. This relation can be further used to compute stresses for arbitrary sea states and operational conditions using the computed wave loads (bending and torsion moments) as input. Based on this proposed method for structure stress analysis, an efficient procedure is formulated and found to be in very good agreement with the full-ship finite element analysis. In addition it is several orders of magnitude more time efficient for fatigue assessment of containership structures.     

Parametric method for the assessment of fatigue damage for marine shaft lines

An original parametric method is introduced for the assessment of the fatigue life of marine shaft lines. The particularity of the method lies in the fact it introduces a relevant load modelling around a limited number of parameters specific to marine shaft lines while depicting the loading complexity (multiaxiality, mean stress effect, non-proportionality of the loading path). The method is designed to allow for assessments, in both in-use and pre-design phase shafts, towards a particular fatigue damage mode. The observations made in this study show two damage modes. On one hand, a damage mode issued from multiaxial cycles and associated with ship maneuvers, corresponding to HCF regime. On the other hand, there is a damage mode in the VHCF regime resulting from bending stress cycles due to the structure rotary bending.     

波致立管疲劳损伤计算方法研究

针对波浪作用下平台运动引起的立管疲劳损伤,进行完整的频域计算方法研究。首先采用势流理论对平台的运动响应进行分析,求得平台的运动频响函数;接着采用有限元法对立管的应力响应进行分析,求得立管的应力频响函数;最后,结合立管材料的S-N曲线,采用谱分析方法对四种不同海况下波致立管振动引起的疲劳损伤进行分析,获得损伤沿立管轴线方向的分布曲线。研究结果表明：波致立管振动疲劳损伤与有义波高密切相关,疲劳损伤值随着波高的增加成指数次方增长;立管的最大疲劳损伤值通常出现在立管的边界区域。     

砰击载荷作用下船艏结构瞬态响应研究

  目的  为了提高砰击载荷对舰船疲劳损伤影响计算的准确性,提出一种计及砰击载荷的疲劳损伤直接计算法。  方法  首先,将非线性载荷的时域计算与线性频域的谱分析方法相结合,基于梁理论得到舰船短期海况下的应力时历;然后,采用雨流计数法和S-N曲线计算出考核点的损伤;其后,参考中国船级社（CCS）指南计算计及非线性砰击载荷对疲劳损伤的贡献度及影响系数,再结合谱分析方法修正对应的应力响应传递函数;最后,对应力谱进行计算,得到计及非线性砰击载荷的疲劳损伤。  结果  结果显示,相比传统谱分析方法,采用计及砰击载荷的舰船疲劳损伤直接计算方法得到的损伤结果其损伤度约增加了10%~50%。  结论  研究表明,对航行于恶劣海况下的舰船进行疲劳强度评估时,使用所提方法可以提高载荷计算的准确性。     

疲劳强度对船体结构设计的影响

船舶中的销轴结构在工作过程中的受力情况类似于一悬臂梁。船舶在航行过程中的激励载荷会使销轴上的支撑结构做轻微晃动,造成销轴末端受到较大应力,导致销轴结构疲劳寿命较差。为了提高某船用销轴结构疲劳寿命,改善其设计质量。本文构建销轴的三维结构,并采用有限单元法建立销轴结构双工况有限元模型,获得销轴结构应力分布以及销轴末端位移参数。并将随机载荷谱、疲劳寿命曲线、有限元法相结合,获得销轴结构在外界随机激励载荷情况下的疲劳破坏点,该方法同样可为船舶其余结构设计提供参考。     

Simplified method for quasi-static collision assessment of a damaged tanker side panel

The paper presents a simplified analytical method to examine the energy absorbing mechanisms of intact and damaged small-scale stiffened plate specimens, quasi-statically punched at the mid-span by a rigid wedge indenter. The specimens scaled from a tanker side panel are limited by one span between web frames and stringers. The influence of the initial damage on the impact response is based on the plastic behaviour of an intact specimen. The initial damage is provoked at one-quarter from the support by the same indenter that, afterwards, punches the specimen at the mid-span. In practice, initial imperfections of this type could be due to minor incidents during ship service operation, such as collision of ships with floating objects. To validate the proposed simplified method, experiments and numerical simulations are conducted. The experimentally obtained force-displacement responses and shapes of the deformation show good agreement with the simulations performed by the explicit LS-DYNA finite element solver. The analytical method derives expressions to estimate the energy dissipated by the intact and the damaged specimens based on the plastic deformation mechanisms, assuming that both the plate and stiffener structural components absorb the incident energy through the rotation of the plastic hinges at the point of contact and at the supports and the membrane tension over the plastically deforming region between the loading and the supports.     

Development of simplified method for prediction of structural response of stiffened plates under explosion loads

During their lifetime, marine structures may be exposed to accidental loadings such as from collisions or explosions, as well as environmental loadings such as from slamming, sloshing and green water. Such loadings can cause damage to structures. Therefore, to minimize such damage, advanced and robust design guidelines should be formulated. Among those loads, in this study, explosions imparting an impulsive pressure loading containing a rapid increase in pressure and a short duration that can cause serious casualties, property losses, and marine pollution were considered. In this paper, a practical and robust method for damage assessment of marine structures exposed to explosion loads based on a single degree of freedom (SDOF) system and numerical simulations is proposed. The SDOF method was improved by introduction of new and better idealization resistance for the system and consideration of the effect of strain-rate, and subsequently was verified by a numerical method developed using the commercial ABAQUS software package. The numerical method was itself validated by comparison with relevant pulse pressure test data available in the open literature (good correlation was shown). Based on the validated numerical models, a rigorous parametric study of the structural response of stiffened plates having actual scantlings of offshore structures was performed. The numerically obtained maximum deformations were compared with the results from the improved SDOF method in a parametric study, and the variation of both methods was verified. Finally, simple yet accurate and reliable formulations for prediction of structural response were empirically derived. These formulations are expected to be usefully employed as a first-hand tool for prediction of damage extent of marine structures (including offshore structures) due to explosion loads.     

小水线面双体船减摇鳍的疲劳强度分析

针对某大型小水线面双体船两种连接方式的减摇鳍,即轴连接式和插入式减摇鳍,分别建立有限元模型,计算应力响应传递函数,确定减摇鳍疲劳累积损伤度,并对比两种类型减摇鳍的损伤结果特性。结果表明:减摇鳍与小水线面双体船下潜体相交各个部位热点应力较大,容易产生疲劳问题,且轴连接式减摇鳍的疲劳损伤比插入式的略为严重大,在设计和建造时应对这些部位重点关注。