Determination of structural stress for fatigue assessment of welded aluminum ship details

The main objective of this paper has been to review and to verify already published hot-spot stress extrapolation procedures for plate structures, and to develop and verify a new and general method for the structural stress extrapolation to be used together with a hot-spot design S-N curve for aluminum ship structures. The proposed extrapolation method has been based on the asymptotic behavior of the stresses adjacent to an idealized notch (‘singularity’). On basis of the fatigue test S-N data obtained in this study, relevant S-N curves to be used together with a proposed extrapolation procedure and with already published extrapolation methods have been suggested. A hot-spot design S-N curve with a characteristic strength of 32 was suggested as a suitable choice for the fatigue assessment of profile ground fillet welded stiffener/bracket connections while a design hot-spot design S-N curve with a characteristic strength of 25 was suggested for as-welded stiffener/bracket connections.     

海上风机在三种风载荷作用下的疲劳分析方法研究

文章提出一种基于等效疲劳载荷的快速有效的结构优化设计方法,首先通过bladed模拟得到时域下的风载荷,然后通过雨流计数法则和等效损伤理论得到相应的疲劳载荷谱和等效疲劳载荷,接着以导管架式海上风机为例,利用AN-SYS对其进行三维建模,选取三种典型管节点和两种非管节点,基于热点应力法计算了其在三种风疲劳载荷作用下的疲劳损伤,通过比较三种载荷作用下的疲劳损伤结果,验证了等效疲劳载荷的可靠性.接着又计算了各等效疲劳载荷分量单独作用下的海上风机焊接节点的疲劳损伤,得出各疲劳载荷分量对疲劳总损伤的贡献,可以为设计者提供更好的载荷设计依据.相比于传统的时域疲劳分析方法和疲劳载荷谱方法,等效疲劳载荷方法更加方便有效.     

On stress concentration factors for tubular Y- and T-Joints in frame structures

This paper is written as a result of some years experience with fatigue analysis of offshore jacket structures where the connections are made as tubular joints. The hot spot stresses at the tubular joints in such analysis are normally derived based on parametric equations for stress concentration factors. These stress concentration factors are normally related to the axial force in the brace. It is observed that the hot spot stresses at the crown positions of the tubular joint in some cases are significantly affected by the local loading on the chord and the bending moment in the chord. In order to use the existing formulae in these cases some engineering effort is required to derive correct hot spot stress. This work can be avoided by using the nominal stress in the chord as the basis for calculating the hot spot stress at the crown position instead of using the axial force in the brace as basis for the analysis. This also extends the validity of the equations for stress concentration factors for T- and Y- joints in design standards. The proposed modification makes it also simpler to include the effect of joint flexibility in a proper way. The basis for a proposed revision of the equations for stress concentration factors for these joints is presented in this paper. It is considered that this modification leads to minor changes of the computer code, but that it will save analysis work for engineers and reduce the possibility of calculating incorrect fatigue lives in tubular frame structures.     

液货晃荡对油船结构性能的影响研究

论文研究了液货晃荡对油船的结构响应影响研究.论文对比研究了考虑液货晃荡与忽略液货晃荡影响的油船结构强度,包括设计波参数对比,最大扭矩、最大垂向弯矩、最大水平弯矩控制参数对应工况下的结构应力分布特征对比.对比研究了考虑液货晃荡与忽略液货晃荡影响的油船疲劳强度,包括满载状态下的疲劳评估结果、疲劳损伤的浪向角分布结果对比.论文针对液货晃荡对油船结构响应影响提出的研究结论,对油船的结构设计及评估具有重要的研究意义.     

Stress concentration factors at welds in pipelines and tanks subjected to internal pressure and axial force

In this paper, analytical expressions for stress concentration factors in pipes subjected to internal pressure and axial force are derived for a number of design cases based on classical shell theory. The effect of fabrication tolerances in simple butt welds is assessed. Analyses based on classical mechanics are compared with results from axisymmetric finite element analyses for verification of the presented methodology. Stress concentration factors are presented for circumferential butt welds in pipes welded together from pipes with different thicknesses, welds at buckling arrestors, welds at flanged connections in pipelines, and welds at ring stiffeners on the inside and the outside of the pipes. It also includes stress concentration factors at end closures in pipes for gas storage. Larger pipes are fabricated from plates with a longitudinal weld. This fabrication process introduces out-of-roundness in the pipes. The actual out-of-roundness is a function of internal pressure. An analytical expression for the bending stress in the pipe wall due to this out-of-roundness is presented. The derived stress concentration factors can be used together with a hot spot stress S–N curve for calculation of fatigue damage.     

船舶热点疲劳强度研究

以一艘71 000 t的散货船为研究对象,基于IACS的《散货船共同规范》和CCS的《船体结构疲劳强度指南》,采用热点应力法来进行疲劳强度的评估。对热点疲劳计算的结果进行分析和比较,指出用热点应力法进行疲劳强度评估时的影响因素。     

Fatigue analysis of welded joints: state of development

The literature on fatigue analysis of welded joints is reviewed, considering mainly papers and books published during the past 10–15 years. After a short introduction, the different approaches for fatigue analyses are covered, i.e. the nominal stress approach, the structural or hot-spot stress approach, the notch stress and notch intensity approach, the notch strain approach and finally the crack propagation approach. Only seam-welded joints are considered, and not the behaviour of spot-welds, which is a very special field. Due to the vast amount of relevant literature, some specific areas are left for other reviews or only touched, i.e. fatigue testing and evaluation, fatigue loading and variable amplitude effects, environmental effects and fatigue reliability.     

圆筒型FPSO上部模块支撑结构疲劳分析

与船型FPSO相比,圆筒型FPSO没有明显的总纵弯曲,上部模块与船体结构之间通常采用刚性支墩来连接,水平运动所产生的弯矩和装/卸载引起的船体垂向变形对模块支撑结构的影响较为显著。因此,以“希望6号”圆筒型FPSO上部模块支撑结构为研究对象,基于DNVGL船级社规范,介绍一种简化疲劳分析方法。以FPSO运动加速度和船体变形载荷作为载荷输入条件,利用SESAM/GeniE软件进行有限元分析,得到结构在所有组合工况下应力的扫描计算结果。根据作业海域各个方向波浪发生的概率,运用简化疲劳分析方法计算得到所关注节点的疲劳损伤和各个工况对结构节点疲劳损伤度的贡献。结果表明,所关注节点的疲劳强度均满足设计疲劳强度要求;同一节点的疲劳损伤对不同浪向的敏感度不一样。该简化疲劳分析方法同样适用于承受周期性载荷的FPSO上部模块主结构和其他型式海洋结构物的疲劳分析。     

Hull-girder reliability of new generation oil tankers

A series of new generation oil tankers is presently under construction. These ships differ from traditional oil tankers by their unusual form and therefore direct hydrodynamic analysis is used to determine design vertical wave bending moments instead of adopting IACS rule values. The purpose of the paper is to quantify changes in hull-girder reliability resulting from the new design features. To achieve this, first-order reliability analysis is carried out with respect to ultimate collapse bending moment of the midship cross section of a new generation oil tanker and of a conventional “rule” designed oil tanker. The stochastic model of wave-induced bending moment is derived from direct hydrodynamic analysis performed according to IACS Recommendation No. 34 Standard Wave Data, Rev 1, 2000. The probability distribution of the still water bending moment is assumed based on the data from loading manuals. The model uncertainties of linear wave loads, non-linearity of the response as well as load combination factors are included in the reliability formulation. The reliability analysis is performed for three relevant loading conditions: full load, ballast and partial load and for two states of the hull: the “as-built” hull and “corroded” hull according to anticipated 20-year corrosion. One of the most interesting conclusions from the study is that the annual hull-girder reliability of new generation oil tanker is increased considerably compared to the conventional oil tanker. Sensitivity and parametric studies are performed with respect to random variables representing modelling uncertainties. The results of a sensitivity study enable sorting of pertinent variables according to their relative importance, while parametric study is used to quantify changes in the reliability indices for moderate variation of input parameters. Furthermore, some other results and discussions are presented pointing out the benefits of introducing the ship reliability methods in design practice, especially if this refers to new designs.     

A design equation for evaluation of strain concentration factor in concrete coated X65 pipelines

An extensive parametric study using detailed nonlinear finite element (FE) models, was conducted in order to develop a design equation for predicting Strain Concentration Factor (SCF) in field joints of X65 concrete coated pipelines under bending. Dimensions of the pipeline and coating, material properties of the coating and anti corrosion layer, as well as the loading level, were included in the design equation. Buckingham's theorem was incorporated to simplify the equation by introducing non-dimensionalized parameters. The design equation was fitted to FE data using the nonlinear least square regression, while additionally, margins of safety were defined using the confidence interval concept for practical applications. The interactive effect of parameters on SCF was studied, leading to the introduction of a single non-dimensional “coating parameter” that can accurately describe the upper limit of the SCF. This study concluded with a definition of a critical “coating parameter” which can be used to assess safe combinations of coating thickness and installation loads, thus preventing excessive strain concentration.     

等壁厚钢管桩横向焊缝应力集中系数的数值模拟

大型海洋工程结构多常用钢管桩,在钢管桩的制作和安装过程中通常会产生环向对接焊缝,由管节偏位引起的环向焊缝应力集中现象是焊缝疲劳热点应力评估和焊缝疲劳安全分析的重要影响因素.通过实用近似分析方法,研究在轴向拉力和平面内弯矩单独作用下等壁厚对接焊缝应力集中间题,得出等壁厚拉弯构件对接焊缝应力集中系数的近似计算公式;根据对钢...     

在非线性波浪载荷作用下计及多种应力组合时船体总纵弯曲累积损伤计算

在时域内计算计及非线性时的船体总纵弯曲应力,与船体局部弯曲应力迭加后,可得纵向构件中合成应力的时历.对该应力时历进行雨流计数,经统计分析获得应力范围的长期分布,进而算出船体总纵弯曲时纵向构件在多种应力成份组合下的疲劳损伤.为方便此时的计算,本文提出了基于等效规则波概念的简化方法.数例表明按严格方法与按简化方法所得的结果比较接近.     

A toughness based deformation limit for fatigue-cracked X-joints under in-plane bending

This study reports a deformation limit for the initiation of ductile fracture failure in fatigue-cracked circular hollow section (CHS) X-joints subjected to brace in-plane bending. The proposed approach sets the deformation limit as the calculated crack driving force in a fatigue crack at the hot-spot location in the tubular joint reaches the material fracture toughness measured from standard fracture specimens. The calibration of the proposed approach and the numerical procedure utilizes two large-scale X-joint specimens with fatigue generated surface cracks. The subsequent numerical investigation covers X-joints with two different brace-to-chord intersection angles, a wide range of geometric parameters and a practical range of material parameters. The development of the deformation limit includes a non-dimensional material toughness, which covers both the geometric parameters and material properties. The lower-bound deformation limit thus developed exhibits a linear relationship with respect to the crack depth ratio and indicates consistent values among X-joints with different brace-to-chord intersection angles.     

浮式平台上部模块非管节点SCF控制方法

应力集中系数（Stress Concentration Factor，SCF）是评估节点疲劳损伤的重要参数。针对浮式平台上部模块非管节点的SCF控制，设计5种加强形式，采用Ansys软件对非管节点进行SCF分析。结果表明：筋板加强可有效改善面外弯矩作用下的应力集中；扣管加强对轴向力、面内弯矩和面外弯矩均具有较好的改善效果。     

Fatigue resistance of welded steel tubular X-joints

High-cycle fatigue experiments are performed on welded tubular steel X-joints, with braces and chord of equal diameter. They are scaled-down joints, used extensively in offshore wind platforms. Three different welding procedures are considered in specimen fabrication: manual, fully-automatic and manual with HFMI post-weld treatment. Τwo possible locations for crack initiation were identified: chord “crown” and “in-between location”, also verified by numerical calculations and fractography of failed specimens. Monotonic loading tests on fatigue-cracked specimens showed good performance in terms of ultimate strength and deformation capacity, despite the presence of through-thickness cracks. The results are compared with predictions from relevant design standards.     

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.     

双体船大开口角隅节点疲劳强度优化方法研究

与同样吨位的单体船相比,双体船具有更大的甲板面积、更大的舱容。在双体船大开口角隅位置,由于应力集中,更容易引起疲劳损伤的迅速积累。因此,在结构设计中,角隅位置的疲劳强度一直是工程界关心的部分。本文使用谱分析方法研究双体船角隅节点的疲劳强度。在疲劳校核基础上对角隅位置进行优化设计,研究增加板厚和增设肘板对角隅处疲劳强度的影响。在此基础上分析不同优化方法的适用性,为船舶设计与安全性能提供适当的建议。     

薄膜型LNG船全船结构屈服和疲劳强度分析

以某薄膜型液化天然气（Liquefied Natural Gas,LNG）船的结构设计为例,开展全船屈服强度校核和基于精细网格的有限元疲劳强度分析。针对5种典型装载状态,基于美国船级社（American Bureau of Shipping,ABS）全船强度直接计算指南,采用ABS-DLA/SFA系列软件,用三维波浪载荷预报程序对波浪随机载荷进行长期预报。基于预报结果,针对每种装载状态计算15个设计波参数组,求解全船结构在各载荷组合工况下的应力分布,继而完成屈服强度校核。以甲板机械室与穹顶甲板相交处的关键节点区域的节点设计为例开展细网格局部强度分析,并通过各种改进设计解决应力集中问题。针对2种常用典型操作装载状态及营运于北大西洋海区疲劳寿命满足40a的要求,基于ABS全船疲劳强度直接计算指南计算2个典型细化位置热点应力传递函数,通过谱分析得到疲劳累积损伤和疲劳寿命,完成疲劳强度校核。采用的全船强度和疲劳分析方法和思路适用于其他超大型船舶的结构分析。     

Geometric stress distribution along the weld toe of the outer brace in two-planar tubular DKT-joints: Parametric study and deriving the SCF design equations

Regarding the research efforts expended so far on the calculation of stress concentration factors (SCFs) in tubular joints, two major shortcomings can be noted: (a) significant effort has been devoted to the study of SCFs in various uni-planar connections. Nevertheless, for multi-planar joints which cover the majority of practical applications, very few investigations have been reported due to the complexity and high cost involved; (b) majority of these research works focused on the study of SCFs at certain positions such as the saddle, crown toe, and crown heel, and they have ignored the hot-spot stress (HSS) at other positions along the weld toe. In the present paper, effects of dimensionless geometrical parameters on the SCF distribution along the weld toe of main (outer) braces in the axially loaded right-angle two-planar tubular DKT-joints are investigated. In order to study the multi-planar effect, SCF distribution in two-planar joints is compared with the distribution in a uni-planar joint having the same geometrical properties. A complete set of SCF database is constructed based on the two-planar DKT-joint Finite element models which are verified against experimental results and the predictions of Lloyd’s Register (LR) equations. The FE models cover a wide range of geometrical parameters. Six new SCF parametric formulae are developed through nonlinear regression analyses for the accurate and reliable fatigue design of two-planar DKT-joints under axial loads. An assessment study of these equations is conducted against the experimental data, the original FEM database and the acceptance criteria recommended by the UK Department of Energy.     

Fracture mechanics analysis for mooring chain links subjected to out-of-plane bending (OPB)

A fracture mechanics (FM) based investigation on the mechanism of out-of-plane bending (OPB) between mooring chain links and its effects on fatigue lives of mooring chain links are conducted. Four types of OPB problems that mooring chain links laying on the chain wheel, chain links passing over the bending shoe, chain links constraint provided by the chain hawse, and chain links constraint provided by the chain stopper are considered. Tension ranges of mooring lines are calculated based on the combined loading process induced by the motions of wave frequency (WF) and low frequency (LF). Initial cracks are assumed to propagate from surfaces of chain links and stress intensity factors are calculated in terms of stress ranges determined by a finite element (FE) analysis. The results show that fatigue lives of mooring chain links are decreased significantly due to OPB effects. In addition, the increase of the number of pockets of chain wheel mitigates OPB effects on fatigue lives of mooring chain links laying on the chain wheel, and the increase of the track diameter would reduce OPB effects on fatigue lives of mooring chain links passing over the bending shoe as well. However, for chain links constraint provided by the chain hawse, the diameter of mooring chain hawse has no significant effect on fatigue lives of the mooring chain links subject to OPB if without the abrupt change of the contact conditions between chain links and chain hawse. For mooring links constraint provided by chain stoppers considering the effect of proof loading test, fatigue lives of mooring chain links would drop significantly with the increase of interlink angles and friction coefficient.