The Stress Concentration of Multiplanar Tubular Joints of Offshore Structure

- The formulation for the stress concentration matrix of multiplanar tubular joint with braces subjected to combined load is established in this paper. The formulations for SCF values of multiplanar tubular joints are compared with those by Wordsworth and Efthymiou. The stiffening effect and load-interaction effect are discussed. A spatial offshore jacket with braces subjected to combined load is computed as a numerical example. The maximum stress of each multiplanar tubular joint and its location are found through SCF matrix. Finally, the difference between the maximum stresses of multiplanar tubular joints and their corresponding uniplanar tubular joints are discussed.     

Local Joint Flexibility Multibrace Tubular Joints

The local joint flexibility matrix of multibrace tubular joints (uniplanar and multiplanar joints) is defined. The formulation for computing the elements of local flexibility factor matrix of multibrace tubular joints by semi- analytic method is presented in this paper. The stiffening effect of unloaded braces and cross-flexibility between braces are discussed. Using the local flexibility of unibrace joints instead of that of multibrace joints in conventional structural analysis will lead the result to an unsafe side.     

Stress Analysis of Spatial Non-Concurrent Tubular Joints

The formulations of analytic-numerical method for the stress analysis of non-concurrent spatial tubular joints are introduced in the paper. The spatial DT joints with different eccentricity in the vertical diametrical plane of chord are computed. Finally the influence of eccentricity on the stress at possible hot spots is discussed.     

Parametric Analysis and Experimental Study on Local Flexibility of TY-Type Tubular Joints

Loal flexibility of tubular joints has important effect on the static and dynamic behaviour of offshore platforms, therefore, the determination of it becomes an important research subject in the field of offshore engineering. In this paper, the local flexibility of TY-type tubular joints, which are widely used in offshore platforms, is calculated by using semi- analytical method. Based on the calculated results, parametric formulae for evaluating element in the local joint flexibility matrix of TY- type tubular joints are derived by regression. A test on PVC models of TY-type tubular joints to measure the local joint flexibility is also reported. A comparison of the results calculated from the parametric formulae presented in this paper with those measured from the model test shows that the parametric formulae are reliable. It is recommended that these formulae be used in the global structural analysis of offshore platforms.     

A study on the Local Joint Flexibility (LJF) of two-planar tubular DK-joints in jacket structures under in-plane bending loads

In the present paper, results of a parametric study conducted on the Local Joint Flexibility (LJF) of two-planar tubular DK-joints under In-Plane Bending (IPB) loads are presented. DK-joints are among the most common joint types in jacket substructure of Offshore Wind Turbines (OWTs). A total of 324 finite element (FE) analyses were carried out on 81 FE models under four types of IPB loading in order to investigate the effect of the DK-joint’s geometrical parameters on the LJF factor (f_LJF). Based on the results of parametric study, the factors leading to the LJF reduction were introduced. Generated FE models were verified using the existing experimental data, FE results, and parametric equations. The effect of the weld profile was also considered. The f_LJF in two-planar DK- and uniplanar K-joints were compared. Results indicated that the effect of multi-planarity on the LJF is quite significant and consequently the use of the equations already available for uniplanar K-joints to calculate the f_LJF in two-planar DK-joints may lead to highly under-/over-predicting results. To handle this issue, the FE results were used to derive a set of parametric equations for the prediction of the f_LJF in IPB-loaded two-planar DK-joints. The proposed equations were checked against the acceptance criteria recommended by the UK DoE and can be reliably used for the analysis and design of tubular joints in OWTs.     

Parametric Equation of Stress Concentration Factor for Circular X-Joints Under Axial Loads

In engineering practice,tubular X-joints have been widely used in offshore structures.The fatigue failure of tubular X-joints in offshore engineering is mainly caused by axial tensile stress.In this study,the stress concentration factor distribution along the weld toe in the hot spot stress region for tubular X-joints subject to axial loads have been analyzed by use of finite element method.Through numerical analysis,it has been found that the peak stress concentration factor is located at the saddle position.Thereafter,80 models have been analyzed,and the effect of the geometric parameters of a tubular X-joint on the stress concentration factor has been investigated.Based on the experimental values of the numerical stress concentration factor,a parametric equation to calculate the stress concentration factor of tubular X-joints has been proposed.The accuracy of this equation has been verified against the requirement of the Fatigue Guidance Review Panel,and the proposed equation is found capable of producing reasonably accurate stress concentration factor values for tubular X-joints subject to axial loads.     

TY型管节点局部柔度的参数分析及试验研究

由于海洋平台管节点的局部柔度对平台结构的静动力性能有重要影响，如何确定各类管节点的局部柔度成为一个重要的研究课题。本文用半解析方法计算了海洋平台中一类常见的管节点──ＴＹ型管节点的局部柔度，并用回归方法得出了计算ＴＹ型管节点局部柔度矩阵中各元素的参数公式。本文还介绍了对一组ＴＹ型管节点ＰＶＣ塑料模型进行的局部柔度试验。参数公式计算结果与模型试验结果的比较表明，本文得出的参数公式是基本可靠的，建议可在海洋平台总体结构分析中使用。     

An Improved Formulation for the Ultimate Static Strength of Tubular Joints subjected to Combined Loads

"The effect of interaction of loads on the ultimate static strength of tubular joints of offshore fixed platforms, is a practical problem. But there is still absence of rigorous theory to explain available experimental data and empirical criteria for the static strength of tubular joints. The idea of yield at hot spot of tubular joints is introduced in this paper. The interaction equations of plastic capacity for the tubular joints under combined loads (two and three different kinds) are derived. Thereafter the Yura's test data and empirical criteria of ultimate static strength for the tubular joints can be explained. The idea of classification of category of loads in accordance with experimental data and the present theory is suggested. Finally, the improved ultimate capacity equations for tubular joints are recommended. The physical significance of the coefficient of plastic reservation Qp is discussed.     

KK型管节点应力集中系数几何敏感性分析

KK型管节点是自升式平台桁架式桩腿中的一种管节点，其应力集中系数是影响桩腿疲劳寿命的重要参数。应力集中系数与管节点的几何形式密切相关，为分析KK型管节点应力集中系数对几何参数的敏感性，利用ANSYS软件对某KK型管节点进行几何参数化建模，利用有限元数值模拟方法对各工况下的热点应力进行分析，并分别计算各相应工况下的名义应力，然后将热点应力与名义应力相比得到不同几何参数下的热点应力集中系数。对计算结果进行整理分析，得到了KK型管节点应力集中系数对无量纲几何参数的敏感性规律。结果表明，应力集中系数与撑杆受力状态、管节点结构形式有关，在满足结构布置、建造工艺和其他安全性指标的前提下，分析结果能够为KK型管节点的结构设计和疲劳分析提供技术支撑。     

Fatigue Test of Homemade Z Direction Steel Welded T Tubular Joints

This paper introduces the process and result of fatigue test of steel (Z direction steel) welded T tubular joints used in offshore engineering. Detailed measurement of stress concentration factor, stress distribution, fatigue life and crack development has been performed. Through analysis, an empirical formula of stress concentration factor for T tubular joints, fatigue S-N curve and crack propagation rule are obtained.     

Parametric study of geometrical effects on the degree of bending (DoB) in offshore tubular K-joints under out-of-plane bending loads

In this paper, results of comprehensive finite element (FE) analyses carried out on 81 generated models of tubular K-joints are presented. Results of FE models were verified against experimental data and parametric equations. The fatigue life of a tubular joint, which is commonly found in offshore industry, is not only dependent on the value of hot-spot stress (HSS), but is also significantly influenced by the through-the-thickness stress distribution characterized by the degree of bending (DoB). Data extracted from FE analyses was used to study the effects of geometrical parameters on the DoB values in tubular K-joints subjected to two types of out-of-plane bending (OPB) moment loads. The determination of DoB values in a tubular joint is essential for improving the accuracy of fatigue life estimation using the stress-life (S–N) method and particularly for predicting the fatigue crack growth based on the fracture mechanics (FM) approach. Geometrically parametric investigation was followed by a set of nonlinear regression analyses to develop DoB parametric formulas for the fatigue analysis of K-joints under OPB loadings.     

Truss Spar平台桁架管节点疲劳分析

Truss Spar平台在风、浪、流的作用下,结构受力十分复杂,准确分析其疲劳强度相当困难。通过建立Truss Spar平台三维有限元模型,利用HydroD软件进行载荷预报、波浪搜索,得到产生最大应力幅响应的最不利浪向和相位角。运用Sestra软件进行整体有限元分析,得到的结果作为桁架管节点细化模型的边界条件,再用Nastran软件进行局部分析求得节点的热点应力,修正壁厚影响。最后根据DNV规范的S-N曲线计算了管节点的疲劳寿命。     

Probabilistic analysis of the DoB in axially-loaded tubular KT-joints of offshore structures

The degree of bending (DoB) characterizing the through-the-thickness stress distribution has a profound effect on the fatigue behavior of tubular joints commonly found in steel offshore structures and the determination of DoB values is essential for improving the accuracy of fatigue life estimation. Probability density functions of the involved random variables are necessary for the fatigue reliability analysis of offshore structures. The objective of present research was the derivation of probability density function (PDF) for the DoB in tubular KT-joints commonly found in jacket-type offshore platforms. A total of 162 finite element (FE) analyses were carried out on 81 FE models of KT-joints subjected to two types of axial loading. Generated FE models were validated using experimental data, previous FE results, and available parametric equations. Based on the results of parametric FE study, a sample database was prepared for the DoB values and density histograms were generated for respective samples based on the Freedman-Diaconis rule. Thirteen theoretical PDFs were fitted to the developed histograms and the maximum likelihood (ML) method was applied to evaluate the parameters of fitted PDFs. In each case, the Kolmogorov-Smirnov test was used to evaluate the goodness of fit. Finally, the Generalized Extreme Value model was proposed as the governing probability distribution function for the DoB. After substituting the values of estimated parameters, six fully defined PDFs were presented for the DoB in tubular KT-joints subjected to two types of axial loading.     

Fatigue Behaviour of Tubular Joints Made of China-Made Platform Steel

In China, a nationwide research program on fatigue behaviour of tubular joints made of China-made platform steel was carried out from 1983 to 1990, which was set by the Tubular Joint Research Committee for Offshore Structures (TJCOS) of China. This paper gives an outline of the basic properties of the China-made platform steel and fatigue test of tubular joints, including test specimens, test particulars and test results. A p-S-N curve based on the test results was established which reveals that the fatigue strength of tubular joints is fairly good. The thickness effect and effect of corrosion on fatigue strength in sea water are briefly discussed.     

基于谱分析法的深水半潜式平台疲劳强度分析

以某新型深水半潜式钻井平台为分析对象,依据三维绕射理论计算单位波幅下波浪诱导载荷,通过建立三维有限元模型计算完成了热点应力响应。运用热点应力的谱分析法计算得到危险节点的疲劳寿命,并进一步分析了各个短期海况和不同浪向对节点总的疲劳损伤度的贡献。研究结果表明,危险节点的疲劳寿命都满足设计寿命要求;在各种中低海况下,疲劳损伤主要来自于平均跨零周期在6 s附近,有义波高转大的短期海况;平台关键连接部位的疲劳损伤主要是在横浪和斜浪状态下产生的,并且对某单一方向的波浪非常敏感。此研究对该类型平台的连接处结构设计和疲劳分析方法有参考意义。     

Stress concentration factors (SCFs) of bulge formed K-joints under balanced axial loads

A novel stiffened joint called bulge formed joint was put forward. Compared with the unstiffened joint, an additional bulge plate is utilized to connect the chord and braces. Based on the finite element (FE) and experimental method, stress concentration factors (SCFs) were investigated for both a bulge formed K-joints and the corresponding unstiffened joint. By the verification of experimental data, the FE models were used to investigate the SCFs of the bulge formed joints. The maximum SCF of the bulge formed K-joint under balanced axial loads is located at the position φ = 105°, which is close to but not exactly the saddle. The SCFs around the intersection weld can decrease remarkably compared with the unstiffened joint by the change of geometrical parameters of the bulge plate. Then 2117 FE analyses were conducted to investigate the geometrical effects on SCFs of the bulge formed joint. These dimensionless geometrical parameters include τ_s, η₁, η₂, θ, β, γ, τ, among which, the first three parameters are typical of the bulge formed joints, while the others are same as the definitions in the unstiffened joints. Finally, a set of SCF parametric formulas were obtained by nonlinear regression analyses.     

张力腿平台局部节点强度可靠度分析方法

局部节点可靠度分析对张力腿平台(TLP)整体安全性评估具有重要意义。提出了一个TLP平台局部节点可靠度的分析方法,首先对TLP平台整体进行分析,统计局部节点各种失效模式并得到典型失效模式及对应的典型荷载工况;然后应用改进的子模型技术对TLP平台局部节点进行极限承载能力分析,确定了局部节点在典型荷载工况下的极限承载力;对局部节点极限承载力及南海某区域的波浪荷载进行统计分析,得到其概率统计特性;在此基础上计算了TLP平台局部节点对应典型失效模式下的可靠度指标。通过算例分析表明,该TLP平台各节点可靠度指标值均大于3.1;立柱与浮箱连接节点在不同荷载工况下失效模式不同,而立柱与甲板连接节点易发生管交汇处剪切破坏。为TLP平台的安全性评估提供了一种有效的分析方法,具有一定的理论价值及实用价值。     

Optimum design of ice-resistant offshore jacket platforms in the Bohai Gulf in consideration of fatigue life of tubular joints

Economy and ice-resistance are two factors that must be considered in the design of offshore oil platforms in the Bohai Gulf. This paper focuses on the optimal design of ice-resistant offshore jacket platforms in the Bohai Gulf in consideration of fatigue life of tubular joints, with the minimum initial weight and satisfying dynamic properties. An efficient fatigue analysis procedure based on spectral analysis is proposed, in which a hybrid finite element model is adopted to simulate the jacket platform and the tubular joints, and the pseudo-excitation method is used to calculate the power spectral density of the hot spot stress. Finally, a practical jacket platform with an ice-breaking cone in the Bohai Gulf is optimized, and the results demonstrate that the fatigue life of tubular joints could be improved and the weight of jackets decreased simultaneously.     

安全叠接管节点在低周循环载荷下的破坏模式研究

采用实验和数值模拟方法研究了一种新型海洋平台管节点在地震载荷下的响应和破坏情况。采用拟静态循环加载模拟地震载荷 ,对真实尺寸的试件进行实验 ,研究局部屈曲和裂纹等对管接点抗震性能的影响。能量分析表明 ,发生在斜支管中的局部屈曲消耗了大部分输入能量。与现在使用的N型管节点相比 ,该新型管节点有较好的抗震性能。采用有限元数值模拟方法分析比较了不同尺寸配置下 ,该型管节点的响应曲线     

非常规管节点疲劳寿命分析与计算

对非常规管节点的疲劳寿命进行了分析研究，用SESAM计算疲劳载荷，应用精细有限元分析计算热点应力，用规范的S—N曲线计算管节点的疲劳寿命。通过实例计算表明，这些处理对非常规管节点疲劳寿命分析与计算是很有效的。