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.     

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.     

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.     

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.     

Design and analysis of tapered stress joint for Top Tensioned Riser system

Stress Joint(SJ) plays a key role in the Top Tensioned Riser(TTR) system for deep water engineering.A preliminary design method of tapered SJ is proposed in the paper,which could help designers obtain accurate design data.After a further sensitive analysis is carried out,the related parameters choice and control methods are recommended in the engineering practice.By taking the extreme environment conditions into consideration,the effects of bending stress reduction and curve control are analyzed,and the 3-D FE models are established by ABQOUS for numerical evaluation to verify the correctness of design results.At last,dynamic analysis and fatigue analysis,based on actual project,are carried out with designed stress joint.The analysis results prove the feasibility and guidance of this method in the practical engineering applications.     

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。     

Regression Formulations for the Stress at Hot Spot of Multiplanar Tubular DT Joints

- Stress concentration analysis of multiplanar tubular DT joints plays an important role in the fatigue design of offshore platforms. A semi-analytic method for stress analysis under the condition of any loads is briefly introduced in the paper. Nineteen multiplanar tubular DT joints with one of two braces of the same dimension subjected to axial loads and out- of- plane bending moments are computed for parametric stress analysis by using the present method. The influence of geometrical parameters on the stresses of multiplanar tubular DT joints is discussed and compared with corresponding uniplanar T joints. The regression formulae for the stress at hot spot of multiplanar DT joints are found by modification of SCF of corresponding uniplanar T joints. The parametric formulae for the maximum stress by superposition. Finally, the influences of stiffening effect and load-interaction effect on the maximum stress of DT joints are discussed.     

Non-symmetric cyclic bending of helical wires in flexible pipes

Unbonded flexible pipes have superior fatigue performance as the internal armor layers are allowed to move relative to each other, leading to reduced structural loading. The main interactions between the internal layers are the contact forces and the frictional forces. Frictional interaction leads to a complex non-linear response of unbonded flexible pipes making prediction of cyclic bending fatigue a demanding task. Nevertheless, detailed understanding of local armor wire stresses and the related fatigue phenomena is of paramount importance as unbonded flexible risers are often operated close to their mechanical limits. This paper presents a method for calculating the tensile armor wire loading and the hysteresis effect on flexible pipes when subjected to tension, and non-symmetric cyclic bending. The effect of non-symmetric cyclic bending with different tensile armor lay angles, and frictional conditions are studied. The analysis uses an efficient repeated unit cell finite element model, allowing the analysis to be performed on a desktop computer. The study shows that the tensile armor wires gradually translate towards the compression side of the pipe bending plane, when the unbonded flexible pipe is subjected to combined tension and non-symmetric cyclic bending. In the analysis, cyclic bending is applied until steady-state in the bending response is achieved over a full bending cycle. The global bending response of the flexible pipe and the tensile armor wire loading conditions for fully stabilized non-symmetric cyclic bending become symmetric around the frictionless state for the mean cyclic bending curvature. An approximate analytic model for the tensile armor in the stabilized cycle, based on symmetric bending about the geodesic curve corresponding to the mean pipe curvature, is proposed.     

Validity and limitation of analytical models for the bending stress of a helical wire in unbonded flexible pipes

Unbonded flexible pipe is one of the important pieces of equipment in floating production systems for transport of oil and gas between floaters and subsea wells located in deep water. To assure safety over a long-term service period, analysis of fatigue behavior under alternative bending is a key requirement. An analytical model for prediction of bending behavior is essential for understanding the mechanism of the local stress distribution in the helical wires of the tension armor layers under alternative curvatures and rapid estimation of the service life of flexible pipes for designers in engineering practice. In this paper, seven analytical models available in the literature are selected and summarized. Although the experimental results reported in the literature are limited, a three-dimensional finite element model is developed for investigation of the seven models, and the validity and limitations of these models for different structural parameters of helical wire are discussed, i.e., the width-thickness ratio of the wire section and helical angle. The analytical model based on spring theory resulted in a satisfactory estimation of bending stress for most cases and is recommended as a tool for the basic design of the helical armor wire structures in flexible pipes.     

基于监测数据的深水水下井口循环弯矩计算方法

水下井口的疲劳完整性是海洋油气田长期安全开采的前提。工业界往往采用贴应变片直接测量水下井口应变来计算弯矩和疲劳损伤,但水下井口应变片粘贴困难且不能长时间连续工作。采用对水下井口监测方法,基于隔水管—防喷器组—水下井口的运动和力学特性,考虑防喷器组惯性力矩建立系统耦合动力学方程,最终形成基于监测数据的水下井口循环弯矩计算方法。以南海某深水水下井口为例,建立隔水管—防喷器组—水下井口系统有限元模型并进行动态分析,提取隔水管底部张力、转角、防喷器组加速度及转角等参数,代入所建立的系统耦合动力学模型,得到水下井口弯矩与有限元计算结果吻合良好。研究表明只需通过在线监测获得所需的输入数据,无需监测水下井口应变即可获取水下井口循环弯矩。建立的系统耦合动力学模型可为水下井口疲劳完整性评估提供理论依据。     

Multiaxial fatigue analyses of stress joints for deepwater steel catenary risers

In the present study, the dynamic and fatigue characteristics of two types of stress joints are investigated under ocean environmental condition. Connected with the riser and the platform, stress joint at the vessel hang-off position should be one of the main critical design challenges for a steel catenary riser (SCR) in deepwater. When the riser is under a high pressure and deepwater working condition, the stress state for the joint is more complex, and the fatigue damage is easy to occur at this position. Stress joint discussed in this paper includes two types: Tapered Stress Joint (TSJ) and Sleeved Stress Joint (SSJ), and multiaxial fatigue analysis results are given for comparison. Global dynamic analysis for an SCR is performed first, and then the local boundary conditions obtained from the previous analysis are applied to the stress joint FE model for the later dynamic and multiaxial fatigue analysis. Results indicate that the stress level is far lower than the yield limit of material and the damage induced by fatigue needs more attention. Besides, the damage character of the two types of stress joints differs: for TSJ, the place where the stress joint connects with the riser is easy to occur fatigue damage; for SSJ, the most probable position is at the place where the end of the inner sleeve pipe contacts with the riser body. Compared with SSJ, TSJ shows a higher stress level but better fatigue performance, and it will have a higher material cost. In consideration of various factors, designers should choose the most suitable type and also geometric parameters.     

套管加强T型管节点在平面外弯矩作用下极限强度分析

对用套管加强的T型管节点在平面外弯矩作用下的极限强度作了参数化分析。同时将相应的套管以及管节点的几何参数对极限强度的影响进行评估,并且和未加强的T型管节点作了比较。在计算结果中可以得出在管节点的弦管和撑管的连接处用合适的套管加强,可以将管节点在平面外弯矩作用下的极限强度提高到3倍多。     

海洋柔性立管弯曲加强器参数敏感性分析

针对海洋立管弯曲加强器所受环境载荷与功能载荷的特点,基于有限元法对某海洋柔性立管弯曲加强器结构进行数值分析。通过有限元分析软件建立了弯曲加强器与柔性立管组合等效模型,重点讨论了弯曲加强器的设计参数对其自身防弯性能的影响,根据弯曲加强器的曲率及柔性立管的应力分布,对弯曲加强器结构进行优化设计,得到结论可供设计参考。     

Fatigue analysis of offshore well conductors: Part I – Study overview and evaluation of Series 1 centrifuge tests in normally consolidated to lightly over-consolidated kaolin clay

An important aspect of deepwater well integrity assurance is conductor fatigue analysis under environmental loads acting on the riser system during drilling operation. Fatigue damage arises from stress changes in a structure due to cyclic loading. In practice, the lateral cyclic soil response is typically modelled using Winkler p–y springs. An appropriate soil model for conductor–soil interaction analysis is the one based on which the absolute magnitudes of stresses and their changes can accurately be predicted for well integrity evaluation. The API recommendations for p–y curves, which are often used for conductor–soil interaction analysis, have originally been developed for piled foundation and are inappropriate for well fatigue analysis. To that end, an extensive study involving four series of centrifuge model tests and FE numerical analyses was conducted to fundamentally study conductor–soil interaction under a wide spectrum of loading conditions. The tests simulated conductor installations in normally to over consolidated clays, and medium-dense clean sands. Soil models were developed specifically for conductor fatigue analysis for each of the soil types. The test results and soil models are presented in two papers. The first paper, Part I, presents an overview of the study and first series tests in normally to lightly over-consolidated kaolin clay and discusses the observations made with regards to monotonic and cyclic soil resistances and their relationship to conductor fatigue modelling. The second paper, Part II, presents centrifuge test results in normally to lightly over-consolidated Golf of Mexico (GoM) clay, over-consolidated natural clay and medium-dense clean sands along with the respective soil models developed for conductor fatigue damage prediction. Overall, the accuracy of fatigue life predictions using these novel soil models is very high – generally within about a few percentage of the measured values.     

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.     

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。     

FEM study on the stress concentration factors of K-joints with welding residual stress

K-joint is widely used on offshore platforms; its strength is very important for safety evaluation of platforms and the welding residual stress is inevitable. According to the thermo-effect of welding process, based on ANSYS, using the birth-death element technology, numerical simulation was performed for the welding process by coupling the temperature field and stress field. The nonlinear changes of material properties were considered and the distribution of temperature and stress was obtained. Considering that annealing treatment is generally performed for K-joint after welding, the process of annealing treatment was also simulated in this paper and the influence of it on welding residual stress was analyzed. Based on this, axial loads were applied on two different K-joint models, one with welding residual stress and the other without welding residual stress while just considering the influence of welding seam structure on the K-joint. The stress concentration factors (SCFs) of these two models were calculated and thus the influence of the welding residual stress on stress concentration factor was analyzed. Furthermore, considering the influence of welding residual stress, 105 models of K-joints with different geometric parameters and loading conditions were generated and the effects of them on the SCFs were discussed, results presented in this paper provide references for safety analysis of K-joints.     

在平台振荡条件下TLP张力腿的涡激非线性响应

给出了预测张力腿涡激横向振动的时域分析,考虑了波浪、海流、张力腿平台的横漂与垂荡诸因素.张力腿平台的垂荡引起张力周期性变化,对张力腿的涡激横向振动起参数激励的作用,使其动力响应更为复杂.讨论了参数激励下涡激响应的共振条件;以尾流振子模型为基础,发展了分析张力腿在动张力作用下的涡激振动的工程方法,并分析了一座TLP实例,给出了在横漂周期内张力腿上质点的相对位移、弯矩、剪力幅值的时间历程.结果表明,平台垂荡使张力腿的涡激响应幅值升高、高频成分增加,因此它对张力腿的疲劳设计是重要的.     

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.     

Dynamic stress analysis of the leg joints of self-elevating platform

Since a self-elevating platform often works in water for a long time,the lattice leg is largely influenced by wave and current.The amplitude of leg joint stresses is a very important factor for the fatigue life of the platform.However,there are not many researches having been done on the mechanism and dynamic stress analysis of these leg joints.This paper focuses on the dynamic stress analysis and suppression methods of the leg joints of self-elevating platforms.Firstly,the dynamic stresses of the lattice leg joints are analyzed for a self-elevating platform by use of the 5th-order Stokes wave theory.Secondly,the axial and bending stresses are studied due to their large contributions to total stresses.And then,different joint types are considered and the leg-hull interface stiffness is analyzed for the improvement of the joint dynamic stress amplitude.Finally,some useful conclusions are drawn for the optimization design of the self-elevating platform.