Fatigue Life Of Tubular Joints Research Articles

Stress Concentration Factors of Concrete-Filled Double-Skin Tubular K-Joints

Tubular joints are important connecting parts of a welded steel tube structure. The S-N curves based on the hot spot stress (HSS) method are often used to evaluate the fatigue life of tubular joints in practical engineering. The stress concentration factor (SCF) is a key parameter to calculate HSS. In this paper, stress concentration tests of hollow-section and concrete-filled double-skin tubular (CFDST) K-joints were carried out, respectively, and then finite element models of K-joints considering the weld were established. The developed models were validated with the experimental results. The influence of key geometrical parameters, such as the diameter ratio of brace to chord β, the diameter to thickness ratio of chord γ, the wall thickness ratio of brace to chord τ, brace angle θ, and hollow section ratio ζ on the distribution and key position of SCFs along the weld toe, was discussed. Parametric studies were conducted to obtain the calculating equations for the SCF values of CFDST K-joints. The results demonstrate that infill concrete can effectively reduce SCFs along the weld on the chord. When the hollow section ratio was reduced to 0.317, the SCF was reduced by 77.2%. Notably, the SCF reduction rate was sensitive to γ and θ, with a decrease observed as γ increased. The hollow section ratio ζ had a less pronounced effect on SCF distribution patterns, but as ζ decreased, the chord’s stiffness improved, suggesting a potential approach to enhance joint performance. The distribution of SCFs is similar for joints of the same type but different geometric configurations. The innovatively integrated hollow section ratio in the CFDST design equation significantly simplifies and enhances the precision of SCF calculations for CFDST K-joints.

Fatigue tests and design of Q460 CHS-CHS T-joints under axial loading and in-plane bending

Previous research has primarily focused on stress concentration factors (SCFs) and fatigue life of tubular joints made from low-strength steel (nominal yield strength of 235–360 MPa). The fatigue behavior of high-strength tubular joints remains insufficiently studied. In this study, a total of sixteen CHS-CHS T-joints fabricated from Q460 steel were designed, eight of which were subjected to axial load and eight of which were under in-plane bending. Static and fatigue tests were conducted to analyze the distribution of hot spot stress, fatigue crack propagation, and fatigue failure modes. The experimental results showed that the crack initiation consistently occurred at location of maximum hot spot stress for all specimens. The maximum SCFs were concentrated at the saddle (90°) for specimens under axial loads and around the position of 22.5° for those under in-plane bending. The modified SCF design formula for in-plane bending was proposed based on regression analysis. Prior to crack initiation, all specimens exhibited a relatively slow stiffness degradation due to cumulative damage from fatigue loading. Through least squares analysis of the test data, the fatigue design Srhs-N curve for Q460 CHS-CHS T-joints was developed. The proposed Srhs-N curve is capable of accurately predicting the fatigue life of Q460 CHS-CHS T-joint subject to axial loads and in-plane bending. Comparison with previous research and existing design guidelines shows that under the same stress amplitude level, the fatigue lives of CHS-CHS T-joints subjected to axial loading of Q460 steel are comparable to that of low-strength steel (WFG36Z). Under in-plane bending, the fatigue lives for Q460 steel are higher than that for low-strength steels (C350L0, t < 4 mm).

Increasing the fatigue life of offshore wind turbine jacket structures using yaw stiffness and damping

In this study, an appropriate proportion of yaw stiffness and damping is proposed to greatly improve the fatigue life at the tubular joints of offshore wind turbine (OWT) jacket support structures. The fatigue analyses using the DTU 10-MW OWT jacket support structure under 1148 IEC-61400-3 loads indicated that the torsion of rotor blades under turbulent wind conditions is the most important load leading to fatigue damage to tubular joints. Other loads, such as the moments generated from rotor blades and wave loads, are less important than the torsion. A Guyan reduction two-degrees-of-freedom model and eleven full finite element models indicated that low stiffness and large damping of the yaw system can significantly increase the fatigue life of tubular joints, and the yaw angular velocity will not increase. However, it is necessary to address the restrictions in the yaw rotation and to avoid Fn3P resonance (Integer multiples of 3P) with the first natural frequency of the structural vertical rotation.

Evaluating frequency domain methods for fatigue analysis of fixed jacket type offshore platform

Fatigue damage in offshore platforms is a major issue since they are exposed to periodic wave loads. Frequency domain methods of fatigue damage calculation are more efficient than time domain method, but they usually provide inaccurate and conservative results. Several alternatives have been developed to modify the results of frequency domain methods. In this paper, accuracy of these methods for application to fatigue analysis of tubular joints of fixed offshore platforms is evaluated by studying a typical platform in the Persian Gulf, considering various sea states and structural dynamic characteristics. The results are compared with those of time domain analysis using rainflow count method. They show that the frequency domain methods generally present conservative results, when the bandwidth parameter of stress spectrum is greater than 0.7. It is also found that the slope of S-N curve has a significant impact on the results. Furthermore, among the existing methods, Dirlik and Tovo-Benasciutti ones while being conservative, provide relatively most accurate results for fatigue life of tubular joints.

Prediction of Tubular T/Y-Joint SIF by GA-BP Neural Network

In fracture mechanics, the fatigue evaluation depends on the precise provisions of the stress intensity factor (SIF), which is a quantitative factor established to evaluate the effect of stresses along the crack front. Stress intensity factor performs a definitive role in reflecting material fracture and estimating fatigue life of tubular joints. Some researchers have proposed a series of parametric equations to predict SIF values; however, it gave arguably inaccurate results. There is still a need to improve the accuracy of the SIF prediction's equation. Thus, to overcome this shortage, this study introduced back-propagation (BP) neural network with conjunction to genetic algorithm (GA) (GA-BP) in predicting the SIF of cracked tubular T/Y-joint. To train and verify the results of the hybrid algorithm GA-BP SIFs' database covers a wide variety of cracked tubular T/Y joints were simulated by ABAQUS and verified against experimental results. Meanwhile, the SIFs' result produced by the GA-BP optimization method were compared with those SIF calculated from the accessible parametric equations. The comparison indicated that the GA-BP neural network optimization method is reliable, precise and capable tool in calculating the SIFs of cracked T/Y joints and it also provides higher accuracy than common methods.

Effects of geometrical parameters on the degree of bending (DoB) in two-planar tubular DT-joints of offshore jacket structures subjected to axial and bending loads

The fatigue life of tubular joints commonly found in offshore structures 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). In the present research, data extracted from the finite element (FE) analyses carried out on 81 two-planar tubular DT-joint models was used to study the effects of geometrical parameters on the DoB values in DT-joints subjected to six different types of loading including two types of axial loading, two types of in-plane bending (IPB) moment loading, and two types of out-of-plane bending (OPB) moment loading. Generated FE models were validated using available parametric equations. The determination of DoB values in a tubular joint is essential for improving the accuracy of fatigue life estimation using the stress-life (SN) 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 three DoB parametric formulas for the fatigue analysis and design of two-planar DT-joints.

Fatigue life estimation of tubular joints – a comparative study

AbstractIn fatigue analysis, the structural detail of tubular joint has taken great attention among engineers. The DNV/GL‐RP‐0005 is covering this topic quite well for simple and clear joint cases. For complex joint and geometry, where joint classification is not available and there is limitation on validity range of non‐dimensional geometric parameters, the challenges become a fact among engineers. The classification of joint is an important factor to consider in fatigue analysis. These joint configurations are identified by the connectivity and the load distribution of tubular joints. To overcome these problems to some extent, this paper compares the fatigue life of tubular joints in offshore jacket according to the stress concentration factors (SCF) in DNV/GL‐RP‐0005 and finite element method employed in Abaqus/CAE. The paper presents the geometric details, material properties and load history of the considered jacket structure. It then describes the global structural analysis and identification of critical tubular joints for fatigue life estimation. Hence, fatigue life is determined based on the guidelines provided in design codes. Fatigue analysis of tubular joints is conducted using the finite element employed in Abaqus/CAE as the next major step. Finally, predicted SCFs and fatigue lives are compared, and these observations tend to conclude that even though the fatigue life, which is calculated based on code given SCFs, provides more realistic prediction to the simple uniplanar joints, there is a doubt for complex joints and geometry, where joint classification is not available. Also, the study emphasized that it is very important to preciously investigate SCFs by considering accurate geometry of complex tubular joints for a good judgement of fatigue life.

Degree of bending (DoB) in tubular K-joints of offshore structures subjected to in-plane bending (IPB) loads: Study of geometrical effects and parametric formulation

The fatigue life of tubular joints in offshore structures 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). 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 using the fracture mechanics (FM) approach. Although the tubular K-joints are commonly found in offshore jacket structures, the DoB in K-joints subjected to in-plane bending (IPB) loads have not been investigated so far and no design equation is available to calculate the DoB for IPB-loaded joints of this type. In the present research, comprehensive finite element (FE) analyses were carried out on 81 generated models of tubular K-joints. Results of FE models were verified against experimental data and parametric equations. Data extracted from FE analyses was used to investigate the effects of geometrical parameters on the DoB values in K-joints subjected to two types of IPB loadings. Parametric study was followed by a set of nonlinear regression analyses to develop DoB parametric equations for the fatigue analysis of IPB-loaded K-joints.

Probabilistic Assessment of Degree of Bending in Tubular X-Joints of Offshore Structures Subjected to Bending Loads

Fatigue life of tubular joints in offshore structures is significantly influenced by the degree of bending (DoB). The DoB exhibits considerable scatter calling for greater emphasis in accurate determination of its governing probability distribution which is a key input for the fatigue reliability analysis of a tubular joint. Although the tubular X-joints are commonly found in offshore jacket structures, as far as the authors are aware, no comprehensive research has been carried out on the probability distribution of the DoB in tubular X-joints. In the present paper, results of parametric equations available for the calculation of the DoB have been used to develop probability distribution models for the DoB in the chord member of tubular X-joints subjected to four types of bending loads. Based on a parametric study, a set of samples was prepared and density histograms were generated for these samples using Freedman-Diaconis method. Twelve different probability density functions (PDFs) were fitted to these histograms. In each case, Kolmogorov-Smirnov test was used to evaluate the goodness of fit. Finally, after substituting the values of estimated parameters for each distribution, a set of fully defined PDFs have been proposed for the DoB in tubular X-joints subjected to bending loads.

Fatigue Life of Repaired Welded Tubular Joints

In this study, the effect of repair on fatigue life of tubular joints is investigated. Six cracked specimens precedently subjected to fatigue loading undergone to weld repair. Two of those specimens were shot peened before primary fatigue loading. It is shown that repair gives rise to about 150% increase in fatigue life for original specimens while the increase of fatigue life for shot-peened and repaired specimens is around 105%. The in depth residual stresses are measured on the repaired joints before and after fatigue loading. It can be stated that repair made a remarkable improvement into fatigue behaviour of tubular joints examined in this investigation. However, if repair is not possible, shot peening can be a good alternative to improve the fatigue life of welded joint but if repair is planned, shot peening may not be financially or technically justifiable.

Spectral Fatigue Analysis of Jacket Stuctures in Mumbai High Field

Fatigue analysis of offshore structures is an integral part of design of offshore structures and shall be carried out with suitable method of discretising the seastate. Historically, for most of the fixed offshore structures, deterministic fatigue analysis found to predict the fatigue damage reasonably well and has been in use for several decades. Fixed structures with small topsides, mostly exhibit a static response characteristics and the natural period may be in the order of less than 2 seconds. Offshore platforms with larger production capacity and deeper water depths may require specialised treatment of seastate due to their dynamic characteristics more vulnerable for fatigue damage. A spectral fatigue analysis has been performed for two different platforms in Mumbai high field (MNP and RS14) and a comparison is made with deterministic analysis. The spectral fatigue analysis indicates that the predicted fatigue life is lower than the deterministic analysis since the dynamic amplification of wave loads are treated approximately in deterministic analysis. Hence for large structures, it recommended to use spectral methods to assess the fatigue life of tubular joints.

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.

Effect of Stress Distribution on Fatigue Life of Tubular Joints in Offshore Engineering

In the assessment of fatigue life of tubular joints in offshore engineering, hot spot stress range, in conventional method, is frequently used to predict the number of cycles that a tubular joint can sustain before failure from corresponding S-N curves. This method only considers the effect of the peak stress at the weld toe on the fatigue life of a tubular joint, but ignores that of the stress distribution along the weld toe. The effect of the stress distribution along the weld toe on the fatigue life of tubular joints can be evaluated indirectly by analyzing a fracture mechanics parameter, namely stress intensity factor. The stress intensity factor values of the surface cracks in the fatigue failure process for tubular joints subjected to different loading types, thus causing the difference of the stress distributions along the weld toe, are investigated from both numerical analysis and experimental measurement. The results show that the stress distribution along the weld toe has remarkable effect on the fatigue life of tubular joints.

Mexico Engineering Experiences and Developments

Abstract This paper presents an overall view of the research and developments achieved during the generation of local risk-based criteria for design and assessment of platforms in Bay of Campeche. The results from two studies on meteorological and oceanographic characterization and wave height attenuation due to soft bottom interaction are first presented and discussed. Then, the results from a study on wave loading on decks is presented followed by a discussion on earthquake characterization. Two more studies on the biases and uncertainties involved in the evaluation of the capacities of tubular joints and pile foundations are next presented. Finally, a brief discussion on fatigue life of tubular joints is presented. In each section, the final uncertainty values considered to set the safety requirements are shown. [S0892-7219(00)00301-0]

Fatigue Life of Cathodically Protected Tubular Joints of Offshore Structures

Steel jacket platforms are widely used for production and processing of oil from offshore fields. Tubular joints of these structures are susceptible to fatigue or corrosion fatigue cracking due to cyclic wave action, stress concentration, and corrosive environment. The submerged part of the structure is cathodically protected against corrosion by fixing sacrificial anodes at various places. The effect of cathodic protection on the fatigue life of tubular joints was studied by conducting fatigue tests on 13 T and Y-joints in air, under free corrosion, and with cathodic protection. The results of the studies are discussed in this paper.

Stress Concentration Factors of DK Square-to-Square Tubular Joints

The fatigue life of tubular joints with many braces, e.g., DK and DTK joints, are commonly determined by treating them as tubular joints with less braces, e.g., T/Y, K, and X joints, based on the joint classification approach recommended by the American Petroleum Institute. The DK square-to-square tubular joint type was selected to verify the reliability and accuracy of such an approach. A parametric stress analysis of DK square-to-square tubular joints subjected to axial loads, in-plane and out-of-plane bending moments has been performed using the finite element technique. The results of this analysis are presented as a set of formulas expressing the stress concentration factor as a function of the relevant geometric parameters for various loading conditions. A comparison is made between the results obtained for DK square-to-square tubular joints and those obtained for X and K square-to-square tubular joints, which are commonly employed to simulate the former when the joint classification approach is adopted. In general, the stress concentration factors for DK joints are significantly higher, which shows that the recommended approach may not be reliable and accurate in dealing with DK joints.

Application of the probabilistic fracture mechanics method of predicting the fatigue life of tubular joints

In this paper fatigue strength of T-type tubular joints subjected to in-plane bending or out-of-plane bending load is investigated. By considering material constants and initial crack sizes as random variables and applying Monte Carlo simulation method, we have given a statistical analysis on fatigue life. Simultaneosly, linear regression analyses of computed results are performed and compared with that of the known experimental data.