Multi-planar Tubular Joints Research Articles

Bending load effects on stress concentration factor distribution of multi-planar DTKY tubular joints

The stress concentration factor (SCF) in tubular joints is an important parameter in calculating the fatigue life of offshore structures. In design practices, estimating the SCF is usually based on standard empirical formulas, providing the majority for uni-planar tubular joints, and could not be applied directly for multi-planar joint cases. Therefore, accurate SCF estimation became critical for specific multi-planar tubular joints commonly found as members of offshore platforms. More detailed analyses of complex designs are possible with the recent development of finite element analysis and supported by advanced digital computers. Thus, this research is intended to analyze the SCF of multi-planar DTKY tubular joints with finite element analysis. The local model of the tubular joint will be simulated for bending moment load cases. The results show stress distribution characteristic of the joint at both brace and chord sides. The results indicated that the SCF of the joint under the in-plane bending (IPB) loading is higher at the brace than the chord side, with values of 2.83 and 1.47, respectively. Meanwhile, under the out-of-plane bending (OPB) loading, the SCF is 2.43 at the chord saddle location and 3.54 at the brace saddle location.

Capacity of unstiffened multi-planar tubular KK-gap joints under anti-symmetric loading

This research presents a mathematical formula for calculating the capacity of anti-symmetric loaded multi-planar KK gap joints. The study was conducted using a well-calibrated and validated finite-element model, with 172 records generated for the parametric study using different geometries, member sections, and material properties. The results of the parametric study were analyzed using the GRG solving technique to develop a final predictive model. The developed model can predict actual joint capacity as a 3D joint, rather than approximating it as a 2D joint as in previous literature. It consists of two equations based on the ratio of the gap (gt/gl) with a transition point of (gt/gl) ≈ 0.9, which distinguishes between two different modes of load-transfer mechanisms: membrane action and direct shear action. The formula results were compared with experimental test results and showed an average accuracy of 91.2%.

Study on hot spot stress distribution of three-planar tubular Y-joints subjected to in-plane bending moment

The three-planar tubular Y-joint (3Y-joint) is the main part of the fatigue assessment of tripod substructures of offshore wind turbines (OWTs). As typical multiplanar tubular joints, 3Y-joints are affected a lot by multiplanar interaction between braces. Moreover, the locations of hot spot stress (HSS) can vary considerably under different load types. Thus, the distributions of stress concentration factor (SCF) and multiplanar interaction factor (MIF) along weld toe curves are necessary to calculate HSS. Considering these requirements, this study focuses on the 3Y-joint considering the wide application of the tripod substructure of OWT. A finite element (FE) analysis method is introduced and validated. Then, a numerical database is established covering common ranges of parameters used in practice. The SCF and MIF of 3Y-joint under in-plane bending moment are analyzed. Distribution formulas are proposed and proved suitable for calculating HSS in engineering design.

Experimental study of hot spot stress for three-planar tubular Y-joint: II. Combined loads

The tripod foundation structure is typically used for offshore wind turbines, in which fatigue design under complex load conditions involving axial force, in-plane bending moment, out-of-plane bending moment, and torque is vital. Experimental data show that the hot spot stress (HSS) obtained using the Det Norske Veritas recommended method may lead to inaccurate evaluation of the fatigue life of multiplanar tubular joints. Therefore, this study investigates the HSS distribution along the weld seams of a three-planar tubular joint under combined load cases using model tests. This is in parallel to the experiment on basic loads reported in the companion paper. The geometric stress at key locations and the peak values of the stress concentration factor under 17 load cases were analyzed. The influence of torque on the HSS was statistically investigated using experimental data. An indirect method to evaluate HSS induced by complex load conditions is proposed based on the summary of the companion paper.

Experimental study of hot spot stress for three-planar tubular Y-joint: I. Basic loads

Various tubular joints of offshore wind turbines (OWTs) have seen rapid development in the past decades. However, experimental studies on multiplanar tubular joints have mainly been underexplored. This study designed and constructed a multiplanar loading experimental system for multiplanar tubular joints. A model test was performed using this system to investigate the hot spot stress (HSS) of a three-planar tubular Y-joint, which is critical for the fatigue design of a tripod substructure. The HSS distribution along the weld seams under the 12 basic load cases was obtained from the test analysis. The geometric stresses at the key positions of the chord and brace sides were obtained and analysed. The peak values of stress concentration factor (SCF) were obtained and compared with those calculated using the method recommended by the codes. An indirect method that explicitly considers the interaction between multiple braces was proposed and verified to calculate the SCF distribution of multiplanar tubular joints.

Parametric study of Unstiffened multi-planar tubular KK-Joints

Multi-planar tubular joints became widely used in steel structures especially in industrial and offshore project. However, the current design procedures are still not accurate enough and depend on studying each plan separately and adjust the joint capacity using some correction factor. The aim of this research is to present a mathematical formula to calculate the capacity of symmetrical loaded multi-planar KK joint. A verified ANSYS model was used to generate 172 records for parametric study with different geometry, member sections and material properties. The outputs of the parametric study were analyzed using (GRG) solving technique to develop the final predictive model. The novelty of the developed model is it can predict the actual joint capacity as 3D joint not as approximated 2D joint as previous literatures. The developed model consists of two equations based on failure mode. One for local chord failure where the chord thickness is less than 25 times its diameter and the other for global chord failure if that ratio is more than 25. The results of the developed formula were compared with experimental test results and showed average accuracy of 92.5%.

Parametric study on SCF distribution along the weld toe of internally ring-stiffened two-planar tubular KK joints under axial loading

The internal ring stiffeners have been commonly introduced in tubular joints to improve the ultimate strength and fatigue life of offshore structures. However, for internally ring-stiffened multi-planar tubular joints which are frequently used in practical application, few investigations about stress concentration factor (SCF) in the joints have been reported so far and no parametric equation of the SCF is currently available for fatigue design. In this paper, the effects of geometrical parameters on the SCF distribution along the weld toe of the internally ring-stiffened two-planar tubular KK joints under axial loading are investigated by performing a set of three-dimensional (3D) FE stress analyses. Firstly, based on the requirements of AWS D1.1-2010, an approximate model of the actual weld profile is proposed. Then, several 3D FE models of various tubular joints are established to verify the proposed model. Subsequently, the effects of multi-planarity and stiffeners on the SCF in two-planar tubular KK joints are investigated. Additionally, the influence of geometrical parameters on both the SCF and the SCF distribution of the joints under axial loading is studied. Finally, SCF parametric equations for fatigue design of the internally ring-stiffened two-planar tubular KK joints under axial loading are developed from nonlinear regression analysis.

A Bracket‐Stiffened Two‐Planar Overlapped Tubular KT‐Joint for Offshore Jack‐Up Fatigue Analysis

There have been a large number of research programs dedicated to the investigation of stress concentration factor and its parametric design formulae for a variety of uniplanar and multiplanar tubular joints. However, for the multiplanar overlapped tubular joints, the related reports are very limited due to the fact that the combining effect between multiplanarity and brace overlapping is quite complex. In this paper, a novel bracket‐stiffened two‐planar overlapped tubular KT‐joint is proposed by adding two brackets between the upper diagonal and horizontal braces to alleviate the stress concentration at the brace‐to‐chord intersection area. As compared with the original KT‐joint and KT‐joint of larger brace gap, stress concentration reduction and fatigue strength enhancement at the most critical point of the proposed KT‐joint are significant under both axial and in‐plane bending (IPB) loads. The superiority of the proposed KT‐joint is further tested with a simplified fatigue life assessment of offshore jack‐up in field transit condition, when the KT‐joint corresponding to the upper guide level is most critical and fatigue‐sensitive. The obtained results with conservative assumptions may serve as guides or references in civil and offshore engineering practices.

Stress Concentration Factors for Multi-planar Tubular Joints Subjected to Axial Loading

The support structure for an offshore wind turbine is subjected to combined hydrodynamic loads and aerodynamic loads. The tubular joints are the weakest component leading to fatigue failure of the whole structure. Based on the multi-pile foundation structure which is used widely in China, the typical threeplanar tubular Y-joints is selected to study stress concentration factor (SCF). Then, the load types subjected to axial loadings of three-planar tubular Y-joints are determined. The finite element models of three-planar tubular Y-joints are established and used to calculate hot spot stresses. The stress concentration factors along the weld of the three-planar tubular Y-joints under the axial forces are obtained. The effects of geometrical parameters on SCFs are studied.

Prediction of stress concentration factor distribution for multi-planar tubular DT-joints under axial loads

The stress concentration factor (SCF) plays an important role in the fatigue life assessment of tubular joints. Although various SCF parametric equations have been proposed for uni-planar tubular joints, only few are focused on multi-planar tubular joints. In the present study, the finite element (FE) method was employed to investigate the SCF distributions along the chord–brace intersection of multi-planar DT-joints. A method of sub-zone mesh generation was developed to ensure good quality and an appropriate number of FE meshes. The effects of the geometrical parameters on the SCF distributions were investigated. A set of equations was proposed based on the results of the FE analysis. Two types of error analysis were used to verify the reliability of developed equations.

Stress concentration factor distribution of inclined brace in multiplanar offshore tubular double kt joints

The majority of tubular joints commonly found in offshore structures are in the form of multi-planar tubular joints, but very few investigations have been reported due to the complexity and high cost involved. The majority of research have been focused on the study of stress distribution at certain position such as saddle and crown points, and the hot spot stress (HSS) at other position along the weld toe of brace/chord intersection have been ignored. In this paper a 60° two-planar double KT (DKT) tubular joint is modelled as a finite element model from an offshore jacket platform. The effect of dimensionless geometrical parameters on the geometrically stress distribution and SCF distribution along the weld toe of inclined brace in axially loaded on the joint are investigated. Non-dimensional parameters that are varied include β, τ, γ and θ. Validation of the finite element model shown a good agreement to the global structural analysis results. The results of parametric studies show that the increase of the β leads to decrease of SCF. While the increase of the τ, γ and θ leads to increase of SCF. The peak SCF mostly occurs at the outer saddle point. The effect of parameters β, γ and θ on the SCF are greater than the effect of parameters τ.

Experimental and Numerical Study on Complex Multi-planar Welded Tubular Joints in Umbrella-Type Space Trusses with Long Overhangs

A test rig with multi-functional purposes was specifically designed and manufactured to study the behavior of multi-planar welded tubular joints subjected to multi-planar concurrent axial loading. An experimental investigation was conducted on full-scale welded tubular joints with each consisting of one chord and eight braces under monotonic loading conditions. Two pairs or four representative specimens (two specimens for each joint type) were tested, in which each pair was reinforced with two kinds of different internal stiffeners at the intersections between the chords using welded rectangular hollow steel sections (RHSSs) and the braces using rolled circular hollow steel sections (CHSSs) and welded RHSSs. The effects of different internal stiffeners at the chord–brace intersection on the load capacity of joints under concurrent multi-planar axial compression/tension are discussed. The test results of joint strengths, failure modes, and load–stress curves are presented. Finite element analyses were performed to verify the experimental results. The study results show that the two different joint types with the internal stiffeners at the chord–brace intersection under axial compression/tension significantly increase the corresponding ultimate strength to far exceed the usual design strength. The load carrying capacity of welded tubular joints decreases with a higher degree of the manufacturing imperfection in individual braces at the tubular joints. Furthermore, the interaction effect of the concurrent axial loading applied at the welded tubular joint on member stress is apparent.

A novel framework for deriving the unified SCF in multi-planar overlapped tubular joints

For the past few decades, a significant number of research efforts have been devoted to the studies of SCF values and parametric design formulae at certain positions in various uniplanar and multi-planar tubular joints. However, very few investigations on the multi-planar overlapped tubular joints have been reported. The main reason may lie in the complexity of combining effects between multi-planarity and overlapping. In this paper, a novel framework for deriving the unified SCFs is proposed by reducing joint modeling from multi-planar out-of-plane overlapping to equivalent uniplanar non-overlapping. By integrating the equivalent beam model with solid finite element modeling for the overlapped tubular KK-joint in offshore jack-ups, the procedures for computing the unified SCF values in the equivalent beam stick model are successfully developed. Verification of the proposed framework is obtained by comparing the unified SCFs based on basic load cases with those based on actual wave loading typically experienced by the jack-up.

Numerical analysis on the HSS and SIF of multi-planar DX-joint welds for offshore platforms

Among offshore platform structures, multi-planar tubular joints are usually in the most dangerous positions and generate fatigue cracks the earliest. In this study, DX-joints were regarded as the research object, and finite element method was adopted to conduct systematic research on the fatigue properties of multi-planar tubular joint welds. Axial, in-plane bending moment, and out-of-plane bending moment loads were applied to explore the stress distribution along the weld path and the hotspot location under different load conditions. Results indicated that the hotspot stress (HSS) location of DX-joints changed with the variation in the combined stress ratio. Through HSS analysis, cracks were prefabricated at the hotspot location of DX-joints to analyze the change rule of the stress intensity factor (SIF). The influence of different loads, crack lengths, and crack depths on SIF was studied to estimate the growth trend of the weld crack and its dominance. The findings can provide theoretical guidance for fatigue damage and safety assessment of offshore platforms.

Experimental and Numerical Analysis of Complex Welded Tubular DKYY-Joints

The design of multi-planar welded tubular joints is still not fully developed and very often it is not covered in design codes due to the complexity of such structures. The joint position for multi-planar tubular structures is especially critical because there are many brace members which are welded onto the chord surface. Thus, many external forces which are transferred from the braces are applied directly to the chord. Among the various multi-planar tubular joint configurations, the DKYY-joint configuration is often used in tubular space structures to form long-span roofs. However, its design, failure modes and calculations of its joint capacity are not provided in any design code. As a result, it is necessary to carry out experimental investigation and finite element numerical analysis to understand further the behavior and failure process of this type of joint. This paper details an actual practical application where a full-scale specimen is investigated experimentally in a test programme. To improve its joint capacity, reinforcements such as strengthening plates are placed inside the brace members. The chord at the joint connection is thickened to avoid local buckling. The stresses at some critical positions on the braces and on the chord are measured from strain gauges and strain rosettes to monitor the failure process and the ultimate joint capacity is determined from experimental load-displacement relationship. Experimental results show that local buckling on the chord surface at the joint is avoided effectively due to the reinforcement to the chord by increasing the chord thickness locally and placing inner circular plate inside the chord. As the stiffness of the chord at the joint is improved efficiently, failure position moves to the brace, and thus the chord is protected. Finally, finite element analysis is conducted and the numerical results are compared against those obtained from experimental measurements.

Fatigue reliability analysis of the jacket support structure for offshore wind turbine considering the effect of corrosion and inspection

Due to the high level of fatigue loads as well as a large number of load cycles caused by wind and wave loads together, fatigue performance of welded connections is a design driving criterion for offshore wind turbine support structures. In this paper prediction of fatigue reliability of welded multi-planar tubular joints of the support structure of a fixed jacket offshore wind turbine designed for a northern North Sea site in a water depth of 70m is performed. Dynamic response of the jacket support structure due to wind and wave loads is calculated by using a decoupled procedure with good accuracy. Hot-spot stresses at failure-critical location of the relative reference brace of the selected tubular joint are derived by summation of the single stress components from axial, in-plane and out-of-plane action, the effects of planar and non-planar braces are also considered. A two-parameter Weibull function is used to fit the long-term statistical distribution of hot-spot stress ranges by combination of time domain simulation for representative environmental conditions (wind/sea states) in operational condition. The main uncertainties associated with the whole procedure of reliability analysis are identified and quantified. The load histories are normalized to ensure that fatigue design criteria based on the SN-Miner-Palmgren approach is satisfied. Hence the reliability estimates obtained refer to fatigue design of tubular joints that satisfy design criteria. The reliability analysis is based on fracture mechanics (FM) analysis of crack growth. The corrosion-induced increased crack growth rate is taken into account by considering the increased hot-spot stress range due to changes of nominal stress and stress concentration factors (SCFs) produced by the thickness thinning (wastage) effects of all braces and chord of the selected tubular joint with a general uniform corrosion model. The geometry function effect and the material degradation effect due to corrosion on the reliability analysis are also investigated. The effects of inspection and repair with and without consideration of corrosion are quantified based on the quality of inspection in terms of probability of crack detection curves. The sensitivity of the reliability index on important random variables is estimated.

Long-term fatigue analysis of multi-planar tubular joints for jacket-type offshore wind turbine in time domain

Long-term fatigue analysis of welded multi-planar tubular joints for a fixed jacket offshore wind turbine designed for a North Sea site in a water depth of 70 m is performed. The dynamic response of the jacket support structure due to wind and wave loads is calculated by using a decoupled procedure with good accuracy (Gao et al., 2010). Hot-spot stresses at failure-critical locations of each reference brace for 4 different tubular joints (DK, DKT, X-type) are derived by summation of the single stress components from axial, in-plane and out of plane action, the effects of planar and non-planar braces are also considered. Both a 2-parameter Weibull function and generalized gamma function are used to fit the long-term statistical distribution of hot-spot stress ranges by a combination of time domain simulation for representative environmental conditions in operational conditions of the wind turbine. A joint probabilistic model of mean wind speed U w , significant wave height H s and spectral peak period T p in the northern North Sea is used to obtain the occurrence frequencies of representative environmental conditions (Johannessen, 2002). In order to identify the contributions to fatigue damage from wind loads, wave loads and the interaction effect of wind and wave loads, 3 different load cases are analyzed: wind loads only; wave loads only; a combination of wind and wave loads. The representative environmental condition corresponding to the maximum contribution to fatigue damage is identified. Characteristic fatigue damage of the selected joints for different models is predicted and compared. The effect of brace thickness on the characteristic fatigue damage of the selected joints is also analyzed by a sensitivity study. The conclusions obtained in this paper can be used as the reference for the design of future fixed jacket offshore wind turbines in North Sea.

Strength Analysis of Large-Scale Multiplanar Tubular Joints with Inner-Plate Reinforcement

For large scale multiplanar tubular joints used in practical engineering, the brace/chord intersection is a critical position as failure usually occurs here due to the weak bearing capacity of the chord in radius direction compared to the strength of the braces in axial direction. To improve the ultimate strength, different reinforcements can be used to strengthen the structures. Inner plate reinforcement is a relatively new strengthening method compared to conventional reinforcing methods. As there is no corresponding guideline which can be used for the design of inner plate reinforcing joints, it is necessary to investigate the failure mechanism of such tubular structures. Both experimental test and finite element analysis are carried out in this study to investigate the static behaviour of multiplanar tubular joint reinforced with inner plate. In the experimental work, the stresses distribution and development of the specimen is monitored, and the failure mode is observed. From the experimental results, both the failure process of the multiplanar tubular joint and the reinforcing efficiency of the inner plate were analyzed. Using finite element analysis, the failure process of the specimen is also analyzed step by step. The finite element results agree reasonably well with experimental measurements.

A General Formulation for the Strength Design of Axially Loaded Planar and Multiplanar Tubular Joints

A database of newly generated numerical strengths of planar and multiplanar joints was used to assess the current offshore design code methodologies and to derive a new design equation, based on the AWS ovalization parameter concept. Within defined geometric limits, the proposed equation has been found to be generally applicable to any axially loaded joint configuration, provided that the braces do not overlap. It negates the need for potentially subjective brace classification to be carried out and accounts fully for planar and multiplanar effects. The proposed equation is fully validated against the planar and multiplanar data in the screened database complied by MSL Engineering, and has been shown to have similar accuracy as the existing design equations advocated by ISO, yet have significantly wider validity.

Finite-Element Modelling of Multi-Planar Offshore Tubular Joints

This paper describes the calibration and validation of a finite-element modelling procedure that was used to conduct an extensive parametric study on the strength of multi-planar tubular joints. Various factors that influenced results were investigated. The modelling procedure was calibrated using the International Organization for Standards (ISO) design equations and the underlying test database for compression and tension loaded simple T and Y joints, and was further validated with balanced loaded K joints. Overall, the proposed procedure has been shown to be adequate in predicting the strength of the basic joint types, thereby giving confidence in its use for more complex joints.