Tubular Welded Joints Research Articles

A new method for predicting stress intensity factors in cracked welded tubular joints

Fracture mechanics is an important tool for the analysis of cracked bodies. It has been established for use in a range of industries including the power generation and the aircraft industry. Fracture mechanics provides the basis for fatigue life prediction, steel selection and tolerance setting on allowable weld imperfections. It can also be used during the operational stage of a structure to make important decisions on inspection scheduling and repair strategies and as a tool for establishing limits on operational conditions. The stress intensity factor is a very important fracture mechanics parameter. Linear elastic fracture mechanics relies on the use of the stress intensity factor concept. Therefore the accuracy of any fracture mechanics model for the prediction of fatigue crack growth will depend very much on the accuracy of the stress intensity factor solution used. It is known that some factors are important in this process. One of these is the crack aspect ratio, which represents a major source of uncertainty in fatigue crack growth prediction. This paper presents a new approach for the prediction of stress intensity Y correction factors for welded tubular joints. The proposed method accounts for crack aspect ratio evolution during crack propagation. The method is based on a statistical model used to quantify and account for the deviation of experimental results from previous semi-empirical solutions and the modified Newman and Raju flat plate solution.

Review of empirical and semi-empirical Y factor solutions for cracked welded tubular joints

The practical use of fracture mechanics has been established for use on large turbine and electric generator rotor components used in the atomic power generation and the aircraft industry. Application areas in the offshore industry have also been identified. Fracture mechanics is currently used at the design stage of offshore facilities. It provides the basis for fatigue life prediction, steel selection and tolerance setting on allowable weld imperfections. Fracture mechanics is also used during the operational stage of a structure to make important decisions on inspection scheduling and repair strategies and as a tool for establishing limits on operational conditions. Linear elastic fracture mechanics relies on the use of the stress intensity factor concept. The stress intensity factor is a very important fracture mechanics parameter. Therefore, the accuracy of any fracture mechanics model for the prediction of fatigue crack growth in offshore structures for example will depend very much on the accuracy of the stress intensity factor solution used. Several empirical and semi-empirical solutions have been developed over the years with varying degrees of accuracy. This paper presents a review of some of these methods and attempts to assess their accuracy in predicting Y factors for welded tubular joints by comparing predicted results with experimental data obtained from fatigue tests conducted on large scale welded tubular joints. The experimental results were conducted under simulated service conditions, using a jack-up offshore standard load history (JOSH). A comparison between the experimental and predicted results shows that there may be other factors, which influence fatigue crack growth under variable amplitude conditions. Some of these factors have been identified and discussed in this paper.

Prediction of degree of bending in tubular X and DT joints

Analysis of the large scale fatigue testing results of offshore welded tubular joints showed that the fatigue life is not dependent on the hot spot stress alone, but is also significantly influenced by the through thickness stress distribution which can be characterised by degree of bending (DoB). Accurate prediction of DoB data in tubular joints is crucial for improving the accuracy of fatigue life prediction using stress-life (S–N) curve and particularly for predicting fatigue crack growth using the fracture mechanics method. However, there is no set of DoB parametric equations available for tubular X and DT joints. For this reason, comprehensive thin shell finite element (FE) analyses were carried out for 330 tubular X and DT joints typical of those used in offshore structures, under six different modes of loading. The results of these FE analyses were used to derive a set of parametric equations to predict the DoBs at the critical positions on both brace and chord toes in tubular X and DT-joints under each mode of loading.

Hybrid Design Concept Using High-Strength Cast Steel Inserts for Tubular Joints of Offshore Structures

In a joint project of a German working group supported by the ECSC and the Studiengesellschaft fu¨r Stahlanwendung e.V., the fatigue behavior of large-scale hybrid tubular joints with inserts manufactured from the high-strength cast steel GS-12 MnMo 7 4 welded into tubular members formed from the fine-grained steel StE 500 were compared to the behavior of large-scale welded tubular joints. The latter were made from medium-strength fine-grained steel StE 355 and high-strength StE 690. In addition, data from hybrid joints with cast steel inserts of medium-strength GS-8 Mn 7 welded into StE 355 tubulars is available for comparison. The tests were carried out under variable amplitude loading in artificial seawater. The results were evaluated for the failure criteria fatigue life to crack initiation (a = 1 mm) and through crack. With medium-strength (Rp0.2 > 355 N/mm2) hybrid tubulars, where by the use of cast steel inserts the welds were removed into areas of lower stress concentration, fatigue lives higher than a factor of 100 were achieved compared to the welded nodes, even those made from StE 690. However, by the use of high-strength (Rp0.2 > 500 N/mm2) cast steel inserts and tubular members of corresponding strength, the fatigue life to crack initiation was improved by a factor of two despite a thickness reduction compared to the medium-strength design. Post-weld treatments of the welded tubulars without cast steel inserts like shot-peening, TIG-dressing, or their combination resulted only in a slight increase of fatigue life. The results of this investigation do not only show how to improve the fatigue life by a new design using cast steel inserts, but indicate also how to revise design codes from the point of damage calculation (damage sum of 0.50 for welded nodes and 0.25 for cast steel inserts instead of the conventional value of 1.00), as well as consideration of fatigue life to initiation of a technically detectable crack with a defined depth e.g., a = 1 mm.

Stress concentration factor parametric equations for tubular X and DT joints

During the design stage, the peak stress is usually needed for estimating the fatigue life of offshore tubular welded joints by an S  N approach. However, for fracture mechanics calculations of remaining life, on cracked joints in service, information is required on the magnitude and distribution of the stress acting in the anticipated crack path, not just the peak stress at one location. Fatigue crack propagation rates are important to reliability-based inspection scheduling: hence the need for this information is becoming more pressing. Parametric equations are available for Y and T joints in terms of peak stress, stress distribution and bending-to-membrane ratio. However, for X and DT joints, there are no parametric equations for stress variation through the thickness and around the intersection. Even for stress concentration factor (SCF), so far there is no full set of parametric equations especially for single-brace loading. Thin-shell finite element analyses have been conducted for 330 X and DT joints typical of those used in offshore structures, subject to six modes of loading. The results from this work have been used to produce a new set of parametric equations as a function of non-dimensional joint geometric ratios α, β, γ, τ and θ by carrying out regression analysis. These equations can be used to predict SCFs at the crown toe, saddle, crown heel and hot-spot positions for each mode of loading, for both chord and brace, as well as the angular location of the hot-spot stress site around the intersection. This set of SCF parametric equations has been assessed by comparing the predicted values with results from steel and acrylic model tests and also with the predictions from existing parametric formulae given in the literature. The degree of bending data, and stress distribution data, will be reported in other publications.

A study on the non-destructive inspection and corrosion fatigue crack growth in tubular welded joints using reliability methodologies : Kam, J.C.P. Proceedings of the 9th International Conference on Offshore Mechanics and Arctic Engineering, Vol. 3, Pt. A, Texas (United States), 18–28 Feb. 1990. pp. 189–196. The American Society of Mechanical Engineers. (1990)

A study on the non-destructive inspection and corrosion fatigue crack growth in tubular welded joints using reliability methodologies : Kam, J.C.P. Proceedings of the 9th International Conference on Offshore Mechanics and Arctic Engineering, Vol. 3, Pt. A, Texas (United States), 18–28 Feb. 1990. pp. 189–196. The American Society of Mechanical Engineers. (1990)

Crack shape evolution in tubular welded joints : Ma, C.N.; Kam, J.C.P. NDT & E International, Vol. 24, No. 6, on. 291–302 (Dec. 1991)

Crack shape evolution in tubular welded joints : Ma, C.N.; Kam, J.C.P. NDT & E International, Vol. 24, No. 6, on. 291–302 (Dec. 1991)

Non-destructive evaluation of tubular welded joints : Dharmavasan, S.; Dover, W.D. OTJ' 88. Conference on Recent Developments in Tubular Joints Technology, Surrey (United Kingdom), 4–5 Oct. 1988, Section 13. 25pp. UEG Offshore Research

Non-destructive evaluation of tubular welded joints : Dharmavasan, S.; Dover, W.D. OTJ' 88. Conference on Recent Developments in Tubular Joints Technology, Surrey (United Kingdom), 4–5 Oct. 1988, Section 13. 25pp. UEG Offshore Research

Local strain computation of crack initiation life of an unstiffened steel tubular T-joint

This paper presents the results of the fatigue crack initiation life estimation for offshore tubular welded joints. The results are obtained by using the fatigue strength exponent, b, and fatigue ductility exponent, c, computed by a backcalculation procedure from an extensive experimental crack initiation estimation of both small scale and medium scale welded steel tubular joints. A general purpose finite element package, ABAQUS, was used for the stress analysis. This allows for the accurate determination of the critical location where the crack/cracks is/are likely to initiate. From the stress analysis, the stress and strain concentration factors (SCF and SNCF) along the intersection of the tube were obtained. The peak SCF and SNCF were used in modified Neuber's rule and the result introduced into a Manson-Coffin model for crack initiation life computation. The results obtained were compared with the experimental results previously obtained at the Canadian Cooperative Fatigue Studies Program.

The prediction of crack shape development for in-service cracks in offshore welded tubular joints

Abstract Due to the severe cyclic environmental loading experienced by offshore structures in the North Sea, fatigue has been identified as one of the most important causes affecting the long-term structural integrity of the welded tubular joints on these structures. Fatigue damage appears in the form of fatigue crack growth. Maintenance of structural integrity therefore concentrates on carrying out periodic in-service inspection and integrity assessment of the intact and cracked structures. The aims of the maintenance programme are to ensure that either no crack is present in the structures, or if cracks are discovered, that the remaining joints are still ‘fit’ for their purpose in terms of fatigue life and static capacity. The key information linking the in-service inspection and integrity assessment is the size of the cracks. Although the crack depth is the controlling variable for fatigue analysis, both depth and length of the surface breaking cracks are needed for the assessment of remaining ultimate capacity. In the past it was.found to be difficult to correlate the analytical prediction of crack aspect ratio (depth to half-length ratio) development with measured values from full-scale test results. Many integrity assessments therefore used a predefined aspect ratio pattern for the analysis. This paper reports an extension of a methodology in order to produce a better correlation between predicted and measured aspect ratio patterns. The methodology is based on fracture mechanics analysis with structural modelling.for cracked tubular joints. The paper reviews briefly the recent progress in the analysis of fatigue crack growth in tubular joints and outlines how the new methodology i.s formulated. The results showed that the methodology was adequate.for predicting aspect ratio growth for cracks deeper than 25% of the chord wall thickness.

A simple reliability model for the fatigue failure of repairable offshore structures

A simple reliability model for fatigue failure of tubular welded joints used in the construction of offshore oil and gas platforms is proposed. The stress-life data obtained from large-scale fatigue tests conducted on tubular joints is used as the starting point in the analysis and is combined with a fracture mechanics model to estimate the distribution of initial defect sizes. This initial defect distribution is hypothetical but it agrees well with other initial defect distributions quoted in the literature and when used with the fracture mechanics model results in failure probabilities identical to those obtained from the stress-life data. This calculated distribution of initial defect sizes is modified as a result of crack growth under cyclic loading and the probability of failure as a function of fatigue cycles is calculated. The failure probability is modified by inspection, and repair and the results illustrate the trade-off between inspection sensitivity and inspection interval for any desired reliability.

A SIMPLE RELIABILITY MODEL FOR THE FATIGUE FAILURE OF REPAIRABLE OFFSHORE STRUCTURES

AbstractA simple reliability model for fatigue failure of tubular welded joints used in the construction of offshore oil and gas platforms is proposed. The stress‐life data obtained from large‐scale fatigue tests conducted on tubular joints is used as the starting point in the analysis and is combined with a fracture mechanics model to estimate the distribution of initial defect sizes. This initial defect distribution is hypothetical but it agrees well with other initial defect distributions quoted in the literature and when used with the fracture mechanics model results in failure probabilities identical to those obtained from the stress‐life data. This calculated distribution of initial defect sizes is modified as a result of crack growth under cyclic loading and the probability of failure as a function of fatigue cycles is calculated. The failure probability is modified by inspection, and repair and the results illustrate the trade‐off between inspection sensitivity and inspection interval for any desired reliability.

Crack shape evolution in tubular welded joints

There has been extensive research carried out for fatigue crack growth analysis in offshore tubular joints. The controlling parameter in these analyses is the leading crack depth. Many NDT techniques used in the inspection of these welded tubular joints, however, measure only surface breaking lengths. This paper presents a method to bridge the gap between inspection and analysis. The method is based on a statistical analysis of an extensive data base of tubular joint crack growth data. The findings include the length-depth conversion equation and the uncertainties associated with the conversion. The latter information is essential for reliability-based analysis.

Fatigue Data for Reliability‐Based Offshore Platform Inspection and Repair

Fatigue of welded fixed offshore platforms is a primary design and operational concern. Environmental, fatigue data, and method uncertainties, as well as inspection and repair activities, are major parameters that influence the welded joint reliability. This paper investigates and establishes the relation between welded tubular joint fatigue life computed from a deterministic design process and the probability of occurrence of: (1) First detectable crack; (2) first through‐thickness cracking; and (3) total joint failure stages. Several experimental fatigue data bases were used to derive the appropriate probabilistic distributions and relations between different joint failure stages. The effect of repair methods, such as repair welding and grinding, on the fatigue life is studied, and parameters that modify the probability of joint failure states are introduced. The data examined show that time to first discernible surface cracking for welded tubular joints is about 10‐65% of the time to through‐thickness cracking, with a mean value of 35%. The total joint failure will occur at about 150% of the time to through‐thickness crack. Simplified relations between crack size and remaining fatigue life are derived based on recent experimental results. Data presented in this paper provide the basic tools for performing a reliability‐based inspection and repair assessment.

Fatigue Fracture Mechanics Modeling and Structural Integrity Assessment of Offshore Welded Tubular Joints

Evaluation of the structural integrity of offshore structures requires information on the reliability of nondestructive testing, the accuracy of fatigue crack growth modeling and other data. The University College London Underwater NDE Centre has been set up to provide information on the effectiveness and reliability of different nondestructive testing methods. To achieve this aim, a large library of cracked specimens will be assembled. In the preliminary phase of producing this library, a series of large-scale welded tubular joints were fatigue tested and the crack growth was fully monitored with the ACPD technique. This paper will describe briefly the background to the crack library and present the data obtained from fatigue tests. It will also describe a new model for fatigue crack growth prediction in tubular joints using fracture mechanics. This model allows the prediction of the size effect noted previously in the stress/life curves for tubular joints.

The stress intensity factors of semi-elliptical cracks in a tubular welded T-joint under axial loading

The stress intensity factors K I, K II and K III of semi-elliptical cracks located near the chord-brace intersection of a tubular welded T-joint have been determined under axial loading conditions. A finite element analysis technique has been used, based on the line-spring concept of Rice and Levy to represent the crack, while the structure has been represented by shell elements. The stiffness of the joint is maintained until the crack penetrates the chord wall when the stiffness decays. The study provides a basis for a fracture mechanics analysis of the fatigue life of tubular welded joints.

Weld crack characterization on offshore structures using AC potential difference and ultrasonics

To assure the integrity of offshore structures greater reliability and accuracy in the detection and sizing of surface-breaking defects in fatigue-affected welded nodes is being demanded. Accurate defect measurements can be used with fracture mechanics based models to assess fatigue crack growth rate and the remaining life of the joint. This paper describes the application of the AC potential difference method to the inspection of offshore structures and also discusses improved ultrasonic techniques developed for this inspection problem. A ‘library’ of tubular welded joints of typical geometries and containing fatigue damage is being collected at the new Underwater NDE Centre established at University College London. Initial trails using some of these specimens to assess detection and crack sizing capability are discussed.

Stress Distribution Formulas and Comparison of Three Stress Analysis Techniques for Tubular Joints

The available data on hot spot stress for tubular welded joints has been reviewed and a database established. Stress analysis techniques in general use for the design of offshore structures and parametric equations have been assessed against this database to determine their accuracy. A set of equations to predict the stress distributions for simple joints has been proposed and has been used to predict mixed mode stress concentration factors. Results obtained from a complex K joint have been examined in detail and the importance of the stress state, when predicting the mixed mode stress concentration factors, is demonstrated.

The Development of Casting Leg Nodes for a Jackup Rig

Ohba, H. Kawasaki Heavy Industries Ltd. Susei, S. Kawasaki Heavy Industries Ltd. Sakai, Y. Kawasaki Heavy Industries Ltd. Atsuta, T. Kawasaki Heavy Industries Ltd. Ohkuma, Y. Kawasaki Heavy Industries Ltd. Ohminami, R. Kawasaki Heavy Industries Ltd. Tamura, A. Kawasaki Heavy Industries Ltd. This paper discusses the development of casting leg nodes for a jackup rig. Two types of nodes were designed, based on theoretical analysis. A low-alloy cast steel with high tensile strength, ample fracture toughness, and weldability was developed. A number of nodes were molded to study the workability in casting, and their strength was confirmed by full-scale tests with three nodes. Introduction With the continuing progress in ocean development and in the oil industry, more efficient, economical and reliable' offshore structures are in demand. Jackup rigs undergo severe meteorological and environmental stress, which requires high-quality material and thorough analysis of the member connections (nodes) of the leg structures.From the points of view of strength and fabrication, the use of casting nodes is of great advantage to directly welded joints. However, only a few rigs have been constructed with casting leg nodes because of (1) the unavailability of suitable cast material to meet the severe offshore requirements, (2) difficulties in molding light and thin-walled tubular joints to meet the weight requirement, and (3) increased costs.To overcome these problems, a low-alloy cast steel was developed with which casting leg nodes were designed for a standard jackup rig for 300-ft water depth, as shown in Fig. 1. Fig. 2 shows a part of the leg structure with two types of casting nodes: Node L (leg chord casting) and Node K (K-brace casting). Leg Nodes Generally, the leg structure of the jackup rig consists of high-strength tubular members whose joints tend to be complicated and heavily welded constructions, causing several problems:highly concentrated stress at toes of weldments,lamellar tearing under bead due to surface traction,property change in heat affected zone,residual stress and strain restraint, andwelding defects and hydrogen-induced cracking. Therefore, when using welded tubular joints, precise analysis, thorough quality control, and a high degree of skill are required in every stage of production, from design and selection of material through fabrication and inspection.By use of casting leg nodes, most of these problems can be solved because of (1) reduction of stress concentration due to smoothly changing wall thicknesses, (2) homogeneous strength inherent to the castings, (3) separation of weldment from the structural discontinuity, and (4) constant quality assured by lot production with standardized inspection at the shop. Thus, with casting leg nodes it is possible to produce highly reliable leg structures as described in the following. Design of Leg Nodes Fig. 3 shows theoretically obtained stress distributions in a welded tubular joint. The hot spots, where stress concentrates and yielding initiates, are located at junctions of members designated by the dots in Fig. 3. It is known that the hot-spot stresses consist mainly of bending stresses in the cylinder walls generated by out-of-plane bending moments along the intersection of tubular members. JPT P. 1271＾

Considerations in the Design of Large-Size Welded Tubular Truss Joints

The rational design of welded truss joints built from tubes has been complicated by the radial flexibility of the chord tube wall. For certain types of joints this flexibility might be efficient, under static axial loads, since it provides, under increasing loads, a load-distributing action between branch members and chord tube. However, for most joints the flexibility is a source of severe stress concentrations. In connections subjected to dynamic loads, as for example offshore well drilling structures, stress concentrations can be most hazardous. Although even an approximate analysis of tubular joints of different configuration is extremely difficult, several designs are presently used and aimed at reducing the influence of the wall flexibility. This objective can be achieved by using thicker-walled tubes, or by inter-welding incoming branch members. In the latter instance the force acting normal to the chord tube can be reduced considerably. A second group of joints incorporates gusset plates or stiffening rings, which are intended to either contain the load transfer primarily within the gusset plate or to provide an element—the stiffening ring—to stiffen the chord wall and to distribute the forces over a larger part of the chord member wall.