Steel Tubular Joints Research Articles

Fatigue crack growth in stiffened steel tubular joints in seawater environment

Welded steel tubular structures commonly used for offshore platforms are susceptible to fatigue failure due to stress concentration at welded joints and environmental loading which is cyclic in nature. Offshore structures contain flaws ranging from microscopic material defects to macroscopic geometric imperfections, due to fabrication and construction process. Such flaws can develop into cracks due to influence of cyclic wave loading in an aggressive marine environment. Due to this and many other causes, maintaining structural integrity of these structures has been a major concern for the oil industry. Hence a model for crack initiation life and crack growth life estimation has been developed by conducting constant amplitude corrosion fatigue tests on internally ring-stiffened steel tubular T and Y joints under freely corroding conditions. Biologically inert synthetic seawater was used as the corrosive environment. Tests in seawater were carried out with a loading frequency of 0.2 Hz. The studies were made with respect to tropical environment similar to Indian offshore. The corrosion fatigue crack growth data obtained during the tests and the Fracture Mechanics (FM) model developed are discussed in this paper.

Fatigue Behavior of Internally Ring-Stiffened Welded Steel Tubular Joints

The use of internal ring stiffeners in offshore steel jacket platforms is a subject of controversy because of their peculiar failure pattern, observed in several tests in the early 1980s. It is estimated that at least 2,000 internally ring-stiffened steel tubular joints are in service in the North Sea. Despite the fact that internal ring stiffeners have been introduced in many offshore platforms, none of the design codes provide quantitative design guidance. Therefore, to create an extensive database on internally ring-stiffened steel tubular joints, experimental investigations were conducted at the Structural Engineering Research Centre, in Chennai, India, on the fatigue behavior of internally ring-stiffened steel tubular joints. Various loading conditions, joint geometries, and environments were considered. In this paper, the experimental results are compared with API and DEn design curves and the results are discussed.

Behavior of Internally Ring-Stiffened Joints of Offshore Platforms

As part of a larger experimental program on assessment and rehabilitation of damaged, internally ring-stiffened, steel tubular joints of offshore platforms, experimental investigation was carried out on undamaged, internally ring-stiffened tubular joints to study their behavior, determine their strength, and establish a benchmark. The paper presents, in detail, the experimental investigation carried out on tubular joints that are stiffened internally with three annular rings under axial brace compression loadings. The chord and brace diameters of the tested T and Y joints were 324 and 219 mm and their thicknesses were 12 and 8 mm, respectively. The tested joints are approximately a quarter of the size of the largest joints in the platforms built in a shallow water depth of 80 m in the Bombay High field in the Arabian Sea. Bending of the chord as a whole was observed to be the predominant mode of deformation of the internally ring-stiffened joints, in contrast to the ovaling and punching shear of the unstiffened joints. Strength of the internally ring-stiffened joints was found to be almost twice that of the unstiffened joints of the same dimensions.

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.

Elastoplastic behaviour of unstiffened and stiffened steel tubular T-joints

An offshore structure such as a jacket-type tower or a semi-submersible drilling platform consists of a very large number of steel tubular joints. Due to high stress concentration near such a joint, plastic deformation often occurs which ultimately leads to the failure. In this paper, static elastoplastic behaviour of two types of stiffened and unstiffened T-joints are studied using assumed strain degenerated shell elements. The variation of stress and flow of plasticity with the increase in load is studied for all the above cases. The results are found to be in good agreement with experimental results.

Evaluation of stress intensity factors for steel tubular T-joints using line spring and shell elements

This paper develops a simplified method for evaluating stress intensity factors in steel tubular T-joints. Line spring elements incorporated into shell models are used for the analysis. A general-purpose finite element package “ABAQUS” is initially used for stress analysis to determine problem spot areas where cracks may develop. The line spring element (LS6) is thereafter utilized in stress intensity factor computations. The results compare favorably with the existing numerical results which are based on the more cumbersome three-dimensional finite element method. This approach will provide an economical tool of analysis for the detection of fatigue cracks, particularly in welded steel tubular joints in offshore structures.

Corrosion Fatigue of Stiffened Offshore Steel Tubular Joints

Corrosion fatigue is a complex phenomenon involving many variables. Welded-steel tubular joints of offshore structures are subjected to corrosion fatigue due to the combined action of corrosive seawater environment and cyclic wave forces. This paper addresses the aforementioned problem, mainly with respect to internally ring stiffened steel tubular joints. The studies were made with respect to tropical seawater environment similar to that in the Bombay High offshore region where bulk of the Indian oil is produced. Corrosion-fatigue tests were conducted on five welded-steel tubular \iT and \iY joints under constant-amplitude axial-brace loading at a frequency of 0.2 Hz. The studies have shown reduction in fatigue life when compared to \iS-\iN curves recommended by the codes that are mainly based on air fatigue tests. Based on crack-growth data obtained from the tests, a model has been proposed to predict the remaining life of stiffened tubular joints under corrosion fatigue.

Structural Efficiency of Internally Ring‐Stiffened Steel Tubular Joints

Welded tubular structures are widely used for offshore platforms. In India, there are more than 120 offshore platforms in the Bombay High region, all of which are jacket‐type structures consisting of welded tubular members. The tubular joints of these structures are subjected to stress concentration, which because of the increased hot‐spot stresses reduces the fatigue life of the structure. Also, the ultimate strength of the tubular joints is very much reduced due to the local buckling effect of the tubular members. Based on the analytical and experimental studies conducted at the Structural Engineering Research Centre, at Madras, it is observed that these deficiencies of tubular joints can be reduced to a great extent by providing internal ring stiffeners in the chord members. Internal ring stiffeners are found to reduce stress concentration, increase fatigue life under cyclic loads, and also improve the ultimate strength of the joints. The results of investigations on internally ring‐stiffened steel tubular T and Y joints are presented in this paper.

Analytical and experimental investigations on internally ring stiffened steel tubular joints: Murthy, D.S.R., Rao, A.G.M., Gandhi, P., Thandavamoorthy, T.S., Pant, P.K. and Murty, V.S.R. Conf. Fatigue and Fracture in Steel and Concrete Structures, ISFF '91, Madras, India, Dec 1991 Vol 2 (Oxford & IBH Publishing Co. Pvt. Ltd, New Delhi, India, 1991) 715–728

Analytical and experimental investigations on internally ring stiffened steel tubular joints: Murthy, D.S.R., Rao, A.G.M., Gandhi, P., Thandavamoorthy, T.S., Pant, P.K. and Murty, V.S.R. Conf. Fatigue and Fracture in Steel and Concrete Structures, ISFF '91, Madras, India, Dec 1991 Vol 2 (Oxford & IBH Publishing Co. Pvt. Ltd, New Delhi, India, 1991) 715–728

Behavior of Unstiffened and Stiffened Steel Tubular T-Joints

Offshore towers standing in deep sea are susceptible to cyclic nature of wave forces. Fatigue failure initiates at the welded tubular joints of these structures where a very high stress gradient and localized stress concentration exist due to abrupt changes in the geometry. Stress concentration plays a vital role in the design of steel tubular joints, as it directly affects the fatigue life of the structures. This paper presents results of an experimental study on the behavior of an unstiffened and a stiffened steel tubular welded T-joint under axial brace loading. The effect of internal ring stiffeners has been studied theoretically and experimentally. In the experimental technique, the specimens were extensively strain-gaged and the stress distribution determined under static load. The analytical results have been obtained by finite element technique.