Formulas for Variation of Stress Concentration Around Intersection in Tubular Joints

1988 ◽  
Vol 110 (4) ◽  
pp. 237-245 ◽  
Author(s):  
R. Sundaravadivelu ◽  
M. Hariharan ◽  
C. Ganapathy
Keyword(s):  
Fatigue Life ◽  
Stress Concentration ◽  
Current Practice ◽  
Saddle Points ◽  
Offshore Structures ◽  
Exploration And Exploitation ◽  
Empirical Formulas ◽  
Tubular Joints ◽  
Fatigue Design ◽  
Semi Empirical

Many of the offshore structures used for oil exploration and exploitation are built of tubular members. The repeated damages in tubular joints have clearly shown that the safety of these structures depends on the safe fatigue design of the joints. The fatigue life of the joint depends very much on the Stress Concentration Factor, SCF. The current practice is to use semi-empirical formulas proposed by various investigators to determine SCF. While some of the formulas give the maximum SCF, others give the SCFs at both the crown and saddle points. The design of fatigue life using the value of maximum SCF is conservative, while using SCFs at crown and saddle is unsafe. The main aim of this paper is to propose a set of formulas for T and Y-joints, which gives not only the magnitude of maximum SCF as in the presently available formulas, but also the variation of SCF around intersection.

Experimental studies to determine strain concentration factors for space tubular joints

1987 ◽  
Vol 22 (4) ◽  
pp. 237-245 ◽  
Author(s):  
R Sundaravadivelu ◽  
C G Nandakumar ◽  
S K Srivastava ◽  
C Ganapathy
Keyword(s):  
Stress Concentration ◽  
Stress Concentration Factor ◽  
Concentration Factor ◽  
Experimental Studies ◽  
Offshore Structures ◽  
Strain Concentration ◽  
Tubular Joints ◽  
Orthogonal Plane ◽  
Concentration Factors ◽  
Semi Empirical

This paper presents studies carried out on grouted and ungrouted space tubular joints, with K in one plane and T in the orthogonal plane. Three models have been tested for different load cases adopting T, K, and both T and K (STK) configurations. The tubular joints used in offshore structures, especially in jacket platforms, are multiplanar and some are grouted and hence their study is essential. The stress concentration factor, SCF, as determined using the semi-empirical formulae which are applicable for planar joints are not appropriate for space, grouted joints. The results of the present study indicates that the SCF as determined by formulae for simple joints is unsafe in the plane of K and conservative in the plane of T for the space joint. The effect of the grout is to reduce the stress concentration.

Stress Concentration Factor for some DTK Joints Based on Finite Element

2011 ◽  
Vol 58-60 ◽  
pp. 1482-1486
Author(s):  
Long Guo ◽  
Xiu Feng Zhang
Keyword(s):  
Finite Element ◽  
Stress Concentration ◽  
Axial Load ◽  
Thin Shell ◽  
Nonlinear Least Squares ◽  
Shell Element ◽  
Offshore Structures ◽  
Empirical Formulas ◽  
Tubular Joints ◽  
The Difference

By research of simple tubular joints, thin-shell element analyses have been conducted for thousands of DTK joints frequently used in offshore structures in this paper. These joints are subjected to axial load, by use of nonlinear least squares fitter, the above results are derived a new set of parametric equations, in terms of several non-dimensional geometric ratios γ, ξ, β, τ and θ with the Origin software. Separate analyses have been conducted for the difference of SCFs in DTK joints and SCFs for TK joint from empirical formulas. The SCF results of these joints could act as a good reference for both further academic studies on SCF of other complex joints and engineering designing works.

Fatigue Life of Cathodically Protected Tubular Joints of Offshore Structures

1998 ◽  
Vol 120 (4) ◽  
pp. 232-236
Author(s):  
D. S. Ramachandra Murthy ◽  
P. Gandhi ◽  
G. Raghava
Keyword(s):  
Fatigue Life ◽  
Stress Concentration ◽  
Cathodic Protection ◽  
Fatigue Cracking ◽  
Offshore Structures ◽  
Fatigue Tests ◽  
Corrosive Environment ◽  
Sacrificial Anodes ◽  
Tubular Joints ◽  
Steel Jacket

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.

FATIGUE LIFE OF WELDED CIRCULAR TUBULAR JOINTS IN OFFSHORE STRUCTURES

1984 ◽  
pp. 535-542
Author(s):  
I. Ryan ◽  
N. Recho ◽  
L. Regnier ◽  
H.P. Lieurade
Keyword(s):  
Fatigue Life ◽  
Offshore Structures ◽  
Tubular Joints

Stress Concentration Factors Calculation: Analytical and Numerical Approaches for Welded Tubular Joints

2020 ◽  
Author(s):  
Nathalia Paruolo ◽  
Thalita Mello ◽  
Paula Teixeira ◽  
Marco Pérez
Keyword(s):  
Stress Concentration ◽  
Oil And Gas ◽  
Hot Spot ◽  
Experimental Tests ◽  
Oil And Gas Industry ◽  
Offshore Structures ◽  
Gas Industry ◽  
Tubular Joints ◽  
Concentration Factors ◽  
Stress Concentration Factors

Abstract In the oil and gas industry, fixed platforms are commonly applied in shallow water production. In-place environmental conditions generates cyclic loads on the structure that might lead to structural degradation due to fatigue damage. Fatigue is one of the most common failure modes of offshore structures and is typically estimated when dimensioning of the structure during design phase. However, in times when life extension of existing offshore structures is being a topic in high demand by industry, mature fields may represent an interesting investment, especially for small companies. Concerning fixed platforms, composed mainly by welded tubular joints, the assessment of hot spot stresses is considered to predict structure fatigue. The estimation of welded joint hot spot stresses is based on the stress concentration factors (SCFs), which are given by parametric formulae, finite element analysis (FEA) or experimental tests. Parametric formulae may be defined as a fast and low-cost method, meanwhile finite elements analysis may be time consuming and experimental tests associated with higher costs. Given these different characteristics, each method is applied according to the study case, which will rely on the joint geometry and associated loads. Considering simple joint geometries several sets of parametric equations found in the literature may be applied. On the other hand, the SCFs calculation of non-studied yet complex joints consider known formulae adapted according to the under load joint behavior and geometry. Previous analysis shows that this adaptation may furnish different results compared to those obtained by FEA. Furthermore, it is observed that even for simple joints the results derived from the different methods may differ. Given their importance for the oil and gas industry, since they are the basis for the assessment of the fatigue life of welded tubular joints which may impact on additional costs related to maintenance and inspection campaigns, the estimation of SCFs must be the most accurate as possible. Therefore, this paper intends to investigate the differences between results derived from parametric formulae and different FEA studies.

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

1998 ◽  
Vol 120 (1) ◽  
pp. 10-19 ◽  
Author(s):  
C. M. Sonsino ◽  
R. Umbach
Keyword(s):  
Fatigue Life ◽  
Crack Initiation ◽  
Large Scale ◽  
Fatigue Behavior ◽  
Cast Steel ◽  
High Strength ◽  
Offshore Structures ◽  
Fine Grained ◽  
Tubular Joints ◽  
Medium Strength

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.

A Multidimensional FEA Approach for Determination of Hot Spot Stresses in Offshore Jacket Structures

2021 ◽  
Author(s):  
Kris Hectors ◽  
Hasan Saeed ◽  
Wim De Waele
Keyword(s):  
Fatigue Life ◽  
Stress Concentration ◽  
Hot Spot ◽  
Wave Spectrum ◽  
Wave Direction ◽  
Tubular Joints ◽  
Jacket Structures ◽  
Concentration Factors ◽  
Stress Concentration Factors

Abstract A new fatigue lifetime assessment approach for offshore jacket structures is presented. It combines a previously developed numerical framework for automated determination of stress concentration factors in tubular joints and a multidimensional finite element modelling approach. The approach is explained based on a case study of an OC4 type offshore jacket. To determine the fatigue life, a directional wave spectrum is combined with the JONSWAP spectrum. The fatigue life of the jacket is assessed for two different sea states. Based on the fatigue analysis the most fatigue critical wave direction is identified. The hot spot stresses in one of the most critical joints are determined and compared to stresses obtained with the Efthymiou equations. The shortcomings of these equations are highlighted and it is shown how the numerical framework can be used to improve the current fatigue design philosophy for offshore jackets which relies on the Efthymiou equations for stress concentration factors in the welded tubular joints.

Issues in structural fatigue design and evaluation

1994 ◽  
Vol 21 (6) ◽  
pp. 903-912 ◽  
Author(s):  
A. C. Agarwal ◽  
P. C. Birkemoe ◽  
A. P. Goel ◽  
R. A. P. Sweeney ◽  
J. C. Thompson ◽  
...  
Keyword(s):  
Stress Concentration ◽  
Transmission Lines ◽  
Crack Detection ◽  
Power Transmission ◽  
Fatigue Loading ◽  
Offshore Structures ◽  
Power Transmission Lines ◽  
Local Boundary ◽  
Fatigue Design ◽  
Stress Concentration Region

Current practices in the design and evaluation of structural components for fatigue are generally semi-empirical. This is because either it is not practical or economical to realistically simulate field conditions in the laboratory testing or the actual conditions are not known precisely. Fatigue life curves based on the constant amplitude cyclic loading may result in unconservative design and evaluations where real load cycles are variable and the loads are of transient nature. There is a need to apply probability-based principles to the fatigue considerations, particularly in the areas of developing representative fatigue loading, and fatigue crack detection. In the area of power transmission lines, there is a need for better understanding of the failure mechanism and rationalize the fatigue design provisions. The state of stress within a stress concentration region depends only on the local boundary geometry, the magnitude of the resultant force, and the material type. This equivalence of stress concentration regions can be exploited to develop design aids for stress concentration factors independent of the global conditions. This paper identifies the above issues in detail and establishes some specific areas where there is a need for improvement in the current practice. Key words: fatigue, structural design, structural evaluation, bridges, power transmission lines, offshore structures.

Automated Identification of Critical Tubular Joints of Offshore Jacket Structure by Deterministic Fatigue Analysis

2017 ◽  
Author(s):  
Shrikarpagam Dhandapani
Keyword(s):  
Fatigue Life ◽  
Fatigue Damage ◽  
Hot Spot ◽  
Offshore Structures ◽  
Fatigue Analysis ◽  
Optimized Design ◽  
Base Shear ◽  
Hot Spot Stress ◽  
Environmental Loads ◽  
Tubular Joints

Fatigue occurs in structures due to the stresses from cyclic environmental loads. Offshore environmental loads being highly cyclic and recurring in nature, fatigue analysis with high degree of accuracy is required for reliable and optimized design of offshore structures. The main aim of this paper is to automate the process of identification of fatigue critical tubular joints of an offshore jacket structure using deterministic fatigue analysis with emphasis on the Hot Spot Stress Range (HSSR), an important measure in estimating fatigue damage, calculated using three different approaches for each tubular joint. The first approach determines HSSR at the time of maximum base shear of the jacket, the second, by calculating the difference between maximum and minimum Hot Spot Stress (HSS) and the third, at all time-instants of the wave cycle. Thus fatigue damage and fatigue life of the tubular joints are estimated using the highest HSSR value and the joints with lower fatigue life are identified as fatigue sensitive joints. This ensures effective identification of critical tubular joints of the offshore jacket structure which needs detailed investigation or redesign for fatigue. The deterministic approach discussed in this paper is applicable to large jackets which contains more number of tubular joints where sophisticated fatigue assessment at the preliminary stage is computationally intensive and manual identification of fatigue critical joints is laborious.

Corrosion Fatigue Tests on Welded Tubular Joints

1985 ◽  
Vol 107 (1) ◽  
pp. 68-73
Author(s):  
T. Iwasaki ◽  
J. G. Wylde
Keyword(s):  
Corrosion Fatigue ◽  
Welded Joints ◽  
Sea Water ◽  
Water Environment ◽  
Offshore Structures ◽  
Fatigue Tests ◽  
Tubular Joints ◽  
Fatigue Design ◽  
Out Of Plane ◽  
The Cost

The corrosion fatigue performance of welded tubular joints is recognized as one of the most important factors in the design of offshore structures. Because of the cost of such tests it has been practice to carry out tests on tubular joints in air and to perform corrosion fatigue tests on simple welded joints. Thus very few corrosion fatigue tests have been carried out on tubular joints. The present paper describes the results of fatigue tests which have been carried out on welded tubular joints both in air and in a sea water environment. The specimens were tubular K and KT-joints with chord dimensions of 168 mm diameter and 6 mm wall thickness. The tests were carried out under out-of-plane bending. The corrosion fatigue tests were carried in sea water at 10° C at a frequency of 10 cycles per min, which is typical of wave periods. The specimens were not cathodically protected. The fatigue results in air and sea water are compared, and discussed in connection with the current fatigue design S-N curves for tubular joints contained in various standards.

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