scholarly journals Fatigue Strength Curve for Tubular Joints of Offshore Structures under Dynamic Loading

Dynamics ◽  
2021 ◽  
Vol 1 (1) ◽  
pp. 125-133
Author(s):  
Sudath C. Siriwardane ◽  
Nirosha D. Adasooriya ◽  
Dimitrios Pavlou
Keyword(s):  
Fatigue Strength ◽  
Hot Spot ◽  
Offshore Structures ◽  
Hot Spot Stress ◽  
Tubular Joints ◽  
Jacket Structures ◽  
Strength Curve ◽  
Tubular Joint ◽  
Stress Concentration Factors

Offshore structures are subjected to dynamic environmental loads (wave and wind loads). A stress-life fatigue strength curve is proposed for tubular joints which are in the splash zone area of offshore jacket structures. The Det Norske Veritas (DNV) offshore structures standards given design T-curve in the air is modified with the environment-dependent parameters to obtain this fatigue strength curve. Validity of the curve is done by comparing fatigue lives given by the proposed curve with experimental fatigue lives of tubular joints tested in seawater under different loading conditions. The fatigue assessment of a case study tubular joint is performed using the proposed curve. Nominal stress ranges of the members, which are connected to the joint, are obtained by dynamic analysis of the jacket structure. Stress concentration factors are utilized with the nominal stresses to obtain the hot spot stress ranges. Fatigue lives are calculated and compared with the conventional approach. Hence the applicability and significance of the proposed fatigue strength curve are discussed.

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.

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.

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.

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

1985 ◽  
Vol 107 (1) ◽  
pp. 60-67
Author(s):  
S. Dharmavasan ◽  
W. D. Dover
Keyword(s):  
Stress Concentration ◽  
Stress Analysis ◽  
Mixed Mode ◽  
Hot Spot ◽  
Offshore Structures ◽  
Hot Spot Stress ◽  
Analysis Techniques ◽  
Mode Stress ◽  
Concentration Factors ◽  
Stress Concentration Factors

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.

Uncertainty Modeling for Fatigue Strength Assessment of Welded Structures

1998 ◽  
Vol 120 (2) ◽  
pp. 97-102 ◽  
Author(s):  
W. Fricke ◽  
A. Mu¨ller-Schmerl
Keyword(s):  
Fatigue Strength ◽  
Hot Spot ◽  
Offshore Structures ◽  
Fatigue Tests ◽  
Published Data ◽  
Hot Spot Stress ◽  
Total Uncertainty ◽  
Strength Assessment ◽  
Characteristic Values ◽  
Fatigue Strength Assessment

The results of fatigue tests are characterized by much scatter. Such scatter is further increased if data from different test series are combined to derive, for instance, characteristic values for individual types of welded joints used in codes. Characteristic values are normally applied to the design of fatigue-resistant ship and offshore structures in connection with the nominal stress approach using S-N curves. More advanced approaches such as the hot-spot stress approach and the notch stress approach are applied to an increasing extent. Such approaches explicitly consider certain influence factors and allow the scatter of these factors to be treated individually. This way, probably even the total uncertainty can be reduced. After reviewing the different approaches used for fatigue strength assessment, the sources of scatter are addressed and assigned to factors considered in the different approaches. Based on published data of fatigue tests and imperfections observed in real structures, an attempt is made to quantify the uncertainties of the different factors and to draw conclusions for their individual consideration in the approaches mentioned.

Probabilistic S-N Fatigue Assessment Methods for Welded Joints in Offshore Structures

2008 ◽  
Author(s):  
A. Muhammed ◽  
A. Stacey
Keyword(s):  
Welded Joints ◽  
Hot Spot ◽  
Offshore Structures ◽  
Assessment Methods ◽  
Nominal Stress ◽  
Stress Range ◽  
Hot Spot Stress ◽  
Fatigue Assessment ◽  
Jacket Structures ◽  
Stress Estimation

A study of probabilistic fatigue assessment methods for offshore structures confirmed that fatigue life predictions for offshore welded joints is dominated by uncertainties in hot spot stress estimation, arising from uncertainties in nominal stress and stress concentration factors. Analysis of data from previous fatigue studies on North Sea jacket structures conducted in the late 1980s suggests that nominal stresses are, in general, overestimated by about 30% and the COV of the bias is about 0.35. The study demonstrated that in-service fatigue failure probability is not only dependent on the COV but also on the median bias of the nominal stress range. A number of nominal stress COVs are recommended with associated median values for general offshore application. Distributions are also suggested for other variables such as Miner’s damage sum and the S-N design curve coefficients. Methods based on long-term stress range distribution with random parameters to cater for uncertainties in stress estimation are described and example calculations are given.

scholarly journals Improvement of trawler hull structure under condition of ensuring fatigue strength

2021 ◽  
Vol 4 (7(112)) ◽  
pp. 50-59
Author(s):  
Leontii Korostylov ◽  
Dmytro Lytvynenko ◽  
Hryhorii Sharun ◽  
Ihor Davydov
Keyword(s):  
Fatigue Strength ◽  
Fatigue Damage ◽  
Hot Spot ◽  
Fatigue Cracks ◽  
Bending Moment ◽  
Theoretical Method ◽  
Corrosive Wear ◽  
Hot Spot Stress ◽  
Hull Structure ◽  
Internal Forces

The structure of the hull of the project 1288 trawler in a region of fore hold was improved to ensure fatigue strength of assemblies of the intersection of main frames with the second bottom. To this end, a study of the fatigue strength of these assemblies was carried out for the original side structure and two versions of its modernization. Values of internal forces at the points of appearance of fatigue cracks in the compartment have been determined for three design versions of the side. It was found that the greatest forces act in the middle of the fore half of the compartment. Calculations of parameters of the long-term distribution of magnitudes of ranges of total equivalent operating stresses according to the Weibull law in the points of occurrence of fatigue cracks for different design versions of the side grillage have been performed. These parameters were determined for the middle of the fore hold of the vessel and for the areas in which maximum values of bending moment ranges are in effect with and without corrosive wear. Values of total fatigue damage and durability of the studied assemblies were determined. Calculations were carried out by nominal stress method, hot spot stress method, and experimental and theoretical method. It was shown that in order to ensure fatigue strength of the assembly under consideration, it is necessary to extend the intermediate frames of the original version of the side structure to the level of the second bottom fixing them to the deck. It is also necessary to attach a cargo platform to the side thus reducing the frame span. As a result, the level of fatigue damage over 25 years of operation will decrease by about 3.5 times. As it was found, approximate consideration of the slamming effect does not significantly increase the amount of fatigue damage to the assembly. The results of the development of recommendations for modernization of the side structure can be implemented both on ships of the 1288 project and on other ships with a transverse side framing system.

Stress concentration factors for non-90° X-connections made of square hollow sections

1998 ◽  
Vol 25 (2) ◽  
pp. 370-375 ◽  
Author(s):  
J A Packer ◽  
J Wardenier
Keyword(s):  
Stress Concentration ◽  
Strong Influence ◽  
Hot Spot ◽  
Steel Structures ◽  
Hot Spot Stress ◽  
Hollow Section ◽  
Measured Stress ◽  
Concentration Factors ◽  
Stress Concentration Factors ◽  
Hollow Structural Sections

A number of fatigue experiments and stress concentration factor measurements on non-90°, square hollow section X-connections have been carried out. Comparison of the measured stress concentration factors with those derived from existing parametric formulae for 90° T- and X-connections showed a strong influence of the brace angle. A tentative extension of the range of validity of the parametric formulae for 90° T- and X-connections for other brace angles has been derived.Key words: steel structures, connections, fatigue, hollow structural sections, hot spot stress, stress concentration factors.

Structural, technological and Aesthetical Considerations for the Detailing of Steel Tubular Joints

2018 ◽  
Author(s):  
Vasil Georgiev Georgiev ◽  
Dimitar Dakov ◽  
Yavor Mihov
Keyword(s):  
Fatigue Strength ◽  
Stress Concentration ◽  
Civil Engineering ◽  
Fatigue Cracks ◽  
High Stress ◽  
Traffic Sign ◽  
Tubular Joints ◽  
Joint Repair ◽  
Tubular Joint ◽  
Supporting Structures

<p>For the majority of steel outdoor facilities (towers, masts, billboards and traffic-sign supporting structures) wind loading is the governing factor for determining their resistance and stiffness. In many cases fatigue-related issues appear, with cracking and failure in the welded connections of tubular joints or in the parent metal adjacent to the welds.</p><p>Structural detailing of the joints in steel tubular structures subjected to repeated cyclic loading is of great importance for their fatigue strength. Sharp changes in the shape, sharp turns in the welds and notches give rise to high stress concentration. The combined effect of discontinuities and stress concentration is the main cause for the formation and propagation of fatigue cracks. When detailing the erection joints it is also necessary to observe technological requirements related to efficiency of fabrication. For the civil engineering works exposed to public it is indispensable to include additional requirements for the aesthetic appearance of their visible structural parts. The design experience shows that applying aesthetic considerations to steel tubular joint detailing may contribute to satisfying the increased fatigue strength requirements.</p><p>The paper presents a study on the wind action on a specific kind of civil engineering works (traffic- sign supporting structures) and the approach used for its determination. The leading structural, technological and aesthetic criteria to be implemented in the detailing of tubular erection joints are formulated. An example of tubular joint destroyed due to propagation of fatigue crack is given and possible options for the joint repair are proposed. Numerical modeling and analyses of the original and repaired joints have been carried out in order to make conclusions for the advantages and shortcomings of the joint repair options.</p>

Sign in / Sign up

Export Citation Format

Share Document