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.

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.

Stress Concentration in Tubular Joints

1977 ◽  
Vol 17 (04) ◽  
pp. 287-299 ◽  
Author(s):  
A.B. Potvin ◽  
J.G. Kuang ◽  
R.D. Leick ◽  
J.L. Kahlich
Keyword(s):  
Stress Concentration ◽  
Computer Program ◽  
Fatigue Cracking ◽  
Offshore Structures ◽  
Parameter Study ◽  
Stress Concentrations ◽  
Element Analysis ◽  
Analytical Technique ◽  
Tubular Joints ◽  
Hostile Environments

Abstract Fatigue cracking caused by stress concentrations in tubular joints has been observed in some fixed platforms installed in hostile environments. platforms installed in hostile environments. Consequently, the ability to assess the magnitude of the stress concentration is a Prerequisite to dealing with the fatigue problem of tubular joints. This paper deals with the problem of computing the stress concentration in three types of simple, nonreinforced joints: T-joints, K-joints, and TK-joints. Semi-empirical equations are presented for estimating the stress concentration due to axial loads and bending moments. Introduction In off shore structures stress concentrations usually occur at the intersections of tubular members (i.e., tubular joints). For some joints, the stress concentration can produce a maximum stress at the intersection as high as 20 times the nominal stress acting in the members. Stress concentrations have aggravated the fatigue of tubular joints in many existing offshore structures. Therefore, an accurate computation of stress concentrations is of utmost importance in a tubular joint design. The first part of this paper presents a discussion of the finite-element analysis method and the associated computer program developed exclusively for the analysis of tubular joints. The second part of this paper describes the parameter study carried out by means of the computer program. Formulas for estimating stress-concentration factors for simple joints commonly used in offshore structures are derived from the results of the parameter study. The usage of the resulting formulas is illustrated by a numerical example. ANALYTICAL TECHNIQUE In offshore structures such as fixed platforms and semisubmersible drilling vessels, tubular members comprise the main load-carrying components. Examples of cracking and even complete separation at the intersections of such members have been cited previously throughout the literature. A typical example of such an intersection is shown in Fig. 1. The member of greatest diameter will be referred to as the chord. The smaller diameter members framing into the chord will be called branches. The sections of the chord wall lying within the branch intersection line (if present) will be called plugs. Fig. 2 shows the various simple joint types referred to in this paper. Because of the relative complexity of the geometrical configuration of tubular intersections as well as the thin-shell theory governing their behavior, reliable prediction of the stresses in such joints by analytical techniques has proven to be costly as well as difficult. Early attempts at analysis ranged from elementary strength-of-materials approaches such as the "punching shear" method to more complicated treatments that solve the governing equations by means of Fourier Series superposition. SPEJ P. 287

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.

THREE FACE ATTACHMENT RETROFIT AGAINST FATIGUE CRACKING AT STEEL GIRDER WEB PENETRATION

2018 ◽  
Vol 5 (2) ◽  
Author(s):  
Chihiro Sakamoto ◽  
Masahiro Sakano ◽  
Hideyuki Konishi ◽  
Takashi Fujii
Keyword(s):  
Fatigue Strength ◽  
Stress Concentration ◽  
Stress Reduction ◽  
Fatigue Cracking ◽  
Highway Bridges ◽  
Lower Surface ◽  
Strength Properties ◽  
Fatigue Tests ◽  
Lower Side ◽  
Steel Girder

Fatigue cracking in steel girder web penetration details is so dangerous that it can break steel girders. Since a number of highway bridges have such web penetration details in Japan, it is of urgent importance to grasp these fatigue strength properties. In this study, we investigate stress reduction effects of three face attachment retrofit through fatigue tests using a large girder specimen with web penetration details where cross beam lower flanges are connected to the lower surface of a slot by welding. As a result, there is very little difference between two and three face attachments about stress reduction effects, while they are more effective than one face attachment. The upper side attachment is more effective than the lower side attachment, while both side attachment is best. Two and three face both side attachments can reduce about 40% of stress concentration, while two and three face upper side attachments can reduce 50– 60%.

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.

The Influence of Corrosion Pits and Cold Expanded Fastener Holes on the Fatigue Life Aluminium 7075-T651

2014 ◽  
Vol 891-892 ◽  
pp. 87-92 ◽  
Author(s):  
Benjamin Withy ◽  
Stephen Campbell ◽  
Glenn Stephen
Keyword(s):  
Fatigue Crack ◽  
Fatigue Life ◽  
Stress Concentration ◽  
Air Force ◽  
Fatigue Tests ◽  
Expansion Process ◽  
Cold Expansion ◽  
Concentration Factors ◽  
Stress Concentration Factors ◽  
Corrosion Pits

The Royal New Zealand Air Force (RNZAF) utilised the split sleeve cold expansion process to increase the fatigue life of fastener holes in the wings of the C130 transport fleet. As part of the validation of the fatigue improvements offered by the process the Defence Technology Agency conducted a series of fatigue tests on cold expanded fastener holes in aluminium 7075-T651, including specimens with corrosion induced after the cold expansion process had been performed. This research conducted an analysis of fatigue crack origins and modelled the stress concentration factors generated as a result of the corrosion pits. These results were used to explain the differing fatigue life and s-n curves produced by corroded and non-corroded fatigue specimens and the location of crack initiation sites around corroded cold expanded fastener holes.

Improvement of performance of bolt-nut connections, Part I: Simulations

2014 ◽  
Vol 228 (17) ◽  
pp. 3069-3077 ◽  
Author(s):  
GH Majzoobi ◽  
M Agh-Mohammad Dabbagh ◽  
P Asgari ◽  
MK Pipelzadeh ◽  
SJ Hardy
Keyword(s):  
Numerical Simulation ◽  
Fatigue Life ◽  
Stress Concentration ◽  
Numerical Simulations ◽  
Load Distribution ◽  
Inflection Point ◽  
Close Agreement ◽  
Three Dimensional ◽  
Fatigue Tests

The performance of bolt-nut connections can be improved by enhancing fatigue life of the connections. This can be accomplished by reducing the stress concentration in the threads of the connection. This investigation consists of two parts. In this part (part I), load distribution in threads of some ISO bolts is computed by three-dimensional numerical simulation and Stockley-proposed relations. The results show a close agreement between Stockley relations and the simulations for nearly all bolt sizes. The results indicate that stress concentration is nearly constant regardless of the bolt size. It is also found that the load percentage carried by the first thread varies from 35% for M6 and reaches to 58% for M20 and M30 ISO bolts. The results suggest that the rate of load distribution changes at a point of inflection, i.e. the rate after the inflection point diminishes as the bolt size decreases, whereas before this point, the trend of the rate is reversed. In part II (to be submitted separately), various techniques are employed for the reduction of stress concentration and enhancement of fatigue life of the connections. The techniques are evaluated by numerical simulations and fatigue tests.

Multiaxial Fatigue of 6061-T6 Aluminum Alloy under Corrosive Environment

2016 ◽  
Vol 853 ◽  
pp. 77-82
Author(s):  
Xu Chen ◽  
Rui Si Xing ◽  
Xiao Peng Liu
Keyword(s):  
Aluminum Alloy ◽  
Fatigue Life ◽  
Stress Amplitude ◽  
Low Cycle Fatigue ◽  
Cyclic Hardening ◽  
Cyclic Softening ◽  
Fatigue Tests ◽  
Plastic Strain Amplitude ◽  
Multiaxial Fatigue ◽  
Corrosive Environment

Aluminium alloys are widely used in the fields of automobile, machinery and naval construction. To investigate the effect of non-proportional loadings and corrosive environment on the fatigue resistance of 6061-T6 aluminum alloy, a set of uniaxial and multiaxial low cycle fatigue tests were carried out. Firstly, the results of uniaxial tests showed that the alloy exhibited cyclic hardening then cyclic softening. With the increase of stress amplitude the cyclic softening became pronounced. The increasing of plastic deformation was basically cyclically stable with small plastic strain amplitude accumulation when the stress amplitude was lower than 200MPa ,while it was increasing rapidly when the stress amplitude was higher than 220MPa. Secondly, it was observed that non-proportional cycle additional hardening of 6061-T6 aluminum alloy was little. While the fatigue life was badly affected by the loading paths. Thirdly ,the fatigue corrosion interactions were also talked about in details by performing the tests under the same loading conditions with corrosive environment. The experiment proved that the seawater corrosion has huge impact on fatigue life under pH 3. Finally, a multi-axial fatigue life prediction model was used to predict the fatigue life with or without the corrosive environment which showed a good agreement with experimental data.

Fatigue Life Properties of Circumferentially-Notched Bar of Austenitic Stainless Steel With Various Concentration Factors

2018 ◽  
Author(s):  
Naoaki Nagaishi ◽  
Michio Yoshikawa ◽  
Saburo Okazaki ◽  
Hisao Matsunaga ◽  
Junichiro Yamabe ◽  
...  
Keyword(s):  
Stainless Steel ◽  
Fatigue Life ◽  
Stress Concentration ◽  
Austenitic Stainless Steel ◽  
Stress Concentration Factor ◽  
Concentration Factor ◽  
Stress Ratio ◽  
Ambient Air ◽  
Fatigue Tests ◽  
Concentration Factors

Fatigue tests were performed using three types of round-bar specimens of Type 304, meta-stable, austenitic stainless steel. The specimens had circumferential notch with stress concentration factors, Kt, of 2, 3 or 6.6. Load controlled fatigue tests were conducted at stress ratio, R, of 0.1 and −1 in ambient air at room temperature. At R of 0.1, fatigue life was decreased with an increase in the stress concentration factor. Conversely, at R of −1, the stress concentration factor had little influence on the fatigue life. To understand the mechanism of the stress ratio effect, local deformation behavior at and beneath the notch root during the fatigue test was computed by means of finite element analysis considering that the plastic constitutive model describes the cyclic stress-strain response.

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