scholarly journals Fatigue of Jacket Platforms: Effect of Marine Growth Modelling

2005 ◽  
Author(s):  
Franck Schoefs ◽  
Morgan Boukinda ◽  
Christophe Guillo ◽  
Antoine Rouhan
Keyword(s):  
Fatigue Behavior ◽  
Offshore Structures ◽  
Hydrodynamic Coefficients ◽  
Growth Modelling ◽  
Growth Effects ◽  
Growth Profile ◽  
Jacket Structures ◽  
Selection Of

After decades of developments of methods and software, reassessment of jacket structures is still a challenge. This paper focus on marine growth effects in case of fatigue behavior for Jacket offshore structures. Two main items are pointed out: selection of marine growth profile and modeling of hydrodynamic coefficients in presence of marine growth. A complete approach is provided and completed with both sensitivity and uncertainty 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.

scholarly journals Probabilistic Assessment of Degree of Bending in Tubular X-Joints of Offshore Structures Subjected to Bending Loads

2015 ◽  
Vol 2015 ◽  
pp. 1-12 ◽  
Author(s):  
Hamid Ahmadi ◽  
Amirreza Ghaffari
Keyword(s):  
Probability Distribution ◽  
Goodness Of Fit ◽  
Accurate Determination ◽  
Offshore Structures ◽  
Fatigue Reliability ◽  
Distribution Models ◽  
Estimated Parameters ◽  
Jacket Structures ◽  
Kolmogorov Smirnov ◽  
Bending Loads

Fatigue life of tubular joints in offshore structures is significantly influenced by the degree of bending (DoB). The DoB exhibits considerable scatter calling for greater emphasis in accurate determination of its governing probability distribution which is a key input for the fatigue reliability analysis of a tubular joint. Although the tubular X-joints are commonly found in offshore jacket structures, as far as the authors are aware, no comprehensive research has been carried out on the probability distribution of the DoB in tubular X-joints. In the present paper, results of parametric equations available for the calculation of the DoB have been used to develop probability distribution models for the DoB in the chord member of tubular X-joints subjected to four types of bending loads. Based on a parametric study, a set of samples was prepared and density histograms were generated for these samples using Freedman-Diaconis method. Twelve different probability density functions (PDFs) were fitted to these histograms. In each case, Kolmogorov-Smirnov test was used to evaluate the goodness of fit. Finally, after substituting the values of estimated parameters for each distribution, a set of fully defined PDFs have been proposed for the DoB in tubular X-joints subjected to bending loads.

Selection of Design Lower Deck Elevation of Fixed Offshore Platforms for Mexican Code

2015 ◽  
Vol 137 (5) ◽  
Author(s):  
Dante Campos ◽  
César Ortega ◽  
Jorge L. Alamilla ◽  
Alberto Soriano
Keyword(s):  
Tropical Cyclones ◽  
Offshore Structures ◽  
Offshore Platforms ◽  
Mean Sea Level ◽  
Maintenance And Repair ◽  
Equipment Replacement ◽  
Expected Values ◽  
Definition Of ◽  
Selection Of ◽  
Lower Deck

This paper presents the definition of lower deck elevation (LDE) of fixed offshore platforms in Mexican part of the Gulf of Mexico (GoM), which were applied in the Mexican code for design and assessment of fixed offshore structures NRF-003-PEMEX-2007. This was obtained as a result of a decision methodology developed here, an optimization algorithm composed of an objective minimization function subject to a safety restriction and by the application of a set of real structural maintenance and equipment replacement costs. Platform projected service life dependent total costs of LDE contemplate expected costs of structural and equipment maintenance after the occurrence of tropical cyclones and northers, and the expected cost of equipment mobilization. This work regards a group of offshore platforms in the Bay of Campeche whose elevation lies between 15.85 and 19.10 m above mean sea level, as well as recent metocean hazard information. Similar expected values of total costs due to maintenance and repair were obtained, while safety constrains demand the LDE not to be less than +19.10 m.

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.

A New Formulation for Equivalent Hydrodynamic Modeling of the Jack-Up Legs

2002 ◽  
Author(s):  
M. Daghigh
Keyword(s):  
Type Equation ◽  
Offshore Structures ◽  
Geometrical Model ◽  
Hydrodynamic Coefficients ◽  
Slender Bodies ◽  
Pile Model ◽  
New Formulation ◽  
Jack Up ◽  
Inertia Forces ◽  
Drag Term

Regulations of offshore structures suggest the application of Morison type equation for the estimation of forces induced by wave and current on the slender bodies of Jacket and Jack-up structures. However, common values of hydrodynamic coefficients are rarely defined in two different regulations. Estimation of global responses of Jack-up structure, the simplified geometrical model is used, therefore we will try to modify the DNV formulations in order to estimate the hydrodynamic forces on equivalent pile. Finally, the forces on the real structure and the equivalent pile model are compared and it has been shown that the approximation of the inertia forces has more accuracy comparing to the drag force, due to the nonlinear effect in drag term.

RMS Control of Response of Jacket Structure Using Tuned Liquid Column Ball Gas Damper Under Random Ocean Waves

2021 ◽  
Author(s):  
Satheesh Jothinathan ◽  
Deepak Kumar
Keyword(s):  
Genetic Algorithm ◽  
Oil And Gas ◽  
Good Control ◽  
Wave Force ◽  
Ocean Waves ◽  
Good Alternative ◽  
Offshore Structures ◽  
Liquid Column ◽  
Jacket Structures ◽  
Gas Damper

Abstract Jacket structures are one of the most important offshore structures for extracting oil and gas. The fatigue life is affected due to the continuous dynamic wave force experienced by the structure. Generally, the structure is designed so that the dynamic response is small, which increases the cost. So, controlling its response is a good alternative to increase its life span. In this work, a simplified jacket structure under a random sea state is controlled for its response using a tuned liquid column ball gas damper (TLCBGD). The jacket structure in a water depth of 60m is modeled in a surge degree of freedom. The parameters of TLCBGD are optimized using a genetic algorithm for achieving better control in response quantities. For the analysis purpose, the wave is considered stochastic and presented by Pierson–Moskowitz (PM) spectrum of significant wave height 10m. In such a case, the jacket structure response can be presented using the root mean square (RMS) values obtained from the Lyapunov technique. Based on the random vibration analysis theory, the Lyapunov method can be employed to obtain the RMS of the system driven directly without solving the governing differential equation. This method requires the system to be driven by white noise. So, in this study, filters are developed to get the required narrow banded ocean spectrum. It is noticed that the response quantity is highly sensitive to the filter parameters. This is because a slight change in excitation parameters and a change in filter parameters near the system’s natural frequency affect the response significantly. Further, it is seen that the use of the genetic algorithm for tuning the TLCBGD gives very good control on the response quantity of the jacket structure.

Effects of chromic acid anodizing on fatigue behavior of 7050 T7451 aluminum alloy

2021 ◽  
Vol 63 (9) ◽  
pp. 805-810
Author(s):  
Çağrı İlhan ◽  
Rıza Gürbüz
Keyword(s):  
Aluminum Alloy ◽  
Stress Ratio ◽  
Chromic Acid ◽  
Fatigue Behavior ◽  
Fatigue Tests ◽  
Constant Amplitude ◽  
Aluminum Oxide Layer ◽  
Fatigue Life Reduction ◽  
Selection Of ◽  
Scanning Electron

Abstract The effect of chromic acid anodizing (CAA) surface treatment on 7050 T7451 aluminum alloy was presented in this study in terms of fatigue behavior. CAA is a treatment against corrosion by producing aluminum oxide layer (Al2O3) at the surface. However, fatigue performance of 7050 T7451 is affected by the coating. In this study, eight different CAA processes were examined with regard to etching stage of pre-treatments by using an alkaline etchant and/or acid etchants with various immersion times. Optical microscopic examinations were applied in order to determine pitting characteristics for the selection of CAA process parameters before fatigue tests. A CAA process was selected among eight processes in terms of pitting characteristics in order to apply fatigue specimens. Four fatigue test groups were determined to investigate bare condition of 7050 T7451 and sub-stages of the CAA particularly. Constant amplitude axial fatigue tests were conducted on specimens at 91 Hz at stress ratio (R) -1 until run-out criteria, which was 106 cycles. Fatigue life reduction was determined due to pretreatments of CAA. Fracture surfaces of the specimens were examined by scanning electron microscope (SEM) to investigate morphology and crack initiation sites.

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