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Collaborating Authors
The Second International Offshore and Polar Engineering Conference
ABSTRACT Recently additional fatigue results from an Italian collaborative research program have become available, permitting are-evaluation of both the effect of cathodic protection and the so-called โthickness effectโ. This data defines the fatigue life of 50 mm base member plate-to-plate and tubular joints in both air and sea water with cathodic protection (CP). When combined with previously presented data for 32 mm base member joints, it permits an assessment of the thickness effect for plate-to-plate joints in air and CP, and for tubular joints in CP. The derived thickness effect in air is of similar magnitude to that reported in the literature. However, the thickness effect for the tubular joints appears significantly less than for the T-butt in air. The thickness effect associated with crack initiation life and crack propagation life is also derived. It is notable that the thickness effect associated with crack initiation is markedly reduced in the case of the tubular joints with respect to the plate joints. The efficiency of CP in restoring total life is assessed for both types of joints and both thicknesses. Further analysis on the plate-to-plate data allows an assessment of the effect of CP on crack initiation and crack propagation. INTRODUCTION Two of the most debated topics in the field of "fatigue life of structures" have been the effect of increasing joint dimensions (the thickness effect) and the effect of cathodic protection (CP) on joint life. These effects have been studied by researchers for more than ten years (Cole et aI., 1990; Cole and Vittori, 1991a; 1991b; DeBack, 1981; Department of Energy, 1977; Gurney, 1979; 1981; Gurney and Sharp, 1991; Hilton and Webster, 1989; Manigrasso et at., 1991; Reynolds and Sharp, 1990; Sablok and Hartt, 1991; Vosikovsky and Bell, 1991; Yagi et ai., 1991).
- Energy (0.69)
- Government > Regional Government (0.34)
ABSTRACT The effect of hydrogen contents on crack nucleation time was conducted by using compact tension specimens of 473K tempered AISI 4340 steel in three kinds of environmental solutions under various electrode potentials. The crack nucleation at the notch root is determined by the electrical potential method. When the crack initiates, the voltage differences between the notch mouth starts to increase. The crack initiation times depend on hydrogen contents and the apparent stress intensity factors. These dependencies are successfully explained by a dislocation pile-up model with the hydrogen atom interaction. 1. INTRODUCTION Considerable works have been done to clarify the crack nucleation mechanisms of hydrogen assisted cracking (Troiano, 1760, Beachem, 1972, Leeuwen, 1975, Oriani, 1974, Page, 1982 and Hirose, 1977). In the present paper, the effect of hydrogen contents on crack nucleation time was investigated for 473K tempered AISI 4340 steel in three kinds of environmental solutions; distilled water, 3.5% NaCl solution and 0.1N H2S04 solution. A new dislocation model is proposed to explain the crack nucleation phenomenon. Experiments are performed to compare with the present theory. 2.EXPERIMENTAL PROCEDURE The material used was AISI 4340 steel (wt.%; 0.39C, 0.74Mn, 1.38Ni, O.78Cr, O.23Mo)" Compact tension specimens (see Fig. 1) with notch radius, ฯ = 0.5mm and thickness, B = 5.5mm were machined from cutting slice at the right angle to the axis from hot rolled round bar with 100mm diameter. The specimens were normalized at 1153K for 1 hour and austenized at 1123K for 1 hour and then oil-quenched and tempered at 473K for 2 hours. After the surface layer was removed by grinding, the notch was made carefully by an electrical discharge machine to minimize the worked layer. The specimens were finally finished by electro-polishing. The mechanical properties of the material after heat treatment are given in Table 1.
ABSTRACT The effect of the modification, produced by microbial process, on the interfacial (biofouling I material) solution are considered from an electrochemical and corrosion point of view. The evaluation of the effects due to pH modification, oxygen concentration, carbonate content, presence or not of sulphides on anodic and cathodic processes and also on the passive film stability, allows to describe the effect of biofouling on seawater aggressiveness. In sea water, the non uniforme spatial distribution of the fouling on the surfaces appears as a concause of prevaling localized corrosion forms with respect to the generalized corrosion forms. Specific effects, direct and indirect, connected to the modifications of the solution composition on the cathodic processes kinetics seem to be the main cause of modification of the coupling galvanic current either between different materials either between anodic area, affected by localized corrosion, and the remaining cathodic area of the same material. Specific effects, connected to the modification of the solution composition on the passivity films stability, seem the main cause that can modify the probability of localized corrosion forms developments on passivable materials. In particular, the decrease of surface pH seems able to enhance the kinetic of cathodic oxygen reduction process on stainless steel and to favour the anodic process on copper alloys. High surface carbonate content seems to sustain the hydrogen evolution cathodic process at surface pH near the neutrality even though, normally, diffusional problems make it be ininfluent. The presence of sulphides appears:โto enhance the cathodic process kinetic of oxygen reduction on copper alloys and inhibit it on stainless steels; โto hinder the localized attack repassivation and consequently to increase the probability of localized corrosion onset;
- Well Completion > Well Integrity > Subsurface corrosion (tubing, casing, completion equipment, conductor) (1.00)
- Production and Well Operations > Production Chemistry, Metallurgy and Biology > Corrosion inhibition and management (including H2S and CO2) (1.00)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Materials and corrosion (1.00)
- Reservoir Description and Dynamics > Improved and Enhanced Recovery (0.66)
ABSTRACT The paper describes the results of one year field tests performed in deep water in the North Sea in order to evaluate the stress corrosion cracking behaviour of high strength steels under cathodic protection. The tests were performed on precracked specimens obtained from base materials and welded joints. The results allowed to measure the environmental threshold stress intensity factor (KISCC) and to draw the effect of the mechanical properties of the steel on the susceptibility to stress corrosion cracking. INTRODUCTION Today there is a general request for low alloy steels with yield strength ranging or exceeding 560โ700 MPa for the construction of high loaded components of offshore structures, as it especially occurs for deep sea applications. Unfortunately, the potential susceptibility of high strength steels to stress corrosion cracking is an important obstacle for their widespread use in seawater, affecting the structure safety. Consequently, the effective resistance of the material must be evaluated if high strength steels have to be utilized. Stress corrosion cracking (SCC) has not been identified as a cause of failure of low alloy and carbon steels with yield strength below 700 MPa in seawater even under cathodic protection. It has been observed on steels with yield strength exceeding approximately 1250 MPa. On lower strength steels, stress corrosion cracking has been observed starting from heat affected zone of welding with high hardness (above 46 HRc) (Carter and Hyatt, 1977). Laboratory data and field tests on smooth specimens confirm such trend, showing an overall resistance to stress corrosion cracking under high stress (75% TYS) for steels with yield strength below 1250 MPa. Vice versa, they demonstrate the general susceptibility of steels with yield strengths exceeding about 1450 MPa (Carter and Hyatt, 1977; Schmitt and Phelps, 1970; Leckie and Loginow, 1968; Reinhart, 1967).
- North America > United States (0.28)
- Europe > United Kingdom > North Sea (0.25)
- Europe > Norway > North Sea (0.25)
- (3 more...)
ABSTRACT The crack propagation characteristics of steel A537. under constant load, were studied at 238K.273K and 303K. using fracture mechanics. The fatigue crack propagation in steel A537 at the above testing temperatures was in accordance with Paris law. As the temperature decreased, the fatigue crack growth rate decreased for most region of the crack propagion and the Paris exponent n greatly increased. This caused a much higher acceleration of growth rates at lower temperatures. The curves of the propagation diagram at different temperatures tend to converge with the increase of stress intensity factor range โK and intersect at a certain high โK level. Thus. after the intersection, the crack growth rate at lower temperatures was inversely higher than that at room temperature. The crack growth rates are represented by the Yokobori formula. The fracture surfaces were observed under a scanning electron microscope. showing that the fracture of steel A537 at all the testing temperatures took the form of microvoid coalescence. which characterizes ductile fracture. INTRODUCTION In cold climates, structures such as bridges, towers and buildings may be subjected to temperatures that fall as low as 220K. The materials used in constructing these structures are low-carbon structural steels, rather than steels designed for low temperatures. Because of reduced fracture toughness of such low-carbon structural steels. fatigue cracks are quickly propagated and become unstable at low temperatures. Therefore. to prevent unstable fractures. reliable data on the fatigue crack growth rates of such steels at low temperatures is required for safely designing and determing the inspection period of these structures. This proposal was for Bohai Oil Company. one of the largest offshore oil companies in China. Bohai's winter temperatures fall to 248K and the Bohai Sea stays frozen for almost three months of the year.
J-Integral Estimation Procedure For Weldment Fracture Analysis
Shim, Yong L. (Ohio State University) | Lee, Sung G. (Ohio State University) | Tsai, Chon L. (Ohio State University) | Kim, Dong S. (Shell Development Company) | Jaeger, John J. (U.S. Army Corps of Engineers) | Chasten, Cameron P. (U.S. Army Corps of Engineers)
ABSTRACT A crack driving force estimation procedure was developed for a welded plate subjected to a combined residual stress and external loading. The finite element method was used to predict a residual stress distribution and the J-integral values for a combined loading. Residual stress was determined for a welded plate without a crack. A crack was introduced in the next step and external loading was applied to evaluate the J-integral. A plane stress analysis was carried out for a welded thin plate with a center crack. A two dimensional cross section was analyzed for multi-pass welding of a 1 inch thick plate with a double V-groove. The J-integral values were calculated for a single edge crack located at the centerline subjected to a combined residual stress and external loading. INTRODUCTION During a fusion welding process, a welding arc produces a nonlinear thermal loading which results in residual stress and distortion after welding. As the plate thickness increases the number of welded passes required to complete a full penetration weld also increase. Therefore the plate is subjected to multiple complex thermal cycles and inelastic strain patterns, creating a more severe and accumulated residual stress distribution through the thickness of the plate. This residual stress increases crack driving force and largely affects the strength and resistance to brittle fracture of the structures. Tensile residual stress around the weld area is a major factor contributing to cracking and fracture problems in heavy structures with welded thick plates. Yield stress is usually used as a residual stress magnitude for as-welded structures in fracture assessment guidelines, which is conservative and sometimes results in unnecessary repair of discontinuities. Therefore it is necessary to accurately assess the magnitude and distribution of residual stresses throughout the thickness of the plate.
- Government > Military > Army (0.47)
- Energy (0.35)
Toughness Required Of Frigid Zone Offshore Structure Steel Plate 400Mpa In Yield Point And A Newly Developed Heavy-Thickness Plate
Yajima, H. (Mitsubishi Heavy Industries, Ltd.) | Tada, M. (Mitsubishi Heavy Industries, Ltd.) | Sakai, F. (Mitsubishi Heavy Industries, Ltd.) | Ebara, R. (Mitsubishi Heavy Industries, Ltd.) | Fushimi, A. (Mitsubishi Heavy Industries, Ltd.) | Sugie, E. (Kawasaki Steel Corporation) | Amano, K. (Kawasaki Steel Corporation)
ABSTRACT The authors address a TMCP-manufactured low temperature, high-tensile steel plate 60mm in thickness and 400MPa-class in yield point with the high-heat-input-weldability, which was newly developed specifically for use in fabricating a cone-type fixed offshore structure intended for the operation in the Arctic Ocean. The salient points reviewed are summarized as follows.Required toughness and its concept Steel plate design concept to ensure required characteristics (particularly the high-heat-input- weldability) Strength characteristics of the base plate and high-heat-input welded joints of the newly developed heavy-thickness steel plate INTRODUCTION The trend with offshore structures built for the operation in the Arctic Ocean and other frigid sea areas is toward the increasingly larger size, and the consequent demand for the reduction in their weight has triggered another trend in the selection of their materials toward the use of steels higher in tensile strength. Also, there is just as strong a demand for shortening the construction period through the employment of high-efficiency fabrication method. For these demands to be met, the use of highstrength steel plate excellent in low-temperature toughness and the employment of high-heat-input welding process for highly efficient welding operation are imperative. On the other hand the Thermo-Mechanical Control Process (TMCP) in the field of steelmaking technology has made an impressive stride to the point where even low-temperature high-tensile steel plate can now be produced for practical application(Yajima et al., 1990, 1991, Bessyo et al., 1991). Mitsubishi Heavy Industries, Ltd. experimentally designed recently a cone-shaped sit-onbottom- type fixed offshore structure for the Arctic Ocean application (see Fig.1), and the authors studied the toughness required of its cone section, the most important structural component, as well as a TMCP-manufactured heavy-thickness steel plate newly developed to satisfy such a toughness requirement.
ABSTRACT This paper investigates temperature rise distribution near a crack tip of CT specimens of a mild steel due to plastic work under intermediate loading rate for the purpose of considering the criterion for crack instability under arbitrary loading rate. The dynamic thermal elastoplastic FEM analysis shows the good agreement with experimental temperature rise distributions by using thermo viewer. The calculation results of yield stress in the process zone in the vicinity of a crack tip fairly keeps constant because of opposed effects of strain rate and temperature on yield stress. Moreover yield stress in the process zone increases with increasing loading rates up to a certain rate due to the increase in strain rate, however, the increase in loading rates over the certain value leads to decrease in yield stress which is caused by local heating near a crack tip due to plastic work because there is not enough time to dissipate heat. From above information, the criterion of fracture initiation under arbitrary loading rate is conducted. Experimental Kc values of HT-50 TMCP steel with various loading rates support quantitatively the criterion. 1 Introduction It is well known that crack instability is strongly affected by loading rate and temperature especially in body-centered cubic metallic material as steel. In other words, fracture toughness decreases with increasing loading rate in the region of intermediate loading rates including quasi-static condition and then reaches a minimum and increases again if the loading rate exceed a certain limit (KalthotJ,1986). In the previous study (Toyosada et.al.,1989), we found out the possibility that fracture toughness depends only on strain rate-temperature parameter R defined by Bennet(196S) if the thickness of material is fixed. 1. A quantitative estimation of strain rate dependency on fracture toughness is plausible in the low toughness range since the plastic work is quite small.