Federal State Educational Institution of Higher Education Gubkin
Federal State Educational Institution of Higher Education «Gubkin Russian State University of Oil and Gas (National Research University) KINETICS OF FATIGUE CRACKING OF FIXED OFFSHORE PLATFORMS FOR OIL AND GAS PRODUKTION The chief of the Department APS Ph. D, Starokon Ivan
The offshore oil and gas construction Sivash Tavrida Peter Godovanets 2
Methods E is the modulus of resiliency of the material. The expression shows a decrease in the strain energy of deformation of the plate due to the presence of a crack in it, and 2 а is the total length of the crack. This expression was obtained under the assumption that, in the presence of a 2 a crack in the plate, the strain deformation energy is absent in the material volume equal to. The value 4 a. T is the surface energy of the crack, taking into account the formation of two surfaces; T is the specific surface energy equal to the work needed to form a unit of a new surface. 3
Crack classification Drw. 1. Crack classification according to the type of load applied Drw. 2. Scheme of elliptical crack 4
Methods 5
Methods E is the coefficien to felasticity; v is the Poisson's ratio; av – impact elasticity on Charpy’s type specimens; k - dimensionless proportionality coefficient calculated by the formula σb and σt are the temporary resistance and yield limit of steel 6
Impactelasticity of welded joints for various combinations of welded steels Welded steels Impact elasticity at +20 0 С Impact elasticity at -40 0 С k. Gm/sm 2 5 2, 5 Carbon-bearing + lowalloyedorchromium-molybdenum steel Carbon-bearing + austeniticsteel Low-alloyedorchromiummolybdenum + austeniticsteel 7
Result А 0 А 1 А 2 А 3 А 4 1, 2114378 -1, 6577755 11, 743555 -16, 672913 9, 7708125 8
Results Drw. 3. Kinetic diagram of fatigue fracture of welded joints of support blocks of fixed offshore platforms Drw. 4. Scheme of a structural element of FOP with an annular crack 9
Conclusion Describes the process of occurrence, development and opening of cracks for the main structural elements and welded joints. The crack development patterns are described and the value of the stress intensity factor is explained. An algorithm has been developed to calculate the stresses acting in cracks. An increase in the crack length per cycle depends on the magnitude of the stress intensity factor. The crack growth rate for elements and welded joints of offshore platforms is uneven, and is described using the kinetic diagram of the growth of fatigue cracks. Important parameters of this diagram are threshold and critical stress intensity factors. This kinetic diagram is described by three sections of the crack growth rate in millimeters per cycle: low (up to 5 ∙ 10 -5), medium (up to 10 -3) and high (over 10 -3). The boundaries of the transition from one section to another are threshold Kth and critical Kfc values of stress intensity factors. The work carried out calculations that made it possible to establish that for the studied welded joint, the value of the threshold stress intensity factor will be 11. 45 MPa ∙ m 0. 5, and the critical 29 MPa ∙ m 0. 5. Based on the research, the author constructed a kinetic diagram of the growth of fatigue cracks for welded joints of the support block. The principles for constructing the kinetic diagram of fatigue described in this article are applicable to the main structural elements of the support block of the offshore platform. 10
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