Assessment Of The Suitability Of Defects In Large Fillet Welds In Crude Qil Storage Tanks

The large fillet weld of storage tank is the key part of the study on the stability and strength of storage tank.Its welding structure and the damage in service will produce the crack defect,and cause the crack to expand under the complicated load.At present,the settlement of foundation generally exists in the tank area,and the uneven settlement of foundation has the most serious influence on the tank.In this thesis,by means of experiment,theoretical calculation and finite element numerical simulation,the crack propagation of in-service storage tank with crack defect after uneven settlement of foundation is studied:（1）The basic mechanical properties of 12Mn Ni VR steel base metal and welded joint for large storage tank were obtained by uniaxial tensile test.The yield strength of the base metal was 628 MPa,the tensile strength was 693 MPa,and the yield strength of the welded joint was 507 MPa,the tensile strength is 639 MPa.According to the method provided in BS7910,the impact energy obtained by Charpy impact test is transformed into the static fracture toughness of the material,which is 197.56 k J/m² at the base metal position and 93.01 k J/m² at the welding joint position（2）A three-dimensional full-scale finite element model of large-scale storage tank was established by using shell-solid coupling technique.The results show that the maximum Mises stress is 360.8 MPa,the maximum axial stress is 426.613 MPa,and the maximum circumferential stress is 408.146 MPa,both occurred in the inside of the toe of the fillet weld.A three-dimensional full-scale finite element model of large-scale storage tank was established by using shell-solid coupling technique.The results show that the maximum Mises stress is 360.8 MPa,the maximum axial stress is 426.613 MPa,and the maximum circumferential stress is 408.146 MPa,both occurred in the inside of the toe of the fillet weld.（3）The finite element model is used to study the foundation settlement.Firstly,the single harmonic settlement is studied to analyze the influence of different harmonic number,harmonic amplitude and liquid level on the stress response of the tank,the stress response of the large fillet weld is analyzed.Under the effect of high order（5～6 order）harmonics,the large fillet weld shows stress catastrophe between wave crest and wave trough,the settlement data of two 10×10⁴m³ tanks in a storage depot in northwest China are fitted as a continuous settlement curve expressed by Fourier series along the circumference of the tanks.Based on the analysis of the stress distribution of the large fillet weld of the storage tank under the action of uneven settlement,the results show that the local stress of the large fillet weld changes sharply when the settlement difference between the two points is large,it may approach or exceed the yield limit of the material,and it is easy to be invalidated.（4）Crack model is established to analyze the influence of crack size and crack location on crack driving force under hydrostatic pressure and non-uniform settlement,and to calculate the j-integral value at the crack depth,the critical value of material fracture toughness is used to judge whether the crack is unstable or not,and when the J integral of the crack reaches the critical value J_（40）C,the corresponding limit foundation settlement difference is used as a criterion to judge the crack instability,the limit values for different crack sizes are given.