Application of Fracture Mechanics Parameters to Spherical Storage Tank Integrity Assessment

: Fracture mechanics parameters have been applied to assess pressure vessel integrity. The pressure vessel analysed here was the ammonia spherical tank (volume of 1800 m 3 , outer diameter 15.12 m, thickness 30 mm) where different cracks in the longitudinal and transverse butt joints were found. The finite element method was used to calculate stress and strain state and to analyse cracks and their influence on spherical storage tank integrity. Toward this end, the stress intensity factors were analytically determined, and compared with the fracture toughness to find out the critical pressure using Failure Assessment Diagram (FAD) in combination with net stress vs. critical stress.


INTRODUCTION
The spherical storage tank for ammonia (volume 1800 m 3 , outer diameter D s = 15.12 m and thickness s e = 30 mm), made of high strength low alloyed steel A36.52 (NIOVAL 47), was tested recently using non-destructive techniques (NDT). The maximum operational pressure was p = 16 bar, whereas test pressure was 30% higher, p i = 20.8 bar. A large number of surface cracks were detected in welded joints, and four critical ones were analysed in this paper, no. 204, 205, 207 and 211, Fig. 1. As explained in [1], it was decided not to repair the storage tank, because of previous experience, indicating that more damage than benefit would be made, [2,3], mainly due to unfavourable microstructure of the steel, used for tank production, [4]. Namely, more cracks were found after repair welding than before, not only because of material, but even more important, due to unnecessary overpressurizing during testing, [5,6].
Therefore, another strategy was used here, based on structural integrity assessment, enabling simple treatment/removal of cracks with a profile milling cutter. Toward this end, the stress intensity factors for detected cracks were calculated and compared with the fracture toughness to assess spherical storage tank structural integrity. Final decision is based on an application of Failure Analysis Diagram (FAD), i.e. combination of brittle fracture and plastic collapse, [7][8][9][10]. Spherical tank is made of A36.52 [1], with chemical composition and mechanical properties given in Tab. 1 and Tab. 2, respectively. According to EN 13445-3 the allowed stress for steel A36.52 is σ all = 240 MPa or 360 MPa, for local conditions, [1]. In that case, minimum thickness for a spherical shell under pressure, p = 16 bar, is t = 26.2 mm, [1].

ASSESSMENT OF THE INTEGRITY OF THE SPHERICAL TANK USING FRACTURE MECHANICS PARAMETERS
During regular periodic inspection of transversal and longitudinal welded joints of the segments of the spherical storage tank for storing liquid ammonia by non-destructive methods, 211 irregularities were detected in the form of unacceptable defects (cracks). Technical regulations do not allow the use of pressure equipment with such defects. Anyhow, since eventual repair would make more damage than benefit, [11,12], these defects were analysed by fracture mechanics using a conservative approach to prove pressure vessel integrity. To calculate stress intensity factors one has to know load and geometry. If the fracture toughness cannot be measured, a conservative value can be used instead, [7,9]. Corrosion and fatigue, as well as residual stresses and proximity of the connections are also taken into account. As the most critical, cracks no. 204, 205, 207 and 211 have been analysed by fracture mechanics principles. The position of detected errors is schematically shown in Fig. 1.
Application of linear elastic fracture mechanics is based on the stress intensity factor, KI, representing load and geometry, including shape and size of crack, on one hand, whereas its critical value, i.e. fracture toughness, K Ic , represents the material resistance to crack growth. Using this interpretation, structure integrity is established as follows: K I ≤ K Ic -the structure integrity is not compromised, K I > K Ic -the structure integrity is jeopardized because of possible brittle fracture.
For the stress intensity factor the following is obtained: Based on the values obtained for the parameters K r and S r for cracks no. 204, 205, 207 and 211 in the Failure Assessment Diagram points are located in the secure part of the diagram shown in Fig. 2. A critical value of crack depth, i.e. the depth at which the stress intensity factor K I reaches its critical value K Ic , is a cr = 8.8 mm.

Finite Element Model and Influence of Processed Cracks on the Stress State
The finite element model of the ammonia spherical tank containing cracks has to be three-dimensional (3D) because of complex geometry. The maximum length for forming the finite element model is calculated  The finite element mesh consists of 2.558.929 tetrahedron-type elements and 544.427 nodes. Finite element size in crack zone is 0.2 mm, and on the part of the spherical shell without a crack, 2 mm. Some details of the finite elements mesh in the spherical shell crack zone are shown in Fig. 4.
The effect of cracks on the spherical tank stresses and strains was simulated by a groove, introduced following the real procedure of taking crack out. Loading was uniform pressure equal to the maximum value of the hydrostatic pressure (p = 16 bar) plus maximum calculated value for the groove. Tab. 3 presents the dimensions of grooves at locations of the cracks and values of maximum stresses. Figs. 5, 6 and 7 define stresses in the groove zone, indicating maximum stress, σ max,s = 382.7 MPa, Fig. 5a, at the bottom of groove no. 204 and 205, where the thickness is reduced from 30 mm to 28.5 mm, which is larger than the allowed local stress σ all = 360 MPa. As the stress proof is not satisfied, the groove dimensions have to be corrected, to reduce the maximum stress to σ max,s = 298.6 MPa, Fig. 5b. Maximum stress at the bottom of the groove no. 207 (thickness reduced from 30 mm to 28 mm is σ max,s = 429.5 MPa, which is larger than the allowed local stress σ all = 360 MPa, Fig. 6a. Therefore, groove dimensions were corrected to reduce the maximum stress to σ max,s = 326.2 MPa, Fig. 6b. Maximum stress at the bottom of the groove no. 211 (thickness reduced from 30 mm to 26 mm) is σ max,s = 380.8 MPa, which is larger than the allowed local stress σ all = 360 MPa, Fig. 7a. Therefore, groove dimensions were corrected to reduce the maximum stress to σ max,s = 325.9 MPa, Fig. 7b.

CONCLUSIONS
Calculation of the spherical tank thickness according to EN 13445-3:2017 is satisfied since all the required thicknesses are greater than the minimum measured thickness. Using fracture mechanics parameters, it was concluded that detected defects do not affect spherical storage tank integrity. Also by using fracture mechanics parameters, the value of the critical error depth was determined which would bring the spherical tank integrity in critical state. By checking the maximum stress in corrected zones using the finite element method it was concluded that the integrity of the spherical storage tank will not be affected at the defined dimensions of the groove.