Large Storage Tanks Foundation Settlement Structural Mechanics Characteristics Analysis

Transcription

1 2017 Joint International Conference on Materials Science and Engineering Application (ICMSEA 2017) and International Conference on Mechanics, Civil Engineering and Building Materials (MCEBM 2017) ISBN: Large Storage Tanks Foundation Settlement Structural Mechanics Characteristics Analysis Ke GONG 1,2,a and Jia-Shun HU 1,b* 1 Research Department of Safety Technology, CNPC Research Institute of Safety & Environment Technology, Beijing, China 2 College of Mechanical, Storage and Transportation Engineering, China University of Petroleum, Beijing, China a gobash@16.com, b hujiashun@cnpc.com.cn *Corresponding author Keywords: Storage tank foundation settlement, Finite Element Structural Analysis, Tank structure, Harmonic component, Uneven settlement of foundation. Abstract. Foundation settlement is one of significant risks in the process of construction or operation of large storage tanks. In particular, the uneven settlement of the foundation has a significant impact on the safe operation of large storage tanks. In this paper, the Fourier series method is used to extract the harmonic data of large storage tank foundation. Based on the actual measurement foundation settlement data of 100,000 m external floating roof tank, the stress and deformation analysis of tank structure is carried out. The stress and deformation of the wall in the dangerous area of the tank is studied, and distribution regulation of the mechanics is revealed. The research results provide some technical guidance for the safety assessment and operation of tank foundation settlement for storage tank enterprise. Introduction China's oil and gas enterprises, oil refining enterprises and the national oil reserve base and commercial oil reserve base has a much number of large oil storage tanks. Large storage tank has the characteristics of high safety, environmental risk, and serious accident consequence. In recent years, the continuous occurrence of the "7 16" Dalian Xingang explosion fire, 6 2 Dalian petrochemical tank explosion, "4 6" Fujian gulei tank farm fire explosion accidents, which caused such enormous negative impact to the state and society. The safety and environmental issues of large storage have attracted unprecedented attention of the state. At present, most of the domestic large-scale storage tanks are built in soft soil, frozen soil, collapsible loess area and other adverse geological conditions, especially in the coastal soft soil area in our country, there are a much number of large oil storage tank. Soft soil foundation have the characteristics of large amount of compression and long consolidation time, although there are foundation reinforcement and foundation settlement observation during the construction, but with the service time of tank going, it is easy to lead to tank safety accidents such as leakage, tanks tilt, chuck and so on which are caused by uneven settlement. In order to understand the harm of tank foundation settlement and ensure the safety of storage tank, in the last century geotechnical engineers put forward the relevant standards to control the settlement of the tank according to the experience and the theory of rock and soil mechanics. Later, based on the theory of thin shell, Kamyab H et al.[2] obtained the stress and displacement formula of the uneven settlement tank wall on the basis of the linear static analysis. Kamyab and Palmer(1989)[] used the Fourier series to decompose the tank bottom settlement curve, and utilized the series superposition method to carry on the storage tank settlement study, and compared with the theoretical solution and test solution. Although there are a lot of researches on the storage tank settlement at home and abroad, the detailed study on the stress of the storage tank is not enough. Many researchers believe that the research on the influence of settlement on the tank will only be used to study the settlement of the tank

2 wall, while ignoring the influence of settlement tank bottom. In the literature [5-7], the influence of tank bottom settlement on storage tank is not considered. Based on the actual measurement foundation settlement data of a 100,000 m external floating roof tank, the stress and deformation analysis of tank structure is carried out by using the finite element numerical simulation. The stress and deformation of the wall in the dangerous area of the tank wall is studied in detail, and dangerous area stress and deformation of each tank layer are discussed. Numerical Analysis of Foundation Settlement of Large Tank Analysis Method of Uneven Settlement of Tank u = u0 + ui cos( iθ + θi ) (1) i= 1 In which: u 0 is the uniform settlement of the whole, i is the harmonic order, u i is the amplitude of the i st harmonic wave settlement, θ i is the i th order harmonic initial phase angle. The static force analysis of tank can use linear analysis method, which individually analyze each order harmonic influence on storage tank, and then the superposition principle is used to stack the internal force and deformation of each order harmonic, finally we get the cloud of tank uneven settlement. Numerical Analysis Model of Large Storage Tank Oil storage tank of 100,000 m is a case study. The tank diameter is 80 m, tank height is 21.8 m, tank wall is composed of 9 wall panels in the height direction, in the top edge there is 100 mm steel angle. The maximum operating level of the tank is 19.5 m. The relevant parameters of the tank are shown in Table 1. Using ANSYS software to model the tank. The shell181 unit is used in tank body, the elastic modulus is 2.06E11 Pa, the Poisson's ratio is 0., and the steel density is set to kg / m. The foundation is simulated by solid element, and the elastic modulus is 1.6E10 Pa.The reinforcing ring, wind girder and steel angle is used the same parameter as the tank wall materials. According to the structure of tank, the influence of reinforcing ring, wind girder and steel angle is considered to establish the model of variable wall thickness tank whose ratio is 1:1. As shown in Fig.1. Table 1. The relevant parameters of the tank. tank wall thickness/ mm panel height/ mm material yield strength/mpa bottom plate 20 / Q25-C 25 1th layer SPV490Q 490 2th layer SPV490Q 490 th layer SPV490Q 490 4th layer SPV490Q 490 5th layer SPV490Q 490 6th layers SPV490Q 490 7th layer MnR 45 8th layer Q25-C 25 9th layer Q25-C 25 steel angle Q25-B 25

3 Figure 1. Finite element model of storage tank. In order to get settlement curve of the tank, the 7-year observation data were processed by Fourier series method. For simulating the actual situation of the tank in service, the liquid level of this paper is set to 80%. The hydrostatic pressure is distributed on the tank wall, and the triangular linear distribution of the liquid static pressure increases gradually from the top to the bottom of the tank. In order to achieve the effect of true settlement, the settlement of the bottom center of the tank is set to the value of the average settlement of observation points [7]. It is assumed that the settlement at the bottom of the tank is the linear distribution from the tank wall settlement to the center settlement of the tank bottom. The formula (2) is the expression of the settlement point ( Ri, ui ) of the bottom of the tank. ui = u 0 + ub u0 Ri R (2) In which: ui is settlement of tank bottom some point, in m. u0 is the settlement of tank bottom center, in m. ub is the settlement of the corresponding point of the tank wall in a radial direction, in m, R is tank radius, in m. Ri is radius of the tank bottom some point, in m. The location of observation points is in the Fig.2 and the measured data of the tank settlement in Table 2 was carried out by using the Fourier decomposition formula (1) and the bottom settlement curve is simulated by Fourier function. The data of Table. is obtained after processing Figure 2. The location of observation points.

4 Table 2. Tank settlement data ( - is the value of tank bump). observation point settlement value/ mm observation point settlement value/ mm u Table. Fourier decomposition of tank settlement data( i / mm, θ i /degrees). i u i θ i Mechanical Characteristics Analysis of Large Storage Tank The analysis is based on the finite element numerical model, and the result is shown in Fig 2. The maximum deformation occurs at the top of the tank, which is 19.2cm, and the maximum stress is 82 MPa. The maximum stress of each layer is shown in Fig. According to the data in Table 4, it was found that the stress and displacement of each tank wall layer increased first and then decreased and finally increased again with the increase of tank height. The maximum stress occurs near the junction between the first and second layers. It also explains the cracking phenomenon of the seam where is near the junction of between the first panel and the second panel. The main stress of the wall is concentrated in one to four wall panels and the top of the tank. Fig 4 shows the maximum displacement of each panels. With the increase of the layer, the displacement of the wall is also increasing. The roof displacement is the largest, and that is to say the tank roof deformation is most serious. The analysis of the tank profile near the maximum stress of the tank is shown in Fig 5. Near the maximum stress of tank, stress situation from bottom to top of tank is shown in Table 5, and the change is shown in Fig 7. The location of the maximum stress concentration is below 4 m. The largest stress of tank is located near 2.4 meter, while the high of first layer is 2.42 m, so the tank body is likely to be broken at the weld of the first and second wall panels. From 11.1 meter to 20.8 meter of tanks, there are 5 layer of reinforcing ring and wind girder, so the stress here is decreased. However, due to the relatively larger tank wall settlement in which is the biggest differential settlement between the two observation points around tank bottom. Large settlement leads to increase stress and deformation in the upper part of the tank, so there is a peak value of stress near 21 of tank wall. So generally serious tank deformation is at the top, even sometimes it leads to stuck floating plate, to prevent the floating plate from up and down movement.

5 Figure. Cloud of storage tank analysis. Figure 4. The tank wall stress distribution of each layer. Figure 5. The maximal displacement of every layer. Figure 6. The section location of large settlement. Figure 7. The section location of small settlement. Figure 8. The stress of large settlement section. Fig. 8 shows the displacements at the section. In the height of near 2 m, there appear a peak displacement first, it shows that in the action of tank pressure the displacement of the tank is larger than that of the nearby tank. The serious settlement will cause serious stress concentration, if so, it will lead to the phenomenon of "elephant foot" on the tank wall. It is found that the higher of the tank wall, the greater the deformation. But from 19.1 m to 20.8 m in this section of the tank displacement reduced significantly, while the height of 19.1 m and 20.8 m is the location of the two wind girder. It can be seen that due to the strengthening effect of the wind girder on the tank, the displacement of the tank decreases obviously. The displacement of the tank in the windshield upwards is markedly increased.

6 height/m height/m displacement/mm stress/mpa Figure 9. The displacement of large settlement section. Figure 10. The stress of small settlement section. However, not every point on the tank is in accordance with the rules above. The section (Fig.6) of the area where the settlement of the tank is small is analyzed, and the results is showed in Fig 9. Because of the bottom settlement and liquid pressure in the tank, the stress of the tank wall is mainly concentrated in the area below 5 meter. Above 11 meter of tank, there are reinforcing ring and wind ring, and it play an important role in reducing the stress, so the stress of tank decrease gradually. Summary According to the analysis results of m tank settlement data, in the area where the settlement is large, the tank is also affected more. There are two peaks in the tank's stress, one is near the weld between the first panel and second panel. This area is also the largest stress of tank area. Another peak appears at the top near the wind girder, and leads to the deformation of the roof. In the area where the tank settlement is smaller, the main force of tank is at the bottom of the tank in the position of below 5 meter. In upper part of the tank, due to the function of strengthen ring and winder girder the stress is reducing gradually. In order to run the tank safely and effectively for a long time, we should try to avoid the uneven settlement of the tank. After analysis, the stress of tank is concentrated in the lower part of the tank. Among them, the first layer and the second layer are the most concentrated, so it should pay more attention to the failure of these parts in the work of security check and analysis. Acknowledgement This research was financially supported by the National Key Research and Development Plan (Project number: 2016YFC ). References [1] LeiXiang. Stability behavior of large tapered steel tanks under differential settlement. [D]. Zhejiang University, [2] Kamyab H, Palmer S C. Displacements in Oil Storage Tanks Caused by Localized Differential Settlement[J]. Journal of Pressure Vessel Technology, 1991, 11:1(1):71-80.

7 [] Kamyab H, Palmer S C. Analysis of Displacements and Stresses in Oil Storage Tanks Caused by Differential Settlement[J]. Archive Proceedings of the Institution of Mechanical Engineers Part C Journal of Mechanical Engineering Science (vols ), 1989, 20(1): [4] Cao Qingshuai. Structural behavior of large steel tanks under harmonic settlement. [D]. Zhejiang University, [5] Yang L, Chen Z, Cao G, et al. An analytical formula for elastic plastic instability of large oil storage tanks[j]. International Journal of Pressure Vessels & Piping, 201, 101(7): [6] Kamyab H, Palmer S C. Analysis of Displacements and Stresses in Oil Storage Tanks Caused by Differential Settlement[J]. Archive Proceedings of the Institution of Mechanical Engineers Part C Journal of Mechanical Engineering Science (vols ), 1989, 20(1): [7] Shi Lei, Shuai Jian, xukui. Assessment of large-scale oil tanks foundation settlement based on FEA model and API 65[J]. China Safety Science Journal, 2014, 24():