WO2019081964A1

WO2019081964A1 - Vertically isolating of liquid storage tanks against seismic loads

Info

Publication number: WO2019081964A1
Application number: PCT/IB2017/056623
Authority: WO
Inventors: Mohammad Ali GOUDARZI; Mohammad Reza NIKOOMANESH
Original assignee: International Institute Of Earthquake Engineering And Seismology
Priority date: 2017-10-25
Filing date: 2017-10-25
Publication date: 2019-05-02

Abstract

The conventional horizontal base isolation system is placed underneath the total base plate area in horizontal direction to reduce the seismic base shear of the tanks. The Innovative method proposed here has two main different features with the available method of liquid tank isolation. First, this method considers the reduction of overturning method by isolating the vertical displacement of tank shell. For this purpose, the tank shell is supported by a ring of vertically isolation system to damp the rocking motion of the tank shell caused by dynamic loads. Therefore, the proposed method is based on the vertical motion of tank shell instead of horizontal motion. Second, the isolation is only required underneath of a tank shell instead of total area of the tank bottom. Therefore, the costs of isolation for placing massive isolation system as well as the demand of rigid base for supporting them are extensively reduced. Moreover, the proposed method can be applied not only for future tanks but for the rehabilitation of existing tanks. The positive effects of the proposed vertical isolation system are experimentally and numerically examined for various tank dimensions and earthquake ground motions. The results of numerical analysis and experimental tests indicate that the idea can efficiently reduce the main seismic design parameters of the tanks including base shear, overturning moment, and seismic stress in a tank shell.

Description

Vertically Isolating of Liquid Storage Tanks against seismic loads

Description

Technical field

[0001] The technical field of this design is related to designing security systems of structures (civil) and liquid storage repository (including oil, natural liquid gas, water and etc.).

Background of the invention

[0002] The idea of using Vertical Isolation of a Tank Shell (VITS) is suggested to reduce the seismic hydrodynamic forces exerted on the liquid tanks during the earthquakes. Liquid storage tanks are considered as the important structures in vital artery networks which their destruction during earthquake will lead to undesired economical, environmental and safety consequences.

[0003] Moreover, liquid storage tanks play a major role in meeting essential requirements of people after earthquake. One of the main damages of liquid storage tanks caused by earthquake in liquid storage tanks is the buckling of a tank wall as a result of hydrodynamic powers caused by the interaction of liquid and structure.

[0004] Isolation of a structure has been introduced during the last three decades to mitigate the seismic force caused by earthquakes. Isolating of a structure changes the natural period of a structure and consequently leads to the reduction of seismic loads absorbed by the structure. In this invention, an innovative method is introduced to effectively isolating of a liquid storage tank in vertical direction.

Problems found in previous methods

[0005] Most of the previous Isolation methods for liquid storage tanks focuses on the isolation of base shear caused by hydrodynamic pressures exerted on a tank shell. However, there are several critical issues regarding to use the conventional horizontal isolation of liquids tanks. First, the net tension produced inside the tanks shell due the seismic overturning moment cannot be properly handled by conventional horizontal isolation system.

[0006] Moreover, the isolation of total area of a tank bottom plate requires a rigid slab underneath the total area of tank bottom which is too expensive. Third, the equipment for restraining the maximum horizontally displacement of a tank is required in horizontal isolation. Finally, making the connected equipment such and interior and exterior pipes to be compatible with the large horizontal displacement of isolation system is the difficult and expensive task.

[0007] The idea of VITS presented here is based on the isolation of overturning moment instead of shear forces. Since the tank shell is isolated in vertical direction in VITS, it has some advantages with respect to the conventional horizontal isolation systems that can properly tackle most of above mentioned limitations. Description of the Prior

[0008] To reduce the seismic response of structures during earthquakes, base isolators are commonly employed. The appliance of the conventional horizontal isolator used for liquid storage tanks is schematically illustrated in Fig.l.

[0009] The total hydrodynamic forces applied on the tank shell by an earthquake is named as shear forces. The mechanism of common available base isolation is to cut off the horizontal motion of liquid tanks by employing several isolators underneath of the tank base). This type of isolation reduces the shear force by changing the natural frequency of the tank which is the main philosophy of using isolators (Fig. l-A&B).

[00010] The seismic shear forces exerted on the tank shell produce the overturning moment which is the main source of vertical stress and strain on the shell of the tank. If the shear force is reduced, the overturning moment is indirectly reduced as well, and the buckling of tank shell is protected. The isolators should be distributed all over the tank bottom. Therefore, the conventional isolation system needs a rigid base that the isolators placed on.

Description of the invention

[00011] The proposed method is based on the vertical motion of tank shell instead of horizontal motion. Second, the isolation is only required underneath of a tank shell instead of total area of the tank bottom. Therefore, the costs of isolation for placing massive isolation system as well as the demand of rigid base for supporting them are extensively reduced.

[00012] Moreover, the proposed method can be applied not only for future tanks but for the rehabilitation of existing tanks. The positive effects of the proposed vertical isolation system are experimentally and numerically examined for various tank dimensions and earthquake ground motions.

[00013] The results of numerical analysis and experimental tests indicate that the idea can efficiently reduce the main seismic design parameters of the tanks including base shear, overturning moment, and seismic stress in a tank shell.

Brief description of the figures

Fig.l: Schematic view of typical horizontally isolating

Fig.2: Vertically isolating proposed in this invention

Fig.3: Connection of tank shell and tank base plate in vertical direction

Detail description of the figures

[00014] To introduce the system of the invention, the hydrodynamic pressure creates an overturning moment exerted on the bottom of the tank shell. The overturning moment results in the tensile and compressive stresses which is vertically exerted on the tank shell (Fig.2-A).

[00015] These vertical seismic stresses are the main source of the elastic and inelastic buckling of the tank shell. Therefore, the isolation of tank rocking motion (vertical direction) can efficiently and directly reduce the seismic stress caused by hydrodynamic overturning moment. The schematic view of the mechanism of VITS is presented in (Fig.2-B). In VITS, limiting vertical displacement of the tank wall is practically well manageable for both slender and broad tanks.

[00016] This is due to the net tensile forces which are affected by overturning moment.

These tensile forces may exceed the resistance of the horizontal isolating system especially for slender tanks. Therefore, the vertically isolating of a tank shell is practically more preferable than the conventional horizontal one.

[00017] One of the main advantages of VITS is that this system requires only isolation of the tank shell instead of isolating the whole area of the tank bottom which needs a concrete and solid floor for proper acting of base isolators. This advantage dramatically reduces the cost of isolating as well as bottom concrete slab. In other words, we can disconnect the shell from the base plate and only let the shell to freely move in vertical direction during the earthquake. Additionally, VITS provides the possibility of applying this system for retrofitting purposes of existing liquid tanks.

[00018] Regarding to the limitation of the system, it should be mentioned that the system can be very effective of the tank shell is disconnected from the base plate. If the disconnection is applied, we have to somehow prevent the possible leakage of liquid from the bottom of tank shell. This is practically challenging but doable. For example, flexible plastic layers that can be adapted with vertical motion of the tank shell can be attached to the tank shell and base plate (Fig.3A). Also, the thin corrugated plate can be used with accordion shape to handle the vertical motion without leakage (Fig.3B).

Clear and precise explanation of advantages of the claimed invention

1- A significant fall in the cost of the isolation caused by the reduction of required isolation system under the tank shell

2- Tackling the tensile stress, especially in slender tanks, which is undesirable in common isolating system.

3- Potentiality of its execution on the available tanks for retrofitting and seismically

strengthening of existing tanks.

4- Higher speed in project running

5- Reduction of the area provided for freely oscillating of a horizontally isolated tank during earthquake.

Explicit statement of application

[00019] Using this invention can sharply reduce the probable seismic damages of the tanks constructed in the area with seismic risk (up to 80 %). Also, the idea can be used for retrofitting and strengthening of existing liquid tanks which are not strong enough against seismic loads.

Claims

1. Liquid Storage Tanks with Vertical Oscillation of against seismic loads comprising: a. Vertical Oscillation

b. Connection of tank shell and tank base plate

2. According to claim 1: the proposed method is based on the vertical motion of tank shell instead of horizontal motion.

3. Liquid Storage Tanks which the isolation is only required underneath of a tank shell instead of total area of the tank bottom.

4. Liquid Storage Tanks which the shell is disconnected from the base plate and only let the shell to freely move in vertical direction during the earthquake.

5. Liquid Storage Tanks which the flexible plastic layers that can be adapted with vertical motion of the tank shell can be attached to the tank shell and base plate.

6. According to claim 5: the thin corrugated plate used with accordion shape to handle the vertical motion without leakage.