RIBBED SLAB FOUNDATION FOR CYLINDRICAL REFRIGERATED TANKS FOR LIQUIFIED GAS STORAGE

Abstract

A foundation for cylindrical refrigerated tanks for liquified gas storage, at locations where the minimum ambient temperature is always greater than 0° C., characterized by a reinforced concrete ribbed slab structure at grade level, where the clear spaces in between the parallel webs of the ribbed slab are configured as air circulation channels to provide ambient air circulation suitable to prevent the ground underneath the foundation itself from reaching freezing temperatures, i.e. ≤0° C., while providing the necessary bearing and structural capacity.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention refers to the Oil & Gas Refining Industry and in particular to an innovative solution for the foundations of cylindrical refrigerated tanks for liquified gas storage, such as Propane, Butane and Ethylene at locations where the minimum ambient temperature is always above freezing (>0° C.), particularly but not exclusively in Tropical and Subtropical Regions.

Description of the Related Art

The internal temperature of this type of tanks, that operate at ambient pressure, is normally and continuously well lower than 0° C. and hence also the foundation soil might be subjected to temperatures below 0° C. whereas the outside temperature is always higher than 0° C. with high humidity.

The main issue with these low temperatures is that they would cause soil freezing and consequent frost heaving and hence the uneven displacement of foundation with damages to either the tank and the piping connected thereto.

Furthermore, this phenomenon is progressive, meaning that freezing of the shallower soil moisture layer usually attracts by capillarity more water that in turn will freeze as well, building-up an ice lens under the tank foundation centre and consequently increasing the strain in the foundation up to a level exceeding its structural capacity or serviceability limits.

In order to prevent this phenomenon, the engineering practice as well as the international reference codes (i.e.: API 625) recommend two alternative solutions:

• • To install electric heaters within the thickness of the disc foundation slab, or
  • To construct the disc foundation slab elevated above grade, on a set of columns.

Both these alternative options have a cost impact: the first one for the capital and operational cost of electric heating, while the second for the capital cost of constructing the elevated support structure.

SUMMARY OF THE INVENTION

The innovative idea of this invention—for cylindrical tank foundations to be erected at sites such as in Tropical and Subtropical Regions where the minimum ambient temperature is always above freezing (>0° C.)—consists in constructing a ribbed slab foundation where the ambient air circulation in the clear spaces in between the webs of said ribbed slab will prevent the ground underneath the foundation from reaching freezing temperatures (≤0° C.) due to the contact of tank bottom with the top of the foundation slab and, at the same time, the webs act as main structural component of the foundation.

In case of presence of non-bearing soil strata under the foundation, then piles are required, and in this case the ribbed slab foundation with its “ribs” directly connected onto the piles allows to implement a “bottom-up” construction sequence, that results much quicker, cheaper and less hazardous in terms of safety, than the usual “top-down” industrial practice. The latter in fact requires first to build a 2.5 meter high (minimum) embankment, to execute the piles through it, then to cast the slab on its top and finally to dig-out the embankment between the slab and the ground surface, thus resulting is an expensive and time consuming activity.

This has been accomplished by designing the foundation as a reinforced concrete ribbed slab with its webs parallel to the tank diameter and supported at grade. The clear spaces in between said foundation webs are open to the ambient in order to ensure a continuous heat transfer from ambient air—which temperature is always higher than 0° C.—into the foundation supporting the refrigerated tank containing the liquified gas, thus preventing the freezing of soil underneath.

In the following description the term “ribbed slab” is meant to indicate a reinforced concrete slab sitting on its webs directly in contact with the soil, where said clear spaces between webs function as air circulation channels.

For a better understanding of the invention, a detailed description is given hereinafter with reference to the enclosed figures that are provided as examples, without any limitation implied therein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following drawings:

FIG. 1 shows in vertical cross section the behavior of a usual tank/foundation/soil system, i.e. without any air circulation channels, and demonstrates that temperatures lower than 0° C. are indeed occurring in the soil underneath.

FIG. 2 shows in vertical cross section the behavior of the tank/foundation/soil system with air circulation channels according to the invention, and demonstrates that temperatures are higher than 0° C. both in the foundation and in the soil underneath.

FIG. 3 shows in isometric view the behavior of the tank/foundation/soil system with air circulation channels as per FIG. 2, and hence demonstrates that temperatures are higher than 0° C. both in the foundation and in the soil underneath.

FIG. 4 shows in vertical view parallel to the channels, the behavior of the tank/foundation/soil system with air circulation channels as per FIG. 2, and confirms that temperatures are higher than 0° C. both in the foundation and in the soil underneath.

FIG. 5 is a plan view of the foundation showing the parallel webs and the resulting channels having rectangular cross section.

FIG. 6 is a vertical cross section showing the air circulation channels in between the foundation webs having rectangular cross section, constructed using removable formworks.

FIG. 7 is a vertical cross section of the reinforcement arrangement in the cross section of the ribbed slab foundation showing the “ribs” that support the slab while integrating channels for the natural ventilation of the structure.

DETAILED DESCRIPTION

According to the invention, the foundation (FV) is provided with a number of air circulation channels (C) (hereinafter called “channels”), evenly distributed in plan and preferably parallel each other and to the bottom of the tank, that are passing from one side of the foundation (FV) to the opposite one with a substantially constant cross section, with a nominal longitudinal slope and with both ends open to the surrounding ambient air.

The ventilation of these channels is preferably, though not exclusively, natural and their minimum transversal cross section dimension is 600 mm to allow their visual inspection.

During the experimental campaign, the real behavior of the tank/foundation/soil (S/FV/T) system has been analyzed by means of a finite elements 3D Model (prepared using the ANSYS software), in order to simulate the heat transfer from ambient air in the channels and the liquified gas inside the tank (S).

This thermal analysis is required during the design stage of the foundation (FV) both to select the size and spacing of air circulation channels (C), and to allow performing the structural design of the ribbed slab foundation (FV) that creates these channels, taking into account the reduction of structural cross section due to the presence of said channels, as well as of any piles required to support the foundation itself.

The ribs and hence the air circulation channels (C) are preferably realized by means of formworks removable after concrete hardening. In order to enhance the stack effect, it is preferable that said channels (C) are prismatic and have a nominal longitudinal slope, these features will also facilitate their inspection and possible cleaning.

According to the invention, the orientation of air circulation channels (C) is preferably perpendicular to the prevailing wind direction at the site where the described foundation (FV) is placed, with the aim to minimize the possibility of sand or dirt being dragged inside these channels.

The next table shows the data relevant to three prototype-foundations constructed for “experimental” purposes for three refrigerated tanks (S) with double containment for liquified gas, having the following main features:

	Tank:	D-0001	D-0002	D-0003
	External	45	46	46
	diameter (meters)
	Internal tank	27	27	27
	height (meters)
	Design	−46° C.	−7° C.	−46° C.
	Temperature
	Product	Propane C3	Butane C4
				3/C4
	Foundation:
	Diameter	46	48	48
	(meters)
	Thickness	1.8	1.8	1.8
	(meters)
	Air circulation	n.12 ×	n.12 ×	n.12 ×
	channels(meters)	1.8 × 1.0	1.8 × 1.0	1.8 × 1.0
	Minimum	+11° C.	+11° C.	+11° C.
	ambient
	temperature
	Note:
	even if the design temperature of tank D-0002 is −7° C. instead of −46° C., like the two others, its foundation has been designed with the same features of the others both for construction standardization, and in view of a possible future change of tank contents and accordingly of design temperature.
	indicates data missing or illegible when filed

FIG. 1 of the finite element 3D model for a usual tank/foundation/soil system without air circulation channels (C) shows clearly that temperatures lower than 0° C. can be reached in the soil (T) underneath the foundation (F).

Whereas, FIGS. 2, 3 and 4 of the finite element 3D model of the tank/foundation/soil system with air circulation channels (C) in accordance with the present invention, show clearly that temperatures lower than 0° C. remain confined within the tank (S), while the soil temperature remains greater than +5° C. with a +11° C. minimum ambient air temperature.

FIGS. 5, 6 and 7 are showing a plan view and partial cross section of the foundation (FV) in accordance with the as-built prototype of the present invention, where the channels (C) have been realized by means of removable formworks.

LEGEND

• • S=tank
  • F=usual foundation
  • T=soil or ground
  • FV=ribbed slab foundation
  • C=air circulation channels
  • P=pile
  • W=web of the ribbed slab

Claims

1. Foundation for cylindrical refrigerated tanks for liquified gas storage, at locations where the minimum ambient temperature is always greater than 0° C., comprising a reinforced concrete ribbed slab structure at grade level, where the clear spaces in between the parallel webs of said ribbed slab are configured as air circulation channels to provide ambient air circulation suitable to prevent the ground underneath the foundation itself from reaching freezing temperatures, while providing the necessary bearing and structural capacity; wherein said air circulation channels are placed within the thickness of said reinforced concrete ribbed slab structure of the foundation, and wherein the cross section of said air circulation channels is calculated by a method for designing the cross section of air circulation channels provided within a foundation for cylindrical refrigerated tanks for liquified gas storage, at locations where the minimum ambient temperature is always greater than 0° C., in order to ensure enough natural ventilation to prevent the ground underneath the foundation itself from reaching freezing temperatures, while providing the necessary bearing and structural capacity, said method including performing a thermal analysis of the real behavior of the tank/foundation/soil system by means of a finite elements 3D Model, in order to simulate the heat transfer from ambient air in the channels to the liquified gas inside the tank; wherein said thermal analysis is performed during the design stage of the foundation both to select the size and spacing of air circulation channels, and to allow performing the structural design of the ribbed slab foundation that creates these channels, taking into account the reduction of structural cross section due to the presence of said channels, as well as of any piles required to support the foundation itself,said method being performed to ensure enough natural ventilation notwithstanding the prevailing wind direction at the site where the foundation is placed.

2. The foundation of claim 1, wherein said air circulation channels are open to the outside ambient to allow the continuous heat transfer from ambient air—which temperature is always greater than 0° C.—to the inside of the foundation supporting the tank containing the liquified gas, thus preventing the freezing of soil underneath the foundation itself.

3. The foundation of claim 2, wherein said air circulation channels are evenly distributed in plan and are substantially parallel to each other and to the upper face of the foundation supporting the tank bottom; where said air circulation channels are crossing the foundation from one side to the opposite one.

4. The foundation of claim 2, wherein said air circulation channels have a 600 mm minimum dimension of their cross section to facilitate inspection and cleaning.

5. The foundation of claim 2, wherein said air circulation channels are prismatic in section with a nominal longitudinal slope in order to enhance the stack effect.

6. The foundation of claim 2, wherein said air circulation channels are oriented perpendicular to the prevailing wind direction at the site where the foundation is placed, in order minimize the possibility of sand or dirt being dragged inside said air circulation channels.

7. (canceled)

8. The foundation of claim 2, wherein, in case foundation with piles is necessary, the foundation is configured to be built in a bottom-up construction sequence: piles followed by ribbed slab; instead of the top-down construction sequence adopted in the usual industrial practice for these cases, namely in construction sequence order: embankment above grade, piles, slab, digging-out of embankment material.

9. The foundation of claim 3, wherein said air circulation channels have a 600 mm minimum dimension of their cross section to facilitate inspection and cleaning.

10. The foundation of claim 3, wherein said air circulation channels are prismatic in section with a nominal longitudinal slope in order to enhance the stack effect.

11. The foundation of claim 4, wherein said air circulation channels are prismatic in section with a nominal longitudinal slope in order to enhance the stack effect.

12. The foundation of claim 3, wherein said air circulation channels are oriented perpendicular to the prevailing wind direction at the site where the foundation is placed, in order minimize the possibility of sand or dirt being dragged inside said air circulation channels.

13. The foundation of claim 4, wherein said air circulation channels are oriented perpendicular to the prevailing wind direction at the site where the foundation is placed, in order minimize the possibility of sand or dirt being dragged inside said air circulation channels.

14. The foundation of claim 5, wherein said air circulation channels are oriented perpendicular to the prevailing wind direction at the site where the foundation is placed, in order minimize the possibility of sand or dirt being dragged inside said air circulation channels.

15. The foundation of claim 3, wherein, in case foundation with piles is necessary, the foundation is configured to be built in a bottom-up construction sequence: piles followed by ribbed slab; instead of the top-down construction sequence adopted in the usual industrial practice for these cases, namely in construction sequence order: embankment above grade, piles, slab, digging-out of embankment material.

16. The foundation of claim 4, wherein, in case foundation with piles is necessary, the foundation is configured to be built in a bottom-up construction sequence: piles followed by ribbed slab; instead of the top-down construction sequence adopted in the usual industrial practice for these cases, namely in construction sequence order: embankment above grade, piles, slab, digging-out of embankment material.

17. The foundation of claim 5, wherein, in case foundation with piles is necessary, the foundation is configured to be built in a bottom-up construction sequence: piles followed by ribbed slab; instead of the top-down construction sequence adopted in the usual industrial practice for these cases, namely in construction sequence order: embankment above grade, piles, slab, digging-out of embankment material.

18. The foundation of claim 6, wherein, in case foundation with piles is necessary, the foundation is configured to be built in a bottom-up construction sequence: piles followed by ribbed slab; instead of the top-down construction sequence adopted in the usual industrial practice for these cases, namely in construction sequence order: embankment above grade, piles, slab, digging-out of embankment material.

19. The foundation of claim 7, wherein, in case foundation with piles is necessary, the foundation is configured to be built in a bottom-up construction sequence: piles followed by ribbed slab; instead of the top-down construction sequence adopted in the usual industrial practice for these cases, namely in construction sequence order: embankment above grade, piles, slab, digging-out of embankment material.

20. The foundation of claim 9, wherein said air circulation channels are prismatic in section with a nominal longitudinal slope in order to enhance the stack effect.

21. A method for designing the cross section of air circulation channels provided within a foundation for cylindrical refrigerated tanks for liquified gas storage, at locations where the minimum ambient temperature is always greater than 0° C., in order to ensure enough natural ventilation to prevent the ground underneath the foundation itself from reaching freezing temperatures, while providing the necessary bearing and structural capacity, said method including a thermal analysis of the real behavior of the tank/foundation/soil system by means of a finite elements 3D Model, in order to simulate the heat transfer from ambient air in the channels to the liquified gas inside the tank; wherein said thermal analysis is performed during the design stage of the foundation both to select the size and spacing of air circulation channels, and to allow performing the structural design of the ribbed slab foundation that creates these channels, taking into account the reduction of structural cross section due to the presence of said channels, as well as of any piles required to support the foundation itself.