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.

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 .

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 2,5meter 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.

In the following drawings:

Figure 1 shows in vertical cross section the behavior of a usual tank/foundati on/soi 1 system, i.e. without any air circulation channels, and demonstrates that temperatures lower than 0°C are indeed occurring in the soil underneath. Figure 2 shows in vertical cross section the behavior of the tank/foundati on/soi 1 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.

Figure 3 shows in isometric view the behavior of the tank/foundati on/soi 1 system with air circulation channels as per Figure 2, and hence demonstrates that temperatures are higher than 0°C both in the foundation and in the soil underneath.

Figure 4 shows in vertical view parallel to the channels, the behavior of the tank/foundati on/soi 1 system with air circulation channels as per Figure 2, and confirms that temperatures are higher than 0°C both in the foundation and in the soil underneath.

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

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

Figure 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.

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 600mm to allow their visual inspection.

During the experimental campaign, the real behavior of the tank/foundati on/soi 1 (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:

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 . Figure 1 of the finite element 3D model for a usual tank/foundati on/soi 1 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, Figures 2, 3 and 4 of the finite element 3D model of the tank/foundati on/soi 1 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.

Figures 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 = ai r circulation channels

P = pile

W = web of the ribbed slab