BASE MAT ASSEMBLY AND METHOD OF CONSTRUCTING THE SAME
A precast reinforced concrete base mat element, a base mat assembly and method for constructing a base mat assembly from a plurality of the base mat elements are described. The base mat element comprises a concrete body having first and second conduits extending there through in first and second generally perpendicular directions and a liner plate anchored in the concrete body. Preferably the concrete bodies have interlocking surfaces formed there on. A plurality of the base mat elements may be juxtaposed together with the interlocking surfaces interlocking with one another and with the tensile members extending through the first and second tensile conduits and being post-tensioned and anchored about the base mat elements to clamp the base mat elements together. The liner plates may then be welded together to form a a generally planar membrane. In one embodiment, at least one of the first and second tensile conduits is located above and one below the horizontal center planes of the concrete bodies to limit the base mat elements from displacing vertically relative to one another.
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The present invention relates generally to LNG (liquefied natural gas) tanks and methods for constructing LNG tanks, and more particularly, to base mat foundations for containment tanks which are used to support LNG tanks.
BACKGROUNDLNG full containment tanks typically comprise an inner primary container (hereinafter “inner tank”) for storing LNG and an outer container (hereinafter “outer tank”) which is designed to contain leaks from the inner tank. Large storage tanks, such as those used with LNG full containment tanks, are typically sensitive to differential settlement in the foundations supporting the inner tank which stores LNG. Consequently, standard practice is to build a rigid foundation with a flat top surface supporting the bottom of the inner tank. For example, the foundation can be a grade supported reinforced concrete mat, a deep foundation with a mat resting on the ground, or an elevated slab foundation. In cases where very stiff soil or soft rock is present at a construction site, a grade supported mat is the preferred type of foundation. The conventional design for the foundation mats utilizes cast in place reinforced concrete.
Construction using an assembly of precast concrete elements is a known approach when construction time at a site needs to be minimized and when site conditions and locations are problematic for casting in place structures. Assemblies made of precast elements are often used for temporary and, in some cases, for permanent pavement.
However, application of precast elements in LNG tank foundations is not a conventional approach. Such foundations would be subject to high forces at connections which makes design and construction of such connections very difficult. Only small allowances can be made for vertical differential displacements across a base mat when an inner primary container or inner LNG tank is to be supported upon an assembly of precast elements forming the base mat. Grade supported mats provide special support to a large tank bottom so as to minimize differential settlements. If a base mat is to be constructed from precast concrete elements, the forces that will occur due to differential settlement or voids in the underlying soil will tend to separate and displace the base mat elements from one another. It is challenging to provide sufficient vertical stiffness to precast base mat elements forming a base mat while keeping the thickness of the base mats within an economically acceptable range.
SUMMARYA precast reinforced concrete base mat element, a base mat assembly or foundation and method for constructing a base mat assembly from a plurality of the base mat elements are described. The base mat element comprises a liner plate and concrete body having first and second tensile conduits extending there through. Preferably the tensile conduits extend in first and second generally perpendicular directions. The liner plate is anchored or cast into the concrete body. Ideally the base mat elements have interlocking surfaces formed thereon.
A plurality of the base mat elements may be juxtaposed together. The tensile members extend through the first and second tensile conduits and are anchored and clamped about the base mat elements utilizing anchors with a planar membrane surface being formed by the liner plates. The liner plates may be welded together to form a membrane which is generally flat and which utilizes the liner plates as a vapor barrier. Ideally, if interlocking surfaces are provided on the base mat elements, the interlocking surfaces can interlock with one another to limit displacement of the base mat elements relative to one another. Preferably, the base mat elements interlock in both horizontal and vertical directions. In one embodiment, the first and second tensile conduits are located above and below the horizontal center plane of the concrete body so that when tensile members are passed through and post-tensioned and anchored about the base mat elements using anchors, the clamped base mat elements resist relative displacement between one another due to the clamping force provided by the post-tensioned tensile members and the additional stiffness provided by the membrane.
These and other objects, features and advantages of the present invention will become better understood with regard to the following description, appended claims and accompanying drawings where:
In a first embodiment, a full containment LNG tank 18 is shown in
In this particular embodiment, outer tank 20 is primarily made of precast reinforced concrete elements which reduce fire risk relative to using a primarily steel outer container or tank. The construction of outer tank 20 involves assembling precast reinforced concrete elements for a foundation 24, including a base mat assembly 100 and an outer ring beam 200, an outer wall or shell 300 and a roof 400. As an example of order of magnitude, outer tank 20 in this embodiment has a diameter of approximately 80 meters, an overall height (to the top of the dome of roof 400) of about 50 meters, and a height to the top of shell 300 of about 40 meters. Those skilled in the art of LNG facilities construction will appreciate that much larger or smaller tanks can be built and still use the design considerations described herein. However, tanks having diameters of at least 25 meters are particularly well suited to the construction methods described herein which utilize precast reinforced concrete elements. These elements ideally can be welded together to form structural welds thereby minimizing the number of cast in place concrete structural joints which have to be made to form an outer containment tank.
Ideally, inner tank 22 can be assembled at the same time as roof 400 is being built to save time and money in the overall construction of inner tank 22 and outer tank 20. Note in
Inner tank 22 can be built in any of a number of ways which may require welding processes to connect individual plates. For example, the plates can be joined using shielded metal arc welding (SMAW) and submerged arc welding (SAW) for 9% nickel tanks of the size described above, i.e. having a diameter greater than 25 meters. In this particular example, inner tank 22 can be friction stir welded (FSW) by plate at a time erection method. A course of plates at a time can be welded with the bottom (thickest) first and then subsequent (thinner) courses welded there above. Scaffolding (not shown) can be used as each course is added. Alternatively, inner tank 22 can be constructed course down either by supporting the tank from the roof to allow insertion of the next course or by supporting on jack stands to allow insertion of the next course. For the last two methods, the scaffolding need be erected only one time. By way of example and not limitation, another alternative method is to use coil material shaped to the curvature required for the tank. Coil tank building occurs from the top course down either using jacking or by roof supported. Expanded pearlite insulation is typically used in the annulus formed between inner tank 22 and outer tank 22. Although not shown, an under bottom insulating layer, which can be made of foam glass, is applied on the top of base mat assembly 100 with inner tank 22 prior to inner tank 22 being placed there on.
Foundation 24, i.e., base mat assembly 100 and ring beam 200, is designed to support inner tank 22 and outer shell 300. Outer shell 300 rests upon and transfers loads directly to outer ring beam 200 preferably through welded connections. A particularly significant load is the torsional load applied to ring beam 200 from outer shell 300. Base mat assembly 100 is secured to and transfers loads to outer ring beam 200 as well through a cast in place joint 270, which is best seen in
Concrete ribs 130 and grooves 132 are also formed on the longitudinally and laterally extending edge surfaces of concrete body 114. Locating projections 106 and ribs 130 and receiving recesses 110 and grooves 132 allow a plurality of base mat elements 102 to be placed in interlocking juxtaposition as suggested in
The rows of tensile conduits are disposed below and above the central horizontal plane 144 of concrete body 114. These laterally and longitudinally extending tensile conduits are designed to receive tensile members 150 there through which allow multiple base mat elements 102 to be clamped together utilizing anchors 152 (
Base mat assembly 100 is constructed by juxtaposing base mat elements 102 in both first and second generally perpendicular directions as seen in
Tensile members 150 are fed through tensile conduits 120, 122, 124 and 126 as base mat elements 102 are being juxtaposed to one another. After the mating surfaces on the base mat elements 102 are properly located relatively to one another, tensile members 150, i.e. cables, are post-tensioned and anchored by anchors 152 to clamp base mat elements 102 together. In this particular example, tensioning can be accomplished such as by using a Williams Strand Anchor System available from Williams Form Engineering of Belmont, Mich., USA. Those skilled in the art will appreciate that other tensioning systems can also be used to post-tension and anchor tensile members 150. After the tensioning is complete, anchors 152 are locked in place on tensile members 150 to maintain the tension in tensile members 150 with anchors 152 bearing upon base mat elements 102.
Liner plates 104 of each of base mat elements 102 are then welded together to form a part of a generally contiguous membrane 140 on the top of base mat assembly 100. As tensile conduits 120, 122, 124 and 126 are located above and below the central horizontal plane 144 of concrete body 114, the top and bottom surfaces of base mat elements 102 are held together and do not separate due to the post tensioning of tensile members 150 with anchors 152 clamping about base mat elements 102, preferably even under construction and operating loads applied to base mat assembly 100.
C. Mounting of Base Mat Elements on PilesBase mat elements 102 can be assembled on a graded, level surface if the underlying surface or soil is sufficiently stiff. However, if the soil does not provide adequate support, base mat elements 102 can be mounted on piles 160. The upper portion of a typical pile 160 is seen in
Also formed within concrete body 220 are circumferentially extending lower and upper tensile conduits 232, 234. In this particular embodiment, there are four such tensile conduits 232, 234. Two of these tensile conduits 232 are generally arranged below and two conduits 234 are arranged above the horizontal center plane 236 of ring beam element 202. Also, two of the tension conduits 232, 234 are arranged beneath base plate 206 and two are located radially inwardly beneath liner plate 204 closer to the inner radial surface 224. These cooperating locations of tensile conduits 232, 234 allow tensioning members 250, i.e., cables, to pass through each of ring beam elements 202 and assist in counter balancing the bending, shearing and torsional loads applied to base plate 206 by outer shell 300. Also, tensile members 250 and anchors 252 cooperate to clamp about ring beam elements 202 to prevent the abutting ring beam elements 202 from displacing with respect to one another. In this particular example, high density polyethylene (HDPE) tubing is used to form the tensile conduits 232, 234 in concrete body 220. Of course, the tensile conduits could be made of other suitable materials such as steel or other structurally strong materials. Downwardly and upwardly opening steps 238 and 240 allow elements 202 to be vertically interlocking as well. Also, located within concrete body 220 is a plurality of reinforcing bars (not shown) which are conventional for adding tensile strength to cast concrete bodies. Extending radially inwardly are reinforcing bars or J-hooks 242 which are later to be included in cast concrete connection 270 which connects base mat assembly 100 with ring beam 200.
Ring beam elements 202 are juxtaposed with respect to one another with locating concrete ribs 226 of one ring beam element being held within locating recess 230 of the adjacent ring beam element. Similarly, cooperating steps 238 and 240 assist in vertical alignment between ring beam elements 202. Tensile members 250 are placed through tensile conduits 232 and 234 as the ring beam elements are being positioned adjacent one another. As seen in
After the individual ring beam elements 202 are aligned and clamped together using tensile members 250 and anchors 252, base plates 206 are welded together along their radially extending abutting edges to form an annular ring which strengthens outer ring beam 200. Finally, liner plates 204 on ring beam elements 202 are also welded together along their radially extending edges to form a continuous membrane 244 on the radial inner side of outer ring beam 200, as best seen in
Each of shell elements 302 includes a pair of tapered carbon steel side plates 304 and a carbon steel liner plate 306 forming a generally U-shaped steel cross-section. In this example, the liner plate 306 is about 10 mm thick and side plates 304 are about 25 mm thick. The width of side plates 304 is about 800 mm at their bottom and 400 mm at the top providing the tapered shape to shell element 302. A base plate 312 connects side plates 304 and liner plate 306 at the bottom of shell element 302. Numerous reinforcing bars 310 are welded to and extend from side plate 304 to the opposing side plate 304 and from base plate 312 to top end cap 314. Base plate 312 is about 50 mm in thickness in this example. A steel end cap plate 314, also about 50 mm thick, is welded to side plates 304 and steel liner plate 306 at the top of shell element 302. Concrete is cast in place in the U-shaped volume defined by side plates 304 and steel liner plate 306 and about reinforcing bars 310 and J-hooks 316 to form reinforced concrete body 318. Liner plate 306 acts as a vapor/gas barrier
B. Constructing Shell 300Shell 300 is constructed by arranging shell elements 302 vertically upon outer ring beam 200. Initially, base plates 312 are arranged radially slightly outside of their final position for welding with shell elements 312 being slightly radially spaced apart. Shell elements 302 are then moved radially inwardly until all base plates 312 and side plates 304 are brought into abutment at the proper radial position atop of base plates 206 of ring beam 202. Erection gear will be used to finally align and pull the side plates 304 closely together to begin welding.
As seen in
All of shell elements 302 are permanently welded together to form shell 300, with the exception of 2-4 special half shell elements 302′. Half shell elements 302′ are similar to shell elements 302 except they are only about half the height of regular shell elements 302. These special shell elements 302′ will be temporarily sealed with shell opening 350 to accommodate air raising of roof assembly 404, as suggested in
As an alternative to permanently closing construction opening 350, liner plates can be welded to close shell opening 350. Then reinforcing bars and steel rib stiffeners can be placed in a form and concrete can be cast in place to form a cast in place shell element similar to that used in the design of the precast shell elements 302. Shell 300 should have great strength when completed due to the welded connections 320 on the inner and outer edges of side plates 304 which cooperate to form a large number of radially extending stiffeners arranged around the circumferential periphery of outer tank 20.
C. Alternate Shell DesignLower shell element 302a includes a concrete body 316a to which a pair of circumferentially spaced apart side plates 304a, liner plate 320a, upper end cap plate 314a and lower base plate 312a are anchored or embedded (anchors and reinforcement bars not shown). Base plate 312a is welded by welds 324a to base plate 206 of ring beam 200. Upper end cap plate 314a has two downwardly depending sockets 362 formed therein.
Upper shell element 302b includes a concrete body 316b to which a pair of circumferentially spaced apart side plates 304b, liner plate 320b, upper end cap plate 314b and lower base plate 312b are anchored (anchors and reinforcement bars not shown). Two downwardly depending and circumferentially spaced apart alignment pipes 360 are welded to lower base plate 312b. As seen in
Referring in general now to
Once enough weld joints have been formed to safely support roof 400, the pressurization within shell 300 is removed. Personnel can then access the inside of the outer tank 20 to construct inner tank 22. Additional welding is done to complete the circumferential weld joint between the roof membrane 412 and shell membrane 340. Special extra strength roof elements 402′ are attached to shell 300 and roof frame 404 forming the radial outermost course of roof elements 402. Then typical roof elements 402 are attached to roof frame 404 with the radial outermost course of roof elements 402 being attached first. Then the successive next radial outermost course of roof elements is added until all the rings of roof elements 402 are in place forming roof 400.
A. Roof Assembly 404Roof assembly 404 is built on the ground in this particular preferred exemplary embodiment. Roof frame 406 comprises a plurality of radially and circumferentially extending wide flange beams 420, as best seen in
Referring now to
Half shell elements 302′ are removed to provide access from within shell 300. Referring now to
Roof elements 402 and 402′ are mounted to roof assembly 404 after roof assembly 404 has been affixed by weldments to shell 300.
Roof element frame 428 is used to construct precast reinforced concrete roof elements 402, as seen in
For the outermost concentric course of roof elements 402′ which secure to both roof assembly 404 and to shell 300, the design is slightly different from that of roof elements 402 disposed on the inner radial concentric courses. As shown in
The outer concentric ring or course of roof elements 402′ are located adjacent shell 300 and this outer ring of roof elements 402′ is first welded to roof assembly 404 and to shell 300. This outer ring of roof elements 402′ adds significant strength to roof 400. Subsequently, second through eighth courses of roof elements 402 are sequentially welded to roof assembly 404 starting from the radially outermost course and then each concentric course of roof elements is added until all rows of roof elements 402 are in place to form a complete roof 400 such as seen in
L-shaped brackets 432 of roof elements 402 are mounted atop and are welded to tubular mounting beams 414 creating welds such as the weld joint 436 seen in
While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to alteration and that certain other details described herein can vary considerably without departing from the basic principles of the invention. For example, any one or more of base mat assembly 100, ring beam 200, outer shell 300 or roof 400 could be constructed using conventional cast in place construction techniques while the other components are constructed using the precast elements as described herein. As another example of an alternative embodiment, the roof assembly could be directly built atop shell 300 such as by using a crane. In this instance, roof assembly 404 would not have to be airlifted and attached atop shell 300. However, ideally even in this embodiment, the shell elements 402 could be welded to the roof assembly concurrently with the construction of the inner tank 22 to save construction time on building outer tank 20 and inner tank 22.
While beam ring 200 described in the above particular exemplary embodiment was used in conjunction with a full containment LNG tank, a beam ring could certainly be used in other applications. For example, the ring beam could be used in cases where neither a liner plate or thick structural base plate is necessary. In this case, the ring beam need only comprise ring beam elements having precast reinforced concrete bodies with embedded tensile conduits which receive tensile members there through so that the tensile members may be tensioned and anchored by anchors to form the beam ring. Although not required, preferably the ring beam elements would be interlocking with one another.
Claims
1. A method of constructing a base mat assembly for supporting a primary container storing liquefied natural gas tank, the method comprising:
- providing a plurality of precast base mat elements, at least some of the base mat elements having: a concrete body having first and second tensile conduits extending there through and a top membrane plate anchored in the concrete body;
- juxtaposing a plurality of the base mat elements;
- inserting tensile members through the tensile conduit members of the base mat elements and tensioning and anchoring the tensile members about the base mat elements to clamp the base mat elements together; and
- welding the membrane plates of the base mats together to form a generally horizontally extending base mat assembly with a generally contiguous membrane.
2. The method of claim 1 wherein:
- the first and second tensile conduits in a concrete body extend in generally perpendicular first and second directions to one another; and
- the tensile members clamp the base mat elements in generally perpendicular directions.
3. The method of claim 2 wherein:
- the first and second conduits include at least two conduits in each of the first and second directions, the at least two conduits being located above and below the horizontal center plane of the concrete body.
4. The method of claim 1 further comprising:
- interlocking the concrete bodies of the at least two base mat elements to one another prior base mat elements being clamped together by the tensile member and anchors, the concrete bodies having interlocking surfaces.
5. The method of claim 4 further comprising:
- interlocking the concrete bodies in a horizontal direction.
6. The method of claim 4 further comprising:
- interlocking the concrete bodies in the vertical direction.
7. The method of claim 4 further comprising:
- interlocking the concrete bodies in a horizontal direction and in the vertical direction.
8. The method of claim 1 further comprising:
- mounting the base mat elements upon piles to provide enhanced stiffness to the base mat assembly.
9. A base mat assembly for supporting a primary container storing liquefied natural gas, the base mat assembly comprising:
- a plurality of precast reinforced concrete base mat elements, at least two of the base mat elements including a concrete body having first and second tensile conduits extending there through in first and second generally perpendicular directions and a liner plate anchored in the concrete body; and
- a plurality of post-tensioned tensile members extending through the first and second conduits and a plurality of anchors which are anchored on the tensile members and clamp the base mat elements together in first and second generally perpendicular directions;
- wherein the liner plates are welded together to form a generally horizontally extending base mat assembly with a generally contiguous membrane on top the concrete bodies.
10. The base mat assembly of claim 9 wherein:
- the concrete bodies of the base mat elements have interlocking surfaces; and
- the concrete bodies are interlocked with one another utilizing the interlocking surfaces.
11. The base mat assembly of claim 9 wherein:
- the base mat elements interlock in first and second horizontal directions.
12. The base mat assembly of claim 11 wherein:
- the base mat elements interlock in a vertical direction.
13. The base mat assembly of claim 11 wherein:
- at least one of the first and second tensile conduits extends horizontally above and at least one extends horizontally below the horizontal center plane of the concrete bodies of the base mat elements.
14. The base mat assembly of 13 further comprising:
- a plurality of piles driven into the earth which support the base mat elements and provide additional stiffness to the membrane formed by the liner plates.
15. A base mat element for constructing a generally horizontally extending base mat assembly, the base mat element comprising:
- a concrete body having first and second conduits extending there through in first and second generally perpendicular directions and a generally planar and horizontally extending liner plate anchored in the concrete body;
- wherein a plurality of the base mat elements may be juxtaposed one another with tensile members extending through the first and second tensile conduits with anchors anchoring about the tensile members to clamp the base mat elements together with a generally planar membrane surface being formed by the clamped together base mat elements.
16. The base mat element of claim 15 wherein:
- the concrete body has interlocking surfaces formed thereon such that the interlocking surfaces of a plurality of such base mat elements can interlock with one another when the base mat elements are juxtaposed one another.
17. The base mat element of claim 15 wherein:
- at least one of the first and second tensile conduits is located above and one below the center horizontal plane of the concrete body so that when tensile members are passed through and anchored about the base mat elements, the clamped base mat elements resist displacing vertically relative to one another.
18. The base mat element of claim 15 wherein:
- the base mat element has interlocking surfaces which allow a plurality of the base mat elements to be juxtaposed one another and interlocked.
19. The base mat element of claim 18 wherein:
- the interlocking surfaces allow a plurality of the base mat assemblies to be interlocked in first and second horizontal directions and in a vertical direction.
Type: Application
Filed: Dec 23, 2008
Publication Date: Jun 24, 2010
Applicant:
Inventors: Lestle R. Shockley (San Ramon, CA), Vincent G. Borov (Martinez, CA)
Application Number: 12/342,503
International Classification: E04H 5/00 (20060101); E04H 7/00 (20060101); E04C 5/08 (20060101); E04C 5/12 (20060101);