U.S. patent number 3,899,988 [Application Number 05/393,896] was granted by the patent office on 1975-08-19 for ships equipped with pressurized cargo tanks supported on continuous shells.
This patent grant is currently assigned to Sener, Tecnica Industrial y Naval, S.A.. Invention is credited to Jaime Torroja Menendez.
United States Patent |
3,899,988 |
Menendez |
August 19, 1975 |
Ships equipped with pressurized cargo tanks supported on continuous
shells
Abstract
A continuous, suitably stiffened skirt is attached by its bottom
edge to the top of the double bottom of a ship and beneath the
shell or shells which constitute the support for each cargo tank.
The top edge of the skirt, which has the same circumference as the
bottom edge of the internal one of the shells which form the cargo
tank support and is opposite the same, is attached to a suitably
stiffened platform, preferably horizontal, which extends between
top edge and the inner shell of the ship's hull. Between the skirt
and platform, there are a number of flat plates perpendicular to
the base plane of the ship and located opposite the double bottom
floors and longitudinal girders. These flat plates are welded at
their top and inside edges to the platform and skirt, respectively,
and at their bottom edge to the top of the double bottom, while at
the external edges, those opposite the double bottom floors are
welded to the inner shell of the ship's hull, whereas the plates
opposite the double bottom longitudinal girders are welded to the
skirt of the adjacent tank or to the terminal transverse bulkheads
of the cargo tank zone.
Inventors: |
Menendez; Jaime Torroja (Las
Arenas, ES) |
Assignee: |
Sener, Tecnica Industrial y Naval,
S.A. (Las Arenas, ES)
|
Family
ID: |
8462103 |
Appl.
No.: |
05/393,896 |
Filed: |
September 4, 1973 |
Foreign Application Priority Data
Current U.S.
Class: |
114/74A;
220/560.05; 114/74R; 220/901; 220/560.12 |
Current CPC
Class: |
F17C
13/082 (20130101); B63B 25/14 (20130101); F17C
2203/01 (20130101); Y10S 220/901 (20130101) |
Current International
Class: |
B63B
25/00 (20060101); B63B 25/14 (20060101); F17C
13/08 (20060101); B63b 025/16 () |
Field of
Search: |
;114/74R,74A,74T,.5T
;220/9LG,9A,14,15 ;105/358 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Blix; Trygve M.
Assistant Examiner: Frankfort; Charles E.
Attorney, Agent or Firm: Fleit & Jacobson
Claims
I claim:
1. In ships having an independent pressurized cargo tank supported
in a tank support comprising continuous shells, the improvement
comprising a continuous skirt positioned on top of the double
bottom floors of the ship and under said support shells, said skirt
being stiffened and joined by its lower edge to said top of said
double bottom and its upper edge being of the same perimeter as the
lower edge of the lowermost internal shell in the tank support and
disposed opposite it, a stiffened platform attached to said skirt
and running between said upper edge and the inner shell of the
hull, a number of flat plates positioned between the platform and
said skirt perpendicular to the base plane of said ship, said
plates being stiffened and facing the floors and longitudinal
girders and being welded by their internal and upper edges to said
skirt and said platform, respectively, and by their bottom edge to
the top of said double bottom, the external edges of said plates
being welded to the inner shell of the ship, with the plates facing
the floors, and to the skirt of an adjacent tank or to the end
transverse bulkhead of the cargo tank zone, with the plates facing
the longitudinal girders, a tray positioned under the tank
independent of the structure of the ship and of the structure of
the support, said tray having an insulating layer and a stiffening
ring and being designed to catch possible small leaks of
transported liquid, and a continuous leak deflection plate joined
by its external edge to the inner shell of the tank support, the
free edge of said tray being positioned next to the inner shell of
the tank support below said deflection plate.
2. The combination of claim 1 in which the inside of the skirt has
a series of brackets with faceplates and intermediate stiffeners,
the said brackets being coplanar with the aforementioned plates and
being joined by their external edges to said skirt and by their
lower edges to the top of said double bottom.
3. The combination of claim 1 in which a secondary skirt is
positioned below said platform and facing the external shell of the
support, said secondary skirt having a considerably lower height
than the main skirt and being joined to said platform by its upper
edge, said secondary skirt also being joined to said plates along
the lines of intersection with the same and to said main skirt by
means of intermediate parts.
4. The combination of claim 1 in which the internal edge of said
platform is slightly inside the upper edge of said skirt.
5. The combination of claim 1 in which there is an intermediate
deck between the outer and inner shells of the ship, facing the
said platform.
6. The combination of claim 1 in which said skirt has a conical
shape.
7. The combination of claim 1 in which said skirt has a cylindrical
shape.
8. The combination of claim 1 in which the double bottom of said
ship is omitted at least in the area located outside said skirt
extending from the vicinity of the lower edge of said skirt, the
lower edge of said plates being joined to the faceplates of the
floors and longitudinal girders.
9. The combination of claim 1 in which the inner shell of said ship
is omitted at least in the area located below said platform, the
external edges of said plates facing said floors being joined to
the faceplates of the transverse web frames.
10. The combination of claim 1 in which said tray has a surface
which at least in its central zone is parallel to the bottom
external surface of said tank so that when said tray is lifted it
rests on the tank over said central zone, through the intermediate
insulation, the peripheral zone of said tray having a conical shape
tangential to the surface of the central zone.
11. The combination of claim 1 in which said tray is equipped,
extending away from its free edge, with a continuous peripheral rim
acting as a retaining element for liquid caught in the said tray
and having a free edge at a distance from the external surface of
said tank.
12. The combination of claim 1 in which said stiffening ring is
located in the lower face of said tray and in the vicinity of the
free edge of said tray.
13. The combination of claim 1 in which said stiffening ring is
located on the lower face of said tray along an intermediate
annular zone.
14. The combination of claim 1 in which said tray is supported
freely on a number of main supports through said stiffening ring
which has external projections which co-operate with lateral
retention elements so arranged as to keep said tray in position and
to allow displacements of the same parallel to the axis of
revolution of said tank as well as radial expansion or
contraction.
15. The combination of claim 14 in which an insulating material is
arranged between said stiffening ring and said supports and also
between said projections and said retention elements, said
insulating material having sufficient compressive strength to
withtand the weight of said tray, the collected liquid, and also
the accelerations parallel and perpendicular to the axis of
revolution of said tank.
16. The combination of claim 14 in which the supports which carry
said tray and said lateral retention elements are joined to the
internal shell of said tank support.
17. The combination of claim 14 in which the supports supporting
said tray and said lateral retention elements are joined to said
skirt.
18. The combination of claim 14 in which the supports supporting
said tray and said lateral retention elements are joined to
brackets on the inside of said skirt.
19. The combination of claim 14 in which there are secondary
supports facing said stiffening ring between which supports and
said ring there are positioned movable jacks which allow the
raising and lowering of said tray.
20. The combination of claim 1 in which the insulating layer of
said tray is on the upper face of the same and at least one
independent tight layer is attached to said tray along its whole
periphery and makes a hermetic seal delimited generally by said
tray and above by said tight layer, there being at least one
opening therein for the introduction of gas under pressure.
21. The combination of claim 1 in which the insulating layer of
said tray is on the upper part of the same and there are disposed
above the insulating layer two tight layers which are mutually
independent, and also independent of said insulating layer, which
are hermetically sealed to each other at their peripheries to form
a bag with at least one opening for the introduction of gas under
pressure, said bag in turn being joined at its periphery to said
tray.
Description
BACKGROUND OF THE INVENTION
The present invention relates to improvements in ships equipped
with pressurized independent cargo tanks, designed especially for
the transportation of liquefied gases.
The transportation by sea of liquefied gases has raised a number of
technical problems stemming chiefly from the movements of the ship
and also from the low temperatures at which the material has to be
transported in the majority of cases. This is the case, for
example, with liquefied natural gas (LNG) which has to be
transported at a pressure close to atmospheric and a temperature of
around -160.degree.C.
All this has meant that the responsible authorities have insisted
that the transportation of LNG should be effected in double-walled
vessels so that in the event of rupture of the "primary barrier"
the low-temperature cargo will be temporarily retained by an
insulated "secondary barrier" thus preventing the temperature of
the hull from reaching such low levels that there is a serious risk
of brittle fracture. However, the construction of double-walled
vessels makes the overall cost of the ship substantially more
expensive so that the cost of transportation of the liquefied gas
increases correspondingly.
Recently, various authorities and bodies concerned with the setting
of standards have decided to accept the elimination of the
secondary barrier subject to the proviso of certain security
conditions. This change of attitude has opened up broad
possibilities for reducing the cost of tanker vessels for the
transportation of LNG, in which the secondary barrier, due to the
enormous amount of labor it involves and the construction as well,
makes up a considerable percentage of the total cost of the ship,
and major efforts have been directed towards the construction of
liquid gas transportation vessels not possessing a secondary
barrier.
The most logical method of constructing such vessels while still
complying with the requisite security regulations, is to design
them as pressure vessels, in the form of unstiffened
solids-of-revolution, perhaps the most suitable shape for which
would be spherical. Tanks of this kind, in the form of
solids-of-revolution, and more specifically spherical ones,
nevertheless present major problems not merely of construction but
also in terms of the system of support in the ship, due to the
large size of the tanks.
Recently, two basic designs have been put forward within the
context of pressure vessels having only a single barrier. Both
designs employ spherical tanks, the chief difference between the
two designs being the system of support. In accordance with one of
the designs which have been developed, the forces acting upon each
vessel are absorbed by a certain number of separate supports
arranged along three great circles. Each individual support
comprises a lever mechanism the purpose of which is to reduce to
the minimum the bending forces transmitted by each support to the
tank envelop.
The other design is basically a maritime version of the well tried
approach according to which the tank is supported upon a continuous
envelope which is a solid-of-revolution. Its chief characteristic
is the special design of the equatorial portion which links the
supporting frame to the top and bottom hemispheres of the tank.
Following this second system, the present assignee has developed a
type of support or bracket, which is described in Spanish Patent
No. 401,734, which comprises a peripheral structure attached to the
tank and to a certain extent forming part of the wall thereof, and
two continuous skirts or shells which are unreinforced, attached by
one of their edges to said peripheral structure and by the other to
the ship's structure.
SUMMARY OF THE INVENTION
One object of the present invention relates to improvements in
ships equipped with tanks of the type referred to, which
improvements are designed to achieve a simple pedestal structure
which uniformly distributes the tank loads, transmitted by the tank
support, to the ship's structure without giving rise to stress
concentrations.
Another object of the invention is to simplify the structural
design of ships intended to carry tanks of the indicated kind.
Yet another object of the invention is to develop a system suitable
for the assembly of large-size tanks for the largest ships
envisaged.
In accordance with the invention, on the top of the double bottom
of the ship, and beneath the shell or shells which constitute the
support for each tank, there is arranged a continuous, suitably
stiffened skirt attached by its bottom edge of the top of the
double bottom of the ship, and at its top edge, which has the same
circumference as the bottom edge of the internal one of the shells
which form the tank support and is opposite the same, there is
attached a suitably stiffened platform, preferably horizontal,
which extends between the top edge and the inner shell of the hull.
Moreover, between the skirt and platforms there are a number of
flat plates perpendicular to the base plane of the ship and duly
stiffened, located opposite the double bottom floors and
longitudinal girders. These flat plates are welded at their top and
inside edges to the platform and skirt, respectively, and at their
bottom edge to the top of the double bottom, while at the external
edges, those opposite the double bottom floors are welded to the
inner shell of the ship's hull, whereas the plates opposite the
double bottom longitudinal girders are welded to the skirt of the
adjacent tank or to the terminal transverse bulkheads of the cargo
tank zone.
With this disposition, the vertical forces are transmitted from the
tank support to the skirt as normal forces and to the flat plates
as shear forces along the whole length of the weld seams. From
these the loads are then distributed to the floors, webframes,
longitudinal girders, transverse bulkheads and the other structural
elements of the ship. The horizontal dynamic forces are transmitted
from the tank support to the ship's structure principally as shear
forces which are distributed over the whole of the horizontal
platform. With this disposition, the ship's structure retains its
simple constitution and at the same time no stress concentrations
are induced into the tank support or the ship's structure.
On the inside of the aforementioned skirt there are a series of
brackets fitted with faceplates and intermediate stiffeners. These
brackets are coplanar with the aforementioned plates. The brackets
are fixed to the skirt by the outside edge and by the lower edge to
the top of the double bottom.
If it is desired to reinforce the joint, a small skirt may be put
under the platform and opposite the external shell of the support,
with a smaller height than that of the skirt and joined by its
upper edge to the platform. This small skirt is also joined to the
abovementioned plates on the lines of intersection of the plates
and skirt by means of intermediate parts. Preferably the internal
edge of the platform will be slightly inside the upper edge of the
skirt.
In accordance with the invention and in order to get a better
transmission of forces, an intermediate deck can be put between the
ship's inner and outer shells, facing the platform. The disposition
of the platform can permit the suppression of the double bottom of
the ship in the zone outside the skirt, away from the proximity of
the lower edge of the skirt. The lower edges of the said flat
plates are then joined to the faceplates of the floors and
longitudinal girders. For the same reason one can dispense with the
inner shell of the ship, at least in the zones situated below the
platform, the external edges of the said flat plates facing the
floors being then joined to the face plates of the transverse web
frames.
The skirt can have either a conical or cylindrical shape. The
cylindrical shape is the better so long as the shape of the ship
permits its use.
With the described construction, the double bottom zone resembles,
under each tank, a flat grillage that has a rectangular shape if
there are brackets on the inside of the skirt, and a circular one
if not. This grillage is supported throughout its entire perimeter
and is subjected to hydraulic pressure which acts upon its lower
face. The flat plates between the skirt and the platform transmit
the forces to the transverse webframes as shear forces and to the
floors and longitudinal girders as compression forces. The brackets
on the inside of the skirt improve the distribution of the loads
and reduce the spans of the double bottom panels.
The horizontal forces which act on the tanks, be they transverse or
longitudinal, are transmitted through the support as shear
stresses. At each level they are distributed following a cosine
function, the maximum being in the plane perpendicular to the
direction of the forces and the minimum (zero) being in the plane
of application of the forces. These shear stresses are carried
through the platform to the skirt and from the skirt to the top of
the double bottom.
For the maritime transportation of liquid products at low
temperatures in pressurized tanks with relatively thin walls, the
competent authorities demand the installation of a partial
secondary barrier or drip tray used to catch any small leaks that
may be produced as a result of the appearance of small cracks over
a period of some 2 weeks so that the structure of the ship does not
find itself subjected to excessively low temperatures.
The measures which have been commonly used up to now consist of
covering the ship's structure, in the zone below the tank, with a
water-tight coat of insulating material able to withstand the low
temperature of the collected liquid. This system presents the
inconvenience that its construction must be performed on board, and
the above work cannot be carried out at the same time as the other,
i.e., the construction of the hull itself, and the mounting of the
tanks and their supports.
In accordance with the invention and foreseeing the possibility of
said leaks, a drip tray is placed beneath the tank which is
independent of the ship's structure and of the tank support. This
system allows the mounting of the tank and its support and
corresponding tray in a single operation. The assembly can be
suspended, in which case the tray is fixed to the support, at least
during the mounting stage, or alternatively the assembly can be
floated by flooding the dock where the ship is being built, in
which case the upper edge of the tray must stay above the flotation
level of the assembly.
The mounting of the tanks by flotation with their corresponding
support and tray can be done without a great deal of water in the
construction dock, before assembling the part of the platform and
skirts situated between each two consecutive tanks in order that
through this gap the tanks can float to their respective locations.
Once the tank is in position the platform and small skirts can be
completed and then the next tank can be mounted.
So that the tray can be constructed without stiffeners and be kept
without danger of buckling during the operation of floating the
assembly, at least the surface of the central part of the said tray
should be parallel to the lower external surface of the tank, so
that the support between tank and tray is achieved through the
corresponding insulation over an area of sufficient magnitude. The
rest of the tray may have a conical shape, tangential to the
surface of the central zone.
The tray will be provided with insulation either below or above it.
In the first case, the tray will be constructed from a metallic
material, capable of resisting the low temperature of the dripping
liquid, preferably the same material as the tank. The insulating
layer should be protected during the flotation by a water-proof
film, for example, a plastic-based film.
In the second case, the tray may be constructed from a material
which does not resist low temperatures, preferably normal steel,
since the insulating layer will protect it from the aforementioned
low temperatures. However in this case the insulation layer must
have a tight covering able to withstand the low temperatures of the
dripping liquid. The tight covering of the insulation layer, when
placed on the upper face of the tray, can be constituted of one or
two superimposed layers of tight material.
In the first case, the layer of tight material can be bonded to the
insulating layer covering it completely, or can be independent of
it, and be attached in sealed fashion along the whole of its free
edge to the insulating layer or to the drip tray and in this way
achieve a tight assembly defined below by the tray and above by the
tight layer. Also this tight layer can be joined to the insulating
layer at its centre. This system allows the detection of leaks and
cracks at any time in the tight layer and in the tray by means of
introducing gas under pressure without having to penetrate the
limited space between the tray and the tank to make a visual
inspection.
If the tight covering is formed by two superimposed layers then
they will be separate from the insulating layer and be joined at
their free edges, forming a bag whose tightness can easily be
checked by introducing gas under pressure. The bag thus formed will
be attached at its contour to the tray and rest on the insulating
layer. Also, in this case the two layers can be joined together, at
a series of intermediate points, thus reducing the volume of gas
needed to test the water tightness, and to the insulating
layer.
In any case, the deformation of the layer or layers, which might be
caused by the pressurized gas will be confined to the defined space
between the tank and the tray. The layer or layers that form the
tight covering must be of a material that will withstand the low
temperatures of any escaping liquid, for example, "Mylar,"
reinforced or not with glass-fiber cloth.
No stresses will be caused in the tight covering by the low
temperature of any escaping liquid, whether it is composed of one
layer or two, because any shrinkage can be absorbed by the
deformation of those parts of the layer or layers which reproduce
the shape of the knuckle between the edge of the tray and the
surrounding retention brim of the latter.
In the case where the insulating layer is on the upper face of the
tray, said layer may be continuous or made of performed parts or
panels. In either case, the layer can be formed of an expanded
polyurethane, polyvinyl chloride, polystyrene, or any other
suitable material that meets the thermal requirements and presents
a certain resistance to compression.
If the insulating layer is formed by pre-fabricated pieces or
panels, these will have a reduced surface area and so shrinkage
caused by low temperatures, away from the upper face, will not
produce grooves of excessive width which might cause bending
stresses in the layer on layers of the tight covering when these
layers have to withstand the liquid pressure coming from a possible
leak in the tank.
With the same purpose, if the insulating layer is continuous,
grooves will be machined in it in at least two different
directions. These grooves should not penetrate the whole thickness
of the material and should define intermediate portions that can
contract under the low temperatures without giving rise to the
formation of excessively wide intermediate grooves which would
result in bending stresses in the layer or layers of the tight
covering caused by the pressure of the escaping liquid caught in
the tray. These grooves, machined in the surface of the insulating
layer, will be filled at room temperature with pre-compressed
insulating material, for example, glass-wool or rock-wool which
will continue to fill out the grooves when the latter widen due to
the shrinkage of the intermediary portions.
The insulating layer may also be provided with a reinforcing cloth
or netting of high mechanical characteristics embedded in the mass
of the insulating layer immediately below the bottom of the grooves
to avoid the appearance of cracks below said bottom.
So that the leaking liquid caught in the tray does not spill due to
the movements of the ship, the tray is provided with a brim around
its periphery pointing upwards and/or towards the inside. The total
collection of any leaks is assured by means of a deflecting plate
which extends from the internal one of the shells forming the tank
support and pours inside the free edge of the retaining brim of the
tray.
The tray is also provided with a continuous exterior stiffening
ring that may be situated on the free edge of the tray, preferably
corresponding with the retaining brim or in an intermediate annular
zone. This ring will normally function in compression. The purpose
of this ring is to absorb membrane stresses that are produced in
the tray due to its own weight, the weight of the collected liquid,
and the vertical accelerations. Once installed in the ship, the
tray will be freely supported on a discrete number of supports
through the stiffening ring.
The stiffening ring will also have external projections running
down or preferably outwards which will co-operate with lateral
retention elements independent of the ring, to avoid the general
displacements of the tray in any perpendicular direction to the
axis of revolution of the tank, but allowing any movement of the
tray parallel to the axis of revolution of the tank, as well as
expansions and contractions of a radial nature.
An insulating material is provided between the stiffening ring and
the supports as well as between the afore-mentioned projections and
the retention elements. This material should have sufficient
resistance in compression to bear the weight of the tray and
collected liquid and also the accelerations parallel and
perpendicular to the axis of revolution of the tank. This material
may not exist if the insulating layer of the tray has been placed
on the upper face.
The tray supports and the lateral retention elements may be joined
to the internal shell of the tank support or to the pedestal
structure. In either case the tray supports and the lateral
retention elements may be either independent of each other or
joined. If the supports are joined to the pedestal structure they
will be joined to the brackets placed on the inside of the skirt or
will go directly to the skirt if these brackets have not been
installed or are of reduced dimensions.
Apart from the afore-mentioned supports hereinafter termed main
supports, there are other secondary supports that can either be
independent or combined with the main supports or lateral retention
elements. These supports are used to install jacks which allow the
tray to be taken up and lowered to test the upper face of the tray
and the insulation of the tank, having first of all removed any
insulating material placed among the main supports and the
stiffening ring.
The object of the present invention, i.e., the ships construction
with respect to the situation of the tanks and the supporting tray,
will be more easily understood from the following description with
reference to the enclosed drawings, which illustrate an embodiment
given by way of non-limitative example.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view that partially shows the structure of
the pedestal, the structure of the ship, and the lower part of the
support.
FIG. 2 is a partial plan view of the ship.
FIG. 3 is a section on the line III--III of FIG. 2.
FIG. 4 is a similar view to FIG. 3 showing a variant
embodiment.
FIG. 5 is a section on line V--V of FIG. 2.
FIG. 6 is a section on the line VI--VI of FIG. 2.
FIG. 7 is a partial, schematic diametral section through the tank
resting on its support with drip tray below.
FIG. 8 is a scaled-up detail of the supports of the drip-tray.
FIG. 9 is a view in the direction A of FIG. 8.
FIG. 10 is a section on the line X--X of FIG. 9.
FIG. 11 is similar view to FIG. 8 showing a variant embodiment.
FIG. 12 is a view similar to FIG. 1, but showing an embodiment of
the invention in which portions of the double bottom and inner
shell are omitted.
DETAILED DESCRIPTION OF THE DRAWINGS
As it can be seen in FIG. 7, tank 1 is mounted on the ship 2 by
means of a support made up of a peripheral structure 3, which forms
a part of the tank shell 1. From this peripheral structure 3, two
skirts or shells 4 and 5 go out. These skirts are stiffened at
their lower part but not at the top. These skirts are used to
transmit the corresponding forces to the ships structure.
In accordance with the invention, the pedestal for the support of
shells 4 and 5 is made up as shown in FIG. 1 of a horizontal
platform 6 and a continuous reinforced skirt 7 that runs between
the platform 6 and the double bottom of the ship, the skirt being
joined by its lower edge to the double bottom 8 and by its upper
edge the platform 6. The upper edge of the skirt 7 is of the same
perimeter as the lower edge of the internal shell 4 of the tank
support and faces said lower edge of the shell 4. The platform 6
runs between the upper edge of the skirt 7 and the inner shell 9 of
the hull.
Making reference to FIGS. 1 and 2, between platform 6 and the skirt
7, there are some flat plates 10 mounted perpendicularly to the
base plane of the ship. These are represented by dotted lines.
These plates are stiffened and face the floors and longitudinal
girders. These plates 10 are welded by their inside edge of the
skirt 7, by their upper edge to the platform 6, and by their lower
edge to the top 8 of the double bottom. On their outside edge, the
plates facing the floors are welded to the inner shell of the ship
9, and the outside edges of the places facing the longitudinal
girders are welded either to the skirt of the adjacent
corresponding tank or to the extreme transverse bulkheads in the
cargo area.
The skirt or external shell 5 of the tank support may reach the
platform 6 or is bottom edge may be spaced away therefrom. In the
first case, when the skirt 5 extends as far as the platform 6,
there is arranged beneath said platform a secondary small skirt 11
as shown in FIG. 3. This small skirt faces the afore-mentioned
external shell 5 of the support. This secondary skirt has a height
substantially smaller than that of the main skirt and is joined by
its upper edge to platform 6. The secondary skirt 11 is also joined
to the flat plates 10 at the lines of intersection with the same.
This secondary skirt is joined to the main skirt using intermediate
parts 12 to which are joined the vertical stiffeners 12' of the
skirt. Preferably, as can be seen in FIG. 3, the internal edge of
the platform 6 will be situated slightly inside the main skirt
7.
As shown in FIGS. 2 and 5 on the inside of the skirt 7 there are a
series of brackets 13 having faceplates 14 and intermediate
stiffeners 13'. for FIGS. 4 and 5 the stiffeners of the hull, outer
shell, double bottom, etc. are not shown as these would be
standard.
FIG. 5 which is a vertical section taken on the line V--V of FIG.
2, shows the arrangement of these brackets 13, their faceplates 14
and the stiffeners 13'. In this figure there can be seen the
arrangement of the plates 10 facing the longitudinal girders.
Plates 10 are between two consecutive skirts 7 and 7' having a
common platform 6.
In FIG. 6 which is a vertical section taken on the line VI--VI of
FIG. 2, the arrangement is also shown of the flat plates 10, in
this case facing the floors, which by their external edges are
welded to the inner shell 9 of the ship. The stiffening elements of
the plates are shown as dotted lines 15, while the stiffening
elements of the platform 6 are shown by numeral 15'.
In FIG. 6 there can be seen the whole of the double bottom 8 and
the ship's inner shell 9, in the area situated below the platform
6. However, the part of the double bottom and inner shell situated
below the said platform can be suppressed in which case these parts
are not reinforced, and then the floors, longitudinal girders and
transverse webframes terminate in faceplates to which the
corresponding edges of the flat plates 10 are welded. This means
that if the double bottom is deleted the lower edges of the flat
plates 10 are welded to the faceplate of the corresponding floor.
If the inner shell 9 of the ship is omitted, the external edges of
the flat plates 10 facing the floors are welded to the faceplates
of the transverse web frames. See FIG. 12. There, it can be seen
that the inner shell 9 is discontinuous below the horizontal
platform 6, degrading into the faceplates 40 of the webframes 42.
Also, except for an annular extension 44, the top of the double
bottom 8 degrades into the faceplates 46 of the double bottom
floors 48.
The skirt 7 may be cylindrical, as shown in FIG. 3, or take the
form of an inverted cone as shown in FIG. 4 in order to match the
shape of the bow and stern of the ship. If the outer one of the
shells 5 that form part of the tank support does not reach the
platform 6, the small skirt 11 shown in FIG. 3 is not used, but
instead only the main skirt 7, complying with the afore-said
requirements, with its stiffeners 12'.
As can be seen in FIG. 7, there is a tray 17 below the tank 1 which
is used to catch any leaks of transported liquid. This tray 17 is
independent of the structure of the ship and of the tank support.
To prevent the liquid caught in the tray from being split by the
motions of the ship a peripheral rim 18 is added. This rim extends
upwards and/or inwards. The collection of any leakage from the tray
17 is assured by means of a deflecting plate 19 that extends from
the internal shell of the support into the inside of the free edge
of the surrounding retention rim 18.
The tray 17 has an exterior continuous stiffening ring 20 that may
be placed at the free edge of the tray as illustrated in FIG. 11,
in which case it will correspond with the retaining rim 18 or be
located in an intermediate annular zone as shown in FIGS. 7 and 8.
The purpose of this ring which would normally work under
compression is to absorb the membrane stresses which are produced
in the tray by its own weight, the weight of the collected liquid,
and the vertical accelerations.
The tray is mounted on supports 21 which can be joined to the
brackets 13 or to the skirt 7 in the case where the structure of
the pedestal is not fitted with such brackets or the brackets are
of small dimension, or, alternatively, to the internal one of the
two shells that comprise the support, this depending on whether the
stiffening ring 20 is situated in an intermediate zone of the tray
or at its free edge.
Referring to FIGS. 7, 8, 9 and 10, the main support 21 is made up
of a continuous platform 22 with reinforcement 23 at its free edge
and supporting elements 24 situated at intervals coniciding with
the brackets 25 which are coplanar with the brackets 13.
Reinforcing ring 20 has a continuous band 26 and also a small
continuous skirt 27 which are perpendicular to one another and
reinforced with brackets 28 and 29. Extending from the ring 20
there are some projections 30 having an inverted profile section 31
between which and the platform 22 of the main support 21 there is
an insulating material 32 of sufficient compressive strength to
withstand the weight of the tray, the collected liquid, and the
accelerations parallel and perpendicular to the axis of revolution
of the tank. On both sides of each projection 30 there is a guide
33 which prevents lateral displacement of the tray but does allow
axial displacement and radial expansion.
Apart from the main support already described, there are other
secondary supports which, in this case, are constituted by the same
platform 22 used for the main support with a bottom reinforcement
34. Between this secondary support and the rim 35 of the ring 20
there are jacks 36 that allow the elevation or lowering of the tray
after extraction of the filling material 32 for inspecting the
upper face of the tray at any given moment. To locate the jacks 36
the secondary supports may be independent of the main supports 21.
Also, the guides 23 can be independent of the supports 21. The
stiffening ring 20 would then have projections housed between the
guides 23 to prevent lateral displacements of the tray.
In the embodiment illustrated in FIG. 11, the supports take the
form of a table or platform 22' joined to the internal shell 4 of
the tank support which also has reinforcements 23'. The lateral
retention is also obtained by means of guides of the same nature as
those illustrated in FIG. 9. The jacks 36' are placed between the
frame 37, shown by dotted lines, and the band 26'.
The main supports and the lateral retention elements or guides 23
can be joined to the internal shell of the tank support or to the
pedestal structure. In both cases, the supports and the lateral
retention elements can either be joined together or be independent.
In the same way, the secondary supports can be independent or can
be combined with the main supports or lateral retention
elements.
The tray 17 will have an insulating layer 38 which, although in the
described example is on the upper face of the tray, could equally
well be located on the lower face. In the first case, the
insulating layer will have a tight covering 39 able to withstand
the low temperatures of any leaks from the tank. Consequently, the
tray can be made from a normal steel or some other non-cryogenic
material. In the second case, if the insulating layer is on the
underside of the tray, the tray should be made of a metallic
material capable of resisting the low temperatures of the liquid
and preferably be of the same material as the tank itself.
As already indicated, the tight covering 39 can be formed by one or
two layers. In the first case, the tight covering is tightly
connected to the drip tray along its whole periphery, so that the
existence of leaks can be detected by introducing gas under
pressure between the two elements. If the tight covering 39 is made
up of two independent layers, these are joined together at their
periphery so as to form a bag which, in its turn, will be joined to
the tray along its periphery. This system makes it possible to
determine, in case of a lask of the gas introduced between the tray
and the adjacent tight covering, whether the leak is in the tray or
in the lower layer of the tight covering. Shrinkage of the tight
covering 39 will be absorbed by the portions of said tight covering
which reproduce the shape of the knuckle between tray 17 and the
surrounding retention rim 18.
The fact that the tray is independent of the tank allows said tank
together with its support and corresponding tray to be assembled in
a single operation. This is done either by lifting the assembly in
which case the tray is attached to the support, at least during
assembly operations, or by floating the assembly after having
flooded the dock where the ship is being constructed. In the second
case, the upper edge of the tray should be above the waterline of
the assembly and if the insulating layer is on the lower face of
the tray it should be protected by a water-proof film.
So that the tray 17 can be constructed without stiffeners and made
thin, without risk of buckling, during flotation of the assembly,
at least the surface of the central part of the tray should be
parallel to the bottom external surface of the tank so that the
support area between tank and tray will be of sufficient magnitude.
The rest of the tray can have a conical shape as shown in FIG. 7
tangential to the surface of the central zone. With the overall
construction described the best transmission of forces through the
support of the tank 1 to the ship is achieved retaining a simple
construction of the ship structure.
As the tray is independent from the structure of the ship and
outside the tank support, it can be built separately from the ship
and installed jointly with the tank and the tank support. The
support system for the tray is of a simple design thus assuring
perfect functioning and allowing inspection of both upper and lower
faces.
In the event that the lateral retention elements or guides 23 are
independent of the main supports, the corresponding insulating
material will be located so that in all conditions the structure of
the ship will be protected against low temperatures.
* * * * *