Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to an offshore platform struc-
ture of the articulated type used for drilling, production, stor-
age, flaring, mooring/loading and other activities related to off-
shore development of sub-sea resources, such as oil.
:
At present, suitable bottom-supported platforms exist ;
for water depths up to approx. 200 m. When water depths exceed
200 m, bottom supported platforms are very expensive. On the ;;~
~; other hand, other kinds of platforms all suffer from technical
problems. For example, floating platforms have problems with
risers, and articulated platforms usually have problems with
the link. Lack of sufficient pay~load capability and storage -
is also a common problem.
This present invention relates to a platform which
has the following advantages:
(1) Pay load and deck area are sufficient, even
for large capacities;
,~, ,
(2) storage is included;
(3) the platform can be made of concrete, with all
~ the well-known advantages of concrete.
:~ 20 (4) the construction technique is very similar ` `~
to the well-known technique of gravity plat~
form construction;
.~ :.: ~
t5) some wells can be drilled before the platform ~;~
is placed, improving the cash-flow; ~`
(6) drilling technique is very similar to conven-
tional technique, allowing the wellheads to be
,, , ~
placed on deck;
(7) the hinge between the bottom template and the
tower is very simple, and the key components
can be placed without any shutdown of drilling/
production;
~ (8) diving is almost not necessary,
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(9) the cost of the platform is moderate,
110) the concept covers a larye depth range,
may be from 100 to lOOOm. ;~:
(11) the plato.rm i5 not vulnerable to earth- ~
quakes, and -.
: (12) if required, the platform can be constructed :
and/or floated in a horizontal position,
~ which limits the necessary water depths.
: In accordance with the present invention, there is
provided a structure fox the development of sub~sea natural ;-.: .
resources, comprising two structural parts, the structural parts
being a template fastened to the sea bed and a tower structure
which is linked to the template, the.link comprising a plurality
of tension members placed near the center of the link, the tension
members fastened to both structural parts and able to take the ~.
vertical tension forces between them, and stiff dowels placed ~:
away from the center of the link, the dowels fastened to both
structural parts, but able to move ln the longitudinal direction
.; relative to one o~ the structural parts, thereby not preventing -
vertical or angular movements in the link, but preventin~ horizontal ;
~- or twisting movements in the link.
:. . : ~
: In the drawings which illustrate preferred embodiments .~. .-
of the invention~
Figure 1 is a vertical section of a first em~odi~
: ment of the platform; ..
Figure 2 is a horizontal section of the first
embodiment, taken along the line A - A
~, .., , -
.: of Figure l;
.: , ,: . :
,. Figure 3 is the ~irst embodiment of the invention ~ .
used as a mooring/loading platform, ~
` ~, ~: ',, ' .
: .. ... : .:: . . . ~ ..... . : . . .. . . .
9 0~
Figure 4 i9 a plan vie~ of the embodiment of the
template according to the present .
invention; i:
:: Figs. 5, 6 and 7 are various vertical sections of
the bottom structure of Figure 4, taken
.~ along the lines B - B, C - C, and D - D
respectively; : :
, . ~ ,
Figure 8 is a detail of one em~odiment of the cable
anchoring structure
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Figure 9 is a vertical section of a second embodi-
ment of the bottom structure according
to the present invention; and
Figure 10, appearing on the first sheet of draw-
ings, is a third embodiment of the plat-
form.
The invention is best described by an example. The ~-
example is related to a water depth of 300 m., and North Sea -
environmental conditions are assumed. ;
As best seen in Figure 1, the platform consists of ~ ~,
three main parts: the template 1, the tower 2 and the dec]c struc- ~-
ture 3. Between the template and the tower there is a link which
permits the tower to oscillate. The template 1, as seen in Fig~
; ures 4 to 7, consists of a core section 4, a slab 5 and a ring
wall 6. From Figure 2, it is seen that the tower consists of ~ .
.:
nine vertical cells, of which eight cells 7 are lying on the
periphery and one cell 8 is in the center. Between the cells is
an open area 17. Further, the deck 3 is a concrete or steel
structure of a conventional design and requires no further des-
cription insofar as the present invention is concerned. Produc- `~
tion and drilling equipment, living quarters, helideck etc., are
placed on the deck in a conventional way.
Before any further description of the platform, the --
construction technique will be described step by step. Initially, ~ `
the concrete template 1 is fabricated ashore, is subsequently
transported to the desired location by a barge, and is lowered
to the sea bed by a crane barge. A semisubmersible moves in and
fastens the template by eight piles 9, illustrated in Figure 5, `~
and also drills some wells. The wellheads (not shown) are placed `
on the slab 5 between the core 4 and the ring wall 6. The wells
are not completed.
Four anchors for the later mooring of the platform -~
: -- ~:
are laid and tested. Approximately, simultaneously with the -
3 ~
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fabrication of the template, a graving dock is made. In this
dock, the bottom raft of the tower is constructed. The dock is
opened and the bottom raft is towed to a deep-water site and
moored. The walls of the tower are slip formed, keeping the top
of the structure at a constant freeboard by water ballasting.
The deck is commenced approx. simultaneously with the fabrica-
tion of the template and the making of the graving dock.
The tower is submerged to a freeboard of six metres;
the deck is towed over and fastened. The tower is raised to a
tow-out draft and towed to the field in a vertical position. The
tower is placed in the pre-laid moorings, lowered down over the
template and fastened to the template by the link.
The pre-drilled walls referred to above are completed.
This could be done either by keeping the wellheads on the tem~
plate (sub-sea completion), or by placing the wellhead on the `- `
deck 3 and just lengthening casings and tubing. ~he remaining
wells are drilled from the deck 3. Sub-sea pipelines are pulled
in on the sea bed and connected to pre-installed risers.
As best seen in Figures 4 to 7, the core structure 4
of the template is made of concrete. There are eight through-
` going holes 10 for the piles 9. The holes may be slightly coni-cal in order to increase the shear capacity between the pile and
the core. The piles consist of steel tubes 11 which are drilled
down to the desired depth, for instance 100 m. Steel reinforce-
ment is lowered down into the steel pipe and extends up to the ~`
top of the template. Finally, concrete is filled in the steel
tube and the holes 10. ~-
The basic object of the core structure is to Eorm the
base for the link. The main part oE the link structure is 14
cables - 12 wlth a total capacity of approx. 10,000 t. The
cables extend through holes 13 and are fastened in a chamber 14.
The upper ends of the cables go through holes 15 and are fasten-
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ed on the upper side of a slab 16.
The tower has an open area 17 inside the peripheral
cells 7.The lower part of this area has one downward truncated
cone 18 and one upward truncated cone 19. These cones are fasten-
ed in a cylinder 20 (of diam. approx. 24 m) which is tangential
to the peripheral cells and which is monolithic cast together with
them. The slab 16 is situated on a cylinder 21 which, in turn,
is constructed on the upper cone 19. The inner tower cylinder 8
goes all the way through these structures from the top to the
bottom of the tower.
The described structures,that is, the piles 9, the
core 4, the cables 12 and the concrete parts 16, 21, 19 and 20
of the tower, together form the part of the connecting link
which is able to take tension. It is easy to see that the link
is a very simple and tough structure which can easily be de~
signed for very large capacities. - ~
It is assumed that the tower will always have a posi- j -
tive buoyancy of a magnitude which is always greater than any ~`
downward force, for instance, caused by wave action. Therefore,
the cables will always be under tension and the tower can freelyoscillate. However, accidents could in some cases cause the
tower to acquire a negative buoyancy. In such a case, the tow-
er would "fall down" on the core, and come to rest on the cen- `~
-: :
tral cell 8. The lower part o~ this cell is strengthened to a
ring foundation 22, which can transfer the forces to the lower
cone 18. An elastomeric bearing 23 is placed on the core just
below the ring foundation 22. This bearing will dampen the fall
of the tower and allow some movements of the tower without over-
stressing the concrete. `~
The tower must also be prevented from moving sideways
in the link and from twisting. These kinds of movements are pre-
vented by two groups of dowels, primary dowels 24 and secondary ~ ~
~` .s ~ S ~ : . ': ~ .
~.
dowels 25. The primary dowels 2~, as seen in FicJure 5, are
fastened in the "star cell" between -the peripheral cells 7 and
the cylinder 20. The secondary downs 25, as seen in Figure 6, are
fastened in the slab 26 and the upper cones 19. Both kinds of
dowels are placed in open holes, (27 for the primary dowels), so
that they can be retracted and replaced, but they are prevented
from falling down. The primary dowe~s extend down into openings ~
28 in the ring wall 6 and the secondary dowels in holes 29 in ,,
the core.
The dowels will then act as if they were fixed to the
tower structure, while permitting limited angular and vertical
movements in the template structure. As a result, the tower is ,! ~,,
prevented from any kind of horizontal movement at this eleva~
tion, but limited angular movements are permitted. The annulus ~
between the dowel and the wall of the hole 27 should be filled ~,
with an elastomeric bearing to provide some flexibllity.
The primary dowels will be those acting under normal
circumstances. The reason for introducing the secondary dowels
is to have an additional safety if the primary dowels fail for ~-
some reason. ~he total capacity o the dowels is very depend~
ent on the conditions in each separate case. It could be as
low as 100 tons, and as high as 5,000 tons horizontal load.
When installing the tower (construction step 12), the primary ~; `,'
dowels are the lowest part. They have to enter the holes 28; ;
and to ease this operation-, cones 30 are fastened to the ring ' `~
wall~ The capacity of one dowel should be lower than the point
resistance capacity of the template. If so, only the dowel
will be hurt if a collision occurs between a dowel and the tem~
plate. The pre-installed wellheads are lower than the,ring wall
6 and are therefore protected.
When installing the tower, the structure is lowered
by water ballasting. The primary dowels enter the holes 28 and -
.
` - 6 ~
~05'~(38
the lowering continues until the tower is resting on the bear-
ing 23 with a small weight. The cables 12, which are pre-instal-
led in the holes 15, are lowered down through the holes 13 and
fastened. Thereafter, water is pumped out until the desired
positive buoyancy is obtained, and the tower is "hanging" in the
cables. Now, the platform can cope with any weather conditions.
Max. horizontal movement at sea level is approx. 15 m, giving a
maximum deviation from vertical of 1:20. ~-
A very important point is access to the structural
parts forming the link, both during installation, maintenance,
and removing of the platform. Access can be obtained by three
methods. Divers can go down in the open area 17, through holes -
(not shown) in the cones 19 and 18, and down on the slab 5). From
here, there is a horizontal tunnel 31, as seen in Figures ~ and
7, and further an opening 32 through which the diver can enter
the chamber 14. Diver access could also be inside the center
cell 8, through openings (not shown) in the cone 19 and the slab
26 down on the core. Further access to the chamber 14 would be `-~
through the holes 34. rrhe tunnels 31 could be closed by water~
tight hatches 35.
The second method for access is made possible by intro-
ducing the slab 36, as seen in Figure 1, in the centre celI.
Compressed air is pumped in the centre cell below this slab and
presses out the water in the centre cell below the slab 36 down -
to the tip of the ring foundation 22. Through a sluice in the -~
slab 36, men can go down and work in the lower part of the cen-
tre cell in air. -~
rrhe third method for access is only possible during
; good weather conditions. The tower is ballasted so that it rests `~
on the bearing 23. ~hen the hatches 35 are closed, it is possi-
ble to pump out all the water in the center cell and in all the
openings in the template core. People have then access through
: ', . .
i~ - 7 -
1~)5'~8
the centre cell to the cables and to the secondary dowels under
atmospheric conditions.
The centre cell is filled with a non-corrosive liquid
to prevent corrosion on -the parts con-tained in this cell. A
membrane 37 is fastened between the ring foundation 22 and the
template core to prevent the non-corrosive liquid to escape. For
the same reason, the hatches 35 are normally closed. A reduced ~-
amount of such liquid is necessary if all openings in the slab
36 are closed and the non-corrosive liquid is filled below this
slab only.
The slab 36 has access openings and openings to per-
mit cables 12 to be lowered directly from the deck into position.
As seen in Figure 8, the cables 12 have steel plates 38 perman-
ently fi~ed at both ends thereof. The holes 15 and L3 have dia-
meters slightly larger than the plates 38 and the cables can
therefore be lowered through the holes. They are fastened by
steel plates 39 which are split and are therefore installed
from the sides. Varying thicknesses of steel plates 39 can be
used to compensate for small differences in cable lengths.
Pipes with curved walls 71 are placed in the annular ;~
spaces between the cable and the hole's wall so as to prevent
sharp bends on the cables when the tower oscillates. The cables
must have a certain length, decided in each separate case, in -
order to achieve the desired flexibility. This is obtained by
employing the cylinder 21, the length of which can be varied for
each separate platform.
The peripheral cylinders of the tower are those which
give the buoyancy. Each has a bottom dome 67 and two internal
domes, a lower dome 39 and an upper dome 40. Seven of the cells -
are storage cells 41, for the storage of,`for instance, oil. The
eighth cell is a utility cell 42 for storing pipes, machinery,
etc. In each storage cell is a lower compartment 43 which is ~ ~
- 8 - ` ;
.. . .. . . . . . ... . .
~0~ 8
used for storaye. The lower compartment 43 is always filled with
liquid, either sea water or oil. A middle compartment 44 is air-
filled, but is also used for ballasting. Ballasting is necessary
for lifting or lowering the tower, as well as to compensate for
different weights. This may be caused by chanying the payload
or by the different specific weight of oil and water, which will
change the weight of the tower according to the amount of oil
filled in the storage. Differential ballas-ting may also be neces-
sary, for example, to compensate for horizontal forces caused
by wind or current, giving the tower a heel, or to give the
tower a wanted stability (GM).
An upper compartment 45 is also airfilled, and is also
used as a water level tank. This is necessary because the whole
tower structure should be kept at a certain underpressure, to
obtain compressive forces in the structure. The storage compart-
ment is in direct communication with the water level tank, and
therefore the storage is kept at the same pressure. In this ~ -~
case, the underpressure in the storage will be approx. 40 m.
In the utility celI, the middle compartment 33 is
used for the main machinery as, for instance, oil pumps, sea
water pumps, etc. The lower compartment 46 contains an inner ~ -
cylinder 47 which houses the ballast pumps and other machinery ?~
that should be placed close to the bottom of the tower. Also,
the upper compartment 48 has an inner cylinder 49. The reason
for this is to protect the machinery against flooding if a `
collision should cause flooding of the upper compartment 48. ``~
A major reason for the compartmentization by the domes
39 and 40 is to obtain acceptable damage conditions. Risers 72
are pre-installed in separate riser cells 50. These cells are
3Q slipformed together with the main cells. At the bottom, the
cell is terminated by a slab 51. As seen in Figure 7, the pipe-
line 52 to be connected to the riser has a bend 53 in the end. ; ~;
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The bend is supported by the sledge 54 and is prevented from tilt-
ing by the sledge. The pipeline is pulled under the tower until
the vertical part of the bend is exactly under the pre-installed
riser. The bend is then lifted and the ver-tical part enters a
hole 55 in the slab 51. The annulus between the slab and the pipe-
line is sealed and the riser cell 50 is pumped dry. Now, the weld
56 between the riser and the pipeline can be done under atmos-
pheric conditions. When this is done, the riser cell is filled,
the seal is removed, the riser lowered and the pipeline sledge
~4 will again rest on the sea bed, as shown in Figure 7.
The area 17 between the peripheral cells 7 and the
centre cell 8 is always waterfilled, and in this area the con-
ductors are situated. Just one conductor 57 is shown in Figure
7. The conductor extends through holes 58 and 59 in the cones,
and through a hole 60 in the template slab. The holes 59 or 58
could be temporarily closed during construction and installation `~
to increase buoyancy, but will be open when the tower lS in-
stalled. Conductor guides (not shown) are instalIed at differ-
, ent levels in the area 17. When movements are sufficiently small,
no link will be lntroduced in the conductors. The movements are -
taken by bendiny only.
Some figures for the embodiment shown shall be given. `-
he water depth is 300 m. The outer diameter of the peripheral ~;
cells 7 is 15 m., giving a diameter of the open area 17 of
` approx. 24m. The diameter of centre cell 8 is 8 m. The total `~ ~`
displacement is more than 400,000 m3, and the concrete volume `
approx. 100,000 m3. The deck load could be in excess of 30,000
t. ;~
More figures could be found directly from the figures. `
30 Figures 1 and 2, which are approximately to a scale of 1:2000;
while Figures 4 - 7 are approximately to a scale of 1:200.
~ ~ second embodiment of the invention is shown in Fig-
.: '~"
- 10 - " ~,~
~.~35,'~ 3 7
ure 9. In thi~s embodiment, only the link design differs from
that of the firs-t embodimen-t. The object of the second embodi-
ment is to find a structure where the link is not only for ten~
sion, but where the vertical forces could be both tension and
: .
compression. ~`-
The cables 12 with anchors are in principle the same ~ -
as for the first embodiment. However, in the centre, the cables
are omitted and a support 61 is placed there. The support con-
sists of an upper steel slab 62, and a lower steel slab 63. The
slabs are curved and are very solid, having a diameter of 1 - 2m.
In this case, the upper slab 62 is dome shaped (convex), and the
lower slab 63 is slightly concave. When the tower oscillates, the
upper slab will roll on the lower slab and form a support for ;;
the tower. - -
The upper slab 62 is fixed on the lower cone 18 and :
the lo~er slab 63 is fixed on the template core 4. The central
~,
part of the core is heightened somewhat, so that the upper part ~ ~-
- is enclosed by the lower part of the central cylinder 8. When
the central-cylinde`r is filled with compressed air, the water ;~;
level will be below the access openings 64. This means that the
: , ~
chamber 65 could also be air filled and that under-water work ~- ;
could be avoided when inspecting and removing the cables 12 and
the support 61. The support 61 should preferably be made remov~
able. This could be done by omitting some of the cables, pull
the whole support 61 to the side and replace it with another
~similar support. During this operation, the tower must be bal~
lasted to positive buoyancy. The cables can be replaced when
. .. ;,
the tower has a negative buoyancy. Then, the support forces will ~;
be taken by the support 61. The support 61 could also be done
in other ways than described, for instance, by elastomeric bear~
ings.
; The dowels are, in principle, the same for the first '
and second embodiments. It might be necessary, however, to move `;~ ~
~ -- 11 -
- '.. '' - ~,':
~os~o~
-the centre of rotation somewhat to get the same centre of rota-
tion as the support 61. The primary dowels must also be longer
for the second embodiment, due to the heightened core.
A third embodiment of the invention is shown on Fig-
ure 10. In this case it is the tower itself that differs from
- the first embodiment. The section shown in Figure 10 corresponds
to that shown in Figure 2. The third embodiment has some small
-~ buoyancy cells 66 in addition to those in the first embodiment.
The smaller cells 66 reach from the bottom of the tower to a
level somewhat below sea level, for instance to an elevation of -
40 m.
The background for the cells 66 is the following. When ~ ~-
a wave passes the structure, there will be pressure variations
in the water. The variations on the lower domes 67 will create
a varying vertical force on the tower and consequently also in
the link. By introducing the cells 66, this force will increase,
but the pressure variations will also act on the upper domes of
- the cells 66, and in the opposite direction. In addition, the
pressure variation is larger et al. - 40 than at el. - 290.
This means that by correct designing the vertical variations can
be considerably reduced.
.
; An example will show the effect of a wave on the cells
66. The lower domes 67 of the periphery cells 7 have an area
of 1400 m2. The domes of the cells 66 have an area of 630 m ,
~` if the cell diameter is 10 m. For a certain wave, the pressure
~` variation may, at 290 m depth, be 3 t/m , and at 40 m depth
8 t/m . In this case, the first embodiment of the invention
will give a pressure variation of 1400 x 3 = 4200 t. The third
embodiment will give 2030 x 3 = 6090 t in one direction and `-
630 x 8 = 5040 t in the other direction. This means that the re~
sultant force, as a result of the introduction of the cells 66,
- will be reduced from 4200 t to 1050 t.
- 12 -
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Figure 3 shows a mooring/loading system connected to the
platform. This is a conventional s~stem and does not require a -
detailed explanation. When a ship 68 swings around the platform
due to the weather, a mooring system 69 and a loading systern 70
follow on rails going around the tower. sy placing the mooring/
loading system directly on the tower, a special loading buoy is
avoided, as is a great deal of subsea work.
It is possible also to place flare structures etc., on
the deck. This means that all the facilities needed for an off-
shore development of a field could be incorporated in one plat-
form. This is a great advantage because, in this case, no sub-
sea pipelines are needed and the amount of diving will be great-
ly reduced. In fact, it is possible that diving could be com-
pletely avoided.
When constructing and towing the tower 2, it has been
assumed that the tower should at all times be in a vertical posi-
tion. This may be difficult in many cases, due to depth limita- ~`
tions. Thus, it should be emphasized that it is also possible
to construct the tower in horizontal position in a dock, tow it
to the field or to a deep-water outfitting site in horizontal "
position, and turn it to a vertical position at the site, by a
proper ballasting of the cell compartments.
It is also possible to construct the tower in a verti-
cal position (which is a great advantage because slipforming can
then be used), turn it to horizontal position for towing over
shallow areas, and then turn it back to a vertical position. A
towing at a certain tower heel could also be imagined~
The template 1 should have protecting caps during ~ `~
instalLation. The central area of the core, inside the piles, `~
should have a steel cap to cover the elastomeric bearings and `;
the area inside during all construction steps until the tower is
to be installed. The area outside should have a steel cap (with :~
;~.: . .
holes for the piles) during the piling period. - -;
- 13 -
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The structure described is well fit to be used in earth-
quake areas. If the horizontal shear force in the link exceeds
the capacity of the dowels, the dowels will yield somewhat. This
is no catastrophy, as it is possible to cut the dowels and to
replace them with new dowels.
Vertical forces from earthquakes will usually be taken
by the deflection of the cables. However, it might be necessary
to place the support 61 on elastomeric bearings to obtain suf-
ficient flexibility in the support. The elastomeric bearing -
should be larger than the support, for instance, 5 x 5 m andshould be stiffened by a steel plate. The steel plate and the
bearings must then have holes for the cables.
The embodiments described are examples only, and
the design could be varied within the scope of the present in-
vention, e.g., by ~sing steel rods instead of cables.
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