Note: Descriptions are shown in the official language in which they were submitted.
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Floatingplatform for operation in re iog ns exposed to extreme weather
conditions
The invention relates to a floating platform for drilling after or production
of
hydrocarbons offshore, comprising a semi-submersible main platform carrying
process
and/or drilling equipment at its upper surface, and which is designed as a
vertical,
essentially flat-bottomed cylinder.
A platform designed as stated above is known from Norwegian patent No.
io 319 971. By designing the platform with a cylindrical platform body, there
is obtained
a substantial reduction of pitching and rolling moveinents. Further, such a
platform has
large carrying capacity with respect to production or drilling equipment, and
a large
storage capacity for oil, fuel and the like. The platform will be direction -
independent
with respect to environmental influence, and therefore may be anchored by
means of a
1s simple fixed anchoring system. Further, risers can be connected directly to
a receiving
system on the platform, without any need for swivel transference or the like.
For operation in arctic regions where there is a danger for icebergs, it is a
requirement that a floating platform must have the possibility for
disconnection of both
anchoring system and risers. When operating in regions where extreme weather
occurs
20 with low frequency, for example in regions exposed to hurricanes, it may be
of great
advantage to be able to disconnect a floating unit, as this will be able to
reduce the
requirements to an anchoring system considerably.
The drawbacks of a conventional disconnection with production of hydro-
carbons where anchoring lines and risers are disconnected individually, are
loss of time
25 and breaks in deliveries. In addition, equipment which is to be able to be
disconnected
is more expensive in order to be able to take into account a possible leakage
together
with picking-up from the sea. A disconnection and a subsequent connection of
anchoring lines and/or risers may involve several weeks of stoppage in the
production
of a production unit. The connection generally will require that a tender
vessel is
30 engaged for picking-up of anchoring lines and risers. The connecting
operation
therefore will be sensitive to the weather conditions.
The object of the present invention is to provide a platform for use in
regions
exposed to extreme weather or where drifting ice and icebergs occur.
The above-mentioned object is achieved with a platform of the introductorily
35 stated type which, according to the invention, is characterised in that the
main platform
is provided with a central, vertical shaft which at its lower end is adapted
for reception
in releasable locking of an anchoring buoy carrying fastening equipment for
anchoring
lines and for risers and umbilicals.
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The invention will be further described below in connection with exemplary
embodiments with reference to the drawings, wherein
Fig. 1 shows the main platform and the anchoring buoy connected together;
Fig. 2 shows how the anchoring buoy is raised to the main platform by chain
winches;
Fig. 3 shows a first embodiment of the anchoring buoy with flexible risers
connected thereto;
Fig. 4 shows a second embodiment of the anchoring buoy with steel risers
connected thereto;
Fig. 5 shows a top view of a set-up of anchoring lines connected to the
anchoring buoy;
Fig. 6 shows the anchoring buoy lowered to a free-floating position after
disconnection; and
Fig. 7 shows pick-up lines extending from the anchoring buoy and floating on
the water surface.
As shown in Figs. 1 and 2, the illustrated embodiment of a platform structure
1
according to the invention comprises a main platform 2 and an anchoring unit
consisting of a submerged anchoring buoy 3. The main platform 2 is designed as
a
vertical, essentially flat-bottomed cylinder body, in a corresponding manner
as in the
introductorily mentioned patent specification. The platform may have a
circular or a
polygonal cross-section.
As distinct from the previously known platform, the cylinder body in this case
is provided with a central, vertical shaft 8 which at its lower end is adapted
for
reception and releasable locking of the anchoring buoy 3. The platform has
double
walls and possibly a double bottom that are used as ballast tanks 4. These are
divided
into a sufficient number of tanks to make sure that damage stability and the
like is
taken care of. Inside of the ballast tanks there are arranged loading tanks 5
for storage
of hydrocarbons. Under a living quarter 18 on the main deck 7 there will be
engine
rooms and different tanks, a cofferdam etc. Equipment for the production of
oil and
gas, drilling equipment and different auxiliary systems are placed as modules
6 on the
main deck.
As appears from Figs. 1 and 2, winches 9 are arranged in the upper part of the
shaft 8, for pulling-in of the anchoring buoy 3 from a submerged position and
up to the
connecting position in the shaft 8. The anchoring buoy has an at least partly
conical
shape, for reception in a corresponding conical portion in the lower part of
the shaft.
When the buoy 3 is in the correct position, it will be securely locked by a
mechanical
locking means (not shown). The winches 9 will also be used for connection, pre-
stressing and after-tensioning of the anchoring lines (anchor chains) 10 for
the
platform. On the main platform there are also installed coupling manifolds for
the
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transfer of well stream, and for oil and gas export. Transfer hoses will be
installed
between the coupling manifold and riser fastenings 20 located on the anchoring
buoy.
The main platform 2 may be a passive unit which has no propulsion machinery
of its own, and which has to be towed by a tender vessel to and from the
position for
connection, or if it has to be moved away from the current location. The main
platform
2 may also be equipped with propellers (thrusters) 11 which will give the main
platform the possibility to transfer itself without any assistance from a
tender vessel. It
may transfer itself to avoid bad weather or icebergs, and it may sail back for
own
machine and couple itself to the anchoring buoy 3.
The anchoring buoy 3 is a large buoyancy tank. The buoyancy tank is divided
into a suitable number of cells in order to ensure that a damage does not have
dramatic
consequences. At the outer edge of the buoy there are placed fairleads 12 for
fastening
of the anchor chains 10. The anchor chains 10 extend from the fairleads and up
to chain
locks 13 at the top the buoy. The chain locks will carry all forces from the
anchor
chains 10, both when the buoy floats freely and when it is connected to the
main
platform 2. Risers and umbilicals 14, 15 will be connected to the buoy. If
flexible risers
14 are used at "normal" water depths, there will be arranged I- or J-tubes
extending
through the buoy hull from the bottom to the top. The risers 14 may then be
pulled in
and locked fixedly at the upper edge of the buoy. Here there will be placed
safety
valves and the like together with coupling flenses 20 for connection of
transfer hoses
etc. At the bottom of the I- or J-tube there may be arranged either a
bellmouth or a
locking means for fastening of bend stiffeners. If steel catenary risers (SCR)
15 are
used, an opening 25 will be arranged in the center of the buoy (see Fig. 4)
where there
is arranged a deck for suspension of the risers. The risers then will hang in
a flexible
joint fastened in this deck. Safety valves, coupling flanges etc. will be
placed above the
deck. With this arrangement the steel risers will hang freely towards the sea
bed. A
similar suspension may also be used for freely hanging flexible risers 14. The
flexible
joint at the top then will not be required.
When installing the platform structure 1, the main platform 2 and the
3o anchoring buoy 3 will be interconnected by a mechanical locking means.
Pulling-
in/adjustment of anchoring lines 10 and tensioning of lines will be as on a
"conventional" unit. After pulling-in of the anchoring lines 10 they will be
locked by
the chain locks 13 located at the top of the anchoring buoy 3. After locking
of the
anchoring lines/anchor chains 10, the chain length which has been pulled on
board will
be placed in chain lockers 16 at the top of the anchoring buoy. At the end of
some of
the chains there will be mounted a pick-up line 17. The number of pick-up
lines 17
depends on how the anchoring lines 10 are arranged. For the arrangement shown
in
Fig. 5, three pick-up lines 17 will be used, i.e. one from each group of
anchoring lines
10. The pick-up lines 17 may be arranged so that they are picked up by a
remotely
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operated vehicle (ROV) in that lines 17 with a buoyancy element are released
by a
hydroacoustic signal, whereafter the lines can be picked up at the water
surface, or at
least one line 23 may be equipped with a floating element 22 so that it lies
on the
surface and can be picked up.
Pulling-in and connection of risers, umbilicals, etc. will also be carried out
with the anchoring buoy 3 connected. After locking of risers 14, 15,
connection hoses
and cables between the top of risers/cables and the manifold arrangement on
the main
platform 2 can be mounted in place. After connection of anchoring lines 10 and
risers
14, 15 the platform structure 1 is ready for operation.
io As mentioned above, the anchoring buoy 3 in the connected position is
locked
to the main platform 2 by a mechanical locking means. Before a disconnection
can be
carried out, all connection hoses and cables between the main platform 2 and
the
anchoring buoy 3 must be disconnected. When this has been carried out, the
anchoring
buoy can be disconnected by opening the mechanical locking means. The
anchoring
buoy is ballasted so that it finds a position of equilibrium at a pre-
determined depth D
below the water surface 24. The anchoring buoy 3 will now float in a position
wherein
it will only to a small extent be subjected to loads from waves. In addition,
the risk for
collision with vessels and possible icebergs will be reduced to a minimum.
When connecting the anchoring buoy 3, the main platform 2 will be brought to
the correct position above the buoy, either unaided or by assistance from a
tender
vessel. A remotely operated vehicle (ROV) picks up pick-up lines 17 which are
brought up inside the shaft 8 in the main platform 2. The pick-up lines 17 are
connected to the chain winches 9 which are used to pull up the ends of the
anchor
chains 10 located in the chain lockers 16 at the top of the anchoring buoy 3,
and the
chains are placed on the chain winches. The pull-in is carried out in that the
chain
winches 9 pull in the anchoring buoy 3 with all risers, umbilicals, cables,
etc.
connected to and extending down to the sea bed. When the anchoring buoy 3 is
in the
correct position, it is locked by the mechanical locking means. Coupling
hoses, cables,
etc. from the main platform 2 is connected to the anchoring buoy 3 and the
production
can be restarted.
Alternatively, the pick-up lines 17 will have floating buoys 21, 22 which can
be released for instance by a hydroacoustic signal or which lie on the water
surface and
can be picked up in a conventional manner. If an arrangement with a floating
pick-up
line 23 is used, a line must be arranged from the shaft 8 and outwards to the
side of the
platform 2. The floating line 23 is then picked up from the side of the main
platform 2
and is connected to said line. The buoyancy elements 21, 22 are disconnected
and the
interconnected lines are again dropped into the sea. The pick-up lines 17 can
now be
pulled up through the shaft 8 and connected to the chain winches 9. Further
pulling-up
and connection will be as with the use of an ROV.
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When the platform structure is to be equipped with an installation for
drilling
after oil, such an installation requires a vertical opening through the main
platform (a
moon-pool) if the installation is to be placed on the deck 7 of the platform.
It will then
be most suitable to use a part of a loading tank 5 as a moon-pool. When using
an
5"internal" moon-pool, drilling risers and the like will be protected against
ice and the
like in exposed regions. Possible oil leakages in the moon-pool will also in a
simple
manner be able to be intercepted and removed.
Alternatively, the drilling installation may be placed on a cantilever
platform
(not shown) at the side of the main platform 2. In case of drilling operations
the
io anchoring lines may be adjusted in order to maintain a correct position of
the platform
when the weather load varies. If the platform is equipped with thrusters 11
these may
be used in addition in order to maintain a best possible position while
drilling.
When connecting and disconnecting the anchoring buoy 3, the drilling
installation will not be involved. Drilling risers etc. will be connected
after that the
anchoring buoy is in the locked position. When disconnecting the anchoring
buoy,
drilling risers with associated valves etc. will be disconnected from the
bottom and
preferably raised and stored prior to disconnection.
