Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: METHOD AND APPARATUS FOR DISCONNECTING AND
RETRIEVING MULTIPLE RISERS ATTACHED TO A
FLOATING VESSEL
BACKGROUND OF THE INVENTION
Reference to Related Provisional Annlication
This application claims the benefit of provisional application serial no.
60/033668 filed December 8, 1996.
Field of the Invention
This invention relates to a method and apparatus for disconnecting and
retrieving multiple risers attached to a floating vessel, and more
particularly to such
an apparatus and method in which the vessel includes a turret having a
releasable
riser buoy with a plurality of flexible risers supported thereon for docking
within the
turret.
Description of thePrior Art
Heretofore, it has been known to provide a vessel having a turret about which
the vessel may weathervane with the turret anchored to the sea floor and with
risers
extending between sea floor wells and the turret for the transport of
production fluids
to the vessel. Also, it has been known to provide a deep water mooring system
in
which a submerged anchor member has been provided to support risers when
disconnected from the vessel.
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The de Baan et al. patent No. 5,044,297 dated September 3, 1991 shows a
deep water mooring system in which submerged flotation tanks support
production
risers or lines in addition to mooring lines. A floating marker buoy includes
a retrieval
line connected to the submerged flotation tanks so that the risers and the
mooring
lines may be easily retrieved for connection to a vessel after being
disconnected from
a previous vessel. A mooring weight on the sea floor may be connected to the
flotation tanks to anchor the flotation tanks at a predetermined water depth.
The
vessel normally includes storage tanks for the production fluids transported
by the
risers. The support buoys of the '297 patent are not received within a turret;
they are
provided primarily for the support of mooring lines. The buoys or flotation
tanks
always remain in a submerged location away from the vessel.
U.S. Patent No. 4,509,448 dated April 9, 1985 shows buoys for supporting
mooring lines of a drillship both in a submerged position when connected to a
vessel
and in a floating position on the sea surface when the mooring lines are
disconnected from the vessel. The buoys may be pulled onto the deck of a
workboat. The buoys do not support risers or riser lines.
U.S. Patent No. 5,306,186 which issued April 26, 1994 shows a
disconnectable mooring system including a buoy disconnectable from a turret
and
in equilibrium depth beneath the surface of the sea, such as 100 feet. They
buoy
supports mooring lines in addition to risers. The buoy is docked against the
lower
end of the turret and is not received within the turret. The entire disclosure
of U.S.
Patent No. 5, 306,186 is incorporated by this reference.
It is an object of this invention to provide a disconnectable riser buoy for
supporting only risers or riser lines, but not mooring lines.
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It is a further object of this invention to provide a disconnectable riser
buoy for
docking within a turret on a floating vessel.
Another object of the invention is to provide a riser buoy which carries
risers
which are connected to flow paths which are removably coupled to vessel
product
lines at a position above sea level.
Another object of the invention is to provide a riser buoy when disconnected
from a turret is maintained at a submerged depth in the sea by a buoy anchor
leg
which is attached to a weight which may be lowered down to the sea floor or
raised
to the bottom of the turret.
SUMMARY OF THE INVENTION
The present invention is for a method and apparatus for connecting,
disconnecting, and retrieving multiple risers or umbilicals for a floating
vessel. The
risers extend to sea floor manifolds or wells. Production fluids are
transported by the
risers to storage areas of the floating vessel. A riser buoy is removably
connected to
the turret which is rotatably coupled with the vessel. The turret is anchored
to the
sea floor independently of the riser buoy. Multiple risers are carried by the
riser
buoy. The risers are grouped in predetermined assemblies which are connected
to
the riser buoy. For example, three groups of riser lines may be carried or
connected
to the riser buoy. The riser buoy is connected to the turret at a location
above the
draft line or sea level surface in order to provide access to the riser
connections
during and after mating of the riser buoy to the turret.
A retractable weight or gravity base is mounted on the riser buoy. Upon
, release of the riser buoy from the turret, the weight is paged out until it
comes to rest
on the sea bed in order to moor the riser buoy and limit the excursions of the
risers
within acceptable limits. By pre-determining the length of the line from the
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retractable weight to the riser buoy, the riser buoy may be positioned at a
precise
water depth when released from the turret to permit the riser buoy and risers
to be
located below the tanker or other vessel traffic and to be beneath a region of
high
wave loads.
The riser buoy of the present invention does not have buoyancy sufficient to
support the mooring lines; thus, it may be substantially smaller than buoys
designed
to support mooring lines simultaneously with risers. The turret receives the
buoy in
a docked position at a height above the sea level within the turret. Quick
disconnectlconnect couplings are provided between the buoy flow paths in the
buoy
for the flexible risers and product conduits on the turret. Thus, workmen are
provided
dry access for coupling and uncoupling the quick disconnect/connect couplings
in the
docked position of the riser buoy.
The lower end portion of the turret is curved. The flexible riser lines may
contact the lower curved portion of the turret when the riser buoy is in the
docked
position in the turret. If such contact occurs, lateral loads are transferred
from the
risers to the turret.
BRIEF DESCRIPTION OF THE DRAWINGS
The objects, advantages and features of the invention will become more
apparent by reference to the drawings which are appended hereto and wherein an
illustrative embodiment of the invention is shown, of which:
Figure 1 is a schematic view showing a vessel having a turret about which the
vessel weathervanes with riser groups releasably connected to the turret;
Figure 2 is a side elevationai view, partly schematic, of the vessel shown in
Figure 1 showing the plurality of risers connected to a riser buoy docked to a
turret
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with the risers extending down to product flow lines on the sea bed for the
transport
of production fluids from manifolds or seabed wells;
Figure 3 is an enlarged sectional view of a turret mounted on a vessel and
showing a riser buoy with risers connected thereto and docked within the
turret and
S with anchor chains connected independently of the riser buoy body to the
upper
portion of the turret;
Figure 4 is an elevational view of the riser buoy comprising the present
invention removed from the turret;
Figure 5 is a top plan of the riser buoy when viewed from section fines 5-5
shown in Figure 4;
Figure 6 is an elevational view, partly schematic, showing the riser buoy and
supported risers of the invention released from the turret and being lowered
toward
the sea floor with a weight suspended from the riser buoy for positioning the
riser
buoy at a predetermined water depth and location relative to the sea floor,
the weight
shown connected to the riser buoy by means of a cable or line shown in broken
lines;
Figure 7 is a schematic view showing the riser buoy with a retrieval line
connected at its upper end to a floating marker buoy, with a dotted fine view
of the
riser buoy, cable, retrieval line and risers illustrated when the buoy is
forced to a non-
vertical position by sea currents or the like; and
Figure 8 is a partial sectional view of an alternative embodiment of anchor
legs
secured to the lower end portion of the turret.
pESCRIPTION OF THE INVENTION
Referring particularly to Figures 1, 2 and 3, a storage vessel 10, is shown
for
the storage andlor collection and transport of production fluids, such as oil
or gas
from sea floor manifolds or wells (not illustrated) . The surface of the sea
is shown
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at 12; the sea bed is shown at 14. A generally cylindrical turret 20 is
mounted within
a vertical opening or moon pool 22 of vessel 10 and is supported on bearing
assemblies 24 so that when anchored to the sea floor 14, the vessel 10 may
rotate
about the turret 20. Alternatively, or in addition, thrusters 16 may be
mounted
externally on the bottom of the hull 18 of vessel 10 for positioning and
maintaining
vessel 10 at a desired position in the sea.
Three groups of mooring lines 17 as shown in Figure 1 anchor the turret 20
to the sea bed 14 for mooring of vessel 10. Mooring lines 17 may not be
necessary
if vessel 10 is positioned solely by thrusters 16. Three groups of risers 19
which are
coupled to subsea wells or manifolds (not illustrated} are positioned between
the
three groups of mooring lines 17 to minimize any interference between mooring
lines
17 and risers 19.
Turret 20 has product conduits or lines 26 placed thereon for coupling to
risers
19. Suitable swivel arrangements as well known (not shown) may be provided for
fluid rotative coupling between conduits 26 and suitable conduits fixed to
vessel 10
which extend to storage areas or tanks within vessel 10.
The lower end portion of turret 20 has a bend shoe or fender shown at 27 with
a beveled or arcuate contour for fairieading riser lines 19 in order to cause
a
substantial portion of the lateral loads from the riser lines 19 to be reacted
directly
by turret 20. In one preferred embodiment, mooring lines or chains 17 extend
through tubular guides 21 on turret 20 and are removable connected by stoppers
23
at the upper ends of guide 21. Stoppers 23 as well known fit about a link of
chain 17.
A retrieval line 25 is connected to a winch 29 for pulling in chain 17.
Mooring lines
17 may be disconnected from turret 20 by removal of stoppers 23. If it is
desired to
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move vessel 10 to another location, a separate retrieval line {not shown) for
each
mooring line 17 may be connected to a marker buoy.
An upper main deck 30 is provided on vessel 10. The operating water level
or sea level 12 is below deck 30 and below the position of quick
disconnect/connect
couplings 32 for product conduits 26 thereby permitting product conduits 26 to
be
connected to buoy flow paths for risers 19 above the sea as explained below. A
walkway 33 for workman is provided for workmen access above sea level 12
adjacent docked buoy 36.
Referring further to Figures 4 and 5, a riser buoy 36 includes a plurality of
product passages or "flow paths" 38 therein connected to lower flexible risers
19.
Upper quick disconnect/connect couplings 40 are provided for releasable
connection
to mating couplings 32 when in the docked position of riser buoy 36 shown in
Figure
3. Suitable quick disconnect/connect couplings 32, 40 are available from
several
manufacturers. A center hawse pipe 42 in riser buoy 36 receives a buoy anchor
chain 44 therein.
Spaced bumper members 46 are arcuately spaced about the outer periphery
of riser buoy 36. Bumper members 46 are preferably of an elastomeric material
for
contacting the inner periphery of turret 20 when docking. For docking riser
buoy 36
at a predetermined location, three recesses or slots 48 having notches 50
therein are
provided about the outer periphery of buoy 36. Locking arms 52 are pivotally
mounted at 54 to turret 20 and are actuated by hydraulic cylinders 56.
Cylinders 56
are actuated to move locking arms 52 into engagement with notches 50.
Buoy anchor chain 44 has a weight or gravity base 58 connected to its lower
end. A light weight retrieval line 60 is connected to the upper end of anchor
chain 44.
Upper retrieval line 60 is connected to a winch 62 for paying weight 58 in and
out
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relative to riser buoy 36. A removable stopper 64 is positioned on the upper
end of
chain 44 for engaging the upper end of hawse pipe 42 when weight 58 is at a
desired
distance from riser buoy 36 for deployment. If vessel 10 is disconnected from
mooring lines 17 and moved to another location, retrieval line 60 may be
disconnected from turret 20 and connected to a marker buoy 66 as in Figure 7
for
floating on the sea surface. If desired, a retrieval line may be connected to
anchor
chain 44 for connection to marker buoy 66. For docking riser buoy 36,
retrieval
line 60 is pulled upwardly over pulley 61 by winch 62 along with weight 58 and
anchor chain 44 until weight 58 contacts buoy 36. Then, buoy 36 and weight 58
move upwardly together. Quick disconnect/connect couplings 32 and 40 are
vertically aligned, and riser buoy 36 is drawn upwardly inside turret 20 until
its top
surface is above sea level 12. Thus dry access is provided for connecting
couplings
32 and 40. When in proper position, locking arms 52 are actuated into notches
50
to secure riser buoy 36 to turret 20. Then, couplings 32 and 40 are connected.
Turret 20 has a lower circumferential fender or shoe 27 as best seen in Figure
3. When risers 19 are angled outwardly into contact with shoe 27, a portion of
the
lateral load from risers 79 is opposed directly by turret 20 and shoe 27
thereby
reducing the load on riser buoy 36 and its connections to turret 20. In the
position
of Figure 3, product may flow from subsea wells through risers 19 and conduits
26
to suitable storage compartments in vessel 10.
After loading of vessel 10 with product for transport to another location (or
for
a FPSO which must be moved because of an approaching storm or ice flow), quick
disconnects 32, 40 of product conduits 26 are disconnected from risers 19.
Next,
buoy anchor chain 44 and retrieval line 60 along with weight 58 are lowered by
winch
62 until a stopper 64 on chain 44 contacts the upper flared end of pipe 42.
Weight
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58, along with buoy 36, then moves downwardly by gravity in the sea until
weight 58
contacts sea bed 14. The length of chain 44 is predetermined in order to
position
riser buoy 34 at a predetermined submerged location as shown in Figure 7.
Retrieval
line 60 is connected between buoy 36 and marker buoy 66. If desired, a second
retrieval line may be connected to a second marker buoy 66 to provide
redundancy
in the event of failure of a marker buoy or retrieval line. Retrieval tine 60
is normally
releasably connected to marker buoy 66 by a workman in a small boat.
When the same or another vessel returns to the vicinity of buoy 36, it may be
positioned near marker buoy 66 for retrieving line 60. Retrieval line 60 is
disconnected from marker buoy 66 and passed over suitable pulleys 61 to winch
62
for lifting riser buoy 36 along with the associated supported risers 19 to
within turret
20.
The broken line view of buoy 36 in Figure 7 shows the excursions of riser buoy
36 and marker buoy 66 resulting from wind, waves, and water currents or the
like or
when it is being retrieved by means of winch 62. Riser buoy 36 may be
positioned
at a water depth of about fifty (50) meters below the surtace 12 which permits
riser
buoy 36 to be suspended below tanker traffic and beneath the region of high
wave
loads. Furthermore, direct contact between risers 19 and sea bed 14 may be
minimized by the length of buoy anchor chain 44 depending on the position of
the
manifolds or subsea wells on sea bed 14.
Weight or gravity base 58 is pulled to the underside of riser buoy 36 by
' retrieval line 60 during docking. Thus, weight or gravity base 58 is
protected by turret
20 when riser buoy 36 is in docked position within turret 20 thereby shielding
weight
58 from hydrodynamic loading and vessel motions.
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The riser buoy arrangment of the present invention for a floating storage
vessel provides an arrangement of apparatus for disconnecting a plurality of
risers
19 from turret 20 while keeping the risers 19 grouped together in disconnected
condition. In addition, the riser buoy 36 allows the risers 19 to be suspended
in the
sea above the sea bed yet below tanker traffic and beneath the region of high
wave
loads. Because risers 19 are supported in the sea while in a disconnected
condition,
contact between risers 19 and the sea bed 14 is reduced or prevented.
Riser buoy 40 may be used with storage vessels that do not require anchor
legs secured to the sea bottom for maintaining the position of the vessel.
Such
vessels may be maneuvered by thrusters only without mooring anchor legs.
Referring to Figure 8, an alternative mooring arrangement for the vessel and
turret is illustrated. Turret 20A is mounted in vertical opening 22A of vessel
10A. A
lower bearing assembly 29A engages the hull of vessel 10A. A chain support
assembly generally indicated at 31A secures the upper ends of mooring or
anchor
legs 17A about a pivot 33A. A release link 35A is provided for each anchor leg
or
chain 17A. Risers 19A are secured to riser buoy 36A as in the embodiment of
Figures 1-7. The sea water level 12A in moon pool 20A is below the top surface
of
riser buoy 36A when in the docked position.
While preferred embodiments of the present invention have been illustrated
in detail, it is apparent that modifccations and adaptions of the preferred
embodiments will occur to those skilled in the art. However, it is to be
expressly
understood that such modifications and adaptations are within the spirit and
scope
of the present invention as set forth in the following claims.
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