Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DETACHABLE MOORING AND CARGO TRANSFER SYSTEM
BACKGROUND OF T E INVENTION
Hydrocarbons can be transferred from an undersea
pipeline, such as one which l~ads to undersea wells, to
tankers at the sea surface, by a variety of types of
offshore terminals. Under severe environmental
conditions, such as where there are intrusions of lce
packs as well as severe storms, it is desirable to
enable a tanker to disconnect from the terminal to sail
away, while much of the terminal sinks a considerable
depth below the sea surface to lie free of much of the
severe environmental conditions. One ty~e of offshore
terminal which can be used under these conditions
includes a riser having a lower end that is loosely
anchored to the sea floor, as through a group of
catenary chains. The chains hold the riser deep under
water but above the sea floor, while enabling the riser
to be easily lifted up to the vessel. Improvements in
such a system which facilitated connection of the top
of the riser to the vessel and which provided improved
mooring at minimal cost, would be of considerable
value.
SUMMARY OF THE INVENTION
In accordance with one embodiment of the present
invention, an offshore terminal is provided,of a type
which includes a riser that is loosely anchored as by
chains, which can facilitate the dynamics of
anchoring. In a detacheable riser, the riser can be
lifted by a line that extends through a central hole in
the connector frame, to lift the upper end of the riser
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into the central hole. The lifting can continue until a
portion of the riser with at least one
sidewardly-facing hole therein lies at the same level
as the holes in the inner portion of a fluid swivel or
S PDU (product distribution unit~ that is held to the
vessel. The rlser can extend most of the height of the
sea and have a lower end that is weighted, to provide
better mooring dynamics. The weight can include a
clump weight hanging from the lower end of the rlser.
The novel features of the invention are set
forth with particularity in the appended claims. The
invention will be best understood from the following
description when read in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure l is a perspective view of an offshore
terminal constructed in accordance with the present
invention, shown with the riser connected to the
vessel.
Figure 2 is a side elevation view of the
terminal of Figure l, showing various steps in the
raising of the riser from a stowed position to a
connected position.
Figure 3 is a perspective view of a mooring boom
of the system of Figure 1, shown in a deployed
position.
Figure 4 is a perspective view of a mooring
riser connector frame assembly of the system of Figure
1, shown with the riser in the connected position.
Figure S is a sectional view of a portion of the
connector frame and riser of Figure 4.
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DESCRIPTION OF THE PREFERRED EMBODIM~NT
Figure 1 illustrates an offshore terminal system
10 for mooring a vessel 12 such as a tanker and for
transferring cargo such as hydrocarbons between a
pipeline 14 at the seafloor and the vessel. The system
includes a riser 16 which has a base or lower end 18
anchored by a group of catenary chains 20 that extend
in catenary curves in different compass, or horlzontal,
directions to the sea floor and along the sea floor to
anchor locations. The upper end 20 of the riser
includes a riser head 22 which is connected through a
mooring riser connector frame 24 and a mooring boom 26
to the vessel. Hydrocarbons pass from the pipeline 14
up to the vessel through a conduit 30 which includes a
submarine hose 32 held off the sea bottom by a buoy 34,
a solid pipe 36 that extends through the riser, a fluid
swive] unit 38 at the top of the riser and a jumper
hose 40 that connects to piping leading to the vessel.
The system is designed for use in regions that
are sometimes subjec~ed to severe environmental
conditions, such as where there are icepacks and/or
severe storms that would endanger a vessel. In the
event of such conditions, the riser 16 can be
disconnected from the vessel 12 and the vessel can be
sailed away. The riser then sinks to the orientation
shown in Figure 2 at 16a, wherein the upper end 20 of
the riser lies at a depth D which is sufficient to
isolate it from the effects of the severe weather
conditions and also so that it lies under the deepest
draft of a tanker hull 12 that is designed to move to
t:he terminal. To avoid damage to the submarine hose and
the riser, the bottom 18 of the riser is maintained in
positlon above the sea floor by the riser buoyancy, the
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weight of portions of the catenary chains, and by a
weight 44 that hangs straight down from the bottom of
the riser and which is supported on the sea floor. A
buoyancy chamber 46 near the top of the riser provides
sufficient buoyancy to support the welght of the riser
and the suspended portion of the catenary chains, and
maintains the riser in a substantially vertical
position.
When a vessel 12 sails to the region of the
terminal, an in-haul line 48 that is attached to the
top of the riser is picked up by the vessel, threaded
- through the connector frame 24, and pulled by a winch
50 on the vessel. As the riser is pulled, it aligns
itself with the connector frame 24 while the frame 24
aligns itself with the riser as at 16b. The riser is
finally pulled up to the position shown in phantom
lines at 16. As shown in Figure 4, the mooring riser
connector frame 24 includes a lower end 54, an u~per
end 56, and a pair of legs 58, 60 that connect them,
with portions of the leg 60 cut away in Figure 4. The
bottom 54 of the frame has a central or riser-receiving
hole 62 which receives the upper end 22 of the riser,
to transmit large forces through the riser for mooring
of the vessel and to provide a fluid coupling between a
pipe extending within the riser and the fluid swivel or
product distribution unit 38. The extreme upper end of
the riser carries an in-haul line attaching means in
the form of a shackel 64. A thimble 66 at an end of the
in-haul line 48 extends from the shackel and around a
sheave 68 (and additional sheaves) to a mooring winch
(shown at 50 in Figures l and 2). The uppPr end 56
(Figure 4) of the connector frame is coupled through a
universal joint 72 to an outer end 74 of the boom. The
universal joint permits pivoting of the frame relative
to the boom about two largely horizontal axes 76, 78.
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When the riser is being pulled upward, the line
48 extends through a central hole guide 90 and a
sleeve 80 (Fig. 4) within the central hole 62 of the
connector frame. The central hole guide 90 is closer to
S the lower end of the frame 24 than its upper end where
it connects to the universal joint 72, which tends to
align the largely vertical axis of the central hole 62
with the length of the riser. ~owever, since the
tensioned line extends around the sheave 68 (so that
the line can extend largely horizontally therefrom to
the winch on the vessel), there is considerable force
on the line against a forward upward location 82 on the
periphery of the sheave. This force produces a torque
that tends to pivot the connector frame to extend
lS vertically rather than at an angle to align itself with
the riser that is being pulled up. However, the sheave
is mounted much closer to the top of the frame 24 near
the universal joint 72, than the bottom, so the
misaligning torque is minimal. The location 82 is
spaced from the joint 72 by less than one-fourth the
height of the frame.
Figure 5 shows details of the coupling of the
upper end of the riser, at the riser head 22 thereof,
to apparatus at the lower end 54 of the connector
frame. As the riser is pulled up into the connector
frame, it is guided by the concave and largely
cone-shaped central hole guide 90 having a
progressively greater width at progressively lower
locations thereon.The guide 90 moves the riser into a
riser-receiving hole 91 in the sleeve 80 of the
connector frame assembly, which is aligned with the
central hole 62 of the frame. The sleeve 80 is
rotatably mounted about a largely vertical axis 92 that
passes through the central hole of the frame, by a
bearing 94. When the head 22 at the top of the riser
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is fully received within the connector frame, latch
dogs 96 are hydraulically actuated to fit into a latch
groove 98 in the riser to lock the riser in the sleeve
and therefore in a fixed position within the connector
frame. While the bearing 94 must withstand
considerable vertical forces to support much of the
riser weigilt, it does not have to withstand
conslderable torque tending to turn the riser about
horizontal axes, since the connector frame can follow
such turning of the riser because of its mounting by a
universal joint on the boom.
The riser head has fluid ports 99 that are
connected to the pipe 36 within the riser through which
hydrocarbons flow. The sleeve 80 and a ring form the
nonrotatable inner portion of the fluid swivel unit or
PDU (product distribution unit) 38 through which the
hydrocarbons pass in moving~to the tanker. The fluid
swivel also includes a rotatable outer portion 104. The
term nonrotatable for the inner swivel portion at lO0
is used merely to indicate that it does not turn
without limit about a vertical axis, although it can
turn to some extent as the riser twists; the outer
portion 104 is rotatable without llmit as the vessel
drifts about the terminal. Ports or holes 106 and 108
in the inner and outer swivel portions communicate with
the fluid ports 99 in the top of the riser, to enable
the flow of hydrocarbons to the outer portion 104 of
the fluid swivel unit, and from there through a jumper
hose flange llO and the hose (40 in Fig. 1) to the
tanker.
The portion of the sleeve 80 (Fig. 5) which
forms the inner fluid swivel portion 100, is sealed to
the riser by a pair of inflatable seals ll~, 113 with
one seal lying above and the other below the fluid
ports 98 and holes 106. The seals 112 which lie in
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grooves, are initialiy not inflated as the riser is
pulled up, to avoid damage to the seals. Once the
riser is at a level where it-is locked in place by the
dogs 95, pressured fluid such as air from a source 111
is applied to the seals 112 to inflate them, to a
pressure that is higher than the pressure of the
hydrocarbons passing through the fluid swivel. Each
seal includes a hollow ring-shaped inflatable member
that extends around the riser.
In some situations, where it is desired to
protect the cylindrical upper region 115 of the riser
head 22, where the fluid ports 99 are located, a
protective sleeve 116 can be provided. The protective
sleeve includes a ring 118 of foam which makes the
sleeve highly buoyant. When the top of the riser lie~
under water, the buoyancy of the sleeve urges it
upwardly until a lower guide ring and stop 120 prevents
further upward movement of the sleeve and the sleeve
covers the fluid ports 99. As the riser is pulled out
of the water during its deployment to a use condition,
the sleeve 116 slides down along the riser head to
scrape away barnacles or other deposits on the head,
while also uncovering the top of the head. The guide
cone 90 will force the sleeve down if it has not
already slid down.
The mooring boom 26 shown in Figure 3 is
pivotally mounted about a substantially horizontal axis
124 on a turntable 126. The turntable 126 is rotatably
mounted by a bearing assembly 128 about a substantially
vertical axis 130 on the forecastle deck 132 of the
tanker. Lifting actuators or rams 134 can lift and
lower an outer end 136 of the boom 26 whose inner end
138 is mounted through the turntable to the vessel. In
the deployed or use position shown in Figure 3, wherein
3s the outer end 136 of the boom lies outboard of the bow
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end of the vessel to hold the connector frame assembly
24 beyond the hull of the vessel, the boom is
preferably supported by a forward support structure
140. When the riser is disconnected from the vessel so
the vessel can sail away, the rams 134 are operated to
lift the boom so that it lifts the connector frame
assembly 24 above the level of the deck. A motor 139
rotates the turntable and boom by about 180, so that
the outer end of the boom and the connector frame lie
inboard of the vessel. The rams 134 are then operated
to lower the boom, to lower the connector frame into a
locker 142 (Figure 1) so the boom and connector frame
are stably supported. The boom is then in the position
shown at 26A. In both the use and stowed positions of
the boom, it extends (an imaginary line between its
ends extends) primarily along the centerline 143 of the
vessel, as seen in a plan view. of course, the boom
could be rigidly fixed in position.
During the hauling up of the riser, as shown in
Figure 2, from the position 16b to the position 16,
there can be considerable friction of the in-haul line
48 on the connector frame 24, because the connector
frame 24 tends to hang downwardly and only tension in
the line 48 pivots the connector frame into alignment
with the riser. Such tension in the line can be reduced
by the use of a counterweight indicated at 144 that is
fixed to the frame 24, so that a minimal torque is
required to pivot the frame into alignment with the
The length of the riser 14 is preferably at
least about 1/2 the depth of the sea, and the bottom of
the riser is heavily weighted especially by the hanging
weight 44, to provide good dynamic mooring of a
drifting vessel. When the vessel drifts as to the
position shown in Figure 2, the riser tilts and its
lower end moves in the direction of vessel drift. The
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tilting riser acts like a long pendulum with a heavy
weight at its bottom, which tends to pivot back towards
the vertical. If the riser has a small height whlch is
much less than half the height of the sea, then only
S the chains 20 will serve to urge the vessel back. This
would require very heavy chains. The clump weight 44
hanging from the lower end of the column, combined with
a long column at least about 1/2 the sea depth, enables
light weight chains 20 to be used. The weight 44 is
suspended by a hanger 45 which may be a chain to
permit the weight 44 to swing relative to the bottom of
the column, and also to permit some collapse of the
hanger 45 when the column at 16a in Figure 2 moves low
enough that the hanger 45 is collapsed slightly (i.e. a
chain hanger would be limp).
one offshore terminal design is for the
production of hydrocarbons in a region threatened by
ice flows in the winter and also by severe storm
conditions, where the depth of the sea is about 25~
feet. The riser is designed for use, under the worse
conditions, with a 120,000 ton dead weight tanker that
is fully laden. The riser has a length of 141 feet, a
buoyancy chamber length of 39 feet and diameter of 8
feet, and a riser diameter of 4 feet.
With the riser free to sink, its bottom would
float 40 feet above the sea floor. When subjected to
maximum operational loading, the tanker would move 135
feet from a neutral position, and the riser would then
be inclined at 35 from the vertical. The
hydrocarbons would be expected to reach a pressure of
225 psi at the top of the riser, and the inflatable
seals 112 were to be inflated to approximately 350 psi.
Thus, the invention provides an offshore
terminal of the type which includes a riser havin~ a
lower end loosely anchored to the sea floor, which
provides efficient mooring and which enables connection
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and disconnection from a vessel. A weight hung from
the bottom of a tall riser, provides a pendulum effect
in mooring a vessel and limits the submersion depth of
a disconnected riser. Minimum modification to the
vessel is required because connection is made through a
connector frame held outboard of an end of the vessel
by a boom, so that no major modification of the vessel
hull is required. When not moored, the boom can be
swung about 180, so its outer end and the connector
frame attached thereto lie well within the vessel and
can be stored thereat. The terminal, which includes a
riser whose lower end is loosely anchored to the sea
floor so that it can be raised from a deep underwater
depth to a position where its upper end is above sea
level, has at least one flow port near its upper end
through which oil can flow into a fluid swivel held at
the connector frame. The connector frame includes a
fluid swivel with an inner end portion that receives
the perforated portion of the riser and which can turn
with it.
Although particular embodiments of the invention
have been described and illustrated herein, it is
recognized that modifications and variations may
readily occur to those skilled in the art and
consequently, it is intended that the claims be
interpreted to cover such modifications and
equivalents.
