Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The invention relates to a system for trans~erring
hydrocarbons produced from an offshore oil- or gas field
to a tanker~ and more in particular to transferring crude
oil to a shuttle tanker or to a storage tanker by means
of a single point mooring device provided with a separator,
; wherein gas present in the crude oil is separated from the
crude oil and the separated oil is loaded into the tanker.
A problem with such a system is the disposal of gases
either continuously or in emergency cases only. A known
simple way of dealing with the gas is to flare or vent it
by means of a stack mounted on ~he single point mooring
device. However, the gases, the heat, fumes, unburned gases,
and possible liquid particles produced by the stack cause
hazards and hindrance to the tanker moored to the single
point mooring device, since stack and tanker orientation
can coincide depending on wind and current direction. Also
~ helicopter operations can be seriously hampered or become
- even impossible when the wind is directed from the stack
towards the helideck. ~urthermore, if a supply boat is
moored to the device, the same problems as with the tanker
may occur.
In order to deal with this problem, a system for
transferring hydrocarbons produced from an offshore oil-
or ~as field to a tanker, comprises according to the in-
vention a single point mooring device provided with a
; mooring element for mooring a tanker thereto, which
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element is rotatable around a vertical axis, a separator arranged on, orclosè to, the single point mooring device, a conduit extending from the
offshore oil - or gas field to the separator, a conduit extending from the
separator and being adapted to be connected with the tanker moored to the
single point mooring device, a stack for flaring or venting gases, which
stack is mounted on the single point mooring device in such a manner that
it is rotatable around a vertical axis relating to the single point mooring
device, so that the stack can be so positioned that the tanker experiences
the least hindrance from the gases leaving the stack, and gas conduit means
between the separator and the stack.
In a possible embodiment of the invention a stack is mounted on
or coupled to a rotatable memberJ which is rotatable around a vertical axis.
In an attractive embodiment of the invention a stack is mounted
on or coupled to the rotatable mooring element.
The invention will be further explained with reference to the
drawings showing two possible embodiments of the invention.
Figure 1 shows a schematic side view of a first embodiment of a
single point mooring device on its location.
Figure 2 shows a perspective of the same device.
Figure 3 shows the same single point mooring device during towage.
Figure 4 shows a schematic side view of another embodiment of a
~ single point moOTing device on its location.
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Fig. 5 shows a side view IV-IV o' the device as shown
in fig. 4.
Fig. 6 shows a schematic top plan view of the device as
shown in fig. 4 on an enlarged scale.
In the embodiment as shown in figures 1 through 3, which
is destined for use on an offshore oil field, a single point
mooring device 1 comprises a buoyancy element 2 and ballast
elements 4 secured to the buoyancy element 2. By means of
a number of columns 5, the buoyancy element 2 carries a
deck 3 and a superstructure 6. The buoyancy element 2 has
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- an annular shape. The ballast elements 4 are applied in
this embodiment in order to obtain adequate stability of
- the device in an efficient manner.
The deck 3 is provided with a rail 7 which is of
annular shape. A mooring element 18 is so arranged on the
annular rail 7 that it is rotatable around a vertical axis
relative to the superstructure 6. A tanker 8 can be moored
to the mooring element 18 in a suitable manner. In the
embodiment as shown in figures 1 and 2 a mooring hawser 9
is used for mooring the tanker 8 to the mooring element 18.
The single point mooring device 1 is anchored to the sea
- bottom 10 by means of conventional anchor lines 11 and
anchors (not shown).
A number of oil wells in the sea bottom 10, of which
one is shown and is indicated by the reference numeral 12,
-~ is connected to a central manifold 13 by corresponding flow
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llnes 14. The central manifold 13 is connected by a number
of risers 1~ to production equipment 16 accommodated in the
superstructure 6. The production equipment or separator 16
serves amon~st others for separating gas from the crude
oil. The gas separated from the oil can be led through a
gas conduit 20 to a stack 21. The oil separated from the
gas can be led through a conduit 22 to a loading arm 23
which is carried by the mooring element 18. Via the loading
arm 23 the conduit 22 is in communication with a loading
hose 24 which is carried by the loading arm 23 and which is
adapted to be connected to the tanker 8.
The s,ack 21 is mounted on a rotatable member or con-
struction 25. The rotatable member 25 is mounted on the
superstructure 6 in such a manner that it is rotatable
around a vertical axis relative to the superstructure 6.
The stack 21 extends upwardly from the rotatable member
25 and laterally away from the superstructure 6 in order to
keep the vented or flared gases 26 leaving the stack 21
away from the single point mooring device 1 and the tanker 8.
;`~ 20 The use of the single point moorir.g system according
to the invention is as follows. The device 1 is towed to
.~ the desired location by a tug 17 while the device 1 is in
the condition as shown in figure 3. In ~his condition the
ballast elements 4 are retracted and locked in such a
- 25 position that minimum drau~ht is obtained. In said position
the ballast elements 4 extend upwardly above the buoyancy
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element 2 as shown. During towage the buoyancy element 2
is deballasted so that it is floating at the water surface
27 as shown.
Upon arrival at the desired location, the ballast
elements 4 are lowered by allowing them to slide down-
wardly thro~gh vertical openings in the buoyancy element 2.
When the ballast elements 4 have been lowered and have
reached the low position relatively to the buoyancy element
2 as shown in figures 1 and 2, the ballast elements 4 are
locked again to the buoyancy element 2. Then the buoyancy
element 2 is ballasted at least partly with water so that
the single point mooring device 1 reaches the desired
draught as shown in figures 1 and 2. The de~ice is anchored
by the anchor cables 11 and conventional anchors (not
shown). Finally, the risers 15 are installed so that a
fluid communication is created between the central mani-
- fold 13 and the production equipment 16.
During the early production phase of the oil field,
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the oil produced by a number of oil wells flows through
flow lines 1~, central manifold 13 and risers 15 to the
production equipment 16. During this early production phase
a shuttle tanker 8 is moored by the mooring hawser 9 to
the mooring element 18 and the loading hose 24 is con-
nected to the tanker 8. In the production equipment or
separator 16 gas is separated from the oil. The gas is
passed via a pipe swivel (not shown) allowing rotation
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around a vertical axis and via gas conduit 20 to the stack
21. At the top of the stack 21 the gas is vented or it is
burned so that a flare 26 is formed.
.~ The separated oil is passed from the production
; 5 equipment 16 via a pipe swivel (not shown) allowing
rotation around a vertical axis, the conduit 22, the
loading arm 23 and the hose 24 to the tanker 8. Since the
mooring element 18 is rotatable around a vertical axis
relative to the single point mooring device 1, the tanker 8
is free to swing around the mooring device 1 in dependence
of current- and wind forces. To protect the tanker 8 from
the hot combustion gases and the radiation produced by the
flare 25 or from the vented gases, the rotatable member 25
is rotated together with the stack 21, relatively to the
mooring element 18 in such a manner that the stack 21 reaches
- a position in which the tanker 8 experiences the le~t hindrance
- of the gases leaving stack 21 or of the combustion gases and
radiation produced by the flare 26. In that position the
-~ rotatable member 25 can be coupled to the mooring element
18 so that the rotatable member 25 and the mooring element
~ 18 will rotate as one unit.
;- When the oil field has reached such a rate of production
that interrupted production could be undesirable from a
technical and an economical point of view, the export shuttle ~ -
~; 25 tanker 8 is replaced by a storage tanker (not shown) which is
permanently moored to the singe point mooring device 1,
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where~ the storage tankc~r can be moGred to the mooring
element 18 b~r means of a mooring yoke (not shown) which
is permanently arranged between the storage tanker and
the single point mooring device. This mooring yoke carries
a suitable conduit for creating a fluid communication
between the single point mooring device and the storage
tanker. When such a storage tanker is permanently moored
to the single point mooring device, care is taken that
the rotatable member 25 together with the stack 21 is
rotated in such a manner that stack 21 reaches a position
wherein the storage tanker experiences the le~ttrouble
from the gases and/or heat and fumes from the stack 21.
Periodically an export tanker is moored to the storage
tanker to be loaded and to transport the oil to a desired
location.
If desired part of the buoyancy element 2 can be used
for temporarily storing oil or condensates therein.
In the embodiment as shown in figures4 through 6, a
number of equivalent elements are indicated by the same
; 20 reference numerals as in ~igures 1 through 3, viz. the
elements 1, 2, 4, 5, 6, 8, 10, 11, 13, 14, 15, 16 and 27
and in connection with these elements reference is made to
' the description of figures 1 through 3. The single point
mooring device 1 is pro~ided with a mooring element 28
which is mounted on an annu]ar rail 35 in such a manner
- that the mooring element 28 is rotatable around a vertical
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a~is. The annular rail 35 is mounted on a deck 34.
- A tanlcer 8 can be moored to the rotatable mooring
element 28 by way of a fastening device 29 in the form of
a mooring yoke.
The oii separated from the gas in the separator 16
can be led through a conduit 30 to the tanker 8.
The Gas separated from the oil in the separator or
production equipment 16 can be led through a suitable
conduit (not shown) to one of two stacks 31, which are
coupled to the rotatable mooring element 28 by means of
coupling elements 36 and annular coupling elements 37.
:. Valve means (not shown) are present to direct gas from
the ~parator 16 to a selected stack 31. The stacks 31
extend upwardly and laterally away from the superstructure 6.
: ~ 15 The single point mooring device is further provided
with a helideck 33, coupled to the rotatable mooring
~A~ element 28 by means of coupling elements 38 and coupling
elements 36 and 37.
The two stacks 31 are symmetrically arranged relative
~ 20 to a vertical plane A-A passing through the ver'cical
:. rotation axis of the mooring element 28 and through the
middel of fastening device 29. In the embodiment as shown
the two stacks 31 are so arranged relative to the fastening
device 29 and the helideck 33 that the free area necessary
for use of the helideck 33 and the approach to the heli-
deck 33 is sufficient and the gases produced by a stack 31
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are kept away as far as possible from the helideck 33 and
from the tanker S.
In normal operation, one of the stacks 31 is selected
for use, depending on wind- and current conditions and the
actual orientation of the tanker in such a manner that the
tanker 8 experiences the least hindrance from the gases
produced by the selected stack 31.
In the above, embodiments of the invention have been
described when applied to an offshore oil field.
Instead, the system according to the invention can be
used as well on an offshore natural gas field. Then con-
densates are separated from the natural gas and the con-
- densates are loaded into a tanker moored to the single
point mooring detJice. The natural gas is transported to
shore by means of apipeline. In case of an emergency natural
; gas is flared by means of the stacks as described.
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