Note: Descriptions are shown in the official language in which they were submitted.
CA 02220092 1997-11-03
WO 96!36529 PCT/N09i6/0~IIS
1
A method of loading and treatment
of hydrocarbons
The present invention relates to a method of: loading
and treatment of a gaseous or liquid hydrocarbon mixture produced
on a:n offshore production platform, a production vessel o=~ a well
installation when producing oil and gas from a reservoir, wherein
the mixture is supplied to a gas treatment vessel via a buoy
loading system comprising a buoy of the STL/STP type, and a.s
o treated on board the vessel for producing liquefied nai~ural gas
(LNG) or an LPG mixture stored in tanks on the vessel.
In offshore production of hydrocarbons ( oil and gas ) it
is known to use production vessels based on the so-ca,la_ed STP
technique (STP = Submerged Turret Production). In this technique
there is used a submerged buoy of the type comprising a central
bottom-anchored member communicating with the topical underground
source via at least one flexible riser, and which is provided
with a swivel unit for the transfer of fluid under a high
pressure to a production plant on the vessel. On the ceni~ral buoy
ao member there is rotatably mounted an outer buoyancy member which
a.s arranged for introduction and releasable secureme:nt in a
submerged, downwardly open receiving space at the bottom of the
vessel, so that the vessel is able to turn on the anchored
central buoy member under the influence of wind, waves and water
as currents. For a further description of this technique rej:erence
may ~s.g. be made to Norwegian laying-open print No. 17E~ 5_29 and
to international patent application No. PCT/N094/00119,.
In offshore loading and unloading of hydrocarbons it is
also known to use a so-called STL buoy (STL = Submerged Turret
3o Loading) which is based on the same principle as the STP buoy,
but 'which has a simpler swivel means than the STP swivel which
normally has several through-going passages or coursers . For a
further description of this buoy structure reference may e.g. be
made to international patent application No. PCT/N092/00056.
3s By means of the STL/STP technique there is achieved
that one is able to carry out loading/unloading as well as
offshore production of hydrocarbons in nearly all weathers, a
connection as well as a disconnection between vessel arid buoy
beinl~ able to be carried out in a simple and quick manner, also
CA 02220092 2007-O1-24
under very difficult weather conditions with high waves . Further,
the buoy may remain connected to the vessel in all weathers, a
quick disconnection being able to be carried out if a weather
limitation should be~ exceeded.
s ~ The ob j ect of the invention. is to provide a flexible
system .for simultaneous loading of oil and gas ; via an STL/STP
buoy to one or more vessels. ~. . .. ,,
For the achievement of the above-mentioned ob j ect there
is provided a method of the introductorily stated type which,
o according to the invention, is characterized in that, simultane
ously with the supply of the hydrocarboin mixture, oil as also .
supplied to the gas treatment vessel via said buoy, the buoy
including an STP connector having pipe courses for the respective
fluids, the oil being transferred directly from the STP 'connector
via a pipeline and an unloading means on the vessel to a tanker
f or storage and transport of the supplied oil .. :. . . . ~ .
Hy :means of the method according to the invention there .
. is obtained, a very flexible , system . for simultaneous loadz ng of
~oil~and gas via a loading buoy to one or'more vessels.. ,F.urther,
Zo there .is achieved that one can load oil and simultaneousl~r can
harvest. LPG (Liquefied Petroleum Gas) and/or. gas which would.
other~rise be reinj ected into ~ the reservoir... . .
According to a first broad aspect of an embodiment of the
present invention, there is disclosed a method of loading and
25Ytreatment of a gaseous or liquid hydrocarbon mixture produced on
an offshore production platform, a production vessel or a well
installation when producing oil and gas from a reservoir,
wherein the mixture is supplied to a gas treatment vessel via a
buoy loading system comprising a buoy of the STZ/STP type., and
30~i5 treated on board the gas treatment vessel for producing
liquefied natural gas (ZNG) or an ZPG mixture stored in tanks on
the gas treatment vessel, wherein
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WO 96136529 ~ PCTlN096/OOI18
2a
simultaneously with the supply of the hydrocarbon mixture, oil
is also supplied to the gas treatment vessel via .said buoy, the
buoy including an STP connector having pipe courses for the
respective fluids, the oil being transferred directly from STP
connector via a pipeline and an unloading means on the gas
treatment vessel to a tanker for storage and transport of the
supplied oil.
The invention~will be :further described below .in
connection with exemplary embodiments with reference to the
z5~accompanying drawings,-wherein~
Fig. 1 shows a'schematic view of an offshore installa-
tion 'and a vessel for..the supply of gas and/or~an LPG mixture;
Fig . Z .shows a schemattic view o.f interconnected vessels .
for carrying out the method according to the~inventiom;..
ao Fig. 3 shows- a longitudinally sectioned view of an STP
connector for use in ~simu~taneous loading of oil and LPG mixture;
and
Fig_ 4 shows a longitudinally sectioned view of a
modified STP connector for loading of oil only...
35 Fig. 1 shows- a - production platform 1, ~ a conventional
floating loading buoy 2 and . a vessel '3 which is anchored ~to ' a
submerged,_ bottom-anchored buoy 4 (the anchoring system is not
shown ) of the introductorily mentioned STL or STP type, the buoy
4 being introduced and secured in a submerged receiving space at
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WO 96136529 PCT/N0961001 Z8
3
the bottom of the vessel 3. A number of risers 5 for transport of
hydrocarbons .from the production platform 1 extend between the
base 6 of the platform and the buoy 4. Similar risers 7 and 8,
respectively, extend between the platform base and the loading
buoy Z .and a production vessel 9, respectively: Further, there
is shown a production well 10 which communicates with a reservoir
(not shown) and which is connected to the platform 1 'through
fbowlines 11.
Previously, it has been customary to reinject LPG
and/or gas, so that the value of this hydrocarbon fraction has
not been utilized.. However, with the system shown in Fig . 1, this
hydrocarbon fraction may be utilized, in that the risers 5
constitute pipe courses for gas and/or LPG of this type, so that
the ,gas or the LPG mixture is supplied to the vessel 3 via the
is buoy 4, the buoy then cooperating with a suitable STP connector»
The vessel 3 may be an hPG or an LNG vessel, for treatment of the
topical gas or LPG mixture. Normally, it will be necessary to
return some of the gas, and this is done via one of the risers
zo ,Fig» .2 shows a system comprising interconnected vessels
for use in carrying out the method according to the invention.
As shown, the system comprises a first vessel 12 which
in this case is presupposed to be an LPG production vessel, and
a second vessel 13 in the form of an oil tanker. The vessel 12
25 is anchored town STP buoy 14 which is secured in a submerged
receiving space 15 at the bottom of the vessel and which is
connected to an anchoring or mooring system comprising mooring
lines 16 connected to chain sections 17 at the sea bed 1B.
Buoyancy elements 19 are attached. to the mooring .lines 16 to
so facilitate the mooring.. In practice the ocean depth may be
several thousand meters with such a system..
A number of risers 20 extend between the sea bed 18 and
the STP buoy 14, the risers at the bottom being connected to
respective fastening or base members 21. In this case the risers
35 comprise a riser 20a for transport of oil, a riser 20b for
transport of LPG or gas, and a riser 20c for return of gas. At
their upper ends the risers are connected to respective pipe
courses in the bottom-anchored central member ( not further shown )
of the buoy 14, which cooperates with an STP connector
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WO 96!36529 PCT/N096100II8
4
(also called rotating connector) which is adapted for. transfer
of the topical fluids to or from the production vessel 12 (see
Fig. 3).. This is shown.to comprise a number of tanks 22 for
storage of the topical product, i.e. LPG mixture in the present
CdSe. ~ ~ ..
Hydrocarbons in liquid or gaseous condition are
supplied to the risers from platforms, production vessels,
production wells or other suitable installations,;for example as
shown in Fig. 1. w
As mentioned, 'the vessel 12 in this case is a produc-
tion vessel for LPG mixture, and therefore it has no capacity for
storage of oil which 3s supplied through the risers 20 simultane-
ously with the hydrocarbon mixture., The supplied oil therefore
is transferred directly from the STP connector via a pipeline 23
15 which is shown to extend along the deck.24 of the vessel, to.an
unloading means 25.: Between the unloading means 2S and the second
vessel, i.e.. the oil tanker 13, there is arranged a pipeline 26,
and the oil as transported through this.pipeline to tanks 27 on
aboard the oil tanker 13. The oil tanker 13 is moored _to the
zo .production vessel 12 , by means of .a mooring line '28: In this
manner it is possible to load or supply oil and gas/LPG to two
different vessels vis one and the same STL/STP buoy 'i4.
In practice it takes a relatively short time, less than
24 hours, to fill an oil tanker, whereas it takes a substantially
25 longer time, several weeks up to months, to fill the tanks on a
vessel processing gas or an LPG mixture from hydrocarbon=carrying
formations.. .
When the production vessel 12 is ready for unloading of
the processed gas, the vessel is disconzaected from the loading
buoy 14 in order to go to the unloading site. The other vessel
13 may then use the buoy 14, this vessel also being presupposed
to be provided faith a submerged receiving space (not shown) for
this purpose. As mentioned, the vessel 1~ is an oil tanker, and
therefore has no possibility for treatment of the gas from the
topical reservoir.. In order to be able to utilize the system for
oil loading in this situation, there is used an insert member or
adapter for modification in connection with the buoy 14, so that
its pipe courses for gas transport are shut off, and so that the
oil-carrying pipe courses of the buoy are used in accordance with
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" WO 96136529 PCT/N096l00118
the conventional STL concept. This modification will be further
described with reference to Fig. 4.
Fig. 3 shows an axial section of a rotating connector'
device (STP connector) 31 of the type disclosed in the aforemen
s tinned international application No. PCT/N094/00119, and to which
reference -is here made for a further description thereof.. Briefly
stated, such a connector device includes a swivel device having
a number of fluid courses for interconnection between a buoy of
the above-mentioned type and a pipe system on the topical vessel,
to wherein the swivel device comprises a female member and a male
member which can be inserted axially into or withdrawn from each
other, the female member being permanently fastened to the ..
bottom;anchored central member of the buoy.- .
In Fig. 3 there is suggested a buoy 30 corresponding to
5 the buoy l4 and which is presupposed to be .introduced into and
secured in a receiving space in a vessel, e.g. the vessel 12.. The
rotating connector 31 includes a female member 32 which is
permanently fitted in the upper end of the central member 33 of
the buoy 30. A male member 34 is introduced into the female
zo member, the male member being raisable and .lowerable :by means of
a hydraulic hack 35 forming part of an operating means 36~ The
rotating connector also comprises a guide sleeve 37 for guiding
of the male member 34. ~ ~.
In the illustrated embodiment the central member 33 of~
zs the buoy comprises three pipe courses for fluid transport, as
described above for the buoy 14. Thus, therelis provided a pipe
course 38 for transfer of oil, a pipe course 39 for transfer of
gas or LPG, and a pipe course 40 f.or return of gas. Additional
pipe courses could be provided according to requirement. The male
so member 34 of the connector device 31 is provided with axially
extending pipe courses 41, 42, 43. The lower ends of these pipe
courses communicate with respective ones of the pipe courses 38,
39, 40 via respective annular spaces 44 arranged between the male
member 34 and the female member 32, whereas the upper ends of the
35 pipe courses communicate with associated pipelines 45, 46,-47 on
the vessel via respective annular spaces 48 arranged between the
male member 34 and the guide sleeve 37. The rotating connector
device 31 thus permits supply of oil and gas/LPG to the vessel,
and return of gas from the vessel, even if the vessel together
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.. WO 96136529 ~ PCT/N096/00l.l8
6
With the outer buoyancy member of the buoy turn about the bottom-
anchored central member of the buoy under the influence of wind,
Waves and water currents.
Fig. 4 shows an axial section of an STP connector 50
which is modified to be used only for oil transfer, i..e. in
accordance with the conventional STL concept, as described above.
In this embodiment an insert member or adapter 51 is inserted
into the female member 32 fitted -in the buoy 30, which adapter
is designed to shut off the pipe courses in the buoy which are
not to be used, ~i.e. the pipe course 39 for gas/LPG and the pipe
course 40 for return of gas. The adapter 51 and the female member
32 define an annular space 52 communicating with the pipe course
38 of the buoy 30 for oil and with a pipe course 53 which extends
axially through the adapter 51 ,and via a convent3.ona1 STL
coupling head 54 communicates with a 'pipe member 55 l.eadirig to
a pipeline for oil transport, e.g.. the pipeline 23 described
above .
In order. for the previous STP connector.. to be com-
patible with conventional STL connectors, the pipe course 53 is
ao arranged centrally in the adapter 51.. The adapter which is
inserted into the 5TP connector, may .be inserted either by the
gas treatment vessel 12 or the oil tanker 13. In order to install
the adapter 51, the male member 34 of the STP connector firstly
must be pulled up from the female member 32 and stowed away.
25 As soon as the gas treatment vessel 22 is back on the field,
it may connect itself to the loading buoy 30. The adapter 51 then
firstly must be withdrawn from the female member 32 in the buoy 30,
and the male member 34 of the original STP coupling, which opens
the pipe courses for transfer of the topical additional fluids,
3p must be, inserted into the female member..
