Note: Descriptions are shown in the official language in which they were submitted.
WO91/18182 PCT/GB91/00747
PROD~CTION SYSTE~S FOR UNDERWATER WELLS 2 0 8 2 6 5 2
This invention relates to production systems for
underwater wells, particularly (but not ~xclusively) oil or
gas wells.
It is known to drill a number of subsea wells using a
; drilling template to provide a subsea satellite production
system, the oil or gas produced being fed to a platform
terminal or shore terminal.
.
Such groups of satellite wells, which are serviced
individually, have drawbacks in that there is considerable
duplication of flowlines and umbilical lines between the
wells and the terminal.
Accordlng to the present invention there is provided a
production system for extracting material from an underwater
deposit using a caisson, characterised in that there are a
plurality of Christmas tree assemblies which can be
accommodated simultaneously in the caisson, a drilling
tempiate for use within the caisson, firstly in the drilling
of wells and subsequently for supporting the Christmas tree
assemblies, manifold means for connecting each Christmas
tree assembly to at least one flowline, which serves all of
the Christmas tree assemblies, and distribution means for
connecting each Christmas tree to a power and control
umbilical which serves all of the Christmas tree assemblies.
The manifold means preferably comprise connections to a
production flowline and a test flowline; and the
distribution means preferably comprise connections to
electrical and hydraulic lines in the umbilical.
The umbilical will normally include chemical injection
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2 ~ 8 ~ 2 - PCT/GB91/007~7
hoses for the transmission of chemicals (for example
methanol), in which case the drilling template will also
support a chemical injection manifold for distributing the
chemicals to the Christmas tree assemblies.
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The manifold means, distribution means and chemical
injection manifold are preferably modularised units which
may be fitted to the template before or after it is located
within the caisson. Each unit preferably has its own
lock-down system to facilitate its installation or removal.
The Christmas tree assemblies are preferably four in
number. Whatever the number, preferably they are
distributed around a vertical axis of the caisson, on which
axis the manifold means lies.
To provide physical protection and minimise the ingress
of sand and silt the caisson may have a lid which is
preferably slightly domed, so as to minimise scouring effect
to optimise hydrodynamic loading, and preferably has a
: removable cover above each Christmas tree and the manifold
means to allow access by a diver.
' .
Although the invention may be carried out in a variety
of different ways one particular embodiment thereof will now
be described, by way of example, with reference to the
accompanying drawings in which
Figures I and IA are perspective views of a subsea
caisson production system according to the invention, Figure
IB being an expioded, cutaway and enlarced view of the
caisson assembly;
Figure 2 is a vertical section through the installed
caisson,
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' WO9t/18182 PCTIGB91/00747
2~2~52
Figure 3 is a vertical section through the caisson
showing its construction in more detail;
Figure 4 is a plan corresponding to Fig. 3 of the
cai3son;
Figure 5 is a plan of the caisson lid;
Figure 6 is a plan of a drilling template;
Figure 7 is an exploded side elevation of the
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components housed in the caisson;
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Figure 8 is an exploded plan view, partly in section,
'~ of the caisson components:
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Figure 9 is an elevation of a Christmas tree installed
in the caisson;
~, Figure IO is a plan of manifold piping, and Figs. IOA
and IOB are corresponding elevations;
Figure 11 is a diagram showing the general arrangement
of the production and control systems: and ~-
Figure 12 is a plan of the caisson with lid removed
;; showing the caisson control system in more detail, Figure
12A being a detail of an umbilical dist-ibution unit and
'adjacent caisson wall. ~ -
As shown in Figs. 1 to 4 a subsea caisson produc-ion
system comprises a caisson 10 supported in an oversized
sloping-sided excavation in the seabed 12 on a stabilised
pre-engineered concrete foundation layer 14 through which
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WO 91/18182 - 4 - PCI'/GB91/00747
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project four conductors l6, and a central conductor l7 (see
Fig. IB) these being surrounded by a protective rock dump
18. The caisson has a floor formed by a drilling template
22 on which are mounted four Christmas tree assemblies
distributed around a central vertical axis ~not shown) of
the caisson and a central manifold module 26 which lies on
the axis and from which a production flowline 28 and a test
flowline 30 run to an on-shore plant 32 via a shore crossing
34 or to a platform. An umbilical 36 which comprises
electric, hydraulic and chemical lines also runs to the
plant 32 in an excavated trench. Pig receivers 3l, a
splitter box 33 and tie-in spools 35 are also shown.
The foundation layer l4 is laid on a fine rock dump 40
at the bot,om of the excavation ll. The excavated area
surrounding the sides of the caisson lO is filled with
selected fine rock dump material 42 covered by a top layer
of coarse material 44 which provides a stabilised gently
sloping surround extending from the top of the caisson lO to
the existing seabed 12 some 2m above.
While the whole of the caisson lO and its lid 20 lie in
a recess in the seabed in the illustrated installation, it
is possible for part of the assembly of caisson and lid to
project out of the recess so that the top of this assembly
is at a higher level than that of the seabed.
The caisson lO has a l2-sided wall 46 comprising twelve
double skin wall sections 48 formed of welded plates 50 and
interconnected by vertical tubular members 52, the sections
48 may be flooded with sea water or illed with other
material when the caisson lO has been lowered into position.
If desired the c~isson lO may be supplied with levelling
mechanisms (not shown).
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WO91/18182 2 ~ 8 2 ~ ~ 2 PCT/GB91/00747
A horizontal internal ring 54 is positioned near the
lower edge of the caisson wall 46 to support the drilling
template 22 and is fitted with a series of drilling template
installation guides 56 and drilling template lockdown
brackets 58. At its upper edge the wall 46 is capped by a
box-section top chord 60 surmounted by a bumper frame 62
which is interrupted for a set of four lifting eyes 64, and
of flowline clamp support inserts 66.
The caisson 10 is fitted with a removable lid 20 (Fig.
5) which comprises a tubular framework 68 covered by a
serrated bar grating 70, and having five removable hatches
72 to provide access to the Christmas tree assemblies 24 and
manifold module 25. Four lifting eyes 74 for connection to
a sling 76 (Fig. 7) are also provided.
The drilling template 22 (see Fig. 6) comprises a
tubular framework 78 including two intersecting parallel
pairs of tubular members 80, each terminating in a buffer 82
which on lowering of the template 22 engages the respec.ive
guides 56 and is then secured by lock down brackets 58 when
it has been levelled (to within 5 cm), which may be
accomplished independently of the caisson 10 which is also
alignable. Four frusto-conical, gimball-type guide funnels
84 are peripherally disposed and provide a facility to land
drilling guide bases 86 (Fig. 7) which, after the drilling
of four wells, support the Christmas tree assemblies 24. A
similar guide funnel 84 is provided centrally to receive a
similar guide base 86 for the manifold 26. Between each
ajdjacent pair of guide funnels 84 there is positioned a pair
of umbilical distribution socket inserts ao incorporating
port sockets which provide a choice of locations for
mounting an umbilical distribution unit 92 (See Figs. Il and
12). Two sets of lifting eyes 94 and 96 are welded to the
frame-work 78.
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WO 91/18182 PCI/GB91/00747
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Figure 9 shows one of the Christmas tree assemblies 24
located in the drilling template 22 by means of the drilling
guide base 86 having upstanding guide posts 98 over which
guide funnels ]00 of a guide frame 102 engage. A tree cap
]04 fits over the top of the frame 102 (see Figs. 7 and 8).
The Christmas tree itself (106) has a minimum of four
fail-safe barriers, which are all arranged to be
automatically shut in the case of alarm or emergency, and
leak detection, system monitoring and data acquisition
facilities, and, as its construction is generally
conventional, it will not be described in detail. Exploded
views of the Christmas tree assembly 24 and a tubing hanger
108 are shown in Figs. 7 and 8.
Centrally disposed on the template 22 is the manifold
module 26 shown in detail in Fig. 10, and generally in the
exploded diagrams of Figs. 7 and 8. The manifold module 26
is mounted on the central guide base 86 by means of a
framework ]10, including four guide funnels ~]2, and is
surmounted by a protective cap 1]4. The functional
components of the module include a pair of tree tie-in
spools 116 for each well which are all connected to a
production flowline ~20 and a test flowline 118 which
communicate with the flowlines 28 and 30, respectively, a
pigging loop 122; and valve actuators 124. Besides allowing
test flow, production flow and well-killing, the manifold
module also includes piping headers which allow a chemical
treatment facility such as methanol dosing to inhibit
hydrate formation, pour-point depressant to prevent wax
formation, corrosion inhibitor dosing and intermittent scale
inhibitor. The chemicals are fed along the umbilical 36 to
the umbilical distribution unit 92 and reach a chemical
injection manifold 125 on the central manifold module 26 via
a chemical injection jumper 126 (Fig. Il). From the
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WO91/18182 2 a ~ 2 PCT/GBg1/00747
manifold 125 the chemicals pass to the Christmas tree
assemblies 24.
The umbilical distribution unit 92 is located on a
desired one of the umbilical distribution socket inserts 90,
being mounted on two guide posts 127 which are engaged by
guide funnels 128 flanking the unit 92. The unit 92 is
connected to the umbilical 36 by a jumper 130, the
connection being made through a pullhead 132 received within
a pulling cone 134 mounted on an insert 90. Hydraulic and
electric jumpers 136 lead to a control pod 138 located by
each Christmas tree assembly 24 to provide power and control
therefor; and further jumpers 140 lead from each pod 138 to
the central manifold module 26 to supply the valve actuators
124.
The general arrangement of the control system is shown
in block form in ~ig. 11 and will not be described in
detail.
It will be noted that the test flowline 118 and the
production flowline 120 are connected by the pigging loop
122 so that "round trip" commissioning may be undertaken by
passing the appropriate pig train through the lines. The
manifold module 26 allows the flow of production
hydrocarbons to either the production flowline 120 or, on an
individual basis, to the test flowline 118 for well-testing
purposes. In normal operation, the oil or gas from two
wells passes down the production flowline 120 and that from
the other two wells passes down the test flowline 118.
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The two unused umbilical dist-ibution socket inserts 90
may be used for further umbilicals and/or flowlines, for
~xample if it is subsequently desirable to use gas lift or
water injec~ion techniques.
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