[0001] This invention relates to a flow line system for use in the transfer of fluid to
or from under water sites.
[0002] In the transfer of fluid to or from an under water site, for example in the extraction
of oil or gas, it is necessary to provide a flow line or lines between the water bed
and production facilities which may be on a fixed, tensioned or moored floating production
platform. Advantageously such flow lines should be easily disconnectable from, and
re-connectable to, the subwater well heads or manifold, particularly in potentially
rough water areas. The problems in providing ease of connection and disconnection
are greater at deep water sites, or where icebergs are a problem and/or if the production
facilities are on a floating production platform. However, whilst disconnection and
connection, when required, should be easy to accomplish, it is also important that
the need to effect such manoeuvres should be minimised.
[0003] In United Kingdom published Patent Application No. 2 024 766 A there is disclosed
a marine production riser system in which a rigid column that extends from a rigid
connection at the water bed to near but below the surface supports a flow line arrangement
that is rigidly supported over the whole length of the column. From the top of the
column the flow line arrangement is flexible as it passes to a vessel floating on
the surface. As the column is rigidly connected to the water bed and is rigid throughout
its length it is unable to absorb anything more than very minor lateral loads and
impacts because such loads of any size would cause excessive stress at the rigid connection
at the water bed. Furthermore, the depth to which such a system can be installed is
limited as an unduly long rigid column is difficult to handle during installation
(and subsequent removal), and if made slim to facilitate such handling would, in situ,
be subjected to excessive deflections and stresses over its length.
[0004] According to the present invention there is provided a flow line system for use in
the transfer of fluid to or from an under water site, the system comprising a first,
flow line arrangement to be installed so as to extend from the water bed to near,
but below, the surface, this arrangement being carried by a buoyant support column
that is for articulated connection to the water bed and that consists of rigid sections
that are connected in series at articulated connections so that the first flow line
arrangement is rigidly supported by - each section but can articulate at the base
of the column and at the connections between the rigid sections; and a second, flexible,
flow line arrangement releasably connectable to the first arrangement, where the first
arrangement terminates near the surface, so as to extend the first arrangement. Such
a system can be installed between a wall head, manifold or other convenient location
at the water bed and production facilities at the surface with the advantage that
the connection between the first and second flow line arrangements is near the surface
and hence readily accessible. The articulated connections at the water bed and between
the rigid sections, with the otherwise rigidly supported first flow line arrangement
able to articulate with the support column at these connections, enables the system
as a whole to cope with extensive movement of floating facilities at the surface so
that the system is suitable for use at considerable depths (extent of surface movement
increases with water depth). The suitability for use at considerable depth is enhanced
by the provision of articulation of the column and the first flow line arrangement
as this gives to the column and the flow line arrangement it supports good ability
to absorb lateral loads and impacts. Also as this is achieved by the provision of
the articulated connections, the column can be made slim which facilitates its handling
during installation and removal.
[0005] For a better understanding of the invention and to show how the same may be carried
into effect, reference will now be made, by way of example, to the accompanying drawing,
in which there is illustrated, in Figure 1, a flow line system between a sea bed well
head and production facilities provided on a semi-submersible production platform.
Figure 2 shows on a larger scale the detail ringed at II in Figure 1.
[0006] Referring to the drawing, the semi-submersible production platform 1 is shown in
position over the sea bed well head 2 with workover guidelines 3 extending between
the platform and the well head. To one side of the platform and well head there is
a sea bed anchorage structure 4 to which is anchored, by an articulated connection
5, a series of buoyed rigid column sections 6. The column sections 6 are connected
to one another in series at articulated connections 5A, and each section incorporates
a buoy 13 at its upper end. The articulated connections can be of any suitable construction
as known in the art giving freedom of movement about two mutually perpendicular axes.
The column made-up by the sections 6 rigidly supports between the connections 5A a
first flow line arrangement 7. One part of this first flow line arrangement 7 is connected
to flow lines 8 from the well head 2. Another part is connected to export flow lines
9. At each connection 5A there is an articulated connection 12 in the flow line arrangement
7 and this arrangement can also articulate at the base of the column. Thus the flow
line arrangement 7 is rigidly supported by the buoyed column sections 6 but can articulate
at the base of the column and at the connections between the column sections. The
articulated connections in the flow line arrangement 7 are effected by flexible flow
line portions 12A interconnecting the otherwise rigid runs of the flow line.
[0007] The column terminates near, but below, the surface of the sea, and at this termination
the first articulated flow line arrangement 7 is releasably connected to a second,
flexible, flow line arrangement 10. This second flow line arrangement 10 extends to
the production platform 1, to which it is releasably connected, and serves for supply
to, and export from, the production facilities.
[0008] The semi-submersible platform 1 can be held on station by a conventional mooring
system or by an automatic jack mooring system, the inclusion of the flexible flow
line arrangement allowing movement of the platform to take place within a predetermined
range. Conveniently the automatic jack mooring system can utilise the top of the column
as a reference point for locating the platform.
[0009] Should weather conditions become such that platform movement cannot be constrained
within the allowable range, or should it be desired to move the platform off-station,
disconnection from the well head and export facilities is'facilitated by the provision
of the releasable connections at the platform and at the top of the column. The latter
connection is near the surface and can be of a form not requiring operation by a diver.
It is to be noted here that not only is it advantageous to have this connection near
the surface from the point of view of ease of actuation, but also by so positioning
the connection the length of the flexible flow line arrangement is kept to a minimum.
A flexible flow line arrangement is required to accommodate motion of the floating
platform, but to have the flexible flow line arrangement extending, for example, all
the way to the water bed could result in a flexible flow line arrangement of such
considerable length and consequent weight as to introduce problems in, for example,
recovery thereof after disconnection from the well head. On the other hand, by having
an articulated but otherwise rigidly supported flow line arrangement terminating below
surface level, adverse effects of wave action, lateral loads and impacts on this arrangement
and its support (the column) are avoided or readily absorbed.
[0010] Preferably at least the connection 5 at the base of the column is releasable, along
with flow line connections to the flow lines 8 and 9, but in the system shown all
the connections 5A and 12 are releasable as well.
[0011] In the illustrated form, the flexible flow line arrangement 10 is provided with buoyancy
11 to cushion the movement of the flexible flow line, especially when it is disconnected
in an emergency. The flexible flow line arrangement 10 also avoids a need for providing
tensioning equipment on the floating platform, as is necessary in constructions where
a tensioned riser flow line arrangement is provided between a well head and a floating
production platform. Also loading on the production platform is reduced as compared
with a tensioned flow line arrangement. The illustrated form also has the advantage
that the flow line arrangements are disposed to one end of the floating platform,
leaving the central moonpool of the platform free for working over the well head.
Workover relative to the flow line arrangements is via a separate derrick mounted
over the flow line arrangements.
[0012] It may not always be necessary to provide buoyancy for the flexible flow line arrangement
10. Also the flexible flow line arrangement 10 need not necessarily be connected to
floating facilities, and the facilities to which it is connected could be other than
production facilities.
[0013] The buoyancy for the column sections 6 can be built-in along the length of each section,
rather than provided by the buoys 13.
1. A flow line system for use in the transfer of fluid to or from an under water site,
the system comprising a flow line arrangement (7,12) to be installed so as to extend
from the water bed to near, but below, the surface, and in flexible manner from the
termination of the column near the surface to the surface; characterised in that the
column is for articulated connection to the water bed and consists of rigid sections
(6) that are connected in series at articulated connections (5A) so that a first part
(7) of the flow line arrangement is rigidly supported by each section (6) but can
articulate at the base of the column and at the connections between the rigid sections
(6); and in that a second part (12) of the flow line arrangement, which is flexible,
is releasably connectable to the first part so as to extend the first part.
2. A flow line system as claimed in claim 1, wherein the first part (7). of the flow
line arrangement consists of rigid runs supported by corresponding rigid column sections
(6), and articulated connections (12) between these runs and corresponding to the
articulated connections (5A) between the rigid column sections (6).
3. A flow line system as claimed in claim 1 or 2, wherein the articulated connection
(5) of the column to the water bed is releasable, and wherein the first part (7) of
the flow line arrangement is releasably connected, at the water bed, to fluid flow
lines (8,9).
4. A flow line system as claimed in claim 2 or claims 2 and 3, wherein each articulated
connection (5A), (12) between column sections (6) and the runs of the first part (7)
of the arrangement is releasable.
