U.S. patent application number 10/512892 was filed with the patent office on 2005-07-21 for flexible riser system.
Invention is credited to Luppi, Ange.
Application Number | 20050158126 10/512892 |
Document ID | / |
Family ID | 28800027 |
Filed Date | 2005-07-21 |
United States Patent
Application |
20050158126 |
Kind Code |
A1 |
Luppi, Ange |
July 21, 2005 |
Flexible riser system
Abstract
Riser tower system connecting a fixed subsea installation to a
floating surface unit, comprising at least one flexible pipe
arranged in a first catenary and extending between the surface
installation and a submerged buoy; at least one riser arranged in a
second catenary between the buoy and the seabed, the buoy being
anchored to the seabed by a tether device comprising at least two
taut tethers, at least two moorings in a catenary and on which
there are provided a return device which exerts on the buoy a
return force that depends on the lateral movement of the buoy. It
has oil industry applications for offshore operation.
Inventors: |
Luppi, Ange; (Nimes,
FR) |
Correspondence
Address: |
OSTROLENK FABER GERB & SOFFEN
1180 AVENUE OF THE AMERICAS
NEW YORK
NY
100368403
|
Family ID: |
28800027 |
Appl. No.: |
10/512892 |
Filed: |
October 28, 2004 |
PCT Filed: |
April 9, 2003 |
PCT NO: |
PCT/FR03/01119 |
Current U.S.
Class: |
405/224.2 ;
166/350; 166/367; 405/224 |
Current CPC
Class: |
E21B 17/015 20130101;
B63B 22/023 20130101; B63B 22/18 20130101; B63B 21/508
20130101 |
Class at
Publication: |
405/224.2 ;
405/224; 166/350; 166/367 |
International
Class: |
E02D 005/54; E21B
007/12 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 29, 2002 |
FR |
02/05378 |
Claims
1-10. (canceled)
11. A riser system for connecting a fixed subsea installation to a
floating surface unit, comprising: a submerged buoy; at least one
flexible pipe arranged in a first catenary and extending between
the surface installation and the submerged buoy; at least one riser
also arranged in a respective second catenary and extending between
the buoy and the seabed; a tether device anchoring the buoy to the
seabed, the tether device comprising at least two taut tethers
between the buoy and the seabed; at least two moorings attached to
the buoy and each in a third catenary; a respective return device
on each third catenary positioned to exert a return force on the
buoy that is dependent on the lateral movement of the buoy with
respect to the seabed.
12. A riser system according to claim 11, wherein each mooring has
a lower portion toward the seabed, and the return device comprises
at least one ballast weight arranged on the lower portion of each
mooring.
13. A riser system according to claim 12, wherein each mooring
catenary has an end at the seabed and has a touchdown point spaced
from the end at which the mooring touches down on the seabed when
the buoy is not displaced from a first lateral position; and the
ballast weight on the mooring is distributed on each side of the
mooring from the touchdown point.
14. A riser system according to claim 13, wherein a part of the
ballast weight is situated above the touchdown point of the mooring
and provides the return force on the buoy.
15. A riser system according to claim 11, further comprising a
bridle connecting each mooring to the buoy, the bridle being
arranged under the buoy and the bridle is positioned for preventing
the buoy from rotating.
16. A riser system according to claim 11, wherein the taut tethers
lie approximately in the mid-plane passing through a longitudinal
axis of the buoy.
17. A riser system according to claim 11, wherein the buoy is of
variable buoyancy.
18. A riser system according to claim 17, wherein the buoy is
comprised of several parts each selectively operable to be buoyant,
whereby the buoy is of variable buoyancy.
19. A riser system according to claim 12, wherein the buoy has
opposite lateral ends; and each of the moorings is connected to one
lateral end of the buoy and the riser is connected to the other
lateral end.
20. A riser system according to claim 12, wherein each mooring is
connected to the submerged buoy at a point lying approximately on
the midplane of the buoy.
21. A riser system for connecting a fixed subsea installation to a
floating surface unit, comprising: a submerged buoy; at least one
flexible pipe arranged in a first catenary and extending between
the surface installation and the submerged buoy; at least one riser
also arranged in a respective second catenary and extending between
the buoy and the seabed; a tether device anchoring the buoy to the
seabed; a mooring attached to the buoy and in a third catenary; a
respective return device on the third catenary positioned to exert
a return force on the buoy that is dependent on the lateral
movement of the buoy with respect to the seabed.
22. A riser system according to claim 21, wherein the mooring has a
lower portion toward the seabed, and the return device comprises at
least one ballast weight arranged on the lower portion of the
mooring.
23. A riser system according to claim 22, wherein the mooring
catenary has an end at the seabed and has a touchdown point spaced
from the end at which the mooring touches down on the seabed when
the buoy is not displaced from a first lateral position; and the
ballast weight on the mooring is distributed on each side of the
mooring from the touchdown point.
24. A riser system according to claim 23, wherein a part of the
ballast weight is situated above the touchdown point of the mooring
and provides the return force on the buoy.
25. A riser system according to claim 21, wherein the buoy is of
variable buoyancy.
26. Method for producing a riser system between a subsea
installation and a floating surface installation and for restoring
a position of the floating surface installation, the method
comprising: submerging a buoy, connecting the buoy to the surface
installation by at least one flexible pipe and selecting the pipe
length and the position of the buoy so that the pipe is arranged in
a first catenary; anchoring the buoy to the seabed by a taut
tether; arranging at least one riser in such a position and of such
length as to define a second catenary between the buoy and the
seabed, and connecting the riser to the at least one flexible pipe;
exerting on the buoy a return force which depends upon the lateral
movement of the buoy from a predetermined lateral position.
27. The method of claim 21, further comprising attaching a mooring
between the buoy and the seabed and forming the mooring of a length
and attaching the mooring to the seabed such that a third catenary
is defined; and wherein the return force is exerted by applying the
return force on the mooring attached to the buoy, wherein the
return force on the mooring moves the buoy laterally.
28. The method of claim 22, wherein the mooring meets the seabed at
a touchdown point, and the return force is applied increasingly as
the mooring is lifted from the touchdown point on the seabed by
lateral movement of the buoy.
Description
[0001] The present invention relates to a riser tower system
intended to connect a fixed subsea installation such as a well head
or a manifold to a floating surface unit such as a platform or a
ship of the FPSO (Floating Platform Storage and Offloading)
type.
[0002] Offshore operation of an oil field is increasingly
complicated as the water depth increases, which depth these days
may be as much as several thousands of metres. The transfer of
product from the fixed installation, situated on the seabed and
usually consisting of a well head, to the floating surface
installation or unit, poses a certain number of difficulties. The
most commonly used transfer systems are those known as riser tower
systems which comprise pipes through which various products to be
transported between the seabed and the surface flow, these products
being, for example, oil, gases, water, etc. Other pipes may also be
used, particularly fluid-injection, charging or electrical and/or
hydraulic control lines.
[0003] In drilling for oil, particularly when the deposits are very
deep down, the areas of turbulence which lie between 50 and 300 m
below the surface of the water may have effects not only on the
surface unit or installation which may move as a result of the
swell and of other phenomena such as pitching, rolling, etc., but
may also have an influence on the riser tower system which
experiences forces due to the waves, to the wind and to marine
currents.
[0004] As a result, riser tower systems are designed to withstand
these stresses the magnitudes of which may vary.
[0005] Various types of riser tower system have been proposed,
these are described for example in U.S. Pat. NoS. 3,111,692,
3,677,302, 4,031,919, 4,188,156, 4,182,584, 4,388,022, 4,400,109
and 4,423,984.
[0006] The main disadvantages with these systems lie in the fact
that it is necessary to use buoys of great buoyancy of at least
2000 tonnes when this buoyancy is distributed over the entirety of
the riser, the elements of the buoy having to withstand significant
pressures. Another disadvantage is that these systems are
manufactured on land and have then to be brought out to and
installed on the site, all of these operations being tricky and
expensive. In addition, it is very difficult to anchor the ends of
the rigid sections to the seabed without using divers or very
sophisticated equipment such as ROVs (Remote Operated Vehicles),
and this introduces not insignificant costs into the placement and
monitoring while the field is being exploited.
[0007] U.S. Pat. No. 5,639,187 describes a system which combines
rigid pipes and flexible pipes to establish the fluid communication
between the fixed subsea installation and the surface unit, the
system comprising a submerged buoy which is anchored to the seabed
by means of four taut tethers, each of the tethers being attached
to the ends of the buoy and to a corner of a kind of rectangle
formed on the seabed, so as to minimize the rotation of the buoy
that could be brought about by horizontal forces and by the weight
of the pipes running between the buoy and the seabed. In fact, the
system is of the tension leg type, these having to withstand
vertical loadings of at least 1500 tonnes and even more in order,
especially during installation, to react the buoyancy of the buoy
which has to be greater than the weight of the pipes, of which
there are a great many in such an application. The submerged buoy
has also to have a reserve of buoyancy so as to give the entire
system the stiffness it needs to limit its lateral movements. What
happens is that the lateral movements of the buoy are undesirable
because they bend the rigid pipes to bend radii such that these
pipes may experience plastic deformation and thus cause the
beginning of crushing by ovalization at the sag bend. The sag bend
lies over the touchdown point which is where the pipes touchdown
onto the seabed.
[0008] The purpose of the present invention is to propose at least
a riser tower system which is better suited to the movements of the
surface unit, simple to produce and to install and less expensive
than the systems of the prior art, for comparable operating
conditions.
[0009] One subject of the present invention is a riser tower system
intended to connect a fixed subsea installation to a surface unit,
of the type comprising at least one flexible pipe arranged in a
catenary and extending between the surface installation and a
submerged buoy, at least one riser arranged in a catenary between
the said buoy and the seabed, the said buoy being anchored to the
seabed by a tether device comprising at least two taut tethers, and
which is characterized in that it comprises at least two moorings
in a catenary and on which there are provided return means which
exert on the said buoy a return force that depends on the lateral
movement of the said buoy.
[0010] One advantage of the present invention lies in the fact
that, when the submerged buoy moves laterally, it is automatically
returned to its initial position or position of equilibrium by the
return means the force of which is variable, that is to say that
they develop a return force which is dependent upon the amplitude
of the lateral movement of the buoy.
[0011] According to another feature of the invention, the return
means consist of a ballast weight which is distributed on each side
of the touchdown point where the mooring touches down on the
seabed. The weighted part of the mooring, situated above the
touchdown point, in the direction of the buoy, mainly constitutes
the return force. Such a structure is simple to produce because the
ballast weight can be of any kind, such as chains, balls, weights
or alternatively clump-weight. In addition, it is easy to determine
the "return" length of the mooring, that is to say the length of
the weighted part, in advance. Finally, it is possible to weight
down a longer length than necessary of the portion of the mooring
lying along the seabed if it is desirable not to anchor the end of
the mooring. In such a case, care should be taken that the mooring
does not move excessively and does not become entangled in the
buoy's taut tethers.
[0012] According to another feature, each mooring is connected to
the buoy by a flange or mooring bridle arranged under the buoy and
intended to prevent or at the very least limit the rotation of the
buoy.
[0013] Other advantages and features will become apparent from
reading the description of one embodiment of the present invention
and from the appended drawings in which:
[0014] FIG. 1 is a schematic view in elevation of the riser tower
system according to one embodiment of the invention.
[0015] FIG. 2 is a schematic view in elevation of the riser tower
system for several lateral positions of the buoy.
[0016] The riser system 1 depicted schematically in FIG. 1 is
intended to connect a fixed subsea installation 2, consisting for
example of a well head, a manifold or some other collector and
delivering a product from an oil deposit or the like, to a floating
surface unit or installation 3 such as a platform or an FPSO, the
distance separating the surface 3 and subsea 2 installations being
possibly as much as several thousands of metres. At a certain
distance from the surface 4 of the water, and generally beyond the
area of turbulence of the stretch of water concerned, is submerged
a buoy 5 which is anchored to the seabed 6 by two tethers 7, 8
which are stretched between the buoy 5 and a deadweight 9 or other
anchoring means (suction pile).
[0017] One or several flexible pipes 10 running in a catenary
between the surface unit 3 and the buoy 5 are connected to one or
several risers 11 extending in a catenary between the buoy 5 and
the fixed subsea installation 2 so that fluid communication is
established between the said installations 2 and 3. The risers
running in a catenary from the buoy on the seabed may be of any
type such as rigid pipes commonly known as SCRs (Steel Catenary
Risers), single-walled or pipe in pipe and even flexible pipes or
hybrid pipes having at least one flexible part and one rigid
part.
[0018] The riser tower system comprises at least two moorings 18,
19 arranged in a catenary between the buoy 5 and the seabed 6. Each
mooring 18, 19 comprises, in the lower portion 14, a part 13 which
is weighted by a ballast weight 12. This ballast weight 12
constitutes return means for the buoy, the return force being
dependent chiefly upon the lateral movement that the said buoy 5
might have and which may be caused by a strong swell, marine
currents and more generally by movements of the surface unit 3. The
ballast weights 12 are distributed on each side of the touchdown
point 15 which is the region or point where the mooring touches
down on the seabed 6. The ballast weights 12 may consist of
weights, balls, chains or clump-weight. The ballast weights are
distributed on each side of the touchdown point when the buoy is in
its position of equilibrium (central position A in FIG. 2).
[0019] In the position A which corresponds to that of FIG. 1, the
tethers 7, 8 are roughly vertical and the weighted parts 13 of the
moorings 18, 19 rest for the most part on the seabed 6. In the
position C, the weighted parts 13 rest more on the seabed whereas
in the position B, the weighted parts 13 are lifted and thus
develop a return force which has a tendency to return the system
towards the position A, the return force varying according to the
weighted length raised off the seabed by the movement of the
corresponding mooring, which movement is brought about by the
lateral movement of the buoy 5 (FIG. 2).
[0020] By way of example, the ballast weight on each mooring 18, 19
consists of 4-inch (.congruent.10 cm) chains spread over a 100 m,
in the knowledge that when the buoy 5 is at the mid-point
(vertical), about one third of the ballast weight is raised off the
bed and produces a tension of about 50 tonnes in each mooring 18,
19. Obviously, these indications are given solely by way of
example, the choice and arrangement of the elements of which the
ballast weight is made being dependent on the particular case
envisaged. However, it is possible to suggest that the weight per
unit length used to weight the catenary mooring 18, 19 is dependent
in particular on the distance between the buoy 5 and the seabed
6.
[0021] Each taut tether 7, 8 is attached to a comer 16 of a flange
or mooring bridle 17 fixed to the buoy at one end thereof, and
which is also intended to prevent or greatly limit the rotation of
the buoy. The points of attachment of the tethers 7, 8 to the
flanges 17 are preferably roughly in the midplane passing through
the longitudinal axis 18 of the buoy 5. In FIG. 1, the midplane
comprising the tethers 7, 8 is embodied in dotted line X-X.
[0022] Each mooring 18, 19 is connected to an end 20 of the buoy 5
which is the opposite end to the other lateral end 21 to which the
riser or risers is or are connected. It may also be connected to
the top 16 of the flange 17 so that the points of attachment of the
moorings 18, 19 lie more or less in the midplane in which the
points of attachment of the moorings 7, 8 to the buoy 5 are
situated.
[0023] In a preferred embodiment of the invention, the buoy 5 has
variable buoyancy and comprises several parts, for example three
parts 22 to 24 each consisting of a hollow cylinder. Such a
structure of the buoy 5 makes it possible to avoid having to
develop very high forces at the buoy 5, which forces are dependent
in particular on the number and weight of risers to be provided
between the seabed 6 and the said buoy 5. Indeed, by virtue of this
compartmentalization of the buoy 5, each cylinder 22 to 24
constitutes a compartment which can be partially or completely
emptied as additional risers are added. Thus, in a first phase, the
compartments are filled with an appropriate fluid such as water.
Then, once the first riser has been fitted, part of a compartment
is emptied and filled with gas, the amount emptied being a function
of the weight of the riser fitted. This procedure then continues
sequentially and in the same way for the other risers.
[0024] According to the described embodiment of the invention, the
risers 11 are connected to the associated flexible pipes 10 by end
fitting connections in a way known per se. These risers 11 are
supported by the buoy by a connection and suspension receptacle
device shown schematically in FIG. 1 and referenced 30, in which
their terminal end fitting is housed. It can be noted that this
device may comprise damping means intended to allow the risers a
certain angular excursion with respect to the buoy at their
connection.
* * * * *