Device for hybrid riser for the sub-sea transportation of petroleum products

A device for transporting petroleum products in deep waters from the seabed up to a floating or semi-submersible surface structure wherein at least one rigid and straight hybrid riser extends vertically. The hybrid riser has a rigid central hollow tubular structure surrounded by a cylindrical block of syntactic material which provides buoyancy and thermal insulation for the riser. A plurality of rigid pipelines are embedded in the syntactic material and surround the central tubular structure for receiving petroleum products from wells on the sea bed. The bottom of the riser is connected to a suction anchor at the sea bed. A submerged float is fixed to the top of the riser and exerts an upward vertical force thereon. Flexible pipelines connect the rigid pipelines in the riser to the floating or semi-submersible structure.

The subject of this invention is a device with a hybrid riser for the 
sub-sea transportation of petroleum products. 
The technical sector of the invention is the field of the construction of 
installations for the extraction of petroleum products from the sub-sea 
sub-soil. 
The present invention relates more particularly to a pipeline system or 
device for transporting petroleum products extracted from one (or more) 
well(s) drilled in the sub-sea sub-soil to a floating or semi-submersible 
(surface) structure, particularly to a floating production, storage and 
off-loading (F.P.S.O.) vessel. 
The invention is particularly applicable to that part of the pipeline which 
extends from the surface of the sea bed up to the floating or 
semi-submersible structure. 
One objective of the present invention is to provide such a device which is 
well suited to great depths, particularly depths of 750 meters or more, 
and to its method of installation. 
In order to raise to the surface petroleum products which have been 
extracted in deep waters, it is known practice to make use of rigid 
pipelines, particularly made of steel, whereas for raising products which 
have been extracted from shallower waters, it is known practice to make 
use of flexible or deformable pipes. 
In particular, the use in deep waters of risers which extend more or less 
vertically and are more or less rigid is known. 
Such risers, sometimes known as hybrid risers, may consist according to the 
invention--as depicted diagrammatically in cross-section in FIG. 9--of a 
vertical bundle of steel pipes which are, at least in part, supported by 
buoyancy means; such risers comprise a straight tubular central structure 
made of steel which extends vertically and may be filled with air in order 
to play a part in buoyancy, and which is surrounded by syntactic buyoancy 
composite material over at least part of its height, for example in the 
form of hollow cylindrical modules stacked (and/or strung) around the 
central tubular structure; this syntactic material contributes to the 
buoyancy; production pipelines transporting the extracted products to the 
surface and service pipelines transporting fluids and sometimes power 
towards the sea bed extend around and along the length of the central 
structure, through the syntactic material; these lengths of peripheral 
pipeline for vertical transport are connected at their lower end to 
essentially rigid and metal pipelines running along the sea bed, down to 
the extraction well, and are connected at their upper end to lengths of 
flexible pipeline extending up to the floating structure, generally via 
gooseneck connections. 
Such structures of risers in which the transport pipelines are surrounded 
by syntactic material are particularly beneficial for raising petroleum 
products from the sea bed because the syntactic material acts as thermal 
insulation, thus limiting the cooling of the "crude" by the cold sea 
water, and thus limits the formation of undesirable products (paraffin, 
hydrates) in the pipes. 
Because these risers, which extend up to within a few tens of meters from 
the surface, are very long (tall), that is to say several hundreds of 
meters long, it is important that their deformation (curvature) resulting 
in particular from the hydrodynamic action of the currents and their 
positioning under the water be controlled, so as to keep the displacement 
of the upper end of the risers to within acceptable limits; failing this, 
excessive loadings on the flexible pipelines connecting these risers to 
the floating structure may result; impacts between two risers located 
close together, and/or undesirable overlapping (or intertwining) of the 
riser and other string-like structures (umbilical cables, other risers for 
example) located close to it may also result. 
The problem posed consists, in particular, in proposing a highly effective 
system for anchoring the base of the riser to the sub-sea sub-soil. 
One objective the invention consists in proposing a system for attaching 
the base of the riser to the sub-sea sub-soil which is easy and 
inexpensive to implement at great depth. 
Another objective of the invention consists in proposing a system for 
attaching the base of the riser to the sub-sea sub-soil which facilitates 
the connection of the riser to the sub-sea sub-soil and, if necessary, its 
future disconnection with a view to the riser being re-used in another 
place. 
Another objective of the invention consists in proposing a system for 
anchoring the base of the riser to the sub-sea sub-soil which allows 
connection between anchoring means and the base of the riser which may be 
either articulated or fixed. 
Another objective of the invention consists in proposing a system for 
anchoring the base of the riser to the sub-sea sub-soil which is of low 
cost (in terms of hardware and operation), so that it can be abandoned 
after use. 
According to a first aspect of the invention, the riser is fixed to the 
sub-soil by a friction anchor; for this, the anchor has a large (and 
preferably ribbed) contact area with the sub-soil. 
According to another aspect, the riser is fixed to the sub-soil by a 
gravity anchor; for this, the anchor has a great mass (several tonnes or 
tens of tonnes at least). 
Preferably, in both cases, the anchor has a more or less cylindrical shape, 
one base of which is open. 
In the case of the friction anchor, the anchor may essentially consist of 
an elongate shell with a ribbed cylindrical wall, of polygonal 
cross-section, which is closed (hermetically) at one end, by a wall, for 
example a planar wall, in the shape of a disc of polygonal contour which, 
in the position of use (of anchorage) forms the upper part; the anchor is 
installed as follows: the anchor is placed on the sea bed via the open 
face provided at the bottom end of the anchor; the internal space 
delimited by the walls of the anchor is partially evacuated (by pumping or 
sucking out water using a pump) and the anchor then sinks (more or less 
vertically) into the soil under the effect of the hydrostatic pressure 
applied to it, until its cylindrical lateral walls have fully (or at least 
substantially) entered the soil; in general, the bottom end of the anchor 
will be sunk at least 5 meters into the soil, and usually will be sunk of 
the order of 10 to 25 meters. Bearing in mind the large area (of the order 
of 100 to 1000 m.sup.2) of the internal face and of the external face of 
the walls of the anchor which are sunk into the soil, significant 
resistance to pulling out (of the order of several tens or hundreds of 
tonnes) is obtained, because of the friction forces exerted by the sub-sea 
sub-soil on these walls; furthermore, because the (sea-water-filled) 
residual cavity, delimited by the lateral or peripheral walls of the 
(bell-shaped) anchor and by the upper wall, is isolated more or less 
hermetically from the sea water surrounding the anchor, an additional 
resistance to pulling out is obtained as a result of a suction or 
suction-cup effect. 
According to another aspect of the invention the riser for transporting 
petroleum products is attached by an anchor which is installed by a method 
in which the anchor is sunk into the sub-sea sea-bed by creating an under 
pressure when partially evacuating the cavity delimited by the soil and by 
the upper part of the bell-shaped anchor. 
In the case of the use of a gravity anchor, this anchor may essentially 
consist of a cylindrical shell of polygonal cross-section, the upper base 
of which is open and the lower base of which is at least partially closed. 
Such an anchor forms a container capable of holding a sizeable amount 
(several hundreds of tonnes) of a heavy bulk material such as a metal ore 
or residue from the processing of such an ore. 
According to another aspect of the invention the riser for transporting 
petroleum products is attached by an anchor which is intalled by a method 
in which a heavy material is deposited under gravity in the shell of the 
anchor by conveying this material through the hollow central tubular 
structure of the riser. 
The numerous advantages afforded by the invention will be better understood 
through the following description which refers to the appended drawings, 
which illustrate, with no implied limitations, some preferred embodiments 
of the invention.

With reference to FIG. 9, the riser 8 comprises a tubular central structure 
23 consisting of a steel tube delimiting a cylindrical cavity 25 which may 
be filled with air in order to contribute to the buoyancy of the riser and 
which may also be used for the transporting, and the dropping under 
gravity, of heavy materials which can thus be transported from the water 
surface (or from the top end of the riser which lies at a shallow depth) 
down to the sea bed which is in deep water, in order to fill the structure 
of a gravity anchor allowing the riser to be anchored. 
This central structure 23 is more or less straight, and has a longitudinal 
axis 26 extending, in the position of use, more or less vertically, as 
illustrated in particular in FIG. 1. 
The central structure 23 is surrounded by more or less cylindrical blocks 
of syntactic material 21, inside which there extend tubes 22 and 24 
parallel to the central tubular structure 23 and distributed around it, so 
that they are insulated by the syntactic material 21; the tubes 22, which 
for example are metal and rigid, are used to raise petroleum products 
extracted from the sub-sea sub-soil, while tubes or umbilical cables 24 
are used to transport service fluids or electrical power, for example, to 
the sea bed. 
With reference to FIGS. 1, 5 and 6 in particular, the riser 8 extends 
vertically in the direction of the axis 26, is attached at its bottom end 
9 to a suction anchor 11 via mechanical means of connection 13, and is 
attached mechanically by its top end 7 to a float 4 such as a container 
full of air which also contributes to the buoyancy of the column, by 
exerting an upwards vertical force on it. 
The pipelines for transporting petroleum products that the riser 8 
comprises, are connected at their top end, via bent gooseneck pipes 6, to 
flexible pipelines 3 extending in a catenary curve between the top end 7 
of the riser 8 and the floating (or semi-submerged) structure 1 at the 
surface 2 of the sea. The flexible pipe 3 are mechanically attached to the 
structure 1 by fastening means 5 illustrated diagrammatically in greater 
detail in FIGS. 5 to 7 in particular. 
The said pipelines for transporting petroleum products are also connected, 
at the bottom end 9 of the riser 8, to the bundles of pipelines 20 which 
run along the surface 14 of the sea bed 10 (and which come from one or 
more extraction well(s)), as follows, illustrated, in particular, in FIGS. 
1 to 3, 5 and 6: 
The bottom end 22a of a pipeline 22 for transporting petroleum products is 
connected to a pipeline 18 forming a sleeve, itself connected to the end 
of a pipeline 20b forming part of the bundle 20 running along the surface 
14 of the sea bed 10; the bundle 20 of pipelines may, for example, consist 
of two pipelines 20b for raising petroleum products and of two pipelines 
20a for service fluids, especially gas or water, in order in particular to 
pressurize or maintain the system of pipelines; the ends of the portions 
20a, 20b of the bundle 20 of pipelines are attached to a mechanical 
structure 19 forming a sled, which is equipped with runners 19a, which 
help it to slide along the sea bed 10 and which may be towed via a hook 
19b, with which it is equipped, when the bundle 20 of pipelines is being 
installed on the sea bed prior to its connection to the riser. 
With reference to FIGS. 2 and 3 in particular, the friction anchor 11 used 
for attaching the base 9 of the riser to the sub-sea sub-soil 10 comprises 
a metal structure consisting of eight ribbed lateral facets 30 forming, 
when viewed from above as illustrated in particular in FIG. 3, a wall of 
octagonal cross-section, of cylindrical overall shape, of axis 26 which is 
vertical when in the position of use; the height of the lateral facets 30 
of the structure of the anchor 11 allows these walls to be sunk to a depth 
31 into the sub-sea sub-soil as illustrated in FIG. 2, for example of the 
order of 10 to 20 meters, the upper portion of the lateral walls 30 
extending above the surface 14 of the bed 10 by a height 32, for example 
of the order of one or more meters; a horizontal upper wall 12, provided 
at the top end of the anchor 11 forms, with these side walls, a sort of 
bell which (when the anchor has been sunk into the sub-soil 10 as 
illustrated in FIG. 2), delimits with the surface 14 of the bed, a 
water-filled residual cavity 33; this makes it possible, through a 
suction-cup effect, to create a resistance of the anchor 11 to pulling 
out, which resistance adds to the resistance to pulling out that results 
from the significant friction forces exerted over the entire area of the 
facets or side walls 30 of the anchor sunk into the sub-soil 10; the 
diameter or width 38 of the anchor 11 is preferably of the order of 
several meters, for example of the order of 5 to 10 meters. 
With reference to FIG. 2 in particular, the base 9 of the riser is rigidly 
attached, for example by welding, to a reinforced tubular length 35, the 
bottom end of which is mechanically secured to a connector 34, itself 
mechanically secured to the planar horizontal top wall 12 of the structure 
of the anchor 11; such attachment by rigid connection makes it possible, 
for example to limit the displacements of the top end 7 of the riser 8 to 
within a cone of apex half-angle 60, of the order of 1 to 5 degrees for 
example, so as to limit the displacement of the said top end 7, in a 
horizontal plane, to a value of the order of one or several tens of 
meters, bearing in mind the significant length (or height) of this riser 
8, which is, for example, of the order of 1000 to 2000 meters; this top 
end 7 of the riser 8 is, for example, located at a depth 61 of the order 
of several tens of meters, for example close to 100 meters, and the 
floating structure 1 is situated, for example, at a distance 62 from the 
vertical axis 26 of the riser 8, also of about 100 meters approximately; 
this makes it possible, with reference to FIG. 4 in particular, for 
several risers 8 relatively distant from one another to be connected by 
corresponding bundles of flexible pipelines 3, allowing a displacement of 
the said end 7 of each of the risers 8, without the latter knocking 
together or becoming intertwined; with reference to this figure, the 
structure 1 is positioned at the water surface by anchoring means such as 
anchoring lines 15 equipped at their end with anchoring means depicted 
symbolically by anchors. 
Whereas, as illustrated in FIGS. 1, 2, 5 and 6 in particular, each riser 8 
may be attached rigidly and in a more or less inset way into the sub-sea 
sub-soil by the friction anchors 11 or, as an alternative, by gravity 
anchors illustrated diagrammatically in FIG. 8, these risers may also, as 
illustrated in FIG. 7, be attached by connecting means allowing these 
risers a greater displacement, that is to say by more or less articulated 
connections, which, as illustrated in FIGS. 7 and 7a, may essentially be 
produced by lengths of metal cable 40, fixed by their first, upper end to 
collars or attachment means provided at the bottom 9 of the riser, on the 
one hand, and attached by their second end to friction anchors 11a 
identical or similar to those described above; in the embodiment 
illustrated in FIG. 7 and 7a, the base 9 of each riser 8 is attached to 
the sub-sea sub-soil by two friction anchors 11a; the three risers 8 
illustrated in this figure, which use common anchors 11a, use a total of 
four anchors 11a for this attachment by cables 40; these risers 8 are 
placed in tension via their top end 7, by means of a common float 4 of 
essentially cylindrical shape of horizontal axis, to which they are 
attached by means 42 illustrated diagrammatically in greater detail in 
FIG. 7b, and constituting kinds of grippers; this float 4 is itself 
connected to the sub-soil 10 by friction anchors 11b sunk into the 
sub-soil in the same way as described earlier, the float 4 being connected 
to these two anchors lib by two cables 39, thus limiting the possible 
displacement of the float 4. 
With reference to FIG. 7a, connection of the base of the riser 9 to the 
bundles 20 running along the sea-bed, is via a bent portion of pipeline 
and via a connection 41 which is preferably a connection that can be 
fitted or activated by a remote-operated underwater vehicle. 
With reference to FIG. 8, the structure of the gravity anchor intended to 
hold a heavy material is similar to the structure of the friction anchors 
described earlier, which structure essentially consists of more or less 
planar and undulated facets 30, together forming a cylindrical structure 
of octagonal cross-section, of longitudinal axis 26 vertical when in the 
position of use, the upper face of which is open and the lower face of 
which is at least partially closed; this structure delimiting the cavity 
33 capable of containing a heavy material is preferably reinforced by 
cross members 50 arranged in a cross in one or more horizontal planes in 
particular. 
FIG. 10 illustrates, as a side view, an alternative form of a head float 
for a riser. 
FIGS. 11 and 12 are respective views on XI and XII of FIG. 10. 
FIG. 13 illustrates the use of the float of FIGS. 10 to 12 for fastening 
the top end of a riser and guiding the flexible pipelines that connect the 
riser to the floating structure. 
With reference to these FIGS. 10 to 13, the float 4 essentially consists of 
two cylindrical caissons 104 of mutually parallel axes 105, which are 
sealed at their bottom and top ends and connected by two tubular portions 
102, the longitudinal axes 103 of which are mutually parallel and 
perpendicular to the axes 105; the lower part of the tubular length 102 
situated in the bottom of FIGS. 10 and 11 has a mechanical articulation 
101 such as a knuckle joint allowing the articulation, about an axis 
perpendicular to the plane of FIG. 11, of an arm 100 allowing the top end 
7 of the riser to be attached mechanically to the float 4; in FIGS. 10 and 
11, only portions of the flexible pipelines 3 are depicted; in FIG. 13 it 
can be seen that the top tubular portion. 102 of the float 4 illustrated 
in FIGS. 10 to 12 is used for guiding that part of the flexible pipelines 
3 located in the vicinity of the connection with the top end of the riser 
8. 
FIGS. 14 to 28 respectively illustrate successive operations of the 
installing of a riser and its attachment to an anchor already placed on or 
sunk into the sub-sea sub-soil; 
FIG. 14: the anchor 11 anchored in the sub-sea sub-soil and emerging via 
its upper part above the sea bed 14 is fitted with a pulley 112 in which 
two strands of cable 111 are engaged and run up to the surface 2 where 
they are fixed to a buoy 110; 
FIG. 15: the riser 8, secured to its float 4, is transported to the site 
where the anchor 11 is situated for attachment, via a towing vessel 113 
connected to the float 4 by a hauling line or cable 115, and by a 
follow-up vessel 114a connected to the end 9 of the riser 8 by a is second 
cable 116; so that the riser 8 can be taken to the installation site, this 
riser is preferably temporarily equipped with buoys 120 that allow it to 
float on the surface 2; 
FIG. 16: on the site, the towing vessel 113 is anchored to an anchor 118 
which may be used later for anchoring the production floating structure 1, 
which anchor 118 may be a suction or friction anchor; this anchoring is 
via a line or cable 117; 
FIG. 17: the follow-up vessel 114a steers towards the buoy 110 connected to 
the pulley 112 with which the anchor 11 is fitted, exerting a pulling 
force 119 on the end 9 of the riser 8, which has been detached from the 
buoys referenced 120 in FIGS. 2 and 3, and which therefore sinks below sea 
level by a height 130, for example of the order of several tens of meters; 
FIG. 18: the top end of the two strands 111a and 111b previously connected 
to the buoy referenced 110 in 
FIG. 14, is connected respectively to the end 9 of the riser 8 in the case 
of the strand 111a, and to a vessel 114b in the case of the strand 111b; 
the end 9 of the riser 8 also remains secured to the vessel 114a via the 
line or cable 116, the paying-out of which is controlled by the vessel 
114a; 
FIGS. 19 to 21: the lengthening or paying-out of the line 116 by the vessel 
114a and the simultaneous pulling of the strand 111b by the vessel 114b, 
cause uniform and controlled submerging of the lower end 9 of the riser 8, 
the end 7 of which remains on the surface by virtue of the float 4 (to 
which it is connected by the articulated connection 100, 101), until the 
riser 8 is in a position stretched out along a vertical axis as 
illustrated in FIG. 21; 
FIGS. 22 to 25: a ship 121 equipped with pumps to allow the float 4 to be 
filled with and emptied of water, is connected for this purpose by 
pipelines and cables 200; the float 4 is gradually and partially filled 
with water so that it inclines and sinks, allowing the lowering of the 
riser 8, the lower end 9 of which remains guided in the direction of the 
anchor 11 by virtue of the action of the lines 111a, 111b attached to its 
bottom end 9 and pulled simultaneously by the vessel 114b in the direction 
of the arrows 120 until the bottom end 9 of the riser 8 is more or less in 
contact with the top of the anchor 11 intended to receive the riser, which 
corresponds to the configuration depicted in FIGS. 24, 25 and 29; 
FIGS. 26 to 28: it is then possible, as illustrated in these figures, 
having detached the cables 111a, 111b from the anchor 11, for example 
using a remote-operated underwater vehicle 131, to engage the bottom end 9 
of the riser 8 in the connection means provided at the top of the anchor 
11, particularly as depicted on a larger scale in FIG. 29; as illustrated 
in FIGS. 27 and 28, it is then possible to connect the flexible pipelines 
3, first of all to the top end of the transport pipelines provided in the 
riser 8, as illustrated in FIG. 27, and then to connect these flexible 
pipelines 3 to the storage and production surface structure 1. 
With reference to FIG. 29, the bottom end 9 of the riser 8 may be fitted 
with a pivot 125 provided at the bottom end of a connection piece 35, 
which pivot 125 has a part which protrudes from the piece 35 and has 
bearing faces 128, capable of coming opposite faces 127 of a connecting 
piece provided at the top part 12 of the anchor 11, which connecting piece 
delimits an opening or notch 124, inside which the part 35 of the 
connecting means can be engaged through a movement of more or less 
horizontal translation, while the pivot or stud 125 engages in the cavity 
126 that extends under the opening or notch 124; as illustrated in this 
FIG. 29, the anchor 11 is provided in its top with a pipe 122 for 
temporary connection to a pump allowing the cavity delimited by the 
bell-shaped anchor 11 to be partially evacuated. 
FIG. 29 illustrates, in diagrammatic perspective, the base of the riser and 
the top of an anchor, before they are secured together. 
Said syntactic material, which is made up of microspheres or macrospheres 
in a matrix of plastic resin such as epoxy resin, polyurethane resin or 
polypropylene resin, can be obtained from plastics traders and 
manufacturers suchc as BTMI (france), Balmoral Marine (UK), or Emerson 
Cuming (USA).