A self standing marine riser is provided which comprises a base, a riser column, a flexible joint between the base and the riser column, and means for providing a loose coupling between the top of the riser column and a vessel, rig or platform on the surface above the location of the riser. The riser column comprises an upper column section which includes at least one buoyancy chamber, and a lower, relatively slender column section. The riser includes, or is adapted to support, at least one conduit for the conveyance of a fluid (e.g. oil or gas) or a control line. The buoyancy provided by the upper section of the riser column is preferably variable, and this facilitates the connection and use of the riser. The riser may be used for drilling operations or for production operations. By employing a riser in accordance with the invention, it is not necessary to use large riser tensions in order to maintain the position and structural integrity of the riser in deep water and rough weather.

BACKGROUND OF THE INVENTION 
This invention relates to a self-standing marine riser suitable for use in 
drilling, and in semi-submersible production operations and with a 
dynamically positioned oil/gas production ship, a chain moored ship with a 
spindle or with a tension leg platform. 
SUMMARY OF THE INVENTION 
According to the present invention, there is provided a self-standing 
marine riser which comprises a base, a riser column, a flexible joint 
between the base and the riser column, and means for providing a loose 
coupling between the top of the riser column and a vessel, rig or platform 
on the surface above the location of the riser, characterised in that (1) 
the riser column comprises a lower, relatively slender column section and 
an upper column section which includes at least one buoyancy chamber, and 
(2) the riser includes, or is adapted to support, at least one conduit for 
the conveyance of a fluid. The fluid can be oil, gas, water, or drilling 
mud. Optionally there may be provided a conduit for conveying solid 
objects, such as tools, from the top of the riser to the base. One or more 
control lines (e.g. electrical or hydraulic lines) may be housed in the or 
one of the conduits. A flexible joint may be provided at the top of the 
riser column between the column itself and a riser bundle connecting with 
the surface structure and through which the conduit for the conveyance of 
a fluid passes. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
It is preferred that the buoyancy provided by the upper column section 
should be variable. This arrangement facilitates the emplacement of the 
riser and its loose coupling to a surface vessel or structure. The 
buoyancy chamber(s) are advantageously such as to enable neutral buoyancy 
to be achieved; in preferred embodiments of the invention, positive and/or 
neutral buoyancy can be achieved for the riser alone, submerged and 
unattached to the surface vessel, or for the combination of the riser and 
the means by which it is loose coupled to the surface vessel or structure, 
or when coupled to the surface vessel or structure. This loose coupling is 
advantageously effected by chains which are removably attached to the top 
of the riser column. Conveniently there can be four such chains when the 
riser is secured to a semisubmersible rig or platform. If the riser is 
secured to a ship, there may be two or four such chains attached to a 
swivelling ring to allow for azimuth variations. 
The riser can include one or more pipes attached to the exterior of the 
riser column at least in the region of the lower section thereof. One or 
more flexible hoses may be provided at the lower end of the riser to 
connect the or each of said pipes to a wellhead or a production outlet at, 
or in the vicinity of, the base of the riser. 
The base itself can be a gravity base or a piled base. Generally, the base 
will be aligned next to or positioned around a sea bottom drilling 
template. If the base is a piled base, it is preferable to install the 
base at the same time as the drilling production template, for reasons of 
wellhead safety. The riser column and flexible joint may then be linked to 
the base with a connector on completion of the well drilling. 
The riser is self-standing and buoyant when standing alone and submerged. 
Two or more wirelines are preferably attached between the upper column 
section and the base. These initially are used as guideline wires to 
emplace the riser and connect it to the base; subsequently when terminated 
and fastened to the upper section of the riser after its emplacement and 
attachment to intermediary points and to the base they act as safety wires 
to avoid accident should the riser break, serving to prevent the riser 
accelerating to the sea surface. The necessary slack in the wires to allow 
freedom of riser column angular movement is advantageously taken up by 
weighted lever devices at the base. In preferred embodiments the riser is 
chained by four chains to a semisubmersible structure through pontoon 
mounted fairleads or to a ship either through spaced hull-mounted 
fairleads (when the ship is dynamically positioned over the riser) or in 
the case of a spindle-moored ship through fairleads which form part of the 
spindle structure. Each chain may be connected to the riser via a swivel 
chain fastener or fasteners that are part of a swivelling ring located 
about the riser top. The first arrangement is preferred for connection to 
semisubmersibles and to spindle-moored vessels while the second is 
preferable for connection to ship-shaped surface structures which are 
dynamically positioned and must weathervane. Preferably, the chains can be 
"quick released" from the riser, the action necessary to achieve this 
being effected on the semi-submersible platform or on the vessel to which 
the riser is loose coupled. 
The conduits for conveying fluids, e.g. oil or gas, along the riser can 
terminate in a submerged riser top manifold/stabbing block. The connection 
between the manifold/stabbing block and the production facility should be 
of the quick-release type, so that in an emergency the well(s) may be 
shut-in, after which the connection to the riser conduit(s) may be 
"quick-released" from the top of the submerged riser, followed by "quick 
release" of the chains from the submerged riser top. The connection from 
the surface vessel to the riser manifold/stabbing block can be in the form 
of a flexible riser bundle. When the self-standing riser is emplaced, it 
will generally be fully submerged. With an arrangement such as that just 
described, the tensioned riser bundle will be supported by tensioners 
which compensate for vessel motion, draught and changes in sea level. 
With the preferred structure described above, if the self-standing 
production riser should break loose while still connected to a 
semi-submersible or vessel production facility, the riser will not float 
to the surface because of the restraining safety wires and the weight 
imposed by the catenary chains. 
In general, the riser will maintain a substantially upright configuration. 
Movement of the vessel, rig or platform to which it is loose coupled as 
well as tidal and current effects may result in the riser moving away from 
the vertical. The riser will accommodate a maximum of 15.degree. tilt from 
the vertical, but it is preferred that the riser should not deviate from 
the vertical by more than 10.degree.. Under normal operating conditions, 
the maximum inclination of the riser is expected to be about 7.degree. or 
less from the vertical. 
A riser constructed in accordance with the present invention does not 
require a complicated tensioning system to hold it in place. The buoyancy 
and stiffness provided in the submerged riser mean that the riser is not 
subjected to stresses as severe as those normally associated with an 
equivalent length tensioned riser. Furthermore, the design is such that if 
the riser breaks free at the bottom while connected to a production 
facility, it will not inevitably come to the surface and/or collide with 
the production facility. Also, if a piled or gravity base is employed, it 
can straddle the well head template (without contacting it) thereby 
providing protection for the well "trees".

Referring now to the drawings, the riser 1 shown in FIGS. 1 and 2 is 
loosely coupled to a semi-submersible production platform 2 via a 
plurality of chains 3. There are four such chains in the embodiment 
illustrated in the drawings. As illustrated, the chains are attached to 
the inboard area of the pontoon; alternatively they may be attached to the 
outboard area. The riser 1 comprises a piled or gravity base 4, e.g. a 
piled steel base which can have two basic configurations. In the first, it 
is mounted over but is not in contact with a circular wellhead template 
(not shown). In the second, the base is connected to one end or to the 
middle of a rectangular or square wellhead template. The circular template 
can accommodate ten wells with one spare slot in its presently envisaged 
form. The number of wells which can be accommodated depends on the 
capability of the riser and manifold system to handle the fluids. In the 
circular wellhead template, the production trees are protected by the base 
4. The riser may also be connected to a satellite production tree or trees 
or a separate manifold well template adjacent the base 4, as indicated by 
line 5 in FIG. 1. 
The riser column comprises a lower slender part 6 connected to the base 4 
by a universal, ball or flex joint 7. Pipe conduits 8 are mounted on the 
outside of lower section 6 of the riser column. Each of conduits 8 is 
connected at its lower end to a flexible hose 9 which in turn is connected 
to the well production tree 10. The lower part 6 of the riser column 
occupies the greater proportion of the total length of the riser. The 
upper portion 11 of the riser column includes both a fixed and a variable 
buoyancy system. Conduits 8 pass through the interior of upper riser 
column section 11. 
At the top of the riser column, there is a riser top manifold/stabbing 
block 12 by means of which a flexible riser bundle 13 may be connected to 
the upper termination of conduits 8. 
The length of each of chains 3 is adjustable. Under normal operating 
conditions, each chain will generally have substantially the same length. 
The connection between the chains 3 and riser 1 is effected at swivelling 
chain fasteners 14 which are attached to the outside of upper riser column 
section 11 at the top part thereof. The length of each chain catenary 
between connectors 14 and the pontoon fairleads of the production platform 
2 will normally be in the range from 20 to 60 meters, preferably about 45 
meters; the length may occasionally be as little as 10 meters. The loose 
chain connections may be made either to the insides or to the outsides of 
the pontoons, and the chain will generally run through fairleads whose 
positions are such as to afford the optimum scope ratio for control of the 
submerged riser. The scope ratio will depend on environmental conditions, 
rig layout, depth of the riser top below sea level and pontoon depth for 
an optimum operation. 
Referring now to FIG. 3, there is shown a mooring arrangement suitable for 
use when a self-standing production riser in accordance with the present 
invention is loosely coupled to a dynamically-assisted vessel, i.e. a ship 
or barge whose mooring position is maintained with dynamic assistance. The 
two chains 3 are attached to the upper section 11 of the riser column at a 
slewing ring 16 which is fitted about the top part of column section 11. 
The chains 3 pass over chain sheaves 17 which preferably can be raised or 
lowered by a predetermined amount in order to adjust the 
vertical/horizontal chain catenary ratio to the optimum for any given 
circumstance. The mooring chains then pass upwardly into chain tubes 18 
within the vessel 20. Alternatively, if four chains 3 are employed, there 
may be four chain tubes 18 positioned on the outside of the hull of the 
vessel. The flexible riser bundle 13 passes through a moonpool 19 and 
terminates at a fluid swivel 21 to which tensioners 22 are connected via 
cables 23. A guide frame 24 holds fluid swivel 21 in position in a 
horizontal plane, and also functions to rotate it. 
Referring now to FIGS. 4 and 5, an arrangement is shown for connecting a 
free-standing marine riser in accordance with this invention to a vessel 
having a turret/chain mooring arrangement. In this case, the mooring chain 
3 can be connected to the top of riser section 11 either by two, three or 
four swivelling chain fasteners or by chain fasteners which are part of a 
slewing ring attached to the outside of upper riser section 11. The 
arrangement illustrated in FIG. 4 shows the first of these two 
possibilities, there being two swivelling chain fasteners 14 attached to 
the outside of upper riser column section 11. The choice between these two 
possible configurations will be decided according to the method of 
equipment installation relative to acceptable weather conditions. The 
outboard ends of mooring chains 3 are connected to wires 30 which pass 
over fairleads 31 held by spreader arms 32. The length of each chain 
catenary between fasteners 14 and the first of the fairleads 31 will 
generally be about 23 to 27 meters in the presently preferred 
arrangement. The spreader arms 32 are structurally connected to a 
cylindrical body 34 forming part of the vessel 33, the interior of body 34 
constituting a spindle or turret. This turret also houses winches and 
mooring line equipment (not shown) and anchoring windlasses one of which 
is shown at 35. When the mooring lines are in place, the turret 34 remains 
on a consistent heading while the vessel itself can weathervane about the 
turret. 
The flexible riser bundle 13 passes through turret 34 and terminates at a 
multi-fluids swivel 21a mounted above the vessel deck. This swivel is held 
in a gimballing table guided by frame 24 attached to riser tensioning 
wires 23 which terminate in tensioning means, such as pneumatic or 
hydraulic tensioners or weights 22. 
The schematic arrangement shown in FIG. 5 illustrates the positioning of 
four double-drum mooring winches or windlasses (MW) mounted on top of 
turntable 36 which is, in effect, the topmost part of turret body 34. 
The riser 1 can be used in deep water conditions, for example at depths of 
90 meters (300 feet) or greater. 
An emergency release system (not shown) is provided to enable chains 3 to 
be separated from riser 1 quickly. The system can comprise a wire attached 
to a locking arm which, when the wire is pulled taut, will cause a locking 
pin holding a respective chain to connector 14 to shear and allow the 
chain to fall free of the riser. 
The loose coupling between riser 1 and platform shown in FIGS. 1 and 2 may 
be effected as follows. Initially, the buoyancy of the riser is adjusted 
so that it is slightly positive. With the riser in this condition, the 
semisubmersible is moored with its moonpool centered over the riser. When 
all is ready for effecting the connection, the buoyancy in upper section 
11 of the riser column is increased and the chains 3 are lowered from the 
semisubmersible for connection to the top of riser 1. This can be done by 
attaching strayline wires to a point a given number of links above the 
hanging chain ends, and paying out the chains as the wires are pulled 
towards the moonpool. The end links or shackles of the chains will be 
locked into the riser swivelling chain fasteners 14, opposing chains 
preferably being connected simultaneously. The strayline wires may then be 
let out and detached from the chain; they can later be used as guidelines 
for guiding the riser sections from the surface to the stabbing manifold 
block at the top of the submerged riser. Next, the chains will be 
tightened to give the desired catenary chain lengths. When the first two 
chains are connected, the procedure will then be repeated for the other 
two opposing chains. When all four chains are connected, the combined 
weight of the coupled chains, the riser and the maximum vertical wave 
force is buoyed, which results in an overall marginally positive buoyant 
system. 
The loose coupling between riser 1 and the vessel 20 shown in FIG. 3 may be 
effected as follows. The vessel 20 is positioned with its moonpool 
centered over the riser 1. When all is ready for effecting the connection, 
the buoyancy in the upper section 11 of the riser column is increased and 
the chains 3 are lowered from the sides of the vessel through 
bilge-mounted fairleads for connection to the top of the riser. The 
procedure for effecting this connection may be substantially the same as 
that described above with reference to FIGS. 1 and 2. However, instead of 
attaching the chains to swivelling chain fasteners 14, they are attached 
to connectors mounted on the slewing ring 16 which is capable of rotation 
about the top of riser section 11. 
The coupling between riser 1 and the turret/chain moored vessel shown in 
FIGS. 4 and 5 may be effected generally as described above with reference 
to FIGS. 1 and 2. The flexible riser bundle 13 is connected to the 
submerged riser section 11 at a stabbing block manifold show 
diagrammatically at 13a in FIG. 4. Sections of the flexible riser bundle 
13 pass through the turret 34 to the deck area of the vessel, where a 
multi-fluid swivel 21a is provided. The top section of swivel 21a is 
affixed to a gimballed plate forming part of the frame 24 and having wire 
connections 23 to tensioning means e.g. weights 22 which are suspended via 
pulleys from a supporting frame 37. This frame is also used to pull and 
lower the riser sections as required. Hard piping or hose 25 having 
terminal swivel joints are connected to the multifluid swivel 21a, there 
being a separate piping line for each fluid which is carried in the 
system. The hard piping is arranged so as to allow the heave of the vessel 
to be accommodated. The pitch and roll of the vessel, and the angular 
offset of the riser sections caused by vessel movement, is accommodated by 
the gimballed plate which forms part of the frame 24. Where line 25 is in 
the form of hard piping, it may advantageously be guided by a sleeve-like 
structure for support (such as that shown in FIG. 3), since a certain 
amount of torque at the multi-fluid swivel will develop with change of 
vessel heading. To protect the riser sections from torque build-up, 
pressure sensing transducers may be employed in conjunction with fluid 
swivel turning motors mounted on the multi-fluid swivel 21a; these are not 
shown in the drawings. 
An alternative method of attaching the mooring chains 3 to the top of the 
submerged buoyant riser will now be described with reference to FIG. 6. In 
this figure, four chains 3 are attached to a circular plate 40 which is 
provided with three or four tapered sockets 41. The plate 40 is suspended 
by wires 42 (conveniently the same in number as sockets 41) which wires 
may be passed through a vessel chain tube or turret as shown in FIGS. 1 to 
5. The top of the riser section 11 is formed with an appropriate number of 
upstanding, fluted posts 43 which are designed to mate with the sockets 
41. The posts 43 may be mounted on a slewing ring (not shown in FIG. 6). 
As plate 40 is lowered, the fluted posts 43 penetrate into sockets 41 from 
which water is forced out. This evacuation of water from within the 
sockets 41 causes an automatic cushioning effect which increases in 
magnitude as the plate 40 approaches surface 44 of riser section 11. This 
passive cushioning effect assists the steady location of the plate 40 onto 
the riser section 11. When the fluted posts 43 are fully engaged in 
sockets 41, plate 40 may be locked hydraulically to the top of the 
submerged riser. 
After connection in the manner just described, the riser bundle with its 
centering probe and a hydraulic connector, flexible joint and riser 
flowline tubes, is lowered and positioned, locked and tensioned, for 
example by use of the riser tensioners 22 as illustrated in FIGS. 3 and 4. 
Buoyancy in the submerged riser section 11 is adjusted when the riser 
bundle is connected thereto. 
A modified arrangement may be adopted at the lower end of the submerged 
riser in order to facilitate well entry through the top of the well 
tree(s). In this modification, a circular well template is provided inside 
the riser base and the flowlines connected from each tree pass up along a 
bell-shaped, gimballed structure attached to the lower riser section at a 
point high up enough to allow as slight an angle of flowline deviation as 
possible; the gimballed structure is also attached low enough on the riser 
so as not, with changing riser angles, to cause too much deflection of the 
flowlines. A flex joint will be provided atop each well tree in order to 
accommodate the changing flowline angles caused by movement of the 
bell-shaped structure as it follows the riser deflections.