Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 022112~1 1997-07-23
MOORING SYSTEMS
The invention relates to a system for mooring a vessel, and is
particularly concerned with a system for mooring a vessel at an
offshore oil production site at which at least one riser from a
production unit is to be connected to the vessel. The vessel may
be a tanker which is to be loaded with oil from the production
unit, or in the case of a field with limited requirements for
production and control lines the vessel may be a production
vessel with processing and well control facilities.
The usual method of mooring a tanker or other vessel at an
offshore oil production site has been to use several separate
anchor lines either directly to the vessel's standard mooring
equipment, to a vessel turret arrangement or to a separate
floating mooring buoy which is connected to the vessel by a
further line. In each case each of the anchor lines is secured
at its proximal end to the vessel or the buoy and is anchored at
its distal end to the sea-bed. The anchor lines are in
equilibrium with each other in order to limit as much as possible
the excursion of the vessel on the surface and hence reduce the
risk of damage to risers or other connections to the production
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unit, particularly if one of the lines should break or drag.
However, with the advent of more robust production equipment,
particularly with regard to loading hoses, it is considered that
the requirement for a mooring system to limit the surface
excursion of a vessel as much as possible is no longer quite so
rigorous, thus permitting the design and use of alternative
mooring systems.
Accordingly, the invention provides a mooring system comprising
a plurality of anchor lines each having means at its distal end
for anchoring said distal end of the anchor line to the sea bed,
an anchor line assembly unit through which passes at least one
of the anchor lines (termed a passing anchor line) and to which
each of the other anchor lines (termed fixed anchor lines) is
secured at its proximal end, and a buoyancy device to which the
proximal end of the or each passing anchor line is attached
either directly or indirectly, the or each passing anchor line
having a stop for limiting the travel of the anchor line assembly
unit along the line in a direction towards its distal end.
Preferably the mooring system will also include a swivel unit
comprising a first part to which is secured a mooring line for
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securing a vessel to the mooring system, and a second part which
is rotatable relative to the first part and to which the proximal
end of the or each passing anchor line is secured, the buoyancy
device being attached to the swivel unit or to the mooring line.
The number of passing anchor lines is preferably the same as the
number of fixed anchor lines when there are an even number of
anchor lines, and one less or one more than the number of fixed
anchor lines when there are an odd number of anchor lines.
Normally from four to eight anchor lines will be the right
choice, although three can sometimes be used, and more may be
provided if necessary, up to a maximum of twelve. Usually, the
anchor lines will be distributed substantially evenly around the
assembly unit.
The fixed anchor lines will all be located in one sector of a
circle centred on the assembly unit, and the passing anchor lines
will be located in the complementary sector of the circle. The
two sectors will generally be approximately 180~ sectors,
although the fixed anchor line sector may be made as large as
possible without causing the fixed anchor lines to prevent the
assembly unit being lifted freely to the surface.
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The anchor lines may be formed by chains or by wire cables or
hawsers. Generally, however, the fixed anchor lines will be
chains, and the passing anchor lines are either chains along
their whole length or are formed by a chain from the distal end
to the stop and by a wire cable or hawser from the stop to the
proximal end.
The assembly unit is preferably provided with a separate fixing
site for each of the fixed anchor lines and a separate passage
for each of the passing anchor lines, each passage converging in
a direction towards the proximal end of the respective passing
anchor line. The stop on each passing anchor line may be able
to enter at least partly into the respective passage in the
assembly unit but not to pass through it. Preferably each
passage and the stop on the respective passing anchor line are
correspondingly frusto-conical in shape, and preferably the
longitudinal axes of the passages are substantially parallel to
each other.
When the mooring system is installed, the anchor lines will
radiate outwardly from the assembly unit with their distal ends
anchored to the sea-bed such as by conventional anchors, suction
anchors, piles or any other suitable anchoring means. When the
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mooring is not in use, the anchor lines and possibly also the
assembly unit will rest on the sea-bed, with the proximal end
portions of the passing anchor lines, and the swivel unit if
provided, suspended from the buoyancy device which floats at an
equilibrium depth determined by the size and buoyancy of the
device and the weight of the suspended portions of the system.
The system will preferably include a messenger line for enabling
a vessel to locate and draw in the mooring system, the messenger
line having at least one surface floating marker buoy. The
messenger line may be attached to the swivel unit or to the
buoyancy device, but usually it will be attached to the end of
the mooring line which is attached to the swivel unit and may
also have a sub-surface buoy for supporting the weight of the
mooring 1 lne .
When a vessel is to be moored the messenger line is picked up and
winched aboard, pulling after it the mooring line and thereby
raising the swivel unit, the buoyancy device and the proximal
ends of the passing anchor lines so that the stops on the passing
anchor lines lift the assembly unit and the proximal ends of the
fixed anchor lines away from the sea-bed. The messenger line and
the mooring line will usually be winched aboard at the bow of the
vessel, which will preferably be provided with a cantilever
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structure for keeping the lines clear of the vessel hull,
although alternative locations and arrangements can be used.
When the swivel unit has been raised to the desired position,
which may be to the cantilever structure, the mooring line will
be secured to the deck of the vessel, for example by a chain
stopper if the mooring line is a chain, and the vessel is thus
moored.
If the mooring is at an oil production site and is to be used by
a tanker or a production vessel, the mooring system will
preferably have a swivel unit including a third part, which may
be integral with the second part, which is rotatable relative to
the first part and to which is connected at least one riser hose
from an oil production unit on the sea-bed, and the first part
has connected to it at least one continuation hose for connection
to the moored vessel, the first and third parts permitting fluid
communication between the or each riser hose and a respective
continuation hose whatever the relative angular positions of the
first and third parts. When the vessel has been moored, the
continuation hoses from the swivel unit are connected to
respective pipework on the vessel and loading or processing can
be commenced after carrying out appropriate checks on the system
and the various connections. The swivel unit, which must be able
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to accommodate the anchoring forces between the mooring line and
the passing anchor lines, serves to prevent the riser hoses or
lines from twisting or rubbing against the anchor lines as the
vessel weathervanes about the mooring.
An important advantage of the invention is that all operations
relating to installation and retrieval of the mooring system can
be performed relatively easily from the deck of a simply equipped
surface vessel. When installing the system, all the anchors and
attached anchor lines are first laid and tensioned, normally by
pulling two opposing lines against each other. The anchor lines
in one sector, which are to form the passing anchor lines, then
have the assembly unit stops fitted to them, and the proximal
ends of the anchor lines in the opposite sector are secured to
the assembly unit. The assembly unit is then threaded on to the
proximal ends of the passing anchor lines and allowed to slide
down them to the stops, preferably under the control of a control
wire attached to the assembly unit. The swivel unit is then
attached to the proximal ends of the passing anchor lines, the
buoyancy device and the messenger line are also attached as
necessary, and the free end of the control wire is connected to
the swivel unit. Finally, the riser line or lines are connected
to the swivel unit and the unit is released to allow the mooring
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to sink to its rest position with the assembly unit on or just
above the sea-bed. It is the extra length of the passing anchor
lines and the sliding arrangement for the assembly unit which
enable the system to be installed from the surface. If this
extra length of each passing anchor line is too short for easy
installation and retrieval, the passing lines can be lengthened
temporarily during the installation and retrieval processes by
attaching additional lengths of line. The extra length of the
passing anchor lines also makes the mooring system a slack system
and thereby a soft acting system as seen from the moored vessel,
the assembly unit moving in a circle about the central axis of
the mooring as the vessel weathervanes around the mooring.
When retrieving the system, the messenger line and mooring line
are first winched in to raise the swivel unit to the retrieval
vessel. The control line is then disconnected from the swivel
unit and used to winch the assembly unit up to the vessel. The
anchor lines can then be retrieved.
As will be appreciated, a mooring system in accordance with the
invention can be assembled using standard, readily available
components, except for the anchor line assembly unit and the
swivel unit, which are in fact novel components in their own
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right forming important additional aspects of the present
invention.
Embodiments of a mooring system in accordance with the invention
will now be described, by way of example, with reference to the
accompanying diagrammatic drawings, in which:-
Figure 1 shows a side perspective view of one embodiment of themooring system installed in the vicinity of an oil production
unit, the system including an oil loading hose rising from the
production unit and the system being shown at rest and not in
use;
Figure 2 is a view similar to Figure 1, but showing the mooring
and loading system in operation with the moored vessel lying in
two different positions;
Figure 3 is a perspective view of an anchor line assembly unit
of a mooring system in accordance with the invention having six
anchor lines;
Figure 4 is a side view, partly in section, of one embodiment of
a swivel unit for a mooring and loading system in accordance with
CA 022112~1 1997-07-23
the invention; and
Figure 5 is a view similar to Figure 4 but showing an embodiment
of a swivel unit for a mooring and production system in
accordance with the invention.
The mooring and loading system illustrated in Figures 1 and 2
comprises four anchor lines 1, 2, 3 and 4, an anchor line
assembly unit 5 to which two of the anchor lines 1, 4 are secured
at their proximal ends and through which the other two anchor
lines 2, 3 pass, a swivel unit 6 to which the proximal ends of
the two passing anchor lines 2, 3 are attached, a buoyancy device
7 attached to the swivel unit, a mooring line 8 also attached to
the swivel unit, and a messenger line 9 attached to the mooring
line. The system further comprises an oil loading hose 10 which
is connected at its distal end to an oil production unit (not
shown), which may be on the sea-bed or a surface unit, and is
connected at its proximal end to the swivel unit 6, and a
continuation hose 11 which is also connected to the swivel unit.
As will be described later in more detail, the loading hose 10
and continuation hose 11 are connected to parts of the swivel
unit which are rotatable relative to each other and which permit
fluid communication between the two hoses whatever the relative
CA 022112~1 1997-07-23
11
positions of the two parts.
The four anchor lines 1, 2, 3 and 4 are laid so that they extend
outwards along the sea-bed from the assembly unit 5 at
substantially equal angular intervals, and each is provided at
its far or distal end with an anchor (not shown) or other
suitable means for anchoring the end of the anchor line to the
sea-bed. The two passing anchor lines 2, 3 are located in one
180~ sector and the fixed anchor lines l, 4 are located in the
opposite 180~ sector. Each of the two passing anchor lines 2, 3
is fitted, on the distal end side of the assembly unit 5, with
a stop member 12 (See Figure 3) which will not pass the assembly
unit 5 and hence limits the travel of the assembly unit along the
passing anchor lines 2, 3 towards their distal ends.
The assembly unit 5 will generally comprise a body having as many
fixing sites as there are fixed anchor lines for the attachment
of the fixed anchor lines, and as many through passages as there
are passing anchor lines for the passing anchor lines. An
example of an anchor line assembly unit for use in a mooring
system in accordance with the invention having six anchor lines
is illustrated in Figure 3, and comprises three frusto-conical
tubular members 13, 14, 15 which are open at their upper and
CA 022112~1 1997-07-23
lower ends to define through passages for the three passing
anchor lines 2, 3, 16, and which are welded or otherwise fixed
at their narrower upper ends to three of the corners of a
substantially square upper support plate 17. The axes of the
passages defined by the frusto-conical tubular members 13, 14,
15 are substantially parallel, and the wider lower ends of the
members are welded or otherwise fixed to a lower support plate
18 to form a rigid structure. The stop member 12 in each of the
passing anchor lines 2, 3, 16 is preferably frusto-conical in
shape and dimensioned to fit into the respective tubular member
13, 14, 15. In Figure 3 the tubular members 14, 15 are shown
with the stop members seated within them, but for illustration
purposes the stop member 12 in the anchor line 2 is shown axially
displaced from the tubular member 13. The assembly unit also
comprises a rib plate 19 which is disposed vertically between the
upper and lower support plates 17, 18 and extends diagonally
between the intermediate tubular member 15 and the fourth corner
of the upper plate 17, the rib plate 19 being welded to both
support plates 17, 18 and to the tubular member 15. Two further
rib plates 20, 21 are similarly disposed between the upper and
lower support plates 17, 18 so as to mutually intersect with the
rib plate 19, the plate 20 extending from the tubular member 13
to a position between the outer edge of the plate 19 and the
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tubular member 14, and the plate 21 extending from the tubular
member 14 to a position between the outer edge of the plate 19
and the tubular member 13. Each of the rib plates 19, 20, 21 has
a portion 22, 23, 24 projecting below the lower plate 18 at its
edge remote from the respective tubular member to form a lug, and
each of these lugs 22, 23, 24 is provided with an eye for
attachment of the proximal end of a respective fixed anchor line
1, 4, 25 by means of a shackle. A lug 26 having an eye is also
provided on the upper surface of the upper plate 17 near its
fourth corner for the attachment of a control wire 27, by means
of a shackle.
In this example the fixed anchor lines 1, 4, 25 are formed by
chains, and the passing anchor lines 2, 3, 16 are each formed by
a chain from the distal end to the stop member 12 and by a wire
hawser from the stop member to the proximal end which is attached
to the swivel unit. If preferred, however, the passing anchor
lines may be formed wholly by wire hawsers or wholly by chains
and the fixed anchor lines may be formed by wire hawsers.
The construction of a swivel unit 6 which may be used in the
mooring and loading system described with reference to Figures
1 and 2 is shown in Figure 4. As can be seen, the swivel unit
CA 022ll2~l l997-07-23
14
6 comprises a first part 30 formed by a substantially cylindrical
central rod-like member 31 and a cylindrical sleeve 32 bolted or
otherwise fixed to the central member 31 and closely surrounding
a portion of the central member near its upper end. The upper
end of the central member 31 projects from the sleeve 32 and is
formed with a lug 33 having an eye for the attachment of the
mooring line 8.
At its lower end the central member 31 iS surrounded by a second
part 34 of the swivel unit, the second part 34 being formed by
an annular member 35 co-axially surrounding the central member
31 and an end cap 36 which is bolted and sealed to the annular
member 35 over the lower end of the central member. The second
part 34 iS axially retained on the central member 31 and is
mounted so as to be rotatable about the central member by means
of bearings 37 and 3 8. The upper end of the annular member 35 iS
closed by a ring 39 which is sealed with respect to the central
member 31 and the annular member 35. The lower face of the end
cap 36 iS provided with a lug 40 having an eye for the attachment
of means for fixing the proximal ends of the passing anchor lines
2, 3 to the second part 34 of the swivel unit.
The swivel unit further comprises a third part 41 in the form of
CA 022ll2~l l997-07-23
a cylindrical sleeve 42 which co-axially surrounds the central
portion of the first part 30 and is rotatably mounted thereon by
means of bearings 43 and 44. The sleeve 42 is axially retained
in position relative to the first part 30, and the upper end of
the sleeve 42 overlaps the lower end of the sleeve portion 32 of
the first part. Opening radially through a port 45 in the sleeve
42 iS one end of a pipe elbow 46 which is welded or otherwise
fixed to the sleeve 42 and which has a coupling flange 47 at its
outer end for the attachment of the riser hose 10. Similarly,
a pipe elbow 48 iS welded or otherwise fixed to the sleeve 32 of
the first part 30 So as to open radially through a port 49 in the
sleeve 32, and the outer end of the pipe elbow 48 has a coupling
flange 50 for the attachment of the continuation hose 11. The
periphery of the central member 31 of the first part 30 iS formed
with two axially spaced circumferentially extending grooves 51
and 52 which register with the ports 45 and 48 respectively and
which communicate with each other via axial slots 53 in the
periphery of the central member 31. In this way the riser hose
10 connected to the pipe elbow 46 will always be in communication
with the continuation hose 11 connected to the pipe elbow 48
irrespective of the relative angular positions of the first and
third parts 30, 41 of the swivel unit.
CA 022ll2~l l997-07-23
16
When the mooring system is installed but idle (i.e. not in use),
the anchor lines 1, 2, 3 and 4, and possibly also the assembly
unit 5, will lie at rest on the sea-bed, with the swivel unit 6
and at least a portion of the passing anchor lines 2, 3 suspended
above the sea-bed by means of the buoyancy device 7. Usually
however, sufficient buoyancy will be provided to ensure that the
whole of the passing anchor lines 2, 3 from the assembly unit to
the swivel unit, and preferably also the assembly unit itself,
will be suspended above the sea-bed as shown in Figure 1. The
upper end of the control wire 27 attached to the assembly unit
will be secured to the bottom of the swivel unit. The loading
hose 10 will preferably be provided with buoyancy elements 54
along at least part of its length in order to keep most of the
hose off the sea-bed, and the continuation hose 11 will either
hang down from the swivel unit or it may be attached to a
buoyancy element 55 as shown. The mooring line 8, which will
usually consist of a chain pigtail, may be provided with the sub-
surface buoyancy element 55 for holding up the end of the mooring
line, and the messenger line 9 which is attached to the mooring
line 8 will generally be provided with at least one, and
preferably at least two, surface floating marker buoys 56 so that
a portion of the line 9 will float on the surface of the sea to
facilitate pick up of the mooring. One of these marker buoys 56
CA 022112~1 1997-07-23
may have a flashing light unit and another may have a radar
reflector to further facilitate location and pick up of the line
When a tanker 57 is to be moored and loaded with oil from the
production unit, the floating portion of the messenger line 9
between the marker buoys 56 is located and picked up from the
surface of the sea by suitable means on the tanker, and is
winched aboard over a fairlead on a cantilever structure 58
provided at the bow of the tanker for keeping the various lines
of the mooring and loading system clear of the hull of the
vessel. The mooring line chain pigtail 8 follows the messenger
line 9 aboard until the swivel unit 6 is raised to a desired
position, whereupon the mooring line 8 is fastened to the vessel
by means of a suitable chain stopper on the deck. In Figure 2
the swivel unit 6 is shown raised to the cantilever structure 58,
but in practice it may be preferred not to raise the unit 6 so
far. The free end of the continuation hose 11 attached to the
first part 30 of the swivel unit 6 is then pulled aboard and
coupled to a loading manifold on the deck. In raising the swivel
unit 6, the proximal ends of the passing anchor lines 2 and 3 are
carried with it and, as a result of the engagement of the passing
anchor line stop members 12 with the assembly unit 5, the unit
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5 and portions of all of the anchor lines 1, 2, 3 and 4 on the
distal side of the assembly unit are lifted from the sea-bed as
shown in figure 2. It should be noted that the length of the
passing anchor lines 2, 3 between the assembly unit 5 and the
swivel unit 6 will be sufficient to ensure that the assembly unit
will always remain below the bottom of the vessel in order to
prevent fouling of the anchor lines by the vessel.
Because the mooring system is a slack anchoring system, the
tanker will be free to weathervane around the mooring in a circle
having a radius determined by the slack in the mooring system,
and figure 2 illustrates the positions adopted by the mooring
system when the vessel is at opposite points of the permitted
weathervaning circle. As the vessel weathervanes, the central
first part 30 of the swivel unit, to which the mooring line 8 to
the vessel is attached, is able to rotate relative to the second
and third parts 34 and 41 to which the anchor lines and the
loading hose are attached, so that the swivel unit 6 acts to
preserve the angular positions of the anchor lines and the
loading hose relative to each other and to the sea-bed. However,
it is possible that power assistance may need to be provided in
order to ensure that the necessary relative rotation of the
swivel unit parts occurs in order to maintain the relative
CA 022112~1 1997-07-23
positions of the anchor lines and loading hose. This of course
ensures that loading can continue irrespective of changes in the
position of the vessel, and also minimizes the risk of damage to
the components of the system.
The length of the loading hose 10 will need to be sufficient to
accommodate excursion of the vessel to the most extreme position
- which is defined as the furthest position to which the vessel
will move under the most hostile environmental operating
conditions and when the most inconvenient anchor line is broken.
The loading hose may be any suitable marine hose when relatively
large flow capacities are to be catered for, but for more limited
flow capacities it would be preferred to use a hose of the Manuli
type.
Although a loading and mooring system has been described, it will
of course be understood that the loading hoses may be replaced
by a production riser system to connect a sea-bed well-head to
a moored production vessel. Such a system will involve several
hoses and lines to accommodate the well flows and controls, and
generally these will be bundled together along most of their
length between the well-head and the swivel unit. At the swivel
unit at least the larger hoses will be separately connected to
CA 022112~1 1997-07-23
the unit for respective communication with continuation hoses
leading to the vessel, and an example of a swivel unit which can
accommodate two separate hose flows is shown in figure 5. As can
be seen, the swivel unit 60 is similar to the unit 6 shown in
figure 4, and corresponding components have been given the same
reference numerals and will not be described again. The main
differences are that the sleeve 42 of the third part 41 has an
additional pipe elbow 61 fixed to it and opening radially through
a port 62 into an additional peripheral groove 63 in the central
member 31, and the sleeve 32 of the first part 30 has an
additional pipe elbow 64 fixed to it and opening through a port
65 in the sleeve. In addition, the central member 31 is provided
with a central axial bore 66 which is blocked at each end, and
with which the peripheral groove 63 communicates via a radial
passage 67 and the port 65 communicates via a radial passage 68.
Thus, in use, the hose connected to the pipe elbow 46 will
communicate with the continuation hose connected to the pipe
elbow 48, and, separately therefrom, the hose connected to the
pipe elbow 61 will communicate with the continuation hose
connected to the pipe elbow 64.
