[0001] The invention relates to a vessel comprising at least two anchor lines connected
respectively at a substantially fixed position near the bow and near the stern of
the vessel.
[0002] From US-A-3,583,354 a bow-stern spread catenary mooring system is known in which
the vessel is maintained in a fixed position by means of four anchor lines, two of
which are connected to the bow of the vessel and two at the stern. By applying the
correct tension to these anchor lines it is possible in normal weather to maintain
the ship in a reasonably fixed position. According to US-A-3,583,354, during storm
conditions the stern anchor lines are slackened, such that the vessel can weathervane
around the bow through 90°.
[0003] From US-A-3,822,663, a weathervaning vessel is known, which is anchored to the seabed
by means of four anchor lines. The vessel is provided with a track which encircles
the hull of the vessel. A plurality of movable carriages are guided along the track,
one anchor line being connected to each carriage. The carriages can be placed at specific
points along the track, each carriage being equipped with a winch for accumulating
or discharging a respective anchor line. By moving the position of the carriages along
the track, and by selectively paying in or paying out the anchor lines, the vessel
can weathervane about a substantially fixed position point located between the bow
and the stern. The known anchoring construction however is relatively complex and
the anchor line arrangement covers a relatively large part of the seabed. Hence the
anchor lines may interfere with the operating area for the risers and leaves little
room for the anchoring spread of a drilling/workover rig when such a rig is anchored
next to, or in close proximity to the drilling vessel.
[0004] It is therefore an object of the present invention to provide for a vessel which
can weathervane within a predetermined area, in a controlled manner, using a relatively
simple anchoring system. It is a further object of the present invention to provide
a vessel comprising such a weathervaning anchoring system having a reduced anchoring
layout and leaving a relatively large seabed area for the risers and for the anchoring
spread of adjacent vessels such as drilling/workover rigs or shuttle tankers.
[0005] Thereto the vessel according to the present invention is characterised in that at
least one anchor line comprises a first line section extending to a connection point
located below keel level of the vessel and at least two branching anchor lines, each
connected with one end to the anchor line in the connection point and connected with
the other end to the sea-bed. With the above anchoring arrangement the vessel according
to the present invention can be easily accessed along its sides. Thereby other vessels
can easily approach and moor aside the vessel according to the present invention,
and sufficient space is available for location of for instance flexible risers.
[0006] By providing at least one anchor line with two branching anchor lines, a very stable
multiple point mooring system (at least a three point system) is provided which will
leave enough play for the vessel to weathervane through angles less than 90° in both
directions. Preferably both the bow and the stern anchor lines each comprise at least
two branching anchor lines.
[0007] In one embodiment, the anchor lines extend substantially vertically downward. Hereby
access of the vessel is further increased.
[0008] As used herein, the term "substantially vertical" shall mean a position from the
first anchor line section being inclined from the vertical position by no more than
45 degrees.
[0009] In another embodiment, a buoyancy member is connected near the branching anchor lines.
In this way, the stiffness of the anchor arrangement can be adjusted for varying the
weathervaning action.
The term "anchor line" as used herein comprises anchor cables, ropes, chains or combinations
thereof.
[0010] In one embodiment, at least one anchor line extends through a generally vertical
shaft extending through the hull of the vessel from deck level to keel level. In this
way the anchor lines are located below the vessel, and the bow and stern are easily
accessible as well.
[0011] Preferably a counterweight is connected to at least one of the anchor lines, such
that the tension in the vertically extending part can be adjusted and the stiffness
of the anchor line system can be varied for adjustment of the weathervaning action.
[0012] Another embodiment of a vessel according to the present invention comprises at least
two anchor lines connected respectively at a substantially fixed position near the
bow and near the stern of the vessel, at least one of the anchor lines being connected
to a substantially fixed-position pulling device for lengthening or shortening of
the anchor line, and is characterised in that the pulling device can vary the length
of the at least one anchor line such that both anchor lines are maintained in a tensioned
state, and the vessel can weathervane around the bow anchor line or the stern anchor
line.
[0013] The invention is based on the insight that during normal operating conditions, a
vessel will be subjected to wind and wave directions which are more or less constant
for larger time periods. During these substantially constant conditions, the vessel
only needs to weathervane through a relatively small angle such as between 0° and
90°. For these conditions, by properly tensioning and slackening at least one of the
anchor lines at the bow and stern of the vessel, an efficient and controlled weathervaning
action can be achieved, without the need of a complex system of displaceable winches.
[0014] In one embodiment the pulling device, such as for instance a winch system, according
to the invention varies the length of the anchor lines simultaneously. Variation of
the anchor lines can take place under the control of a mean position sensor which
actuates the winch system on the basis of the mean position of the vessel, the wind
or wave directions, etc. The change in one of the above environmental characteristics
will change the control signal and hence the stiffness of the anchor line system.
[0015] Preferably each anchor line is connected to the pulling device via a sheave located
generally at the centre line of the vessel. By the placement of the sheaves on the
centre line of the vessel, a symmetric weathervaning action to both sides is achieved.
[0016] According to an embodiment of the vessel according to the present invention the anchor
lines are connected to a single pulling device or winch. By operation of this single
pulling device, bow and stern anchor lines can be simultaneously slackened or tightened
to obtain a proper weathervaning position of the vessel. In one embodiment, a single
anchor line passes from the bow, via the pulling device to the stern of the vessel,
such that the length of the anchor line that is for instance taken in at the bow corresponds
with the length of anchor line payed out at the stern.
[0017] By passing the anchor lines through a vertical shaft or hawser pipe extending from
deck level to keel level, the sides of the vessel are easily accessible, and a shuttle
tanker can be moored alongside the vessel according to the present invention without
interference with the anchor lines.
[0018] By connecting at least one of the anchor lines below sea level to a counterweight,
which in turn is connected to the seabed, the vertical arm of the weathervaning system
can be adjusted and thereby the stiffness of the weathervaning system can be varied.
Preferably the vessel according to the invention is connected to a subsea hydrocarbon
structure via at least one riser. The riser is connected to the side of the vessel
near midship where dynamic movements are minimum thereby increasing the longevity
of these risers. The riser may pass through the vessel via the moonpool. As the movements
of the vessel are limited to rotations of approximately 90 degrees, the risers also
do not require complicated swivels at their attachment points as the riser itself
can twist this angle.
[0019] Some embodiments of a vessel according to the present invention will be explained
in detail, by way of example, with reference to the accompanying drawings. In the
drawings:
Figure 1a shows a schematic side view of a vessel according to the present invention
wherein the anchor lines are connected to the bow and the stern,
Figure 1b shows an anchoring arrangement according to figure 1 comprising buoyancy
means,
Figure 2 shows a schematic side view in which the front most anchor line is connected
to a pulling device, both front and rear anchor lines extending through a vertical
shaft in the vessel,
Figure 3 shows a schematic side view of a vessel wherein the anchor lines at the bow
and at the stern are mutually connected to a cable, the bow anchor line being connected
to a winch,
Figure 4 shows a schematic side view of an embodiment of a vessel according to the
present invention wherein each anchor line is connected to a separate winch,
Figure 5 shows a schematic side view of an embodiment wherein the anchor lines are
connected to a single winch,
Figure 6 shows an embodiment wherein the anchor lines are passed through vertical
shafts in the vessel,
Figures 7 and 8 show a schematic plan view of the vessel according to the present
invention in different weathervaning positions, and
Figure 9 shows a schematic perspective view of the embodiment using a single winch
and a single anchor line as shown in figure 8.
[0020] Figure 1a shows a vessel 1 which at its bow 2 and at its stern 4 is connected to
anchor lines 3 and 5. The anchor lines 3 and 5 comprise a generally vertically extending
section and two branching sections 11,11', 12,12' attached to the vertical parts.
[0021] In the embodiment of figure 1b, submerged buoys 13,14 are attached to the stern and
bow anchor lines. By positioning of the buoys 13,14, the stiffness of the anchoring
system can be varied.
[0022] In the embodiment shown in figure 2, the bow anchor line 3 is connected to a fixed
position winch 6. Both stern and bow anchor lines are connected to counter weights
9 and 10. Each anchor line extends through a vertical shaft 19,20 extending from deck
level to keel level. The vessel will weathervane around the anchor line which is stiffest.
If both anchor lines have the same stiffness and the winch 6 pays out bow anchor line
3, the vessel will weathervane around the stern anchor line 5 because this anchor
line is relatively stiffer than anchor line 3. In reverse, if winch 6 takes in bow
anchor line 3, then the vessel will be weathervaning around anchor line 3 because
this anchor line will be stiffer than the stern anchor line 5.
[0023] In the embodiment shown in figure 3, the bow anchor line and stern anchor line are
mutually connected by a cable 13. By adjusting the length of the bow anchor line 3
by operation of the winch 6, the stern counter weights 9 will be lifted or lowered
and the stiffness of the system will be varied for adjustment of the weathervaning
action thereof.
[0024] Figure 4 shows a vessel 1 with at its bow 2 and its stern 4 respective winches 6,7.
To each winch 6,7 an anchor line 3,5 is connected. The anchor lines 3,5 may be comprised
of ropes, chains or cables. Each anchor line 3,5 in the present embodiment is connected
to a counterweight 9,10 which counterweight in turn is connected to the seabed by
means of chains or cables 11,12. The stiffness of the weathervaning system according
to the present invention may be adjusted by changing the weight of the counterweight
9 and 10 or by changing the length of the vertical arm (vertical part of the anchor
line 3,5). The use of the counterweights 9 and 10 however is optional, and instead
thereof buoys could be used as in figure 1b.
[0025] Risers 8 are connected near midship of the vessel 1, which risers are connected to
a subsea oil or gas structure which is not shown in the drawings.
[0026] Figure 5 shows an embodiment wherein the anchor lines 3,5 are connected to a fixed
position sheave 16,17 at the stern and bow of the vessel 1. The anchor lines are connected
to a single winch 15 on the vessel 1.
[0027] In the embodiment of figure 6, each anchor line 3,5 is passed through a vertical
shaft 19,20 extending from deck level to keel level through the hull of the vessel
1. In this way, the anchor lines 3,5 stay clear from the sides of the vessel and do
not interfere with other vessels which need to moor alongside the vessel 1 according
to the present invention.
[0028] As can be seen in figure 7, the sheaves 15 and 16 and the winch 15 are all located
on the longitudinal centre line of the vessel 1. In this way, a symmetric construction
is obtained and weathervaning to both sides of the centre line of the vessel can take
place around the bow anchor line 3 or the stern anchor line 5. As can be seen from
figure 4, in this embodiment each anchor line 3,5 is connected to the seabed by means
of three chains or cables 11,11',11'' and 12,12',12'' which extend from the counterweights
9,10 to the seabed.
[0029] Figure 8 shows a situation in which the anchor line 3 at the bow is payed out and
the anchor line at the stern is taken in, such that the vessel 1 will weathervane
around the stern anchor line 5 in the direction of the arrow as indicated. In this
construction it is possible to use a single anchor line for the bow and stern anchor
lines 3,5, such that the anchor line which is payed out at the bow corresponds to
the length of anchor line which is taken in at the stern, as shown in figure 6.
1. Vessel comprising at least two anchor lines (3,5) connected respectively at a substantially
fixed position near the bow (2) and near the stern (4) of the vessel, characterized in that, at least one anchor line comprises a first line section extending to a connection
point located below keel level of the vessel and at least two branching anchor lines,
each connected with one end to the anchor line in the connection point and connected
with the other end to the sea-bed.
2. Vessel according to claim 1, characterised in that, the anchor lines extend from their fixed position substantially vertically downward.
3. Vessel according to claim 1, characterised in that, a buoyancy means is connected to at least one anchor line at or near the connection
point.
4. Vessel according to claim 2, characterized in that, at least one anchor line extends through a generally vertical shaft extending through
the hull from deck level to keel level of the vessel.
5. Vessel according to claim 1,2 or 4, characterized in that, at least one anchor line is at or near the connection point connected to a ballast
weight.
6. Vessel according to claim 1,2,3,4 or 5 characterized in that, both anchor lines are connected to respective branching anchor lines.
7. Vessel according to any of the previous claims, characterized in that, at least one of the anchor lines is connected to a pulling device for lengthening
or shortening the anchor line, wherein the pulling device can vary the length of the
at least one anchor line such that both anchor lines are maintained in tensioned state
and the vessel can weathervane around the bow anchor line (3) or the stern anchor
line (5).
8. Vessel (1) comprising at least two anchor lines (3,5) connected respectively at a
substantially fixed position near the bow (2) and near the stern (4) of the vessel,
at least one of the anchor lines (3,5) being connected to a substantially fixed-position
pulling device (6,7,15) for lengthening or shortening of the anchor line, characterised in that, the pulling device (6,7,15) can vary the length of the at least one anchor line
(3,5) such that both anchor lines are maintained in a tensioned state, and the vessel
can weathervane around the bow anchor line (3) or the stern anchor line (5) by paying
in or out at least one of the anchor lines.
9. Vessel according to claim 8, characterized in that, the pulling device is a winch system (6,7,15).
10. Vessel according to claim 8 or 9, characterized in that, each anchor line is connected to a pulling device.
11. Vessel (1) according to claim 8,9 or 10, characterised in that, the pulling device (6,7,15) varies the length of the anchor lines (3,5) simultaneously.
12. Vessel (1) according to claim 8,9,10 or 11, characterised in that, each anchor line (3,5) is connected to the pulling device (15) via a sheave (16,17)
located generally at the centre line of the vessel (1).
13. Vessel (1) according to any of claims 8 to 12, characterised in that, the anchor lines (3,5) are connected to a single pulling device (15).
14. Vessel (1) according to claim 13, characterised in that, an anchor line extends from the bow (2) via the pulling device (15) to the stern.
15. Vessel (1) according to any of claims 8 to 14, characterised in that, the anchor lines pass (3,5) from the pulling device (15), through the hull of the
vessel to a respective anchoring point.
16. Vessel (1) according to claim 15, characterised in that, the vessel is at least near its bow (2) or stern (4) provided with a generally vertical
shaft (19,20) extending through the hull, at least one anchor line extending through
the shaft (19,20).
17. Vessel (1) according to claim 16, characterised in that, the vessel comprises a shaft (19,20) near the bow (2) and near the stern (4).
18. Vessel (1) according to any of the claims 8 to 17, characterised in that, at least one of the anchor lines is connected below sea level to a counter weight
(9,10), which in turn is connected to the seabed via at least one anchor line.
19. Vessel (1) according to any of claims 8 to 17, characterised in that, a buoyancy member is connected to at least one of the anchor lines.
20. Vessel (1) according to any of the previous claims, characterised in that, the vessel is connected to a subsea hydrocarbon structure via at least one riser
(8).
21. Vessel according to any of the previous claims, characterised in that, the vessel does not comprise a turret structure.