[0001] The invention relates to a mooring assembly for mooring a floating structure, such
as a vessel, to a further floating or fixed structure, comprising a connecting structure
acting with the vessel as a mass spring system with respect to at least the surge
motion between the floating structure and the further floating or fixed structure.
[0002] In such a mooring assembly the connecting structure enables the floating structure
to maintain a position with respect to the further floating or fixed structure which
may vary within predetermined margins (defining a 'position window'). The floating
structure may be a vessel, whereas the further floating or fixed structure may be
a buoy, a further vessel or a fixed tower anchored to the seabed. The mass spring
system character of the connecting system provides a reciprocating surge motion (motion
along the longitudinal axis of the floating structure). Such a reciprocating surge
motion can become undesirably large when the mass spring system (connecting structure
with vessel) is excited in or close to a natural frequency (generally as a result
of environmental loads, such as current, waves and wind).
[0003] Thus it is an object of the present invention to provide an improved mooring assembly
of the above-mentioned type.
[0004] According to the present invention the mooring assembly is characterized by a damping
system for damping at least said surge motion.
[0005] The damping system provides damping forces for damping the reciprocating surge motion
of the floating structure relative to the further floating or fixed structure, such
that said surge motion is kept within acceptable limits. As such the damping system
in a very simple, yet nevertheless reliable manner provides an additional functionality
to the mass spring system which, on itself, does not provide adequate damping.
[0006] In one embodiment of the mooring assembly according to the present invention the
damping system only dampens the surge motion of the floating structure away from the
further floating or fixed structure. This means that only when the reciprocating surge
motion is in a phase in which the floating structure (such as a vessel) moves away
from the further floating or fixed structure (such as a fixed tower) damping forces
are generated to dampen the surge motion. In this embodiment the complexity of the
damping system can be kept at a minimum.
[0007] The above, however, does not preclude the provision of a damping system which creates
damping forces during substantially the entire surge motion (generally no damping
forces will be created at the transitions between the surge motion of the floating
structure away from and towards, respectively, the further floating or fixed structure)
which will make the damping more effective but generally will lead to a more complicated
damping system.
[0008] Preferably the damping system is devised for transforming kinetic energy of the surge
motion into heat.
[0009] It is possible that the damping system is devised for transforming the surge motion
into a displacement of a fluid through a restriction means and thus for transforming
the displacement of the fluid into the generation of heat.
[0010] The surge motion will force a fluid (such as for example a hydraulic fluid) through
the restriction means, as a result of which the kinetic energy of the fluid is transformed
into heat which can be drained to the environment. As a result part of the kinetic
energy of the surge motion is transformed into heat, thus limiting the resultant surge
motion.
[0011] In an effective embodiment of the mooring assembly according to the invention the
restriction means is a narrowed orifice in a line for the fluid, which line is in
communication with a piston chamber of a cylinder-piston assembly of which the piston
is movable in correspondence with the surge motion.
[0012] The surge motion (or part of the surge motion in case the damping system only dampens
the surge motion of the floating structure away from the further floating or fixed
structure) leads to a corresponding motion of the piston, thus displacing fluid from
the piston chamber into the fluid line and through the narrowed orifice. It is noted
that 'corresponding motion' not necessarily means that the surge motion and the motion
of the piston are equally large.
[0013] Preferably the cylinder of the cylinder piston assembly is connected to the floating
structure, whereas the piston of the cylinder piston assembly is connected to a point
externally of the floating structure of which the distance to the floating structure
varies as a result of the surge motion. Thus, the main part of the damping system
is part of the floating structure and can be used in combination with several further
floating or fixed structures and is always at hand when needed.
[0014] Basically there are two possibilities then. Firstly, it is conceivable that said
point externally of the floating structure is part of the further floating or fixed
structure. For example this may mean that the piston of the cylinder piston assembly
is connected to a fixed tower. It is noted that in this respect 'connected' is to
be understood as comprising an indirect connection too, for example by means of intermediate
constructive members (which, however, generally will have a stationary position relative
to, in this example, the fixed tower).
[0015] The second possibility comprises embodiments in which said point externally of the
floating structure is part of the connecting structure. For example, when the connecting
structure comprises first and second parts movable relative to each other and moveable
connected to a vessel and fixed tower, respectively, said point may be located at
the interconnection between said parts. Then, not only a damping of the surge motion
of the vessel, but also a damping of the movement of components of such connecting
structure is obtained.
[0016] Mechanically, the transformation of the surge motion into the motion of the piston
may be obtained, for example, when the piston carries a pulley around which a flexible
member, such as a cable, rope or alike, extends of which a first end is connected
to said point externally of the floating structure and of which the second end is
attached to the floating structure.
[0017] This embodiment is an example of a damping system which only dampens the surge motion
of the floating structure away from the further floating or fixed structure. As a
result of the provision of the pulley the magnitude of the piston motion is half,
or even less, the magnitude of the surge motion, thus keeping the piston motion within
acceptable limits.
[0018] Preferably the piston of the cylinder piston assembly is provided with a pretension
such as to keep the flexible member taut during the entire surge motion. As a result
the damping system will operate in a most effective manner.
[0019] Such a pretension of the piston, for example, may be caused by a hydraulic actuator
connected to the cylinder piston assembly. Such a hydraulic actuator may be active
but also passive (in the latter case, for example, comprising a pressurised accumulator
with large volume).
[0020] In other embodiments the damping system is devised for transforming the surge motion
into a relative motion between friction plates, or the damping system is devised for
transforming the surge motion into the flow of an electric current through an electrical
resistance. One solution could be the use of a pretension winch with hydro- or electromotors.
[0021] Although the mooring assembly may be of any type, in one embodiment it is provided
with a connecting structure of the type comprising pendulum members and a rigid arm
assembly, which at a first end are hingedly interconnected and each at a second end
are connected to the floating structure and further floating or fixed structure, respectively,
and further comprising ballast weights at the interconnected ends.
[0022] Hereinafter the invention will be elucidated while referring to the only figure illustrating
schematically and in a side elevational view an embodiment of the mooring assembly
according to the present invention.
[0023] A vessel 1 (for example a tanker) is moored to a fixed tower 2 anchored to the seabed
3. The mooring assembly for mooring the vessel 1 is of the type comprising pendulum
members 4 and rigid arm assemblies 5, which at a first end 6,7 are hingedly interconnected
and each at a second end 8,9 are hingedly connected to the vessel 1 and fixed tower
2, respectively, and further comprising ballast weights 10 at the interconnected first
ends 6,7. In the illustrated embodiment (and preferably) the ballast weights 10 are
rigidly connected to the first end 7 of the rigid arm assemblies 5. The pendulum members
4 at their second ends 8 are connected to support arms 11 mounted on the vessel 1.
The connection between the second ends 9 of the rigid arm assemblies 5 and the fixed
tower 2 occurs through a turntable 12 rotatably supported on the fixed tower 2 around
a vertical axis 13.
[0024] The mooring assembly defines a connecting structure acting with the vessel as a mass
spring system with respect to at least the reciprocating surge motion (movement along
the longitudinal axis) between the vessel 1 and the fixed tower 2 in a manner known
and thus not described in detail here. It is noted only that such a reciprocating
surge motion can become undesirably large when the mass spring system (connecting
structure with vessel) is excited in or close to its natural frequency (generally
as a result of environmental loads, such as current, waves and wind).
[0025] To prevent such large surge motions, the mooring system is provided with a damping
system 14 for damping said surge motion. As will be described later, in the illustrated
embodiment the damping system 14 only dampens the surge motion of the vessel 1 in
its stage away from the fixed tower 2 (which, of course, results in damping the entire
surge motion and, thus, also any swaying motion of other components, e.g. the ballast
weight 10).
[0026] In the illustrated embodiment the damping system 14 comprises a cylinder piston assembly
with a cylinder 15 mounted to the vessel 1 and a piston 16 movable relative to the
cylinder 15. The piston 16 at its free end is provided with a pulley 17 around which
a flexible member 18 (cable, rope etc.) extends. A first end 19 of the flexible member
18 is attached to the fixed tower 2 (or to the turntable 12 thereof) whereas a second
end 20 of the flexible member is attached to the vessel 1.
[0027] A bypass line 21 ends extends between a piston chambers at one end of the piston
head 22 and a hydraulic control unit 25, and comprises a restriction 23 (e.g. a narrowed
orifice). When fluid (preferably hydraulical fluid) is forced through the bypass line
21 and through the restriction 23 its kinetic energy is transformed into heat.
[0028] When the vessel 1 moves away from the fixed tower 2 the flexible member 18 moves
the piston 16 with piston head 22 according to arrow 24 thus forcing the fluid in
the cylinder through the bypass line 21 and restriction 23. Therefore, the damping
system is devised for transforming the surge motion (movement of the vessel 1 away
from the fixed tower 2) into a displacement of a fluid through said restriction 23
and thus for transforming the displacement of the fluid into the generation of heat.
Thus this movement away from the fixed tower is dampened (and therefore the entire
surge motion).
[0029] For the damping system to be effective the piston 16 of the cylinder piston assembly
has to be connected to a point externally of the vessel 1 of which the distance to
the vessel varies as a result of the surge motion. In the illustrated embodiment said
point externally of the vessel 1 is part of the fixed tower 2. However it is possible
too that said point externally of the vessel 1 is part of the connecting structure,
and for example located at the ballast weights 10 or rigid arm assemblies 5. In such
a case also a sideways motion of such components could be dampened by the damping
system.
[0030] The piston 16 of the cylinder piston assembly is provided with a pretension such
as to keep the flexible member 18 taut during the entire surge motion, and especially
when the vessel 1 closes in to the fixed tower 2. In the illustrated embodiment the
pretension of the piston 16 is caused by the hydraulic control unit 25 connected to
the cylinder piston assembly in a manner known per se and indicated only schematically
here.
[0031] It is also possible to replace the cylinder-piston assembly by friction plates which
are moved relative to each other through an element corresponding to piston 16. Or
one could use a means generating an electrical current flowing through an electrical
resistance, for example.
[0032] The invention is not limited to the embodiments described before which may be varied
widely within the scope of the invention as defined by the appending claims. For example,
the cylinder piston assembly also may be attached to the fixed tower 2 mutatis mutandis.
Further the invention, for example, also applies to a situation in which pendulum
members are provided which are hingedly connected to the fixed tower in stead of to
the vessel.
1. Mooring assembly for mooring a floating structure, such as a vessel, to a further
floating or fixed structure, comprising a connecting structure acting with the vessel
as a mass spring system with respect to at least the surge motion between the floating
structure and the further floating or fixed structure, characterized by a damping system for damping at least said surge motion.
2. Mooring assembly according to claim 1, wherein the damping system only dampens the
surge motion of the floating structure away from the further floating or fixed structure.
3. Mooring assembly according to claim 1 or 2, wherein the damping system is devised
for transforming the kinetic energy of the surge motion into heat.
4. Mooring assembly according to claim 3, wherein the damping system is devised for transforming
the surge motion into a displacement of a fluid through a restriction means and thus
for transforming the displacement of the fluid into the generation of heat.
5. Mooring assembly according to claim 4, wherein the restriction means is a narrowed
orifice in a line for the fluid, which line is in communication with a piston chamber
of a cylinder-piston assembly of which the piston is movable in correspondence with
the surge motion.
6. Mooring assembly according to claim 5, wherein the cylinder of the cylinder piston
assembly is connected to the floating structure, whereas the piston of the cylinder
piston assembly is connected to a point externally of the floating structure of which
the distance to the floating structure varies as a result of the surge motion.
7. Mooring assembly according to claim 6, wherein said point externally of the floating
structure is part of the further floating or fixed structure.
8. Mooring assembly according to claim 6, wherein said point externally of the floating
structure is part of the connecting structure.
9. Mooring assembly according to any of the claims 6-8, wherein the piston carries a
pulley around which a flexible member, such as a cable, rope or alike, extends of
which a first end is connected to said point externally of the floating structure
and of which the second end is attached to the floating structure.
10. Mooring assembly according to claim 9, wherein the piston of the cylinder piston assembly
is provided with a pretension such as to keep the flexible member taut during the
entire surge motion.
11. Mooring assembly according to claim 10, wherein the pretension of the piston is caused
by a hydraulic actuator connected to the cylinder piston assembly.
12. Mooring assembly according to claim 3, wherein the damping system is devised for transforming
the surge motion into a relative motion between friction plates.
13. Mooring assembly according to claim 1 or 2, wherein the damping system is devised
for transforming the surge motion into the flow of an electric current through an
electrical resistance.
14. Mooring assembly according to any of the previous claims, wherein the connecting structure
is of the type comprising pendulum members and a rigid arm assembly, which at a first
end are hingedly interconnected and each at a second end are connected to the floating
structure and further floating or fixed structure, respectively, and further comprising
ballast weights at the interconnected ends.