[0001] The invention relates to a floating object provided with a permanent mooring system
for securing said floating object to at least one anchor on a seabed, wherein said
mooring system comprises at least one mooring line and at least one guidance member
arranged to guide the mooring line from the exterior of said floating object to the
interior of said floating object such that the upper end of said mooring line can
be secured to said floating object at a location other than the exterior contact location
between said mooring line and said floating object. Said floating object may for instance
be a floating marine energy converter platform, a single point mooring buoy or a floating
offshore oil or gas platform.
[0002] "Permanent" in this context means "for a longer period", for instance several or
many years.
[0003] Permanently moored floating objects require a mooring system in which the mooring
connection point between each mooring line and the floating object is located well
underneath the water line to achieve a better force balance. However, this poses the
person skilled in the art with the following challenges to solve:
submerged connection makes the connection procedure complex and sensitive for failure;
multiple connections and disconnections during lifetime; and
short installation window, especially in the case of tidal energy converters which
are for its purpose always located in areas with very high current velocities.
[0004] An important requirement of mooring lines which are used to fix a floating objet
to a seabed, is that they have a certain elasticity to accommodate differences in
their length, caused by waves and different water levels at different tides.
[0005] Applying synthetic lines in a mooring system can resolve the mentioned challenges,
as these materials lower the mooring loads and reduce the weight compared to chain
and steel wire mooring lines. However, conventional synthetic mooring lines are prone
to wear and contact damage. In permanent mooring lines applications, this problem
is normally avoided by connecting a synthetic line to the floating object with either
a shackle to an (underwater) pad eye, or a (double pivoting) chain stopper in combination
with an upper section of chain to secure the lines.
[0006] The shackle/pad eye solution is straight forward but does not allow post installation
adjustment of the mooring line length. The use of an upper section of chain allows
adjustment of the line length but requires heavy equipment for handling of the chain
section.
[0007] Furthermore, both concepts require underwater activities to make the connection and
are difficult to access for release/disconnect or maintenance. Moreover the concepts
are not designed for quick connection-disconnection, because it's is not normally
a concern for those applications.
[0008] It is an object of the invention to solve any of the above mentioned problems, and/or
to provide a solution which allows a quick hook-up and release of the floating object
once the mooring lines are installed without diver assistance or underwater activity.
[0009] According to the invention said mooring line comprises: a relatively long main mooring
line section of a material having a relatively low axial stiffness (and the required
elastic properties); and
a relatively short upper mooring line section of synthetic material having a relatively
high axial stiffness; wherein in use the lower end of said main mooring line section
is secured to said anchor, the upper end of said main mooring line section is secured
to the lower end of said upper mooring line section, said upper mooring line section
extends though said guidance member and the upper end of said upper mooring line section
is secured to said floating object.A high axial stiffness of the upper mooring line
section will reduce wear because strain is low so the sliding motions of the pendant
through the guidance member are minimized.
[0010] Preferably said upper mooring line section has a smaller diameter than the main mooring
line section, which has the advantage that it allows for a small design of the guidance
member, and decreases the bending stiffness of the upper mooring line.
[0011] Preferably said synthetic material having a relatively high axial stiffness is an
ultra-strong fibre material. More preferably said ultra-strong fibre material is chosen
from the group consisting of: an ultra-high-molecular-weight polyethylene, for instance
Dyneema
™ or Spectra
™, an aramid, for instance Twaron
™ or Kevlar
™, a thermoset liquid crystalline polyoxazole, for instance PBO
™ or Zylon
™, a thermotropic liquid-crystal polymer, for instance Vectran
™ or Zenite
™, carbon, a polyester. The core of said upper mooring line section may be a different,
axially stiff material than the outer layer, which may be a wear resistant material.
[0012] Preferably said guidance member comprises a mooring line contact surface which is
made of a wear resistant material relative to the upper mooring line or a wear protective
sleeve around it, such a s a coating or a cladding. More preferably said wear resistant
material is chosen from the group consisting of: epoxy, polymer, polyamide, polyester,
polyurethane, or said wear resistant synthetic material comprises a composite synthetic
material matrix with a filler of different materials, such as PTFE, FEP or graphite
particles. Said wear resistant synthetic material is preferably a low friction material
relative to the upper mooring line or a wear protective sleeve around it. Preferably
said guidance member comprises a trumpet shaped outer end, on the inner surface of
which said line contact surface is present. Such a guidance member as such is known
for example from international patent application
WO 2009/127832, which is incorporated herein by reference. The geometry of the guidance member has
said trumpet like shape to support the upper mooring line section in at least two
degrees of freedom. The geometry is designed to reduce bending fatigue. The material
is selected to provide minimal wear in the contact area with the upper mooring line
section.
[0013] In the preferred embodiment, in use, the outer end of said guidance member is below
the upper water level. And preferably the location where the mooring line is secured
to the floating object is above the upper water level. This allows for a dry and direct
connection of the synthetic mooring lines at the deck of the floating object while
the point of action of the force remains under the water line.
[0014] If necessary a part of the upper mooring line section which is intended to contact
the mooring line contact surface is provided with a protective sleeve.
[0015] Although the main mooring line may be a steel wire or chain, in the preferred embodiment
the main mooring line section is also made of a synthetic material . In one preferred
embodiment both the main mooring line section and the upper mooring line section are
ropes of intertwined yarns, wherein the upper end of said main mooring line section
is secured to the lower end of said upper mooring line section by intertwining yarns
of both ends. In another preferred embodiment the main mooring line section and the
upper mooring line section are connected by a splice eye and shackle connection.
[0016] The invention also relates to a mooring line for use in the mooring system of the
floating object, comprising: a relatively long main mooring line section of a material
having a relatively low axial stiffness; and a relatively short upper mooring line
section of synthetic material having a relatively high axial stiffness; wherein one
end of said main mooring line section is secured to one end of said upper mooring
line section.
[0017] The invention will now be illustrated by means of a preferred embodiment, as shown
in the figures, wherein:
Figure 1 is a front view of a floating object;
Figure 2 is a side view of the floating object of Figure 1;and
Figure 3 shows a cross sectional perspective side view of a detail of the mooring
system in the hull of the floating object of Figure 1.
[0018] According to Figures 1 and 2 the part of a floating object 1 which is relevant for
the current invention is shown, said object 1 comprising a substantially cylindrical
hollow body 2.
[0019] The floating object 1 is secured to anchors 8 on the seabed by four mooring lines
9, two at each end of the hollow body 2.
[0020] In order to accommodate the difference in distance between the seabed and the floating
object caused by the range of tide, the mooring lines 9 need to have a certain elasticity.
[0021] A cross section of the upper part of the mooring line system is shown in Figure 3.
The mooring line 9 consist of two different parts: a relatively long main mooring
line section 9b of a material having a relatively low axial stiffness and a relatively
short upper mooring line section 9a of synthetic material having a relatively high
axial stiffness.
[0022] The high strength and axially stiff material of the upper mooring line section 9a
can for instance be any fibre based rope as for example, but not limited to, ultra-high-molecular-weight
polyethylene, aramide, liquid crystal polymer, polyester, Dyneema
™, PBO
™, Zylon
™, Spectra
™, Vectran
™, Twaron
™, Technora
™, Kevlar
™, Zenite
™, carbon, etcetera.
[0023] The material of the main mooring line section 9b can be any suitable synthetic line.
However other materials for the main mooring line 9b can be used as well, for example
steel wire or chain.
[0024] In the shown embodiment a the upper mooring line section 9a and the main mooring
line section 9b are ropes of intertwined yarns, and the upper end of the main mooring
line section is secured to the lower end of said upper mooring line section by intertwining
yarns of both ends over a suitable length in the intermediate section 9c. However,
both ends of the sections 9a and 9b may also be connected by means of for instance
spliced eyes and a shackle.
[0025] The mooring line connection system consists of a mooring hawser pipe 10 protruding
obliquely through the hull 11 of the hollow body 2. The mooring line 9 is pulled through
the hawser pipe 10 and connected at deck level to connection point 14 by means of
spliced eye 15.
[0026] The guidance member 12 at the lower end of the hawser pipe 10 supports the upper
mooring line section 9a under tension and limits the bending radius of the upper mooring
line section 9a. The part of the upper mooring line section 9a which is in contact
with the trumpet mouth 16 of the guidance member 12 is covered with a braided jacket
13 to limit wear. Furthermore the trumpet mouth 16 of the guidance member 12 is covered
with a plastic lining 17, made of a high wear resistant and low friction material
relative to the outer surface of the upper mooring line.
[0027] The mooring line 9 will be installed by pulling the upper mooring line section 9a
thought the hawser pipe 10 using a messenger line and for example a winch. On deck
the upper mooring line section 9a can be secured by a suitable connection point 14.
Alternatively the upper mooring line section 9a can be guided to a tensioning system
to (automatically) adjust the line length after installation.
[0028] The contact area between the upper mooring line section 9a and the guidance member
12 will be most affected by wear and fatigue. The lifetime of the upper mooring line
section 9a can be extended by changing the point of contact of the upper mooring line
section 9a with the guidance member 12. This can be achieved by for example using
connection points 14 on the floating object 1 at different heights or having different
length spliced eyes 15 in the upper mooring line section 9.
[0029] Inspection and replacement of the upper mooring line section 9a can be done without
disconnecting the floating object 1. The upper mooring line section 9a can be released
using the pull-in line attached and inspected above water. Alternatively the upper
mooring line section 9a together with the main mooring line section 9b can be pulled
through the hawser pipe 10 for inspection.
[0030] This concept offers specific advantages for (semi) permanent moored floating objects
1 suitable for wave and tidal energy converter systems. It may also offer a cost effective
solution for the oil and gas industry as the concept is free of any additional moving
parts under water as no equipment is required other than the main mooring line section
9b and upper mooring line section 9a to resolve issues with (bending) fatigue and
wear in this invention.
[0031] The invention has thus been described by means of a preferred embodiment. It is to
be understood, however, that this disclosure is merely illustrative. Various details
of the structure and function were presented, but changes made therein, to the full
extent extended by the general meaning of the terms in which the appended claims are
expressed, are understood to be within the principle of the present invention. The
description and drawings shall be used to interpret the claims. The claims should
not be interpreted as meaning that the extent of the protection sought is to be understood
as that defined by the strict, literal meaning of the wording used in the claims,
the description and drawings being employed only for the purpose of resolving an ambiguity
found in the claims. For the purpose of determining the extent of protection sought
by the claims, due account shall be taken of any element which is equivalent to an
element specified therein.
1. A floating object provided with a permanent mooring system for securing said floating
object to at least one anchor on a seabed, wherein said mooring system comprises at
least one mooring line and at least one guidance member arranged to guide the mooring
line from the exterior of said floating object to the interior of said floating object
such that the upper end of said mooring line can be secured to said floating object
at a location other than the exterior contact location between said mooring line and
said floating object;
said mooring line comprising:
a relatively long main mooring line section of a material having a relatively low
axial stiffness; and
a relatively short upper mooring line section of synthetic material having a relatively
high axial stiffness;
wherein in use the lower end of said main mooring line section is secured to said
anchor, the upper end of said main mooring line section is secured to the lower end
of said upper mooring line section, said upper mooring line section extends though
said guidance member and the upper end of said upper mooring line section is secured
to said floating object.
2. The floating object of claim 1, wherein said synthetic material having a relatively
high axial stiffness is an ultra-strong fibre material.
3. The floating object of claim 2, wherein said ultra-strong fibre material is chosen
from the group consisting of: an ultra-high-molecular-weight polyethylene, for instance
Dyneema™ or Spectra™, an aramid, for instance Twaron™ or Kevlar™, a thermoset liquid crystalline polyoxazole, for instance PBO™ or Zylon™, a thermotropic liquid-crystal polymer, for instance Vectran™ or Zenite™, carbon, a polyester.
4. The floating object of any of the previous claims, wherein said guidance member comprises
a mooring line contact surface which is made of a wear resistant material relative
to the material of the upper mooring line section or a protecting sleeve around it,
such as a coating or cladding.
5. The floating object of claim 4, wherein said wear resistant material is chosen from
the group consisting of: epoxy, polymer, polyamide, polyester, polyurethane.
6. The floating object of claim 4, wherein said wear resistant synthetic material comprises
a composite synthetic material matrix with a filler of different materials, such as
PTFE, FEP or graphite particles.
7. The floating object of claim 4, 5 or 6, wherein said wear resistant synthetic material
of the guide member is a low friction material relative to the material of the upper
mooring line section or a protecting sleeve around it.
8. The floating object of any of the previous claims, wherein said guidance member comprises
a trumpet shaped outer end, on the inner surface of which said line contact surface
is present.
9. The floating object of any of the previous claims, wherein in use the outer end of
said guidance member is below the upper water level.
10. The floating object of any of the previous claims, wherein in use the location where
the mooring line is secured to the floating object is above the upper water level.
11. The floating object of any of the previous claims, wherein a part of the upper mooring
line section which is intended to contact the mooring line contact surface is provided
with a protective sleeve to avoid wear.
12. The floating object of any of the previous claims, wherein the main mooring line section
is made of a synthetic material.
13. The floating object of any of the previous claims, wherein both the main mooring line
section and the upper mooring line section are ropes of intertwined yarns, wherein
the upper end of said main mooring line section is secured to the lower end of said
upper mooring line section by intertwining yarns of both ends.
14. The floating object of any of the previous claims, wherein said floating object is
a floating marine energy converter platform, a single point mooring buoy or a floating
offshore oil or gas platform.
15. A mooring line for use in the mooring system of the floating object in accordance
with one of the preceding claims, comprising:
a relatively long main mooring line section of a material having a relatively low
axial stiffness; and
a relatively short upper mooring line section of synthetic material having a relatively
high axial stiffness;
wherein one end of said main mooring line section is secured to one end of said upper
mooring line section.