Note: Descriptions are shown in the official language in which they were submitted.
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A floating object provided with a permanent mooring system
and a mooring line
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.
"Permanent" in this context means "for a longer period", for
instance several or many years.
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.
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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.
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.
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.
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.
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
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once the mooring lines are installed without diver
assistance or underwater activity.
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.
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.
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 DyneemaTM or SpectraTM, an aramid,
for instance TwaronTm or KevlarTM, a thermoset liquid
crystalline polyoxazole, for instance PBOTM or ZylonTM, a
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thermotropic liquid-crystal polymer, for instance VectranTM
or ZeniteTM, 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.
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.
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
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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.
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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.
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.
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.
The invention will now be illustrated by means of a
preferred embodiment, as shown in the figures, wherein:
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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.
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.
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.
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.
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.
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-
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molecular-weight polyethylene, aramide, liquid crystal
polymer, polyester, DyneemaTMr PBOTMr ZylonTMr SpectraTMr
VectranTM, TwaronTm, TechnoraTm, KevlarTM, ZeniteTM, carbon,
etcetera.
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.
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.
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.
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
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high wear resistant and low friction material relative to
the outer surface of the upper mooring line.
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.
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.
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.
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
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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.
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.
