Note: Descriptions are shown in the official language in which they were submitted.
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Heave Compensation System
The invention pertains to a heave compensation system, a vessel fitted with a
heave
compensation system and a method for continuously compensating heave between a
floating vessel and a buoyant body.
Floating vessel, as traditional floating platforms called semi submersible
platforms
has its limitations when it comes to being operable during large waves and
harsh
weather conditions. This has resulted in that drilling rigs for the i.e.
Barents Sea
have to be built very large and to a high cost to try to have acceptable
operation
time during these conditions. This lack of operability means large economic
losses
for the operator of the platform. Therefore has there for some time been a
wish to
provide good systems which may increase the operative time for a floating
structure
which.
Another aspect of the problem is that ordinary heave compensation systems
generally are large in size and weight, which both factors, size and weight,
are
crucial for floating structures. An aim for the present invention is to
provide a heave
compensation system which provides large operational windows for the floating
structure. Another aim is to provide a floating vessel which undergoes less
heave
motions, which for a floating oil/gas platform results in less variation in
vertical
forces in risers and eventual anchoring systems, and therefore reduced or
eliminated
use for other heave compensation systems specifically for instance for the
risers. It
is also an aim for the present invention to provide an improved heave
compensation
system usable for other floating structures than semi submersibles, as
pontoons,
foundations for cranes and offshore oil buoy loading systems, bridge
foundations,
riser hangoffs, turrets etc. It is an aim to provide a system that affects a
drilling
rig's ability to operate under harsh weather conditions.
The aims of the invention are obtained with a system, vessel and method as
described in the following independent claims. Additional features of the
invention
are apparent from the dependent claims and the following description and
drawings.
The invention pertains to a heave compensation system for continuous operation
on
a floating vessel. The system comprising at least one buoyant body, at least
one
lever arm for arrangement in a generally horizontal plane, pivotal means
capable of
limited rotation about a horizontal axis and arranged to connect one end of
the lever
arm to the buoyant body. The lever arm has also at least one hinge connected
to an
intermediate point on the lever arm and capable of allowing limited rotational
movement of the lever arm in a vertical plane and means to attach the hinge to
the
floating vessel. On the lever arm there is at least one counterbalance weight
connected to the lever arm on the side of the hinge away from the buoyant
body,
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whereby the inertia of the counterbalance weight resists heave movements of
the
buoyant body relative to the floating vessel.
The heave compensation system is applicable to a large range of floating
vessels as
semi submersibles, pontoons, foundations for cranes, floating bridges etc. as
this is
the case is the description of the floating vessel made as a general
description
covering the different embodiments.
By connecting a drilling module of a floating platform to a guided buoyancy
tanlc
connected to the main floater of the platform via balance arm, will the
drilling
module get its own behaviour and heave period relative to the main floating
object,
i.e. the platform.
The system will for some applications only require one balance arm, for others
many. Many may be used where the space is limited, i.e. inside vessels. The
ballast
will mainly consist of solids such as steel, but can also consist of a fluid,
for
instance water, if there is a need to have variable ballast.
The heave compensating system according to the invention will reduce the
vertical
forces in the buoyancy-support system and will under ideal conditions be +l-
zero.
A drilling rig has a normal heave compensation "stroke" of 48 feet (ca. 14,5
meter).
The present invention may reduce this requirement or even make the whole
existing
compensations system redundant.
The buoyancy tank in the heave compensation system may be open in the centre,
have a "moon pool" allowing sub sea equipment (i.e. BOP and sub sea production
equipment) to be lifted to and from the seabed in a clam environment. For
other
applications, this opening will be limited or even zero.
The water plane inside the buoyancy tanlc can be altered by use of water pumps
or
compresses air. The buoyancy chamber may be partially or fully open in the
bottom
to reduce the effect of the "added mass" due to the exposure from the sea. The
capacity of the buoyancy thank for the drilling rig purpose of a normal size
today
will have to be in the order of 200 to 400 tonnes. However, there will be
different
diameters and capacities for the various uses of the heave compensation system
according to the invention.
In a preferred embodiment of the invention is the buoyancy tank in the heave
compensation system guided by use of guiding means for instance rollers or
gliders
mounted in the mains structure. These guiding means should be mounted on at
least
two levels, one of these levels may be below the water surface. The system
according to the invention will compensate the vertical forces acting on the
system
and maintain the module in a horizontal position. The lower guiding means may
be
operates by winches and or hydraulic cylinders and will constantly move the
lower
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guiding means to keep the buoyancy tanlc vertical. The guide means are the
elements which will connect the heave compensation system to the main floating
structure.
The lever arm may in one embodiment comprise additional weights in form of for
instance clumps. These weights may be used to alter the vertical support
capacity of
the lever arm.
There may in another embodiment of the invention be hydraulic cylinders used
in
addition to the lever arm, to improve the effect of the lever arm or improve
the
accuracy of the system.
The invention also comprises a vessel when fitted with a heave compensation
system as described above. The vessel may as earlier mentioned be any lcind of
floating structure from a oil/gas platform to a foundation for a floating
crane or
bridge or used for parts of a system of floating structures.
The invention also comprises a method of continuously compensating heave
between a floating vessel and a buoyant body comprising the steps of
constraining
the buoyant body for movement generally vertically with respect to the
floating
vessel, disposing at least one lever arm in a generally horizontal plane on
the
floating vessel with a hinge at an intermediate point on the lever arm which
connects the lever arm to the floating vessel, so that the lever arm is
capable of
limited rotational movement in a vertical plane, connecting one end of the
lever arm
to the buoyant body to allow angular movement there between about a horizontal
axis, and connecting a counterbalance weight to the lever arm on the side of
the
hinge away from the buoyant body, whereby vertical movement of the
counterbalance weight resists heave movements of the buoyant body relative to
the
floating vessel.
The invention will now be explained in more detail with preferred embodiments,
with references to the drawings where:
Fig. 1 shows a side sketch of the invention according to one embodiment in
connection with a semi submersible platform,
Fig. 2 shows the embodiment of the invention as shown in fig. 1 without the
main
parts of the platform,
Fig. 3 shows a side sketch of another embodiment of the invention with a
floating
vessel indicated with dotted lines,
Fig. 4 shows the embodiment in fig. 3 seen from above.
Fig. 1 shows the invention for dampening the motion of a floating structure in
the
water in one embodiment, where the floating vessel 1 is a semi submersible
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platform with a buoyant body 2a and several supporting columns 2b supporting a
main deck structure 2c. On the deck structure is it arranged cranes, drilling
derrick
4, flare booms 3 and other equipment necessary to perform the tasks of the
semi
submersible platform. The drilling derrick 4 with the main drilling area 4a is
in this
embodiment arranged in the middle of the floating vessel. From the drilling
area 4a
are there risers 5 running down through the body of water to the seabed, where
they
are connected to a manifold 6 and template 7. The drilling derriclc 4 with
connection
of the risers 5 are arranged according to the invention in connection with a
buoyant
tank 11. The buoyant tank 11 may have ballast 11 a and buoyancy 11 b systems
to
regulate the buoyancy of the buoyant tank 11. In the embodiment shown in fig.
1
has the tank 11 a moon pool 20 wherein the risers 5 are arranged. The buoyant
tanlc
11 is in this embodiment in connection with two lever arms 10, through an
interface
point 14 at one end of the lever arms. The lever arms 10 have a mainly
horizontal
orientation, and may rotate about the interface point 14. At the opposite end
of the
lever arm is arranged a ballast tank 12, which work as counterweight. At a
point
between the ballast tank 12 and the interface point 14 is the lever arm 10
hinged to
the floating vessel 1 through a hinge connection 13, around which the lever
arm
may rotate around a mainly horizontal axis. The buoyant tank 11 and the
floating
vessel 1 are to a certain extent free to move relatively to each other in the
mainly
vertical direction. The ballast tank 11 is normally affected by vertical guide
system
to lceep its movement in relation to the floating vessel 1 mainly vertical.
The guide
system in this embodiment comprises guide rollers 17 arranged by an upper and
lower part of the buoyancy tanlc 11. The guide rollers may be regulates by
hydraulic
cylinders, winches or other equipment to keep the buoyancy tank 11 constantly
in
the most vertical position as possibly. As one can see from this embodiment
may
the system comprise two lever arms 10 facing opposite directions, and it may
be
arranged beneath the floating vessel's 1 main deck structure 2c.
As shown in fig. 2 the heave compensation system according to the invention
comprises at least one lever arm 10 connected to a buoyancy tank 11 at
interface
points 14. There is arranged a ballast tank 12 at the opposite end of the
lever arm
10. The effect of the ballast tank 12 on the system may be changed either by
changing the ballast itself or changing its position along the lever arm. The
same is
the case for the buoyancy tank which may comprise both ballast and buoyancy,
which may be variable, with compressed air systems and or water pumps etc.(not
shown in figures)
At an intermediate point between the interface point 14 and the ballast tanlc
12 is the
lever arm 10 connected to the floating vessel 1 at a hinge connection 13.
There is shown another embodiment of the invention in fig. 3 and 4. The
floating
vessel 1 is shown with dotted lines, and shown as anchored to the seabed with
mooring lines 9. According to the invention is the lever arm 10 connected to
the
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buoyancy tank 11 at an interface point 14. The lever arm also comprises
ballast tank
12, and is further equipped with extra clump weight 15. The lever arms 10, two
shown in fig. 3 is in this embodiment arranged above the main part of the
floating
vessel 1. There is also arranged an additional hydraulic cylinder 16 between
the
5 floating vessel 1 and the lever arm 11 at a distance from the hinge
connection 13.
The buoyancy tank 11 is further equipped with mooring lines 19 from the lower
part
of the tanlc 11 to subsea clump weights 18, arranged on the seabed. These
clump
weights 18 may be other kind of foundations or anchors and they may also be
used
for anchoring the floating vessel 1 as indicated with the mooring lines 9
running
from the floating vessel 1 down to the weights 18. The mooring lines 19 helps
keep
the buoyancy tank 11 in its position. The vessel has in this embodiment also a
moon
pool 20 for the not shown risers, umbilicals drillstring etc.s
As shown in fig. 4 which is the system shown in fig. 3 see from above,
comprises
the heave compensation system in this embodiment two pairs of lever arms 10,
arranged at 90 degrees intervals around the floating vessel.
The invention is now explained with embodiments showing details on how to
implements the invention, but the invention should not be limited to those
embodiments. A person skilled in the art will be able to perform a lot of
alterations
and other embodiments which are within the scope of the invention as defined
in the
following claims.
