Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SHALLOW WATER PLATFORM
The invention concerns a hull that is generally symmetrical about a centre
axis
and arranged to be placed stationary and resting on a seabed. The invention
also
concerns a method of positioning such a hull on a seabed.
Platforms that are used e.g. for exploration, production and storage of oil
and
gas in/from submarine structures and are floating both during transportation
and
operation, may also be used when resting on the seabed in shallow waters. In
particular, platforms having a circular, or substantially circular, horizontal
cross-
section (e.g. of the type known from the applicant's own patent publication
W002090177), where the buoyancy volume is rather large and the platform
therefore can be submerged a relatively great amount without having too small
a
freeboard, are particularly suitable.
The disadvantage of having platforms of this type rest on the seabed is that
the
platform might assume an inclined position as its orientation is depending on
the
seabed conditions. If this is to be avoided, the seabed must be prepared, for
example by filling and levelling, an operation which is time consuming and
costly.
So-called jackup platforms, which in an operative position are resting on
vertically displaceable legs and where the entire platform hull is above sea
level,
are not dependent of such demanding and costly preparation of the surface they
will be placed upon, by that very fact that the position of the platform can
be
adjusted with individual regulation of each leg by means of built-in jackup
mechanisms. The disadvantage with this type of platform is its low load-
carrying
capacity during transportation in a floating condition, and the shape of the
hull
and the positioning of the legs makes is vulnerable for ice, such that it is
not
suited for use for example in arctic regions, with its combination of
demanding
ice conditions, shallow waters and rough seas during bad weather.
The object of the invention is to mitigate or to reduce at least one of the
disadvantages with the prior art, or at least provide a useful alternative to
the
prior art.
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The object is achieved by features indicated in the description below and in
the
following patent claims.
It is provided a floatable hull which is symmetrical or essentially
symmetrical
about a vertical centre axis, for example cylindrical or polygonal, and is
provided
with a plurality of support means that at least in an operative position
protrudes
downwards from a hull bottom and that during lowering of the hull towards a
seabed is arranged to provide an adjustable support of the hull relative to
the
seabed.
The support is provided by the support means resting against the seabed or
penetrating into uncompacted material below the seabed, as the support means
are pressed and/or sucked down into the uncompacted material until sufficient
friction force between the uncompacted material and the support means is
provided for stable support of the hull.
In the following, the terms "suction foot/feet" are used for the support means
of
the type that in principle works as in a per se known suction anchor, as the
support means are formed as a cup with a downward facing open end, and
where the cup is connected to an evacuation system that can provide an
underpressure inside the cup such that it is sucked down into the seabed on
which it is resting. Both water and uncompacted material can in principle be
evacuated from the vessel, in order for the suction foot to be brought
sufficiently
down into the uncompacted material. It is obvious that the penetration is also
affected by the mass of the hull and the buoyancy of the hull, i.e. the net
downward pressure of the hull.
The expression "support leg" refers to a support means configured to be
resting
on a supporting surface, for example the seabed or on prepared foundation
materials, without penetrating much into the surface. Typically, a lower part
of
the support means in this shape will have an expanded, downward facing end
surface.
The support means can be formed as vertical columns, or similar. The
adjustment can be provided by the support means being formed as suction feet
which by differentiated internal underpressure in the suction feet are pressed
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into the uncompacted material below the seabed. Alternatively, the support
means can be formed as support legs resting on the seabed. When the support
means are formed as support legs arranged for contact with the seabed, the
support means are preferably arranged to vertical displacement relative to the
bottom of the hull. Vertical displacement can also be applied when the support
means are formed as suction feet. Each vertically displaceable support means
comprises means for individual vertical displacement. Thereby, the support
means can be adjusted individually, and the hull can thereby be supportedly
arranged against the seabed with the vertical centre axis independent of the
slope and evenness of the seabed.
In a first aspect, the invention relates more specifically to a hull device,
essentially symmetrical about a centre axis and arranged such that a hull side
exhibits a predetermined freeboard, characterized in that the hull is provided
with several support means which in an active position project downwards from
a hull bottom and are arranged to maintain the centre axis of the hull in a
specified position by each support means being arranged for vertical
displacement in relation to one or both of the hull and the seabed.
The support means can be provided with suction feet arranged to be pressed
into uncompacted material below the seabed, and which is rigidly connected to
the hull bottom.
The support means can be provided with suction feet configured to be pressed
into uncompacted material below the seabed, or may be provided with support
legs arranged to about against the seabed, and which are individually
vertically
displaceable in relation to the hull bottom.
The support means may in a retracted position be at least partly accommodated
by respective pockets inside the hull. The outer shape of the hull is thereby
maintained, as the hull and the support means show less vulnerability against
harmful impacts of ice.
The vertical displacement of each support means is provided by one or more
actuators associated with the hull and the support means.
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The hull may comprise a ballasting system arranged to be able to lower the
hull
until a hull side exhibits a specified freeboard.
In a second aspect the invention relates to more specifically a method for
positioning a hull, that is essentially symmetrical about a centre axis, on a
seabed, characterized in that the method comprises the steps of:
a) arranging the hull floating on a specified location above the seabed;
ballasting the hull such that a hull side exhibits a predetermined
freeboard; and
b) displacing several support means vertically downwards to contact with
the seabed, possibly into uncompacted material below the seabed to
obtain stable support of the hull.
The method may comprise the further step of:
dl) displacing the support means provided with suction feet in
relation to the seabed during ballasting of the hull by applying a
differentiated
underpressure to the suction feet.
Alternatively, the method may comprise the further step of:
d2) displacing the support means provided with support legs in
relation to the hull bottom by means of one or more actuators.
The method may comprise the further step of:
e) locking each of the support means relative the hull by means of the
respective actuator.
In the following, an example of a preferred embodiment is described,
illustrated
in the attached drawings, where:
Fig. 1 shows in perspective, at an angle seen from below, a principle
drawing of a hull according to a first embodiment of the invention,
as the hull is provided with fixed support means in the form of
suction feet;
Fig. 2 shows in perspective, at an angle seen from below, a principle
drawing of a hull according to a first embodiment of the invention,
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as the hull is provided with vertically displaceable support means in
the form of suction feet;
Fig. 3 shows in a side view a principal sketch of a hull according
to prior
5 art arranged on a prepared surface on a sloping seabed;
Fig. 4 shows in a side view a principal sketch of the hull
according to a
first embodiment of the invention supported on a sloping seabed by
means of supporting means in the form of fixed suction feet pressed
partly into a uncompacted material below the seabed; and
Fig. 5 shows in a side view a principal sketch of the hull
according to a
first embodiment of the invention supported on a sloping seabed by
means of supporting means in the form of vertically displaceable
support feet resting against the seabed.
In the figures, the reference number 1 indicates a cylindrical hull with a
vertical
centre axis 11 in its operative position and partly with an inclined outwardly
sloping upper portion la. Even though the hull 1 here is shown symmetrically,
the invention also covers other hull shapes which are essentially symmetrical
about a vertical centre axis, for example a polygonal hull. Due to stability
considerations during the floating condition of the hull 1 the hull 1 is
provided
with a hull skirt 13 of a per se known type.
In a first embodiment, illustrated in figures 1 and 4, the hull 1 is provided
with
several fixedly arranged support means 17 protruding downwards from a hull
bottom 12. In this embodiment the support means 17 are provided with suction
feet 17' which by means of a vacuum plant (not shown) a differentiated
underpressure may be applied such that the suction feet 17' by means of the
internal underpressure are sucked down into the uncompacted material 31 below
a seabed 3. By controlling the underpressure individually for each suction
foot
17', the hull 1 can be levelled even if the seabed 3 is sloping.
In a second embodiment, illustrated in figures 2 and 5, the hull 1 is provided
with several pockets 15, each accommodating a support means 17 provided with
support legs 17" arranged to rest against the seabed 3 and which is vertically
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displaceable between an inactive position essentially retracted into the hull
1 and
an active position partly protruding downwards from the hull bottom 12 through
respective openings 121 in the hull bottom 12. The vertical displacement of
each
of the support means 17 is provided by means of one or more actuators 16,
shown here schematically as one actuator, as the actuators form a connection
between the hull 1 and the support means 17.
The support means 17 with suction feet 17' according to the first embodiment
may be combined with the vertical displacement of the support means 17
according to the second embodiment. This combination will be able to provide
better certainty that the hull 1 being capable of being levelled also when the
condition of the uncompacted material obstructs or restrains the penetration
of
the suction feet 17'.
Even though the embodiment according to figures 1 and 4 and according to
figures 2 and 5 is shown with three support means 17, the effect of the
invention
can in principle be achieved by means of only two support means 17, as a
portion of the periphery of the hull bottom 12 can constitute a support means
abutment portion which rests against the highest area of the portion of the
seabed 3 on which the hull 1 is to be supported, while the adjustment of the
inclination of the hull 1 is provided by the two support means 17 by suction
or
vertical displacement or a combination thereof.
The hull 1 may be adapted to the condition of the seabed 3 by providing the
hull
with support means 17 of the type most suitable for the existing conditions.
This
is especially obvious in the embodiment where the support means 17 are
vertically displaceable.
The hull 1 is in a per se known way provided with a ballasting system 14 for
trimming the hull 1, including giving the hull 1 sufficient stability in a
floating
condition during transportation between locations, and to submerge the hull 1
until a hull side 18 exhibits a specified freeboard 181 in an operative
condition
where the hull 1 is to be supported by the seabed 3.
Devices (not shown) for operation of the actuators 16 and pumps etc. (not
shown) for providing a suction effect when the suction feet 17' are placed or
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released, is not shown. These can be arranged in the hull 1, but they can also
be
arranged on tender vessels (not shown) if for example there is a lack of space
in
the hull 1.
