Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Floating platform and method for operation thereof
Area of the invention
The present invention relates to floating platforms and particularly floating
platforms applicable for operation in exposed waters.
Background of the invention and prior art
For drilling for hydrocarbons, production and storage of produced
hydrocarbons at sea and other applications, a wide range of concepts are used.
One of
the concepts is to use a floating installation, which could be a vessel, a
semi-
submersible or floating platform. In the Patent publication NO 319971 is
described an
offshore platform for drilling for, or production of, hydrocarbons. Further it
is
is described a platform designed as vertical, mainly flat bottomed cylinder,
characterized
by the platform body in the lower part of the cylinder is equipped with at
least one
peripheral, circular cut out defined by a ring member below the cut out and
that the
diameter of the platform body is considerably larger than its draught and
buoyancy
centre of the submerged part of the platform is lower than the centre of
gravity of the
platform. Such a construction has shown to have advantageous large capacity
both for
storage of oil and load capacity on deck. Further the cost of the construction
is low, the
assembly period short and it is achieved large flexibility for different
applications.
Such a platform may be positioned by scattered anchoring and neither a turret
nor a
swivel are needed to handle risers/hoses and anchoring lines. The round or
mainly
rounded cross section is beneficial due to that rotation according to weather
is not
necessary and it has been shown that the movement and stresses on the platform
is
surprisingly low compared with other types of floating installations. Thus the
level of
stretch and tension are limited. The form of the hull provides a compact
construction
which contribute to the wave loads only having a limited degree of influence
on tension
3o and stretch forces.
There is, however, a need for a further improved version of such a floating
platform, particularly a platform which is particularly suitable for
utilization in icy
waters, in addition to other waters.
Summery of the invention
The above mentioned need is meet by the present invention providing a
floating platform for drilling, production, storage or other applications,
particularly in
icy waters, the platform comprising
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a hull with outer sidewalls mainly positioned rotation symmetrically around a
vertical centre axis in the platform and in a lower part closed by a bottom,
a deck in the upper end of the hull equipped suitably according to the
intended
application,
the platforms draught being considerably less than the diameter of the
platform
and the buoyancy centre of the platform for the submerged part is positioned
lower
than the centre of gravity of the platform.
The platform according to the invention is characterized by
its mainly symmetrically outer side of the hull includes at least three
sections
1 o counted from the upper end of the hull:
a waterline section with diminishing diameter in the direction downwards
along the centre axis, in which water level section the water level of the sea
is to be
located during operation in icy waters,
an intermediate section with cylinder form and
an under section with increasing diameter in the direction downwards along the
centre axis.
Since the hull is mainly symmetrical around a vertical centre axis of the
platform, implies that the hull is round or nearly round in the form defined
by the outer
sidewall. An outer sidewall of polygonal form, such as an assembly of many
joining,
flat sheets along the peripheral, is meant to be comprised in the term mainly
symmetrical around a centre axis.
The ratio between the platforms draughts and diameter at water level is
preferably 0.2-0.3 during operation in non-icy waters, were the water level
can be
added to the intermediate section of the hull. By operation in icy waters the
ratio
between the draughts of the platform and diameter at the water level is
preferably
approximately 0.3-0.4. A generally preferred ratio is about 0.3.
The water level section has preferably an inclination inward in downward
direction of about 45 , which is considered preferable with regard to
icebreaking and
the prevailing forces. The inclination in the under section is preferably
approximately
45 outwardly, as seen in the direction downwards, which is considered to be
preferable with regard to the handling and transport of ice radial away from
the
platform. The under section contribute to give the ice a movement preventing
it to be
led under the hull. Other inclinations may, however, also be applicable. The
transition
between the sections can be sharp or gliding, so that the form can resemble an
hourglass or an inner part of a laying U. Typical dimensions of the sections
is a water
level section with height of 10-15 m, an intermediate section with height 5-15
m and an
under section with height 2-4 m. The dimensions of the sections can be beyond
the
above and depend on the ice thickness and other expected ice conditions in the
scheduled operational area as well as the size and draught of the platform.
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For a production and/or storage device the platform comprises preferably a
removable, downward extending body (connecting member), coaxially located with
the
platforms vertical centre axis, withdrawn from the lower edge of the outer
sidewall for
attachment and/or through-guiding of anchoring lines, chains, risers, and/or
cables and
with inherit buoyancy. For icy waters such a removable member is preferred,
because
risers, houses, cables, anchoring lines and chains are pulled away from the
pherimeter
and thus protected from the ice and exposed connecting area is pulled down a
distance
under the bottom of the platform. Any ice entering under the platform must be
moved a
fair distance inward towards the centre of the platform for the area of
attachment is
1o reached so that the ice probably must be raised to the bottom of the
platform and not
hitting any risers, anchoring lines etc. Should a large iceberg arrive, the
connecting
member can be detached; whereupon it will sink down to the safe depth
determined by
the balance between the buoyancy of the connecting member and the weight of
attached devices. The removable connecting member is extending at least 10 in
below
the bottom of the platform before the area for attachment/through-guiding of
the risers
is met.
For some applications of the platform, particularly at deep waters, it is not
necessary with anchoring and for some applications neither any removable
connecting
member. Dynamic positioning may be used for positioning of the floating
platform for
some applications, for instance during drilling at deep waters.
With the present invention a method is also provided for operation in icy
waters with a floating platform according to the present invention,
characterized by the
platform is ballasted so that the water level is situated in the water level
section during
the operation of integrated devices for ballasting.
Comprehensive testing of the floating platform according to the invention in
various scales and for a wide spectrum of conditions, has shown surprisingly
positive
results.
Figures
The present invention as well as advantageous of it, are illustrated by means
of
four figures, of which:
Fig. 1 shows a view of a floating platform according to the invention,
figures 2, 3 and 4 show comparable data between the round, vertically standing
Sevan-platforms of which the present platform is one type, a
semi-submersible platform and a vessel, respectively, in that
fig. 2 illustrates data for heave movement,
fig. 3 illustrates data for pitch movements and
fig. 4. illustrates data for rolling movements.
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Detailed description
Reference is made to fig. 1 which in a side view illustrates a floating
platform
according to the present invention. Further is illustrated a floating platform
1,
comprising a hull 2 which is mainly symmetrical around a vertical centre axis
in the
platform and in a lower end closed by a bottom 3. A deck 4 is illustrated in
an upper
end of the hull equipped suitably according to the intended use. It would
clearly appear
from the figure that the draught of the platform is considerably less than the
diameter
of the platform. What is not so obvious is that the centre of buoyancy for the
submerged part of the platform is lower than the centre of gravity of the
platform. It
io clearly appears that the-rotation symmetrical outer side of the hull 2
includes at least
three sections counted from the upper end of the hull, namely:
a water level section 5 with diminishing diameter in the direction downwards
along the centre axis, in which water level section, the water level of the
sea is situated
during operation in icy waters,
a intermediate section 6 with a cylinder form, and
an under section 7 with increasing diameter in the direction downwards along
the centre axis.
In the direction downward the water level section inclines inwardly towards
the centre axis approximately 45 , while the under section inclines outwardly
about
45 . The ratio between draught and diameter of the platform at the water level
is
approximately 0.3. Further is illustrated a removable, downwardly extending
member
(connecting member) 8, coaxially located with the platforms vertical centre
axis and
withdrawn such that it is positioned far from the outer side walls lower edge
of the hull.
The connecting member 8 is for connecting risers, anchoring lines/chains,
hoses, cables
and similar, as needed. The connecting area for risers is at least 10 m lower
than the
bottom of the platform, which is beneficial in icy waters.
During operation in icy waters, the platform is ballasted so that the water
level
is at the water level section. Further, it is considered beneficial to
position the water
level so that an upper edge of the level is meeting in the upper part of the
water level
section. During operation in non-icy waters the ballasting can be such that
the water
level is in the intermediate section 6 having a cylinder form, in that a
cylinder form
with vertical sides at the water level gives less movements for the platform.
The floating platform can have many applications and is equipped
conveniently according to the intended application both on deck and inside.
Further,
the platform can be used as an FPSO (Floating Production Storage Offloading),
a FPU
(Floating Production Unit), a MODU (Mobile Offshore Drilling Unit), a MSV
(Multipurpose Support Vessel), a FLNG (Floating Liquified Natural Gas
Production), a
GTW (Gas Through Wire, that is an offshore power plant), a FDPSO (Floating
Drilling
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Production Storage Offloading), a FAU (Floating Accommodation Unit, that is a
residential quarter), or other applications.
The generally known advantages of the platform construction with regard to
movement in turbulent waters, is illustrated in the figures 2, 3 and 4. In
figure 2 is
5 viewed curves of heaveheave-movements for standing rotational symmetrical
platforms
(Sevan) and semi-submersible platforms and vessel with waves coming in from
the
front from the side of the vessel, respectively. In figure 3 is illustrated
pitch under
similar conditions for a Sevan platform, a semi-submersible platform with sea
coming
in from the front and a vessel with sea coming in from the front and clearly
that the
io general Sevan construction is beneficial under many operating conditions.
In figure 4 is
viewed rolling of similar, floating installations at corresponding conditions
and it is
clearly seen that the Sevan construction has very beneficial properties
closely followed
by the semi-submersible installation while a vessel has considerably more
rolling,
comparably.
Due to very limited storage and load capacity, as well as little applicability
in
icy waters, semi-submersible platforms cannot be compared with the present,
floating
platform, because the functionality is insufficient.
As mentioned it was found at comprehensive testing that the properties for the
floating platform according to the invention is surprisingly beneficial in icy
waters.
Further testing was conducted with ice driving towards a platform model in the
scale of
1:40. In icy waters it is, as mentioned, compulsary that the water level is at
the water
level section, which means that the ice naturally will be broken down in the
downwards
direction towards the hull. At the same time the hull will endure a force
which has a
component in the direction upwards. Without the desire to be bound by any
theory, it is
assumed that incoming ice is applying a force against and accumulating towards
the
platform so that the platform is lifted somewhat on the side facing the
driving ice
(windward side) until the moment due to buoyancy of the platform becomes
stronger
than the moment applied by the ice. The platform is thus rocking or pitching
around a
horizontal axis, but the construction of the platform entails that by rocking
the platform
the moment due to buoyancy is increasing considerably faster than the moment
applied
by the ice, which entails very moderate movement and this effect is considered
to be
particularly pronounced with the water level in the water level section. When
a certain
amount of ice is accumulated towards the side of the water level sections, a
power
balance will be achieved but the inherent ability of the platform to correct
the moment
due to buoyancy will be changed considerably by the centre of buoyancy being
moved
considerably to the side (the distance between the centre of gravity and
buoyancy
centre is increasing), which entails that the platform is rocking/rotating
back to the
starting position while the ice is broken up and diverged in the direction
downwards.
The movement of the water current and the platform motion guide the ice
downwards
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along the intermediate section whereupon the ice by the water current is lead
further
down along the under section and diverged in the direction away from the
centre axis,
along the outer surface of the under section. Thus the ice is bent/broken
down, is
guided down and guided in the direction back against the direction of the
driving ice
whereupon the ice is floating up again as smaller fragments and lead around
the
platform by means of the increased velocity of the water current near the
platform wall.
The ice is broken very efficiently and moved efficiently around the platform
without
doing any damage. The moderate rocking movements of the platform contribute to
a
reduced friction between the ice fragments and the platform in that a radially
outward
io directed water current is created when the platform swings back again. The
swinging
movement or the heave movement seems to adapt a natural frequency. The
velocity of
the water current is higher around the platform than the surrounding water
because the
water will have to follow the way around the platform. This contributes to the
ice
fragments being transported on a "water cushion" around the platform. However,
the
water currents hitting the platform can also be split in a current passing
under the
platform, particularly for a large platform, because it means a shorter or
easier path for
the current then all the way around the platform. This may lead to ice
fragments under
the platform, which is undesirable, but the shape of the under section has
proved to be
efficient to prevent ice under the platform as mentioned above, in addition to
contribute
to an improved "water cushion" effect.
The behaviour of the platform under icy conditions has been measured and
filmed, which approves that it typically has heave movement not exceeding 6
even at
100 years first year ice condition in Arctic and very small accelerations in
all degrees
of freedom. It should be mentioned that other floating platform concepts have
proved
to have large unacceptable movements at similar ice loads in the form of
"jumping"
movements with high accelerations at times. Both movements and amount of ice
accumulating towards the present platform are very low, in that "jumping"
movements
and high accelerations are almost absent.
The platform is preferably equipped with propulsion for operating on
3o assembling, which propulsion preferably also is adapted for utilization for
propeller
washing in the area around the under section with effect towards the surface
at the
wind board side and outwards towards the sides.
