Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 022l0302 l997-07-ll
WO g6123690 PCT~095JOOl)23
A floating device
The inventlon relates to a floater, having a ~ubmerged
buoyancy sectlon of concrete, a column section comprlsing one
5 or a plurality of concrete column(s) protruding up from the
buoyancy section, and a deck section of steel supported above
the surface of the water by the column section, sald one or
said plurality of concrete column(s) being extended to the
deck section as a hollow steel column ready for equipment.
A floater is an installation floating on the sea for the
purpose of exploiting the resources in and below the ocean.
It may be dynamically positioned or anchored. Typical
floaters are maritime installations such as drilling
15 platforms, production platforms, loading buoys, etc.
It is natural that weight and stability are problems
encountered in connection with floaters. Thus, the tendency
to enlarge the so-called topside (deck section) and otherwise
2~ place loads on high levels has become a steadily increasing
problem. This is, inter alia, connected with unanticipated
welght increases in deck structures and modules, a common
experience during the process from initial design to the
realized idea.
The stability and general movement characteristics of a
floater are closely connected with the height of the material
centre of gravity in interaction with the centre of buoyancy
and the metacentre distance above the centre of buoyancy.
Thus, the height of the metacentre plus the centre of
buoyancy shall be defined positively greater than the height
of the material centre of gravity if the floater is to
achleve a satlsfactory stabillty. It is thus clear that
there will be great optimization gains connected with having
~5 the whole material centre of gravity lowered as much as
po~sible. This also means that while the heavy structure of
a concrete floater is an undisputed advantage with respect
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to its bottom elements, the opposite wlll be true with
respect to the upper part of the floater. A deck sectlon
formed as a steel structure contrlbutes ln a posltlve
dlrection to stability.
When the cholce is made of comblnlng steel and concrete ln
floaters, as mentloned by way of introduction, the reasons
are partly economic and partly technical. Concrete ls
competltive in terms of price and is believed to have several
o advantages in relation to steel. The relatively numerous and
dlfferent types of installations constructed ln the North Sea
up till now have so far, with a minimum of maintenance and
wlthout special protection, proved highly resistant to
corrosion. A concrete floater is therefore assumed to have
15 an advantage in terms of durability. Another important
advantage is the sturdiness of concrete structures, a feature
believed to make them particularly well suited ln hlghly
weather-exposed maritime environments and for heavy deck
lnstallations.
Studies have shown that the lnteractlon between steel and
concrete is one of those design aspects that create problems.
The problem arises when the considerable and concentrated
static and dynamic loads between the deck section and the
25 column section are to be transferred to the support in the
concrete structure. These large concentrated loads may lead
to a fracturing of the concrete, and such areas will in
addition be highly exposed to fatigue. In order to have
these forces distributed over a larger area and thus reduce
30 the stress level in a satisfactory manner, it will therefore
be necessary to reinforce the concrete by means of steel
structures. However, such reinforcements result in a
relatively large and undesirable increase in weight, particu-
larly in view of the fact that the weight increase occurs
35 high above the material centre of gravity, with concomitant
adverse effects on stability.
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The magnitude of these compressive loads wlll obviously
depend upon the deslgn, i.e., upon the size of the deck
section and upon the prlnciples and embodiment selected for
the interaction between the underlying support system and the
5 column section of the floater. It will thus be possible to
reduce the magnitude of the concentrated compressive loads by
a careful selection of design based solely upon this concern.
~owever, such a selection will entail an obvious limitation
of the possibilities of technical and economic optimization
o of a floater design.
It is an ob~ect of the present invention to propose measures
which may contribute to improve the stability and general
movement characteristics of a floater, while simultaneously
15 making possible an optimization of construction and equipment
time.
This ob~ect can be achieved by exploiting the advantages of
concrete, with respect to sturdiness, heaviness and corrosion
20 resistance, in the underwater, lower parts, i.e., the portion
of the floater located below the surface of the water, in
combination with the elasticity/plasticity of steel and its
resulting, well documented stress levelling and distribution
power, in all parts above the surface of the water.
According to the invention, a floater as mentioned by way of
introduction is therefore proposed, characterized in that the
dividing line between concrete and steel in the column is
located at a distance from the deck support (the load's point
30 of impact) where the concentrations of stresses from the
concentrated loads on the deck support (the loads' point of
rimpact) are distributed along the shell of the steel column
to a low and relatively even level.
35 By means of the invention, the advantages of both steel and
concrete can be exploited in a suitable manner, i.e., the
area of steel/concrete interaction is positioned in such a
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way that optimal use can be made of the area where the
forces are distributed.
The use of steel will contribute to weight reduction. At the
5 same time the area of steel/concrete interaction will be
advantageously moved, providing the opportunity for a
desirable utilization of a certain area where forces are
distributed, extending down along the columns. The dis-
tribution area should in principle extend as far down the
o columns as possible. In practice, the dividing line in a
floater may advantageously be located at a distance in the
magnitude range of 20 to 30 m from the deck ~upport. For
practical reasons the interaction level may preferably be
located some distance above the surface of the water, partly
15 to prevent exposure of the interaction area to high, external
water pressure and thus a theoretical danger of leakage, and
partly to secure access for maintenance and corro~ion
inspection, procedures considered essential since the floater
may last as long as 50 years.
By employing the inventive idea, it is possible to distribute
the considerable, concentrated compressive loads of the deck
~ection via specially allocated reinforcement parts,
constructed in steel, across to the cylindrical steel shell
25 of the floater (the steel columns). From here, the compres-
sive stresses will be spread further down the cylindrical
steel ~hell in a fan-shape having a double angle of about
45~. Calculations show that both compressive stresses and
the tensile stresses induced by eccentricity moments will
30 range from an asymptotic infinite value at the load's point
of impact, to a low, constant level at a distance approxima-
tely equal to the column diameter, from the top of the column
and further down. Ideally, the interaction steel/concrete
should therefore be placed at a reasonable, yet shortest
35 possible, distance upward from this elevation. For a typical
floater the column diameter will be about 25 m. According
to the invention, the length of the steel column should
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therefore be within the same magnitude range, at the same
time as the concrete/steel connectlon should be located about
5 m above the anticipated water line, thus offering reaso-
nable opportunities for periodic inspection and malntenance.
.,
For a floater of said combined embodiment concrete/steel,
two separate construction sltes may be used, one for the
concrete part and one for the ~teel part. These two construc-
tion sites will be able to work toward a common milestone
o (date) for the completion of the work. The use of steel
columns ls instrumental in reducing the time required for
completing the pro~ect, a time reduction corresponding to the
reduced work load with respect to the concrete portlon.
15 Since it is assumed that all decks for various mechanical
equipment will be enclosed inslde the upper part of the
column sectlon, and these decks therefore are built,
installed, equipped and in addition fully completed and
tested, the work on the steel portion wlll be relatlvely
20 extensive and time-consuming. The ma~or portion of this
manufacturing and completion period represents reductions in
the total manufacturing period in relation to a uniform
embodlment. The concrete/steel deslgn saves a great deal of
time in addition to offering the benefits of separated
25 construction sites, such as better general access (availabi-
lity of crane~, etc.) and more space per operator, cir-
cumstances which contribute to increased safety and a more
efficient use of personnel and equipment, to a reduction in
the number of work disciplines within a restricted area,
30 which is of essential importance for productivity, and to
less vulnerability to design changes late in the pro~ect
rsince the production of the steel portion starts later than
that of the concrete portion.
35 A floater according to the invention will also offer the
advantage that the winches of the floater's anchoring sy~tem
can be mounted in one or several of said steel column~.
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Thus, this part of the anchoring system may be finished and
ready for use as soon a~ the steel column is mounted.
Typical anchoring systems for floaters are use of ordinary
slack anchorlng cables or tenslon stays.
Partlcularly in this connectlon lt would be advantageou~ lf
the floater accordlng to the lnvention comprised two
dlametrlcally opposed steel columns mounted in the column
section, since thls would make posslble an anchorlng system
10 where only the two mentloned steel columns were equlpped wlth
the anchoring system of the floater's buoyancy section
(winches, tension devices, etc.) Such a simplifled anchoring
system is assumed to have an independent inventive signifi-
cance.
A floater accordlng to the lnventlon may have many different
structural embodiments. Thus, the column section may
advantageously consist of a number of closely grouped
columns, an embodiment which might, for example, be especial-
20 ly appropriate for a floater planned as a loadlng buoy. Inthis connectlon, in particular, the floater according to the
invention may have the type of design where the submerged
floater section is lncorporated ln the column sectlon.
Moreover, the deck section may also be greatly reduced and
25 simply consist of a top part of the column sectlon. Thus, a
floater according to the invention can conceivably be built
as a maritime structure where the individual floater sections
cannot, in terms of appearance, be distinguished from each
other.
The invention shall now be further explained with reference
to the drawings, in which:
Fig. 1 shows the mounting of a steel column on a
concrete column,
Fig. 2 shows a perspective view of a possible
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embodiment of a floater according to the
invention,
Fig. 3 shows another posslble embodiment of a
floater according to the invention,
5 Fig. 4 shows a partly cros~-sectional view of a
steel column used in the floater of Fig. 3,
Fig. 5 shows, in an elevational view, the steel
column in Fig. 4 mounted on the underlying
concrete column,
10 Fig. 6 shows an enlarged section from Fig. 5,
taken from the interaction area con-
crete/steel, and
Fig. 7 shows a corresponding section of a modi~ied
embodiment.
In Fig. 1 the upper terminating portion of a concrete column
1 is shown. This concrete column 1 represents a part of a
floater and protrudes, as shown, up through the water surface
2. A steel column ~ is shown while being lifted into
20 position on top of the concrete column 1 by means of two
crane barges 4, 5.
The combined column 1, 3 may, for example, be one of the
elements of the floater shown in Fig. 2. The floater in Fig.
25 2 is of a tgpe where the submerged buoyancy section is
incorporated in the column section, or vice versa, and no
clear division thus exists between the submerged buoyancy
section 6 and the floater's column section 7. A deck section
8 is indicated by dotted lines. This deck section may be of
30 many different designs and may even be so small that it
practically disappears, for example, in the case of a
~ loading buoy existing only in the form of a helicopter
platform or a sultable termination of the top of the column
section.
~5
The floater, as shown, is constructed of closely grouped
columns 1, 9, 10 and 11. The concrete part is cast as a
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continuous structure, here up to a level above the surface
of the water 2, and then extends upward in the form of ~teel
columns 3, 12, 13 and 14. The dlviding lines between
concrete/steel are indicated by reference numerals 15, 16 and
5 17.
A floater such as thls may be constructed by using two
separate construction sites, one for the concrete part and
one for the steel part. The steel columns can be almost
o fully completed before they are mounted on the concrete
columns (Fig. 1). Thus, each steel column may be finished
with all its decks ready for various mechanical equipment,
and the necessary equipment may also be placed in the steel
columns prior to their installation in the floater. The
15 slackly anchored floater in Fig. 2 will, as soon as the steel
columns are mounted, have its anchoring system accessible.
This means that the floater in Fig. 2, for example, in this
case may have the necessary anchoring winches 18, 19 in its
equipped ~teel columns 3 and 13, so that the sugge~ted
20 anchorage may readily be established by means of the slack
anchoring cables 20 - 23. From Fig. 2 it is apparent that
the anchoring system conceivably can be activated by using
only two steel columns, diametrically mounted in the column
section viz. the steel columns 3 and 13. Moreover, it is not
25 required that all columns should have terminating steel
portions, as in Fig. 2. Thus, when it is considered useful
or sultable, steel columns 12 and 14 may be omitted, and the
concrete columns 9 and 11 will thus be terminated at the
dividing line 16 or possibly higher or lower than this
30 dividing line. Such a group of columns may obviously also
consist of a larger or smaller number, of separate or more or
less fused, columns.
In Fig. 3 another possible embodiment of a floater according
35 to the invention is shown, here in the form of a tension stay
platform. The floater in Fig. 3 has a submerged buoyancy
section 25 of concrete, designed as a frame structure (seen
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ln the plan view), having concrete columns 26, 27, 28 and 29
protruding from each corner of the frame. The concrete
columns 26-29 extend through the surface of the water 30 up
to a certain level ~2, 3~, 34, 35. From here, the individual
column continues as a steel column 36, 37, 38 and 39. The
steel columns carry supporting structures/framework 40 for
~upporting deck modules (not shown) and for binding the
columns together.
As previously mentioned, the floater in Fig. 3 is a tension
~tay platform. The necessary tension stays are indicated by
reference numerals 41, 42, 43 and 44, and the handling/tigh-
tening equipment for the tension cables is mounted in the
respective steel columns. This equipment is in Fig. 3
indicated by reference numerals 45, 46, 47 and 48. The
connection between the tension stays and the floater i~ not
shown in further detail.
A typical steel column, as used in the floater in Fig. 3, is
20 shown in Fig. 4 in partial cross section. As apparent from
Fig. ~, the support structure 40 of the deck section is such
that the support of the modules (not shown) of the deck
~ection will be eccentrlc ln relation to the centre line of
the columns of the floater. Therefore, the steel columns
25 have in this case a special design, a reinforcement bulkhead
50 being extended from the periphery of the column and a
bulkhead 51 introduced parallel to this under the support
sy~tem 40 (Fig. 3). Similarly, two parallel bulkheads 52, 59
are introduced between the bulkhead pairs 50, 51. These
30 structural reinforcement parts will primarily functlon as
elements for distribution of stress and moment from the
- support system 40 to the steel column. At the same time,
these parallel bulkheads might be used as, for example,
storage tanks for water and diesel oil, since they might be
35 designed with considerable inner storage volume.
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Moreover, it is seen from Fig. 4 that the required number of
~teel decks 54, 55 can be constructed inside the steel
column.
In Flg. 5 the dividing line between concrete and steel is
shown, and Figs. 6 and 7 show in detail a possible interac-
tion between concrete and steel, the sections being taken
from the section area 56 indicated in Fig. 5. In Fig. 5 the
~~ concrete column is indicated by reference numeral 27 (see
also Fig. 3), and the steel column is indicated by reference
numeral 37 (see also Fig. 3).
The interaction area which is shown in detail for two
15 possible embodiment forms in respectively Figs. 6 and 7,
comprises a thick steel plate 57 placed on top of and conti-
nuously around the upper part of the concrete column 27.
Under the steel plate there are welded bolts of reinforcement
steel or other types of bolts 58, which are embedded in the
20 concrete. The number and dimensions of these bolts will
depend upon existing tensile/compressive forces. Between the
bolts there is welded a shearing plate 59 continuously around
the circumference. This has the triple function of receiving
and transmitting horizontal shearing forces, safeguard
25 against water leakage and, additionally, being made of H-
profiles, receiving and distributing vertical compressi-
ve/tensile forces. In Fig. 7 the connection is shown in an
alternative embodiment, and the bolts are replaced by two
plates of H-profiles 60.
It will be a definite advantage with respect to leakage
protection to have the H-profiles and steel plate be a
continuous welded connection around the whole circumference,
but lnstalling the elements in a continuous ring will create
~5 technical problems in terms of handling. Sectors suitable
for installation purposes should therefore be prefabricated,
the sectors being collected and welded at a suitable distance
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above the concrete edge, te~porarily suspended, for example
in pulleys. After the welding, the ring can be lowered lnto
the final, accurately ad~usted (levelled) posltion. The
steel plate/top plate 57 may have suitably spaced holes for
5 in~ection of (optionally epoxy-based) concrete. It will of
course be possible to use other installation procedures to
achieve circular continuity. The steel column 37 has, as
apparent from Fig. 5, a somewhat smaller diameter than the
concrete column 27. This difference has partly a reinfor-
o cement (concrete-technological) function, but it will also
provide a spatial installation tolerance for the steel column
in relation to the very strict building tolerances normally
set.
5 The dividing line between concrete and steel in the column
should ideally be positioned at a reasonable yet shortest
- possible distance upward, calculated from an elevation where
the stresses which are due to the compressive loads from the
deck section have reached a low, primarily constant, level.
20 This elevation can be calculated, it being assumed that the
compressive stresses ~pread down the cylindrical steel shell
of the steel column in a fan shape. Based on an assumed
stress distribution sector, and by using generally known
formulae for compressive and tensile stresses as a function
25 of a concentrated load and the thickness of the cylindrical
~teel shell in the steel column, it will be possible to set
up a stress distribution diagram showing that both compressi-
ve stresses and the induced tensile stresses from eccentrici-
ty moments will range from an asymptotic infinite value in
30 the load's point of impact to a low, constant level at some
distance from the top of the column and further down. For a
column top diameter of 25 m said distance will be approxima-
tely be equal to the diameter. Another second requirement
which ought to be satlsfled is to place the dlvldlng llne at
~5 a ~ultable height above the waterline of the structure, for
e~ample about 5 m above it, since such a placement will
provide reasonable posslbilities for inspection and mainte-
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nance. This will be a great advantage since it is expedient
to have the whole steel column accessible for inspection and
maintenance even though the connection concrete/steel
supposedly is ~ealed against leakage, taking into account
5 that a floater can have a specified expected operational life
of as much as 50 years.
By means of the invention, the advantages of the concrete
version are exploited with respect to sturdiness, heaviness
and corrosion resistance in the underwater, lower parts,
i.e., those parts of the floater that are below the surface
of the water, in combination with the elasticity/plasticity
of steel and its resulting, well documented power to level
and distribute stress, in all parts above the surface of the
15 water. The stability and general movement characteristics
are improved because the material centre of gravity i~
lowered as much as possible. It is also possible fully to
exploit the advantage of having two building sites, including
the particular advantage of having the steel parts full~
20 equipped before the connection with the concrete structure is
established.
The considerable and concentrated static and dynamic loads
between the deck section and the column section will be
25 distributed over a larger, suitable area, providing a very
advantageous reduction of the stress level and a satisfactory
interaction between steel and concrete.
