Note: Descriptions are shown in the official language in which they were submitted.
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01 FIELD OF mHE INVENTION
_
G2 The present invention relates to offshore s.ructures fcr
03 use in arctic and o';her ice-infested waters, and, more particu-
04 larly, to an offshore structure which is able to ~-ithstand the
05 forces imposed thereon by impinsing ice sheets and other larger
06 ice masses.
0 7 BACKGROUND OF THE INV~NTION
08 In recent years, offshore exploration and production of
09 petroleum products has been extended into arctic and other ice-
infested waters in such locations as northern Alaska and Canada.
11 These waters are generally covered with vast areas of sheet ice 9
12 ~onths or more out of the year. Sheet ice may reach a thickness
13 of 5 to 10 feet or more, and may have a compressive or crushing
14 strength in the range of about 200 to 1000 pounds per square inch.
Although appearing stationary, ice sheets actually move laterally
16 with wind and water currents and thus can impose very high forces
17 on any stationary structure in their paths.
18 A still more severe problem encountered in arctic waters
19 is the presence of larger masses of ice such as pressure ridges,
rafted ice or floebergs. Pressure ridges are formed when two
21 separate sheets of ice move toward each other and collide, the
22 overthrusting and crushing of the two interacting ice sheets
23 causing the formation of a pressure ridqe. Pressure ridges can be
24 very large, with lengths of hundreds of feet, ~idths of more than
a hundred feet and a thickness of up to 50 feet. Consequently,
26 pressure ridges can exert a proportionally greater force on an off-
27 shore structure than ordinary sheet ice; thus, the possibility of
28 pressure ridges causing extensive damage to an offshore structure
29 or the catastrophic failure of a structure is very great.
A structure built stron~ enough to resist the crush-
31 ing force exerted thereon by impinging ice, that is, strong enough
32 to permit the ice to be crushed against the structure, enabling
33 _ ~ _
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01 the ice to ~-low around it, would likely be very massive and corre-
02 spondingly expensive to construct. Therefore, it has been
03 proposed heretofore that structures which are to be used in ice-
Q4 infested waters shouid be built with a sloping or ramp-like outer
05 surface rather than with a surface which is vertically disposed to
06 the impinging ice. As the ice comes into contact with the sloping
07 outer su!face, it is forced upwardly above its normal position
08 which causes the ice to fail in flexure by placing a tensile
0~ stress in the ice. Since ice has a flexural strength of about 85
pounds per square inch, a correspondingly smaller force is imposed
11 on the structure as the ice impinging thereon fails in flexure
12 rather than compression.
13 Several forms of conical offshore structures having
14 sloping outer surfaces are illustratéd in a paper by J.V. Danys
entitled "Effect of Cone-Shaped Structures on Impact Forces of Ice
16 Floes", presented to the First International Conference on Port
17 and Ocean Engineering under Arctic Conditions, held at the
18 Technical University of Norway, Trondheim, Norway, during August
19 13-30, 1971. Another paper of interest in this respect is that
presented by Ben C. Gerwick, Jr., and ~onald R. Lloyd, entitled
21 "Design and Construction Procedures for Proposed Arctic Offshore
22 Sttuctures", presented at the Offshore Technology Conference in
23 Houston, Texas, April 1970.
24 As an ice sheet moues relative to and in contact with
the sloping outer surface of a conical structure, it will be
26 elevated along the slo~ing surface. The elevation of the ice
27 sheet causes initial cracks to be formed in the sheet, which
28 radiate outwardly from the poïnt of contact. Circumferential
29 cracks then form and cause the ice sheet to break up into wedge-
shaped pieces. The approximate total force exerted on a conical
31 structure then consists primarily of the force required to fail
32 the impinging ice sheet in flexure, that is, the force required to
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01 for~ the initial radial or subsequent circumferential cracks, and
02 the force ca~sed by the hroken ice pieces riding up on the outer
03 surface of the structure and interacting therewith.
04 The orce associated with the formation of initial and
05 circumferential cracks in the ice sheet is primari1y a function of
06 the particular mechanical and geo~etrical ~ro~erties of the ice
07 impinging on the structure. The ride-up force is due to the
08 broken ice pieces interacting with the structure and thus is depen-
09 dent upon the surface area of the structure above the water line.
Therefore, to reduce the total ice forces i~posed on a conical
11 structure, it is always desirable to keep the waterline diameter
12 of the structure as small as possible.
13 Larger ice masses such as pressure ridges impacting a
14 conically shaped structure will be lifted along the sloping outer
surface of the structure to cause the ridges to fail in flexure.
16 As with ice sheets, a radial crack will form in the ridge at the
17 point of impact; the formation of a radial crack is followed by
18 the formation o. hinge cracks that occur at a relatively greater
19 distance Erom the structure. As the ridge continues to move into
the structure, it will break into large blocks of ice which fall
21 away from the structure.
22 As indicated above, the force imposed on a structure by
23 an impinging pressure ridge is much greater than that of an
24 impinging ice sheet. The approximate total force exerted on a
conical structure by a pressure ridge is a combination of the
26 force required to fail the impinging ridge in flexure and the
27 force caused by the broken ice pieces, formed by the failure of
28 the ice sheet advancing ahead of the presure ridge, riding up on
29 the outer surface of the structure and interacting therewith. The
large blocks of ice formed when a pressure ridge fails in flexure
31 tend not to ride up the outer surface of the structure; there-
32 fore, the ride-up force is essentially a result of pieces of sheet
33 ice riding up the structure's outer surface.
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Gl Since structures located in -~aters in which larger ice
0~ masses are present are exposed to relatively greater ice forces,
03 they must be built strong enough to withstand these greater ice
04 forces. Utilizing present bottom-supported conical structure
05 designs requires supporting the structure by means of additional
06 foundation support, such as piling; however, this would increase
07 the cost and time of installation of the structure. Without
08 additional foundation support, the structure would have to be made
09 larqer and stronger to resist the greater ice forces, which would
necessitate increasing its waterline diameter. This, however,
11 would increase that component of the total ice force associated
12 with the ride up of ice pieces on the structure, since the ride-up
13 force is proportional to the surface area of the structure above
14 the waterline. For a very large cone waterline diameter, this
component o~ the force would be substantially greater than the
16 force required to fail the impinging ice in flexure.
17 Accordingly, present conical structures built strong
18 enough to withstand the forces associated with larger ice masses
19 would be correspondingly more expensive to construct and install
than one merely designed to withstand the forces associated with
21 an impinging ice sheet. In fact, such str~ctures could be so
22 massive as to be impractical and economically prohibitive to
23 build. The present invention is directed to an offshore structure
24 which is able to withstand the forces associated with large
implnging ice masses, and at the same time is feasible from an
26 economic and si~e standpoint.
27 SUMMARY OF THE INVENTIO~
28 Broadly speaking, the present invention comprises an off-
29 shore structure which is designed for operation in an arctic
offshore environment in which sheet ice and other larger masses or
31 ice, such as plessure ridges, are present. The offshore struc-
32 ture of this invention includes a lower portion in the shape of
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a truncated cone coaxially positionable on top of a base
portion. An upper portion of the structure is in the shape
of a second truncated cone and is coaxially positionable on
top of said lower portion. The walls forming the upper and
lower portions of the structure are inclined at an angle to
the horizontal to receive ice masses moving relative to and
in contact with the structure in order to cause the ice masses
to fail in flexure. The angle of inclination from the
horizontal of the walls of the upper portion is greater than
that of the lower portion. The cross-sectional diameter of
the bottom of the second truncated cone forming the upper
portion is substantially equal to the cross-sectional
diameter at the top of the first truncated cone forming the
lower portion.
The angle of inclination of the walls of the
upper portion is between approximately 26 and 70 from the
horizontal, with the preferred range being between approxi-
mately 54 and 58 from the horizontal. The angle of inclina-
tion of the walls of the lower portion is between approxim-
ately 15 and 25 from the horizontal, with the preferred
range being between approximately 19 and 23 froM the
horizontal.
The above offshore structure configuration
permits the structure to be utilized in waters which contain
ice sheets and relatively larger ice masses without unneces-
sarily increasing the mass and the cost of a structure.
Thus, in accordance with one aspect of this
invention there is provided an offshore structure for use
in a body of water that contains ice masses, comprising:
a lower portion substantially in the shape of
a first truncated cone sloping inwardly and upwardly so that
the walls of said lower portion are inclined at an angle
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between about 15 to 25 to the horizontal to provide a ramp-
like surface to receive ice masses moving relative to and in
contact with said structure;
means for affixing said lower portion to the
bottom of a body of water; and
an upper portion coaxially positionable above
said lower portion, said upper portion substantially in the
shape of a second truncated cone sloping inwardly and upwardly
so that the walls of said upper portion are inclined at an
angle between about 26 and 70 to the horizontal to provide
a ramp-like surface to receive ice masses moving relative
to and in contact with said structure, and the cross-sectional
diameter of the bottom of said second truncated cone forming
said upper portion being substantially equal to the cross-
sectional diameter at the top of said first truncated cone
forming said lower portion.
In accordance with another aspect of this
invention there is provided a marine structure for use in a
body of water that contains ice masses, comprising:
a base portion;
means for affixing said base portion to the
bottom of a body of water;
a lower portion coaxially positionable on top
of said base portion for joining thereto, said lower portion
substantially in the shape of a first truncated cone sloping
inwardly and outwardly so that the walls of said lowerportion are inclined at an angle between about 15 and 25
to the horizontal to provide a ramp-like surface to receive
ice masses moving relative to and in contact with said
structure; and
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an upper portion coaxially positionable on
top of said lower portion for joining thereto, said upper
portion substantially in the shape of a second truncated
cone sloping inwardly and upwardly so that the walls of said
upper portion are inclined at an angle between about 26 and
70 to the horiæontal to provide a ramp-like surface to receive
ice masses moving relative to and in contact with said
structure, and the cross-sectional diameter of the bottom of
said second truncated cone forming said upper portion being
substantially equal to the cross-sectional diameter at the
top of said first truncated cone forming said lower portion.
In accordance with another aspect of this
invention there is provided an offshore structure for use
in a body of water which becomes frozen through natural
conditions, comprising:
a supporting base portion positioned in a body
of water;
means securing said base portion to the under-
water bottom;
a lower portion directly joined to and rigidly
supported on said base portion, said lower portion forming
a first circumferential wall which converges upwardly and
inwardly from said base portion at an angle of 15 to 25
from the horizontal to receive and support an edge portion
of a sheet of ice or other ice mass which moves in contact
with said lower portion so as to elevate said ice above its
natural level an amount to cause said ice to fracture con-
tinuously adjacent said offshore structure;
an upper portion directly joined to and rigidly
supported on said lower portion, said upper portion forming
a second circumferential wall which converges upwardly and
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inwardly of said lower portion at an angle of 26 to
70 from the horizontal to receive and support an edge
portion of a sheet of ice or other ice mass which moves
into contact with said lower portion so as to elevate
said ice above its natural level an amount to cause said
ice to fracture continuously ad~acent said offshore struc-
ture, said second circumferential wall having a bottom
base diameter substantially e~ual to the top diameter
of said first circumferential wall.
In accordance with another aspect of this invention
there is provided a marine structure to be located in
a body of water that contains ice masses, comprising:
a base portion;
means for securing said base portion to the
5 bottom of a body of water;
a lower portion coaxially positionable on top
of said base portion, said lower portion substantially
in the shape of a first truncated cone so that the walls
of said lower portion are inclined to the bottom of the
body of water at an angle of between approximately 15
and 25 from the horizontal to provide a first ramp-
like surface to receive ice masses moving relative to
and in contact with said structure; and
an upper portion coaxially positionable on top
of said lower portion, said upper portion substantially
in the shape of a second truncated cone so that the walls
of said upper portion are inclined to the bottom of the
body of water at an angle of between approximately 26
and 70 from the horizontal to provide a second ramp-
like surface to receive ice masses moving relative toand in contact with said structure, the cross-sectional
diameter of the bottom of said upper portion being no
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greater than the cross-sectional diameter at the top
of said lower portion so that the walls of said upper
portion and said lower portion form a continuous ramp-
like surface; and
a cylindrical throat portion supported on said
upper portion.
Another aspect of this invention is as follows:
A method of reducing the ice forces imposed
on an offshore structure which contains ice masses, com-
prising:
constructing an offshore structure with a lower
portion substantially in the shape of a first truncated
cone so that the walls of said lower portion are inclined
at an angle between about 15 to 25 to the horizontal
to provide a ramp-like surface to receive ice masses
moving relative to and in contact with said structure;
and
positioning an upper portion of said structure
coaxially on top of said lower portion, said upper portion
substantially in the shape of a second truncated cone
so that the walls of said upper portion are inclined
at an angle between about 26 to 70 to the horizontal
to provide a ramp-like surface to receive ice masses
moving relative to and in contact with said structure,
the cross-sectional diameter of the bottom of said second
truncated cone forming said upper portion being no greater
than the cross-sectional diameter at the top of said
first truncated cone forming said lower portion.
Another aspect of this invention is as follows:
A method of reducing the ice forces imposed
on an offshore structure which contains ice masses, com-
prising:
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3~
constructing an offshore structure with a base
portion so that said base portion may be affixed to the
bottom of a body of water;
positioning a lower portion of said structure
coaxially on top of said base portion, said lower
portion substantially in the shape of a flrst truncated
cone so that the walls of said lower portion are
inclined at an angle between about 15 to 25 to the
horizontal to provide a ramp-like surface to receive ice
masses moving relative to and in contact with said
structure;
joining said lower portion to said base portion;
positioning an upper portion of said structure
coaxi.ally on top of said lower portion, said upper
portion substantially in the shape of a second truncated
cone so that the walls of said upper portion are
inclined at an angle between about 26 to 70 to the
horizontal to provide a ramp-like surface to receive ice
masses moving relative to and in contact with said
structure, the cross-sectional diameter of the bottom of
said second truncated cone forming said upper portion
being no greater than the cross-sectional diameter at
the top of said first truncated cone forming said lower
portion; and
joining said upper portion to said lower portion.
Other aspects of this invention are as follows:
A transportable offshore structure adapted to be
deployed in a body of water having moving ice masses of
various thicknesses, said structure comprising:
a superstructure for conducting working operations;
and a transportable suhstructure supporting said
superstructure, said substructure having a base and
being sized to extend from the floor oE the body of
water to above its surface with said base resting on
said floor,
said substructure including an inwardly and
upwardly sloping outer wall to engage the ice masses,
said wall having a waterline and a plurality of ice
engaging wall sections with different slopes, the slopes
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of said wall sections becoming progressively steeper
towards the upper end of said substructure, and the
slope of a least one of said wall sections, intended to
lie completely below the waterline, being between 15
and 25 from the horizontal.
A transportable offshore structure adapted to be
deployed in a body of water having moving ice masses of
various thicknesses, said structure comprising:
a superstructure for conducting working operations;
and a transportable su~structure supporting said
superstructure,
said substructure having a base and being sized to
extend from the floor of the body of water to above its
surface with said base resting on said floor,
said substructure including an inwardly and
upwardly sloping outer wall to engage the ice masses,
said wall having a waterline and a plurality of ice
engaging wall sections with varying slopes, the slopes
of said wall sections becoming progressively steeper
towards the upper end of said substructure, and all
points on the slope of at least one of said wall
sections, intended to lie completely below the
waterline, being between 15 and 25 from the
horizontal.
A transportable offshore structure adapted to be
deployed in a body of water having moving ice masses of
various thicknesses, said structure comprising:
a superstructure for conducting working operations;
and a transportable substructure supporting said
superstructure,
said substructure having a base and being sized to
extend from the floor of the body of water to above its
surface with said base resting on said floor,
said substructure including an inwardly and
upwardly sloping outer wall to engage the ice masses,
said wall having a waterline and a plurality of ice
engaging wall sections with different slopes, the slopes
of said wall sections becoming progressively steeper
~ towards the upper end of said substructure, and the
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slope of the lowermost wall section, adapted to lie
completely below the waterline, being between 15 and
25~ from the horlzontal.
A transportable offshore structure adapted to be
deployed in a body of water having moving ice masses of
various thicknesses, said structure comprising:
a superstructure for conducting working operations;
and a transportable substructure supporting said
superstructure,
said substructure having a base and being sized to
extend from the floor of the body of water to above its
surface with said base resting on said floor,
said substructure including an inwardly and
upwardly sloping outer wall to engage the ice masses,
said wall having a plurality of ice engaging wall
sections with different slopes, the uppermost ice
engaging wall section having a slope not exceeding 70
to the horizontal and having the structure's waterline
located on it, the slope of a lower ice engaging wall
section being no greater than 25 to the horizontal and
the lowermost ice contacting wall section having a slope
of no less than 15 to the horizontal, the slopes of the
wall sections becoming progressively steeper towards the
upper end of the substructure.
A transportable offshore structure adapted to be
deployed in a body of water having moving ice masses of
various th:icknesses, said structure comprising:
a superstructure for conducting working operations;
and a transportable substructure supporting said
superstructure,
said substructure having a base and being sized to
extend from the floor of the body of water to above its
surface with said base resting on said floor,
said substructure including base, an internal frame
assembly and an inwardly and upwardly sloping outer wall
to engage the ice masses, said wall having a waterline
and a plurality of ice engaging wall sections with
different slopes, the slopes of said wall sections
becoming progressively steeper towards the upper end of
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said substructure, and the slope of at least one of said
wall sectlons, intended to lie completely below the
waterline, being between 15 and 25 from the
horizontal.
A transportable offshore structure adapted to be
deployed in a body of water having moving ice masses of
various thicknesses, said structure comprising:
a superstructure for conducting working operations;
and a transportable substructure supporting said
superstructure,
said substructure having a base and being sized to
extend from the floor of the body of water to above its
surface with said base resting on said floor,
said substruc~ure including an inwardly and
lS upwardly sloping outer wall to engage the ice masses,
said wall having two ice engaging wall sections, with
slopes arranged so that the slopes of said wall sections
become progressively steeper towards the upper end of
said substructure, the uppermost ice engaging wall
section having a waterline and a slope not exceeding 70
to the horizontal, the slope of the lowermost ice
engaging wall section, adapted to lie completely below
the waterline, being between 15 and 25 to the
horizontal.
A transportable offshore structure adapted to be
deployed in a body of water having moving ice masses of
various thickness, said structure comprising:
a superstructure for conducting working operations;
and a transportable substructure supporting said
superstructure,
said substructure having a base and being sized to
extend from the floor o.E the bod~ of water to above its
surface with said base resting on said floor,
said substructure including an inwardly and
upwardly sloping outer wall to engage the ice masses,
said wall having three ice engaging wall sections with
slopes arranged so that the slopes of said wall sections
become progressively steeper towards the upper end of
said substructure, the uppermost ice engaging wall
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section having a waterline and a slope not exceeding 70
to the horizontal, the middle ice engaging wall section
having a slope of approximately 30 to the horizontal
and the lowermost ice engaging wall section having a
slope of at least 15 to the horizontal.
A transportable offshore structure adapted to be
deployed in a body of water having moving ice masses of
various thicknesses, said structure comprising:
a superstructure for conducting working operations,
said substructure having a base and being sized to
extend from the floor of the body of water to above its
surface with said base resting on said floor,
said substructure including a waterline and an
inwardly and upwardly sloping outer wall to engage the
ice masses, said wall having a plurality of ice engaging
wall sections with different slopes arranged so that the
slopes of said wall sections become progressively
steeper towards the upper end of said substructure, each
of said wall sections being curved and merging into its
adjacent wall section to define a continuously varying
outer ice engaging wall surface with at least one point
on the slope of said wall sectio,ns below the waterline
being between 15 and 25 to the horizontal and no point
on the slope of said wall sections below the watexline5 being less than 15 or more than 70 to the horizontal.
PRINCIPAL OBJECT OF THE INVENTION
An object of an aspect of the present invention is
to provide an offshore structure which is able to
withstand the forces imposed thereon by impinging ice
sheets and larger ice masses and which incorporates less
structural material and which is correspondingly less
massive and costly.
Additional objects and advantages o~ the invention
will become apparent from a detailed reading of the
specification and drawings which are incorporated herein
and made a part of this specification.
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BRIEF DESCRIPTION_O~ THE DRAWINGS
FIG. 1 is a schematic side elevational view, partly
in section, illustrating the preferred embodiment of the
invention;
FIG. 2 is a schematic illustration in section taken
along line 2-2 of FIG. l;
FIG. 3 is a partial perspective view showing the
upper and lower conical portions and the throat portion
as being fabricated from steel plate; and
FIG. 4 is a schematic side elevation view, partly
in section, illustrating another embodiment of the
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, FIG. 1 represents a
marine structure 15 located in a body of water 30 and
particularly designed for installation in arctic waters
upon which thick sheets of ice 20 and larger masses of
ice such as pressure ridges 22 may be formed. The
structure is held in place on the underwater bottom 12
by its own weight plus the weight of any ballast, as
will be discussed in more detail below, added to the
structure. In unusually severe ice conditions, to
assist in holding the structure in place against the
horizontal forces imposed thereon by an impinging ice
mass, piling 18, as illustrated in FIG. 4, may be driven
through internal guides, not shown, in the base portion
2 and into the underwater bottom 12. The piles may also
be used to support the vertical loads imposed on the
structure. Such piling, of course, if used, is detached
from the structure prior to moving the structure to a
new drilling site.
A work platform 10 of structure 15 is illustrated
in FIG. 1 with a drilling rig 45 located on its deck 42;
other conventional drilling equipment, which is not
illustrated, may also be located on work platform 10.
The invention, however, is not restricted to offshore
structures used to support drilling rigs. It is
suitable for any type of offshore operation conducted in
7-
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01 arctic waters in which there is a need for r,rotection against ice
02 masses formed on such waters.
03 T~e work pl~tform 10 ~ay actually contain several addi-
09 tior.al levels of decks 40 and 41 w~,ich serve as living quarters
05 and working areas for the personnel on the structure. The decks
06 may be enclosed and heated to provide a reasonably comfor~able
07 working environment which offers protection for ~en and equipment
08 during winter weather, during which temperatures may drop to the
09 range of -60F. The interior of the structure may also contain
storage and equipment comparments which are illustrated generally
11 by reference numeral 60.
12 Offshore structure 15 is constructea to be readily estab-
13 lished with full operating capacity at a selected drilling site
14 and with the ability to be moved from one drilling site and estab-
lished at another in operating condition without delay. To this
16 purpose, ballast tanks 62 are integrally built into the intecior
17 of the structure to provide appropriate stability when the struc-
lg ture is being towed and to enable the structure to be lowered
19 through the water and into contact with the sea bottom. The
ballast tanks may, of course, be trimmed as necessary to compen~
21 sate Eor any uneven distribution of weight within the structure.
22 The ballast tanks are each provided with appropriate means, such
23 as sea cocks and a blowdown pipe, neither of which is illustrated,
24 for remotely controlling the amount of water in the tanks so that
the buoyancy of the structure is adjustable.
26 As indicated above, a drill rig 45 is located on decks 42
27 along with other conventional drilling equipment, not shown, for
28 use in drilling a well bore 90 within the subsuraces. A moon-
29 pool Ol drillway 50 thus extends from deck 42 down through the
structure to water bottom 12 so that drill string g2 may be
31 extended into wellbore 90. Since it is both expensive and diffi-
32 cult to construct and install a structure in arctic waters, it is
33 desirable that the structure be provided with the capability to
34 _ ~ _
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01 drill a number of wells at any particular site. For example, a
02 structure may be designed to drill two or more wells to a depth of
03 approximately 20,000 feet. Accordingly, the structure must be
04 made large enough to accommodate the e~uipment necessary for this
05 purpose.
06 An offshore structure large e~ough to carry out the
07 above-described drilling activities will weigh several thousand
08 tons before it receives any of the equipment necessary for the
09 drilling operation. Moreover, the weight o~ existing designs for
bottom-supported structures might increase proportionately as the
11 structure is designed to withstand greater natural ice forces sùch
12 as those associated with larger ice masses such as pressure
13 ridges. Since the weight of the structure is directly related to
14 its costs, the cost will proportionately increase as the weiqht
increases. The present invention is directed toward an offshore
16 structure conEiguration for minimizing the forces imposed on the
17 structure by impinging ice sheets and larger masses of ice, and,
18 at the same time, permitting less structural material to be incor-
19 porated in the structure and correspondingly reducing its mass and
cost.
21 As discussed hereinabove, an ice sheet that moves into
22 contact with the sloping surface of a conically shaped offshore
23 structure will fail in flexure resulting in the ice sheet being
24 broken into wedge-shaped segments. As the ice sheet continues to
move a~ainst the structure, the wedge-shaped pieces of ice will
26 ride up the outer surfaces of the structure and ideally fall away
27 from and be swept around the structure. As the ice masses
28 impinging on the structure become larger, the forces imposed
29 thereon are likewise increased. To prevent failure of the present
designs for bottom-supported conical structures, when a larger
31 mass of ice such as a pressure ridge moves into contact with the
32 structure, several things may possibly be done. First, the base
33 _ 9 _
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01 diamete~ of the structu~e and thl~s i.s size may be increased ~o
~2 {esist the larger ice forces. Second, the StLUCtUre may be
03 provided with a rather gently sloping surface, which also
04 increases its size, to receive the impinging pressure ridge; this
05 has the effect o~ reducing the tot31 ice force imposed on the
06 structure by the impinging ridge, since that component of total
07 force due to flexural failure of a ridge decreases as the angle of
08 inclination from the horizontal of the sloping surface decreases.
09 Third, the structure may be supported by piling; however, this is
undesirable because the cost and time of installation for the
11 structure at a selected drilling site would be increased.
12 To resist the greater forces associated with larger
13 impinying ice masses, the size of present designs for bottom-
14 supported conical structures would then have to be increased,
which necessitates incorporating more structural material in the
16 structure which increases its mass and thus its cost, making it
17 prohibitively expensive to build. Moreover, as these structures
13 are built larger to resist the forces oî large impinging ice
19 masses, the total ice force imposed on the structure increases.
~s pointed out previously, the total ice force exerted on a
21 conical offshore structure~ essentially con~:ists of the force
22 requireA to fail the impinging ice mass in flexure and the force
23 caused by broken pieces of sheet ice riding up the outer surface
24 of the structure and interacting therewith, This ride-up force
depends upon the weight of the ice pieces as well as the force of
26 friction existing between the ice and the outer surfaces of the
27 structure. Thus, it can be seen that ride-up ice force is propor-
28 tional to the surface area of the conical structure above the
29 waterline. Therefore, as the size of the structure is increased,
the ride-up force imposed on the structure is li~ewise increased,
31 and for conic~l structures having relatively large waterline
32 diameters, the ride up .orce ma~ well exceed the force required to
33 fail the impinging ice mass in flexure.
34 - 10 -
123C~7~i
01 Accordingly, there i5 provided in accordance with the
02 present invention an offshore structure which is able to withstand
03 the forces imposed thereon by an impinging ice heet 20 or some
04 other larger mass of ice such as a pressure riZqe 22 wherein the
05 ~ass and cos~ of the structure is not unnecessarily increased.
06 This structure basically has, as illustrated in FIGS. 1-3, a lower
07 conically shaped portion 4 and upper conically shaped portion 6
08 coaxially positioned with respect to one another to for~ a
09 continuous external shell which is adapted to receive ice masses
moving relative to and in contact with the structure. It is con-
11 templated that the external shell of the structure is to be con-
12 structed from steel plate, as illustrated in FIG. 3, but other
13 materials, such as prestressed concrete, may be used.
14 The upper portion 6, as can be seen, is in the shape of
a truncated cone wherein the walls form a ramp-like surface 16
16 which is inclined at an angle to the horizontal so that surface 16
17 converges upwardly and inwardly of lower portion 4. The lower
18 portion 4 of the structure likewise is in the shape of a truncated
19 cone, but is of larger cross-sectional diameter than upper portion
6; that is, the base diameter of the cone forming upper portion 6
21 is no greater than the top diameter of the cone-forming lower
22 portion 4. The walls of lower portion 4 converge upwardly and
23 inwardly of base portion 2 to form a ramp-like surface 14 which is
24 inclined at an angle to the horizontal, hut at an angle of incli-
2S nation rom the horizontal which is greater than that of lower
2~ portion 4.
27 Thus, the waterline diameter of uppel section 6 is ke?t
28 as small as practicable to reduce the ride-up forces acting on the
29 structure On the other hand, to enable the structure to with-
stand the forces associated with larger impinging ice masses, a
31 relatively large lower section 4 with a reduced ansle of inclin-
32 ation is provided. The reduced angle of inclination of lower
-- 1 1 --
~30'~5
section 4 offers the advantage of reducing the forces
imposed on the structure by the flexural failure of a
pressure ridge. Additionally, the relatively large
lower section 4 decreases the likelihood of foundation
failure of the structure, as well as improving its
flotation stability.
The base portion 2 of the structure may also have a
conical shape so that its walls converge upwardly and
inwardly of the underwater bottom 12, with the top
diameter of the base portion being approximately equal
to the bottom diameter of lower portion 4. This
particular shape is useful from the point of view that
it imparts additional stability to the structure when it
is being moved through the water. In addition, the
ramp-like surface of base portion 2 may assist in
failing an impinging pressure ridge. Of course, base
portion 2 may have other appropriate shapes, such as
that of a cylinder, so that walls of the base portion
are vertically disposed to the underwater bottom.
By way of illustration, offshore structure 15 for
installation in waters having a depth of between twenty
and sixty feet may have a base portion with a bottom
diameter of approximately 250 feet and height of
approximately five feet. The particular value for the
base diameter is essentially a function of the flotation
characteristics of the structure and the desired ability
of the structure to resist failure when large ice forces
are imposed on the structure. The lower portion 4 may
have a height of approximately 25 feet and the upper
portion 6 may have a height of approximately 40 feet.
In waters having a depth of between approxi.mately
sixty and thirty feet, larger ice masses, such as
pres~ure ridge 22, extend a considerable distance below
the surface of the water; therefore when they move
relative to and in contact with structure 15, the edge
portion of the ridge 22 will be received by wall of the
lower portion 4 and lifted along surface 14, causing
-12-
~3q:~7~i
01 the rid~e to ~all in flexure. ;~.s the p~ eSSU~;Q ridqQ lS eievated
02 along sur'ace 14, it breû'~s irto blocks of ice which tend to slide
{)3 beneath the ice sheet a~vancillc, behind the ridae; the blocks of
04 ice are thQn swept laterally around the structure. Surface 16 of
0.~ upper portion 6 will receive the ice sheets impinging on the struc-
06 tu~e, and, as described, cause them ~o fail in flexure.
~7 If the structure were lo~ated in reiatively shallow
08 waters, that is, ~aters having a depth of less than thirty feet,
09 the lower conical portion 4 would receive and fail in flexure ice
sheets and smaller pressure ridges impinging on the structure.
11 The only force imposed on the upper portion 6 would be that asso-
12 ciated with the ride up of ~ieces of sheet ice on surface 16.
13 To assist the movement of ice relative to and over the
14 outer surfaces of the upper portion 6 and lower portion 4 of the
structure and to prevent ride-up ice pieces from freezing to these
~r~e-ez~
6 surfaces, appropriate~~fTeeZe,prevention apparatus should be
~e
17 used. ~t~F~e prevention procedures include heatinq the outer
18 surfaces 1~ and 16 of the structure, as disclosed in Chevron
19 R~search ~omlpany'c. U.S. Patent 3,831,385, or coating the surfaces
with a material that reduces ice adhesion, as disclosed in Chevron
~1 ~esearch Company's U.S. Patent 3,972,199.
22 The angle of inclination of the walls ~f the lower
~3 portion 4 and the upper portion 6 of the structure are indicated
24 by ~1 and a2, respectively. These two angles are acute angles
which should be steep enough to cause failure of an ice mass in
2G IlexLIre. The value of ~1 needs to be small enough so that the
27 force associated with the flexural failing o a large ice mass is
28 minimized. However, the ~alue of ~1 should not be too small, as
2~ the base of the structure would then be .oo large, ma~;ing the cost
of the structure economically prohibitive. ~he value f ~2 is
31 large enough such that the surface area of the structure above the
32 waterline is minimized, but nol so large as to cause ar; impinging
33 - 13 -
~3~'7~5
ice sheet to fail in compression rather than flexure.
In most multi-angle conical structures, ~1 and ~2 may
range between approximately 15' and 25' and 26' to 70'
from horizontal, respectively. The preferred range of
~ 1 is between approximately 19' to 23' from the
horizontal, and the preferred range of ~2 is between
approximately 54' and 58'. The preferred angle for ~1'
and ~2~ respectively, is essentially dependent upon
three factors, namely, the range of water depths in
which the structure is to be located, the expected size
of ice sheets and pressure ridges in these waters, and
the soil characteristics of the sea floor on which the
structure is to be supported. Therefore, if the
structure is to be operated in the near shore areas of
the waters off northern Alaska at a water depth which
may range between twenty and sixty feet, the preferred
angle for ~1 is approximately 21' from the horizontal
and the preferred angle for ~2 is approximately 56' from
the horizontal.
As illustrated in FIG. 1, the throat portion 8 of
the structure, which has a cylindrical shape, is
coaxially positioned on top of and vertically abuts
upper portion 6 and extends work platform 10 above the
surface of the body of water 30 to a height sufficient
to avoid contact with pieces of sheet ice riding up the
structure. An inverted truncated cone section 9 may be
positioned between the throat portion 8 and the work
platform 10. Section 9 deflects pieces of sheet ice
riding up throat portion 8, preventing them from causing
damage to work platform 10 and from increasing the total
ice force imposed on the structure. Alternately, AS
illustrated in FIG. 4, the work platform itself may be
in the shape of an inverted truncated cone, so that ice
pieces riding up the structure are prevented from
contacting the uppermost deck 42 of the structure and
from increasing the ice force imposed on the structure.
I The angle of inclination of the walls of the inverted
truncated cone section 9 and the inverted truncated cone
work
-14-
~ 3~ S
01 platform 10 is ~epicted b~ ~. In most structure àesigns, ~ may
32 range between approximately 25 and 70 from the vertical.
03 While it is conte~plated that structure 15 will be towed
04 tO the drilling si~e in a completely asse:nbled condition with no
05 additional construction at the site being necessary, it would cer-
06 tainly be possible and perhaps desirable to tot~ individual sec-
07 tions of the structure from their place of fabrication to the
08 drilling site for assembly. For example, base portion 2 could be
09 brought to the drilling site and placed on the underwater bottom
12. Lower portion 4 could then be brought to the drilling site
11 and positioned in abutting relationship on top of and joined b~
12 appropriate means to base portion 2. Likewise, upper portion 6
13 would be brought to the drilling site and positioned on top of
14 lower portion 4 and joined to lower portion 4. In a like manner,
the other components of the structure could be assembled at the
16 drilling site.
17 The advantages of this disclosure can also be realize~
18 by minor variations in the configuration of the structure wherein
19 the ramp-like outer surface of the structure has a multi-cone
geometry of~more than two conical sections or a continually curved
2i geometry such as a portion of a hyperboloid oE revolution.
22 A model of the multi-angle conical stcucture of the
23 present invention has been tested in an ice laboratory under simu-
24 lated arctic conditions. One of the purposes of the test was to
study the forces imposed on the structure by an impinging ice
26 sheet. l'he model was built on a scale factor of 1:50, and all
27 other scale factors for the test, such as ice sheet thickness and
28 effective water depth, were based on â corresponding scale of
29 1:50. It is interesting to note some of the observations made
during the course of these tests.
31 In prior tests of monocone structures, a noted pheno~-
32 enon has been the formation of fields of ice rubble in front of
33 - 15 -
~L~3~'7~5i
01 the stru_ture between its outer ~urface an~ the advancing ice
02 sheet. These ~lelds a~e formed when the b~oken seg~.ents of sr,eet
03 ice ride up the outer surface of the structure and fall bac~ in
04 front of the structure. The rubble ice formed between the
OS advancing ice sheet and the conical structure increases the total
06 ice force imposed on the structure. This phenomenon, however,
07 does not occur with respect to the multi-angle conical structure
08 of this invention. Instead, the ice pieces have a tendency to
09 ride up and around the outer surface of the upper conical portion
of the structure. Apparently, this is due to the fact that the
11 smaller dia;neter of the upper conical portion and the relatively
12 shallow an~le of the lower conical portion facilitate the movement
13 of ice pieces around and away from the structure.
14 Another interesting result observed durins tests of the
multi-angle conical structure is the reduction, as compared with
16 monocone structures, in the vertical component of the oscillatory
17 force imposed on the underwater bottom on which the structure is
18 supported. The total vertical force imposed on the surface
19 beneath the structure i5 approximately the sum of the weight of
the structure and the vertical component of the total ice force
21 imposed on the structure. The oscillating vertical force i5
22 related to the number of times in which an edge portion of an
23 advancing sheet of ice is failed. The reduction in the oscil-
24 lating vertical force reduces the magnitude of cyclic soil loading
which reduces the likelihood of foundation failure over the life
26 of the structure.
27 Although certain specific embodiments of the invention
28 have been described herein in detail, the invention is not to be
29 limited to only such embodiments, but r~ther only by the appended
claims.
31 - 16 -
