JPS6218683B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an offshore structure for use in the polar ocean and other ice-ravaged waters, particularly an offshore structure capable of withstanding the forces exerted by the impact of ice sheets and other large ice masses. It's about the body.

Recently, offshore exploration and production of petroleum products has extended to the polar oceans and other ice-affected areas such as northern Alaska and Canada.
These areas are generally covered with vast areas of sheet ice for nine months or more of the year.
The ice sheets may reach a thickness of 1.5 m to 3 m or more and have a compressive or crushing strength in the range of about 15 Kg/cm ² to 70 Kg/cm ² . Although apparently stationary, the ice sheets are actually moving laterally due to wind and water currents, thus creating extremely high forces on any stationary structure within the path of these ice sheets. can be added.

An even more severe problem experienced in frigid waters is the presence of large ice masses such as pressure ridges or ice floes. Pressure ridges are formed when two separate ice sheets move toward each other and collide. That is, a pressure ridge is formed by the overlapping and crushing of two interacting ice sheets. Pressure ridges can be quite large, measuring over 100 meters long, over 30 meters wide, and up to 15 meters thick. Therefore, pressure rigs can apply proportionately more forces to offshore structures than ordinary ice sheets. Therefore,
There is an extremely high possibility that pressure ridges will cause extensive damage to offshore structures or even permanently destroy them.

Strong enough to resist the crushing forces exerted by the impact of the ice, i.e., strong enough to cause the ice to strike the structure and fracture, allowing the ice to flow around the structure. A structure built in this area would likely be very large and bulky, and would be correspondingly expensive to build. Therefore, structures to be used in ice-infested marine waters are constructed with sloped or ramp-like external surfaces without surfaces that are disposed perpendicular to the impinging ice. Should. When ice comes into contact with this sloped outer surface, it is forced upward from its normal position and subjected to tensile stress, causing it to bend and break. Ice is approximately 6Kg/cm ²
, the forces exerted on this structure are correspondingly relatively small when ice that impinges on the structure breaks in bending rather than in compression.

Several types of conical offshore structures with sloping external surfaces were used in harbor and marine applications in extremely cold conditions, held at the Norwegian Institute of Technology, Trondheim, Norway, between August 13 and 30, 1971. This is exemplified in a paper by JV Dennis entitled ``Effect of conical structures on the impact force of floating ice'' submitted to the First International Conference on Engineering. Another paper of interest in this regard was presented to the Offshore Technology Conference in Houston, Texas in April 1970, entitled ``Design and Construction Methods for Proposed Extremely Cold Offshore Structures,'' by Ben C. This is a paper submitted by Ronald R. Lloyd.

As the ice plate contacts and moves relative to the sloped outer surface of the conical structure, the ice plate will be lifted along the sloped surface. This lifting of the ice creates initial cracks in the ice sheet that radiate outward from the point of contact. Circumferential cracks then occur, causing the ice sheet to fracture into wedge-shaped ice pieces. The approximate total force exerted on the conical structure is primarily the force required to bend and fracture the impacted ice sheet, i.e. the initial radial or subsequent circumferential crack. , and the forces generated by the crushed ice flakes rising onto and interacting with the outer surface of this structure.

The forces associated with the formation of initial and circumferential cracks in an ice sheet are primarily a function of the specific mechanical and geometric properties of the ice impacting the structure. The uplift force results from the interaction of the broken ice pieces with the structure and thus depends on the surface area of the structure above the water surface. Therefore, it is always desirable to keep the waterline diameter of the structure as small as possible to reduce the overall ice forces applied to the conical structure.

Large ice blocks, such as pressure ridges, which strike the conically shaped structure are lifted along the sloped outer surface of said structure and are broken in flexure. As with ice sheets, radial cracks form within this pressure ridge at the point of impact;
Following the formation of radial cracks, hinge cracks are formed at a relatively large distance from the structure. As the pressure ridge advances into the structure, the ridge will break off into large blocks of ice that will fall from the structure.

As mentioned above, the force exerted on the structure by an impinging pressure ridge is much greater than that of an impinging ice sheet. Substantially all of the force exerted on the conical structure by the pressure ridge is the force required to break the ridge that it impinges in flexure, plus the force that advances in front of said pressure ridge and is deposited on the outer surface of this structure. This is the sum of the force caused by the broken ice flakes formed by the breaking of the interacting ice sheet. The large blocks of ice that form when a pressure ridge fails in flexure have no tendency to climb up onto the outer surface of the structure; therefore, the uplifting forces are essentially This is the result of ice flakes from the ice sheet climbing onto the outer surface.

Structures positioned underwater where relatively large ice masses are present will be exposed to relatively large pieces of ice, and these structures will have sufficient capacity to withstand these relatively large ice forces. It must be made exactly as it should be. If this bottom-supported conical structure is utilized, it is necessary to support this structure with additional foundation support, such as pilings. However, this would increase the cost and time required to install the structure. Without additional foundation support, the structure would have to be made larger and bulkier to resist greater ice forces, necessitating an increase in waterline diameter. However, this would increase the component of the ice force associated with the heave-up of ice chips on the structure, since the heave-up force is proportional to the surface area of the structure above the water line. . For very large cone waterline diameters, this component of the force will be significantly greater than the force required to break the impacting ice in flexure.

Therefore, this conical structure made strong enough to withstand the forces associated with a relatively large ice block will correspondingly be made strong enough to withstand the forces associated with an impacting ice sheet. It will cost more to manufacture and install than a structure designed in the United States. In fact, this type of structure would be so large that it would be impractical and economically unviable to construct it. The present invention is directed to offshore structures that are capable of withstanding the forces associated with large impacting ice blocks, while being economically and economically viable.

Generally speaking, the present invention comprises an offshore structure designed to serve in extremely cold offshore environments where ice sheets and other large ice masses such as pressure ridges are present. The offshore structure of the present invention includes a frusto-conically shaped lower portion that is positionable coaxially over the top of the base portion. The upper portion of the structure has a second frustoconical shape and is positionable coaxially over the top of the lower portion. The walls forming the upper and lower parts of the structure are oriented at an angle to the horizontal plane in order to receive the ice mass moving against and in contact with the structure, causing it to break in flexure. It is slanted. The angle of inclination of the walls of the upper part with respect to the horizontal plane is greater than that of the lower part, and the cross-sectional diameter of the upper part is not greater than that of the top of the lower part.

The angle of inclination of the walls of said upper part is approximately 26
° to 70 °, the preferred range is approximately
The angle is between 54° and 58°. The angle of inclination of the wall of the lower portion is approximately 15° to 25° with respect to the horizontal, with a preferred range of approximately 19° to 23° with respect to the horizontal.

The relative arrangement of the parts of the offshore structure allows the structure to be used in seawater containing ice sheets and relatively large ice blocks without unnecessarily increasing the mass and cost of the structure. .

A particular object of the present invention is to provide an offshore structure that is capable of withstanding the forces exerted on the structure by impacts from ice sheets and large ice blocks, and that uses relatively little structural material and is correspondingly not too large and inexpensive. It's about giving your body.

Further objects and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings.

FIG. 1 of the accompanying drawings shows a thick ice plate 20 and a relatively large ice block, such as a pressure ridge 22.
A marine structure 15 is shown positioned in seawater 30 specifically designed to be installed in the frigid waters where marine life is formed. The structure is held in place on the seabed by the weight of the structure itself plus the weight of the ballast added to the structure, as discussed in more detail below. In unusually severe ice conditions, a peg 18 as shown in FIG. It can be driven into the seabed 12 through an internal guide (not shown) in 2. These piles can also be used to support vertical loads applied to the structure. This type of pile, when used,
Of course, this structure is separated from the lever structure prior to moving it to a new excavation location.

The working platform 10 of the structure 15 is shown in FIG. 1 with the drilling rig 45 positioned on the deck 42. Other conventional drilling equipment, not shown, may also be positioned on work platform 10. However, the invention is not limited to offshore structures used to support drilling rigs. The present invention is suitable for any type of offshore operation carried out in frigid seawater that requires protection against ice packs that form in frigid seawater.

In reality, the working platform 10 includes several additional level decks 40 that serve as living and working areas for individuals on this structure.
and 41. The deck can be enclosed and heated to provide a reasonably comfortable working environment that provides protection for personnel and equipment during winter weather when temperatures drop to the -50°C range. The interior of said structure is also generally designated 6 in the drawings.
Storage and equipment compartments, shown at 0, may also be included.

The offshore structure 15 is easily configured to have full operational capacity at a selected drilling site and the ability to be moved from one drilling site and placed into operational condition without delay at another drilling site. The structure is set to . To this end, a ballast tank 62 is attached to the structure to provide adequate stability when the structure is towed and to enable the structure to be lowered through the water into contact with the seabed. It is integrated inside. The ballast tank can be balanced as necessary to compensate for uneven weight distribution within the structure. Each ballast tank is equipped with suitable devices for remotely controlling the amount of water in the tank, such as a seawater tank and blow-off pipe (none of which are shown), so that the buoyancy of the structure can be adjusted. There is.

As mentioned above, a drilling rig 45 is positioned on deck 42 along with other conventional drilling equipment for use in drilling wellbore cavities within the earth's surface. Thus, a borehole 92 extends from the deck 42 through the structure to the seabed 12 so as to extend into the wellbore 90. Since it is both uneconomical and difficult to build and install structures in the frigid ocean waters, it is desirable that the structures have the ability to drill multiple oil wells at any particular location. For example, the structure can be designed to drill two or more oil wells to a depth of about 6100 meters. Therefore, the structure must be constructed of sufficient size to accommodate the equipment necessary for this purpose.

An offshore structure large enough to carry out the drilling operations described above will weigh several thousand tons before hosting the equipment necessary for the drilling operations. Additionally, the current design weight of bottom-supported structures is such that if the structure is designed to withstand the forces of natural ice, such as those associated with relatively large ice blocks such as pressure ridges, It will increase accordingly. Since the weight of the structure is directly proportional to its construction cost, the construction cost increases in proportion to the increase in weight. The present invention minimizes the forces exerted on this structure by impacts from ice sheets and relatively large ice blocks, and also reduces the amount of structural material used within the structure and correspondingly reduces the mass and cost of the structure. It is directed toward geometries of offshore structures to reduce the

As mentioned above, an ice sheet that moves and comes into contact with the inclined surface of a conical offshore structure will fracture in flexure and thus fracture into wedge-shaped fragments. As the ice sheet continues to move toward the structure, the wedge-shaped ice flakes will heave onto the outer surface of the structure and ideally fall away from and around the structure. It can be wiped away. As the ice mass impinging on the structure becomes larger, the force exerted on the structure increases as well. Several things could possibly be done to prevent the failure of current designs for bottom-supported conical structures if large ice masses such as pressure ridges came into contact with this structure. can. First, the base diameter of the structure, and therefore its size, will be increased to resist relatively large ice forces. Secondly, the structure can be provided with a fairly gently sloped surface to receive impinging pressure ridges, but this also increases its size. This reduces the overall ice force exerted on this structure by the impinging pressure ridges, since the component of the total force due to bending failure of the pressure ridges decreases as the angle of inclination of the sloped surface with respect to the horizontal decreases. It has an effect. Third, the structure can be supported by pilings; however, this is undesirable since it increases the installation cost and time for the structure at the selected excavation point.

In order to resist the relatively large forces associated with relatively large impacting ice blocks, the size of current designs of bottom support conical structures would therefore have to be increased. This requires the use of more structural material in the structure, thus increasing the mass of the structure and its cost, making it so expensive that it becomes impossible to construct. As previously pointed out, the total ice force exerted on the structure offshore of the cone is essentially the force required to break the impacting ice mass in bending plus the force exerted on the outer surface of the structure. and the forces caused by the interacting ice flakes. This lifting force depends on the weight of the ice flakes as well as the frictional forces that exist between the ice and the outer surface of the structure. Thus, the ice lifting force is proportional to the surface area of the conical structure above the water line. Therefore, as the size of the structure increases, the uplift force applied to the structure increases as well, and for conical structures with relatively large waterline diameters, This uplift force will be well in excess of the force required to bend and break the impact ice chips.

Thus, an offshore structure is provided by the present invention that is capable of withstanding forces exerted by an impacting ice plate 20 or other large ice masses, such as a pressure ridge 22, without unnecessarily increasing the mass and cost of the structure. . As shown in Figures 1 to 3, this structure has an essentially conical lower part 4 which is positioned coaxially with one another to receive the ice mass. It has a conical upper part 6 forming a continuous outer shell. Although it is contemplated that the shell of the structure will be constructed from sheet steel as shown in FIG. 3, other materials may be used, such as prestressed concrete.

As can be seen in the drawings, the upper part 6 is frustoconically shaped and has a ramp-like surface with walls inclined at an angle to the horizontal such that the surface 16 converges above and inwardly into the lower part 4. 16 is formed. Similarly, the lower part 4 of this structure is frustoconically shaped but has a larger transverse diameter than the upper part 6; that is, the diameter of the base of the upper part 6 forming the cone is smaller than that of the cone. is not larger than the diameter of the top of the lower part 4 forming the. The walls of the lower part 4 converge above and inwardly of the base part 2 so as to form a ramp-like surface 14 inclined at an angle to the horizontal plane. However, the angle of inclination from the horizontal plane is greater than that of the lower part 4.

The waterline diameter of the upper part 6 is thus made as small as practicable in order to reduce the uplift forces acting on this structure. On the other hand, a relatively large lower part 4 with a reduced angle of inclination is provided so that the structure can withstand the forces associated with relatively large impacting ice blocks. The reduced angle of inclination of the lower part 4 has the advantage that the forces exerted on this structure due to bending failure of the pressure ridge are reduced. Moreover, the relatively large lower part 4 not only improves the floating stability of the structure, but also reduces the risk of foundation failure of the structure.

The base portion 2 of the structure may also have a conical shape with its top diameter approximately equal to the bottom diameter of the lower portion 4 and its walls narrowing above and inward to the seabed portion 12. This particular shape is useful in that it provides additional stability to the structure when being moved through water. Furthermore, the ramp-like surface of the base part 2 can assist in breaking the impinging pressure ridge. The base part 2 may have any other suitable shape, such as the shape of a cylinder, such that its walls are arranged perpendicular to the sea bed.

By way of example, an offshore structure 15 for installation in water having a depth of 6.1 m to 18.3 m may have a base diameter of approximately 76.2 m and a height of approximately 1.5 m. Substantially, the particular value of this base diameter is a function of the floating properties of the structure and the desired ability of the structure to resist failure when large ice forces are applied to the structure. . The lower part 4 can have a height of about 7.6 m and the upper part 6 can have a height of about 12.2 m.

In water having a depth of between 18.3 m and 9.2 m, relatively large ice masses, such as pressure ridges 22, extend for a considerable distance below the water surface; these pressure ridges therefore come into contact with and move with respect to the structure 15. If you do, Ritsuji 22
The edge portion of the lower part 4 is caught by the wall of the lower part 4 and lifted along the surface 14, thus causing the ridge to break in bending. As the pressure ridge is lifted along the surface 14,
This pressure ridge is broken into a block of ice which has a tendency to slide under the ice sheet and advance behind the ridge. The surface 16 of the upper part 6 receives ice sheets that impinge on the structure and breaks them in bending as described above.

This structure is located in relatively shallow water, i.e.
If positioned in water with a depth of less than 9.14 m, the lower conical portion 4 will catch and break up ice sheets and relatively small pressure ridges that impinge on this structure. The only forces applied to the upper portion 6 will be those associated with raising the ice flake onto the surface 16.

Suitable anti-icing devices are provided to resist movement against and over the outer surfaces of the upper part 6 and lower part 4 of the structure and to prevent ice chips from freezing on these surfaces. should be used. The method for preventing freezing and sticking is patented in the U.S. Patent No.
Heating the outer surfaces 14 and 16 of the structure as disclosed in U.S. Pat. No. 3,831,385 or coating the surfaces with a material that reduces ice fixation as disclosed in U.S. Pat. No. 3,972,199. Contains.

The angles of inclination of the walls of the lower part 4 and upper part 6 of this structure are designated α ₁ and α ₂ respectively. These two angles are acute angles that are steep enough to cause the ice block to break upon bending. The value of α ₁ must be small enough so that the forces associated with bending failure of large ice blocks are minimized. However, the value of α ₁ should not be made too small. That is, if it is too small, the base of the structure will become too large, making the cost of the structure economically unfeasible. The value of α ₂ is large enough so that the surface area of the structure above the waterline is minimized, but not so large that the impinging ice sheet breaks in compression rather than in bending. In most multi-angular conical structures, α ₁ and α ₂
are approximately 15° to 25° and 26° to the horizontal, respectively.
It may be in the range of 70° to 70°. Inclination angle α ₁ , α ₂
The optimal numerical range for is essentially dependent on three factors: the depth of the ocean in which the structure is located, the expected size of ice sheets and pressure ridges in those oceans, and the depth of the seafloor soil supporting the structure. It depends on the characteristics. In order to determine the optimum angle, the inventor of the present application created a complicated simulation calculation program using a computer, and confirmed the reliability of the program through a model experiment applying strict similarity rules. The model ice plate used in this experiment was made of a material with special strength and rigidity in order to satisfy the law of similarity. As input information to this program, the shape of the structure including a wide range of inclination angles α ₁ and α ₂ , the expected shape of the ice mass, the physical properties of the ice mass, the water depth, etc. are input to the program. As a result of calculations, the preferred range of α ₁ is between approximately 19° and 23° from the horizontal, which would best reduce the ice force applied to the structure, and the preferred range of α ₂ is between approximately 19° and 23° from the horizontal. The results show that the range is between about 54° and 58°.

As illustrated in FIG. 1, the throat portion 8 of this structure, which has a cylindrical shape, is positioned coaxially on the top of the upper portion 6 and abuts perpendicularly to this upper portion 6 and is oriented in this direction. the working platform 1 above the surface of the seawater 30 to a height sufficient to avoid contact with fragments of ice sheets climbing onto the structure;
It extends to 0. An inverted frusto-conical section 9 may be positioned between the throat section 8 and the working platform 10. Section 9 deflects ice sheet fragments that build up on throat section 8 and prevents these fragments from damaging work platform 10 and increasing the overall ice force exerted on this structure. Otherwise, as illustrated in FIG. 4, the working platform itself would be in the shape of an inverted truncated cone, such that ice chips climbing up the structure would contact the uppermost deck 42 of the structure. The structure may also be designed to prevent the formation of ice and increase the ice forces applied to the structure. The angle of inclination of the walls of the inverted frusto-conical section 9 and the inverted frusto-conical working platform 10 is designated by θ. For most constructions, the range of θ will be about 25° to 70° to the vertical.

Although it is contemplated that structure 15 will be towed to the excavation site in a fully assembled state without the need for any additional construction on site, the individual parts of this structure may be removed from the manufacturing site. It would certainly be possible, and perhaps desirable, to tow the vessel from the point of excavation to the point of excavation for assembly. For example, the base portion 2 is brought to the excavation point and
can be placed on top. Next, the lower part 4
is brought to the point of excavation, positioned in abutting relationship on the top of the base portion 2, and connected to the base portion 2 by suitable means. Similarly, the upper part 6 is brought to the point of excavation and positioned on top of the lower part 4 and connected thereto. Similarly, other components of the structure can be assembled at the excavation point.

The advantages described herein may also be such that the geometrical arrangement of this structure is slightly modified so that the ramp-like outer surface of the structure has a multi-cone shape consisting of two or more conical sections or a portion of a hyperboloid of revolution. It can also be realized by slight changes in the shape of the structure with a continuously curved shape, such as.

One model of the multi-gonal conical structure of the present invention was tested in an ice laboratory under similar cryogenic conditions. One of the objectives of this Tet was to study the forces exerted on this structure by impinging ice plates.

In previous tests of monoconical structures, one notable phenomenon was the formation of a square of ice fragments in front of the structure between the outer surface of the structure and the advancing ice sheet. These squares are formed when pieces of the ice sheet lift up onto the outer surface of the structure and then fall back down in front of the structure. The ice chips that form between the advancing ice sheet and the conical structure increase the overall ice force applied to the structure. However, this phenomenon does not occur in the case of the multigonal conical structure of the present invention. In fact, these pieces of ice tend to build up around the outer surface of the upper conical part of the structure. Apparently this is due to the relatively small diameter of the upper conical part and the relatively small angle of the lower conical part facilitating the movement of ice chips away from it around this structure. are doing.

Other interesting results observed during testing of this multigonal conical structure, compared to a single conical structure, were that the vertical component of the vibrational force applied to the seabed supporting the structure is decreasing.
The total normal force applied to the surface below the structure is approximately the weight of the structure plus the normal component of the total ice force applied to the structure. The oscillating normal force is related to the number of times the edge portion of the advancing ice sheet is broken. This reduction in vibratory normal forces reduces the magnitude of the cyclic loading on the soil, reducing the risk of foundation failure over the life of the structure.

Although specific embodiments of the present invention have been disclosed above, it goes without saying that the present invention is not limited to these embodiments, but only to the scope of the claims.

[Brief explanation of the drawing]

1 is a schematic side view, partially in section, of a preferred embodiment of the invention; FIG. 2 is a schematic sectional view taken along line 2--2 of FIG. 1; and FIG. 4 is a partial perspective view showing the upper and lower conical portions and the throat portion; and FIG. 4 is a schematic side view, partially in section, of another embodiment of the invention. 2... Base part, 4... Conical lower part, 6... Conical upper part, 8... Throat part, 14... Ramp-like surface, 15... Offshore structure, 16... Ramp-like surface.

Claims (1)

[Scope of Claims] 1. A ramp-like surface containing an ice mass and having a support device for supporting a structure at the bottom in seawater, the support device receiving the ice mass moving into contact with the structure. an offshore structure, wherein the ramp-like surface is supported by a lower portion having a peripheral wall converging upwardly and inwardly at an inclination angle of 15° to 25° with a horizontal plane; 26° with the horizontal plane
An offshore structure characterized in that it has an upper section having said peripheral wall converging upwardly and inwardly at an angle of inclination of 70° and having a bottom portion approximately equal in size to the top of said lower portion. 2. An offshore structure according to claim 1, wherein the upper portion has a generally first frustoconical shape and a second generally frustoconical shape located coaxially with the lower portion. , wherein the angle of inclination of the upper part from the horizontal plane of the wall is greater than the angle of inclination of the lower part, and the cross-sectional diameter of the upper part is not larger than the cross-sectional diameter of the top of the lower part. Offshore structures. 3. In the offshore structure according to claim 1, the angle of inclination of the lower part with respect to the horizontal plane of the wall is 21 degrees, and the angle of inclination of the upper part with respect to the horizontal plane of the wall is 56 degrees. Characteristic offshore structures. 4. In the offshore structure according to any one of claims 1 to 3, the lower part is supported by a base part that acts to support the structure on the seabed. An offshore structure featuring: 5. An offshore structure according to claim 4, characterized in that the base portion has the shape of a truncated cone with a diameter at the top of the base portion approximately equal to a diameter at the bottom of the lower portion. offshore structures. 6. An offshore structure according to any one of claims 1 to 5, in which the upper part is arranged such that the cylindrical throat part supports a working platform above sea level. An offshore structure characterized by being located coaxially on top of the