JPS5913612B2 - Offshore structures and methods of reducing forces acting on them - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to offshore structures for use in the Arctic and other icy oceans, and more particularly to offshore structures capable of withstanding the forces exerted by impinging ice sheets and other large ice masses. Involved in structures.

In recent years, offshore exploration and production of petroleum products has extended to the Arctic and other icy waters in places such as Alaska and northern Canada.

These oceans are covered by ice sheets the size of a dog for more than nine months of the year.

The ice sheet reaches a thickness of 1.5 meters (5 ft) to more than 3 meters (10 ft), and
It has a compressive knot or crush strength ranging from 200 pounds per square inch to 70 kilograms per square inch (iooo bonds per square inch).

Although ice sheets appear stationary, they actually move sideways with wind and ocean currents, and can thus exert enormous forces on any fixed structure in their path.

A more serious problem encountered in Arctic waters is the presence of large ice masses such as pressure ridges, polymerized ice masses or boulders.

A pressure ridge is created when two separate ice sheets move toward each other and collide, creating a pressure ridge when the two ice sheets push up against each other and break apart.

Pressure ridges can be very large, hundreds of feet long, more than 100 feet wide, and more than 50 feet thick.

As a result, presure ridges can exert greater forces on offshore structures than ordinary ice sheets, and thus presure ridges can cause significant damage to offshore structures or result in catastrophic failure of the structures. Gender is huge.

Structures built to be strong enough to resist the crushing forces exerted by striking ice, allowing the ice to strike the structure, break it up, and flow around it, are very large and It will cost money to build accordingly.

It has therefore previously been recommended that structures used in icy seas should be built with sloped or beveled exterior surfaces, rather than surfaces that lie perpendicular to the ice they encounter. .

When the ice contacts the sloped outer surface, it is forced upwardly from its normal position, which causes the ice to flex and break by creating tensile stresses in the ice.

Since the bending strength of ice is approximately 85 pounds per square inch (5.98 kg/cm²), less force is applied to the structure when ice that strikes a structure breaks by deflection rather than compression.

Several forms of conical offshore structures with sloping exterior surfaces were presented at the First Conference on Port and Marine Engineering under Arctic Conditions held at the Norwegian Institute of Technology, Trondheim, Norway, August 13-30, 1971. "Effect of conical structure on impact force of water basin" submitted to international conference
Titled J., V.

This is explained in a paper by Daniz.

Another interesting paper on this is the 1970 April
Submitted by Ben C., Jarwyk II, and Ronald R., Lloyd, entitled ``Proposed Arctic Offshore Structure Design and Construction Methods,'' presented to the Offshore Science and Technology Conference, Hyeuston, Texas, May. .

When the ice sheet moves against and contacts the sloped outer surface of the conical structure, it is lifted along the slope.

As the ice sheet lifts, it develops initial cracks that radiate outward from the point of contact.

Peripheral cracking then occurs, breaking the ice sheet into wedge-shaped pieces.

Almost all the forces exerted on the conical structure are then primarily due to the force required to cause the colliding ice sheet to break due to deflection, i.e., the force required to cause initial radial cracking and subsequent circumferential cracking, and the force required to cause the structure to flex. consists of the force exerted by pieces of crushed ice that slide up and interact with the outer surface of the ice.

The forces involved in the initiation of initial cracks and peripheral cracks within an ice sheet depend primarily on the individual mechanical and geometrical properties of the ice striking the structure.

The sliding force is due to ice fragments interacting with the structure and is therefore proportional to the surface area of the structure above the waterline.

Therefore, in order to reduce the overall ice forces exerted on a conical structure, it is always desirable to make the waterline diameter of the structure as small as possible.

Large ice masses, such as presure ridges, that strike conical structures are lifted along the sloped exterior surfaces of the structures and cause them to break due to deflection.

As in the case of ice sheets, radial cracks form on the pressure ridge at the point of impact, and radial crack initiation is followed by hinge cracks at relatively far distances from the structure.
occurs.

As the pressure ridge continuously moves towards the structure,
It breaks into large pieces of ice and falls off the structure.

As mentioned above, the force exerted on a structure by an impinging pressure ridge is much greater than the force exerted by an impinging ice sheet.

Almost the total force exerted on a conical structure by a presure ridge is the force required to deflect and break the impinging presure ridge, plus the force required to deflect and break the presure ridge in front of the structure, riding up and interacting with the outer surface of the structure. This is combined with the force exerted by ice debris created by the ice sheet's destruction.

Large pieces of ice that form when a presure ridge breaks due to deflection tend not to slide up onto the exterior surface of the structure;
The uplift force is therefore essentially due to ice sheet debris sliding onto the exterior surface of the structure.

Structures placed in the ocean where large ice masses exist are subject to relatively large ice forces, so they must be built strong enough to withstand these large ice forces.

When utilizing current bottom-supported conical structure designs, additional foundation support must be provided, such as by piling, which increases the cost and time of installing the structure.

Without supporting the foundation in this way, the structure must be built larger and stronger to resist large ice forces, and this requires increasing the waterline diameter.

However, this increases the component of the total ice force that is responsible for the ice flakes sliding up onto the structure.

This is because the uplift force is proportional to the surface area of the structure above the water line.

For a very large waterline diameter of the cone, this component of the force is considerably greater than the force required to deflect and break the ice that it hits.

Moreover, since these structures are designed for use in deep water, their overall size also increases.

Therefore, this conical structure built for deep sea use and built strong enough to withstand the forces associated with large ice masses is correspondingly more expensive to construct and install.

In fact, such structures become so large that construction becomes impractical and economically prohibitive.

The present invention is directed to offshore structures that can better withstand the forces associated with striking large ice masses, and at the same time are viable from an economic and size standpoint.

Briefly stated, the present invention is designed for operation in icy waters and is particularly suited for use in deep water where ice sheets and other large ice masses such as presure ridges are present, but including but not limited to offshore structures.

The offshore structure of the present invention includes a lower portion in the form of a truncated cone that can be mounted coaxially on the top of the base.

The upper part of the structure has the shape of a second truncated cone and can be placed coaxially on top of said lower part.

The walls forming the upper and lower parts of the structure are inclined at an angle to the horizontal in order to receive and flex ice blocks that move against the structure and cause them to break.

The angle of inclination from the horizontal of the walls of the upper part is greater than the angle of inclination of the walls of the lower part, and the diameter of the cross section of the upper part is smaller than the diameter of the apex of the lower part, so that the upper and lower conical parts There are steps or discontinuities between the walls.

The angle of inclination of the walls of the upper portion is between about 26° and 70° from horizontal, with a preferred range between about 54° and 58° from horizontal.

The angle of inclination of the walls of the lower portion is between about 15° and 25° from horizontal, with a preferred range between about 19° and 23° from horizontal.

The geometry of the offshore structure described above allows the structure to be used in relatively deep oceans containing ice sheets and relatively large ice masses without unnecessarily increasing the size and cost of the structure.

A particular object of the invention is to obtain an offshore structure that can better withstand the forces exerted by impinging ice sheets and large ice masses, incorporates less structural material, and is correspondingly smaller in size and less costly. That's true.

Other objects and advantages of the invention will become apparent from a detailed study of the specification and the accompanying drawings that form a part thereof.

Turning now to the drawings, Figure 1 is located in the ocean 30 and is specifically designed to be installed in the Arctic Ocean where thick ice sheets 20 and large ice masses such as the Pressure Ridge 22 are formed. A marine structure 15 is shown.

The structure is held in place on the seabed 12 by its own weight and the weight of any ballast attached to the structure, which will be discussed in more detail below.

A working platform 10 of a structure 15 has a drilling rig 45 placed on its pole 42 as shown in FIG.
placed on top of.

However, the invention is not limited to offshore structures used to support drilling equipment.

It is suitable for any type of offshore operation carried out in Arctic waters that requires protection against ice masses forming on the sea surface.

The working platform 10 actually includes several other decks 40 and 41 which are used as living quarters and working areas for personnel on top of the structure.

Those decks have a temperature of -51,1℃ (-60'F)
Enclosed and heated to create a comfortable working environment that protects personnel and equipment during winter weather that dries down.

The interior of the structure also includes a storage and equipment compartment, designated generally by the numeral 60.

The offshore structure 15 can be easily installed at a selected drilling location to full operating capacity, and can be moved from one drilling location and easily placed into another location in ready-to-operate condition. Built to be installed.

For this purpose, the ballast Cunkro 2 is used to provide adequate stability when towing the structure and to enable the structure to be lowered into the water and into contact with the seabed.
It is built inside the structure and integrated with it.

Of course, the ballast tanks are balanced as necessary to compensate for any uneven weight distribution within the structure.

Each of the ballast tanks is equipped with suitable devices, such as seawater valves and blowoff pipes, both not shown, for remotely controlling the amount of water in the tank so that the buoyancy of the structure can be adjusted.

As described above, drilling rig 45 is placed on deck 42 along with other conventional drilling rigs (not shown) for use in drilling well hole 90 underwater.

A moon-pool or cutway 50 extends from the deck 42 down through the structure to the seabed 12 such that the cutout 92 extends into the wellbore 90.

Because constructing and installing structures in arctic waters is expensive and difficult, it is desirable that the structures have the ability to drill multiple wells at any location.

For example, a structure has a length of approximately 6,000 meters (20,000 meters).
A well may be designed to drill more than one well to a depth of 0.000 ft).

Therefore, the structure must be sized to accommodate the equipment necessary for this purpose.

Offshore structures of a size capable of carrying out the above-mentioned drilling operations weigh several thousand tons even before containing any equipment required for the drilling operations.

Moreover, the weight of current designs for bottom-supported structures is such that the structures are used in deeper waters.
and increases as they are designed to better withstand large natural ice forces, such as those associated with large ice masses such as Presure Ridge.

The weight of a structure is directly related to its cost, so the cost increases with increasing weight.

The present invention is particularly adaptable to, but not limited to, deep sea use, and minimizes the forces exerted on the structure by impinging ice sheets and large ice masses, while at the same time providing structural materials incorporated into the structure. are directed to the shape of offshore structures that can reduce the size and cost of offshore structures.

As mentioned above, an ice sheet that moves into contact with the slope of a conical offshore structure is flexibly fractured and broken into wedge-shaped sections.

As the ice sheet continues to move toward the structure, wedge-shaped pieces of ice slide up the exterior surface of the structure and then fall off and flow around the structure.

As the size of the ice block that hits the structure increases, the force exerted on it also increases.

When large ice masses like Presure Ridge move into contact with structures, perhaps several things could be done to prevent the failures of current designs for bottom-supported conical structures.

First, the diameter of the base of the structure, and thus its size, can be increased to resist large ice forces.

Second, the structure can include gently sloped surfaces to receive impinging pressure ridges, which also increases the size of the structure.

This has the effect of reducing the overall ice force exerted on the structure by the impinging pressure ridge.

This is because the component of the total force due to the deflection failure of the pressure ridge decreases as the angle of inclination from the horizontal of the slope decreases. This is because this decreases.

Third, structures can be supported by piles;
This is undesirable as it increases the cost and time of installing the structure at the selected excavation location.

To resist the large forces associated with large striking ice blocks, the size of current designs for bottom-supported conical structures must therefore increase, which requires the incorporation of more structural material into the structure. and increases its size and cost, making it extremely expensive to construct.

Additionally, the size of structures tends to increase as they are designed for use in deeper waters.

As these structures are built larger, the total ice force applied to the structures increases.

As noted earlier, the total ice force exerted on a conical offshore structure is essentially the force required to cause the colliding ice block to break due to deflection, and the force required to shear up and interact with the outer surface of the structure. It consists of the force exerted by the acting ice sheet fragments.

This sliding force is due to the weight of the ice flakes and the frictional forces that exist between the ice and the outer surface of the structure.

Thus, it can be seen that the force of ice sliding up is proportional to the surface area of the conical structure above the water line.

Therefore, as the size of the structure increases, the sliding force applied to it also increases, and for conical structures with relatively large waterline diameters, the rising force causes the colliding ice blocks to break due to deflection. far exceeds the force required.

Accordingly, the present invention can better withstand the forces exerted by an impinging ice sheet 20 or other large ice masses, such as plescia ridges 22, can be adapted for deep sea use, and can reduce the size and cost of the structure. It is possible to obtain an offshore structure that does not unnecessarily increase the

This structure basically has a lower conical section 4 and an upper conical section 6 placed coaxially with each other to create a continuous skin, as shown in Figures 1-3. ,
The skin has a discontinuous portion 200 and is adapted to receive ice mass that moves against the structure and comes into contact with it.

The skin of the structure is made of steel plate as shown in Figure 3, although other materials such as prestressed concrete can also be used.

As can be seen, the upper part 6 has the shape of a truncated cone;
The wall has a sloped surface 1 inclined at an angle to the horizontal such that the surface 16 tapers upwardly and inwardly from the lower portion 4.
Making 6.

The lower part 4 of the structure is also truncated, but has a larger cross-sectional diameter than the upper part 6.

That is, the diameter of the base of the cone forming the upper part 6 is smaller than the diameter of the top of the cone forming the lower part 4, such that a step 200 exists between the walls of the upper part 6 and the walls of the lower part 4. do.

The walls of the lower part 4 taper upwardly and inwardly from the base 2 to create a beveled surface 14 that slopes at a smaller angle to the horizontal than the walls of the upper part 6.

The waterline diameter of the upper part 6 is thus made as small as possible in order to reduce the sliding forces used on the structure.

On the other hand, a relatively large lower part 4 is provided with a small angle of inclination so that the structure can better withstand the forces associated with large ice blocks that strike it.

The small angle of inclination of the lower part 4 has the advantage of reducing the forces exerted on the structure by flexural failure of the pressure ridge.

Moreover, the relatively large lower part 4 reduces the possibility of damage to the base of the structure and improves its flotation stability.

Moreover, the discontinuous part existing between part 4 and part 6,
Thus, the stepped portion 200 reduces the overall thickness and therefore cost of the structure, allowing it to be used in deep water.

The base 2 of the structure also has a conical shape such that its walls taper upwards and inwards from the sea bed 12, and the diameter of the bottom apex is approximately equal to the diameter of the bottom of the lower part 4.

This particular shape is useful in terms of giving the structure more stability when it is being moved underwater.

Moreover, the beveled surface of the base 2 helps to break up the impinging pressure ridges.

Of course, the base 2 may be of any other suitable shape, such as cylindrical, so that its walls lie perpendicular to the sea bed.

In the deep ocean, large ice masses like Presure Ridge 22 extend for considerable distances below the surface.

Therefore, when they move into contact with the structure 15, the edge portions of the pressure ridges 22 are received by the walls of the lower part 4 and are lifted along the surface 14, breaking the pressure ridges in flexure. let

As the presure ridge is lifted along the surface 14, it tends to break into ice fragments and slide under the advancing ice sheet behind the presure ridge, which are then swept laterally around the structure. It will be done.

The surface 16 of the upper part 6 receives the ice sheet impinging on the structure;
Then, as described above, they are destroyed by deflection.

If the structure is placed in relatively shallow water, the lower conical section 4 will cause the ice sheets and small pressure ridges that strike the structure to break up in deflection.

The only forces applied to the upper part 6 are those responsible for the sliding of ice sheet fragments onto the surface 16.

Appropriate anti-icing equipment must be used to aid in the movement of ice on the external surfaces of the upper part 6 and lower part 4 of the structure and to prevent ice flakes that have slipped up from freezing to these surfaces.

Freeze protection methods are disclosed in Chevron Research Company, U.S. Pat. No. 3,831,385, which
4 and 16 or coating their exterior surfaces with a material that reduces ice adhesion, as disclosed in Chevron Research Company, US Pat. No. 3,972,199.

The angles of inclination of the walls of the lower part 4 and upper part 6 of the structure are designated α1 and α2, respectively.

These two corners are sharp enough to cause the ice block to break due to deflection.

The value of α1 must be small enough so that the forces involved in the flexural failure of the ice mass are minimized.

However, the value of α1 must not be too small, since the base of the structure will become too large, making the cost of the structure too high.

The value of α2 is such that the surface area of the structure above the waterline is minimized, but not so large that the impinging ice sheet collapses in compression rather than in deflection.

For most polygonal conical structures, α1 and α2 are between approximately 15° and 25° and 26° and 70° from horizontal, respectively.

Inclination angle α1. The optimal numerical range for α2 essentially depends on three factors: the depth of the ocean in which the structure is located, the expected size of ice sheets and pressure ridges that exist in these oceans, and the depth of the ocean floor supporting the structure. It depends on the characteristics of the soil.

In order to determine the optimal angle, the inventor of the present application created a complex simulation calculation program using a computer, and confirmed the reliability of the program through a model experiment applying strict similarity rules.

The model water plate used in this experiment was made of a material with special strength and rigidity in order to satisfy the law of similarity.

This program has a wide range of tilt angles α as input information.
1. The shape of the structure containing α2, the predicted shape of the ice block,
As a result of inputting the physical characteristics of the ice mass, water depth, etc. with a considerable range of variation based mainly on the situation off the coast of northern Alaska, and calculating the preferred range of α1 that would best reduce the ice force applied to the structure. is between about 19° and 23° from the horizontal, and the preferred range of α2 is between about 54° and 58°.

As shown in FIG. 1, the throat part 8 of the structure, having a cylindrical shape, is placed coaxially on top of the upper part 6 and abuts perpendicularly thereto, and raises the working platform 10 above the surface of the sea 30. In addition, they are extended high enough to avoid contact with ice sheet debris that may be lifted up onto the structure.

It is envisaged that the structures 15 may be pulled into position fully assembled so that no additional assembly is required at the excavation location, but the individual parts of the structures may be removed from their manufacturing site in order to assemble them. It would certainly be possible, and perhaps desirable, to pull into a digging position.

For example, take the base 2 to the excavation position and place it on the bottom 1.
It can be placed at 2.

The lower part 4 is then brought to the excavation position and placed against the top of the base 2 and joined thereto by suitable equipment.

Similarly, the upper part 6 is also brought to the excavation position, placed on top of the lower part 4 and joined thereto.

Similarly, other components of the structure can be assembled at the excavation site.

The advantages of the present disclosure can also be realized by making small changes to the shape of the structure, where the beveled outer surface of the structure has a polyconical geometry with more than two conical sections, or have a geometry comprising two stepped, continuously curved surfaces, such as sections of a hyperboloid of revolution.

Although specific embodiments of the invention have been described in detail herein, the invention is not limited to such embodiments, but only by the scope of the following claims.

[Brief explanation of drawings]

1 is a schematic side view, partially in section, of a preferred embodiment of the invention; FIG. 2 is a schematic cross-sectional view taken along line 2--2 of FIG. 1; and FIG. FIG. 4 is a partial perspective view showing the upper and lower conical portions and the throat portion; Reference numeral 2 in the drawings indicates a base according to the claims; 4 is a lower part; 6 is an upper part; 8 is a cylindrical throat; 10 is a working platform; 12 is a sea bed; 14 is a first sloped surface. or a first perimeter wall, 15 is an offshore structure or marine structure, 16 is a second sloped surface or second perimeter wall, 2o is an ice sheet, 30 is the sea, 200 is a stepped portion or Discontinuous part, α1. α2 indicates the inclination angle.

Claims (1)

[Scope of Claims] 1. An underwater device containing an ice mass, having a support device for supporting a structure on the seabed, the support device having a sloped surface for receiving the ice mass moving into contact with the structure. In an offshore structure for use, the sloped surface has a lower portion having a peripheral wall converging upwardly and inwardly at an inclination/inclination angle of 19° to 23° with a horizontal surface, and is supported by the lower portion. , having peripheral walls converging upwardly and inwardly at an inclination angle of 54° to 58° with the horizontal plane, and having a stepped portion between the upper portion and the lower portion, being smaller than the top of the lower portion. An offshore structure characterized in that it comprises an upper part having a bottom part of such size. 2. In the offshore structure described in claim 1,
The upper portion has a generally second frusto-conical shape coaxially located with the lower portion, which is approximately in the shape of a first frusto-cone, and extends from a horizontal plane of the circumferential wall of the upper portion. is greater than the angle of inclination of the circumferential wall of the lower portion from a horizontal plane, and the cross-sectional diameter of the upper portion is such that the cross-sectional diameter of the upper portion is such that the top portion of the lower portion has a stepped portion between the upper portion and the lower portion. offshore structure characterized by having a cross-sectional diameter smaller than that of 3. In the offshore structure described in claim 1,
The angle of inclination of the lower part with respect to the horizontal plane of the wall is 210
An offshore structure characterized in that the upper portion has an inclination angle of 56° with respect to the horizontal plane of the wall. 4. In the offshore structure according to any one of claims 1 to 3, the lower part is supported by a base that acts to support the structure on the seabed. Characteristic offshore structures. 5. In the offshore structure described in claim 4,
An offshore structure characterized in that said base is in the shape of a frusto-cone having a top diameter of said base approximately equal to a bottom diameter of said lower portion. 6 Any one of claims 1 to 5
An offshore structure according to claim 1, characterized in that a cylindrical throat section is located coaxially at the top of the upper section to support a working platform above the sea level.