In recent years, offshore exploration and production of petroleum products has been extended into arctic and other ice-infested waters in such locations as northern Alaska and Canada. These waters are generally covered with vast areas of sheet ice 9 months or more out of the year. Sheet ice may reach a thickness of 5 to 10 feet or more, and may have a compressive or crushing strength in the range of about 200 to 1000 pounds per square inch. Although appearing stationary, ice sheets actually move laterally with wind and water currents and thus can impose very high forces on any stationary structure in their paths.
A still more severe problem encountered in arctic waters is the presence of larger masses of ice such as pressure ridges, rafted ice or floebergs. Pressure ridges are formed when two separate sheets of ice move toward each other and collide, the overthrusting and crushing of the two interacting ice sheets causing the formation of a pressure ridge. Pressure ridges can be very large, with lengths of hundreds of feet, widths of more than a hundred feet and a thickness of up to 50 feet. Consequently, pressure ridges can exert a proportionally greater force on an offshore structure than ordinary sheet ice; thus, the possibility of pressure ridges causing extensive damage to an offshore structure or the catastrophic failure of a structure is very great.
A structure built strong enough to resist the crushing force exerted thereon by impinging ice, that is, strong enough to permit the ice to be crushed against the structure, enabling the ice to flow around it, would likely be very massive and correspondingly expensive to construct. Therefore, it has been proposed heretofore that structures which are to be used in ice-infested waters should be built with a sloping or ramp-like outer surface rather than with a surface which is vertically disposed to the impinging ice. As the ice comes into contact with the sloping outer surface, it is forced upwardly above its normal position which causes the ice to fail in flexure by placing a tensile stress in the ice. Since ice has a flexural strength of about 85 pounds per square inch, a correspondingly smaller force is imposed on the structure as the ice impinging thereon fails in flexure rather than compression.
Several forms of conical offshore structures having sloping outer surfaces are illustrated in a paper by J. V. Danys entitled "Effect of Cone-Shaped Structures on Impact Forces of Ice Floes", presented to the First International Conference on Port and Ocean Engineering under Arctic Conditions, held at the Technical University of Norway, Trondheim, Norway, during Aug. 13-30, 1971. Another paper of interest in this respect is that presented by Ben C. Gerwick, Jr., and Ronald R. Lloyd, entitled "Design and Construction Procedures for Proposed Arctic Offshore Structures", presented at the Offshore Technology Conference in Houston, Texas, April 1970.
As an ice sheet moves relative to and in contact with the sloping outer surface of a conical structure, it will be elevated along the sloping surface. The elevation of the ice sheet causes initial cracks to be formed in the sheet, which radiate outwardly from the point of contact. Circumferential cracks then form and cause the ice sheet to break up into wedge-shaped pieces. The approximate total force exerted on a conical structure then consists primarily of the force required to fail the impinging ice sheet in flexure, that is, the force required to form the initial radial or subsequent circumferential cracks, and the force caused by the broken ice pieces riding up on the outer surface of the structure and interacting therewith.
The force associated with the formation of initial and circumferential cracks in the ice sheet is primarily a function of the particular mechanical and geometrical properties of the ice impinging on the structure. The ride-up force is due to the broken ice pieces interacting with the structure and thus is dependent upon the surface area of the structure above the water line. Therefore, to reduce the total ice forces imposed on a conical structure, it is always desirable to keep the waterline diameter of the structure as small as possible.
Larger ice masses such as pressure ridges impacting a conically shaped structure will be lifted along the sloping outer surface of the structure to cause the ridges to fail in flexure. As with ice sheets, a radial crack will form in the ridge at the point of impact; the formation of a radial crack is followed by the formation of hinge cracks that occur at a relatively greater distance from the structure. As the ridge continues to move into the structure, it will break into large blocks of ice which fall away from the structure.
As indicated above, the force imposed on a structure by an impinging pressure ridge is much greater than that of an impinging ice sheet. The approximate total force exerted on a conical structure by a pressure ridge is a combination of the force required to fail the impinging ridge in flexure and the force caused by the broken ice pieces, formed by the failure of the ice sheet advancing ahead of the presure ridge, riding up on the outer surface of the structure and interacting therewith. The large blocks of ice formed when a pressure ridge fails in flexure tend not to ride up the outer surface of the structure; therefore, the ride-up force is essentially a result of pieces of sheet ice riding up the structure's outer surface.
Since structures located in waters in which larger ice masses are present are exposed to relatively greater ice forces, they must be built strong enough to withstand these greater ice forces. Utilizing present bottom-supported conical structure designs requires supporting the structure by means of additional foundation support, such as piling; however, this would increase the cost and time of installation of the structure. Without additional foundation support, the structure would have to be made larger and stronger to resist the greater ice forces, which would necessitate increasing its waterline diameter. This, however, would increase that component of the total ice force associated with the ride up of ice pieces on the structure, since the ride-up force is proportional to the surface area of the structure above the waterline. For a very large cone waterline diameter, this component of the force would be substantially greater than the force required to fail the impinging ice in flexure. Additionally, as these structures are designed for use in deeper waters, their overall size would likely increase.
Accordingly, present conical structures built strong enough to withstand the forces associated with larger ice masses would be correspondingly more expensive to construct and install than one merely designed to withstand the forces associated with an impinging ice sheet. In fact, such structures could be so massive as to be impractical and economically prohibitive to build. The present invention is directed to an offshore structure which is able to withstand the forces associated with large impinging ice masses, and at the same time is feasible from an economic and size standpoint.