This paper discusses the importance of environmental driving forces in determining ice loads on Arctic offshore structures. The concept of a limited driving force is discussed and methods arepresented to estimate the magnitude of the driving force.
Most techniques for calculating ice loads on structures assume that sufficient driving force is available to sustain failure of the design ice feature against a structure. This paper assesses limits to the environmental driving force in a first-year ice field by considering analytical models of ridge formation, wind and current driving forces and irregularities such as leads and tidal cracks. The predicted forces are compared to available in-situ measurements.
An example is presented to illustrate calculation of maximum rubble pile loads on a conicalstructure. Implications of the concept to selection of design ice loads for structures in exposed, dynamic ice areas are considered.
Average driving forces are estimated to be approximately 10-50 kips/ft for far-field, with some local concentration near fixed structures. These values for the average limited driving force should be considered only as order of magnitude estimates. However, they are sufficiently less than failure loads for many ice features to indicate that consideration of limited driving forces in an ice field may reduce estimated ice loads on Arctic structures.
The design of safe, economical structures in the offshore Arctic requires accurate predictionof ice loads for a wide variety of ice conditions. Ice conditions offshore the northern coast of Alaska can generally be classified as a fast ice zone atransition zone, and pack ice. Near the shoreline the fast ice is relatively stable during the winter with limited movement and deformation. A transition zone, beginning at the outer edge of the fast ice, extends out into the Beaufort Sea where it merges with the arctic Pack. The transition zone, typically beyond 45-60 foot water depths, is characterized by more movement than the fast ice with intense ice deformation. Design ice loads increase significantly with water depth because larger ice features can occur without grounding, and the larger movements indicate a higher probability of encountering a large ice feature.
Gravel islands have been constructed and used for exploratory drilling in water depths of up to approximately 65 feet in the nearshore Beaufort Sea. As offshore development advances in the transition and pack ice zones, reducing the uncertainty in ice load prediction will provide more reliable and cost-effective designs.
The prediction of a design ice load involves several components. First, the design ice feature must be selected and the properties of the ice feature specified. The load required to fail the ice feature against the structure is calculated.
