Massive ice interactions with offshore structures

Abstract

The interaction between a multiyear sea ice floe of variable thickness, and an offshore structure, has been examined using a 3-dimensional finite element method. Elastic response within the ice floe was assumed initially, and a uniform loading of the ice floe by the adjacent pack ice was used. As an example of the results for a frozen boundary condition at the ice/structure contact zone, with a central region of the ice floe having its thickness reduced to 50% as compared to the floe thickness at the structure ($\Delta$t/t = 0.5), tensile cracks first form at the top surface in the thinnest area of the floe. The total force on the structure was 108 MN, as compared with 1500 MN which would be present in the case of an ice floe of uniform thickness. Parameters varied were ice/structure contact zone (located in the centric or the eccentric region), the sliding boundary condition, two-dimensional ice thickness variation, variable ice elastic modulus as a function of depth, and viscoelastic ice behavior. Cases of rigid and of compliant structure and foundation were included. In a second part of the study, the ice island loads acting upon a cylindrical rigid structure were analyzed by this 3-dimensional finite element method. A force of 6600 MN was computed to be acting on the structure with a maximum penetration distance of 8.2 m. A different theoretical method based upon multiyear ice field data resulted in a force of 336 MN and a maximum penetration distance of 75 m. The ice forces on the structure are reduced by ice floe thickness variations, and also are affected by the geometries at the ice floe/structure and ice floe/pack ice boundaries. The reduced elastic modulus in the warmer. lower part of an ice sheet promotes ice bending failure and causes lower structure loads.