Study of Shape Factor for Reducing Ice Force on Arctic Structure and Ice Force Prediction for Combined Ice Failure
Authors: Jo C.H. , Machemehl J.L. 
Published: October 2008

Indentor shape plays an important role in reducing ice load on Arctic offshore structures. Ice forces on single wedge indentors are investigated for crushing failure mode. The maximum indentor wedge angle, ?, of different diameters at diverse ice attack angles, ?, are obtained. using the same ice attack conditions, the ice crushing force on the optimized single wedge is found to be near 30% less than the force on the cylindrical indentor.

Two approaches are introduced to predict ice forces for combined failure modes. They are the "local ice boundary method", and the "proportional failure method". Based on the above two methods, ice forces are predicted and compared with the measured data from the Hans Island programme.

INDENTOR SHAPE STUDY

INTRODUCTION

The methods for minimizing ice load or forces on Arctic offshore structures are centered in the dissipation of ice kinetic energies. In order to dissipate ice kinetic energy, various structural shapes and indentors have been used.

Circular, semi-circular, arch, and wedge indentors are designed to diminish ice impact loads by any combination of ice failure modes. Conical sided structures are also used to reduce the magnitude of ice forces by breaking icebergs with bending and shear forces induced by the sloping sides.

Though the indentor shape plays an important role in reducing ice load on the structure, little research has been conducted. In this study, the ice forces on circular and wedge indentors are compared. The maximum wedge angles, ?, for different ice attack angles, ?, are investigated for the case of crushing failure mode.

SHAPE FACTOR

The shape factors on crushing failure mode were defined in [1] as 1.0 for fiat indentor, 0.9 for semi-circular indentor and 0.85(sina)0.5 for piers with wedge angles of 2? between 60° and 120°. For the wedge shaped structure on splitting failure mode, the shape factors were presented in [2]. However, the wedge shape factor is for small ice fioes and very sharp-edged structures.

Figure 1 shows the shape factors of wedge indentor for both crushing and splitting failure modes. The upper line is based on crushing failure mode which can be explained as the maximum value because the ice load on the structurefrom crushing failure mode is higher than from any other failure mode. The equation of the upper line is 0.85(sina)0.5. The lower line from splitting failure mode can be said to be a minimum shape factor value because the splitting mode occurs from ice tensile strength which is much smaller than compressive strength.

As the combination of ice failure occurs on the boundary between level ice and vertical indentor, the shape factor will be between the upper and lower lines in Figure 1. For example, if 70% of ice energy dissipates on crushing and near 30% on splitting, the moderated shape factor can be expressed as O. 75(sina)0.65 as shown in Figure 1 which isalso valid for wedge angles between 60° to 120° (2?). The failure modes and dominant failure mode can be approximated from the ice failure maps in [3] and [4].