In this part, the ice loads from IACS UR and the mechanics model are compared both globally and locally. To make the comparison equatable, the normal frame angleβ0is set
4.2 Ice Load Comparison as small as possible to reduced the bending failure influence in the IACS UR. The main focus is the high polar class to reveal the uncertainties.
4.2.1 Global Ice Load comparison
A new global pressure-area relationship of the crushing ice during the interaction is gotten using the ice mechanics model. Applying the same reference speed, the pressure-area curves can be compared for the certain polar class with the IACS UR as shown in the Figure4.5. All the polar classes of the new ice load model and theP C1,P C2,P C3and P C7of the IACS UR are shown.
Figure 4.5:Comparison of the global pressure-area relationships between new ice load model and IACS UR. The pressure-area curves of IACS UR are plotted with the solid lines. The pressure-area curves of the ice mechanics model are plotted with the dashed lines.
With the same contact area, the ice mechanics model would give smaller pressure than the IACS UR for the high polar class such as theP C1toP C3. When it is with the low polar class, the pressure from the ice mechanics model may be larger such as theP C7. For two adjacent polar classes, the gap between the pressure-area curves of the IACS UR is
Chapter 4. Comparison between the Ice Loads
getting larger for higher polar class due to the uncertainty. When the formulas in the IACS UR is derived, the theoretical justification is limited, so more conservatism is included in the factor for the higher polar class. As for the ice mechanics model, the pressure change with the ship speed/polar class is milder.
4.2.2 Local Ice Load comparison
When the local ice load is compared, it is difficult to find the most suitable area. Ideally, the bow area with the high polar class and nearly vertical geometry should be used. It could satisfy the requirements of the direct contact, high polar class and ice crushing failure.
But, such an area is not included in any sets of the IACS UR. The comparison is applied in the introduced sets assigning least difference with the ideal case.
General Bow Area Set
In this set of the IACS UR, the direct contact with ice and the high polar class could be satisfied. The normal frame angle is set as small as possible to reduce the influence of the ice bending failure,β0= 15.1◦. Four cases with different ship displacement is included as shown in the Figure4.6. In the IACS UR, the final applied area and corresponded pressure are not on the pressure-area curves. The reason is the transformation from the nominal load patch to the design load patch. As for the ice mechanics model, the pressure-area points are still the maximum contact area points for the ship with certain displacement and polar class. TheP C1toP C3are compared.
Figure 4.6:Comparison of the local pressure-area relationships for the General Bow Area Set. The pressure-area curves are plotted in solid lines. The design pressure-area points from the IACS UR are marked with the circles. The maximum contact pressure point from the ice mechanics model are marked with the triangles. The cases with different displacement are distinguished by colors.
The design pressure of the design points from the rules is larger than the pressure of the
4.2 Ice Load Comparison maximum contact area points from the ice mechanics model for almost all polar classes and displacements. If the same contact area as the design points from the IACS UR is taken (with the same x coordinate in the figure), the pressure from the ice mechanics model is still smaller for larger ships. Thus, in this set, the rules would generate larger pressure during the design stage compared to the ice mechanics model. With the higher polar class and the larger ship, the difference increases. If the same pressure value as the design points from the rules is assigned, the contact area should be nearly0.03m2to1m2 in the ice mechanics model.
It is also noticed that the trend of the pressure with the increasing displacement is different between the rules and the ice mechanics model. The pressure is getting larger for the IACS UR, but a smaller pressure would be generated with a lager displacement for the ice mechanics model. It is because that, if the ship is larger, the maximum contact area becomes larger to dissipate the kinetic energy, so the corresponding pressure is getting smaller. The reason of larger design pressure for a larger ship with same velocity in the IACS UR is unknown.
Special Bow Area Set
The vertical bow shape in this set could limit the ice failure mainly to the crushing, but the applicable polar class only consists theP C6and theP C7. Using the given formulas, the design pressure-area points from the rules are plotted in the Figure4.7. The maximum contact area points with the pressure-area curves from the ice mechanics model are also shown. The normal frame angle is set asβ0= 0.1◦.
Figure 4.7:Comparison of the local pressure-area relationships for the Special Bow Area Set. The pressure-area curves are plotted in solid lines. The design pressure-area points from the IACS UR are marked with the circles. The maximum contact pressure point from the ice mechanics model are marked with the triangles. The cases with different displacement are distinguished by colors.
Chapter 4. Comparison between the Ice Loads
From the figure, the design pressure from the IACS UR is larger than the pressure of the maximum contact area points from the ice mechanics model. The design pressure-area points are also far away from the pressure-area curves for the rules. Though the applied polar class is low, the design pressure in this case is even larger than the value from the General Bow Area Set of higher polar class.
In the calculation process in this set, the given formulas and factors are different with the General Bow Area Set. Considering the large difference between the design points and the curves, it is possible that a different load patch derivation or impact scenarios are used with the rule modification. The comparison in this case seems improper because of the uncertainties.
Other Hull Area Set
Within the areas other than the bow, the pressure from the IACS UR is sub-region dependent. The pressure value is multiplied by a factor to be applied on the sub-region. A pressure range instead of a pressure value would be gotten for a certain polar class. The definition of the maximum contact area point from ice mechanics model doesn’t make sense because there is no direct contact in this set. Thus, only the range of the design pressure from the rules and the pressure-area curves from the ice mechanics model are shown in the Figure4.8. TheP C1toP C3are plotted to compare.
Figure 4.8: Comparison of the local pressure-area relationships for the Other Hull Area Set. The maximum pressure and the minimum pressure from the rules are plotted with the dashed lines and the dot-dash lines, respectively. From right to the left for one class, the design pressure range of ship with displacement of 300kt, 200kt, 80kt and 6kt is displayed by patterned arrows.
The maximum pressure, minimum pressure and pressure range of the IACS UR are all increasing with a higher polar class. The same trend is found with a larger ship displace-ment. This is mainly because of the conservative consideration. When it is related to the larger ships with higher polar class, the uncertainties increase, so the empirical factors are
4.3 Summary and Discussion