Section 1 General

1.1.1 Global forces are generally to be derived using manufacturers’ data to correlate the pressure area relationship assumed for the global ice loads. The loads provided in the Rules are based on past design and approval practice.

1.1.2 Alternative recognised methods for determining the global forces may be accepted, based on the following:

1. Full scale measured data.

2. Correlated laboratory tests.

3. Soil mechanics theories (for the partially consolidated/ unconsolidated keel load).

4. Pressure-area relationships (for strut crushing load).

1.1.3 Detailed requirements for propulsion units operating stern first-year assigned ice classes PC1, PC2 and PC3 have not been provided. For these ships it is considered that the ridge keel load is not applicable. As guidance, it is recommended that the propulsion unit be dimensioned using the impact with a multi-year ice sheet/block with nominal thickness corresponding to the Polar Class assigned in Table 4.2.2 Nominal ice thicknesses of the Rules. Validation is to be provided with respect to these ice loads and ice loads on the propulsion unit propeller blades.

1.2.1 The strut force may be assumed to be as a result of brittle crushing of an ice sheet against the strut. For both the longitudinal and transverse components the contact load area will typically have an aspect ratio < 10,0. Consequently a relationship between the force, contact area and contact area aspect ratio is proposed. See Ch 7, 2.1 References to published papers 2.1.1 for further details.

1.2.2 The relationship in Table 4.1.1 Determination of strut load F _C is considered valid for expected strut geometries; however, the correction factor K will continue to be revised as further operational experience is gained. Currently, K is a factor derived from calibrating a range of strut geometries using Ch 7, 2.1 References to published papers 2.1.1; this is to present the load calculation in a simplified manner.

1.3.1 The propulsion unit body force may be assumed to be as a result of the unconsolidated layer of the ridge collecting against the propulsion unit body/propeller. Table 4.1.2 Determination of keel load F _K may be used to calculate this force.

1.3.2 The force is derived from a Dolgopolov model which assumes a Mohr-Coloumb material behaviour. See Ch 7, 2.1 References to published papers 2.1.1 and Ch 7, 2.1 References to published papers 2.1.1.

1.3.3 Typical expected values based on a first-year ice ridge are also indicated in Table 4.1.2 Determination of keel load F _K .

Table 4.1.2 Determination of keel load F _K

F K = A C P K is the total keel load on the propulsion unit body in MN
where P K = is the keel pressure corrected for keel load distribution in MPa
P T = is the total global ridge keel pressure in MPa
A T = w h K is the total keel load area in m2
A C = w h P is the contact area in m2
F T = μ h K w is the global ridge keel force in MN
where γe = (1 – n K) (ρ w – ρ ice) g is the effective buoyancy of the unconsolidated layer
μ = tan is the passive pressure coefficient
Symbols
= angle of internal friction, in degrees	40
ρw = water density, in kg/m3	1025
ρice = ice density, in kg/m3	920
n K = porosity of keel	0,2
g = gravity, in ms2	9,81
h K = keel depth, in metres	12–20
h P = load height, in metres	Longitudinal (axial) case – propeller diameter (2–7 m) Transverse case – propulsion unit body and lower strut height (3–6 m)
w = load width, in metres	Longitudinal (axial) case – propeller diameter (2–7 m) Transverse case – propulsion unit body length (4–7 m)

1.4.1 The pyramid of strength approach required in Ch 4, 2.2 Propulsion unit structure 2.2.9 of the Rules is to extend to any propulsion unit appendages. Failure of the appendages is defined as yielding and implies that the appendages are to be sacrificial.

1.4.2 For application of PC Rules to the local structure of the propulsion unit, direct calculations may be employed, using the non-bow ice load patch, the bow intermediate Area Factor and the class factors in Table 3.2.1 PC Rule Area Factors for bow and bow intermediate regions applied to the stern of SFIC ships of the Rules.

1.5.1 It is recommended that the propeller blade strength criteria adopted for the propeller dimensioning be validated by LR.

1.5.2 Initial dimensioning of the propeller for estimation purposes may be carried out in accordance with Table 4.1.3 Initial dimensioning for propeller strength calculations.

1.5.3 Final dimensioning and approval of the propeller will require a Finite Element model of the blade to be submitted with the load cases defined from the relevant ice class Rules in addition to any specific load cases determined from the load scenarios.

1.6.1 Typical prescriptive Rule requirements for powering may not be suitable for Stern First Ice Class Ships, therefore ice model testing is recommended.

1.6.2 The assumptions in typical model tests for icegoing ship performance (for example, FSI Rule requirements for performance in a brash ice channel) may not be suitable for verifying independent performance in ice stern first.

1.6.3 It is considered that ice model testing should be carried out by a competent ice model basin, and that the model testing schedule/report should reference the standard and/or load scenarios. Such has been the typical practice for previous SFIC ships.

1.6.4 Typical ice model test reporting for Stern First Ice Class Ships may include:

1. Ice-breaking capability in level ice, ahead and astern;

2. Resistance astern in rubble field;

3. Ice ridge penetration capability astern;

4. Extraction force from ridge;

5. Performance and out-breaking capability in frozen channels.

1.6.5 LR may, if requested, review the ice model test report to verify that the standard load scenarios have been considered in the test program.

1.6.6 It remains the responsibility of the designer/test basin to verify that ice model performance is consistent with the design basis for operating stern first and to demonstrate such to the National Administration if required.

1.7.1 For shaft coupling bolts the maximum load due to bending moment F _E may be calculated by a number of different approaches, however the following clarification for the derivation is provided for reference:
F _E is the maximum bending moment:

where

M T	=	the bending moment at the point of consideration due to shaft weight
x i	=	the distance from the axis to each bolt, in mm
x ai(max)	=	the maximum distance from the axis to the extreme bolt, may be taken as pitch circle diameter, in mm
A a	=	minimum sectional area of bolt, in mm2
I A	=	moment of inertia of bolt array, in mm4
	=

1.8.1 There are a number of different approaches to the calculation of propeller bolts, however, an approach would be expected using the propeller blade break load, and the following clarification is provided for reference.

1. Determine moment for yielding blade (Blade break moment, M _bladebreak).

2. Determine distance from furthest bolt, D.

3. Derive Inertia of bolt array about neutral axis, I _ice (note the neutral axis should align with the angle at which the blade break load is applied (i.e. the pitch angle at radius R1)).

4. Determine stress in bolt from blade break load:

   

5. Derive total load on joint from ice and centrifugal forces in combination (Ftotal):

   

   	=	where
   D boltmin	=	minimum diameter of blade bolt, in mm
   D boltint	=	internal diameter of blade bolt, in mm
   F C	=	centrifugal load per bolt based on blade alone, in N.

6. Determine resultant force on bolt = tightening load of bolt + Ftotal (k1/k1 + k2).

7. Determine resultant bolt stress (resultant force/area).

8. Factor of safety is 1.5; the ratio of determined bolt stress to bolt yield stress.