Section 2 Application of ice class to hull structures

2.1.1 The basic philosophy of the ice-strengthening requirements in the Rules is to apply the ice class bow requirements to the stern of the ship: this is termed ‘stern as a bow’, and follows past design and approval practice. See Table 3.2.1 Stern as a bow concept.

Table 3.2.1 Stern as a bow concept

(a)	Direct application of ‘stern as a bow’ Assigned notation: Ice Class PC5
(b)	Application of ice-strengthening requirements for SFIC notation, assuming standard scenarios Assigned notation: Ice Class PC5, SFIC
(c)	Application of ice-strengthening requirements for SFIC notation, including scenario for bow first in multi-year ice Assigned notation: Ice Class PC2, (PC4 Stern First), SFIC
	Note Requires Scenario document to be developed and submitted
(d)	Application of ice-strengthening requirements for SFIC notation, bow form optimised for ice and open water Assigned notation: Ice Class PC6, (PC4 Stern First), SFIC
	Note Requires Scenario document to be developed and submitted

2.1.2 LR has carried out a series of design reviews and damage studies for the stern areas of Stern First Ice Class Ships currently operating and a comparison of damage cases for conventional ships operating bow first in ice.

2.1.3 The application of ice class Rules to the hull structure in the Rules incorporates the design and damage review. This has led to an arrangement of ice-strengthening validated with service experience for the assignment of SFIC and a reduction in the Area Factor accordingly applied to the stern. The background for this ice-strengthening arrangement is given in Ch 3, 2.2 Ice-strengthening extents.

2.2.1 The SFIC requirements for stern ice-strengthening differ from the direct stern as a bow application.

2.2.2 The area above the ice belt at the stern has been reduced. No damage has been recorded in this area on SFIC ships operating. This addition above the ice belt, usually for the bow, is related to ice impacts at high speeds in relatively open pack ice. This is not considered an operational scenario for SFICs.

2.2.3 The extent aft of the bow region has been reduced. No damage has been recorded in this area on SFIC ships operating. The reduction more closely aligns with the hull form shape at the stern end for SFIC ships.

2.2.4 The extent of the stern shoulder (bow intermediate) region has been increased. Damage has been recorded in this area for both conventional ships and ships equipped with podded propulsion units. The extent of the bow intermediate region has been redefined, based on damage analysis and the typically shallower flat of side angles associated with stern shapes.

2.2.5 For FSICR application additional strengthening has been added below the ice belt and at the foot of the skeg. This reflects strengthening practice for SFIC ships operating strengthened to FSICRs and accounts for the increased probability of ice impacts from the ridge keel when operating stern first.

2.3.1 The SFIC requirements for stern ice-strengthening level have been reduced when compared to a direct ‘stern as a bow' application.

2.3.2 For application of PC Rules, the Area 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 have been reduced by effectively one ice class strength level lower than a direct ‘stern as a bow’ application (one step lower). The strength level for the stern area is expected to be higher than the application of the requirements of the ‘stern as a stern’.

2.3.3 The justification for the reduction in Ch 3, 2.3 Ice-strengthening level 2.3.2 is based on a comparison of existing SFIC strengthening levels and the strengthening level associated with a direct PC Rule ‘stern as a bow’ application, assuming a common level of structural capability for the midship region, stern hull form shapes and angles and damage data. This has been further supported by full scale measurement data. This adjustment will be refined as more operational data on SFIC ships becomes available.

2.3.4 For application of FS Rules a similar ‘stern as a bow’ strengthening level to Ch 3, 2.3 Ice-strengthening level 2.3.3 is given. This is based on previous experience of SFIC ships and full scale data which indicate that a lower ice-strengthening level for stern as a bow application for FSICRs strength level is generally appropriate when assuming a common level of structural capability for the midship region.

2.3.5 Further information on the evaluation of strength levels for SFIC ships may be found in Ch 7, 2.1 References to published papers 2.1.1.

2.4.1  Figure 3.2.1 Application of bow and stern first ice waterlines (note: exaggerated for illustrative purposes) shows how bow first and stern first ice waterlines are to be considered when applying ice class Rules to the hull structure.

Figure 3.2.1 Application of bow and stern first ice waterlines (note: exaggerated for illustrative purposes)

2.5.1 The requirements in the Rules may be applied to icebreakers. This is on the general assumption that icebreakers equipped with podded propulsion units or azimuthing thrusters may use the directional thrust for such manoeuvres as stern first operation through first year ice ridges. However, it is acknowledged that icebreakers also use directional thrust for other activities, such as:

1. Clearing/widening channels;

2. Close manoeuvring around beset ships;

3. Management of large ice features.

2.5.2 Where the notation Icebreaker is desired, it is recommended that a scenario document be developed, in order to determine any other critical scenarios which may influence the strengthening requirements due to specific operational characteristics, in addition to the standard scenarios used in the Rules.