Clasification Society Rulefinder 2016 - Version 9.25
Clasification Society Rules and Regulations - Rules and Regulations for the Classification of Offshore Units, January 2016 - Part 4 STEEL UNIT STRUCTURES - Chapter 2 Materials - Section 4 Steel grades

Section 4 Steel grades

Parent topic: Chapter 2 Materials

4.1 General

4.1.1 The grades of steel to be used in the structure are, in general, related to the thickness of the material, the structural category and the MDT. The grades of steel to be used in the construction of the unit are to be determined from Pt 4, Ch 2, 4.1 General 4.1.6, see also Pt 4, Ch 2, 4.1 General 4.1.5 and Pt 4, Ch 2, 2 Structural categories. Material thicknesses greater than those shown in Pt 4, Ch 2, 4.1 General 4.1.6 may be specially considered by LR, e.g., legs of self-elevating units.

4.1.2 Special consideration will be given to the use of higher tensile steel grades with a minimum yield stress greater than 390 N/mm2, e.g., legs of self-elevating units.

4.1.3 Material where the principal loads or welding stresses are perpendicular to the plate thickness should have suitable through thickness properties. The application of through thickness property Z25 or Z35 grade material is required where tensile stresses exceed 50 per cent of the Rule permissible stress, with plate thickness in excess of 15 mm. In general, Z25 grade would be required; however, for critical joints, Z35 will be required. For certain critical joints with a restricted load path this criterion would be subject to special consideration, for example, mooring fairlead attachments and anchor line or hawser connections.

4.1.4 Steel grades for units required to operate in severe ice conditions will be specially considered. Temperature gradient calculations may be required to assess the design temperature of the structure, see also Pt 3, Ch 6 Units for Transit and Operation in Ice.

4.1.5 Minor structural components such as guard rails, walkways and ladders, etc., are, in general, to be constructed of Grade A steel, unless agreed otherwise by LR, see also Pt 4, Ch 2, 4.1 General 4.1.4.

4.1.6 For components listed in Pt 4, Ch 2, 4.1 General 4.1.6, special consideration may be given to material grades with other impact properties than those required by Pt 4, Ch 2, 4.1 General 4.1.6. In such cases, written agreement is required prior to manufacture. This evaluation is to be based on critical engineering assessment involving fracture mechanics testing on welded material from the intended supplier and proposals are to be submitted which include full details of the application, minimum temperature, design, design stresses, fatigue loads and cycles, welding procedures that will be applied and inspection procedures.

Table 2.4.1 Thickness limitations for hull structural steels for various application categories and design temperatures for use in welded construction

Structural category Required steel grade Maximum thickness permitted (mm) for
various minimum design temperatures, see Note 8
0°C –10°C –20°C –30°C
Secondary A 30 20 12,5 X
B 60 50 25 10
D 100 100 80 50
E 150 150 120 100
AH 50 40 25 10
DH 100 100 70 50
EH 150 150 100 80
FH 150 150 150 120
AQ 50 40 25 10
DQ 100 100 70 50
EQ 150 150 120 80
FQ 150 150 150 120
Primary A 20 12,5 X X
B 25 25 12,5 X
D 50 50 30 20
E 100 100 65 40
AH 25 25 12,5 X
DH 50 50 30 20
EH 120 100 65 40
FH 150 150 150 100
AQ 25 25 X X
DQ 50 50 30 20
EQ 120 100 65 40
FQ 150 150 150 100
Special A 12,5 X X X
B 15 12,5 X X
D 30 30 20 10
E 100 75 35 30
AH 20 12,5 X X
DH 30 30 12,5 10
EH 100 75 35 25
FH 150 100 80 50
AQ 15 12,5 X X
DQ 30 30 12,5 10
EQ 100 75 30 25
FQ 150 100 80 60
NOTES
1. X indicates that the material is not permitted for that design temperature and structural category.
2. Materials are to comply with the requirements of Ch 3 Rolled Steel Plates, Strip, Sections and Bars of the Rules for Materials.
3. Q grades refer to quenched and tempered grades (Ch 3, 10 High strength quenched and tempered steels for welded structures of the Rules for Materials).
4. The thicknesses refer to as constructed thicknesses (e.g., design thickness plus any allowances such as corrosion allowance).
5. Requirements for minimum design temperature lower than –30°C will require special consideration which may include the use of fracture mechanics assessments.
6. Thicknesses greater than those shown in this Table may be used subject to special consideration by LR and may include fracture mechanics assessment.
7. The interpolation of thicknesses for intermediate temperatures may be considered.
8. For LNG installations where the minimum hull shell plating temperature used in the design is the result of heat conduction from the cargo rather than environmental conditions, the material thicknesses shall be in accordance with Pt 11, Ch 6, 1.4 Requirements for metallic materials 1.4.1 in Pt 11, Ch 6 Materials of Construction and Quality Control .

Table 2.4.2 Application where fracture mechanics may be considered to permit alternative grades of steel

Application
Lattice type leg structures
Cylindrical legs
Footing and mats
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