Section
4 Steel grades
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 Table 2.4.1 Thickness limitations for
structural steels for various application categories and design temperatures
for use in welded construction including plates, bars, sections forgings,
and castings, see also
Pt 4, Ch 2, 4.1 General 4.1.5 and Pt 4, Ch 2, 2.2 Column-stabilised and tension-leg units. Material thicknesses greater
than those shown in Table 2.4.1 Thickness limitations for
structural steels for various application categories and design temperatures
for use in welded construction including plates, bars, sections forgings,
and castings 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 For materials where the principal loads or welding stresses are
perpendicular to the plate thickness, the requirements for through thickness
properties must be considered. The specific requirements for through thickness
testing and/or for low sulphur level should be determined using either EN 1993-1-10
or, subject to the agreement of Lloyd’s Register, an alternative recognised national
or international standard.
In addition to the above, special consideration shall be given to the
requirements for through-thickness testing for critical joints with a restricted
load path, 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 Table 2.4.2 Application where fracture
mechanics may be considered to permit alternative grades of steel, special consideration may
be given to material grades with other impact properties than those required by
Table 2.4.1 Thickness limitations for
structural steels for various application categories and design temperatures
for use in welded construction including plates, bars, sections forgings,
and castings. 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
structural steels for various application categories and design temperatures
for use in welded construction including plates, bars, sections forgings,
and castings
Structural category
|
Required steel grade, see Note
2
|
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. Plate materials are to comply with the requirements of Ch 3 Rolled Steel Plates, Strip, Sections and Bars of the Rules for the Manufacture, Testing and Certification of Materials, July 2022. Castings
and forgings shall satisfy the same Charpy V-notch requirements
as the equivalent plate grades in this table.
|
3.
Q grades refer to quenched and tempered grades (Ch 3, 10 High strength steels for welded structures of the Rules for the Manufacture, Testing and Certification of Materials, July 2022).
|
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 Table 6.1.5 Plates and sections for
hull structures required by 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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