Offshore Structure Loads and Strength - Offshore Structures: Design, Construction and Maintenance - page 78

Most of the platform ' s structural members are made from Group I steel, as

●

shown in Table 2.31 .

Group II are intermediate-strength steels with specified minimum yield
strengths of over 279 MPa through 360 MPa. These steels require the use
of low-hydrogen welding processes.

●

Group III are high-strength steels with specified minimum yield strengths in
excess of 52 ksi (360 MPa).

●

Table 2.31 presents the steel groups, their yield strength ranges and their
charpy toughness.
Steels may be used provided that each application is investigated with
regard to:

1. Weldability and special welding procedures that may be required.
2. Fatigue problems that may result from the use of higher working stresses.
3. Notch toughness in relation to other elements of fracture control, such as fab-
rication, inspection procedures, service stress and environmental temperature.

The steel should be made by the basic oxygen or basic electric arc furnace
process. All steel should be fully killed and made to fine grain practice. Rim-
ming steel should not be supplied. The minimum rolling reduction ratio of
material used for plates should be 4:1.
In most offshore structures using ASTM A572 Grade 50 the material should
conform to the requirements of ASTM A572 and ASTM A6, except as noted
below. All steel should be supplied in the normalized condition.

TABLE 2.31 Materials Category and Selection for Structure

Yield Strength
Range, MPa

Charpy
Toughness

Steel Group Structural Element

I 220 - 275 20 J Primary and secondary bracing

Brace end stubs at node
Legs
Piling
Conductor panels
Boat landings and walkways
Stiffener elsewhere

II 280 - 395 35 J Joint cans

Brace end stubs at node
Legs
Stiffener at nodes
Piling with thick wall at sea floor

III 400 - 455 45 J Legs

Bracing in area of high collision