Section 1 General requirements
Clasification Society 2024 - Version 9.40
Clasifications Register Rules and Regulations - Rules and Regulations for the Classification of Offshore Units, July 2022 - Part 4 Steel Unit Structures - Chapter 5 Primary Hull Strength - Section 1 General requirements

Section 1 General requirements

Parent topic: Chapter 5 Primary Hull Strength

1.1 General

1.1.1 This Section defines the overall strength requirements of the unit and the permissible stresses in all operating modes.

1.1.2 The design loads are to be in accordance with Pt 4, Ch 3, 1 General and the design conditions are to be based on the most unfavourable combinations of gravity loads, functional loads, environmental loads and accidental loads.

1.1.3 Specific requirements for structural unit types are also defined in Pt 4, Ch 4 Structural Unit Types.

1.1.4 The local strength of the unit is to comply with the requirements of Pt 4, Ch 6 Local Strength.

1.1.5 The limiting design environmental and operational conditions for each mode of operation is to be defined by the Owner/designer and included in the Operations Manual, see Pt 3, Ch 1, 3 Operations manual.

1.2 Structural analysis

1.2.1 A structural analysis of the primary structure of the unit is to be carried out in accordance with the requirements of Pt 4, Ch 3 Structural Design and the resultant stresses determined.

1.2.2 The loading conditions are to represent all modes of operation and the critical design cases obtained.

1.2.3 The structure is to be analysed for the relevant load combinations given in Pt 4, Ch 3, 4.3 Load combinations.

1.2.4 For the combined load cases applicable to all unit types, see also Pt 4, Ch 4 Structural Unit Types.

1.2.5 The permissible stress levels relevant to the combined load cases defined in Pt 4, Ch 5, 1.2 Structural analysis 1.2.3 are to be in accordance with Pt 4, Ch 5, 2 Permissible stresses.

1.2.6 Special consideration is to be given to structures subjected to large deformations.

1.3 Primary structure

1.3.1 Local stresses, including those due to circumferential loading on tubular members, are to be added to the primary stresses to determine total stress levels.

1.3.2 The scantlings are to be determined on the basis of criteria which combine, in a rational manner, the individual stress components acting on the various structural elements of the unit. The stresses are to be determined with corrosion allowance deducted from the gross scantlings in accordance with Table 3.7.1 Corrosion rate for one side of structural member in Pt 4, Ch 3 Structural Design, see also Pt 3, Ch 1, 5 Corrosion control.

1.3.3 The critical buckling stress of structural elements is to be considered in relation to the computed stresses, see Pt 4, Ch 5, 3 Buckling strength of plates and stiffeners and Pt 4, Ch 5, 4 Buckling strength of primary members.

1.3.4 Fatigue damage due to cyclic loading is to be considered in the design of the unit in accordance with Pt 4, Ch 5, 5 Fatigue design.

1.3.5 When computing bending stresses, the effective flange areas are to be determined in accordance with ‘effective width’, concepts derived from accepted shear lag theories and plate buckling considerations.

1.3.6 Where appropriate, elastic deflections are to be taken into account when determining the effects of eccentricity of axial loading, and the resulting bending moments superimposed on the bending moments computed for other types of loadings.

1.3.7 When computing shear stresses in bulkheads, plate girder webs or hull side plating, only the effective shear area of the plate or web is to be considered. For girders, the total depth of the girder may be considered as the web depth.

1.3.8 Members of lattice type structures may be designed in accordance with a recognised Code as defined in Pt 12, Ch 1, 19.1 Recognised codes and standards for general structural items (skids, support frames and trusses etc.).

1.4 Connections and details

1.4.1 Special consideration is to be given to structural continuity and connections of critical components of the primary and special structure, such as the following:
  • Bracing intersections and end connections.
  • Columns to lower and upper hulls.
  • Jackhouses to deck.
  • Legs to mat or footings.
  • Turret areas.
  • Yokes and mooring arms.
  • Mooring line attachments.
  • Swivel stack supports.

1.4.2 Critical joints which depend upon the transmission of tensile stresses through the thickness perpendicular to the plate surface of one of the members are to be avoided wherever possible. Where the stresses perpendicular to the plate surface exceed 50 per cent of the Rule permissible stress and the thickness exceeds 15,0 mm, plate material with suitable through thickness properties as required by Ch 3, 8 Plates with specified through thickness properties of the Rules for the Manufacture, Testing and Certification of Materials is to be used.

1.4.3 Welding and structural details are to be in accordance with Pt 4, Ch 8 Welding and Structural Details.

1.5 Stress concentration

1.5.1 The effect of notches, stress raisers and local stress concentrations is to be taken into account in the design of load-carrying elements.

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