Section 3 Limit states of design
Clasification Society 2024 - Version 9.40
Clasifications Register Rules and Regulations - Rules and Regulations for the Classification of Offshore Units, July 2022 - Part 9 Concrete Unit Structures - Chapter 1 General Requirements and Design Principles - Section 3 Limit states of design

Section 3 Limit states of design

Parent topic: Chapter 1 General Requirements and Design Principles

3.1 Ultimate Limit State (ULS)

3.1.1 The strength of the structure is to be sufficient to ensure that under the worst combination of wave loads, still water loads and mooring loads, the structure will not collapse, buckle or implode, see also Pt 9, Ch 3, 4.2 Analysis of sections for ULS.

3.1.2 Individual sections are to be checked for rupture. Consideration is also to be given to the mode of failure. In general, the initiation of failure of primary members by compression or shear is to be avoided.

3.2 Serviceability Limit State (SLS)

3.2.1 The serviceability limit is selected to ensure that the structure will meet the requirements for deflection, durability, liquid tightness and cracking under service conditions, see also Pt 9, Ch 3, 4.3 Analysis of sections for SLS.

3.2.2 The deflection of the structure or any part of the structure is to be limited such that it does not adversely affect the efficiency of the structure. Deflections are to be compatible with the degree of movement acceptable for the operation of services, etc. Any particular requirements should be specified by the Owner.

3.2.3 The durability of the structure is dependent upon the mix design, the concrete cover, control of cracking by the reinforcement, and exposure conditions. Requirements for concrete mix and cover are given in Pt 9, Ch 4 Materials and Durability.

3.3 Fatigue Limit State (FLS)

3.3.1 The designer is to demonstrate that the structure is not susceptible to fatigue failure. Agreement is to be reached with LR on the areas of the structure which are potentially vulnerable to fatigue. In particular, the oil storage tank area and the turret area are to be specially considered.

3.3.2 A fatigue analysis of critical areas is to be carried out based on the principle of cumulative damage, or fracture mechanics, see also Pt 9, Ch 3, 4.4 Analysis of sections for FLS.

3.3.3 The dynamic behaviour of the unit is to be investigated to determine whether the increase in load effects due to dynamic amplification is significant.

3.4 Accidental (ALS) and Progressive Collapse Limit State (PCLS)

3.4.1 The layout of the structure and the interaction between the structural members are to be such as to ensure a robust and stable design.

3.4.2 Consideration is to be given to redundancy and the possibility of progressive collapse. The designer must ensure that there is sufficient strength or redundancy to prevent this occurring. This requirement relates particularly to accidental or exceptional loads. Consideration is to be given to both the intact and post damaged condition.

3.4.3 Environmental return periods for use in post damaged conditions are given in Table 2.2.1 Basis for selection of return periods for environmental loads.

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