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 5 Primary Hull Strength - Section 5 Fatigue design

Section 5 Fatigue design

Parent topic: Chapter 5 Primary Hull Strength

5.1 General

5.1.1 Fatigue damage due to cyclic loading is to be considered in the design of all unit types. The extent of the fatigue analysis will be dependent on the mode and area of operation.

5.1.2 Where any unit is intended to operate at one location for an extended period of time, a rigorous fatigue analysis is to be performed using the long-term prediction of environment for that area of operation with the unit at the intended orientation. Due allowance is to be made of any previous operational history of the unit.

5.1.3 The two basic methods of fatigue analysis available are Deterministic Fatigue Analysis and Spectral Fatigue Analysis. Both are acceptable to LR.

5.1.4 Factors which influence fatigue endurance and should be accounted for in the design calculations include:
  • Loading spectrum.
  • Detail structural design.
  • Fabrication and tolerances.
  • Corrosion.
  • Dynamic amplification.
5.1.5 The following important sources of cyclic loading should be considered in the design:
  • Waves (including those which cause slamming and variable-buoyancy effects).
  • Wind (especially when vortex shedding is induced, e.g., on slender members).
  • Currents (where these influence the forces generated by waves and/or induced vortex shedding).
  • Mechanical vibration (e.g., caused by operation of machinery).

5.1.6 Where a fine mesh finite element analysis is carried out to determine local geometric stress concentration factors, selection of associated S-N curves will be specially considered. Account is to be taken of fatigue stress direction relative to the weld. In general, the element mesh size adjacent to the weld detail under consideration is to be of the order of the local plate thickness. Mesh arrangement and analysis methodology are to be agreed with LR.

5.1.7 In general, stresses are to be determined using net scantlings, i.e., no corrosion allowance included, see also Pt 3, Ch 1, 5 Corrosion control.

5.2 Fatigue life assessment

5.2.1 Fatigue life assessment of all relevant structural elements is required to demonstrate that structural connections have a fatigue endurance consistent with the planned life of the unit and compliance with the minimum requirements. The following structural elements are to be included:
  1. Column-stabilised and tension-leg units:
    • Bracing structure.
    • Bracing connections to lower hulls, columns and decks.
    • Column connections to lower hulls.
    • Column connections to deck.
    • Mooring structure and associated hull structure integration.
    • General structural discontinuities.
  2. Surface type units:
    • Hull longitudinal stiffener connections to transverse frames and bulkheads.
    • Toe area of main structural brackets.
    • Hopper knuckle connections.
    • Main openings in the hull envelope.
    • Mooring structure and associated hull structure integration.
    • General structural discontinuities in the primary hull structure.
  3. Self-elevating units:
    • Lattice legs and connections to footings.
    • Leg support structure.
    • Raw water towers.
  4. Other unit types:
    • Special consideration will be given to the hull structure of other unit types on the basis of this Section.
  5. General: Hull, deck and supporting structure in way of topside facilities, e.g:
    • Module support.
    • Process plant support stools.
    • Crane pedestal.
    • Flare structures.
    • Offloading station.
    • Drilling derrick and substructures.
  6. General: Other structures subjected to significant cyclic loading.

5.2.2 Fatigue life is normally governed by the fatigue behaviour of welded joints, including both main and attachment welds. Structure is to be detailed and constructed to ensure that stress concentrations are kept to a minimum and that, where possible, components may deform without introducing secondary effects due to local restraints.

5.2.3 The minimum design fatigue life of a unit is to be specified by the Owner, but is not to be less than 25 years, unless agreed otherwise by LR. See also Pt 10 SHIP UNITS for ship units.

5.3 Fatigue damage calculations

5.3.1 The fatigue damage calculations are to be based on the long-term distribution of the applied stress ranges. A sufficient number of draughts and directions are to be included.

5.3.2 An appropriate wave spectrum is to be used and representative percentages of the total cumulative spectrum included for each direction under consideration. When using a limited number of directions, account is to be taken of symmetry within the structure.

5.3.3 Cumulative damage may be calculated by Miner’s summation:

where

s = number of stress range blocks
= actual number of cycles for stress range block number ‘i’
= corresponding number of cycles obtained from the relevant S-N curve for the detail under consideration

5.3.4 Cumulative damage for individual components is to take into account the degree of redundancy, accessibility of the structure and also the consequence of failure.

5.3.5 Fatigue life estimation is normally to be based on the Miner’s summation method given in Pt 4, Ch 5, 5.3 Fatigue damage calculations 5.3.3, but consideration will be given to the use of an appropriate fracture mechanics assessment.

5.3.6 Where wave scatter diagrams are used for the calculation of fatigue damage for mobile offshore units and transit voyages of floating offshore installations at a fixed location, the scatter diagram is to contain at least one year of data

5.4 Joint classifications and S-N curves

5.4.1 Acceptable joint classification and S-N curves for structural details are contained in Pt 4, Ch 12 Appendix A Fatigue – S-N Curves, Joint Classification and Stress Concentration Factors.

5.4.2 Consideration will be given to the use of alternative methods; detailed proposals are to be submitted and agreed with LR.

Table 5.5.1 Fatigue life factors of safety for structural components

Inspectable/repairable Fatigue life factor
Consequence of failure
Non-substantial

Substantial

See Note 1

Yes, dry

See Note 2

1 2

Yes, wet

See Note 3

2 4
No 3 10
NOTES
1. Substantial consequences of failure include, inter alia, loss of life, uncontrolled outflow of hazardous or polluting products, collision, sinking. In assessing consequences, account should be taken of the potential for progressive failure. This factor will be applicable for bottom structure of oil storage tanks of single bottomed units and side structures of oil storage tanks of single sided units.
2. Includes internal and external structural elements and connections which can be subjected to dry inspection and repair.
3. Includes external structural elements and connections situated below the minimum operating draught of the unit or structure which can only be inspected during in-water surveys but dry repairs could be carried out subject to special arrangements being provided.

Table 5.5.2 Fatigue life factors of safety for anchor line and tether components

Inspectable/replaceable Fatigue life factor
Yes, dry 3
Yes, wet 5
No 10
NOTE
Anchor line or tether components include chains, steel wire ropes, and associated fittings such as shackles, connecting links, rope sockets and terminations.

5.4.3 Full penetration welds are normally to be used for all nodal joints (i.e., tubular brace to chord connections). For full penetration welded joints, fatigue cracking would usually be located at the weld toe. However, if partial penetration welds have to be used where weld throat failure is a possibility, fatigue should be assessed using the ‘W’ curve and a shear stress estimated at the weld root.

5.4.4 For nodal joints, the stress range to be used in the fatigue analysis is the hot spot stress range at the weld toe. For any particular type of loading (e.g., axial loading) this stress range is the product of the nominal stress range in the brace and the appropriate stress concentration factor (SCF).

5.4.5 The hot spot stress is defined as the greatest value around the brace/chord intersection of the extrapolation to the weld toe of the geometric stress distribution near the weld toe. This hot spot stress incorporates the effects of overall joint geometry (i.e., the relative sizes of brace and chord) but omits the stress-concentrating influence of the weld itself which results in a local stress distribution. Hence, the hot spot stress is considerably lower than the peak stress but provides a consistent definition of stress range for the design S-N curve (curve ‘T’ shown in Pt 4, Ch 12 Appendix A Fatigue – S-N Curves, Joint Classification and Stress Concentration Factors). Stress ranges both for the brace and chord sides are to be considered in any fatigue assessment.

5.4.6 For all other types of joint (e.g., welded stiffeners or attachments, including those at nodal joints) the joint classifications and corresponding S-N curves are to take into account the local stress concentrations created by the joints themselves and by the weld profile. The relevant stress range is then the nominal stress range which is to include any local bending adjacent to the weld under consideration. However, if the joint is also situated in a region of stress concentration resulting from the gross shape of the structure, this is to be taken into account.

5.4.7 In load-carrying partial penetration or fillet-welded joints, where cracking could occur in the weld throat, the relevant stress range is the maximum range of shear stress in the weld metal. For details which are particularly fatigue-sensitive, where failure could occur through the weld, full penetration welding is normally to be used.

5.4.8 Geometric stress concentrations may be determined from experimental tests, appropriate references, semi-empirical or parametric formulae or analytical methods (e.g., finite elements analysis). See also Pt 4, Ch 12 Appendix A Fatigue – S-N Curves, Joint Classification and Stress Concentration Factors.

5.4.9 Normal fabrication tolerances according to good workmanship standards as given by the Rules are considered to be implicitly accounted for in the S-N curves.

5.5 Cast or forged steel

5.5.1 Fatigue life calculations for cast or forged steel structural components are to include details of the fatigue endurance curve for the material, taking account of the particular environment, mean stress and the existence of casting defects, and the derivation of any stress concentration factors.

5.6 Factors of safety on fatigue life

5.6.1 The minimum factors of safety on the calculated fatigue life of structural components are to be in accordance with Pt 4, Ch 5, 5.4 Joint classifications and S-N curves 5.4.2. For mooring systems, see Pt 4, Ch 5, 5.6 Factors of safety on fatigue life 5.6.2.

5.6.2 The minimum factors of safety on the calculated fatigue life of anchor lines and tether components of mooring systems are to be in accordance with Pt 4, Ch 5, 5.4 Joint classifications and S-N curves 5.4.2.

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