Section 7 Fatigue Strength Assessment

701. General

1. Fatigue failures are one of the most important structural defects that need to be assessed in the

design. Such failures may be a more significant problem in an offshore vessel compared to ing carrier. The fatigue evaluation should include.

a trad-

(1)

(2)

Evaluation of critical areas: both from experience and fatigue screening Assessment of the loads at the intended site of operation

(3) Calculations of the structural response based on the actual loads

(4)

(5)

Accounting for the design life in the calculations

Fatigue design for design life with increased design fatigue factors

(6)

(7)

(8)

(9)

Internal structure directly welded to submerged part

External structure not accessible for inspection and repair in dry and clean condition Non-accessible areas inside the vessel

Very limited repair possibilities

(10) Critical areas in the hull structures will typically include.

(A) Topside supports, riser connections, flare tower supports etc.

(B) structure typical details such as longitudinal end connections, deck attachments

(C) LNG containment system specific details

2. Fatigue calculations are performed using as input environmental action of the waves and wind.

Wave actions will induce dynamic loads in the hull. For slender structures in the topsides, vortex

shedding from wind loads may also be a fatigue load of importance. The wave scatter diagram is used as input to the hydrodynamic analysis which will then give the motion and load response of the vessel in the waves. Modeling of the structure will typically be refined modelling with a fine

mesh at the critical locations. These include:

(1)

(2)

(3)

(4)

(5)

Moonpool areas

Topside support structures Cargo tank supports

Support of spherical tank skirt and tank cover

Hull adjacent to membrane tank

702. Fatigue Analysis Process

1. This guidance provides a guideline for a simplified fatigue analysis method and a direct fatigue analysis method (See Fig 5.2)

2. In case of requiring more precise fatigue strength assessment, the direct fatigue analysis method is to be applied to assess the fatigue strength. A spectral fatigue analysis method or a transfer func- tion method may apply to the direct fatigue analysis.

3. Other equivalent methods may be applied to assess the fatigue strength when deemed appropriate by the Society.

Fig 5.2 Fatigue analysis method

703. Additional Application

Other various matters on major element in the evaluation process of hull fatigue strength are to be as specified in Pt 3 of Rules for the Classification of steel ships.