PSI - Issue 19

J. Rudolph et al. / Procedia Structural Integrity 19 (2019) 575–584 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Where misalignments exist, local increases in pressure-induced stresses in shells can be determined either by taking into account the shape imperfections when defining the geometry of the F.E. model used or by applying adequate corrections to the stresses determined on the basis of the nominal geometry of the vessel. The clause provides a conservative analytical procedure to estimate those adequate corrections where the stresses are determined on the basis of the nominal geometry.

6. Fatigue strength of steel bolts

The design curve for steel bolting was based on fatigue data obtained from steel bolts. Thus, the design curve already incorporates the stress concentration effect of the thread root. It is used in conjunction with the nominal axial

stress on the minimum bolt cross-section due to applied tension and bending. The design has to take into account the thickness and temperature correction.

7. In-service monitoring of vessel operating in fatigue

When a vessel operating in fatigue significant cyclic loading has entered service, informative Annex M gives guidance on monitoring at the relevant fatigue critical locations , which must be reported in the operating manual of such vessels. If this guidance is followed, then it may be possible to justify further operation beyond the standard allowable fatigue lifetime (D=1) with additional inspection and repair of defects. While Annex M is informative, it can be noted that the calculated allowable fatigue lifetime (D=1) should be regarded as a minimum of the fatigue lifetime that may actually be possible. 8. New annexes NA, NB, NC and ND For a more detailed discussion of the new annex NA “Instructions for determining structural hot -spot stress by finite element analysis using shell and solid elements”, NB “Cycle counting and determination and determination of equivalent stress range”, NC “Fatigue assessment of partial penetration welds” and ND “Table of stress concentration factors K t ” see references [5], [6], [7] and [13].

9. Implementation of a critical plane approach

The procedure is based on seeking for the plane which is the most loaded at the point of the vessel under consideration. The degree to which a material plane is loaded is measured by the damage due to the fluctuations of the shear stress acting in this plane.

The main steps of the procedure are as follows:

I.

Selection of an analysis plane as shown in Figure 6.

II.

Determination of the history of: - the shear stress components in the analysis plane, - the stress normal to the analysis plane, over the design interval as shown in Figure 7.

Selection of an origin point in time (N 0 as shown in Figure 8).

III.