Page 365 - Marine Structural Design

Chapter I7 Fatigue Capacity 34 1

where,

SCF, = Stress concentration factor due to the gross geometry of the detail considered
SCF, = Stress concentration factor due to the weld geometry
SCF;, = Additional stress concentration factor due to eccentricity tolerance (nominally
used for plate connections only)
SCF,= = Additional stress concentration factor due to angular mismatch (normally
used for plate connection only)
SCF, = Additional stress concentration factor for un-symmetrical stiffeners on
laterally loaded panels, applicable when the nominal stress is derived from
simple beam analysis.
The best-known SCF formulae for the fatigue assessment of offshore structures are those of
Efthymious (1988). There are various parametric equations in the literature for the
determination of SCFs, for instance:
SCF equations for tubular connections: AF'I RP2A-WSD, NORSOK N-004 (NTS 1998)
and Efthymiou (1988). In addition, Smedley and Fisher (1990) gave SCFs for ring-
stiffened tubular joints under axial loads, in-plane and out-of-plane bending. For
rectangular hollow sections, reference is made to Van Wingerde, Packer, Wardenier, Dutta
and Marshall (1993) and Soh and Soh (1993).
SCF equations for Tube to Plate Connections: NORSOK N-004 and Pilkey (1 997).
SCF for girth welds: NORSOK N-004 (NTS, 1998).
The SCF equations from the references mentioned in the above have been summarized in
DNV (2000).
It should be indicated that the parametric equations are valid only for the applicability range
defined in terms of geometry and loads. A general approach for the determination of SCFs is
to use the finite element analysis, see the sub-section below.
17.3.4 Hot-Spot Stress Calculation Based on Finite Element Analysis

The aim of the finite element analysis is to calculate the geometric stress distribution in the hot
spot region so that these stresses can be used to derive stress concentration factors. The result
of finite element analysis of SCFs largely depends on the modeling techniques and the
computer program used. The use of different elements and meshes, modeling of the welds, and
definition of the chord's length substantially influence the computed SCF (Healy and Bultrago,
1994).
By decreasing the element size, the FEM stresses at discontinuities may approach infinity. In
order to have a uniform basis for comparison of results from different computer programs and
users, it is necessary to set a lower bound for the element size and use an extrapolation
procedure to the hot spot.
Stresses in finite element analysis are normally derived at the Gaussian integration points.
Depending on the element type it may be necessary to perform several extrapolations in order
to determine the stress at the weld toe. In order to preserve the information of the direction of
principal stresses at the hot spot, component stresses are to be used for the extrapolation.