Offshore Structure Platform Design - Offshore Structures: Design, Construction and Maintenance - page 171

Brace saddle SCFs under OPB: obtained from the adjacent chord SCFs using
SCF = τ − 0 : 54 γ − 0 : 05

4

ð 0 : 99 − 0 : 47 β + 0 : 08 β Þ × SCFchord (3.144)

where SCFchord = Equation (3.140) KT1 or Equation (3.143) .
In joints with short chords (

12), Equations (3.140), (3.143) and (3.144)
can be reduced by the short chord factor F4, where
F4 = 1

α <

2.4 ].
Effect of weld toe position. Ideally, the SCF should be invariant, given the
tubular connection

1.88 exp[

γ − 1.06 − 1.07 β − 0.16 α

). This is how the Efthymiou and
all the other SCF equations are formulated. Hot-spot stress is calculated from
the linear trend of notch-free stress extrapolated to the toe of the basic standard
weld profile, with nominal weld toe position as defined in AWS D1.1 Figure 3.8 .
When this is done, size and profile effects must be accounted for in the S-N curve,
regardless of the underlying cause. This is how the previous API rules were set up.
International thinking tends to suggest that weld profile effects, mainly the
variable position of the actual weld toe, should be reflected in the SCF, rather
than in the S-N curve. This is consistent with how experimental hot-spot
stresses were measured to define the basic international S-N curve for hot-
spot fatigue in 16 mm thick tubular joints. One tentative method for correcting
analytical SCF for weld toe position was presented in Marshall (1989) . Based
on Marshall et al. (2005) , a more robust formulation is now proposed:
SCF corr =

' sgeometry( γ , τ , β , θ ,and ζ

L Þ /L mp (3.145)
where SCF corr = the correction factor applied to Efthymiou SCF; L a = the actual
weld toe position for typical yard practice; L = the nominal weld toe position;
and L mp = the moment persistence length (distance from nominal toe to reversal
of shell bending stress).
Various expressions for L mp are shown in Table 3.11 as a function of joint
type, load type and hot-spot orientation.

1 − ð L a −

TABLE 3.11 Expressions for L mp

Circumferential stress at saddle
All loading modes

L mp = (0.42 − 0.28 β ) R

Angle = (24 − 16 β ) degrees

Longitudinal stress at crown

L mp = 0.6( RT ) 0.5

Axis symmetric

L mp = lesser of 0.6 ( RT ) 0.5 or g/2 Gap (g) of K joint

L mp = 1.5( RT ) 0.5

Outer heel/toe, axial

L mp = 0.9( RT ) 0.5 In-plane bending