Offshore Structure Platform Design - Offshore Structures: Design, Construction and Maintenance - page 158

TABLE 3.9 Safety Factor for Fatigue Life

Failure Criticality Inspectable Not Inspectable

No 2 5

Yes 5 10

For transportation where long-term wave distributions are used to predict
short-term damage, a larger safety factor should be considered.

Stress Concentration Factors
The stress concentration factor (SCF) differs from one joint geometry to
another. It is known that the applied loads on tubular joints cause stresses at cer-
tain points along the intersection weld to be many times the nominal stress act-
ing in the members. The SCF is a multiplier applied to the nominal stress to
reach the peak or maximum stress at the hot spot.
The hot spot is the location in the tubular joint where the maximum applied
tensile stress occurs. To do a fatigue analysis of certain selected tubular joints
in an offshore structure, the stress history of the hot spots in those joints must
first be determined. Three basic types of stress contribute to the development of
a hot spot. Primary stresses are caused by axial forces and moments resulting
from the combined truss and frame action of the jacket. As shown for the in-
plane tubular joint in Figure 3.38 , hot spots in locations 1, 2, 3, 4 and 6 are
most affected by axial forces and in-plane bending moments, while locations
2 and 5 are most affected by the axial forces and circumferential moments in
braces.
Secondary stresses are caused by structural details of the connection, such
as poor joint geometry, poor fit-up, local variation within the joint due to
rigid reinforcement or restraint of the braces by circumferential weld, and
these secondary stresses will amplify the primary stresses. Secondary stresses
are also caused by metallurgical factors, such as faulty welding practice,
insufficient weld penetration, heavy beading, weld porosity or varying cool-
ing rates. These stresses mainly affect location 1, but their effect is also sig-
nificant at locations 3, 4 and 6. Because metallurgical factors are essential in
fatigue stresses on tubular joints, the quality control (QC) procedure for con-
structing this connection should be given more attention, as is described in
Chapter 5 .
The most fatigue-sensitive areas in offshore platforms are the welds at
tubular joints, because of the high local stress concentrations. Fatigue lives at
these locations should be estimated by evaluating the hot-spot stress range
(HSSR) and using it as input into the appropriate S-N curve.