Offshore Structure Platform Design - Offshore Structures: Design, Construction and Maintenance - page 157

the column buckling characteristics determine the diameters of the various
jacket braces. The initial wall thicknesses of chords and braces are determined
by structural analysis. The next cycle of calculation involves increasing the
chord wall thicknesses with heavy joint cans to ensure sufficient static strength
to meet code or specification requirements. The next iteration involves calculat-
ing the fatigue strength of the joint to determine if it is compatible with the ser-
vice life requirement of the platform. Depending on the method of fatigue
analysis used, allowable stress concentration factors must be either specified
for each joint or built into the method of analysis.
For each location around each member intersection of interest in the struc-
ture, the stress response for each sea state should be computed, giving adequate
consideration to both global and local stress effects.
The stress responses should be combined into the long-term stress distribu-
tion,whichshouldthenbeusedtocalculate the cumulative fatigue damage
ratio, D, where

D =∑ð n/N Þ (3.107)

where n = number of cycles applied at a given stress range and N = number of cycles
for which the given stress range would be allowed by the appropriate S-N curve.
In most cases, the damage ratio will be calculated for each sea state and
combined to obtain the cumulative damage ratio.
In general, the design fatigue life of each joint and member should not be
less than the intended service life of the structure multiplied by a safety factor.
For the design fatigue life, D should not exceed unity.
For in-situ conditions, the safety factor for fatigue of steel components
should depend on the failure consequence and its impact on cost, the environ-
ment and the like, and in-service inspectability.
Critical elements are those whose sole failure could be catastrophic. In lieu
of a more detailed safety assessment of Category L-1 structures, which are
manned and non-evacuated, a safety factor of 2.0 is recommended for inspect-
able, non-failure-critical connections. For failure-critical and/or noninspectable
connections, increased safety factors are recommended, as shown in Table 3.9 .
A reduced safety factor is recommended for Category L-2 and L-3, which are
manned evacuated or unmanned structures, respectively. For conventional steel
jacket structures, on the basis of in-service performance data, SF = 1.0 for
redundant diver or ROV inspectable framing, with safety factors for other
cases being half those in the table.
When fatigue damage can occur due to other cyclic loadings, such as trans-
portation, the following equation should be satisfied.
∑ i SF i D i < 1 : 0 (3.108)

where D i is the fatigue damage ratio for each type of loading and SF i is the asso-
ciated safety factor.