Offshore Structure Platform Design - Offshore Structures: Design, Construction and Maintenance - page 172

In air
Cathodic
protection

10000

500

200

100

50

20

10

1000 10,000 100,000 1000,000 10,000,000 100,000,000

No. of cycles to failure

FIGURE 3.40 Tubular joint S-N curve for T = 16 mm (from API RP2A).

R and T are radius and thickness, respectively, of the joint-can. Consistency
in format with the rules for strain-gauge placement at crown and saddle position
may be noted.
Attempts to produce an improved as-welded profile often result in over-
welding. As such, high estimates of L mp , which are the low estimates of local
stress gradient, will produce conservative corrections. This approach assumes
that the weld is not so massive as to change the overall load distribution in
the joint-can, and that local hot-spot stresses are dominated by shell bending
stress. The relation between the hot-spot stress and the cycles of load until failure
is presented in Figure 3.40 , where the thickness of the chord is equal to 16 mm.
Failure is expressed as damage or fatigue life damage, so the fatigue life
damage is the number of cycles of a particular stress range divided by the allow-
able number of cycles for that range from the S-N curve.
Table 3.12 is an example of a fatigue analysis, with the stress range and the cor-
responding number of cycles of stress occurrence and the allowable number of
cycles based on the S-N curve. Assuming a point is subject to 5 cyclic stress ranges
(due to wave), D 5 =10 10 6 =0.2,D 10 =0.1,D 20 =0.2,D 50 =0.05and

D 90 = 0.025. Total damage = 0.575.
So, if these waves occurred over 10 years, then the fatigue life = 10/0.575 =
17.4 years.

10 6 /50

× ×

Jacket Fatigue Design
Dynamic analysis should be carried out to predict the fundamental periods of
the platforms in order to confirm the sensitivity of the structure to wave-induced
excitation. The fundamental sway periods should be used to derive the dynamic
amplification for the in-place analysis loading conditions.