Offshore Structure Platform Design - Offshore Structures: Design, Construction and Maintenance - page 156

FIGURE 3.37 Tearing in the tubular joint.
Therefore, there should be a guideline for tubular joint detailing, so joint can
precautions and detailing are as shown in Figure 3.30 .
3.7.3 Fatigue Analysis
Experience over the last 60 years and many laboratory tests have proven that a
metal may fracture at a relatively low stress if that stress is applied a great num-
ber of times. The offshore structure, particularly its tubular joints, must resist
progressive damage due to fatigue that results from continuous wave action dur-
ing the 20- to 30-year design life of the structure. Over the years, platforms are
subjected to a wide variety of sea states and within each sea state, the structure
experiences many cycles of stress at various levels.
The purpose of fatigue analysis is to account for the fact that the number of
cycles of stress that a structural component can withstand varies with the mag-
nitude of the cyclic stress.
Dynamic analysis is used in the fatigue evaluation to predict the number of
cycles and magnitude of stresses that occur in various sea states. As in the
extreme wave analysis, dynamic effects become increasingly important for
deep water structures with a heavy deck load.
Fatigue cracks grow because of tensile stresses; corrosion of a metal is
accelerated if the metal is subjected to tensile stress. Thus, the effects of corro-
sion and fatigue are combined in the case of an offshore platform.
Kuang et al. (1975) described the design of tubular joints for offshore struc-
tures as an iterative procedure. The process begins with the sizing of the jacket
piles according to the requirements of the specific soil and foundation needs of
the platform. Sizing determines the diameter of the jacket legs and allows clear-
ance for the piles to go through them. Once the truss work geometry is selected,