Offshore Structure Platform Design - Offshore Structures: Design, Construction and Maintenance - page 109

9. The piling interacts with the surrounding soil in an inelastic manner, and
the computer linearizes its response to generate the equivalent elastic
spring, as shown in Figure 3.14 .
10. In the case of sea ice abrasion, an allowance of 0.1 mm/year is to be con-
sidered for all steel between the elevations of

1.3 m to 3.0 m and usually
results in a minimum decrease in thickness of 2.5 mm over the life of the
platform.

−

The process of applying the wave load and current to the offshore structure
is illustrated in Figure 3.13 , as described in API RP2A.
The horizontal members in the wave zone should be designed for wave slam
forces, in accordance with API RP2A. Bending stresses due to both horizontal
and vertical slam forces should be investigated. However, the current velocity
components should not be included in the wave kinematics when calculating
wave slam loading. For X braces, members are assumed to span the full length.
Member lengths are reduced to account for the jacket leg ratio. Wave slam cal-
culations are carried out during detailed design, not basic design.
The static structure analysis for the offshore structure is the same as in nor-
mal structures, because the software uses the stiffness matrix to calculate the
deflection and then the internal forces and stresses on each member. However,
for offshore structures, the problem is the interaction between the structure and
the piles, as the structure will be elastic and the piles will be inelastic. So the
structure analysis steps are:

1. Set-up of the geometric of the jacket with material specifications and pre-
liminary member sections with the dimensions. The software calculates the
stiffness matrix for the jacket excluding the piles.

Moment
spring

Lateral
spring

Axial
spring

FIGURE 3.14 Foundation piling model.