Offshore Structure Loads and Strength - Offshore Structures: Design, Construction and Maintenance - page 62

less than about 200 ft (61 m), and overlying rock-like materials with standard
penetration test for cohesionless soil layers provide higher than 50 blows
per foot.

Soil type C: Deep, strong sands, silts and stiff clays with thicknesses in
excess of about 61 m (200 ft) and overlying rock-like materials with
undrained shear strength from 50 to 100 kPa with standard penetration
test from 15 to 50 blows per foot.

●

Table 2.21 provides the values of the seismic factor, S C , which are related to
the seismic zone and the soil type. For design purposes, these values are used to
obtain the response spectra for the required location.
Both the strength level (SLE) and ductility level (DLE) relative to earthquakes
should be considered in the design, and a period of 200 years should be used
for SLE.

Strength Requirements
Twice the SLE peak accelerations should be used in the DLE analysis. The
design of the platform for an earthquake load depends on the dynamic analysis.
The mass used in the dynamic analysis should consist of the mass of the
platform associated with gravity loading, the mass of the fluids enclosed in the
structure and the appurtenances and the added mass. The added mass may be
estimated as the mass of the displaced water for motion transverse to the longi-
tudinal axis of the individual structural framing and appurtenances. In motions
along the longitudinal axis of the structural framing and appurtenances, the
added mass may be neglected.
The analytical model should include the three-dimensional distribution of
platform stiffness and mass. Asymmetry in platform stiffness or mass distribu-
tion may lead to significant torsional response, which should be considered.
In computing the dynamic characteristics of braced, pile-supported steel
structures, uniform model damping ratios of 5% of critical structural members
should be used for an elastic analysis. Where substantiating data exist, other
damping ratios may be used.

TABLE 2.21 Seismic Factor S C

Seismic Zone

Soil Type 1 2 3 4 5

A 1.0 1.0 1.0 1.0 1.0

B 1.44 1.39 1.33 1.36 1.40

C 1.5 1.45 1.42 1.5 1.45