Page 151 - Marine Structural Design

Part I

Structural Design Principles

Chapter 7 Limit-State Design of Offshore Structures

7.1 Limit State Design
In this Section, the concept of limit state design is introduced to allow an assessment
considering the following limit-states:
ULS - Ultimate Limit States - Ultimate strength behavior
FLS - Fatigue Limit States - Fatigue and fracture behavior
SLS- Serviceability Limit States -Displacements and deflections
ALS- Accidental Limit States - Collision, fire, blast, dropped object, etc.
In general, the structure will need to be checked for all groups of limit states to ensure that
there are sufficient safety margins between the maximum likely loads and the minimum
resistance of the structure.
The general safety format for limit state design is expressed as:
S, I R, (7.1)
Where,
S, = CS, . y, Design action effect
= zR$,
R, Designresistance
S, = Characteristic action effect

R, = Characteristic resistance
y, = Action (Load) factor

y, = Material factor (= the inverse of the resistance factor)
Both the load and resistance factors may comprise a number of sub-factors reflecting the
uncertainties and safety requirements in the load effects and resistance.
Extreme care is required in the finite element analysis to ensure that the correct load and
resistance factors have been applied, particularly when several models are being used and the
results are linearly superimposed.
In the marine specific finite element programs, the relevant code of practice can be selected by
the analyst. This allows the appropriate design formulae to be chosen and the material factor to