Page 490 - Marine Structural Design

466 Part IV Structural Reliability

requirements may be targeted. These targets may vary both in time and geopolitical location
and further, may be continuously affected by technological changes and market forces. To deal
with such design targets in structure design, formal procedures of optimization are required to
make decisions about materials, configuration, scantling, etc. In the optimal design process,
therefore, the key stage is the specification of optimum design targets. General types of design
targets may be cost (initiaYoperational), functional efficiency and reliability.
By using LCC design, it is possible to express the total costs of a design alternative in terms of
mathematical expression, which can be generically described as follows:
TOTAL (NPV)=CAPEX(NPV) + OPEX(NPV)+RISKEX(NPV) (26.4)
Where,
CAPEX = the capital expenditure of initial investment
OPEX = the operational costs
RISKEX = unplanned risk costs
NPV = net present value
One main difficulty that often arises is identification of the costs to include accidental
situations such as grounding or collision of ships. In this case, safety is the primary design
objective while economy takes on the role of important side constraints. One of the ways to
deal with this particular situation is introduction of high cost penalty for certain failure modes,
e.g. high value of CF in the following equation
C, =C, +P,C, =C, +R (26.5)

or
R = P,C, = C(P,C, )= zRi (26.6)
is
where PA is the failure rate of a particular mode i, and CF~ the cost penalty associated to that
failure mode.

26.3 Reliability-Based Design
26.3.1 General

The role of safety factor in traditional deterministic design is to compensate for uncertainties
affecting Performance. Such safety factors evolved through long term experience. Experience,
however, is not always transferable from one class of structure to another, nor can it be readily
extrapolated to novel structures. Further, any single class of a traditionally designed structure
has been typically found to have a large variability in actual safety levels, implying that
resources could perhaps have been more optimally used. Particularly in the context of the
present trend toward reliability-based design, reliability methods are suitable to bridge such
gaps in traditional design. This is because performance uncertainty can be considered both
directly and quantitatively with reliability methodology.
Relative to a conventional factor of safety code, a probability-based design code has the
promise of producing a better-engineered structure. Specific benefits are well documented in
the literature.