Page 296 - Practical Design Ships and Floating Structures

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assessed on a comparative basis, applying the consequence analysis tools and the risk-based techniques,
including methods to analyse the profitsllosses involved. Alternative design solutions will therefore be
suggested, their selection being based on a sound economic application of the QRA-based design
procedure.

3 ELEMENTS OF THE PROCEDURE

All design decision models developed so far, are based on a single economic criterion of merit,
incorporating several constraints related to performance indicators, lately with the addition of some
safety indicators of any type (deterministic, probabilistic or performance-based). In the context of the
above described design goal, the current approaches are not sufficient, due to the fact that safety is not
an integral part of the design process, but is taken into consideration as a design periphery issue at best,
if not as a design afterthought. For this reason, the association of increased safety leading to increased
incurred costs is considered to be the norm.

A clear and complete statement of the goal for the problem at hand is that a design procedure is sought
to “derive effective arrangements and layouts that maximise safety, whilst minimising the incurred
costs”. This is a multiple criteria problem, the design solution of which depends on multiple design
attributes, which in turn derive from the attained performance and characteristics of the alternatives
under consideration.

To achieve such a goal a structured formulation of the criteria, parameters, constraints, objective
function and mathematical models, needs to be developed. This will be based on the observation that
an alternative to a single criterion of merit is the consideration of pair wise comparisons that employ
valid criteria, which can also be extended to the consideration of a hierarchically decompositioned
objective function that reflects and combines economic, performance and safety aspects. Such a
formulation will lead to the following innovative aspects:

The criteria to be considered need only be design-related, and not of conformance nature. More
importantly, these criteria can be incorporated in the formulation of the objective function, as
described below.
The formulation allows for the development of a practically unconstrainted problem, in the sense
that the various indicators (economic, performance, safety) are included in the objective function.
The mathematical models ought to bc of pcrformance nature, at least for the assessment of damage
survivability. This will allow the incorporation of first-principles approaches in the design process,
which when combined with an overall QRA framework, accounts for an objective safety
quantification process.

In the following, the formulation of the decision making process for the determination of the most
effective internal configuration, given the vessel’s hull, will be described. The focus will be on
monohull passenger Ro-Ro vessels.

3.1 Development of the Objective Function
The objective function will be developed based on a method proposed by T.L. Saaty, known as the
Analytic Hierarchy Process (AHP), Saaty (1980). The method consists of three principles:
decomposition, comparative judgement, and priority synthesis.
Decomposition is the description of the problem in a hierarchical form. The elements of each level are
independent of succeeding levels. The hierarchical structure starts at the top with a statement of a
decision goal. The next lower levels contain the criteria by which the alternatives are measured by the