Page 451 - Marine Structural Design

Chapter 23 Basics of Structural Reliabiliv 427

practice. Therefore, it can be used as a launching point for code revision and calibration. Any
adjustments in the implied safety levels should be for the purpose of creating consistency in
reliability among the resulting designs according to the reliability-based code.
23.9.3 Recommended Target Safety Indices for Ship Structures
Recommended target safety levels for hull girder (primary), stiffened panel (secondary) and
unstiffened plate (tertiary) modes of failure and the corresponding notional probabilities of
failure are summarized in the table below (Mansour, 1997). It should be pointed out the values
of the target safety index are also dependent on the methods used to calibrate the reliability
levels.

Table 23.2 Recommend Target Safety Indices for Ship Structures

Primary (ultimate) 3.5 (2.3E-4) 4.0 (3.2E-5)
Secondary 2.5 (6.2E-3) 3.0 (1.4E-3)
Tertiary 2.0 (2.3E-2) 2.5 (6.2E-3)

23.1 0 Software for Reliability Calculations
The following is a few selected computer programs for the calculation of structural reliability,
PROBAN: A probabilistic analysis tools for general structures developed by DNV. It is
part of the SESAM packages.
STRUREL: A general structural reliability analysis software including component
reliability calculation (COMREL), system reliability calculation (SYSREL),
statistical analysis of reliability data (STAREL), etc., developed by RCP
consulting group in Germany.
ISPUD: Specially designed for structural reliability calculation by use of Monte Carlo
Simulation.
CALREL: A general structural reliability software developed by U.C. Berkeley. Its
capabilities include: a) failure probability estimate for components; b) failure
probability estimate for systems; c) FORM and SORM analysis; d) direct MCS
analysis and e) sensitivity analysis.

23.1 1 Numerical Examples

23.11.1 Example 23.1: Safety Index Calculation of a Ship Hull
Problem
The sketch in Figure 23.6 shows the probability density hctions (PDF) of the load
and strength of ship hull girder in terms of applied bending moment and ultimate
moment capacity of the hull, respectively. Both the load S and the strength R are