Page 500 - Marine Structural Design

476 Part IV Siructural Reliability

based on practical engineering judgment should be applied to the calibrated safety factor. The
existing experience with use of safety factors for the specified tubular joints should be
considered in the judgment.

26.6 Numerical Example for Hull Girder Collapse of F’PSOs
With a reference to Part 11 Chapter 13 and Part IV Chapter 25, this Section presents a
reliability-based calibration of hull girder collapse for FPSO(Sun and Bai, 2001). As an
illustration, the bending moment criteria may be expressed as
(26.23)
Y,M, + YlVV,Mw * 4UMU
where 5, yw and 4 are partial safety factors.
The selection of target reliability levels is a difficult task and should be based on the
consequences of failure, reliability formulation, and accessibility to inspection and possibility
of repair.
There are three methods that have been applied (Mansour, 1997):
(1) Agreeing upon a “reasonable” value in the case of novel structures without prior
experience;
(2) Calibrating reliability level implied in currently used design codes (commonly used for
code revision);
(3) Cost benefit analysis. Target reliability is chosen to minimize total expected costs over the
service life of the structure. This method is preferred but is impractical due to the data
requirements of the method.
Mansour (1997) reviewed the sources of information on target reliabilities and suggested that
the reliability index for collapse strength of commercial ships be set at 3.5. Guedes Soares et a1
(1996), suggested that the tentative reliability indices against hull girder collapse are set at 3.7
for the “as built” state and 3.0 for the lower limit of corroded hulls. This is based on their
investigation of worldwide causalities and structural safety level implicitly built-in to present
ship design practice. The corroded state was defined as such that the section modulus is 90%
of the original (“new-built”). Two methods can be used to evaluate the partial safety factors:
E, yw and 4 are given by the ratio of the design value of the variables to the corresponding
nominal value. The design value is the most likely failure point as calculated by first order
reliability method. The following relationships can be derived (Mansour, 1997),

where x* is design value and X“ is the nominal value;
For a given target reliability index PO, characteristics for the strength (cov) and probability
distribution of load effects, the partial safety factors, and minimum required strength can be
determined by first-order reliability method.
Table 26.3 is used to define guidance for the hull girder strength design.