Page 483 - Marine Structural Design

Chapter 25 Reliability of Ship Structures 459

25.5.3 Conclusions

Time-variant structural reliability assessment of an FPSO hull girder relative to the ultimate
strength requires the consideration of the following three aspects: (1) load effects and their
combination, (2) the hull ultimate strength, and (3) methods of reliability analysis.

The environmental severity factors are introduced to fit the the wave-induced bending
moments accounting for the specific-site conditions. The Ferry-Borges method is applied to
combine stochastic processes of still-water and wave-induced bending moments and to
evaluate time-variation of the maximum combined bending moment.
The mean value first order second moment method was applied to calculate failure
probability of ship structures.

A procedure for time-variant reliability analysis has been developed.
An effective response surface approach is used to evaluate the failure hnction at sampling
points. A modified Monte Carlo simulation technique is applied to evaluate the failure
probability.
The time-variant reliability and parametric analysis for an FPSO hull girder are quantified.
It is found that the steady corrosion rate, combination of SWBM and VWBM,
environmental severity factors and transition time in the present corrosion model are very
important in estimating the reliability of the hull girder.
It is concluded that the load combination factors obtained from this method are dependent
on mean arrival rate of SWBM, service lifetime and the environmental seventy factors.

25.6 References
1. ABS (2000), “Guide for Building and Classing Floating Production Installations”,
American Bureau of Shipping.
2. Bucher, C.G., Bourgound, U.A., (1990), “A Fast and Efficient Response Surface
Approach for Structural Reliability Problems”, Structural Safety, Vol. 7, pp. 57-66.
3. Casella, G., Rizzuto, E., (1998), “Second-level Reliability Analysis of a Double-hull
Oil Tanker”, Marine Structures, Vol. 11, pp.373-399.
4. Frieze, P.A. and Lin, Y.T. (1991), “Ship Longitudinal Strength Modeling for
Reliability Analysis”, Proc. of Marine Structural Inspection, Maintenance and
Monitoring Symposium, SNAME, Arlington, VA.
5. Ghose D.J., Nappi N.S., Wiemicki C.J., (1995), “Residual Strength of Marine
Structures”, Ship Structure Committee, SSC-381.
6. Gordo J.M., Guedes Soares C., Faulkner D. (1996), “Approximate Assessment of the
Ultimate Longitudinal Strength of Hull Girder”, Journal of Ship Research, Vol. 40( l),
pp.60-90.
7. Guedes Soares, C., (1 984), “Probabilistic Models for Load Effects in Ship Structures”,
Department of Marine Technology, Norwegian Institute of Technology, Trondheim,
Norway, Report No. UR-84-38.