Inherent Uncertainties in the Ultimate Hull-Girder Capacity
Authors: van der Cammen J.J. , Krekel M.H. , Vink J.H. 
Published: October 2008

The paper will discuss the application of reliability based analysis for determination of the ultimate strength of the hull girder of an FPSO. In specific, a newly developed method to calculate inherent uncertainties in the ultimate hull girder capacity is presented. The method is based on Monte Carlo simulations and a simplified approach to calculate the ultimate bending capacity of the hull-girder in hogging and sagging. The results of a parametric study will be presented, which show insight in the inherent uncertainty of the ultimate hullgirder bending capacity.

Introduction

One of the issues for FPSO design is the bending capacity of the hull-girder as this is commonly governing for the strength of an FPSO. This capacity should always be larger than the largest bending moment applied at the vessel during its life. These bending moments are induced by wave loads (wave bending moments) and by the distribution of cargo oil, ballast water and weight of vessel and process equipment (still water bending moment).

In traditional rule-based ship design the bending capacity of the hull is calculated as a deterministic value, which is kept larger than the maximum bending moment induced by the combination of the wave and still-water bending moment. A safety factor, based on experience, is applied on the bending capacity to ensure adequate safety.

A FPSO is operated differently than a trading tanker. A turret-moored FPSO, as is commonly used on the North Sea, is permanently moored at a fixed location for a multitude of years. Therefore a FPSO can not shelter or run from severe storms, where a traditional trading tanker can. Consequently in harsh environments like the North Sea the maximum bending moments induced on the hull-girder may be higher than those on trading tankers.

The concept of a FPSO is relatively new in comparison with that of trading tankers. Also the number of FPSOs is far smaller than that of trading tankers. Consequently little experience exists with FPSOs in comparison to trading tankers. Because FPSOs are operated differently and will encounter a different loading regime than traditional trading tankers the same safety factors as used in traditional shipbuilding design can not be extrapolated one to one to FPSO design.

In traditional rule-based design of trading tankers the hull-girder bending capacity and the extreme wave and still water bending moment are calculated as being deterministic values. Safety factors based on experience are necessary for this approach. Since these safety factors are not available in FPSO design, reliability analyses ought to be carried. Safety factors for FPSO design can be calculated back from reliability assessments.

The ultimate hull-girder capacity and the maximum bending moments are stochastic quantities rather than deterministic quantities. Both the ultimate bending capacity and the wave and still water bending moment contain inherent uncertainties and model uncertainties. Inherent uncertainties are uncertainties associated with the inherent variability of parameters.