Fatigue Analysis of Converted Floating Production Facilities

ABSTRACT One important aspect of converting an existing semi-submersible into a Floating Production Facility (FPF) is its remaining fatigue life at the new location. This paper presents a practical yet rigorous method of calculating this fatigue life. The method is based on spectral techniques applied to fatigue analysis. Hydrodynamic loads are generated using 20 diffraction theory. Dynamic mooring loads from the catenary lines are included in the analysis. Stress peaks are assumed to follow a Rice distribution instead of the conventional Rayleigh distribution. Short crestedness of the waves is taken into account using a higher-order cosine function. Finally, fatigue life is computed using Minerls cumulative damage rule. The method was applied to a semi-submersible to calculate the fatigue lives remaining after 10 years of North Sea service. It was assumed that the unit would be deployed in a new North Sea location as an FPF. A parametric study was performed to study the effects of payload variation, hull modification, change in wave scatter diagram, change in SF data, increasing the number of wave directions, including wave spreading, variation in spectral parameters and structural modeling techniques. The results showed very conclusive trends that can significantly influence the design process. INTRODUCTION The recent drop in oil prices has encouraged offshore field developers to look for cost effective concepts for producing oil. One of these concepts being considered by many oil companies is conversion of an existing semisubmersible into an FPF. Given a field, the choice of the most optimum semi-submersible for conversion depends on many factors. Two factors that play an important ro1e are the hull modifications required to provide buoyancy to support the vessell s modified payload and the structural integrity of the modified vessel at the new site. The structural integrity is ensured by demonstrating that the vessel has adequate strength and acceptable fatigue life according to the regulations of the new geographical location. The calculated fatigue life of an offshore structure is very sensitive to the method of analysis used to predict the lives. The analytical method must be rigorous enough to be reliable and at the same time practical so that it can be applied to the various vessels being considered for conversion. This paper presents a spectral fatigue analysis method that satisfies both these requirements. Once a vessel has been chosen, the simplicity of this method allows the designer to select a hull and topside modification scheme that maximizes fatigue lives. METHOD OF ANALYSIS General The fatigue damage to the vessel is calculated in five basic steps (shown in Figure 1):The 6 × 6 rigid-body mass matrix is computed;The frequency domain motions equations are solved to produce hydrodynamic loads and rigid-body accelerations;Based on member mass properties and rigidbody accelerations, member inertial loads are generated;Stiffness analysis is carried out using the hydrodynamic loads and inertia loads;