Gulf of Mexico's First Ship Shape FPU: Overview of the Global Strength Analysis

摘要

Normal procedure for assessing the hull strength of a ship shaped Floating Production Unit (FPU) is to use a rule basedrnapproach recommended by the classification societies. In this approach the vertical and horizontal wave bending momentsrnand shear forces are calculated based on formulae prescribed in the rules. The difference between the hull’s total carryingrncapacity and the rule prescribed wave related quantities provide the envelope of the still water moments and shear forces. Thernloading conditions are developed such that the still water envelope is not exceeded. Although this method can reasonablyrnpredict the longitudinal hull strength the following important issues are not addressed.rn1. The transverse strength of the hull.rn2. The stresses at the interface of the hull and add-on structures like living quarters, riser porch, heavy productionrnmodules etc which are common in a FPU.rn3. The stresses at the corner of large deck openings.rnThe above issues can only be addressed effectively through a Finite Element (FE) based approach. In this approach a coarsernmesh integrated model of the hull including the add-on structures is prepared. The envelope values of the wave dynamicrnpressures, accelerations etc. are obtained from the formulae presented in the rules. These are then multiplied with appropriaternfactors also prescribed in the rules to obtain their simultaneously occurring values. These loads along with the gravity loadsrnare first applied on a beam model representing the hull to obtain the actual moments at the target locations. The differencernbetween the target moment and actual moment is applied as concentrated end moments to achieve the target values. The areasrnof high stress reported in the coarse mesh analysis are further refined and studied in a local analysis by employing boundaryrndisplacement methods.rnIn this paper the above procedure has been discussed in detail and selected results have been presented to establish therneffectiveness of this analysis procedure.rnA detail hydrodynamic based analysis is necessary for an optimum site specific strength assessment of a FPU. But a costeffective,rnalthough slightly conservative assessment (since the rule based design strength quantities correspond to northrnAtlantic conditions) can be done by the rule based approach using finite element.rnThe authors believe that the methodology and results reported in this paper for a specific FPU will help in understanding thernrule based analysis procedure of other ship type FPUs.