As a case study, a complete stochastic spectral fatigue analysis of a representative turret-moored FPSO design to be situated in a severe environment has been performed. After a global fatigue screening, three longitudinal-to-web frame connections in the outer shell were chosen for in-depth fatigue analysis. For each connection, fatigue lives were computed and are compared for alternative designs in which the cross section of the longitudinal stiffener, the profile of the web frame cutout, the presence of brackets, the offset of the web frame stiffener, and the position of the lug plate are varied. This paper comprises Part I, in which the analysis procedures, the particulars of the alternative details, and a description of the finite element modeling are presented.
The connections of transverse web frames and the longitudinal stiffeners that line the inner and outer hulls of ocean-going vessels often experience fatigue cracking as the result of dynamic stress fluctuations due to environmental loading. A common detail for which stress cracking is commonly a problem is shown in Fig. 1. Such a detail is characterized by a cutout in the web frame through which the longitudinal, often a bulb section, passes. An overlapping lug plate connects the longitudinal to the web frame in order to compensate for the lost cutout area when the outer (inner) hull is subjected to external (internal) pressure. Often a stiffening bar supports the web frame in the vicinity of the cutout, and this bar may be welded to the longitudinal or offset and sniped before it reaches the inner or outer hull. An offset web frame stiffener will continue to resist buckling of the web frame but will not cause fatigue cracks where it would otherwise be connected to the web frame cutout or the longitudinal stiffener.
