Follow-up to the survey report [1], containing in-depth study of this problem, basically, since 1965, in the present report the almost 100-year’s experience of usage of the ultimate plastic strength criterion (U.S.C) in the Russian and Ukrainian practice of design of hulls of ship of different destination is reviewed. On the basis of comparison of results of computational and experimental estimations of an ultimate bending moments (Mult) and characteristics of their variability the expediency of usage of the (Mult) lower boundary by Boobnov-Papkovich-Shimansky method with a more precise estimation of carrying capacity of longitudinals by the generalized method of C. Smith is substantiated. These recommendations may be used also for estimation of the hull ultimate strength of FPSO (Floating Production Storage and Off-loading Units), the structure of which is similar to the hulls of modern tankers with the double bottom and double sides.
Practically relevant procedures of construction of the short-term and long-term distributions of wave loads and combined stresses at two- and three-dimensional waves is based on the rather cumbersome schemes of definition of the standards “σyi”, effective periods “Tyie” and correlation coefficients “ryiyi” of wave loads “yi”. These calculations can be made more visible and essentially simplified on the basis of the shape stability of the doubly-normalized amplitude-frequency (AFC) and mutual-frequency response characteristics (MFRC). All specific features of these characteristics are determined by their maximum values and frequencies of maxima. In the report the convenient formulae for estimation of σyi, Tyie and ryiyj are derived, and graphs of functions, determining them, for different wave loads are plotted. These relations allow to reveal major influencing factors and use them for construction of the short-term and long-term spectra of wave loads and stresses for full transport ships (CB > 0.75), including FPSO ships, for which non-linear and dynamic effects are expressed rather weakly.
It is shown in the report, that the trial-and-error requirements of the Rules of classification societies to the minimum section modulus of transport ships, including the FPSO-type ships, are based, mainly, on the maintenance of their strength, the lack of which in riveted ships exhibits in the form of weakening of rivet joints, and in welded ships in the form of cracks. The analysis of proposals for analytical formulation of the fatigue strength criterion (F.S.C.) during 150 years of development of steel shipbuilding confirms this thesis. It is shown, that modern recommendations of IACS (1999) do not provide stable reserves of fatigue resistance for ships of different standard-sizes, especially those constructed from high-tensile steels. In the report the ways of perfecting of the requirements to fatigue strength are offered.
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