Assessment of Cumulative Lifetime Seaway Loads for Ships

Abstract

This paper presents a reliability‐based design methodology for primary seaway loadings on surface ships. The goal for such a design methodology is to have one consistent method for developing extreme primary hull girder loads for reliability‐based strength design criteria as well as lifetime exceedance loads for reliability‐based fatigue strength design criteria. The primary hull girder loads, i.e., vertical bending, lateral bending, and torsion, include contributions from both the low frequency (wave‐induced) and the high frequency (slam‐induced) domains for vertical and lateral bending. The contributions of secondary loads, such as hydrostatic and dynamic pressure loads, are important but are beyond the scope of this paper. A method is presented for calculating lifetime seaway loads as functions of ship and seaway parameters. The method is embodied in a PC based computer program called SPECTRA.

Structural Life Expectancy of Marine Vessels: Ultimate Strength, Corrosion, Fatigue, Fracture, and Systems

“…The computations of the wave bending moments can be based on identification of operational profile, computation of ocean wave statistics, calculation of extreme wave-induced bending moment, and application of the largest extreme wave bending moment in analysis [3]. Michaelson [14] provides a computer program for Transactions of the ASME performing these computations, called SPECTRA, that produces annual loading that can be represented by a Weibull probability distribution as provided in Table 4 [15]. The combined wave and dynamic vertical bending moment produced by SPECTRA is for a single encounter, whereas the combined wave and dynamic vertical bending moment [3] is the annual extreme value.…”

System Reliability of Ship Hull Structures Under Corrosion and Fatigue

“…In this article, still water and wave-induced bending moments are considered as load effects, which can be frequently encountered under ship operational conditions at sea. As mentioned previously, it is noted that dynamic bending moment can be included in the load combination for still water and waveinduced bending moments (Ayyub et al 2002b;Sikora et al 2002). By using the IACS (2006) recommendation, the still water bending moment, M sw , is estimated for sagging and hogging conditions as follows:…”

Reliability-Based Operational Performance Metrics for Ship Structures

“…The total bias of dynamic bending (whipping) moment prediction, B^, is the ratio of the experimental result to the whipping prediction. Sikora et al (2002) reports the sensitivity of SPECTRA to input variations. The study concluded that the bias between experimental data and the whipping prediction is normally distributed with a mean of 0.9705 and a standard deviation of 0.2465 (COV = 25.4%).…”

“…The modeling bias for the wave-only bending moment prediction, B^, is the ratio of the experimental result to the prediction based on response-amplitude operators. Normal distribution parameters representing the probabilistic characterization of 5^ are shown in Table 46, as reported by Sikora et al (2002). The total bias of dynamic bending (whipping) moment prediction, B^, is the ratio of the experimental result to the whipping prediction.…”

“…(Sikora 1998b;Michaelson 2000). The Universal RAOs are used to map the seaway environment to a hull girder bending moment.…”