A methodology concerning how to determine the correlation factors between individual long-term dynamic load components for the prediction of the combined total stress is presented. The combined stress in short-term sea states can be expressed by the square root of sum of squares of the load components with corresponding stress factors and relevant correlation factors. One of advantages for this formulation is that the correlation factors do not depend on the relative magnitudes of the individual stress components. The long-term stress is expressed by an equation similar in format to one derived for the short-term sea states. The weighted average values of the correlation factors over wave scatter diagram entry and wave headings are defined as the long-term load combination factors. For illustration, the direct calculations of the load combination factors are performed for end longitudinal connections in the midship tank of a modern crude oil tanker. The calculation results show that the load combination factors for short-term sea states strongly depend on wave heading and average zero-up crossing wave period. For long-term multiple sea states, it is shown that the load combination factors are not sensitive to the selected probability of exceedance level. It is found, however, that the long-term load combination factors can be noticeably different depending on the cargo loading and environmental wave conditions.
In seas, ship structures experience various types of loads. In most situations, the structural members are subjected to loadings attributed to multiple load effects. These include hydrostatic load, wave-induced dynamic load, internal tank pressure, transient slamming load, thermal load and so on. For local scantling, strength evaluation and fatigue analysis of the ship structures using the prescriptive rule loads, the proper combination of the individual load components is very critical.