Combining low- and high-frequency loads acting on large floating structures

Loads acting on large floating structures usually consist of high-frequency and low-frequency loads. The high-frequency loads are associated with the hydroelastic behavior of the structure and excitation of the natural frequency modes. The low-frequency loads are associated with the body motion of the structure and the wave profile. In design analysis, extreme values of these loads must be combined taking into consideration the correlation between them. This paper discusses a methodology for combining the extreme loads, and proposes a simple formulation suitable for use in reliability analysis. A proposed load combination factor K was found to depend on the correlation coefficient of the two loads, the ratio of their standard deviations and the frequency content of the processes from which the loads are determined. The correlation coefficient was found to depend on the complex frequency response functions of the loads and the input wave spectrum. The paper also discusses characteristic extreme values of slightly nonlinear loads acting on large floating structures.Extreme loads may be based on a storm condition with a specified return period. Since very large floating structures are expected to have a long operational lifetime, the return period must be selected carefully. The paper discusses a method for selecting return periods based on the expected operational life of the structure and encounter probability.