scope:

Introduction

Paragraph 3-1-2/1.5 requires that "Consideration is to be given to waves of less than maximum height where, due to their period, the effects on various structural elements may be greater." For a twin-hull semi-submersible, large waves with long periods are not necessarily critical to the design of the primary structural elements. Only the waves with critical periods and headings (as characterized in 3-2-A2/3) that generate a significant amount of hydrodynamic loads are critical to the design of the primary structural elements.

In addition, the wave load distributions on the submerged hulls of a MODU are required in the global structural response analysis. Both stochastic and regular wave approaches can be utilized in the response analysis. The stochastic approach usually follows a frequency domain procedure via a spectral analysis. In the stochastic approach, the vital distribution information on simultaneous load and stress is lost. In contrast, the regular wave approach retains the flow of load and stress, and therefore is more useful in the structural design evaluation.

For each characterized load case, the response analysis determines the critical wave period that generates the maximum hydrodynamic loads. It further finds an equivalent regular wave height which will generate the same amount of the hydrodynamic loads as calculated in the response analysis. The found wave period and wave height, noted as "Design Wave", are to be used for the design of the global strength of a twinhull semi-submersible.

The response analysis can be performed using a stochastic or deterministic approach, depending on the wave information available. This Appendix covers the details of the response analysis using these two approaches.

For MODU classification, ABS requires the Owner to select the wave environment for which the semisubmersible unit will be designed.