Morison's Equation and the Wave Forces on Offshore Structures

摘要

The origin and limitations of the Morison-O'Brien-Johnson-Schaff (MOJS) equation, the nature and decomposition of the time-dependent in-line force, the speculative generalizations of the MOJS equation to body shapes other than circular cylinders, to yawed cylinders, to wave-current combination, and to dynamic response of structures are discussed in detail. The background and the limitations of the existing data are reviewed and the data from sinusoidally-oscillating planar flow about smooth and rough circular cylinders are chosen to critically assess the MOJS equation. Six methods are examined to delineate the limitations of the MOJS equation and to devise a new force expression. The final method used (the Fourier analysis of the residues) led to the formulation of a three-term and a four-term MOJS equation. It is shown through numerous examples that the new MOJS equation reduces the residue significantly for both smooth and rough circular cylinders, particularly in the drag inertia dominated regime. Finally, the applicability of the new equation to the ocean conditions and the effect of spanwise coherence are discussed and numerous research projects are recommended for consideration. The suggestion is made that further improvement and understanding of the MOJS equation rest not only with carefully conducted laboratory investigations but also with additional ocean test designed to shed light on several complicating influences such as the spanwise coherence of vortices and vortex interactions. (Author)