This paper summarizes some key points about the Morison formula, its background and some experiences on its application. An attempt is made to categorize the different types of flows-pile combinations for which the Morison formula applies. For irregular, high Reynolds number flows it is attempted to identify the parameters that characterize a specific flow case, and how to define these parameters. Some new analyzes are presented. The most important feature of this paper may be in the area of directional waves where an extremely simple method, the so-called "principal flow approach", is shown to model the directionality of the waves so that the unidirectional results can be used to predict the actual wave force. This approach proved to be extremely accurate in the range tested, namely for Keulegan-Carpenter numbers up to 35.


The Morison formula for prediction of the forces on a long circular cylinder was originally proposed by Morison et A1 (1950) on an ad hoc basis by considering two limiting cases. The first case was one in which the fluid motion was small relative to body dimensions, so that the flow remained unseparated, and potential theory was valid, yielding a force equal to twice the displaced mass. (Provided the cylinder diameter is short compared to the spatial variation of the flow). In the other limiting case the fluid motion in each direction was very large, so that the flow could be considered as piecewise steady and the force was given empirically as a so-called form drag force. Denoting cylinder radius, water density and velocity and acceleration as R, ρ, u, and ù respectively the <Morison equation>, then predicted a force per unit length equal to the sum of these two terms, but adjusted them by two empirical coefficients Cm and Ca (so-called force coefficients), viz.

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