A More Efficient Form of the Method of Moments for Derivation of Morison’s Force Coefficients

Morison’s equation is the most widely used method of predicting wave forces on slim cylindrical members of offshore structures. The equation assumes that the total force of the fluid is made up of a drag force and an inertial force, where the drag component is due to water particle velocity and the inertial component is due to water particle acceleration. In practice, the force uses two empirical coefficients, which are usually referred to as the drag and inertia coefficients. The values of these empirical coefficients are determined from laboratory and/or field experiments. In a typical wave load investigation, the wave force together with corresponding water particle velocity and acceleration are measured. The measured data is then analysed to calculate constant values for drag and inertia coefficients. One of the methods used in derivation of these coefficients is the method of moments. However, the coefficients obtained from this method show considerable scatter due to large sampling variability. The purpose of this paper is to introduce a more efficient form of the method of moments, which will lead to more accurate estimates of Morison’s coefficients by reducing their sampling variability. Simulated data has been used to compare the new method of moments with the conventional one. The results indicate that the new method is superior to the conventional one. This is particularly the case for drag-dominated forces.