Abstract
The Morison equation has been widely used to describe flow-induced loads on slender structures in waves and currents. However, as far as the load magnitudes and their probabilistic distributions in irregular seas are concerned, it is more effective to formulate a hydrodynamic load prediction model using an equivalent drag expression because this avoids the necessity to define the added mass and calculate the flow acceleration. Since the inertia effect contributes to the total load, the equivalent drag coefficient is constructed to account for this influence. One obvious advantage of this alternative approach is that the equivalent drag coefficient can be calibrated against the results of total hydrodynamic load measurement or CFD calculations without having to measure or quantify the added mass coefficient and the inertia contribution.
In this paper, the equivalent drag approach is introduced in the regular wave condition as a general method for calculating the amplitude of the total wave loading on any slender structure cross-section. In an irregular sea state, the most probable maximum (MPM) value of the wave load amplitude is expressed in terms of the MPM value of the relative flow velocity and the equivalent drag coefficient which is dependent on a newly defined period number χ. The short-term probability distribution of the wave load magnitudes is represented by a Weibull distribution as a function of the equivalent drag coefficient. The long-term magnitude distribution can be developed by summing the short-term distribution of each sea state weighted by the probability of occurrence. As a special application, the equivalent drag coefficient is determined by fitting a modified hyperbolic relation to the model test results of the measured total wave load on the bilge keel of a floating production vessel (FPSO) in irregular sea states. An example of bilge keel fatigue assessment is presented based on the long-term wave load prediction, while load sensitivity is demonstrated.
