Contributions to Second Order Directional Sea Simulation and Wave Forces

Abstract

We briefly describe a method for simulating second order directional seas and associated wave forces. We present wave force calculations for simplified forms of real offshore structures. Compared to unidirectional wave force calculations, this method reduces total design wave force. Complex subharmonic motion of large floating structures can be easily understood within the framework of this simulation method. A real sea is first represented by discrete linear waves of many frequencies traveling in many directions. Then, second order effects are calculated using equations derived for this purpose. The sum of linear and nonlinear waves is used to calculate wave forces on example offshore structures. The simulation method has been applied to calculate total wave forces on a single pile and a 4-pile group. Simulation method calculated wave forces on a single pile and a 4-pile group on a 60 ft square array are only 61% of wave forces using unidirectional wave methods. These results are the same as those obtained by a so-called hybrid method (Dean 1977) for the drag dominant case. When the pile separation is increased to 300 ft (similar to a TLP or a semisubmersible) the corresponding reduction factor varies from 0.79 for a unidirectional random sea to 0.61 for an omnidirectional random sea. Large offshore structures are relatively insensitive to the linear frequencies but could have very large response to the subharmonic frequencies in the simulation method. This is consistent with the field and laboratory observations.