DTM 511 - Second order hydrodynamic forces on offshore structures by using the potential theory

Course Objectives

There are basically two different approaches for evaluating the nonlinear hydrodynamic wave forces acting on a vertical cylinder in water of finite depth : The Morison’s Equation and Diffraction (or) potential flow theory. The Morison’s Equation is an attempt to describe the effects of the potential and viscous flows and therefore it is expressed in terms of the appropriate coefficients obtained from model tests. Thus, the Morison’s Equation is a semi-emprical formulation and it includes a non-linear term (viscous drag term). When CD, viscous drag force coefficient is determined from the model tests, it is assumed that all non-linear effects are included in the viscous term and non-linearities due to the potential flow are not taken into account separately. According to the importance of diffraction effects, most of the research divide the offshore structures into two main groups. The first group is called “hydrodynamically transparent structures”. Semi submersibles, bridges and dock legs are examples of this group. In this group the diffraction effects are small enough to be neglected and the Morison’s Equation is valid. In the second group, “hydrodynamically compact structures” the viscous effects are negligible and the diffraction theory has to be used. Winch Buoys, Jack-up platforms in transit, Foundation Plates are examples of this group. In order to choose between these two methods, the ratio of the characteristic length of the body to wave length is calculated and when this ratio is around 1, it is thought that diffraction effects are too large to be neglected. The diffraction theory does not give good results when the linearization is used especially in steep and short waves . Because of this, some researchers for the last fifteen years, have focused their attention on the non-linear diffraction theory.

Course Description

Nonlinear wave effects on a pile of cylindrical cross-section driven in water of finite depth is solved theoretically .