On the importance of understanding ocean wave kinematics for calculation of dynamics and loads on offshore structures.

For calculation of hydrodynamic loads on offshore structure, Morison's equation is normally being used in the case of slender or relatively slender structure, and diffraction theory is being used in most other applications. While the Morison equation is empirical, diffraction theory is linked to the basic hydrodynamic principles. None of the methods, however, seem to be capable of explaining the dynamic phenomenon occurring when steep waves interact with offshore structures causing a phenomena like ringing of these structures. Attempts have therefore been made to modify the load calculation methods, generally by including higher order forcing terms in the case of Morison equation, and higher order forcing terms as well as a higher order expansions of the governing hydrodynamic equation and the boundary conditions in the case of diffraction analysis. These aspects are discussed further in the paper, see also Faltinsen, 1990. The paper emphasis on that the accurate calculation of the loading is contingent upon the accurate knowledge of ocean wave kinematics. Focus will in this respect be put on the numerical values of the kinematics as well as the contribution of the kinematics from linear and higher order components of the wave motion. It's believed that this approach will lead to an improved understanding of the dynamics of offshore structures and some current research topics in this field will be reviewed. Furthermore, an attempt will be made to point to directions of future research in this field with the ultimate objective of accurate calculation of dynamics and loads on offshore structures.