ABSTRACT The analysis of the complex interaction of wave forces and structural response is in its very early stages of development. Formulating the equation of motion for the structure under the action of wave forces involves the following uncertain parameters: [1] virtual water mass, which is a function of the relative velocities of the wave and the structure, and [2] drag forces, which are also a function of the relative velocities. Basic research is now needed to formulate a satisfactory equation of motion for the structure, based on these parameters. A computer model was developed to predict the dynamic response of structure inthe wave environment. Models were built and tested to check the computer model. The analytical and experimental results demonstrated the importance of the virtual mass and drag forces in formulating the equation of motion. INTRODUCTION The reliable prediction of the response of an offshore platform to wave action requires knowledge about the interaction of the wave and the structure during motion. With the structural system defined for its topological and mechanical characteristics and with the wave known or estimated, response follows from application of proper theory. The design and construction of harbors is one of the oldest engineering applications. Ever since the port of A-ur was built on the Nile prior to 3000 B.C., there has been a continuous search for an understanding of the forces exerted by waves on marine structures. The forces exerted by waves on circular piles, plates, barge-like structures, vertical walls and breakwaters have been predicted by analytical methods and measured in the laboratory. The consensus of the investigators is that water particle velocity and acceleration are the governing parameters of the hydrodynamic forces exerted by waves on structures. It should never be overlooked that the investigators always mean relative velocity of the water particle with respect to the body upon which the force is exerted. The same statement is true with regard to accelerations. This fact is often overlooked because most structures used in harbor engineering are very rigid and consequently the water velocities and accelerations with respect to the body are numerically equal to the velocities and accelerations with respect to ground. This is not the case for some offshore platforms that are flexible enough to have velocities and accelerations in the same order of magnitude as the water velocities and accelerations. Fig. la presents a cylinder rigidly attached to a stiff support. The force F exerted by the wave on the cylinder can be computed by the formula proposed by Morison, O'Brien, Johnson and Shaaf.2F = Cl V2 L + C2 A L,..... [1] where V and A are the water particle velocity and acceleration, respectively; Cl and C2 are constants defined by the geometry of the body, in this case a cylinder; and L is the length of the cylinder. Since the cylinder is rigidly attached to the supports of infinite stiffness, its displacement under the action of the wave forces is zero.