Hydrodynamic Characteristics of Prismatic Barges

ABSTRACT The forces and moments acting on prismatic hulls are presented, for both wave excitation and motions. Using these charts a platform designer can easily calculate the motions of a barge in head seas or beam seas. An example is given to illustrate the procedure for calculating motions. In addition, predicted motions for several barge configurations agree well with corresponding model experiments. INTRODUCTION During the past several years, a number of ocean platforms have failed structurally and in many cases utilization of platforms has had to be curtailed because of loads or motions induced by ocean waves. Under the sponsorship of the Sea Grant Program, three years of study at Stevens Institute has led to the development of a reliable analytical method for predicting motions in the design stage of ocean platform development. This report describes the analytical technique briefly, and presents results with which platform designers can easily predict motions of prismatic barges without resorting to lengthy digital computer programs. It is hoped thereby that use of these methods in the design stage can reduce operational delays and failures due to wave induced platform motions. In addition, by comparing motions of several alternate configurations, an optimum operational quality can be achieved. In the representation of the hydrodynamic forces and moments, use has been made here of some of the most recent theoretical developments in marine vehicle motions analysis. The recent historical development of these analytical techniques is briefly described as follows. The vertical wave-exciting forces on Lewis cylindrical forms (two-dimensional body with a ship-like cross-section) restrained in beam seas were calculated by Grim. This method is based on the assumption that the disturbance of an incident wave caused by the ships hull can be represented by the potential used in describing the flow around the same hull oscillating in calm water. This disturbance potential plus the incident wave potential are superposed with the disturbance potential strength adjusted to satisfy the kinematic boundary condition on the restrained hull. This approach was extended by Tamuraz to the calculation of sway and roll exciting forces and moments on Lewis form cylinders, restrained in beam seas. The fundamental approach. developed by Grim was extended by Kind to include a much greater variety of cross sections through the use of the two-dimensional "close fit" disturbance potential developed by Frank. 4 This method can be used to evaluate the wave-exciting forces on the widely varying configurations of ocean platforms. In addition, the development presented by Frank is used to evaluate the hydrodynamic forces induced by the motions of the vehicle itself. These two hydrodynamic force systems are then combined in the equations of motion to arrive at a procedure for predicting wave induced platform motions.