An investigation was conducted at texas A&M University regarding the interaction of waves and currents, and their relationship to the forces on submerged pipelines. A model pipeline in a wave current test facility was used to obtain experimental drag force values which were compared to values predicted by the Morison equation. The major input parameters required by this equation were the water particle kinematics of velocity and acceleration and the coefficients of drag and inertia. The horizontal velocity fields of the combined wave-current condition and the coefficient of drag were investigated specifically due to the drag dominated condition caused by the small cylinder in a wave field of relatively long wave lengths. Each of the parameters was investigated individually in order to insure the elimination of the compensating errors in the force calculations.
The coefficient of drag was determined from the simultaneous measurements of the horizontal water particle kinematics and the fluid force on the cylinder. The superposition of the velocity fields of a wave in still water and the measured current were used to predict the total velocity field used by the Morison equation. These velocity fields were compared directly to experimentally determined values. The coefficient of drag was also computed from the Morison equation in conjunction with the superposition principle. The values were compared to the coefficients determined directly from the simultaneous measurements.
The primary purpose of this paper is not to provide explicit prototype design criteria, but to provide the design engineer with an insight into the parameters of fluid force of interacting waves and currents. The results are presented in a manner that demonstrates the apparent acceptability of superposition of the velocity fields for the wave and current to predict the total velocity field for the determination of the fluid-force by the Morison equation.
Offshore industry has long recognized the importance of underwater pipelines as a vital link for the transmission of materials between onshore and offshore facilities. Although these pipelines are costly at sea, they remain the most feasible and therefore the most economical method for transporting materials continuously over relatively short distances. To insure their continuous operation, adequate design criteria are required. In some cases a lack of the necessary information for their design has led to high safety factors which in turn have increased the cost. Additional information concerning the hydrodynamic loads can help reduce the uncertainty and therefore the cost.
The hydrodynamic loads on an offshore pipeline resting on the ocean bottom are a function of parameters associated with waves and currents acting near the pipeline. There have been many studies conducted to develop the criteria needed to estimate the hydrodynamic loads imposed by waves and currents. Many of these studies have investigated the effect of these phenomena individually, but to date only limited research has been directed towards evaluating the combined effect.
