An experimental study of the properties of hydrodynamic forces upon a marine pipeline is presented in this paper, in the equilibrium scour conditions for various Keulegan-Carpenter numbers and various initial relative gaps between pipeline and the erosive sandy seabed. The tests are conducted in a U-shaped oscillatory water tunnel with a sand box located at the bottom of the test section. According to the experimental results, the maximum horizontal forces on the pipelines with an initial gap to seabed will decrease to some extent due to scouring process. For engineering appliances, it seems safer to estimate wave induced forces on pipelines under the assumption that seabed is plane. However, it should be noticed that great changes would be brought to the frequency properties of lift forces because of the sandy scour beneath the pipeline, which occurs for certain KC numbers.
Under the complicate ocean environments with variable waves, currents and erosive seabed, to evaluate properly the hydrodynamic force on submarine pipelines is a key problem in designing pipeline systems. Actually, the periodical vortex shedding and the recip-rotating movements of wakes may induce sunken pits in the seabed around the pipeline, and remarkable changes may be brought to the relative positions between pipeline and bed by the action of scour. Some pipelines installed upon seabed can probably become suspended above the seabed. All those may result in the complicate properties of hydrodynamic forces on the pipelines and even jeopardize the safety of the pipelines. To meet the requirement of the engineering design of marine pipelines, numerous theories are developed for calculation of the hydrodynamic forces on a pipeline. This formula treats the wave force in the horizontal direction as drag force proportional to the square of the water velocity and the inertia force proportional to acceleration of the water.