Experimental Study Of Wave Boundary Layer And Wave Forces On Small Diameter Pipelines Over Rough Beds

On-bottom stability of submarine pipelines is one of the key safety factors for pipeline design, construction and service, which mainly depends on the hydrodynamic loads induced by waves. During the past, a majority of research work were focused on the wave forces on the submarine pipelines over flat seabed. However, the actual seabed is generally coarse due to the existence of pebbles, corals and marine growth, which have an important effect on the characteristics of wave boundary layer and the consequent wave forces on submarine pipelines. Hence, the research works shed light on the wave boundary layer and wave forces of submarine pipelines under different seabed roughness possess significant scientific research and engineering application values.In the boundary layer, the turbulence on rough seabed is generally more intense than that on the smooth bed. Compared with a submarine pipeline with large diameter, a submarine pipeline, especially with the diameter has the similar order with the boundary layer thickness, may be submerged by the wave boundary layer. Hence the characteristics of the turbulence in the wave boundary layer cannot be ignored. In the present work, the wave boundary layer characteristics were investigated through physical model tests. In the model test, four different seabed roughness were employed, namely 0.0cm,2.65cm,5.28cm and 6.78 cm, respectively. Compared with smooth bed condition, the characteristics of boundary layer over rough seabed in the range of a/ks∈[0.1~10] was discussed in detail, where, a is the amplitude of orbital motion out of boundary layer and ks is the bottom roughness.After obtaining the characteristics of wave boundary layer, we shed more light on the wave forces of submarine pipeline with a small diameter. In the present work, the relative value of the pipe diameter is expressed by β, which is the ratio between wave Reynolds number Re*=Uma/v, and Keulegan-Carpenter number KC=UmT/D, where Um is the maximum horizontal component of velocity out of the boundary layer, v is the water kinematic viscosity, Tis the wave period and D is the diameter of model pipe. In the range of β∈[70～140], the velocity profile in the boundary layer and wave forces on the model pipe were examined in detail.The present investigation demonstrated that the boundary layer on rough bed is thicker than that on the smooth bed. Under rough bed situations, it is found that the turbulence intensity in the boundary layer is larger than that out of the boundary layer. Moreover, the Reynolds shear stress and the mean shear stress have the similar order. For the experiments about the wave forces on the model pipe, it is observed that the wave boundary layer thickness is the main factor accounted for in-line forces on the model pipe. The corresponding experiment results revealed the lift forces on the pipe model under rough seabed are smaller than that on the smooth bed. Further research work indicated that the lift force on the pipeline may be related to the turbulent properties in the wave boundary layer.