Local scour around a pipeline sleeper system under different flow directions

Abstract

Local scour around subsea pipeline has become an important problem in the marine environment. In this research study, the scour around a pipeline sleeper system in steady current is investigated by carrying out water flume experiments under clear water condition. The experiments were conducted for sleeper system at three different flow direction angles with sleeper β of 0°, 45°, 90°, where 0° and 90° represent the cases where the flow direction is parallel and perpendicular to the sleeper, respectively. Each test was conducted for 24 hours and sand bed profiles were measured at different stages of the scour. The experimental results show that maximum scour depth occurs at β = 45° along the downstream long boundary of the sleeper plate. The maximum scour at β = 90° is near the two upstream corners and the scour pattern is similar to the scour around a very low vertical structure. The scour depth is found to be the shallowest at β = 0° because the sleeper is in a streamlined arrangement. The sleeper rod with a recession on its two ends plays an important role to reduce the scour depth by weakening the flow in the wake of the sleeper. The recession also mitigates the reduction of the contact area between the sand and bottom surface of the sleeper plate for all the three flow approaching angles.