Riprap protection of propeller-induced scour at quay structures

The increasing global trade demands have led to a scaling up of ship dimensions and engine power in recent years. The higher flow velocities from propeller jets have consequently caused greater erosion of the seabed, leading to structural instability and potential failure in berthing structures. Design guidelines for berthing structures include the use of rock riprap apron. However, engineers are still limited by the lack of design guidelines as many existing design guidelines are outdated, or newer ones are still under development and not widely applied. The Permanent International Association of Navigation Congresses (PIANC), have proposed some methods in their 2015 publication, termed as PIANC (2015). Although it claims to be one of the first comprehensive design guidelines for scour, their effectiveness has not been thoroughly evaluated. For example, the propeller vertical clearance is not often accounted for in estimation of bed velocities, even though studies have clearly shown that existing equations underestimate propeller velocities with low vertical clearance. Hence, this study chose to focus on the impact of vertical clearance on the effectiveness of the design guidelines for rock riprap. All experiments were conducted in the Hydraulics Modelling Lab in Nanyang Technological University. The actual efflux velocity required to cause shear failure of a rock in the riprap layer was compared against the efflux velocities predicted by the PIANC (2015) design guidelines. A total of 25 conditions were tested. The propeller’s vertical clearance was set at five varying heights (in cm): ℎp = 7.5, 10.0, 12.5, 15.0 and 17.5. Five rock sizes were used (in mm): R50 = 3.96, 5.85, 9.18, 10.90, 16.42. For each rock size, experiments were conducted at five propeller vertical clearances. The results suggest that most of the design guidelines can be considered conservative. The propeller vertical clearance has been shown to have a visible impact on the critical efflux velocities, especially for smaller-sized rocks. Additionally, the critical efflux velocities were found to be inversely proportional to the propeller vertical clearance. A possible explanation could be that the propeller jet is unable to develop fully due to its proximity to the riprap layer. Upon impinging onto the bed, the propeller jet is unable to reach its maximum efflux velocity and the resultant near-bed velocities are also underestimated. A higher propeller frequency is hence required to attain the critical velocities at the riprap layer to cause shear failure of the rocks. This limiting effect increases for higher propeller frequency as the propeller’s cone of diffusion increases.Bachelor of Engineering (Civil