Manoeuvre analysis and simulation to prevent seabed scour due to ship propellers

Abstract

Erosion of the seabed is produced by cargo and passenger vessels when manoeuvring in a harbour basin due to the high-speed jet flow generated by the main propellers. In case of twin propeller vessels, the ability to perform the whole manoeuvre with no tugs makes the master to use the main propulsion system even close to berthing walls and other structures. Near quay walls, the main variables leading to sediment scour are both related with the propeller behaviour -speed of rotation, pitch- and with the relative position of the propeller to the quay wall and the seabed -axial distance to the quay wall and vertical distance to the seabed-. Therefore, there exists a need to understand the manoeuvre in detail in order to predict the actions of the propellers to the seabed. In this article, AIS data is used to study the target manoeuvre to later reproduce it in the ship simulator. With the output of the simulator, timeseries of the propeller delivered power and the position of the propellers during the manoeuvre are obtained. With these data, the existing formulae in maritime engineering can be applied to obtain the efflux velocity at every ship position. By linking the efflux velocity with the manoeuvre characteristics, -i.e., ship position, ship heading and distance to the quay walls and seabed semiempirical formulation to obtain the flow velocity over the seabed and the expected scour depth is also applied. The numerical simulator turns out to be a very useful tool to analyse any manoeuvre and obtain the locations where it is potentially more harmful, the expected flow velocity and the maximum expected scour depth.