Ship manoeuvring effects on propeller induced erosion

Abstract

The shipping industry evolution over the last decades has led to growing structural and operational problems, in particular, for old marinas with lower depths to hosts present ships and with higher drafts and power of engines. The increase in capacity, size, power and self-propulsion of present ships is the main cause behind morphodynamic sea bed changes in harbour basins. This is producing two different but linked problems: scouring effects near the structures, affecting their stability, and sedimentation of the scoured material in other areas of the basin that reduces the average depth of the basin and the ship motions become uncontrollable. The present contribution aims to compare two different manoeuvring situations in a confined scenario, using laboratory data. Laboratory experiments have been carried out at the Marine Engineering Laboratory facility LaBassa. The two manoeuvring situations, modelled at bollard pull condition are permanent docking/backward and permanent undocking/forward using three different propeller revolutions at a fixed pitch angle and blade area ratio and two fixed locations of the propellers from the wall. The potential erosion is measured through an ecosound scanner of the control area over a homogeneous sandy bed. The evolution of the maximum erosion depth behaviour is completely different for undocking and docking operations. Undocking/backward results show higher depths at the initial stages reaching an almost asymptotic state at the end of the experiments whereas maximum erosion depth during forward operations increases monotonically over time.