Bowthrusters and the stability of a riprap revetment

Abstract

Because of problems with the design guidelines produced by PIANC for armoured slopes under attack by bowthrusters, additional work has been done in the Netherlands. On the basis of this work computational rules have been developed. However, because of the increase of bowthruster power, more detailed knowledge is needed on the effect of a bowthruster on the stability of a riprap revetment. Especially large, fast vessels will cause problems to shore protection. Recently P&O put into operation the “Pride of Rotterdam”, a luxury ferry with two bowthrusters with a capacity of 2000 kW each. When the captain uses both thrusters simultaneously, there is a considerable risk of damaging the rock of the underwater slope protection. But also with relatively small vessels for inland navigation problems arise when skippers use their bowthrusters. For the calculation of the effect of a bowthruster at this moment the common methodology is to use the hydraulics of a plain jet. This is not correct because the propeller in the tube causes quite some extra turbulence. This extra turbulence will cause extra damage to the shoreline protection. So in a good design formula for the determination of stability in a bowthrusterflow, he effect of additional turbulence of the propeller has to be included.