Stability of outfalls during installation

Abstract

Outfalls are built for the purpose ofdischarging sewage, storm runoff or cooling water into the sea. During the installation, outfalls are exposed to relatively large hydrodynamic forces compared to their own weight in case of adverse weather or high currents. To resist these forces, a heavy concrete weight coating has to be applied. With increasingly stringent environmental directives, it is important to situate the end of an outfall further offshore. This is possible by optimising the conservative schematisation of the offshore technology common practice of pipeline stability applied to outfalls. This optimisation can lead to a reduction of the outfall weight coating, which offers perspectives to the construction of longer outfalls and an improved economic design in general. Therefor, research into a less conservative approach of calculating the resultant hydrodynamic forces on outfalls has been carried out as a graduate project at Delft University of Technology. The graduate project concentrates on the most common situation of an outfall built in shallow water, approximately at right angles to the depth contours. Trenching and application of a heavy weight coating have to assure stability during installation. The impact forces on the outfall generated by breaking waves will not be considered.