Breaking wave impact force on a vertical and inclined slender pile—theoretical and large-scale model investigations

Many offshore and harbour structures may be considered as slender cylindrical structures or are composed of cylindrical slender members. Wave forces on such slender structures are usually calculated by the Morison equation. In this paper, the special case of breaking waves acting on a slender cylindrical pile is examined. The wave load is splitted into a quasi-static and a dynamic (slamming) component. The quasi-static load is reasonably approximated by the Morison equation, whereas the dynamic component is associated with the impact of a mass of water on the cylinder. The time history and the intensity of this impact force are analysed. The theoretical description is based on the 2D-model of Wagner (1932) [Wagner, H., 1932. Über Stoß- und Gleitvorgänge an der Oberfläche von Flüssigkeiten. Zeitschrift für angewandte Mathematik und Mechanik 12 (4), 193–215 (in German)] for the peak pressure, which is extended to account for the total duration of the impact and for the 3D-case. As a result, an analytical description for the impact force is developed. Based on large-scale experiments the theoretical 2D-model is verified through comparison with measured local pressures on the tested cylinder. The theoretical 3D-model is verified through comparison with the total force, which is determined by the measured reaction forces at the bearings. As a result of this comparison, the curling factor, which is included in the theoretical model, is determined. The experimental investigations are restricted to plunging breakers. A classification of the breaker types, and thus of the loading cases, is suggested which is related to the distance between breaking location and cylinder. The dependency of the impact force on the yaw angle of the cylinder in and against wave direction is determined in five test series.