Characterization of Wave Slamming Forces for a Truss Structure within the Framework of the WaveSlam Project

Abstract

One increasingly popular type of foundations for offshore wind turbines in shallow water are truss structures. These are exposed to wave slamming forces from breaking wave events. These forces depend, among other parameters, on the slamming factor Cs that typically takes values between π -2π. So far, several studies about slamming forces have been done for monopile structures, but not much work has been done regarding truss structures such as offshore jackets. This paper is based on the WaveSlam project in which an instrumented multi-membered truss model has been subjected to regular and irregular waves in the large wave flume at FZK Hannover in 2013. The structure was built at scale 1:8 and equipped with force transducers along the bracings and columns that measured the structural response from the breaking waves. The goal of this study was to characterize the breaking wave forces acting on the front bracings of the structure, in order to estimate the respective slamming factors. The tested structure has been modelled and validated with a finite element model in ANSYS and a wave run test is analyzed. The wave loads have been defined as uniform loads with a triangular force time history acting along the bracings. Through a fitting procedure, the initial responses from ANSYS and from the data were matched with a relative error of 3% for one single wave test. From this, a slamming factor of 4.78 is found in the highest part of the front bracings. The characterization of more breaking wave loads is recommended in order to get an estimate of the largest slamming factor, which is relevant for design purposes.