Motion response analysis of large-scale structures with small-scale cylinders under wave action

Abstract

The influence of small-scale cylinders is always ignored in classical models for hydrodynamic calculations of truss spar platforms and other large-scale structures in published research. This study is focused on this influence on the motion response under the wave action so that the response can be forecast more accurately. A numerical model is proposed to calculate the wave-body interactions under the second-order frame by utilizing the time-domain higher-order boundary element method. The wave loads on small-scale cylinders are calculated by the Morison formula, in which the wave field disturbed by a large-scale structure is taken into account. Three-dimensional elastic rod theory is chosen to analyze the cable force to a platform, and the motion equation for the mooring system and hull are solved simultaneously using the Newmark-β method. A moored truss spar platform is modelled, and the numerical results demonstrate that the existence of small-scale cylinders may lead to a decrease of the motion response of the platform, while the natural frequency will move to a lower frequency. This phenomenon explains why small-scale cylinders, which can be modelled as truss spar play an important role in damping and change the added mass of the platform. Therefore, in order to get more accurate results for motion response, the wave loads of small-scale components cannot be ignored.