Nonlinear slamming-induced response of floating truss structures

Abstract

This thesis investigates a floating truss structure’s local and global response to breaking waves slamming loads. The case study of interest is the supporting structure of SolarDuck’s (SD) platform FPV. Their design consists of different triangular modules, with the solar panels at a safe height from wave impacts. In future designs, the truss supporting structure underneath them, made of non-circular aluminum braces, may be exposed to slamming events.

The literature research concluded that the current state-of-the-art lacks methods to estimate local and global load on truss structures accurately, and the most relevant physical aspects to correctly represent such phenomena are uncertain. Among them, hydroelasticity, in particular, may play a role. Consequently, the research is structured into three parts, and, in each of them, two cross-sections are considered for the structure, SD’s one and an equivalent circular hollow tube (HT) one.

In the first one, the local analysis on individual braces is performed and four methods are applied to compare the relevance of different assumptions. The structure is expected to show a hydroelastic response at the local level according to the Bereznitsky criterion. Faltinsen’s method takes hydroelasticity takes into account, and provides lower values for the response, being the best method to calculate the actual deformation of the structure. Nevertheless, good estimations of the deflections were offered also by the General Wagner Method and Modified Logvinovich Method for the asymmetric loading, assuming the brace as rigid.

These two methods are included in the global analysis: one triangular module is modeled in ANSYS, and a transient analysis is performed. The structure shows mainly a local behavior, and the non-circular cross-section geometry shows double values for the deformation. The local analysis can be used to approximate the global one, even though additional proof is needed.

A CFD simulation is performed to verify the analytical models through COMFlow, and the MLM applied to the symmetric load case offers the closest response to the numerical calculations. From this analysis, the local slamming coefficient is also derived to be equal to 4.78 for the circular cross-section and 10.38 for the square one.