Rational analysis of Nordlaks' "Havfarm" aquastructure concept for exposed waters

Abstract

A global analysis of Nordlaks Havfarm concept has been conducted. In order to do so three equivalent design waves targeting the ultimate limit state response of the vertical bending, horizontal bending and torsional moment have been estab- lished. There have been built both a hydro and a structural model of the Havfarm in order to be able to perform a seakeeping analysis and to evaluate the capacity of the structure. The modelling process has by far been the most time consuming part of this thesis. Models have been subjected to continues updates. Best engineering judgment have been used during the modelling of the Havfarm in order to best be able to represent the real physics of the structure while still trying to include special characteristics of the structure. That is the lice skirts and the nets of the Havfarm. The lice skirts was neglected at an early stage as it was clear that there was no good way to include them in the seakeeping analysis. However, an effort was made to include the lice skirts, unfortunately it turned out that the only way to model them would change the hydrodynamic properties such as the water plane area and the GML in a way that the model no longer would represent the physical case in a god way. Thus the lice skirts was also neglected. The hydro and the FE model have been modelled as detailed as possible according to the structural drawnings provided by NSK Ship Design. This for both main dimensions, scantlings and weight distribution of steel weight and ballast tanks. Bureau Veritas software have been used to preform the seakeeping analysis and to transfer loads from the hydrodynamic model to the FE model. The seakeeping analysis was performed in the frequency domain with headings ranging from 0o - 180o with 15o increment and periods ranging form 3 - 30 second. Based on the seakeeping analysis RAOs for internal loads were established which in turn was used to determine the ULS response of the internal loads. The results was used to establish the following three EDWs used to evalueate the strucutres ULS capacity. When subjected to the EDWs the FEA results of the Havfarm indicates that the top and bottom pontons will experience von Mises stresses exceeding 400[MPa] over large areas, and high stress concentrations in sharp connectiuons exceeding 600 [MPa]. This indicates that the Havfarm do not have sufficient capacity.