Ultimate Limit State Analysis of Havfarm

Abstract

Marin fish farming is in rapid development. Dimensions are expected to increase and locations are being moved to areas exposed to more energetic waves and stronger current. With increasing dimensions and more challenging environmental conditions the consequences of of failure increases. The industry have met the theses challenges by moving from trial and error to focusing on thorough engineering. One of the outcomes of this development is the "Havfarm" concept developed by NSK Ship Design and Nordlaks.

In this thesis the intention have been to investigate if the proposed concept are able to maintain its structural integrity under extreme environmental loading by using the time domain software USFOS, and the usability of this software for such analyses. The focus have been placed on establishing reliable finite element models for the load carrying structure, net- pens and mooring lines. Consecutively performing ultimate limit state analysis in accordance with applicable rules and regulations. Not all structural drawings have been available and sound engineering judgment have been applied throughout the process, with the objective of establishing a realistic representation of the structural behaviour.

Regular wave analysis and 60 3-hour irregular stochastic simulations have been performed and compared to establish the elastic utilization of elements under extreme environmental loading, and to investigate the applicability of the recommended parameters used for regular waves when performing ULS analyses. Forces on net-pens have been implemented by the use of a Modified Morison Model, and investigations related to the effects of current load- ing have been performed to some extent.

The resulting utilization factors have been plotted on Gumbel probability paper and the 90%-fractile have been established for the irregular sea states. The regular and irregular analysis have shown good compliance and the utilization have been found to be well below the recommended limits for all of the controlled elements, resulting in a structure capable of maintaining its structural integrity. It has however been challenging to establish a model giving reliable results, and there are large uncertainties involved in the achieved results.