ABSTRACT

In this study a typical gravity cage with frame anchor system is presented and a set of equations of motion for this flotation structure are derived using lumped mass method. The purpose of this study is to investigate the gravity cage dynamics under irregular wave conditions using numerical model. The numerical results are discussed focusing on the motions of the float collar and the maximum mooring-line forces. The results show that the mooring lines located upstream are most loaded, and the spectra of wave elevation, surge and heave motion response of float collar have a same dominant frequency.

INTRODUCTION

Global shortage of seafood is becoming more serious. To satisfy the global need for seafood, the marine aquaculture is becoming very popular all over the world. The expansion of near-shore aquaculture is becoming more difficult due to coastal multi-use issues and environmental impact concerns. In the future, more fish farms will be located at the offshore area, but current and severe waves may cause some trouble. This means an increase the necessary of engineered technology employed in the design of net cage. To investigate the performance and reliability of the open ocean net cage, there were several research efforts on net cage involving numerical modeling, scaled physical modeling and field measurements. To our knowledge, Kawakami (1964) proposed relative reliable semi-empirical formula to calculate the drag force acting on net. The formulation includes the drag force caused by flow velocity, mesh size of net and material characteristics. Aarsens et al. (1990) further divided the external forces on net cage into drag force and lift force, considering the angle between normal direction of net plane and current velocity, based on both theoretical work and comprehensive model tests. Their work has laid a foundation for further study of the net cage dynamics.

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