Aquaculture becomes an important industry and associates with many targets of sustainable development goals. As the aquaculture industries go forward offshore, the design of the floating net cage must be able to withstand extreme sea conditions. One of the vulnerable parts of floating net cage is the floater, especially at the bracket. There is load transfer in the bracket from net drag force, mooring tension, and horizontal force of the wave. Thus, this study aims to analyze the bracket under current and wave using the finite element method. Hydrodynamic force is based on the Morison formula. The numerical model was validated with the data obtained from corresponding experimental and numerical tests. On the basis, using the environmental condition in Pangandaran, Indonesia, the structure is simulated globally to obtain the deformation and tension. The simulations of the fish cage consist of single and double pipes. Then, the bracket at the floater will be analyzed locally to get the deformation and stress distribution. The maximum stress occurs at the connection between the horizontal and outer pipe of the bracket.
World fish production is increasing every year. This is mainly because of the trend to use floating net cages in aquaculture. One of the common net cage types is the collar cage. The net cage system must withstand the environmental and accidental loads, particularly on the mooring system as its function to maintain the position. Thus, this study focuses on analysis in mooring tension of the fully scaled net cage using numerical methods. The model scale data obtained from a previous experimental study and then scaled up to obtain the fully scaled net cage. After validation, current and wave data at Pangandaran bay, Indonesia is adapted to the simulation. Morison’s hydrodynamic force formula is used. Configuration of the mooring system is a rectangular array with a variation of spread angles of mooring lines between 90°, 60°, and 30°. The load cases used for simulation considering the operation and extreme conditions also the directions in lines and between lines. The result shows that the smallest mooring tension and offset is the configuration 30° in which the mooring lines spread evenly in each direction.
Accelerating marginal field development must consider the economic factor. While the structural strength must remain capable and robust when subjected to environmental loads. To meet the desired objective in design phase, optimization is used. With the rapid growth of computing technology, the optimization method is developed as more advanced and reduced iteration time. However, the structural evaluation of jacket structure is a complex problem. The usual process of structure evaluation is through finite element analysis, and it is still time-consuming. Thus, surrogate models can evaluate the structure, lowering computational time. This study optimizes the jacket structure to get an affordable and robust minimal jacket structure. Sizing optimization will be performed on the jacket's leg and bracing thickness. For single-objective optimization, weight structure is considered the objective function, and multi-objective optimization adds production cost as the second objective function. The surrogate model uses the radial basis function to predict the relation between design variables and ultimate limit strength. The functions generated from the surrogate model will act as behaviour constraints in the optimization process. For consideration, X-type and V-type bracing configurations are compared. Different results were obtained from the single objective and multi-objective optimization process.
One of the main problems of floating net cages when operating is the escape of fish caused by breaking or tearing of the net. This is due to the influence of environmental loads that have been studied by several researchers. However, the influence of fish on net strength has not been considered. The schooling of fish in the cage can make the hydrodynamic flow change downstream of the cage. The collision of fish in groups on the net can increases 10–28% of the environmental load received by the net. Besides, fish bites on the net reduce the net cross-sectional area and lead to axial stiffness reduction. So, this study focuses on the analysis of the net strength to determine the net maximum tension, deformation, and the location of each parameter by considering the influence of fish in the net cage Simulations are conducted out using finite element method software with cage models based on physical and numerical experiments from prior studies. The results show that the net could withstand operating conditions. Meanwhile, in extreme conditions, the net could only survive in the 0° loading direction. The largest deformation reached 2.5 times the diameter of the cage. The collision of fish in groups on the net showed that the tensile force increased significantly when adding stress loading factor. While the effect of fish bites showed that the tensile force significantly decreased on all threads with a reduction in cross-sectional area.
A strategy to increase the production of oil and gas is through acceleration the development of marginal fields. Economic factors largely determine the marginal field. Currently, regarding structural design, there are still many results of offshore structures that are overdesigned at a certain level. Thus, structural optimization is important in the design phase. It leads to production cost minimization. Therefore, this study analyses the optimization of a jacket structure to get an economical and reliable minimum jacket structure. Dimension optimization will be carried out on the main structure of the jacket and the objective function in the form of minimizing material costs. Surrogate model is used to evaluate the structure. It is constructed by radial basis function. The optimization process uses the the non linear programming. This research is expected to recommend a minimum jacket structure optimization model approach that can be used especially for Indonesian waters.
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