Experimental investigation and numerical methods in analyzing the ocean current displacement phenomena of longlines
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- Institutt for marin teknikk 
As Norwegian fisheries deploy demersal longlines at greater water depths in steep continental slope areas, ocean current displacement—the horizontal movements of longlines from the position at which the anchors are dropped to their positions on the seabed after sinking—may adversely affect this industry, primarily in two ways: o Economic: By loss of longlines, fishing opportunities and catching efficiency o Environmental: Marine pollution due to the loss of fishing gear The main factors resulting in ocean current displacement are: o Current Speed o Water Depth o Longline Sinking Speed: A complex factor influenced by the longline’s materials and composition which experimental research suggests is difficult to measure o Initial displacement of a longline The aim of the present work is to analyze the sinking speed of longlines on various parameters—thickness, materials, anchor weights, baits, shooting speed and shooting method—using numerical methods. Describing an underwater flexible structure like longline fishing gear in three dimensions requires the structure to be divided into finite elements. In the present work, the longline is modeled as mass points connected by springs. First, the modeling incorporates non-active points into a conventional mass-spring model to improve the efficiency of the calculation and the graphical representation. The modeling consists of the following: o Active mass points o Non-active points: These points are not involved in physical calculation but function to adjust the position of structures through a geometrical transformation. Implementing non-active points demonstrably enhances the efficiency of the calculation and graphical representation when compared to modeling only the active points. o Springs Experiments were conducted in a flume tank (still water) to measure the properties of different baits. This was done to ensure the correct resistance coefficients, depending on the shape, sinking orientations and sizes, which were implemented in the simulations. This research suggests that the bait’s resistance coefficient depends on the following: o The projected area of the bait, created by the method used to cut the bait o The method of attaching the bait to the hooks Full-scale longline sinking experiments were conducted to verify the results of the numerical methods, including varying the longline sinking speed with different anchor weights and without rigged snoods and hooks. These early experiments suggested that anchor weight does not have much influence on sinking speed for the middle part of longlines, though we suspected that the measured impact of the anchor weight might have been more pronounced had we conducted these experiments in deeper water. When we conducted full-scale longline sinking experiments with snoods, hooks and baits, we witnessed good agreement between the numerical method and experimental results. The behavior of different positions along the longlines was simulated with various parameters, such as bait properties, material properties (i.e., thickness and density), anchor weights, shooting speeds and shooting methods. To assist fishermen at sea (where the time required to analyze the results of the simulation would cause the loss of fishing opportunities), the results of these simulations were used to establish simplified equations for deriving the longline sinking speed as a function of depth and the parameters mentioned above. Finally, in order to calculate the current displacement, the current speed and direction were analyzed, considering the seasonal conditions, for the most common longlining area in Norway. This analysis resulted in the standardizing the displacement calculations for the longlines at current speeds and directions common for the Ormen Lange area. This study, therefore, has introduced another way to handle ocean current displacement phenomena in a manner directly helpful to Norway’s demersal longline fisheries.