Drag on Nets Fouled With Blue Mussel (Mytilus Edulis) and Sugar Kelp (Saccharina Latissima) and Parameterization of Fouling

Biofouling is a serious problem in marine finfish aquaculture with a number of negative impacts. Marine growth obstructs net openings, thereby reducing water exchange through the net and affecting fish welfare and health, as well as the spreading of dissolved nutrients, particles and pathogens. Furthermore, additional water blockage leads to increased hydrodynamic forces on fish cages, which potentially threaten the structural integrity of the fish farm. However, detailed knowledge about the effects of biofouling on the flow past, and the resulting forces on fish cages, is limited and systematic investigations of the effects of different types of fouling have been called for. This study investigates the effects of different amounts and sizes of two important fouling organisms in Norwegian aquaculture, blue mussel (Mytilus edulis) and kelp (Saccharina latissima) on the drag on net panels. Drag forces on a number of clean and fouled nets were measured in a flume tank at a flow speed of 0.1 m/s. Net solidity was calculated from images acquired of all nets in the current. The relationship between net solidity and drag was then found for clean nets and for each type of fouling, and biofouling was parameterized by comparing clean and fouled net results: for a given fouled net, a clean net can be found that experiences the same drag. The latter can then be used in numerical models to estimate the effect of fouling on net drag. That means existing models can be used to model the drag effect of fouling. This study found a solidity increase due to mussel and kelp fouling to affect drag roughly at the same rate as an increase in clean net solidity at a flow speed of 0.1 ms−1 and within the tested fouling size range for two net types. Therefore, existing models, describing the relationship between net solidity and drag, can be used directly or with minor alterations (especially at high solidities) to estimate effects of additional mussel and kelp fouling on drag. In contrast, wet weight seems to be unsuitable as a measure to estimate drag on nets fouled with seaweed or mussels. It should be noted that these findings are only valid under similar conditions, and that other fouling types and sizes, as well as test parameters and tank size can affect the relationship between solidity and drag.

A Study on the Operation Strategies for Bottom Otter Trawling in Ise Bay

In order to simulate the bottom trawl fishery in Ise Bay, a typical semi-enclosed bay in Japan, a fishery simulator has been developed that consists of a fish behavioral model and a fishing operations model. The fish behavioral models are developed for two major target species, conger eel and mantis shrimp, which consider the fish migration as well as the growth and the population change. The fish behavioral models are combined with a fishing operation model, which predicts the behavior of trawling boats based on economic conditions and resource distribution as well as the fish catch amount by each boat. The model is applied to evaluate the effects of fishery management, and it is investigated that how the control of fishing efforts or number of owned fishing boats affects the profit of the target fishery. The mesh size of the trawling net which maximizes the total catch is estimated considering both conger eel and mantis shrimp. At the target ports, reduction of boats number will increase the profit because the effect of fixed cost change exceeds the impact of fish catch decrease. However, the optimum reduction rate is different in each port and interactive effect should be also considered.

Interaction Between Advanced Spar and Regular Waves

Advanced spar type of the floating wind turbine with a short spar and a cylindrical column floater has been developed and tested recently. However, numerical methods to accurately simulate the interaction between the advanced spar and waves have not been established yet. In this study we simulated the free surface flow around an advanced spar in regular waves using open source computational fluid dynamics (CFD) software OpenFOAM to examine its applicability. We used olaFOAM which equipped with the functions to set the boundary conditions of wave generation at the inlet and wave absorption at the exit. An experiment of the advanced spar model fixed in space in the regular waves with various wave periods was also conducted to obtain an experimental database on the horizontal and vertical forces acting on the structure and pressure distribution on the floater surface. The results of the forces obtained by the numerical simulation, experiment, Morison’s equation were compared to examine the validity of the numerical model. Numerical and experimental results of the horizontal and vertical forces as well as pressure distribution on the floater surface were in good agreement, which confirmed the validity of the present numerical method. Then, we evaluated numerically the effects of the edge of the column by simulating a sharp-edged and a chamfered column floater. The result clarified that a chamfered edge decreased the wake which reduced the forces acting on the floater structure.

A Fundamental Research on Countermeasure of Disaster Mitigation and Impact Force to Cause Drifting Ship

The damage investigations that followed the Great East Japan Earthquake revealed that automobiles, shipping containers, and even ships themselves had been caught up in the resulting tsunami before being hurled into various port facilities, causing immense damage. The damage characteristics of such collisions must be an integral part of disaster mitigation measures aimed at reducing damage due to drifting ships. When considering the impact force of massive vessels on port facilities, it is necessary to take into account the changes in the drift velocity of the vessels due to waves reflected from the facilities and other coastal structures. In previous studies, the moving particle semi-implicit (MPS) method has been adopted to examine cascading tsunami flotsam damage by means of numerical simulations. In the present study, we use the MPS method to examine the drifting behavior and impact force of ships moored to an actual harbor, taking into account waves reflected from coastal area structures. Based on the results, we discuss the applicability of this method to disaster mitigation measures.

A Hydrodynamic Analysis of Motion Coupling Effect of Floating Storage Tank Supported by Marine Fenders

This study concerns a new concept of floating oil storage facility, to be deployed in coastal waters, in which separate oil storage tanks float in an array, separated by a mooring fender system. In this paper, hydrodynamic properties of a single module are investigated numerically. The effects of different mooring fender parameters including fender stiffness and fender position on the coupled motions are studied. Design criteria and a design approach for the marine fender selection are proposed. Next, time-domain simulations under random waves are performed. Finite water depth effects are taken into consideration. Then a brief parametric study on sloshing phenomenon in fender-supported tanks is conducted. Results show that a carefully designed marine fender will help reduce the roll and pitch motions of the storage tank, and thus function as a stabilizer. This analysis is the basis of a global hydrodynamic response analysis for multiple tanks, including the effects of multibody hydrodynamic interactions between tanks in the future.

Environmental Description in the Design of Fish Farms at Exposed Locations

This paper addresses the description of exposure from waves and currents in coastal regions for design of marine fish farms. Representative descriptions of environmental conditions are important inputs to the design and dimensioning of reliable fish farm structures. A trend with moving production to more exposed sites and introduction of new and novel fish farm structures increase the need for more precise descriptions of the marine environment to keep control of uncertainties in design. Dedicated field measurements at two exposed aquaculture sites from February to December 2016 are presented. Results from statistical analyses of the measurement data demonstrate that common practice for characterization of exposure in design of fish farms has several deficiencies that should be improved to reduce uncertainties in design.

Assessment of the Motion of Wave Power Generation by Water Tank Test

Installing devices of marine renewable energy is planned off Kamaishi city, Iwate prefecture, Japan. A device of wave power generation was settled at this site as a test case. It is important to coexist the power generation device and local fishermen. Since Kamaishi is known as a good fishing area, acceptance of fishermen is required to run the wave power generation at the test site. Especially, local fishermen in Kamaishi have a concern about how wave power generation moves along with ocean waves and currents. To acquire an acceptance from local fishermen in terms of setting wave power generation, a model test was conducted to understand the device of wave power generation. It consists of float, spar, middle float, gimbal mechanism and anchor. Middle float, gimbal mechanism and anchor are mooring. The submerged weights of the model almost satisfy the targeted values calculated by the scale ratio and actual submerged weights. The experiment was conducted for 1/125 scale model at a water tank in marine environmental and ecosystem laboratory, the university of Tokyo. We monitored the motion of the model under several conditions of regular waves and currents. The motions of the model were recorded by a video camera and were analyzed. The model was moved with respect to the surface waves and currents. We assessed the motions of the float based on the experiment. The experimental results will be used to explain local fishermen.

Wave Energy Extraction From Multiple Buoys Supporting a Flexible Runway

Integrating an array of buoys type converters with a flexible runway can be a viable option for cost-sharing between wave energy capturing devices and ocean space utilization structures, and thus enhance the cost-effectiveness of wave energy utilization. In this study, a configuration of multiple buoys supporting a runway is proposed. Hydrodynamic interactions among the buoys are analyzed using an exact algebraic method based on linear wave theory in the frequency domain. A parametric governing equation of compound wave energy converter referred to as a wave farm is formulated by using Hamilton’s principle which can be discretized by using Galerkin method. The effects of wave condition and the parameters of PTO on the wave energy absorption and dynamic characteristics of a runway are analyzed. This research work is aimed to provide a theoretical guideline for wave energy converters design.

Operation of the Under Water TV on Chikyu

The Under-Water TV (UWTV) is used as a reentry support system for the scientific drilling by the drilling vessel “Chikyu”. The “Chikyu” UWTV performs scientific operations cost-effectively where the normal Remotely Operated Vehicles (ROVs) in the market can’t be available. The UWTV in the 2nd generation has been tested in a sea trial and operated in the subsequent scientific drilling project. The shape of the UWTV frame has been modified after the trial and the behavior of the UWTV during its dives has been improved. The tension data in the operations has been measured and the effect of the dynamic behavior is extracted from the data. Then the extracted dynamic effect is used as the indicator to see the UWTV situation under the sea surface and to prevent the UWTV from the operational failure.

Observing Fish Using Underwater Camera at the Test Site Before Installing Ocean Power Generation

Assessment of local ocean environment is significant when devices of marine renewable energy are installed at a demonstration site. A concern about how these devices affect the existence of fish is remained among local fishermen. To confirm variation of fish before and after installing the devices of marine renewable energy, we investigated fish using an underwater camera before installation as a main objective of this paper. First, fish-eye video camera was tested to monitor in a fish cage off Kamaishi city, Iwate prefecture, Japan. We aimed to observe the abundance of fish and to specify the species of fish. The fish-eye video camera is composed of a dome-shaped lens, a container, and a digital video equipped into the container. The recorded video showed outlines of fish, consequently it may be possible to know the abundance of fish. However, specifying the species of fish has a difficulty if we use fish-eye camera since the recorded image is unclear enough to identify correctly. In addition, we examined horizontal transparency of fish-eye camera using a fish-shaped target. Before recording by the video camera, vertical transparency was measured using a white plate. Then we recorded the images by varying the water depth and the distance between the camera and target. When the target was close to the camera, it can be displayed clearly, nevertheless the outline of the target was smeared with increasing the distances between them. Our measurement suggested that the horizontal transparency might be relating to the parameters of water depth and vertical transparency. The investigation was conducted under the one condition, thus we need to obtain further data to understand horizontal transparency qualitatively. Next time, we will conduct another test using additional underwater optical video camera, which is attached on the fish-eye video camera. The combination of these two types of video cameras may enable to observe the abundance of fish as well as to specify the species of fish.