Judson DeCew

A case study of a modified gravity type cage and mooring system using numerical and physical models

A modified gravity-type cage, developed by SADCO Shelf Ltd., was examined using numerical and physical models to determine if the cage and mooring system is suitable for an exposed site south of the Isles of Shoals, NH. The 3000-m/sup 3/ SADCO Shelf Submersible Fish Cage has angled stays between the upper framework and the ballasted bottom rim (in addition to net) to resist the horizontal shear deformation. The mooring system consists of three legs-each made up of a taut vertical chain and an angled rope, both leading to deadweight anchors. Normalized response amplitudes (response amplitude operators) were found for motion response in heave, surge and pitch, and load response in the anchor and bridle lines, in regular (single frequency) waves. In addition, a stochastic approach was taken to determine the motion and load transfer functions in random waves using a spectrum representative of seas at the selected site. In general, the system motion had a highly damped response, with no resonant peaks within the wave excitation range of 0.05 to 0.45 Hz. The anchor line force response was at all frequencies below 5 kN per meter of wave amplitude. The physical model tests showed consistently more conservative (larger) results compared to those for the numerical model.

Dynamics of a Floating Platform Mounting a Hydrokinetic Turbine

A two-dimensional mathematical model was developed to predict the dynamic response of a moored, floating platform mounting a tidal turbine in current and waves. The model calculates heave, pitch, and surge response to collinear waves and current. Waves may be single frequency or a random sea with a specified spectrum. The mooring consists of a fixed anchor, heavy chain (forming a catenary), a lightweightelasticline,andamooringballtetheredtotheplatform.Theequationsof motion and mooring equations are solved using a marching solution approach implemented usingMATLAB.Themodel was appliedto a 10.7-m twin-hulled platform used to deploy a 0.86-m shrouded, in-line horizontal axis turbine. Added mass and damping coefficients were obtained empirically using a 1/9 scale physical model in tank experiments. Full-scale tests were used to specify drag coefficients for the turbine and platform. The computer model was then used to calculate full-scale mooring loads, turbine forces, and platform motion in preparation for a full-scale test of the tidal turbine in Muskeget Channel, Massachusetts, which runs north-south between Martha’s Vineyard and Nantucket Island. During the field experiments, wave, current, and platform motion were recorded. The field measurements were used to compute response amplitude operators (RAOs), essentially normalized amplitudes or frequency responses for heave, pitch, and surge. The measured RAOs were compared with those calculated using the model. The very good agreement indicates that the model can serve as a useful design tool for larger test and com

Engineering Overview of the University of New Hampshire's Open Ocean Aquaculture Project

Aquaculture products are projected to play an important role infilling the global demand for seafood in the world marketplace. In the US, stiff resistance to near shore aquaculture sites (where most farms are located) will drive the industry to more exposed locations. In an effort to better understand open ocean aquaculture challenges, the University of New Hampshire (UNH) has been investigating the biological, engineering, environmental and economical issues. This overview focuses on the engineering approach utilized by UNH to determine aquaculture system loads, motions and operational logistics by utilizing a variety of tools including numerical and physical models and field experimentation. Numerical modeling is performed with Aqua-FE, a finite element analysis (FEA) program developed to study aquaculture type systems, MSC.MARC/Mentat, a FEA structural modeling program, and FLUENT, a computational dynamics program. Scaled physical model tests are performed in the UNH wave/tow tank. In addition, an extensive field program experiments with the use of biofouled net panels, telemetry and control systems, feed buoys, scaled cages and various environmental monitoring equipment. Biofouled net panels were tested to determine the blockage effect due to the biological growth. Feed buoys, with telemetry and control options, have been deployed and tested. A new 20 ton capacity feed buoy has been designed and is currently under construction. A scale, experimental, submersible net pen has been designed, built and deployed to determine the feasibility of various components. Environmental measurements are collected with a surface buoy and the data is transmitted to shore. The resulting information from these experiments can help move the near shore aquaculture industry to more exposed locations

Assessment of a submerged grid mooring in the Gulf of Maine

The University of New Hampshire (UNH) developed and maintained an offshore aquaculture test site in the Western Gulf of Maine, south of the Isles of Shoals in approximately 50 m of water. This site was designed to have a permanent moored grid to which prototype fish cages or surface buoys could be attached for testing new designs and the viability of the structure in the exposed Gulf of Maine. In 1999, the first moorings deployed consisted of twin single bay grids each capable of each securing one fish cage. These systems were maintained until 2003. To expand the biomass capacity of the site, the single bay moorings were recovered and a new four bay submerged grid mooring was deployed within the same foot print of the previous twin systems. This unique system operated as a working platform to test various structures, including surface and submersible fish cages, feeding buoys and other supporting equipment. In addition, the expanded capability allowed aquaculture fish studies to be conducted along with engineering and new cage/feeder testing. The 4 bays of the mooring system were located 15 meters below the surface. These bays were supported by nine flotation elements. The system was secured to the seafloor on the sides with twelve catenary mooring legs, consisting of Polysteel® line, 27.5 m of 52 mm chain and a 1 ton embedment anchor, and in the center, with a single vertical line to a 2 ton weight. To size the mooring gear, the UNH software package Aqua-FE was employed. This program can apply waves and currents to oceanic structures, predicting system motions and mooring component tensions. The submerged grid was designed to withstand 9 meter, 8.8 second waves with a 1 m/s collinear current, when securing four fish cages. During its seven year deployment, the site regularly experienced extreme weather events, most notably a storm with a 9 m significant wave height, 10 second dominate period in April 2007. The maximum currents at the site were observed during internal solitary wave events when 0.75 m/s currents with 25 minute periods and 8 m duration were observed. The mooring was recovered in 2010 after 7 years of continuous deployment without problems. The dominate maintenance requirement of the mooring was the cleaning once a year of excessive mussel growth on the flotation elements and grid lines. No problems of anchor dragging or failure of mooring components were documented during the deployment. Upon recovery, critical mooring components were inspected and documented, focusing on items with wear or other areas of interest. The mooring proved to be a reliable, stable working platform for a variety of prototype ocean projects, highlighting the importance of a sound engineering approach taken in the design process.

Practical Applications of Numerical Modeling using Aqua-FE: A Case Study

A numerical model of an American Soybean Association (ASA) cage system was constructed using a finite element program developed at the University of New Hampshire (UNH) called Aqua-FE. The small volume, high density aquaculture system was modeled to determine how the system will operate in normal and extreme environmental conditions. The goals of the study were to determine the maximum loads in the system during tropical storm conditions and determine a similar cage systems response under specified environmental criteria. The cage is currently deployed in Weitou Bay, China. The system consists of a 100 m3 cage (2 m times 4.5 m times 7 m) secured in a single point mooring. The rigid HDPE cage is held to the mooring by two sets of bridle lines, attached to the upper and lower cage framework. Chain ballast hangs below the lower cage rim providing a restoring force. A deadweight anchor secures the system to the seafloor. A 90 kg float suspends the single point mooring and serves as a tie-up location for servicing vessels. Aqua-FE can apply wave and current loading on truss and buoy elements by utilizing the Morrison equation adopted for analysis of aquaculture net pen systems. The algorithm employs a nonlinear Lagrangian formulation to account for large displacements of structural elements. In addition, the unconditionally stable Newmark direct integration scheme is adopted to solve the nonlinear equations of motion. Hydrodynamic forces on the structural elements are calculated using the Morison equation modified to account for relative motion between the structural element and the surrounding fluid. Maximum loads in the mooring gear approached 56 kN during the storm events. When various current velocities were applied, the cage submerged to a maximum depth of 16.4 meters

Distance detection of Unmanned Underwater Vehicles by utilizing optical sensor feedback in a leader-follower formation

This paper proposes an optical detection system between a leader and a follower Unmanned Underwater Vehicle, specifically Remotely Operated Vehicles (ROVs). Cost efficient photodetectors and a single LED light source are used to develop distance detection algorithms to detect translational motion in x-and y-axis directions. Analytical simulations are performed where light is modeled as a first order Gaussian function and integrated into the nonlinear ROV dynamics. The stability of a proportional derivative (PD) controller is shown via Lyapunov stability, as in Fossen [7]. Both leader and follower ROV motions are simulated and experimental results from the distance detection algorithm are shown for proof of concept. In this stage of research, all experiments are performed out of water. Initial results indicate that the proposed detection system shows promise as a precursor stage to underwater testing.

Design of a 20-Ton Capacity Finfish Aquaculture Feeding Buoy

A design for a 20-ton capacity buoy was developed to feed fish in four submerged cages at an exposed site south of the Isles of Shoals, New Hampshire, USA. The buoy was designed to contain all the equipment necessary to accomplish the feed dispensing tasks as well as have the strength and stability to remain on location in a variety of sea states. New feed handling and distribution systems were developed and tested. To evaluate seakeeping response a Froude scaled physical model was constructed and tested at the Ocean Engineering wave/tow tank at the University of New Hampshire (UNH). The mooring system was designed using the UNH developed finite element analysis program called Aqua-FE. The prototype buoy is now under construction, and is scheduled for deployment in late summer 2006

Experimental Studies and Numerical Modeling of Copper Nets in Marine Environment

Copper alloy netting is increasingly used for offshore aquaculture, harbor protection and other marine applications. Its advantageous characteristics include high resistance to biofouling and increased strength compared to polymer nets. However, the hydrodynamic properties of copper nets are not well studied. In this paper, the results of experimental studies of drag forces on copper alloy net panels are reported. Based on these studies, empirical values for drag coefficients are proposed for various types of copper nets, and compared to the corresponding data for polymer netting. It is shown that copper nets exhibit significantly lower resistance to the current flow which corresponds to lower values of drag coefficient. Coefficients obtained from the experiments are incorporated into the finite element program Aqua-FE, developed at the University of New Hampshire for analysis of flexible structures subjected to waves and currents in marine environment. The results of the numerical simulations for a small volume fish cage, subjected to two different sets of environmental conditions, are analyzed to compare how introduction of copper netting instead of traditional nylon nets affects the dynamic response of the system.Copyright

Study of a Three Point Mooring of an Open Ocean Aquaculture Feed Buoy

Open ocean aquaculture will be important in supplying fish as natural fish stocks decline due to environmental changes and overfishing. However, the technology needs to be developed to provide the equipment and methodology for such offshore operations. The University of New Hampshire has been developing and testing systems in the open Gulf of Maine, and has a number of years experience with a feed buoy moored to a fish cage via a stretch hose and elastic tethers. This configuration has survived the Gulf of Maine currents and waves, and continued to feed fish autonomously. The unique tri-mooring is studied via modeling, and observing the wear and behavior for years. The system is robust and has functioned well over the years and is a good candidate for aquaculture operations.

A field study to understand the currents and loads of a near shore finfish farm

An extensive field study was conducted to investigate the current velocities and mooring system tensions in a 20-unit net pen fish farm located in Eastport Maine, USA, near the Bay of Fundy where extreme tides create strong currents loading fish farm components. To understand the flow characteristics at the site, currents meters were deployed at external and internal farm locations during three distinct operational conditions: (1) clean nets for smolts, (2) clean nets for standard grow out and (3) nets for standard grow out at the end of the stocking schedule, when the gear is fouled with biological material. Also, several load cells were deployed at the same time on important anchor leg and net pen attachment components to measure system loads. The current meter data sets provided evidence of velocity flow reduction through the farm by examining the current meter statistics and tidal harmonic constituents. Tidal analysis also showed substantial tidal harmonics or shallow water tides. The load cells measured maximum loads of 104 kN for anchor leg and 11 kN for net pen lines. The results are significant because few extensive fish farm engineering data sets exist, especially for different combinations of nets and levels of biological fouling. These results are being used in complementary studies to quantify flow reduction, to validate Morison equation type numerical models and as a baseline for specifying offshore fish farm gear