M. Robinson Swift

Finite element modeling of net panels using a consistent net element

A consistent finite element is proposed to model the hydrodynamic response of net panels to environmental loading. This equivalent net element is constructed to reproduce the drag, buoyancy, inertial and elastic forces exerted on the netting by current and waves. To evaluate the accuracy of the proposed finite element modeling, numerical predictions have been compared with the experimental observations and (simplified) analytical results of other authors. This new modeling technique has been applied to evaluate the performance of a tension leg fish cage in the open ocean environment.

Fish cage and mooring system dynamics using physical and numerical models with field measurements

Abstract As wild fish stocks decline, marine aquaculture is expected to play an increasing role in satisfying the global need for seafood. Since the expansion of near-shore aquaculture is becoming more difficult because of multi-use issues and environmental impact concerns, the feasibility of moving aquaculture into the open ocean is being studied. To enable the optimum design and evaluation of fish cage and mooring performance in the energetic open ocean, physical and numerical modeling techniques are being utilized. To validate these methods, the dynamics of a fish cage and mooring system deployed in the Gulf of Maine are simulated with physical and numerical models and compared with field observations. Assuming that the system can be modeled as a linear system, a stochastic approach was used to analyze the motion response (heave, surge and pitch) characteristics of the fish cage and the load (tension) response in an anchor line to wave forcing. Transfer functions were calculated from field observations for storm events and used to understand the system dynamics and to validate the models for the deployed system. Fish cage heave and surge motions were found to be overdamped, while pitch exhibited a resonance at low frequencies (less than 0.1 Hz). Transfer functions for anchor line tension were consistent with observations over most of the wave frequencies. The physical model clearly revealed the pitch resonance, while the numerical model was better at predicting mooring line tension. Results also provided insight concerning dynamical processes that require further study, including fluid–net panel interaction, transfer function amplitude dependence and the nonlinear relationship between steady current and wave fluid velocity on drag and its effect on system geometry and therefore response.

Distribution of bottom stress and tidal energy dissipation in a well-mixed estuary

Abstract Estimates of area-averaged tidal bottom stress are made for four channel segments of the Great Bay Estuary, N.H. Current and sealevel measurements are used to estimate acceleration and pressure gradient terms in the equation of motion, while the equation of motion itself is used to infer the remaining stress term. Dynamic terms, bottom stress values, friction coefficients and energy dissipation rates are estimated for each site. The analysis shows that while throughout the estuary the principal force balance is between the frictional stress and the pressure gradient forcing, RMS values of total bottom stress range from 2·67 to 10·38 Nm−2 and friction coefficients vary from 0·015 to 0·054. Both stress and energy dissipation are largest in the seaward portion of the estuary with an order of magnitude decrease in dissipation at the most inland site. These distributions of stress and energy dissipation are consistent with cotidal charts of the principal semi-diurnal tidal constituent (M2) which indicate that the estuary is composed of a highly dissipative more progressive tidal wave regime seaward and a less dissipative standing wave regime landward.

Open ocean aquaculture engineering : System design and physical modeling

An open ocean aquaculture net pen system was developed and deployed for an exposed demonstration site south of the Isles of Shoals, New Hampshire in 55 meters of water. This component of the project is part of an interdisciplinary effort at the University of New Hampshire involving engineers, biologists, economists and commercial fishermen. Initially, two cages were specified for the growout of summer flounder (Paralichthys dentatus). The design process included physical model testing conducted in a wave/tow tank using 1/22.5 scale models. To select an optimum system, experiments were performed using gravity-type and central spartype cages. Vertical taut line and catenary moorings were evaluated. Data acquisition included drag and wave forces on the cages, mooring line forces and heave, pitch and surge motion of the cages. After comparison of the results and holding a design review including outside experts, a central spar configuration was selected for both cages. This system exhibited both reduced force loadings and less extreme motion, and its rigid frame would resist volume changes under storm conditions. In the final mooring design, each cage was separately deployed using a four anchor system to ensure redundancy. A mid-depth, square, horizontal grid was employed in order to reduce anchor footprint area, which is very expensive under New Hampshire permit rules. Mooring system components were sized to meet loadings scaled up from the physical model results. The system was deployed in June of 1999 and has performed well in all weather forcing conditions to date.

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

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.

Shoal formation in the Piscataqua River, New Hampshire

The formation of a shoal was investigated in the Piscataqua River, New Hampshire, which is a well-mixed channel with low freshwater flow and tidal currents up to 2.3 m s−1. Observations of sediment characteristics, bathymetry, and bottom current were made, and theory was used to predict bedload transport. Sediment sampling showed the bottom material to be coarse sand and gravel, and sidescan sonar revealed large sand waves directed upriver at the shoal. Bottom current measurements were made along transects upriver and downriver of the shoal and downriver of an adjacent deepwater area that was also studied for comparison. Bedload flux inferred from current measurements using the Brown-Einstein theory indicated that transport is generally directed upriver. Sediment budget calculations showed the shoal area to be depositional before, immediately after, and subsequent to a dredging operation at rates of 0.36 m yr−1, 1.06 m yr−1, and 0.35 m yr−1, respectively. Predredge and subsequent rates were consistent with the historical record of removal by dredging at the shoal.

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

The dynamic testing of a collision tolerant pile structure

The Collision Tolerant Pile Structure has been under development by the University of New Hampshire and U.S. Coast Guard for ten years. The structure is designed to take the place of standard wooden piles used by the U.S. Coast Guard to mark navigational hazards in narrow ship channels. The CTPS consists of a steel pile that is connected to an omnidirectional hinge at the mudline. This design allows the structure to survive collisions with ships or barges where wooden piles would otherwise fail. Two prototypes deployed in the Houston Ship Channel have survived numerous collisions to date. A third prototype installed at Portsmouth, NH is presently being monitored to determine the fatigue design life. The CTPS is outfitted with an ENDECO/YSI data acquisition field station. This system handles inputs from several meteorological sensors and a pressure sensor in order to obtain continual real time data. Data acquired includes wind speed and direction, air temperature, wave height, and two angles of tilt. This data is acquired and temporarily archived on site. Two way, packet radio communication between the field station and a base station PC located on shore enables adjustment of study patterns and sensor selection. Software running on the base stations PC calls for the field stations data routinely alloying it to be transmitted to the base station packet receiver and archived on the PC. Data is subsequently transferred to UNH or any other location using remote control communications software and a phone line. This software enables the user to remotely control the base stations PC and also allows one to download the data archived there. Sample rates and sensor selection can also be accomplished by this link. Data comes in raw form and, is subsequently processed into engineering units. Initial results are being used to correlate CTPS dynamic response to environmental forcing. Subsequent results will yield data adequate for analytical model verification and stress analysis.

Design of a mooring system for an inertia tube wave energy collector

The dynamics of a point absorber wave energy collector (WEC) were investigated using the University of New Hampshire (UNH) developed finite element computer program Aqua-FE and tank testing. The WEC design considered here makes use of a buoy rigidly connected to a long, vertical inertia tube which is open at the top and bottom. A piston-rod assembly is enclosed and connected to the power take-off (PTO) mechanism. Due to inertia of water within the tube, relative motion between the piston and buoy-inertia tube structure occurs, and this drives the PTO. The Aqua-FE model was then used to design a slack mooring system sufficient for holding the WEC on station while minimizing interference with its energy absorption function. The Aqua-FE model was created and validated by comparison to wave tank measurements made using a 1/9.4 scale physical model in experiments conducted in the UNH 36.6 m long by 3.66 m wide by 2.44 m deep wave tank. The mathematical model was then applied to predict full scale response to seas representing extreme storms expected at UNHs offshore test site south of the Isles of Shoals, NH. Predicted mooring loads were used to specify mooring system hardware. The Aqua-FE model for this system was evaluated using scale model results for free-release tests in heave (vertical displacement), pitch (angular motion), as well as heave in a series of single frequency waves. Wave periods, Froude-scaled to full size, spanned the range of periods observed at the UNH site.