Walter Paul

Numerical modeling of nonlinear elastic components of mooring systems

This paper deals with the finite-element analysis of compliant structures containing nonlinear elastic components and subjected to loading by waves and currents. An efficient procedure to model such components is developed and implemented in a computer program. It can be applied to analyze nonlinear elastic ropes, rubber tethers, risers, and hoses. The elastic modulus is assumed to depend on the amount of strain in a component. The Newton-Raphson iteration scheme is modified to account for the change of elastic properties on each step of time integration procedure. The approach is applied to develop feed buoy mooring systems for the University of New Hampshire Open Ocean Aquaculture site. The complex nonlinear and viscoelastic load-elongation behavior of suitable high stretch mooring elements for this application is modeled to obtain realistic numerical predictions at sea conditions. Various designs and environmental loading scenarios are considered. Numerical simulations provide predictions of the overall dynamics of the system and maximum values of tensions in critical components.

Instrumentation for open ocean aquaculture monitoring

Fudning was provided by National Oceanic and Atmospheric Adminstration for the Open Ocean Aquaculture Project under Contract No. NA86RG0016 to the University of New Hampshire and under Subcontracts 00-394 and 01-442 to the Woods Hole Oceanographic Institution.

Mooring developments for autonomous ocean-sampling networks

Two general-purpose mooring designs have been developed to support autonomous underwater vehicle (AUV) operations in autonomous ocean sampling networks (AOSNs). These moorings provide two-way communications between investigators and AUVs docked on the moorings or conducting survey operations some distance from the moorings. A deep-water design that incorporates an AUV dock and recharging station was built for use in the Labrador Sea during the winter of 1997/1998. This severe winter environment required a robust design that could operate unattended for six months while isolating the dock from surface wave motion. A much lighter, easier-to-deploy design was developed for use in coastal waters to extend the nearshore AOSN operating area by extending the communications network. This coastal design has been deployed without the dock component and has typically been configured for use in a small network of moorings maintained with a small research vessel. The deep-water mooring has been deployed successfully on two occasions, for short periods of time. The coastal moorings have been deployed a number of times and have proven to be quite effective. This paper describes the two moorings in detail and provides information on their performance so that interested investigators can utilize the technology where it meets their needs.

Coastal mooring design: taut elastomeric and chain catenary surface buoy moorings

A comparison is made of the behavior of a buoy anchored with an elastomeric tether and a chain catenary mooring at the same water depth and under the same environmental conditions. The results include numerical predictions of the statics and dynamics of the two moorings under identical ocean current and sea state forcing. The at-sea recorded tensions in an elastomeric tension member buoy system are also studied. The results show that elastomeric tethers respond with one third of the dynamic tensions of a chain catenary mooring in severe sea states. The chain moorings also experience considerable frictional wear due to chain links interacting with the sea bottom. A drawback of elastomeric tether moorings is that they can be damaged or cut when entangled with fishing gear.

Design and Performance of a Horizontal Mooring for Upper-Ocean Research

Abstract This paper describes the design and performance of a two-dimensional moored array for sampling horizontal variability in the upper ocean. The mooring was deployed in Massachusetts Bay in a water depth of 84 m for the purpose of measuring the horizontal structure of internal waves. The mooring was instrumented with three acoustic current meters (ACMs) spaced along a 170-m horizontal cable that was stretched between two subsurface buoys 20 m below the sea surface. Five 25-m-long vertical instrument strings were suspended from the horizontal cable. A bottom-mounted acoustic Doppler current profiler (ADCP) was deployed nearby to measure the current velocity throughout the water column. Pressure sensors mounted on the subsurface buoys and the vertical instrument strings were used to measure the vertical displacements of the array in response to the currents. Measurements from the ACMs and the ADCP were used to construct time-dependent, two-dimensional current fields. The current fields were used as in...

The use of snubbers as strain limiters in ocean moorings

Buoy based seafloor observatories require lightweight synthetic strength member electro-optical anchor cables to be feasible. Typically these cables have maximum elongations of around 0.6% before damage occurs to the copper and optical elements and therefore provide minimal compliance to absorb wave and current forces acting on the surface buoy and cable. A stretchy mechanical system known as a snubber has been developed at WHOI for absorbing wave energy and protecting the buoy electro-optical cable from excessive strain. Results are presented from field trials of three different ocean mooring designs that all use snubber hoses as a key design element.

DEEPWATER MOORING DESIGNS FOR OCEAN OBSERVATORY SCIENCE

This paper describes the current state-of-the-art in mooring systems appropriate to the deepwater ocean observatory context. The technological challenges that need to be addressed in order to realize moored ocean observatories as envisioned for the next generation of ocean observing systems are outlined.

Coil-Cord Conductors on Compliant Elastic Moorings

High stretching rubber tethers are one means of supplying the required compliance in coastal buoy moorings. However, a major disadvantage of the elastic tether approach has been that electrical signals and power could not be carried along the elastic tether to allow data from all depths to be sent to the surface buoy for telemetry to shore. To overcome this difficulty, a coil-cord design concept was tested as part of an ongoing observatory/monitoring effort 13 km off the coast of New Hampshire in 55 m of water in the open Gulf of Maine. In this application, the rubber tether with an added coil-cord was located near the top of the mooring to allow the buoy to move more freely with the sea state to obtain better wave statistics. An environmental sensor package located below the rubber tether was hard-wired through the coil-cord to the surface platform, allowing its data output to be regularly telemetered to shore together with the buoys meteorological, sea surface temperature and salinity and accelerometer data. Near real-time access to the mid-water data is much more useful for the nearby aquaculture than access to archived data collected by internally-recording in-water sensors which are available only every three to four months when the mooring is serviced. The coil-cord/compliance assembly was constructed with four parallel elastic tethers, each 14 m long, with the coil-cord spiraled around one of them. The coil-cord was terminated at each end with standard underwater connectors, and attached to a bridle and the tether splice to prevent movement relative to the tether at each end. Also, the coil-cord was securely attached to the tether with self-vulcanizing tape at two places along the tether. This is done to prevent the coil-cord from working its way down the tether, causing uneven stretch and loading response and possibly local abrasion of the rubber tether around which the coil-cord is wrapped. The top connector on the coil-cord was attached to the surface buoy and the bottom connector to a Sea-Bird SBE-16Plus Seacat measuring temperature, conductivity, dissolved oxygen, optical backscattering and chlorophyll-a. This data collected routinely by the Seacat (15 minute samples) was recorded internally and also sent through the coil-cord to the surface buoy for telemetry to shore. The first deployment in the winter 2005-06 (mid-December through February) worked well. The coil-cord and tether showed no wear, or signs of failure. It was cleaned and redeployed again on 30 March and retrieved mid-July 2006 after the telemetry up the coil cord-cord stopping sending signals. The post-deployment evaluation of the mooring is ongoing. The telemetry stopped when the Seacat battery prematurely ran down. A similar Seacat with identical sensors ran for the full 3.5-month deployment. While the conductors were still in intact, first indications are that the cable was flooded (lower resistance between conductors) through a rope strength member. There was no visible mechanical damage to the rubber tether and coil-cord assembly. We hope to improve coil-cord cable design to prevent flooding and conductor wetting in the future. If this proves successful, the high-stretch conductive mooring element will be a viable candidate for powering sensors down the cable (lowering underwater instrumentation costs by not having to provide power in the sensor) and recording data that is telemetered up the wire to the buoy (again reducing underwater instrumentation costs by not having to store the data). This will enable the buoy to then telemeter the data collected from depths to shore with other information/data collected on the buoy. Also, the high-stretch conductive mooring element can be used for buoy-based observatories where the advantages of elastic compliance can be realized and data can be routinely telemetered to shore for distribution on the Web

The Determination of the Elastic Modulus of Rubber Mooring Tethers and their use in Coastal Moorings

Funding was provided by the Gulf of Maine Ocean Observing System (GoMOOS under ONR grant N0014-01-1-0999), NOAA-UNH CINEMAR (NOAA Grant Number NA16RP1718), and GLOBEC (NSF OCE93-13670 and OCE02-27679).

Editorial Special Issue on Open Ocean Aquaculture Engineering

PEN ocean aquaculture engineering is a relatively new area of study. Only in the last 10 to 15 years have engineers started to develop techniques to design and analyze marine aquaculture systems to support the production of a few selected species. Today, a larger variety of fishery products are being considered for grow-out in exposed open ocean environments. Many new types of systems and aquaculture applications are being proposed. Oceanographic conditions, the use of optimized equipment and marketable species will determine the design criteria, which will vary from region to region and country to country. The goal of this Special Issue is to present a sampling of open ocean aquaculture engineering from a global perspective. Articles and Technical Communications were submitted from Canada, the United States, Taiwan, Norway, New Zealand, and Spain. The individual approaches are unique and reflect different sets of engineering considerations. However, the lessons