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Long-Term Autonomous Data Buoys For Monitoring the English Channel
Recently Overhauled Buoy Systems Measure Parameters Missed by Boat Surveys, Enhance Collection of Biogeochemical Data

Dr. James Fishwick
Bio-Optical Oceanographer
Paul Mason
Marine Technologist
Dr. Chris Gallienne
Scientist and Marine Systems Engineer
Plymouth Marine Laboratory
Plymouth, England

For more than a century, biogeochemical parameters have been measured in the English Channel off Plymouth, England. This sampling has intensified over the past two decades, leading to the formation of the Western Channel Observatory, hosted at the Plymouth Marine Laboratory (PML). Boat sampling, conducted on a weekly basis, measures numerous parameters, spanning chemical, physical and biological components from zooplankton to bacteria. These parameters, combined with atmospheric, meteorological and gas exchange measurements, serve to enhance the complex ecosystem modeling and remote sensing work at PML. Due to the harsh and dynamic nature of this environment, boat sampling is unable to provide the temporal frequency required.

PML has therefore developed two autonomous data buoy systems to continually monitor biogeochemical parameters of the western English Channel. The data buoys have been deployed at two long-term sampling stations. The first, termed L4, is a coastal station located six nautical miles south of Plymouth. The second, termed E1, is located at an open-shelf environment 20 nautical miles south of Plymouth. The high-frequency data collected are crucial for the long-term sustained observations in the western English Channel.

Overcoming Challenges
Scientists and technologists from PML designed a platform capable of sustaining a suite of high-frequency core measurements and being a platform of opportunity for more detailed studies and instrument development. The western English Channel is a hostile environment for the long-term deployment of oceanographic equipment. It can be subject to strong winds at any time throughout the year, with gales and storms being commonplace, especially in the winter months. The prevailing wind direction is from the southwest and, due to the long fetch, this can generate large swells and waves on the order of around five meters. The tides are moderately strong (0.6 meters per second maximum surface stream at mean spring tide) with a tidal range of almost six meters during extreme spring tides.

In addition, the English Channel is a major thoroughfare for commercial shipping, and Plymouth itself is a naval port home to a sizeable fishing fleet. Any fixed moorings need to be well characterized to avoid collisions while also being strong enough to withstand the elements.

Buoy Development
From the outset of this project, the design team was determined to develop a large, highly visible platform that was as collision-proof as possible. The buoy platforms stand more than seven meters tall, with more than 4.5 meters of the platform above the waterline. Floatation is provided by 3.1-meter-diameter, closed-cell polyethylene foam that is coated with a polyurethane elastomer skin. The float also has the capability to be compressed by more than 70 percent and still return to its original shape, creating a large fender around the buoy structure. The buoys are marked on the navigation charts, and a campaign was initiated with fliers and handouts to raise local awareness of the buoys amongst marine users.

Radar reflectors with cross-sections of 24 square meters and an automatic identification system (AIS) were fitted to each buoy. The AIS transmits every three minutes on Message 21, indicating position, identification and warning of subsurface moorings. If a buoy drifts more than 300 meters from its nominal position, a warning transmits to Message 14 every minute. To maximize the benefits of this technology, the PML team utilized a shore-based receiver to receive the AIS messages. Software written by the PML team will detect if the buoys are drifting and send text messages to several mobile telephones, giving an off-position warning. This allows a vessel to be mobilized, if necessary, to recover the buoy.

One of the L4 buoy's wind turbines undergoes some maintenance. The platforms are designed to allow full access to all components at sea, allowing for ongoing maintenance and troubleshooting.

Holding Station
PML carried out considerable research in designing the most suitable mooring configuration for the application. The design had many constraints from the environment, vessel capability and scientific ideals. These included the combined effects of both tide and waves on seal level change, which in extreme events could measure up to 11 meters, requiring the mooring to have sufficient spring.

The mooring design was required to hold the buoy in constant position and orientation. This configuration prevents structural shading of the optical sensors and has the additional benefit of keeping the solar panels facing toward the south to maximize the amount of direct sunlight. Each buoy is moored using a two-point mooring in an east-west orientation, which is designed to coincide with the dominant tidal stream. PML's research vessel, the Plymouth Quest, can only lift three tonnes at one time. For this reason, the chain clumps at the foot of each mooring leg are three tonnes. A combination of chain and rope rises from the weights up to a subsurface float, maintained 15 to 20 meters below the water surface, and another combination of chain and rope connects to the buoy. This subsurface float provides lift to take up any slack in the lines whilst allowing the lines to pull straight if extra height is required.

During a winter storm, an incident occurred where the buoy managed to drag one of the clump weights across the seabed. To counteract this, a one-tonne embedment anchor was placed 100 meters from each clump connected by a ground line; this has proven to be successful.

Power System
In the temperate regions of the western English Channel, there were concerns that solar charging alone may not give the power returns that had been specified by the project. To harvest as much power as possible from the available environmental resources, wind turbines have also been fitted to the buoys.

Both buoy platforms house two independent power systems, which are charged by SunWare's (Duisburg, Germany) 12-volt, 69-watt solar panels and a Marlec (Northamptonshire, England) Rutland 913 wind turbine that is mounted on each system and is capable of providing up to 300 watts. The solar panels are mounted at approximately 30' from vertical to maximize charge efficiency, with one system's panels facing south and east and the other system's facing south and west.

Each of these power systems charges a 12-volt, 230-amphere-hour, sealed lead-acid gel battery (so each buoy has two batteries). The batteries are housed at the base of the above-water structure and are connected together in parallel to provide power to the onboard systems.

Sensor Configuration
The construction of the bespoke platforms centered on the scientific goals of the project, enabling the instrumentation to be optimally and safely deployed. The in-water sensors are housed within a central tube measuring 900 millimeters in diameter, extending 2.5 meters below the water line. Flushing is ensured with a series of 100-millimeter-diameter holes in the sides of the tube. The tube is accessed through an opening in the center of the buoy structure and allows the sensor cage to be winched to the surface for maintenance. The above-water sensors are mounted on the top of the buoys, free from any interference by the structure itself. To continue this article please click here.

Dr. James Fishwick is a bio-optical oceanographer working at the Plymouth Marine Laboratory. He has more than 11 years' experience in marine optics and scientific instrumentation. His research interests include bio-optical interactions in the marine environment, and he is presently part of the observatory group, with responsibilities for all instrumentation and the running of the buoy project.

Paul Mason joined the Plymouth Marine Laboratory as a marine technologist with a background as an ROV pilot and technician for the offshore marine industry. He is a member of the observatory group specializing in the fabrication of bespoke solutions for instrument deployment and platform design.

Dr. Chris Gallienne is a marine scientist and technologist at the Plymouth Marine Laboratory. He is currently part of the observatory group and has specialized in the electronics and software control within the buoy project.

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