DEOS MOORED BUOY OBSERVATORY DESIGN STUDY
R. Detrick, D. Frye, J. Collins, J. Gobat, M. Grosenbaugh, R. Petitt, A.
Plueddeman, K. von der Heydt, F. B. WoodingWoods Hole Oceanographic Institution, Woods Hole, MA 02543
J. Orcutt, J. Berger, R. Harriss, F. Vernon
Cecil & Ida Green Institute of Geophysics and Planetary Physics
Scripps Institution of Oceanography, La Jolla, CA 92092
J. Halkyard, E. Horton
Deep Oil Technology, Houston TX
Moored ocean buoys are a technically feasible approach for making sustained timeseries observations in the oceans and will be an important component of any long-term
ocean observing system. Because of the broad spectrum of scientific needs identified in
DEOS (Dynamics of Earth and Ocean Systems) planning documents, and described in
the recent U.S. National Academy of Sciences/National Research Council report“Illuminating the Hidden Planet: The Future of Seafloor Observatory Science,” it is clearthat there is no single buoy or mooring design that will meet all of these needs while atthe same time minimizing costs. We have, therefore, considered a range of specifications
and designs in this study, and evaluated the trade-offs in performance and costs for thesedifferent systems. We have found that it is possible to meet the specifications that have

Figure 1 Conceptual diagram of a moored ocean buoy observatory1been proposed in these planning documents by using currently available technologies,
although the cost will be significant. The installation of the infrastructure for twenty
systems will cost as much as a Class I UNOLS vessel (ca. ~$50M) and the annual
maintenance and operating costs of these twenty systems will be the equivalent ofoperating a pair of major UNOLS ships (ca. ~$10M/yr). However, the establishment of anetwork of moored ocean buoy observatories will be an important enabling technologythat will lead to fundamental new understanding of the oceans and Earth.
The most important specifications driving buoy design decisions are the telemetry
rate that the buoy system will support for communication to shore, and the amount ofpower delivered to the seafloor. The only cost-effective alternative for continuous highbandwidth communication (64-128 kbps or higher) currently available is a C-Band orKu-Band satellite system. However, the power requirements for operating thesetelemetry systems and for the delivery of power to the seafloor lie in the kilowatt range
and require a buoy equipped with diesel generators. At present, compact, low-powersatellite communications systems that can be powered entirely by solar cells and batteries
are limited to modest data rates (<64 kbps) and high tariffs that will limit datatransmission to ~5 Mbytes/day. Solar cells can only provide at most a few tens of wattsof power to instruments on the seafloor or on a mooring. Diesel generators can provide
several hundreds of watts to the seafloor, providing much greater potential for system
expandability in the future. Thus, the size and payload capacity of the buoy is strongly
dependent on the telemetry rate specified for the system, as well as the power supplied toinstruments on the seafloor or on the mooring. The effects are highly nonlinear; that is,reducing power requirements to the seafloor and telemetry rates by a factor of two does
not translate into a buoy which is half the size and cost.
We have evaluated both single-point, discus buoy moorings and tri-moored sparbuoys for use as ocean observatories. Three specific buoy and mooring designs havebeen analyzed and are compared on the basis of their costs and capabilities. The primary
option investigated was a cable-linked, high-bandwidth observatory employing either aspar or discus buoy that uses an electro-optic cable to connect seafloor and moored
instruments to the surface. This system has been designed to deliver 500 W of power tothe seafloor and utilizes a C-Band satellite telemetry system that can send ~500 Mbytesof data (or more) each day to shore. We have also evaluated a low-bandwidth discus
buoy system that uses acoustic modems to transfer data from instruments on the seafloor
or mooring to the buoy, and can deliver about 5 Mbytes/day of data to shore. No power
is delivered to the seafloor and solar panels (with batteries) are used to power the satellite
telemetry system.

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Figure 2 High-Bandwidth, Cable-Linked Moored Buoy Observatory with a fiber
optic connection from the buoy to the seafloor and nearly continuous 64 kbps
two-way communication between the buoy and the shore via satellite.

Figure 3 Low-Bandwidth, Cable-Linked Moored Buoy Observatory. Uses
acoustic communication to link instruments on the seafloor or the mooring withthe buoy and delivers ~5 Mbyte to shore via a solar-powered, low-power satellite
telemetry system.

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The acoustically-linked approach has the advantage of lower cost and lesscomplicated logistics at the expense of lower data rates, and inability to provide power tothe seafloor. The cabled buoy systems are more expensive to build, install and operate,
but offer much higher data rates and can provide significant power to operate seafloor
instruments. Costs for the two approaches averaged over the ten year operational life of
the systems differ by about a factor of 2 to 3, while data rates differ by a factor of morethan 100. Cable-linked, moored buoy systems will be well-suited to applications which
require high-bandwidth and for which observations over a long time period (a decade ormore) are needed. Acoustically-linked moored buoy systems will be the preferredsolution when power does not need to be supplied to sensors, the data telemetry
requirements are modest, and systems need to be relocatable or deployed rapidly inresponse to transient events. It is likely that a mix of capabilities will best meet broad
observatory needs.
For high-bandwidth applications both discus (single-point mooring) and spar buoys(tri-moors) are suitable. The discus approach is less expensive initially and simpler toinstall. For the MOMAR and NAZCA sites a discus system costs ~$1.9-2.3M/node tobuild and install (including ship time) versus ~$3.2-5.2M/node for the spar. However, aspar buoy with a 3-point mooring may be more reliable in terms of satellite telemetry.
Amortized over the 10-year operational life of the systems the cost to build, install,
operate and maintain a spar system are only about 15-20% higher than for a discus buoy.
The appropriate choice of approach will depend on anticipated weather conditions,location (latitude, water depth), required system lifetime, the power and telemetry
requirements of the sensors, and the need for continuous real-time data. The greater thewater depth, the larger the cost differential between the discus and spar options. The sparbuoy system provides a more stable platform for telemetry, making it the preferredsystem for high-latitude, bad-weather sites. At lower latitude, better weather sites, adiscus system may be a cost-effective solution for observatories requiring highbandwidth. The decision on whether a spar or discus approach is preferred for a specifichigh-bandwidth application must be addressed by a careful, site-specific and applicationspecific analysis.
Assuming the costs given for the MOMAR and NAZCA sites are representative of
the average costs for establishing observatories at DEOS sites, an array of 20 nodes
including 10 high-bandwidth spar systems (for high latitude sites), 5 high-bandwidth
discus buoy systems (for lower latitude sites) and 5 low-bandwidth systems (forapplications with modest data telemetry requirements or the need to be relocatable) could
be built and installed for a cost of ~$38M. Annual maintenance and operations costs
would be ~$500K node for the high-bandwidth systems and ~$200K/node for the low-4bandwidth systems or ~$7M/yr for 20 buoys. If all twenty nodes were high-bandwidth
systems an initial investment of ~$33M (discus design) to ~$54M (spar design) would be
required and annual operating and maintenance costs would be ~$10M/yr. None of thesefigures include ship costs; these costs are difficult to estimate accurately without knowing
specific deployment sites. As a rule of thumb, total costs will approximately double if
ship costs are included. These cost estimates also do not include scientificinstrumentation or labor associated with the scientific use of the observatories; in the caseof the low-bandwidth system each instrument builder will have to supply power and onboard data storage, and in most cases the instruments will have to be recovered andserviced annually. These are infrastructure costs, though some of the cruise costs would
likely be shared between the science users and the infrastructure providers. We anticipate
that the scientific uses and instrumentation will evolve rapidly over the initial decade ofthe program.
There are several important remaining questions regarding the design anddevelopment of moored buoy ocean observatories that should be addressed through aseries of engineering studies or prototype testing. These include:• The reliability of the acoustic communication system in the low-bandwidth system
in deep water. What average data rates can be achieved and what environmental
factors determine acoustic link performance? Does the large watch circle of a discus
buoy on a single-point mooring present problems for directional acoustic telemetry?
• The effectiveness of discus and spar buoys as platforms for Inmarsat B (lowbandwidth) and C-Band (high-bandwidth) satellite telemetry systems. Up to what seastates will these systems operate reliably in practice? How sensitive is performanceto buoy/mooring design?
• The expected operational lifetime of the electro-optic cables for single-point andthree-point moorings. How do factors such as sea state, currents and mooring design
affect cable reliability?
• The service interval required for diesel-powered buoys with C-Band telemetry
systems. Is a maintenance interval of ~12 months realistic?
In view of the engineering issues discussed above and the substantial resource
commitment necessary to build and operate a global network of ocean observatories, werecommend a carefully phased program to develop the infrastructure needed to begin
operating buoy-based observatories. The goal of this program will be to build, install andoperate both a prototype high-bandwidth, cable-linked observatory and a low-bandwidth,
acoustically-linked observatory. When completed, these two systems will demonstrate
the full range of solutions that are available for buoy-based ocean observatories. We5recommend that the high-bandwidth prototype be based on the tri-moored spar design,
which we believe is the more conservative approach based on its inherently lowerdynamics. The low-bandwidth prototype should be implemented using the acoustic linktechnology combined with the single-point discus mooring configuration to optimize thesystem for remote applications and to minimize overall costs. While it is possible todesign and construct these systems, the long-term reliability, over year-long periods, ofthe communications and power systems will have to be evaluated through experience.
This two system approach will result in tested designs that are suitable for use in most of
the envisioned applications at deep ocean sites.
The full copy of the DEOS Buoy Design Report is available on-line at
http://obslab.whoi.edu/buoy.html

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