Why PICO?

Historical perspective

Moored platforms have been around as long as man's recorded history, and the 'Principle of Archimedes' is fundamental to our understanding of floating objects. The middle of the last century saw an increased effort to moor scientific packages in deep water. The instruments were large and heavy and the buoys were properly sized, but the early success rate was poor for a number of factors. We presently have good results with oceanographic moorings, but we are still using many of the same components that were selected and used several decades ago (such as large disc buoys, toroid floats, 8 strand plaited nylon, etc.), while the payloads have shrunk by a factor of 20 or more. We still assemble moorings on the deck of oceanographic ships with entire crews dedicated to highly labor intensive procedures.

Limitations of the status quo

The surface buoys deployed in deep water by NDBC, Environment Canada, several European countries, and other institutions all have similar characteristics. They are large, expensive, and typically do not sample the subsurface. The IMET buoy from WHOI is capable of making quality measurements at great cost per deployment. The ATLAS buoy, with extensive sampling capability as used in the TAO array, was designed and built with available technology in the early 70s in a logical and cautious manner, but never with the concept of a large array with hundreds of deployments. A dedicated NOAA ship plus other vessels are required to maintain the array of 70 buoys in the equatorial Pacific. Yearly recovery and redeployment at each site are necessitated by the design of the system.

Conventional buoy deployments are manpower intensive, and can be dangerous when the weather is bad and seas are rough.

A taut-line buoy moored on the surface of the ocean in the typical depth of 5000 meters is designed around a series of compromises where the proverbial tail wags the dog. The mooring line is the dominant feature in any static or dynamic model. The predicted hydrostatic drag forces and non linear response functions of synthetic lines are used to 'tune' a mooring design for specific applications but with a significant level of uncertainty due to limited knowledge of real world forcing functions. We have always erred on the conservative side due to the high value of deployed systems and limited deployment opportunities, possibly with a resultant 'overkill'. As the size of the instrumentation has shrunk with the age of electronics we find we have a ratio of payload to gross displacement of approximately 2 to 5%. This might be appropriate if we are trying to leave the pull of the earth's gravity field as NASA must do, but for this effort, something closer to 15 to 20% makes more sense. Decreasing the size of the floatation requires a smaller mooring line, which produces less drag, which allows for a smaller buoy, etc.

In an ongoing effort at PMEL to support the TAO array, the DART program, FOCI PEGGY buoys, and NOPP buoys deployed in the north Pacific, we have attempted to address the core issues of 1) losses due to vandalism, 2) increasing system costs, and 3) deployment operation requirements. However, we have been constrained to making only incremental changes and improvements and have now come to the virtual limit of what can be done with these conventional systems. This situation necessitated taking a fresh look at surface mooring technology and developing new concepts and methodologies.

PICO design requirements

Frames from a video tape of a PICO buoy deployment from a ship.

The requirements that drive the PICO concept present significant engineering challenges and little precedence for solutions. Consequently the project is not without risk. However it is instructive to look back on the development of the ATLAS buoy that spawned the TAO array. Prior to 1980 there were no oceanographic surface moorings that could reliably bring subsurface data to the buoy and transmit that data ashore. A prolonged engineering effort eventually achieved the desired goal, but not without a few setbacks, doubters, and difficult times. The PICO concept deserves a more focused engineering effort - the technology is more complex and the cost constraints are greater, but the potential for an enhanced ocean observation capability has never been greater.