Mooring (oceanography)

A mooring in oceanography is a collection of devices connected to a wire and anchored on the sea floor. It is the Eulerian way of measuring ocean currents, since a mooring is stationary at a fixed location. In contrast to that, the Lagrangian way measures the motion of an oceanographic drifter, the Lagrangian drifter.

BrnBld MooringSketch
Mooring as deployed in Fram Strait with top buoy, a CTD-sensor, two rotor current meters, acoustic release and train wheels as anchor.

Construction principle

The mooring is held up in the water column with various forms of buoyancy such as glass balls and syntactic foam floats. The attached instrumentation is wide-ranging but often includes CTDs (conductivity, temperature depth sensors), current meters (e.g. acoustic Doppler current profilers or deprecated rotor current meters), and biological sensors to measure various parameters. Long-term moorings can be deployed for durations of two years or more, powered with alkaline or lithium battery packs.

Components

Top buoy

Surface buoy with meteorological sensors
WHOI moored surface buoy with meteorological sensors and satellite transmitters[1]

Surface buoys

Moorings often include surface buoys that transmit real time data back to shore. The traditional approach is to use the Argos System. Alternatively, one may use the commercial Iridium satellites which allow higher data rates.

Syntactic foam sphere
Syntactic foam sphere used as a subsurface float

Submerged buoys

In deeper waters, areas covered by sea ice, areas within or near shipping lines or areas that are prone to theft or vandalism, moorings are often submerged with no surface markers. Submerged moorings typically use an acoustic release or a Timed Release that connects the mooring to an anchor weight on the sea floor. The weight is released by sending a coded acoustic command signal and stays on the ground. Deep water anchors are typically made from steel and may be as large as 100 kg. A common deep water anchor consists of a stack of 2–4 railroad wheels. In shallow waters anchors may consist of a cinder block or small portable anchor.

The buoyancy of the floats, i.e. of the top buoy plus additional packs of glass bulbs of foam, is sufficient to carry the instruments back to the surface. In order to avoid entangled ropes, it has been practical to place additional floats directly above each instrument.

Instrument housing

Prawlers

Prawlers (profiling crawlers) are sensor bodies which climb and descend the cable, to observe multiple depths. The energy to move is "free," harnessed by ratcheting upward via wave energy, then returning downward via gravity.[2]

Depth correction

Similar to a kite in the wind, the mooring line will follow a so-called (half-)catenary. The influence of currents (and wind if the top buoy is above the sea surface) can be modeled and the shape of the mooring line can be determined by software.[3][4] If the currents are strong (above 0.1 m/s) and the mooring lines are long (more than 1 km), the instrument position may vary up to 50 m.

See also

References

  1. ^ Toole, John M.; McCartney, Michael S.; Hogg, Nelson; Weller, Robert A. (2000). "Outposts in the Ocean". Oceanus Magazine. Woods Hole Oceanographic Institution. 42 (1).
  2. ^ "Prawlers, Engineers, and the Future of Oceanography at Sea. Retrieved 27 Jan 2013".
  3. ^ Dewey, Richard K. "Mooring Design & Dynamics - A Matlab Package for Designing and Testing Oceanographic Moorings And Towed Bodies". Centre for Earth and Ocean Research, University of Victoria. Retrieved 2012-09-25.
  4. ^ Dewey, Richard K. (1 December 1999). "Mooring Design & Dynamics—a Matlab® package for designing and analyzing oceanographic moorings". Marine Models. 1 (1–4): 103–157. doi:10.1016/S1369-9350(00)00002-X.
Acoustic release

An acoustic release is an oceanographic device for the deployment and subsequent recovery of instrumentation from the sea floor, in which the recovery is triggered remotely by an acoustic command signal.A typical release consists of the hydrophone (see dark gray cap in the figure), the battery housing (long gray cylinder), and a (red) hook which is opened to release the anchor by high-torque electrical motor.

Benthic lander

Benthic landers are observational platforms that sit on the seabed or benthic zone to record physical, chemical or biological activity. The landers are autonomous and have deployment durations from a few days (for biological studies) to several years (for physical oceanography studies).

Benthic landers come in a variety of shapes and sizes depending upon the instrumentation they carry, and are typically capable of working at any ocean depth.

Biological pump

The biological pump, in its simplest form, is the ocean's biologically driven sequestration of carbon from the atmosphere to the ocean interior and seafloor sediments. It is the part of the oceanic carbon cycle responsible for the cycling of organic matter formed mainly by phytoplankton during photosynthesis (soft-tissue pump), as well as the cycling of calcium carbonate (CaCO3) formed into shells by certain organisms such as plankton and mollusks (carbonate pump).

Cabled observatory

Cabled observatories are seabed oceanographic research platforms connected to the surface by undersea cables. Such cables supply both power and telecommunications to instruments. By removing the limitations of undersea power sources and sonar or RF communications, cabled observatories allow persistent study of underwater phenomena. A single cable can support multiple observation sites via individual "drops;" multiple or branching cables may then provide data in 2D or 3D.

The extent of coverage is limited by the high cost of laying dedicated undersea cable. Initial experiments used abandoned communications cables; efforts are in progress to extend observations at lower cost by accessing more such cables.

Despite their advantages, cabled observatories can (and do) relay compromised data to scientists, particularly when located in remote parts of the ocean. Factors such as instrumental malfunction and biofouling are often responsible for this. Systematic improvements, to lessen the impacts of such factors, are currently being studied by groups such as Ocean Networks Canada.

Mooring (disambiguation)

A mooring is any structure to which a vessel may be secured by means of cables, anchors, or lines.

Mooring may also refer to:

Mooring (surname)

Mooring (North Frisian dialect)

Mooring (oceanography), a collection of devices connected to a wire and anchored on the sea floor

The Mooring, a 2012 film

The Moorings, New York, a private community

Sediment trap

Sediment traps are instruments used in oceanography to measure the quantity of sinking particulate organic (and inorganic) material in aquatic systems, usually oceans. This flux of material is the product of biological and ecological processes typically within the surface euphotic zone, and is of interest to scientists studying the role of the biological pump in the carbon cycle.Sediments traps normally consist of an upward-facing funnel that directs sinking marine snow towards a mechanism for collection and preservation. Typically, traps operate over an extended period of time (weeks to months) and their collection mechanisms may consist of a series of sampling vessels that are cycled through to allow the trap to record the changes in sinking flux with time (for instance, across a seasonal cycle). Preservation of collected material is necessary because of these long deployments, and prevents sample decomposition and its consumption by zooplankton "swimmers".

Traps are often moored at a specific depth in the water column (usually below the euphotic zone or mixed layer) in a particular location, but some are so-called Lagrangian traps that drift with the surrounding ocean currents (though they may remain at a fixed depth). These latter traps travel with the biological systems that they study, while moored traps are subject to variability introduced by different systems (or states of systems) "passing by". However, because of their fixed location moored traps are straightforward to recover for analysis of their measurements. Lagrangian traps must surface at a pre-determined time, and report their position (usually via satellite) in order to be recovered.

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