Things you should consider
When designing or selecting a bottom frame, there are a few things you should take into consideration before you drop your gear in the water.
- Stability The frame should sit on the bottom and not move or rock back and forth. One common solution is to use a tripod leg configuration. Stability naturally improves with a larger frame "footprint" and a lower profile. Make sure there is space below the instrument's gimbal; an uneven bottom or settling frames will have reduced clearance and could cause the gimballed instrument to tilt.
- Mobility Clearly the frame should not move while on the bottom. Movement can come from strong mean currents or wave generated currents. Wave generated currents can be particularly troublesome in shallow waters or environments where waves are large and long. The frame must also be firmly attached to the bottom if the bottom type is relatively soft. Note that sifting sediment or sand can undermine bottom fasteners.
- Ease of deployment This should be considered if the frame is to be deployed by divers or handled by a small boat. If a large amount of weight is used to eliminate mobility, it may not be manageable by a small boat or crew.
- Orientation The instrument should be placed vertically and in the case of the AWAC, the AST performance deteriorates quickly for tilts greater than 10 degrees. A gimbal is the best solution if you do not use divers to ensure the proper orientation. Since a gimbal uses gravity to keep the instrument upright, it is necessary to have a counter weight underneath the instrument. This is particularly true of the AWAC since it is "top heavy".
- Retrieval This is perhaps more of a deployment consideration, however, some method should be kept in mind for a retrieval system that does not interfere with the instrument's performance. Surface buoys floating above the instrument can interfere with the acoustic beams, moreover they may be dragged away or lost due to shipping traffic or curious mariners. Alternative methods of retrieval include: acoustic releases (from bottom weight), pop up buoys with releases, drag lines and/or offset mooring systems.
- Trawl resistance Bottom fishing is perhaps one of the most challenging issues for bottom mounted systems. There are a variety of trawl resistant frames with different profiles. These frames reduce the effects of a direct hit, but cannot ensure that they are not hauled up or moved some distance. Sometimes the best solution is to choose your deployment location wisely.
- Multi-sensor mounting Since Nortek instruments provide the possibility to integrate external sensors, this should be considered when deciding on a frame.
- Ease of shipping Some frames can be separated into sections for easy shipping. This of course reduces operational costs when testing is performed at different locations.
- Corrosion resistance All material on the frame must be corrosion resistant. Materials such as fiberglass, stainless steel (316), aluminum, and plastic are good alternatives
- Burial This is also a challenging issue for bottom mounted instruments. Low profile frames may not be the best for locations with large sediment deposits or shifting sands. Sometimes it is just a matter of time as the frame slowly sinks in the bottom or sand builds up; in these situations some users have found sensors measuring distance from the instrument to the bottom useful. If mean currents and wave induced currents are managable at the bottom, then an instrument may be mounted on a subsurface buoy that is just above the bottom.
Further discussion on deploying bottom frames, the design, and other considerations is encouraged on the Forum.
Below are two commonly used bottom frames with a description taken directly from the manufacturers. There are several others designed for particular evironment/threats.
Ocean Science Sea Spider The platform is built around a thick fiberglass casting that's designed for long-term immersion in seawater at any depth. It has lead ballast feet and corrosion-free plastic instrument clamps. All fasteners are 316 stainless steel.
The standard Sea Spider weighs 20 kg without lead ballast. A modular option allows the assembly to be shipped in a box rather than on a pallet.
The Sea Spider can carry multiple instruments and a popup buoy for diverless recovery. Gimbals are available for deployment on unlevel seafloors. This bottom frame is suitable for the AWAC, Vector, and all members of the Aquadopp family of instruments.
Miniaturized Trawl Resistant Bottom Mounts is designed for protecting oceanographic instrumentation from trawler gear. These instrument platforms are ideally suited for use with up looking Doppler Profilers and provide sufficient
space for extra battery housings and other instrumentation. Optional diver serviceable AWAC brackets and acoustically activated pop-up buoys are available to suit specific applications. Bridles can be configured for lowering using an acoustic release or slip line.
The basic construction of our MTRBM consists of a truncated rectangular fiberglass cover over a fiberglass grate. Purge holes in the upper perimeter of the cover and handles are standard. Holes in the top surface are placed as needed to meet instrument mounting requirements.