Shallow-water messenger-line recovery system

Shelf and estuarine deployments of bottom mounted instruments generally require complete recovery of the instrument, including anchors. Subsurface instruments may have lift lines for recovery, often on acoustically commanded release of a float. The lift line and float are large for heavy instruments and this creates a flow disturbance that distorts the environment being measured. When redundancy in recovery lines is added, the volume of lines and floats may become unacceptable. Light weight messenger lines with small messenger floats are less flow disturbing and can be added to provide redundancy with less compromise to the measurement. A set of four messenger lines with floats was used in the Hudson River in 1995 to recover a massive quadrapod deployed on the bottom for several weeks. The messenger lines, with ample scope, were used to pull, by hand, one end of a short, strong lift line to the surface for recovery of the quadrapod. In this deployment, each messenger line went to an independent lift line, but several messenger lines could be joined to a single lift line. Redundancy is needed for the most vulnerable elements of a system and in shallow water this is the lift line itself. In deeper water, the extra complexity of connecting several messenger lines to a single lift line is offset by the substantial savings in volume by eliminating a redundant lift line. Experience in two recoveries with this recovery system shows that the burnwire used to release the messenger line float works very well but can become fatigued in shipping, line fouling can trap the float in the launch silo, and floats can rise but fail to surface in strong current because Froude drag increases near the surface. The authors have yet to learn if biofouling and heavy sediment deposition are a problem The benefit of redundancy has been noted in numerous other experiments where loss occurred when lift lines were cut by propellers, bilge keels, and guard buoys, and when tangles prevented a float from coming all the way to the surface. The benefit of small, low drag messenger lines would have been substantial in deployments in deep, high current regimes where a scope of two to one made the lift line package quite large. Four independent messenger line recovery packages took less space than the single primary lift line system that it replaced