Deploying and Recovering Ocean Instruments
with a Helicopter
by David D. McGehee
To fulfill its mission of designing, building, and maintaining coastal project, the US Army Corps of Engineers relies on accurate data about the dynamic ocean. Measurements of waves, currents, and other parameters are essential elements of informed decisions and efficient designs. Sophisticated sensors may be lowered into the water for one minute or left in place for years, and the deployment techniques must be adapted to a variety of environments. The tradition platform for this work is a boat or barge, an approach that is appropriate if sea conditions are relatively mild. But, there are times and places where conditions can render this form of deployment unsafe or economically unfeasible. Installing instruments in the surf zone is particularly challenging, even under the calmest of conditions. Amphibious vessels have proven useful in many situations, but for some locations, such as the Northern Pacific coast of the United States, dangerously high surf is normal. It is in these situations that the US Army Engineer Waterways Experiment Stations Coastal and Hydraulics Laboratory (CHL) has utilized helicopters to accomplish the data collection mission.
Previous uses of helicopters for ocean measurements include surveying (Graig and Team, 1985), deploying permanent floating and bottom-mounted gages, laying cables through the surf zone (McGehee and Welp, 1994), and taking spot current measurements (Pollock et al, 1995). In each of these cases, the overwater hover time was relatively short - on the order of a few minutes. While many measurements can be obtained with a short-duration insertion, a wave measurement requires a sample length of about 20 minutes during which the instrument must remain stationary on the sea floor. The two options for the helicopter are either (1) remain attached to the instrument in a prolonged, near stationary hover, or (2) drop and recapture the instrument after the measurement period. A manually controlled 20-minute hover is severely taxing on the pilot over land where there are visual references to the helicopters position. It is nearly impossible offshore, where visual references are limited and/or distant. Therefore, a unique procedure was developed for that permitted repeated release and recovery of the load. The Chinook was selected for this mission because of its dual, releasable cargo hooks, lift capability, multi-hour flight duration, and high maneuverability
This Note describes how to use a helicopter to deploy an instrument frame through the water column to the sea floor, in shallow waters. Depending on the length of the desired measurement, the frame can be immediately withdrawn and repositioned, or released and subsequently recovered with the helicopter. The advantages of the technique relative to traditional sampling from a vessel are significantly higher operational thresholds for waves and currents and much shorter transit times between stations. The technique was demonstrated during two experiments, conducted in the summer of 1996 and the winter of 1997, offshore of the Mouth of the Columbia River (MCR). Confluence of the largest river entrance on the western side of both North and South America, large, semidiurnal tidal range, and an unrestricted fetch across the north Pacific, makes the MCR one of the roughest coastal regions in the world.
Figure 1 System suspended while in flight
Breaking waves occur at the study site under even moderate wave conditions. (Routine occurrence of breakers in the inlet prompted the US Coast Guard to locate its school for motor lifeboat operators here). The winter experiment successfully collected data from ten sites, in water depths ranging from 10 to 50 m, when surface currents exceeded 6 knots and breaking waves exceeded 6 m in height. Safe navigation of a vessel, let alone over-side operations such as deploying instruments, would not have been possible under these conditions.
Figure 1 shows the system in flight, suspended from the helicopter. The principal components of the system are: the instrument frame; a mooring line; a surface buoy; a buoyant recovery line; a stopper buoy; a triple-hook grapnel; and a lift line. To deploy the instruments, the assembly is lowered until the surface buoy is floating and the recovery line is slack. For a short (on the order of minutes) measurement, the pilot maintains a hover, without tensioning the recovery line. For a longer measurement, the frame is released by continuing downward until the stopper buoy is floating and the grappling hook disengages. Recovery is accomplished by approaching the streaming recovery line perpendicularly, with the grappling hook just below the surface (Figure 2). Continuing forward and upward slides the recovery line through the hook until the stopper ball is reached. At that point the recovery line is secure, and the load is picked up for repositioning or return to shore.
Instrument Frame: The instrument frame has a 1.5 m square base, constructed of 3 in. (7.5 cm) aluminum H-beam and bolted connections.
In addition, a transponding acoustic beacon is usually included to provide a means of locating the frame in the event it became separated from the surface float. A 4-part lift bridle, made from 1 in. ( 2.5 cm) diameter double-braided Dacron line, is secured at each corner with 2 in. (5 cm) shackles. The four lift lines converge to a 1 in. (2.5 cm) section steel D-ring, approximately 1 m above the base of the frame. A subsurface buoy secured to the D-ring prevents the slack bridle from becoming entangled in the frame or instruments during a measurement. The float should have about 15 kg of positive buoyancy, and be rigid, so it will maintain buoyancy at depth.
Mooring Line: The mooring line is a 1 in. (2.5 cm) diameter synthetic line, with an aramid braided core and a double braided polyester jacket for abrasion protection. This is a torque-balanced construction that combines high strength (25,900 kg breaking) with extremely low stretch (< 1 per cent at 30 per cent of rated strength). The length is adjusted to about twice the maximum water depth expected - about 65 m for the MCR experiment. Soft eyes (i.e., without thimbles) are back-spliced in each end for connecting to 2 in. (5 cm) shackles.
Surface Buoy: The surface buoy is a 4-ft (1.2 m) diameter spherical buoy made from rolled 1/4 in. (6 mm) steel plate (Figure 3). A 1.8 m long, 4 in. (10 cm) diameter schedule 80 steel pipe forms a strain member through the central, vertical axis. An internal padeye on the bottom of the pipe accepts a 2 in. (5 cm) shackle. The weight of the buoy is about 275 kg. To improve pitch/roll stability of the buoy, an additional 360 kg of 2 in (5 cm ) anchor chain was attached to the bottom of the pipe as external ballast, providing a metacentric height of approximately 15 cm. The attachment point for the recovery line is a welded bail of 1 in. (2.5 cm) steel bar at the top of the central pipe. A battery-powered light can be placed under the bail if the system is to be left at sea overnight.
Recovery Line: The buoyant recovery line is a 30 m length of 2 in. (5 cm ) diameter, three strand, braided polypropylene line. The line is attached to the bail on the surface buoy with a 2 in. (5 cm) shackle and a 50-ton (45,000 kg) rated crane swivel.
Stopper Buoy: A 2-ft (61 cm) diameter spherical buoy of 1/4 in. (6 mm) steel serves as the stopper to capture the grappling hook as it slides down the recovery line (Figure 3). A 3 in. (7.5 cm) schedule 40 steel pipe is welded flush through the center, as a guide for the recovery line, and as compressive reinforcement for the impact loads from the grappling hook. At the opposite side, a 1 in. (2.5 cm), round steel bar is bent into a V-shape and welded to the buoy. The bitter end of the recovery line is pushed through the guide pipe, and a soft eye was back spliced around the V-shaped bar. This arrangement ensured the stopper buoy can not disengage or slide freely down the recovery line. Since chafing of the line against the guide pipe is a concern, the exit hole is carefully radiused and smoothed, and the line wrapped with tape at that point.
Grappling hook: The shaft of the hook is a 1.5 m long,
3 ½ in. (8.9 cm), double X steel pipe (Figure 4). Three
hooks are cut from 3/4 in (1.9 cm) steel plate and welded to
the shaft. Sections of 1 in. (2.5 cm ) pipe are slotted to fit
over the inside surface of the hooks as fairlead, to prevent
abrasion of the recovery line. A 6-cm-diameter hole in the upper
end accepts the 2 in (5 cm) shackle; next is another 50-ton (45,000
kg) rated crane swivel, to prevent torque transferring up the
lift line to the cargo hook.
This paper describes a technique for deploying and recovering an instrumented frame through the water column - to the sea floor, if desired - in shallow waters with a CH-47 Chinook helicopter. Depending on the length of the desired measurement, the frame can be immediately withdrawn and repositioned or released and subsequently recovered. The advantages of the technique relative to traditional measurements from a vessel are significantly higher operational thresholds for waves and currents and much shorter transit times between stations.
The techniques described in this paper were developed with the assistance of Mr. David Castel of the Scripps Institution of Oceanography, and Mr. Steel Clayton of Global Safety Services, Inc. The experiments conducted at the MCR were performed by the 158th Aviation Regiment US Army Reserve, and with the able assistance of the Oregon Graduate Institute. The US Coast Guard Group, Astoria, provided additional helicopter support for documenting the experiment. Staging and ground support facilities were provided by the Astoria Airport. The dedication and professionalism of all of the various aircrews and support personnel were essential in refining and proving this concept.
Graig, R. and Team, W. (1985). "Surf zone and Nearshore Surveying with Helicopter and a Total Station."Proceedings US Army Corps of Engineers Survey Conference. US Army Engineer Waterways Experiment Station, Vicksburg, MS.
McGehee, D.D. and Welp, T.L. (1994). "Helicopter Deployment of Oceanographic Instruments." Video file # 94039, US Army Engineer Waterways Experiment Station, Vicksburg, MS.
Pollock, C.E., McGehee, D.D., Neihaus, R.W. Jr., Chesser, S.A., Livingston., C. (1995). "Effectiveness of Spur Jetties at Siuslaw River, Oregon; Report 1 Prototype Monitoring Study." Technical Report CERC-95-14, US Army Engineer Waterways Experiment Station, Vicksburg, MS.
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