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OVERVIEW
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.
COMPONENTS
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.
Figure 2 System
recovery |
A 0.6 m high "roll cage" is made from 2 in. (5 cm)
square aluminum tubing to protect the instruments. The frame
is also the anchor for the surface buoy, so it must be heavy
enough to maintain position under the expected conditions. To
hold the surface buoy in high-current, surf-zone regimes, eight
trapezoidal sections of 1 in. (2.5 cm) thick lead plates are
bolted to the frame, bringing its total weight to about 1,350
kg. Brackets for individual instruments are bolted to the base
of the frame. Typically, multiple instruments are used in one
deployment; their combined weight can exceed 100 kg. |
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.
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Lift Line: The lift line is a 30 m length
of the same Spectra line used for the mooring line with soft
eyes. At the upper end, a special hi-tensile steel shackle, a
piece of standard hardware for the CH-47, makes the connection
to the helicopter’s cargo hook. |
Figure 3 Surface
Buoy and Stopper Buoy |
Figure 4 Grappling
Hook |
DISCUSSION
Several important points should be considered when planning
this operation.
The ability to measure coastal waves and
currents in situ at multiple sites, under extreme wave
and current conditions, has been demonstrated at the Mouth of
the Columbia River. However, meteorologic restrictions prevent
this from being an all-weather option. This is strictly a VFR
(visual flight rules) operation - a ceiling of at least 500 m
is required. Though it can fly in stronger winds, the CH-47 cannot
start its engines in winds above 30 knots. If the winds exceed
60 knots, the aircraft cannot remain outdoors, but must be secured
in a hanger.
With practice, each wave measurement should
require 30 - 45 minutes. If a landing area is available in the
vicinity, 4-5 sites per fueling are possible. Refueling takes
about as long as one wave measurement.
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The process is logistically challenging. Functioning aircraft
and instrumentation have to coincide with personnel schedules
and operational flying conditions for a successful day of measurements.
Adequate time must be allowed for crew training and aircraft
maintenance on top of the "normal" field delays. A
full week should be allowed for most data collection experiments.
The expertise of the air crew is critical to the success
of the mission. Refinements in the hardware and procedures can
reduce the reliance on crew skill, but precise hovering and positioning
over rough water will always push the envelope of piloting skills.
More so than most field operations, advanced planing and
design of every component are critical to success. Literally
every nut and bolt must be examined in light of its marine, aeronautical,
safety, and measurement function.
SUMMARY
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.
ACKNOWLEDGMENTS
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.
REFERENCES
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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