up te
ofc
ere:
"” fathoms for about 2 months (Reference 33). The operational and theoretica: aspects
ving have been set forth by SIO in its report on Operation Redwing activities Ref-
However, the anchoring problem as it existed for Redwing was complicated in this
operat... jy the stringent limitations on accuracy of placement, by the fact that mooring had to
be effected on steep bottom slopes (Table 1.1), and by .he increased horizontal drag forces due
to the larger Sail area of the coracles.
Although errors in placement as large as 2! nautical mile could be tolerated on past opera-
tions, a consideration of predicted radiation fields (Reference 22) and cloud diameters (Reference 17) for Operation Hardtack indicated that on each event approximately 70 percent of the
total coracle array had to be more accurately located. Despite operational limitations on the
accuracy of placement, the project attempted to keep the placement error within +300 feet, an
error representing nearly half the predicted distance between upwind and crosswind isodose
contours (Appendix F} differing by one order of magnitude. Other factors requiring high accu-
racy of placement were the estimated arrival time of the base surge, which influenced the
dropping time of the underwater detectors, and the predicted magnitude of underwater snock
(References 23 and 68 through 70).
Consequently, the nominal positions shown in Table 1.1 were intended to represent the actual
poSition of the coracle relative to surface zero, and not the point of contact of the moor with the
bottom. Therefore, the excursion of the subSurface floats was calculated, using experimentally
determined currents for the area in question, and the probable position of the coracle relative
to the subsurface float was estimated from known coracle parameters. The most desirable
point of bottom contact was thus estimated and used in the installation of the deep moors.
2.3.1 Calculation of the Deep Moor. Briefly, the basic principle of deep anchoring is the
reduction of the horizontal excursion of the moored element by the application of vertical tension to the mooring cable. Cable tension is obtained by means of a submerged float placed so
that the orbital motion is less than 1 percent of the surface waves.
.
The buoyancy of this float is adjusted so that maximum cable tension {s obtained without
reducing the net reaction of the anchor with the bottom below the total horizontal drag forces
for the complete system. Buoyant mooring lines are used above the Subsurface float to damp
periodic motion due to surface waves and to reduce abrasion at the junction with the surface
element. The entire system is determined by a series of graphical approximations, and the
exact excursion of the submerged float is determined by iterative vector addition from the
bottom to the surface.
All proposed deep moors for Hardtack were therefore calculated using known coracle characteristics and the following regional information:
1.
Bottom: On the ocean side of Eniwetok Atoll, the steeper portions of the atoll slope are
hard, barren, coralline ridges whereas the flat plateau extending southwest is thinly covered
to a depth of about 24, feet with coral sand and debris interrupted by occasional large coral
blocks fallen from the atoll] slope (Reference 71). Neither surface affords sufficient loose
bottom material for a Danforth anchor to develop maximum holding power. The coralline
ridges run at right angles to the normal direction of wind and current in the region, however;
thus, considerable holding power due to fouling was expected. The lagoon bottom was assumed
to be the usual thick covering of foraminiferal debris dotted with occasional coral heads.
2. Currents: Normal surface currents in the area were assumed to be 1 knot in the Wahoo
target area and 0.4 knot in the Umbrella target area (Reference 53). The idealized subsurface
currents used are given in Table 2.2. Tidal currents were not considered in the calculations,
since all proposed mooring sites were sufficiently distant from the Shoreline.
3. Wind and Waves: Surface winds of 15 to 25 knots are considered normal to both target
areas at the time of the proposed shot schedule. Ocean waves in the lee of Eniwetok Atoll were
expected to range between 3 and 8 feet in height with periods from 2 to 9 seconds (Reference 53).
Long swells reaching the atoll from southern winter storms were not considered important.
Wave conditions within the lagoon were expected to be worse because of a short chop with heights
67
b3