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The history of mooring in deep water is limited. It generallyis considered, for example,
that 2 depth of 100 fathoms limits the installation of military moored mines. Cable-laying craft
casionally have moored in water as deep as 2,000 fathoms and commonly installed temporary
of rkers at depths somewhat shallower than this.
Usually, they attempt to isolate the cable be-
ween two shallower points, however, and not to work at extreme depths.
Oceanographic vessels
nave beet moored in depths up to about 2,900 fathomis using their tapered dredging cables. Both
Scripps and Woods Hole oceanographic institutions have dropped slack moorings in water of all
depths. Such moorings have not been particularly useful, principally because their great scope
has made it difficult to ascertain whether or not they were dragging (thus limiting their use as
reference navigational markers and current measuring platforms) and because of their short
fe engendered by surging and chaffing.
These two difficulties can be met by the installation of taut moorings, where the principal
tensile stresses are carried by a submerged float belowthe limit of the wave motion of sea and
swell. Scripps installed about four such moorings on the 700-fathom seamounts to the north of
Eniwetok prior to Shot Mike, Operation Ivy (Reference 11).
These platforms bore wave recorders,
put it was evident that they also acted as fallout collectors, although no fallout instrumentation
was installed thereon. During Operation Castle, Scripps also had experience with mooring skiffs
in the lagoon for long periods of time and had solved some of the chaffing problems associated
with this type of mooring subjected to extensive wave action. These two areas of experience
were combined for Operation Wigwam, in which about seven skiffs were taut moored in approximately 2,000 fathoms. These skiffs bore strobe lights and were installed for use as navigational
aids. They survived heavier weather than that for which they had been designed and performed
their task, despite a high mortality from engagements with some of the heavier elements of the
United States Navy.
On the basis of these experiences, it was decided to moor a moderate number of platforms
in the deep ocean to the north of Bikini Atoll during Operation Redwing for the documentation of
fallout.
4.3 THEORY
One of the technical problems of installing a taut mooring in water more than 2,000 fathoms
deep is shown in Figure 4.2. Here is depicted the ultimate tensile strength necessary for a steel
wire to be used at any depth in the sea when bearing an additional load equal to its weight. Also
shown is the percentage of ocean area at a depth greater than the ordinate. Using the allowable
depth of mooring as a criterion, it is apparent that wire with an ultimate strength of 100,000 psi
can be safely used to a depth of about 1,700 fathoms, or in about 30 percent of the ocean.
A
wire with an ultimate stress of 180,000 psi, however, can be employed to 3,000 fathoms, or used
in 99 percent of the sea area. The wire used during Redwing had an ultimate strength of about
260,000 psi.
The above relates solely to the quasi-static stresses produced in lowering the mooring wire.
Other dynamic stresses become important as soon as the anchor reaches bottom. Also, an
allowance must be made for weakening of the wire by handling and by corrosion.
The area presented by such a wire to the horizontal drag forces can be quite large. For example, 15,000 feet of ¥,-inch wire, as used during Operation Redwing, presents a projected
area of about 160 ft? of form drag area, or about that of a large barge. Fortunately, water velocities at great depths are low; hence, the large area presented is not a great problem.
Such horizontal forces must be resisted at the anchor; thus, as horizontal forces increase,
the anchor weight and the lowering stresses must be increased. Also, the excursion of the mooring and changes in float submergence are functions of the tensile stresses and the horizontal
drags, Hence, the strength-to-drag ratio of the wire is important. This can be expressedas:
R=
orR » st for constant range of Cd
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