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Calif.) Naval Civil Engineering Laboratory (Port Hueneme.

Concepts for drilling and excavating in and below the ocean bottom online

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Walking Barge. The Walking Barge was built as an amphibious cargo
carrier and tested at NCEL shortly after World War 1 1. This large floating
barge was divided longitudinally into three segments or pontoons, the center
one with a bottom area equal to the sum of the bottom areas of the two side
pontoons. When the barge was driven ashore and grounded, it proceeded up
the beach by resting on the center pontoon, moving the two outside pontoons
up, forward, and down until the weight was transferred to them. Then the
center pontoon moved up, forward and down to complete the walking cycle.
This was repeated to provide forward motion over the beach (Figure 38).

At first glance, an apparent
advantage of the walking barge was
its large, flat bottom which provided
a large ground contact area and low
ground pressure. In operation,
however, all the weight was usually
transferred to one-half the bottom
area, thus doubling the apparent
ground pressure. On a sandy beach,
this was no problem, and the barge
moved steadily forward.

In tests on the mud flats at
Point Mugu, the walking barge sank
several feet into the mud but stayed
afloat and moved slowly forward.
After a few dozen feet of progress,
however, the weight-bearing surface
sank so deep in the mud that the
other pontoon, or pontoons, dragged
in the mud surface on their forward
travel; this counteracted the rearward
drag and caused the unit to stop its
forward movement. It was proposed
to put hinged flaps on the bottom of
the pontoons which would fold flat
against the bottom during the pontoon's forward movement and drop down
into a vertical position during the pontoon's stationary period to prevent
their slipping backward. This proposal was never carried out.



P




Figure 38. Walking barge principle
developed at NCEL in
early 1950's.



47



Tractive Effort

The force developed in the soil to propel a vehicle is called tractive
effort. This force is made possible by the shearing strength of the soil (Bekker,
1956, p. 255).

In Equation 8, it was shown that the soil shearing strength, s, is equal
to the soil cohesion, c, plus the normal pressure of the shear plane times the
tangent of the angle of internal friction, 0:

s = c + ptan0 (8)

The shear occurring at the ground contact area between the soil and
a vehicle, at the point close to a stall, is a large-scale replica of the shearing
process produced in the laboratory by means of the shear-box under the
conditions expressed by the above equation.

Figure 39 shows this analogy and implies that the horizontal tractive
force, H, and vehicle weight, W, acting upon the ground may be related by
the same equation as above,

H = Ac -I- Wtan0 (12)

where A = the shear area, which is approximately equal to the track ground
contact area.

In a purely frictional soil such as dry sand, c = and the tractive effort
becomes

H = Wtan0 (13)

In this case the tractive effort is proportional to vehicle weight, W.

In a purely cohesive soil such as soft, saturated clay, however, where
= 0, we have seen that s = c (Equation 9) and only the soil cohesion contri-
butes to the shear strength. In this case, the tractive effort becomes

H = Ac (14)

Here the vehicle weight does not enter the formula and the tractive effort
basically depends only on the ground contact area, A. Thus, the criterion
for track design for use on the saturated plastic soils found on the ocean
bottom, is track size; the larger the ground contact area, the better the
traction.




I H (shearing force)



approximate line
of shear



approximate shear
area A




(a)



I ^ 1— approximate line



(b)



Figure 39. Graphical analogy between laboratory soil shear box (a) and a
track on undisturbed soil, (b).



It is extremely difficult to predict tlie traction that will be available
in a saturated submerged soil. Although the shear strength of an undisturbed
soil sample may be determined, the effect of agitation caused by a vehicle
moving through the soil— water interface is to mix the soil and water. In the
case of soft clays whose particles are very small and light, the whole mass of
disturbed soil can be mixed with the water and become liquid. In this state,,
the cohesion is destroyed and no traction will be available. Hydromechanical
methods may then be the only means of locomotion under these circumstances
(Bekker, 1956, p. 142).

Propellers produce thrust, or tractive effort, by changing the
momentum of the fluid in which they are submerged. A vehicle working
on the ocean bottom using propellers for propulsion would not have to
depend on the highly unpredictable bottom soil for movement. Another
advantage of a submerged propeller is that it can be made to produce a
dependable amount of thrust in any direction, including the vertical.

Another hydrochemical device for producing thrust is the water
jet. This is essentially a pump that creates a water jet and by so doing has
a thrust exerted upon it which is the propelling force. In fact, a propeller
does the same thing and thus is one form of water jet propulsion (Streeter,
1948, p. 114).

The propelling force, F, of a water jet unit is



pQAV



115)



in which p is the fluid density, Q is the quantity of fluid moved in cubic feet
per second, and AV is the absolute velocity of the fluid.



M9



It can be shown that the theoretical mechanical efficiency, e^, of a
water jet system is the same as that for a propeller and can be expressed as

^ (16)



1 +

2V,



where AV is the absolute fluid velocity in the jet and V^ is the speed of the
boat or vehicle being propelled.

Other things being equal, AV/V^ should be as small as possible. This
indicates that the water jet is most efficient on a high-speed boat where V, is
large. For low speeds, however, V^ must be very small. At any given speed,
the resistance force, F, is determined by the body and the fluid in which it
moves; therefore, in Equation 15 for AV to be very smal I , p Q must be very
large. For this reason, a water jet unit on a slow moving vehicle would require
a very large jet with a low velocity discharge. The type of pump best suited
for large flow at small head is the axial flow propeller pump. In effect, a
propeller is an axial flow pump with the pump casing and jet pipe removed.
Therefore, for a slow moving underwater vehicle, propeller propulsion would
be more efficient and lighter than a comparable water jet propulsion system.

A water jet system can be justified for slow moving craft in applications
where a propeller would be vulnerable to damage from collision or grounding.
In the deep ocean, however, there is no reason why propellers could not be
mounted above the deck or protruding from the sides of the craft where they
would not come in contact with the bottom.

On tugs and towboats where high thrust at slow speed is required,
the Kort nozzle is very effective. This is a nozzle or ring of airfoil section in
which the propeller rotates. In effect, it provides a cross between a propeller
and a large discharge area water jet. It has the additional advantage of providing
a protective ring which may assist in keeping foreign objects such as ropes or
cables from becoming entangled in the propeller.

Summary and Future Plans

It is apparent that probably no one traction method will function on
all possible ocean bottom materials. On solid or rough surfaces, large tires
should prove most practical. For most soft bottoms typical of the deep ocean,
tires will not be functional, and some form of low-ground-pressure track will
be necessary. Interchangeability of tires and tracks on the same machine may
prove the best approach.



50



Of the track systems, the low-ground-pressure pontoon with a ladder
grouser system is predicted to be the most successful, and will be functional
on all but rocky bottoms. Ground pressure can easily be adjusted to the soil
conditions using buoyancy.

During the preparation of this report, a single test track conforming
to the conditions listed in the summary was designed and fabricated; the
schematic from the contract proposal is shown in Figure 40.




^ — stainless si



stainless skid plate, track guide and ballast tray
optional: Teflon or Tem-PR-Glas tape
forced water lubrication



adjustable height drawbar



Figure 40. Experimental test track witli water lubricated rubber belt sliding on
stainless steel skid plate (Nuttall, 1970).

The actual equipment is shown in Figure 41 , suspended prior to tests
in shallow water of Chesapeake Bay, Maryland. Results of these early tests
are described in Nuttall (1970a) and in Nuttall (1970b). The unit is being
tested extensively in submerged bottom materials by the Naval Civil Engi-
neering Laboratory, with results expected to be published in a technical
report by or before mid-1971. Preliminary analysis of results indicates
that for bottoms other than sand, the maximum traction that can be
achieved can be developed at low ground pressures and essentially zero
slip, and that higher pressures and greater slip than these minimum values
tend to destroy the structure of the bottom material. The tracks then dig
in rapidly, an undesirable consequence. The cleated track is carried on a
rubber belt which is in contact with the smooth steel bottom. A novel
feature of this design is that water can be forced under low pressure
between the track and the belt, resulting in a very low friction. The
unit, hydraulically powered, is to be tested in a variety of conditions
including shallow water in muds, sands, and loose sediments. Provision
is made for adjusting the ground pressure and measuring the drawbar pull
developed. Optimum use of buoyancy to adjust the total ground pressure
will be incorporated in the final designs.



51




Figure 41. Test track described in Nuttall (1970a) in water at Chesapeake Bay,
supported between the pontoons of WNRE's test rig.



52



BUOYANCY SYSTEMS

The ability to employ suitable buoyancy to support large weights in
the ocean is probably the single greatest plus in working there over terrestrial
locations. On the surface, simple open hulls are sufficient. At shallow depths,
either a pressure- resistant hull or a filling material (probably air) at the local
pressure of operation or higher must be used to prevent collapse. At succes-
sively greater depths, either the pressure-resistant shell must be made stronger
and heavier or the pressure of the gas must be increased. Spherical aluminum
floats are used to a few thousand feet, at which point the shell thickness nec-
essary is such that the net buoyancy is greatly reduced and the buoys are no
longer economically attractive. Spherical and cylindrical shapes typically fail
in buckling from instability rather than by compression when made of ductile
materials. Nonductile materials and those of comparatively low strength per-
form well over certain pressure ranges, particularly if the density of material
is low enough to allow thick walls, which are stable against buckling. Glass is
a particular case of a very strong, nonductile material which actually increases
in strength when under compression over practically the entire pressure range
to be met in the world's oceans (Perry, 1963).

While concerted efforts along the lines outlined above are being made
under other assigned portions of the Deep Ocean Technology project and
other ocean-related research and development tasks, it does not appear that
economically feasible developments will immediately be ready for use. Because
large volumes of relatively low-cost buoyancy materials capable of withstanding
full ocean pressure will be needed to build an adequate low-ground-pressure
chassis for soft bottom materials, a limited effort in development is outlined
here. Features to be sought in order of their importance are:

1. Low cost per unit buoyancy

2. Pressure resistance of completed buoyancy units

3. Low density

4. Ability to withstand rough usage as fabricated

5. Low thermal coefficient of expansion

The so-called syntactic foams, in which very small beads of either
glass or a strong plastic are embedded in a strong plastic binder such as epoxy,
have been widely used in small systems and considered for at least one large
application (Bechtel, 1965). They are good in factors 2 and 4, but are very
expensive and marginal in density and expansion traits. Figure 42 (Bechtel,
1965) illustrates the form and application of massive floats to a buoyancy
system for lowering a nuclear reactor to the ocean bottom.



53



bridge crane



gantry



main lowering line -



rigger's stores and
buoy storage -
upper float -



upper pendant with
one loop released



lower pendant

attached to main

float without load



- gantry




retracted beam



reactor clear of ship with
weight on heavy lift system



Figure 42. Massive buoyancy system as
proposed for lowering reactor
in ocean (Bechtel, 1965).




chain assembly



bottom weight



54



All of the above factors but the last may be obvious; a low coefficient
of expansion is critically necessary to keep the total buoyancy system simple.
If the buoyancy material changes volume drastically with depth for any reason,
then the buoyancy will not be constant. If the modulus of compressibility is
low, the net buoyancy decreases rapidly with depth. Acting in the same direc-
tion would be a loss in volume with cooling, which can be expected to be an
important factor in all but polar seas. Whatever the surface temperature of
the water, in most situations the deep water will be colder, and flotation
materials will lose further buoyancy on cooling at the bottom. This may
take considerable time but will eventually occur. Some form of trimming
control on the bottom might be necessary but would be complicated. An
alternative which is only slightly less attractive would be to precool the mass
by soaking in a cold bath at the surface.

A method of improving wood to obtain the desired strength charac-
teristics for fabricating buoyancy shapes has been proposed (Beck, 1967),
but to date the necessary development and testing has not been done. Of
the 13 references in the proposal, the most important for discussion are
Barnes (1964) and Kukacka and Manowitz (1965). The former gives an
excellent condensed account of the status of wood as a production material.
The latter discusses the change in compression strength and hardness which
may be accomplished by impregnating hard woods with highly penetrating
monomers and then irradiating them with gamma radiation. Figure 43
(Barnes, 1964) shows a greater than two-fold increase in strength with heavy
impregnations.











.


/






/


y^ ^


olyvinyla


cetate




A


olymeth^


^Imethacr


tflate




y^


y







10 20 30 40 50 60

Parts polymer/I 00 parts wood

Figure 43. Compressive strength of sugar maple filled with plastic monomers and
polymerized by gamma radiation (© Barnes 1964. Used by permission of
Machine Design.)



55



A less costly concept for employing wood in buoyancy shapes is
illustrated in Figure 44. Typically the compressive strength of timber per-
pendicular to the grain is many times less than that parallel to the grain.
For coast-type Douglas fir at 12% moisture content, for instance, the
relative values are 870 to 7,430 psi, respectively, a ratio of over 1 to 8
(USDA, 1955). This suggests the fabrication configuration shown in
Figure 44, in which the fibers are selectively oriented to take advantage
of the anisotropy. This relatively inexpensive wood with minimum water-
proofing of sheathing should be useful at least 12,000 feet in the ocean.
Other even more advantageous arrangements could probably be developed.
For one thing, it appears that some collective reinforcement should be pos-
sible with suitable preferred fiber orientations, so that the measured stiffness
in the complex structures would be greater than calculations based on indi-
vidual components. This phenomenon is referred to as "jamming."

A recent development in buoyancy, not yet fully tested in a pressure
environment, is described by Madden (1970). Figure 45 from that source
shows closely packed spheres formed from brazed hemispheres in a possible
replacement for the well-established honeycomb core material developed for
industrial and aerospace applications. The material offers promise both as a
buoyancy and a structural material.



end grain




Figure 44. Composite wood buoyancy shape, with end grain exposed on all
faces for greater strength.



56




Useful, inexpensive, and
simple static buoyancy systems
appear easily achievable for the
short-term development of a low-
ground-pressure or buoyed chassis
for bottom drilling and excavation.
For the long-term development
program for depths to 20,000 feet,
added complications in the form of
adjustable buoyancy appear to be
justified if it can be developed. One
possible approach is described in a
proposal (Beck, 1964), in which the
goal would be the development of
an active system capable of keeping
a massive system at a predetermined
level, constantly adjusting buoyancy
to achieve this. Some formidable
obstacles can be anticipated, not
the least of which is the continuous
power drain. Small submersibles
represent a compromise in the com-
bination of vertical propellers,
deballasting and ballasting (compressing a ballast tank partially filled with
gas by pumping in seawater), and use of forward motion reacting against
horizontal hydrodynamic surfaces. For the massive earthmoving systems
envisioned here, the problems appear to be less awesome. Vertical accelera-
tions would be less, both because of mass and extended horizontal surfaces.
Power intrinsic to the operation of the machinery would always be available
in large quantity, unlike the situation in small submersibles and in the Buoy-
ancy Transport Vehicle developed by the Navy Undersea Research and
Development Center, Hawaii. Both of these applications use batteries for
power and extensive use of power for ballast control will limit their range.

A final vertical stabilizing influence arises from the proposed vehicle's
purpose — bottom work. The forceful contact with the bottom suggests that
flotation trim will not be critical so long as some portion of the vertical force
is taken in the tool contacting the bottom.

It may well prove that for many bottoms, shear strength will be so low
as to negate the effectiveness of wheels, tracks, belts, etc.; in those cases, a
barge "floating" at the water— bottom interface and reacting through a pro-
peller in the water above would afford the necessary lateral movement (Figure
46).



Figure 45. Top view of expanded

sphere core structure prior
to tests.



57




soft bottom
Figure 46. Concept of a barge-mounted work system for local excavation.



Summary

Adequate pressure-resistant buoyancy systems are available, but
costs for depths to 20,000 feet are inordinately high, suggesting investigation
of new approaches such as benef iciated wood.

Future Plans

No active research and development are planned in the area of
buoyancy, except for limited, informal small-scale pressure chamber trials
of a few simple systems for impregnating and sheathing wood.



SHALLOW DRILLING AND EXCAVATING

The operations described in this section are those which in surface
operations normally would be done by bulldozing, scarifying, scraping, shallow
auger drilling, and loading and removal of the spoil from the site, or using it
for backfill. These are all specific operations requiring great operator skill and,
to achieve accuracy, repeated checking of grade, tamping, etc. There is at pre-
sent no reasonable hope of performing these same functions on the ocean
bottom in the same manner. Fortunately, there appears to be no need to use
established terrestrial methods.



58



The precise excavation foreseen as necessary for ocean-floor sites
will be largely accomplished by a nnachine that will function similarly to a
numerically controlled milling machine (Figure 47). However, instead of
moving the workpiece below the mill with respect to a stationary cutter, it
will be necessary to move the milling head with respect to a movable platform
(Figure 48),* or with respect to a fixed clump (Figure 49). Where preparation
of the immediate area is needed for installation of a prefabricated structure
or other assembly, the massive clump would be lowered with the earth-milling
equipment in place. Upon completion of the excavation phase, the machinery
would be detached and raised to the surface, leaving a precisely formed area
to accept the permanent installation.

In conventional metal milling (Figure 47) cuttings are flushed away
or removed by hand. Soil or rock cuttings, on the other hand, will be immersed
in water and relatively finely divided. It remains only to cut them in such a
way that they can be sucked into the rapidly moving fluid stream and pumped
either into a fast moving current or to a site away from the operation, where
they will form a harmless sediment. Use of water for spoil removal is the big
plus in underwater operations.

At least two evident mechanisms for traversing the cutting head or
mill over the work area are available (Figure 50a and 50b). In the arrangement
of Figure 50a, the cutter would be traversed over an area ahead of the support-
ing barge by movement of tubular ways. A second set of ways would traverse
it at right angles, and the vertical movement would be accomplished through
a vertically moving column bearing the mill, pump and hose as necessary. This
would be essentially duplicating the motion of the ways and milling head of
a vertical mill (Figure 47). From a control standpoint, it would be desirable
to use this arrangement. The standard programs and control techniques for
numerically controlled machines could be used with little modification with
such a machine in conjunction with three-dimensional rectangular coordinates.

Development of suitable controls for the type of machine illustrated
in Figure 50b, on the other hand, would probably require greater effort, as
instructions would have to be translated from the normal engineering rectan-
gular coordinates to a combination of polar coordinates in the horizontal plane
and a linear coordinate in the vertical, resulting in a combination which might
be called cylindrical coordinates. Mathematically, this is not difficult, but the
change does introduce one additional step in instructing the machine. The
resulting machine would be mechanically far simpler than that diagramed in



* See also Figure 46 and related text.



59



Figure 50a, and more flexible. It would not require accurate alignment to
the work site as would the more complicated machine, because its control
system could be indexed to compensate for angular displacement of the
chassis from the desired geographical direction. It could work all around
itself, giving a much greater working area without relocation, simplifying
the control problem further.

An additional simplification in machine development would be
accomplished by adapting a commercially available hydraulic crane as shown
in Figure 51 . However, the control problems would be further complicated
in that this machine would use full polar coordinates — it would see the world
as a sphere. Within limits it would be able to excavate even under itself.

The preceding discussion of the earth milling (or controlled dredging)
concepts by no means proposes to eliminate the possibility of adapting con-
ventional earthmoving techniques and machines to the ocean floor. At least
one remotely controlled bulldozer is known to be available from Japan (Anon-
ymous, 1968). It would be comparatively simple to power a small bulldozer with
individual hydraulic motors (Nuttall, 1970a) on the tracks. The combined
problems of turbidity during the work period, inaccessability for control,
and the inability to drill shallow holes for piling, etc. inherent in conventional
earthmoving makes this approach relatively unattractive in comparison to
milling. Bulldozing is a cut-and-try process relying heavily on the operator's
abilities and the making of successive cuts to achieve grade. The potential for
simple and accurate profiling is obviously not there.

It is a major thesis of this study that accurate profiling of work site
to accept the prefabricated structures is needed for installation of prefabri-
cated installations. In the realm of manufacturing, this type of control is
routinely and precisely obtained with such machines as mills and surface


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Online LibraryCalif.) Naval Civil Engineering Laboratory (Port HuenemeConcepts for drilling and excavating in and below the ocean bottom → online text (page 4 of 8)