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P. K Rockwell.

Deep ocean cable burial concept development online

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for many -years and lately in ocean bottoms. This section discusses the
variety of techniques used to accomplish burial tasks, examines their
advantages and disadvantages, and, where applicable, references burial
systems using these techniques underwater.

Trenching /Excavat ing

Trenching and excavating are the most common methods used for install-
ing buried cables and pipelines. Typical equipment used includes backhoes,
chain /bucket trenchers, and excavating wheels. In the mid-1960s, a Belgian
firm modified an excavating wheel trencher to bury 600 feet of power cable
at depths to 40 feet in the river Scheldt [5]. The trencher was capable
of excavating a 5-foot-deep, 20-inch-wide trench at a rate of 150 ft/hr.
About the same time, a conventional backhoe was modified for underwater
use that could excavate 200 feet of 4-foot-deep trench, 18 inches wide,
in a day. Recently, a commercial cutter wheel trencher was modified and
used in 120 feet of water to trench cable in sandstone. CEL recently used
a similar trencher in coral (Figure 2).

Trenchers such as these are attractive in that they can be used in
material as hard as granite. Wheel and chain/bucket trenchers can be
equipped with cable feed mechanisms that allow placing the cable while the
trench is being excavated. Backhoes require a three-step operation - trench-
ing, placing the cable, and backfilling. In soft or sandy materials, a
means to keep the trench from slumping in must be provided until the cable
is installed. The major drawback for the trenching /excavating technique
is that it is an inherently slow process. Supply power, usually in the
50-to-150-hp range, is well within the range considered feasible for deep-
ocean cable burial.

Plowing

Plowing in cables and small -diameter pipelines was developed princi-
pally to increase the efficiency of installation. The cable can be in-
stalled through a feedshoe that immediately follows the plowshare. Little
or no surface restoration is required since very little earth is forced
out of the slot. Plowing cables has been proven feasible for deep -ocean
cable burial by the Sea Plow (discussed in the Introduction) and by two
Japanese firms that have developed plows. Repeater handling has been accom-
modated by lowering auxiliary plowshares to widen the ditch, or by plowing
a repeater-sized ditch over the entire cable route. The hardware required
for plowing cables is relatively simple, a particularly attractive feature
for deep-ocean application.



Table 1 . Specific Operational Requirements of Cable Burial System

Threat

Depth of operation, D (fm) 5 through 1,000

Depth of burial (ft) 6 if D <20 fm

3 if D >20 fm

Environment

Sea conditions NORMAL SEVERE

Wind speeds (kt) 11 to 16 40

Significant wave height (ft) .... 6 to 7.5 12
Periods (sec)

Average 4.8 to 5.4 6.8

Swell 6.8 to 7.6 9.6

Average wave lengths (ft) 79 to 99 158

Currents (kt) 0.2 to 2 5

Soil Conditions

Shear strength (psi) 0.18 to 2.2 4 to 7

Bulk unit weight (pcf) 78 to 116

Angle of internal friction (deg) .... 30 to 42 —

Slopes (deg) 5 to 1 20

Operations

Speed of advance (kt) >0.5 if D <20 fm

>1 if D >20 fm

Cables to be buried:

Type Caged armor coaxial, 21

Quad, SF, SD

Minimum bend radius (ft) 10 (21 Quad)

Range of sizes — OD (in.) 0.66 to 4.41

Length of cable run (nm) up to 1,000

Repeaters to be buried SB repeater SD repeater

Weight

in air (lb) - 636

in water (lb) - 353

Length (in.) 288 41.5

Diameter (in.) 6 13




Figure 2. Trencher modified for shallow-water coral trenching.

The disadvantages associated with plowing center on the high force
required to penetrate the soil, both vertically and horizontally. In order
to effect initial plow penetration, and to keep the plow in the soil,
ocean plow machinery has been very heavy (19 to 23 tons). To support the
plow on the seafloor, large skids are used. The skid drag coupled
with the force required to plow can be as high as 100,000 pounds. Since
deep -ocean plowing systems are towed from the surface, the high towing
force, high weight, and slow speed of operation impose requirements on
the surface support ship that are not easily met . The high drawbar pull
requirement was recognized as a problem for land cable plows when the
undergrounding of services for older residential communities was increasing
Tractors, required because of their high drawbar pull capability, caused
surface damage which had to be repaired. Analytical and experimental in-
vestigation of vibrating the plowshare showed that up to a 99% reduction
in drawbar pull could be achieved [6-14], allowing the use of smaller,
rubber-tired machines. Roughly half of the total power requirement is
supplied to vibrate the plow, and the other half for running the machine.
This approach worked well on experimental plows, and now most major equip-
ment manufacturers supply vibratory plow equipment. To date, vibration
has not been employed for deep -ocean plows.

Water Jetting

Water jetting is used mainly for burying offshore oil pipelines.
The jetting machine straddles the pipeline and extends into the seafloor



10



to the desired depth of burial. Water or an air-water mixture is supplied
to the machine from the surface. These systems are generally high-flow,
medium-pressure systems (20,000 to 30,000 gpm at 1,000 to 2,500 psi) that
are towed along the pipeline [15]. The jets break up the soil, and
then air or water eductors lift the soil/water mixture out of the trench.
The pipeline settles into the trench after the machine passes, and natural
action eventually backfills the trench. The main disadvantage to jetting
a trench for the cable to settle in is the large amounts of power required.
Comparing four operational pipeline jetting systems working at capacity,
the average power supplied per unit excavation rate is [17,18,19,16]:

p ^ 11.6 hp*
avg ft /min

Using this power -excavation rate density figure for a deep -ocean cable
burial system would require over 1,000 hp. These systems normally operate
at 5 to 30 ft /min (1 kt = 101 ft /min) and work best when guided by a stiff
pipeline. They also are constrained to work in a relatively firm soil so
that the excavation will not fill in before the pipeline settles into place.

Analytical studies for pure jetting (i.e., where no equipment pene-
trates the seafloor) have shown that the power-excavation rate density
can be as low as 0.4 hp/ft^/min [20]. Using this figure, 40 hydraulic
horsepower would have to be supplied to jet a 3-foot-deep, 4-inch-wide
trench at a speed of 1 knot. No information was encountered which discussed
the effect of depth of cut and speed of advance on power requirements.
Pure jetting is a simple technique that has been used with some success
by the Pisces Submersible and the Alcoa Seaprobe. Disadvantages of pure
jetting are that (1) the amount of material which must be excavated depends
on the angle of repose of the soil, (2) there is no positive means of en-
suring the desired burial depth, and (3) backfill depends on re-
sedimentation of the excavated soil.

Jet Plowing

As the name implies, jet plowing combines the features of both water-
jetting and conventional plowing. This technique has been used quite
successfully by the Harmstorf Hydro jet and Aquatech cable plow for
shallow water and river crossing [17-25]. In essence, the water jets
loosen the soil in front of the plowshare, reducing the frontal resistance
on the plowshare. The soil is kept in suspension until the plowshare and
cable guide pass, whereupon it settles. Very little soil is actually remov-
ed from the ditch, and no backfill is required. The Harmstorf unit is
equipped with a vibration means to help break up competent soil. Jet plows
are usually pulled with winches from a barge or from shore. Total power
required ranges up to 1,500 hp. These systems historically have required
supervision and inspection by divers, which is not to say they cannot
be redesigned to operate without first-hand supervision.



In Reference 26, this function is referred to as Nominal Overall
Specific Energy /in /lb
in. 3

11



Dredging

Dredging, which combines a rotating cutting head and suction pump
for spoil removal, is a very effective means of removing soil. Two similar
devices, the Mole [27] and the Gopher [28], are pipeline machines that
straddle the pipe and have mechanical cutters on each side of the pipe
angled towards each other. The cutters dislodge the soil which is then
removed by a suction dredge pump. The Gopher also has water jets and air-
lift pipes to help remove the spoil. The most technically advanced dredging
system for burying pipelines is currently under development by Tecnomare
(Italy) [29] . It is a tracked crawler machine with two dredge cutters
mounted on articulated arms. The system may be programmed to dredge a
prearranged path, or it may be manually controlled from the surface. Dredge
spoil is pumped to the rear of the machine to bury the pipeline after
it has settled in the trench. The system can be made neutrally buoyant
and is supplied from the surface with 1,300 hp. Dredging is a proven under-
water excavation technique, but generally requires large amounts of
power, excavates more soil than necessary for burying a cable, is slow,
and does not lend itself well to backfilling.

Fluidizing

Fluidizing is a technique where water is pumped into the soil at
such a rate that as it flows out of the soil, the individual soil particles
are buoyed up by the water. The soil /water mixture achieves a fluid
or ''quick'' condition which will not support applied shear forces.

Shell Laboratories (The Netherlands) has developed a fluidizing system
for burying pipelines [30] . The soil is fluidized under a predetermined sag
length, and the weight of the pipe and fluidizing device causes the
pipeline to "sink" into the fluidized soil. This technique works in
sandy (noncohesive) soil, but to date it has been stymied by cohesive
(clay) soils as the intergranular forces cannot be overcome and the
soil will not fluidize.

Related Techniques

Other techniques which have been used or proposed for burying cables
and pipelines include cavitation cutting, high-pressure water jetting,
directional drilling, and piercing tools.

Cavitation cutting is basically a forced erosion process that depends
on the formation and violent collapse of bubbles in a fluid. The cavitation
erosion is caused by the shock wave produced when the bubble collapses,
and the energy density is sufficient to erode materials such as rock
and metal. The intensity of the cavitation, and, therefore, the penetration
rate, increases with hydrostatic pressures [31]. Cavitation cutting
development is still in its Infancy, and acoustic transducers powerful
enough to produce the necessary threshold energy levels for high ambient
pressures have not been developed. This technique produces localized
energy densities effective for drilling through rock.



12



High-pressure water jetting, or ''water cannon,'' is a technique that
has particular application for fracturing rock. To generate high pressures,
a rapidly moving piston impacts on a slug of water and extrudes it through
a nozzle, producing a very high velocity water impulse. The water impulse
jet of a prototype underwater unit used for cleaning scale from steel is
about 1/2 cm in diameter, and the device requires 250 hp [32]. The applica"
tion of high-pressure water jetting or cavitation cutting for high -volume
excavation in soft materials has not been reported.

Directional drilling is a technique reported on by Valent [33] for
installing the nearshore end of a cable system. Its attractive feature
is that a shore -based drilling rig can drill under the surf zone and rocky
areas to a distance offshore where nearshore effects have dissipated.
Piercing tools, such as the Pneuma Gopher, have been developed to dig
their way from one point to another when trenching is undesirable, such
as under a busy highway. Both of these techniques are suitable for producing
a relatively short path through which cabling can be led after the hole is
made.



CONCEPT DEVELOPMENT

The major problem areas associated with existing ocean cable burial
systems are the machine/soil interface, and the machine/surface support
control and propulsion interface. These problem areas are quite closely
related; for example, the large forces experienced in the machine /soil
interface cause problems in propulsion and control for the support ship.
To approach a solution to these general problem areas, three major catego-
ries were analyzed, and the resulting information combined in various
ways to formulate concepts. These categories are:

1 . Excavation Subsystem

2. Propulsion Subsystem

3. Running Gear Subsystem

To provide a common basis for comparision of the various techniques
of burying cables, a set of parameters was selected from the specific
operational requirements which represent maximum normal operating condi-
tions. Each concept was analyzed to determine the power required and
the resistance force produced by operating under these conditions. Maximum
allowable target values for size, weight, and force required were also
assigned as they must be used for some of the power and force calculations.
The values are shown in Table 2, and power conversion efficencies are
shown in Table 3.

The burial machine weight and size were selected as desirable maximum
values to allow convenient handling from ships of opportunity. The size
affects water drag on the system and bottom stability. Machine weight
impacts on the running gear /soil interface forces and allowable ground
pressure. The machine speed and current profile create a drag force on the



13



umbilical cable, which is at maximum when the current profile adds to
the system speed at maximum operating depth. The trench dimension is
typical for most of the burying operations. The system must slow down
or consume more power for repeater burial or deeper burial. Finally,
the soil characteristics have a large impact on power and force. For
tougher materials, the system must slow down or consume more power, and
for weaker soils the speed can be increased or the power reduced.

Several excavation techniques that have been used or suggested for
burial of objects in the seafloor are inappropriate for deep-ocean cable
burial because of their inability to meet some of the basic operating
requirements. Therefore, they could be eliminated without performing a
detailed power analysis.

FVu'ld'lzat'ion . The fluidization process is not applicable since it is
intended for cohesionless materials (i.e., sands) and depends on the ab-
sence of intergranular attractive forces for successful operation. Recent
tests with Shell's fluidizer showed that the system stalls when clay
is encountered [34]. Many of the seafloor soils which will be encountered
are cohesive, and switching burial equipment in mid-operation is not
an acceptable solution to the deep-ocean cable burial problem.

Blasting. Blasting is not an appropriate method of burying long cable
runs in sand and clay since it is best used for fracturable materials such
as rock and coral, is a batch (rather than continous) process, and to
date, requires divers to prepare blast holes and set the charges.



Table 2. Design Parameters

Burial machine weight 1 tn (max)

Burial machine size envelope 12 ft wide x 25 ft long

x 10 ft tall (max)

Motion resistance force 5 tn (max)

Power 500 hp (max)

Speed 1 kt (101 ft/min)

Current 2 kt at surface

at 300 ft

Trench dimensions 36 in. deep x 4 in. wide

Umbilical cable 3-in. diameter

6,500 ft long



Soil characteristics Clay



Undrained shear strength,

S = 4 psi
u

Bulk density,

p = 100 Ib/ft^



14



Input




Table 3. Power Conversion Efficiencies



0.39



Cavitation Cutting and Eigh-Pvessicpe Jetting. Both of these techniques
use the principle of focusing moderate amounts of energy to achieve ultra-
high energy densities to cut, fracture, or erode materials such as rock
and metal. As such, they are not suitable means of excavating a trench
in soft materials. High-energy density water jets achieve 100,000 to
5,000,000 psi in a jet 1/16 inch in diameter. The optimal cutting range
is 20 nozzle diameters, and the jet pressure should be at least 10 times
the material strength [35]. Extrapolating this information to digging
a 3-foot-deep trench in a typical (4-psi) seafloor soil suggests a nozzle
size of 1-1/2 inch and jet pressure of 40 psi (minimum). Thus, it can
be seen that the high-pressure water jetting technique provides nominally
2,500 times the pressure required to cut seafloor soil, and the jets
are so small that only a localized area of soil would be excavated. Extra-
polation of high-pressure water jet theory to soil excavation leads to
standard (low-pressure) jetting techniques. Cavitation cutting results
in pressures and cutting volumes similar to high-pressure water jetting.

Direct Insertion. Using this method, the cable is simply forced into
the soil with, for example, a heavy wheel. The wheel must be forced through
the soil while penetrating 3 feet into the bottom. Preliminary analysis
showed that, even if the wheel were water lubricated such that an 80%
reduction in frictional resistance could be attained, the forward force
required to push the wheel through the soil ranges from 4,400 pounds for
a 4-inch-thick wheel to 7,900 pounds for a 1 6-inch-thick wheel. In addition.



15



the force on the wheel required to achieve 3 feet of penetration ranges
from 7,000 pounds to 28,000 pounds. This approach was eliminated because
full penetration is not assured, tracking the cable is a difficult process,
and the probability of damaging the cable is very high since the cable
could be forced into a rock or other hard surface.



SUBSYSTEM ANALYSIS

The following sections present force and power analyses and discuss
the subsystem candidates which appear to be most appropriate for a deep-
ocean cable burial system.

Excavation Subsystems

Plowing . Plowing cables into the soil is a relatively simple and
quite effective means of burying cables. Plowing has been used extensively
and very successfully on land and has had some success underwater. The
basic problems with cable plowing are the high force required to move the
plow through the soils (drawbar force) and the force required to achieve and
maintain plow penetration. Appendix A and Reference 36 discuss drawbar
force predictions for a plowshare.

Appendix A shows that the total drawbar force required to move a
plowshare through the soil is larger for clay than for sand, and is
velocity dependent .

F = C F + F
TOT V s I

where FrpQ^ = total resistance

C = a velocity coefficient determined from Figure A-1

F = force due to static soil resistance

^ =SA+SNA^

us u c f

F = an inertial term

= (1/2)Ps Aj Cjj v2

S = undrained shear strength

A = side area of plow

s ^

A^ = frontal area of plow

N = dimensionless coefficient '\' 1
c

P = soil mass density

C = drag coefficient Oi 1.5

V = velocity



16



To guide the cable to the bottom of the 3-foot ditch without exceeding
a minimum bend radius of 5 feet, the feedshoe/plow length must be 10
feet (Figure A-2). Using the design parameters discussed previously,
the total force required to pull the plowshare at a speed of 1 knot is
F = 44,000 pound. This formulation for predicting drawbar pull compares
favorably with results published for Sea Plow III [37] .

References 8 through 14 present analytical and experimental results
of using plowshare vibration to reduce the drawbar force required to move
the plowshare through the soil. In particular [13], it has been shown that
for vibratory plowing, the use of a raked, wedge-shaped blade with machined
grooves (Figure 3) reduced the average horizontal plowing force in a silty
sand by 98 to 99% when the plowshare was vibrated at a frequency of 20 to
40 Hertz at an amplitude of 3/8 inch. In addition, vibrating the plowshare
aids in achieving and maintaining depth of penetration. In the case above,
a 95% reduction in drawbar force gives



TOT



= 2,200 lb



reduced



which is well within the target requirement of 10,000 pounds. (Note:
Other contributions to drawbar force will be discussed later.) This reduc-
tion in drawbar force will impact significantly on the support ship power
requirements for towing, and ease control problems.

The power required to vibrate the plowshare is also an important
consideration. Appendix A shows that the power required to produce vibra-
tions is 14 hp . Water drag and added mass effects on the power required
for vibration are negligible. To move the vibrating plowshare at 1 knot:



Figure 3. Raked plowshare with
machined grooves.




17



P = Fv = (2,200 lb) (1.69 ft/sec) = 7 hp

^Total = 21 hp
Without vibration,

P = F V = (44,000 lb) (1.69 ft/sec) = 135 hp
total '^

So in addition to a significant decrease in drawbar force, vibrating
the plowshare also results in a reduction in net power requirements.

Trenching . Two types of trenchers are considered, the endless
chain-bucket trencher and the cutter wheel trencher. Both types of
trenchers normally rotate such that the cutting action is in the direction
of machine travel (upmilling) (Figure 4). The machine, then, must supply
sufficient drawbar force to overcome the cutting resistance. If, however,
the trenching means rotates in the opposite direction (climbmilling) ,
cutting resistance acts to push the machine forward, and to lift the
device out of the trench. Appendix B presents a force and power analysis
of both chain and wheel trenchers.

For a wheel trencher or chain trencher, the bucket comes in contact
with the soil and, when forced through the soil, fails it in a manner
similar to the plowshare discussed previously. The total force required
to cut the soil is



S.. A,



F„^„ = S A^ N
TOT u f c



where the first term represents the soil bearing resistance force, and
the second a shearing resistance force [38] . N^ in this case is a dimen-
sionless factor 2^ 3 because of free surface effects.

For a chain-bucket -type trencher with the boom angled 60 degrees
below horizontal the analysis in Appendix B shows that the maximum power
required to excavate a 4-inch-wide, 36-inch-deep trench at 1 knot is

P = 108 hp
max

For upmilling the forces on the unit are

F^p = -2,675 lb

F__ = -2,620 lb
FWD

With the system operating in the opposite direction (climbmilling), the
forces are

F = 2,560 lb
^FWD = 2,620 1b



18




L^^




/?^3^



(a) Upmilling.



(b) Climbmilling.



A. Chain Bucket Trencher.




(a) Upmilling.



(b) Climbmilling.



B. Wheel Trencher.



Figure 4. Trenching modes.



19



and the power requirement is the same. For the cutting wheel trencher,
a parallel analysis results in



For upmilling



and for climbmilling



P = 94 hp

max ^



UP



FWD



•3,515 lb
-4,430 lb



P = 94 hp
max



UP



3,350 lb



F = 4,430 lb
FWD



Climbmilling appears attractive in that, for the same power as upmilling,
a drawbar assist of 2,600 to 4,400 pounds is available to overcome the
running gear /soil interaction forces, cable drag, and water drag on the
machine. The machine must weigh greater than the upward force to keep the
trencher from digging itself out of the trench. However, the incidence of
stiff clays or rocks may cause the cutting wheel or trencher to climb out
of the trench, resulting in instability of the machine and possible
damage to the machine and the cable. Shock absorbing, braking, and possibly
other control systems must be incorporated into the trencher. It may also
be necessary to direct a water stream on the buckets to loosen and remove
trenched soil.

For upmilling, the system can be very light (neutral if desired)
since the cutting force provides a significant downward force, but the
machine must provide 2,600 to 4,400 pounds of drawbar force in addition
to the other forces acting against the system's forward progress. Power
requirements in both cases are high due to the high digging rate required
for a 1 -knot speed of advance.

Water Jetting. Although water jet excavation is the most common
means employed for pipeline burial, very little analytical or experimental
information was encountered in the literature. References 39 and 40
discuss research performed on jetting in sand. The trench depth is related
to the jet flow parameters by



= C,



Q(p + C2)



1/2



20



where d
Q
P



trench depth

flow rate


2 4 5 6

Online LibraryP. K RockwellDeep ocean cable burial concept development → online text (page 2 of 6)