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DEVELOPMENT OF A CABLE BURIAL SYSTEM FOR A BROAD RANGE OF SOILS IN UP TO
DEVELOPMENT OF A
CABLE BURIAL SYSTEM FOR A BROAD
RANGE OF SOILS
IN UP TO
2500
METERS OF SEAWATER
J.E.
P.E.
Perry Tritech, Inc.
821
Jupiter Park Drive
Jupiter, Florida
407-743-7000
EXECUTIVE SUMMARY
An ROV deployed post lay cable burial system has
been developed. This system has been
demonstrated to bury cable in a single pass to a
depth of cover of 1 meter in a wide range of soil
conditions. The burial tool's capabilities include
burial in cohesive soils up to 50
shear strength,
as well as in sandy soils without changing the tool.
Burial rates of
50
meters per hour in
50
cohesive soil and 350 meters per hour in sandy
soil have been achieved. One hundred fifty
waterpump shaft horsepower are absorbed on the
jetting skid to achieve this performance.
An extensive design, laboratory test, field trials and
offshore trials program was conducted in order to
I
meet the performance requirements.
The trend in cable burial systems is deeper burial
in deeper water depths. One recent client, Cable
and Wireless (Marine) wanted an integrated ROV
cable burial system for post-lay burial to meet his
current and anticipated burial needs. Their pertinent
system specifications were:
Water depths
to 2500 meters
Burial Depth
to 1 meter cover
Cable Types
lightweight and armored
Max. Cable Dia.
100
non cohesive1200
50
cohesive150
gravel to 30
The burial rates cited were desired to be
accomplished in a single pass of trenching. In
addition, deeper water round trip times point to the
fact that a multipurpose trenching tool would save
jetting tool changes when widely varying soils are
encountered and thus save overall job time.
-
Avenue
So.
DESIGN TRADEOFFS
Numerous
design
issues
must be
considered
for
any
trencher.
Some
of
the most
serious
are
briefly
discussed
below.
Mechanical trenching is energy efficient, but
requires heavy tools, generates large forces and
vibrations, and is more suited to heavy, bottom
crawling
Mechanical trenching was not
seriously considered in this application.
Although energy inefficient, waterjetting can
generate the required forces and does not capture
or
damage the cable to be buried as
does mechanical trenching. Because of the
technique's inherently fewer moving and wearing
mechanical parts, system reliability is greaterthan
a mechanical system. Waterjetting was selected
for this application.
A
force
balanced
jetting
tool
was
desired,
to
minimize
the
reactive
forces
required
by
the
ROV.
Previous
systems
have
employed
variable
ballast
systems
or
continuous
downward
thrust
to
react
these
forces,
but
both
solutions
cause
additional
problems.
Downward-firing
waterjets
are
difficult
to
balance
and
also
maintain
any
efficiency, since
the
reactive
waterjets
fire
in
a
useless
direction
and
their
energy
is
used
only for
balance. As
an
alternative, forward-firing
water
used
for
cutting
into
the
soil
can be
counter-balanced
by
firing
water
jets
used
for
trenched
spoil
removal.
This
technique
was
chosen
for
this
application.
Cohesive soils require substantial soil destruction
-they are not suitable for undermining and collapse
techniques used in jetting non-cohesive soils. Very
large amounts of energy are required for fixed
nozzles to obtain 100% coverage in a developing
trench in cohesive soil. As will be demonstrated in
the following section, a much more
method is sweeping the nozzles across the area
to be trenched. Oscillating nozzles were chosen. Spoil removal is accomplished by means of a
backward-firing
This technique has the
advantages of simplicity, reliability, and also serves
in the force balance considerations mentioned
above. Dredge pumps have also been used to
more finely control the effluent deposition, at the
expense of reliability and system weight and bulk.
Dredge pumps were not seriously considered for
this application.
Previous systems' operational experience has
clearly
the advantages of a dual-arm
system which straddles the cable being buried.
The arms place the jets properly in the trench and
also laterally confine the cable being buried,
simplifying the operator's control requirements.
Approximately 150 shaft horsepower would be
available at the waterpumps, delivered from a
hydraulic system which supplies the entire ROV
and
skid. Oscillation would use additional
power, and would only be used when necessary.
This practice would make available more power
for propulsion in the lower soil strengths, when
more propulsion is necessary.
Previous systems' experience also dictated the
proper angles for jetting across and ahead in the
trench for a fixed nozzle system. This nozzles
geometry would be used for those soils not
requiring the oscillating features of the system.
PERFORMANCE ANALYSIS
Jet Kerfing
Model
et al. (1 978) carried out an extensive series
of tests on jet kerfing in a simulated cohesive
marine clay with a shear strength of 16
This
data was fitted to an emperical equation for jet
kerfing in soft sediments. This trenching system
is designed to operate in stiffer clay so a series of
kerfing tests in tests in 35
Kaolin potters clay
were carried out. A new formulation for jet kerfing
which accounts for the shear strength of clay
sediments,
in a way consistent with Crow's
(1973) theory of jet cutting was developed. The
kerf depth, h, is given by
where
is nozzle diameter and
is the jet
pressure. The model was recast in the following
non-dimensional, linearized form to find the values
of the constants,
Figure 1 shows both the Hurlburt data and the
Kaolin kerfing data plotted in this non-dimensional
form. The best fit to the data is found with
and
x
1
This model
provides a good fit to both data sets and accounts
for the effects of sediment shear strength and
a
broad range of traverse rates including both the
fixed and oscillating jet configurations for this jet
trenching system.
Kerfing Tests
Figure
1.
Non-Dimensional, Linearized Jet
Kerfing Data with
Best
Model Fit
The volumetric rate at which the jets remove
sediment can be found from the kerf depth, width
and the jet traverse rate. The Kaolin clay
experiments showed that the kerf width was
approximately equal to
1.5
times the jet
diameter. The rate at which jets remove sediment
increases with traverse rate because the velocity
exponent, in the kerfing equation is less than
The volumetric removal rate can be converted to a
trenching rate by assuming that the jets must
remove all of the sediment in the trench. The
trenching rate is thus an increasing function of
traverse rate as shown in Figure 2. A jet traverse
rate of at least
(278
is required to
trench in
50
clay at
50
This result
demonstrates that the jets must be mechanically
oscillated in
meet the system performance
specifications. 0
200
400
600
Traverse
Rate,
Figure 2. Estimated Trenching Rate for A
63
hhp,
151
Psi System
An oscillating jet system provides more efficient
volume removal by increasing the jet traverse rate
and by ensuring that the jets provide uniform cov-
erage over the trench face. The configuration of
the oscillating jet trenching system analyzed here
shown in Figure 3. The system includes two arms
straddling the cable to be buried and inclined at an
I
angle from the horizontal. A total of
40
jet nozzles
are distributed equally on the two arms. The arms
are oscillated through an angle of 100 degrees to
remove all of the material in the trench.
Figure 3. Trenching and Clearing
Jet Configuration
Eductor Analysis
The clearing jet arm illustrated in Figure 3
incorporates a panel-shaped eductor which
provides an efficient means of removing gravel
from the trench bottom while balancing the
trenching jet thrust. The panel shape allows the
eductorto fit behind the jet kerfing arm. The height
of the duct panel was limited to
12
inches by space
limitations within the Jetter Skid Package while the
length of the duct panel was limited
by
the need to
project the gravel behind the ROV. Each eductor
is driven by four primary jets which discharge into
a panel shaped mixing duct