Slideshow image
Mfg. of Water well Drilling Rigs, Dth Hammers and Button Bits...
Water Well Drilling Rigs, Dth Drilling Rigs, Rotary Drilling Rigs...
Blast hole Drills, Water Well Drilling Rig, Mud Pumps, Vertical Turbine Pumps...
klrsai deepagetechjcr


Gerald J. Ruschkofski, P.E., and Thomas J. Meinhart, P.E.
South Carolina Electric & Gas Company (SCE&G) of Columbia, SC., recently required a new installation of a 115-kV submarine electric power cable beneath the Cooper River in Charleston, SC. This crossing was a part of an overall project that entailed installing the 115-kV electrical power cable from the Charlotte Street Substation in Charleston, SC to a terminal structure in Mt. Pleasant, SC. a distance of approximately 2 miles.
Jacobs Civil Inc. (Jacobs) was engaged by SCE&G to perform a feasibility analysis for this project to
determine if Horizontal Directional Drilling (HDD) could be utilized to facilitate the installation of the line.
An 8.625-inch steel conduit would be installed by HDD nearly 7,000-ft across the Cooper River with
approximately 3,300-ft of open-cut required for the tie-ins. The results of the Feasibility Report
determined that the proposed HDD crossing was feasible.
When design was initiated this trenchless crossing would have been the longest completed HDD crossing in the United States. Construction began in fall 2003 on the open-cut section of the project with the HDD construction beginning in March of 2004.
Under an Engineering Procurement Construction Management (EPCM) contract, Jacobs also acted as a general contractor at risk and was responsible for the installation of the 115-kV line. This paper will discuss the design and construction challenges of one of the longest compound curve HDD installations completed to date. The following aspects will be examined; purpose of the project, geotechnical investigation, design decisions, drilling of the pilot hole, tracking of the drill bit, pipe fabrication and laydown, and pullback. The paper will show that the Owner and Manager provided many innovative solutions to promote the successful design and construction of this challenging project.

In July of 2001, the State of South Carolina Department of Transportation decided to construct a new
bridge across the Cooper River between the City of Charleston and Town of Mount Pleasant. The new bridge will replace two existing bridges to provide additional clearance over the river for container ships using the Port of Charleston which is located upstream of the existing bridges. SCE&G was required to replace an existing 115-kV overhead transmission line, which was attached to one of the bridges that spanned the Cooper River.
The existing 115-kV transmission line was installed on the Grace Memorial Bridge in the 1970’s and has
been a vital tie line between the City of Charleston and the high growth area of Town of Mount Pleasant
and East Cooper, South Carolina. The new Cooper River Bridge is currently under construction and it is
expected to be completed in May of 2005. Once the new bridge is completed, the Grace Memorial Bridge
and the other existing bridge paralleling it will be removed. The existing bridges crossing the Cooper
River are configured in two separate spans over the two shipping channels. The new bridge consists of a
single span which passes diagonally over a low point between the separate spans on the existing
bridges. As construction on the new bridge progressed over the low point on the old bridges, the existing
overhead line had to be lowered and removed from normal service. Eventually the overhead line had to
be removed totally from service due to clearance issues with the structural frame of the new bridge.
Because the existing electrical tie line between the two service areas was so important, SCE&G began
reviewing options in 2002 on how to replace it. An overhead line alongside the new bridge was
considered, but was rejected due to permitting issues and heavy commercial ocean ship traffic using the
Port of Charleston. Installing a power cable on the new bridge was also not considered feasible, because
the new bridge would not be completed in time for the cable to be energized before the old overhead tie
had to be taken out permanently of service. Therefore, SCE&G believed that the only viable alternative
was an underwater cable crossing downstream of the new bridge, which is in the narrowest section of
the Cooper River.
Although this is the narrowest section, the Cooper River is in excess of 7,000 feet wide at this point.
Laying the cable directly on the river bottom or burying it in a shallow trench was considered, but rejected
for several reasons. The main reason is that this area of the river is considered part of Charleston Harbor
and it is continuously being dredged to open the channels for the large commercial container ships.
SCE&G was concerned that river dredging presented a great risk of a dredge damaging the cable. Also
as container ships get larger, future dredging operations could get deeper. Another concern was that with
the large volume of commercial marine traffic in this area of the river, dragging anchors or other objects
falling in the river could also damage the cable lying directly on the bottom or buried in a shallow trench.
Therefore, SCE&G investigated the use of Horizontal Directional Drilling (HDD) to install the proposed
transmission line. SCE&G has successfully used this method to install other pipe-type underground
transmission lines in the Charleston and Beaufort areas, but those crossings were only approximately
5,000 in length.

Therefore in June of 2002, SCE&G engaged Jacobs to evaluate the technical feasibility of using HDD to
install the proposed electric transmission line beneath the Cooper River. The feasibility report included a
review of the existing site conditions and subsurface information and a technical feasibility evaluation of
using trenchless construction methods for the proposed project. The technical memorandum also
included a qualitative risk assessment.
Three potential alignments across the Cooper River were studied with the length of the routes ranging
from 6,500 to 6,800 ft. The project team also reviewed existing geotechnical data which consisted of a
geologic profile produced for the Cooper River Bridge in 1961 as well as discussions with a local
geotechnical firm with extensive experience in the Charleston area.
The feasibility report findings indicated that the proposed crossing of the Cooper River which HDD
construction techniques was technically feasibility however the project would be classified as high-risk
due to the proposed length of the crossing. During preparation of the feasibility study, a 7,000 ft. crossing
installed by means of HDD had not been completed in the United States. The geologic conditions which
were reviewed appeared to be acceptable and within the technical envelope for the HDD operations.
Consequently, the project team believed that the project could be successfully completed by an
experienced drilling contractor with sufficient upfront design and planning. To manage the risk, the
project team agreed that it was imperative to conduct an in-depth geological exploration followed by a
detailed design so that the SCE&G could undertake this challenging project with minimal unknowns and the construction documents could be developed to permit bidding by multiple experienced drilling
Although it may be possible to install the cable pipe with HDD construction methods, another technical
challenge was the installation of the proposed cable system once the cable pipe was installed beneath
the Cooper River. Power Delivery Consultants Inc. (PDC) performed preliminary cable pulling
calculations to determine the pulling force, which would be required to install the cables within the cable
pipe beneath the Cooper River. The preliminary calculations determined that the estimated pulling
tensions were near the limits of the cable system as well as approaching the pulling capacity of the
contractor’s installation equipment. Therefore it would be imperative that during the subsequent design
activities, the profile of the proposed cable pipe be analyzed not only for the installation stresses but also
for potential impacts to the required cable pulling tensions to ensure that the proposed profile does not
result in cable pulling forces which exceed the cable and installation equipment capacities.
A detailed subsurface investigation was conducted during the preliminary design activities to better define
the existing subsurface materials and identify the preferred subsurface materials in which the proposed
pipeline would be installed by means of HDD. The geotechnical investigation for the river crossing
included eleven deep soil borings. Samples were obtained by Standard Penetration Test using Split-
Spoon Samplers and undisturbed sampling methods using Shelby Tube Samplers. Samples were
collected from the soil borings to determine the geotechnical and thermal properties of the subsurface
materials. The geotechnical properties were used in the design of the HDD installation and the soil
resistivity values were used to compute the theoretical ampacity that could be carried by the proposed
electric transmission line.
The borings which were conducted in the river were drilled to approximate elevations -123 and -125. In
general the stratigraphic column for the proposed river crossing consisted of between 2 and 76 ft of
organic silt (OH) and/or interbedded organic silt (OH), inorganic silt (MH), inorganic clay (CH) and fine
sand (SM-SP) and occasional marl overlying the Cooper Marl. SPT-N values in the organic silt are
generally weight of hammer and N-values in the interbedded material range from weight of hammer to an
extreme of approximately N=29 in fine sand with a high shell content. The top of the Cooper Marl
appeared to rise to the north with a minimum elevation of approximately -81ft to a maximum elevation of
approximately -42 ft. elevation. The Cooper Marl is described as a soft to stiff brownish-green calcareous
clayey silt with slight sand content; N-values range from 3 to 22 with an extreme N value of 55 in one
Based upon a review of the subsurface information it was determined that the Cooper Marl strata would
be the preferred material for the proposed HDD operations. The results of the geotechnical investigation
were compiled and provided to the drillers as part of the subcontract bidding documents.

One side of the proposed Cooper River crossing consisted of a confined work area located within the
historic streets of the City of Charleston, SC. In addition to the work area constraints, there was known
groundwater and soil contamination resulting from a previous manufacturing facility located immediately
adjacent to the proposed drilling site. The possibility of encountering contaminated soil and groundwater
during the construction activities required additional design and construction considerations.
It was important that the potential for encountering soil and groundwater contamination was identified
early in the project and a Worker Protection Plan was mandated and specific procedures were provided
to the subcontractors in the Contract Documents. The procedures addressed the notification process and
disposal procedures if contaminated materials were encountered during the subsequent construction
activities. The project team also supplied qualified personnel to direct the subcontractors with the required procedures. The bidding documents also required the subcontractors to included unit prices to
cover any potential construction delays if the presence of contamination adversely impacted the
construction activities.
The use of HDD also provides a means of construction which resulted in no environmental impact to the
Cooper River or the existing wetlands area on the Mount Pleasant side of the river where the
underground line passed beneath. Conventional dredging techniques would have impacted both the
Cooper River and the existing environmentally-sensitive areas adjacent to the river.

The feasibility study determined that the proposed crossing of the Cooper River would be approximately
7,100 ft. in length and would require a complex HDD installation. Therefore the proper selection and
design of the cable pipe was a critical factor for success of the project. The proposed 115-kV
underground transmission line which would be installed beneath the Cooper River would be a high
pressure gas filled pipe-type cable system. This system consists of a carbon steel cable pipe which
houses the electric cables and the interior of the cable pipe is pressurized with Nitrogen gas.
Cable pipes differ from traditional straight end carbon steel pipes in that the ends of each pipe length are flared in the manufacturing facility prior to shipment to the project site. The flared ends are used with a short cylindrical section of pipe called a “chill ring” which is used to join the two flared end sections of pipe and also prevent the intrusion of welding material into the interior of the cable pipe which may damage the cables during the installation of the electric cables in the cable pipe. The internal diameter of the chill ring is the same as the internal diameter of the cable pipe thereby providing a smooth joint. The cable pipe which was specified for the Cooper River crossing consisted of an 8.625-inch Outside Diameter pipe with a 0.375-inch wall thickness manufactured in accordance with ASTM A523.
The proposed alignment of the Cooper River crossing required a compound curve due to the preferred entry and exit points on the opposing sides of the Cooper River. The design team acquired sufficient hydro graphic survey prior to the commencement of the final design activities to ensure that the survey would incorporate any possible compound alignment. Due to the length of the crossing the turning radii for the horizontal and vertical curves were increased to facilitate the installation of the steel cable pipe using HDD. A minimum radius of curvature for the curves was limited to 3,000-ft for the horizontal curve and 2,000-ft and 3,000-ft for the two vertical curves respectively. The anticipated pipelines stresses were evaluated for both the operational and installation scenarios to ensure that the 7,100 ft. crossing and compound curve would not subject the steel pipeline to unacceptable stresses. The proposed alignment was analyzed with the use of DrillPath™ software to estimate the stresses which would be imparted to the cable pipe during the pullback operations. Figure 1 depicts the results of the stress analysis which was performed on the cable pipe for the proposed Cooper River crossing alignment.

Figure 1. Stress Analysis Generated from DrillPath™ Software

It was determined that the use of an 8.625-inch O.D. pipe with a 0.375-inch wall thickness and a 30,000 psi Tensile Strength would be adequate for the proposed crossing. The stress analysis as shown in Figure 1 estimated that the maximum tensile stress during the pullback operations would be
approximately 16,230 psi as compared to the pipe’s tensile capacity of 30,000 psi.
The proposed cable pipe was to be installed within an easement within the limits of the Cooper River and
space within this easement was to be reserved for a future cable pipe crossing adjacent to the proposed
cable pipe. Therefore, the Contract Documents specified a drilling tolerance of ± 6 feet from the
centerline of the pipeline in both the horizontal and vertical planes to ensure that adequate space was
provided within the easement for a future cable pipe if required by SCE&G.
The integrity of the cable pipe was very important given the critical infrastructure which is houses;
therefore an interior and exterior coating system was specified for the cable pipe. The interior surface of
the cable pipe was lined with 2 mils of epoxy and the exterior surface of the cable pipe received 12 mils of
fusion-bonded epoxy with a final exterior coating of 20 mils of polymer concrete. The polymer concrete
provides an abrasion resistant protective coating for the fusion-bonded epoxy coating. A cathodic
protection system which included a deep well anode installed on the Charleston side of the crossing was
also specified to protect the cable pipe from corrosion.
After the horizontal and vertical alignment was defined, the cable pulling calculations were prepared to
determine if the resulting alignment design which included a compound curve would result in estimated
cable pulling loads which exceeded the cable capacity. The cable pulling calculations estimated that the
proposed 2250-kcmil copper conductors would have sufficient strength to withstand the theoretical pulling
loads and the anticipated pulling loads were within the capacity of the pulling equipment which is
commercially available. The cable pipe will house the electric circuit which includes three 2250-kcmil
copper conductors rated at 960 amps or 191 MVA.

SCE&G needed to expedite the design and construction of the project to provide system redundancy prior
to the peak summer months. Therefore; after the 50% design documents were completed, the use
alternative contract delivery methods were evaluated by the project team. It was decided that in order to
expedite the delivery of the project an Engineer/Procure/Construction Management (EPCM) contract
would be issued to Jacobs Construction Services. The design team of Jacobs and PDC continued with
the completion of the design documents with emphasis placed upon the development of technical
specifications and drawings for the coated steel cable pipe and the cable. Both of these items are longlead
items due to their lengthy fabrication and delivery times. These long-lead items would be procured
by Jacobs prior to the completion of the design and issuance of the construction subcontracts.
Jacobs served as General Contractor for the project and had overall responsibility for the construction
project. Due to the scope of the project it was decided that two primary construction subcontracts would
be let by Jacobs. The first construction package included the cable pipe fabrication for the river crossing,
open cut installation of the cable pipe, electrical work and cable pulling construction activities whereas the
second construction package included the installation of the cable pipe beneath the Cooper River by
means of HDD. Contractors were pre-qualified for the Horizontal Directional Drilling and Electrical/Cable
Pulling Subcontracts to ensure that the bidders possessed the requisite skills to successfully complete a
project of this complexity. The prequalification process included a review of the contractor’s safety
record, previous similar project experience and financial capabilities. After the design was completed,
two construction packages were prepared and sent to potential subcontractors who had met the project’s
prequalification requirements.
The HDD subcontract was let to Michels Corporation of Brownsville, Wisconsin, and the Electrical/Cable
Pulling subcontract was let to UTEC Constructors of Boston, Massachusetts.
The project required close coordination between Michels and UTEC due to the fact that UTEC was
responsible for fabricating the cable pipe which would be installed by Michels by means of HDD. The
drilling operations were scheduled to ensure that the cable pipe would be fabricated, tested, and ready for
the pullback operations.
SCE&G wished to minimize the technical and construction risk associated with this complex infrastructure
project. Therefore by implementing an EPCM delivery approach, the project team was able to prequalify
several HDD and Electrical subcontractors to ensure that the complex underground project would be
constructed by contractors with the requisite experience to successfully complete a project of this
magnitude. Secondly, Jacobs served as the General Contractor with ultimate responsibility for the
procurement of the long-lead materials, construction subcontractors and the overall project schedule.
The EPCM approach also proved beneficial to SCE&G due to the fact that the final project cost was
substantially less than the project budget developed during the preliminary stages of the project prior to
the receipt of competitive bids for the procured materials and construction subcontracts.

The construction of the Cooper River Crossing project included several construction challenges. One of the primary challenges was the limited construction staging area which was available on the Charleston side of the crossing which served as the drill entry site. The construction staging area was located near the edge of the Cooper River within an existing street and bound by a major theatre complex to the South and the Port of Charleston property immediately to the North. Figure 2 depicts the limited staging area on the Charleston side of the crossing.

Figure 2. Construction Staging Area for Drill Entry

The project team provided close coordination with the City of Charleston to ensure that the infrastructure
work did not adversely impact the local businesses adjacent to the work area. The entry area for the
HDD operations was also confined to a limited staging area of approximately 18,000 square feet to
prevent blocking access to a local connector street which served the major theatre complex.
Another construction challenge included screening the drill cuttings for impacted soil and water. The drill
cuttings were screened by a member of SCE&G’s environmental staff. The drill cuttings which were
impacted were then placed in a separate stockpile and then transported offsite in accordance with the
special provisions contained within the Contract Documents, whereas the drill cuttings which were not
impacted were recycled as is the common practice in HDD operations. This requirement resulted in the
disposal of approximately 5,500 cubic yards of impacted material which required offsite disposal.
During the pilot hole drilling operations, the initial 290 ft of subsurface material prior to encountering the
Cooper Marl material was very soft alluvial material which resulted in problems with maintaining the
proposed grade of the pilot hole. Therefore, Michels elected to install approximately 300 ft of 16-inch
diameter steel casing pipe to stabilize the soft subsurface materials and reduce the problems with
maintaining grade until the drill entered the desired subsurface material known as Cooper Marl.
Monitoring of the 12.25-inch diameter pilot hole was conducted by use of a TruTracker™ system and was
augmented by using a remote monitoring device and the collection of readings from a barge when drilling
beneath the Cooper River. This technique proved favorable and the pilot hole was successfully drilled in
nine days within the specified horizontal and vertical tolerances and minimum radii of curvature as
required in the Contract Documents.
Perhaps the greatest challenge of the crossing was the limited Temporary Construction Easement (TCE)
for the pipeline fabrication and staging area available on the Mount Pleasant side of the crossing. The
existing site constraints did not allow for adequate easement to fabricate and laydown the pipeline in a
single pipe string prior to pullback. The TCE consisted of approximately 2,000 linear feet to fabricate the
nearly 7,100-ft of steel cable pipe. The cable pipe was fabricated in four distinct segments each of which
extended the full length of the available TCE. The TCE extended across several driveways which
required uninterrupted public access to local businesses as shown in Figure 3.

Figure 3. Pipeline Fabrication and Laydown Area

Therefore, when crossing the driveways the construction team laid corrugated plastic drainage pipe
across the driveways, with the fabricated steel conduit housed inside, and then compacted granular
material on top of the drainage pipe creating a ramp that allowed vehicular traffic to drive over the plastic
lines which housed and protected the steel conduit. This method allowed the conduit to extend across
the driveways without impeding vehicular access during the fabrication and pullback operations while
utilizing the entire 2,000 of available TCE. The complexity of the pullback operations were greatly
increased due to the fact that the drilling subcontractor was required to stop the pullback operations three
times to perform the tie-in welds of the four conduit segments. The design and construction team worked
to minimize the length of the pullback operations as much as possible by planning ahead, having all of the
required equipment and auxiliary equipment on site, and by utilizing an alternate coating for the girth
welds which required a shorter cure period.
Michels mobilized for the Cooper River Crossing on February 20, 2004 and the 7,029 ft. of cable pipe was
successfully installed under the river during a 30-hour pullback operation which was completed on March
26, 2004.
After the cable pipe was successfully installed beneath the Cooper River, the project team was presented
with another challenge which included the installation of the 2250-kcmil copper conductors within the
cable pipe beneath the river. Although the calculations indicated that the pulling tensions were within the
capacity of the conductor, there was still some concern. The conductor for the river crossing section
arrived from Okonite on special 14 foot diameter reels. Each reel weighed 85,000 pounds and had to be
hauled on special heavy duty flat bed trucks from Okonite’s New Jersey plant to the project site. UTEC
had to construct special reel stands to support the heavy big reels. After a couple of delays due to
weather, the underwater portion of the conductor was successfully pulled in on June 24, 2004.
Upon completion of the Cooper River Crossing portion of the project the remainder of the 3,300 ft. of
cable pipe was installed by open-cut construction methods on both sides of the Cooper River, the cable
installation and construction of the terminal structures were completed, and the project reached
substantial completion on August 3
The installation of this underground transmission line satisfied the public requirement that a reliable
source of electrical power be constructed to serve the Mount Pleasant , East Cooper area while not
visually impacting the area of the new bridge. The construction of this transmission line greatly improved
the reliability of the transmission system. This was proven by the fact that a couple of weeks after the
new line energized; it was relied upon to supply a major portion of the electrical load to the service area
due to storm damage on the overhead transmission lines that presently feed the area.
The Cooper River Crossing demonstrated that with proper upfront planning and design combined with the
use of alternative contract delivery methods, trenchless construction methods, namely HDD, can be
successfully used while reducing Owner risk in a cost-effective manner. The project team faced many
project challenges and developed innovative approaches to meet the project challenges.
This complex HDD crossing which included a compound curve with a length of 7,029 ft. is believed to be
the second longest HDD crossing successfully completed in the United States and one of the longest

pipe-type cable systems ever installed using HDD construction techniques.
Drilling Today Contact