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Federal Register / Vol. 86, No. 148 / Thursday, August 5, 2021 / Rules and Regulations Hilcorp would begin constructing the LDPI during the winter immediately following construction of the ice road from the mine site to the island location.
Sections of sea ice at the islands location would be cut using a ditchwitch and removed. A backhoe and support trucks using the ice road would move ice away. Once the ice is removed, gravel would be poured through the water column to the sea floor, building the island structure from the bottom up.
A conical pile of gravel hauled in from trucks from the mine site using the ice road would form on the sea floor until it reaches the surface of the ice. Gravel hauling over the ice road to the LDPI
construction site is estimated to continue for 50 to 70 days and conclude mid-April or earlier depending on road conditions. The construction would continue with a sequence of removing additional ice and pouring gravel until the surface size is achieved.
Following gravel placement, slope armoring and protection installation would occur. Using island-based equipment e.g., backhoe, bucketdredge and divers, Hilcorp would create a slope protection profile consisting of an 18.3-m 60-ft-wide bench covered with a linked concrete mat that extends from a sheet pile wall surrounding the island to slightly above medium lower low water. The linked concrete mat requires a high-strength, yet highly permeable, woven polyester fabric under layer to contain the gravel island fill. The filter fabric panels would be overlapped and tied together side-byside requiring diving operations to prevent the panels from separating and exposing the underlying gravel fill.
Because the fabric is overlapped and tied together, no slope protection debris would enter the water column should it be damaged. Above the fabric under layer, a robust geo-grid would be placed as an abrasion guard to prevent damage to the fabric by the linked mat armor.
The concrete mat system would continue at a 3:1 slope another 26.4 m 86.5 ft into the water, terminating at a depth of 5.8 m 19 ft. In total, from the sheet pile wall, the bench and concrete mat would extend 44.7 m 146.5 ft.
Island slope protection is required to ensure the integrity of the gravel island by protecting it from the erosive forces of waves, ice ride-up, and currents. A
detailed inspection of the island slope protection system would be conducted annually during the open-water season to document changes in the condition of this system that have occurred since the previous years inspection. Any damaged material would be removed.
Above-water activities would consist of
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a visual inspection of the dock and sheet pile enclosure that would document the condition of the island bench and ramps. The below-water slopes would be inspected by divers or, if water clarity allows, remotely by underwater cameras contracted separately by Hilcorp. The results of the below-water inspection would be recorded for repair if needed. No vessels would be required. Multi-beam bathymetry and side-scan sonar imagery of the below-water slopes and adjacent sea bottom would be acquired using a bathymetry vessel. The sidescan sonar would operate at a frequency between 200 and 400 kHz. The single-beam echosounder would operate at a frequency of about 210 kHz.
Once the slope protection is in place, Hilcorp would install the sheet pile wall around the perimeter of the island using vibratory and, if necessary, impact hammers. Sheet pile driving is anticipated to be conducted between March and August, during approximately 4 months of the icecovered season and, if necessary, approximately 15 days during the openwater season. Sheet pile driving methods and techniques are expected to be similar to the installation of sheet piles at Northstar during which all pile driving was completed during the icecovered season. Therefore, Hilcorp anticipates most or all sheet pile would be installed during ice-covered conditions. Hilcorp anticipates driving up to 20 piles per day to a depth of 7.62
m 25 ft. A vibratory hammer would be used first, followed by an impact hammer to proof the pile. Hilcorp anticipates each pile needing 100
hammer strikes over approximately 2
minutes 100 strikes of impact driving to obtain the final desired depth for each sheet pile. To finish installing up to 20 piles per day, the impact hammer would be used a maximum of 40
minutes per day with an anticipated duration of 20 minutes per day.
For vibratory driving, pile penetration speed can vary depending on ground conditions, but a minimum sheet pile penetration speed is 0.5 m 20 in per minute to avoid damage to the pile or hammer NASSPA 2005. For this project, the anticipated duration is based on a preferred penetration speed greater than 1 m 40 in per minute, resulting in 7.5 minutes to drive each pile. Given the high storm surge and larger waves that are expected to arrive at the LDPI site from the west and northwest, the wall would be higher on the west side than on the east side. At the top of the sheet-pile wall, overhanging steel parapet would be
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installed to prevent wave passage over the wall.
Within the interior of the island, 16
steel conductor pipes would be driven to a depth of 49 m 160 ft to provide the initial stable structural foundation for each oil well. They would be set in a well row in the middle of the island.
Depending on the substrate, the conductor pipes would be driven by impact or vibratory methods or both.
During the construction of the nearby Northstar Island located in deeper water, it took 5 to 8.5 hours to drive one conductor pipe Blackwell et al.
2004. For the Liberty LDPI, based on the 20 percent impact hammer usage factor USDOT 2006., it is expected that two cumulative hours of impact pipe driving 4,400 to 3,600 strikes would occur over a 10.5 non-consecutive hour day. Conductor pipe driving is anticipated to be conducted between March and August and take 16 days total, installing one pipe per day. In addition, approximately 700 to 1,000
foundation piles may also be installed within the interior of the island should engineering determine they are necessary for island support.
The LDPI layout includes areas for staging, drilling, production, utilities, a camp, a relief well, a helicopter landing pad, and two docks to accommodate barges, a hovercraft, and small crew boats. It would also have ramps for ice road and amphibious vehicle access. An STP would also be located at the facility to treat seawater and then commingle it with produced water to be injected into the Liberty Reservoir to maintain reservoir pressure. Treated seawater would be used to create potable water and utility water for the facility. A
membrane bioreactor would treat sanitary wastewater, and remaining sewage solids would be incinerated on the island or stored in enclosed tanks prior to shipment to Deadhorse for treatment.
All modules, buildings, and material for onsite construction would be trucked to the North Slope via the Dalton Highway and staged at West Dock, Endicott SDI, or in Deadhorse.
Another option is to use ocean-going barges from Dutch Harbor to transport materials or modules to the island during the open-water season.
Depending on the season, equipment and material would be transported via coastal barges in open water, or ice roads from SDI in the winter. The first modules would be delivered in the third quarter of Year 2 to support the installation of living, drilling, and production facilities. Remaining process modules would be delivered to
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