Voglio sapere a riguardo di…

8494

Federal Register / Vol. 86, No. 23 / Friday, February 5, 2021 / Notices
jbell on DSKJLSW7X2PROD with NOTICES2

components could be picked up directly in the marshalling port by the main installation vessels.
Within the SFWF, the main installation vessel would upend the monopile with a crane, and place it in the gripper frame, before lowering the monopile to the seabed. The gripper frame, depending upon its design, may be placed on the seabed scour protection materials to stabilize the monopiles vertical alignment before and during piling. Scour protection is included to protect the foundation from scour development, which is the removal of the sediments near structures by hydrodynamic forces, and consists of the placement of stone or rock material around the foundation. The scour protection would consist of engineered rock placed around the base of each monopole in a 68 m 222 ft diameter circle, using either a fallpipe vessel or stone dumping vessel. Once the monopile is lowered to the seabed, the crane hook would be released, and the hydraulic hammer would be picked up and placed on top of the monopile.
All monopoles would be installed with an impact hammer. Impact pile driving entails the use of a hammer that utilizes a rising and falling piston to repeatedly strike a pile and drive it into the ground. Using a crane, the installation vessel would upend the monopile, place it in the gripper frame, and then lower the monopile to the seabed. The gripper frame would stabilize the monopiles vertical alignment before and during piling.
Once the monopile is lowered to the seabed, the crane hook would be released and the hydraulic hammer would be picked up and placed on top of the monopile. A temporary steel cap called a helmet would be placed on top of the pile to minimize damage to the head during impact driving. The largest hammer South Fork Wind expects to use for driving monopiles produces up to 4,000 kilojoules kJ of energy however, required energy may ultimately be far less than 4,000 kJ. As described in the Proposed Mitigation section below, South Fork Wind would utilize a sound attenuation device e.g., bubble curtain during all impact pile driving.
The intensity i.e., hammer energy level of impact pile driving would be
gradually increased based on the resistance that is experienced from the sediments. The strike rate for the monopile foundations is estimated to be 36 strikes per minute. Two pile driving scenarios for 16 11 m piles, were considered for SFWF Table 1. The standard pile driving scenario would require an estimated 4,500 strikes for the pile to reach the target penetration depth, with an average installation time of 140 minutes for one pile. In the event that a pile location presents denser substrate conditions and requires more strikes to reach the target penetration depth, a difficult-to-drive pile scenario was considered, in which 8,000 strikes and approximately 250 minutes would be required to install 1 pile.
Impact pile driving activities at SFWF
will take place between May 1, 2022
and December 31, 2022. There are two piling scenarios that are considered possible within the current engineering design. The standard scenario assumes that a pile is driven every other day such that 16 monopiles piles would be installed over a 30-day period. A more aggressive schedule is considered for the maximum design scenario in which six piles are driven in a week 7 days such that the 16 piles are installed over a 20-day period. Only one pile would be driven per 24 hours hrs, irrespective of the selected scenario. Please see Table 1
for a summary of impact pile driving activity.
Installation and Removal of Temporary Cofferdam Before cable-laying HDD begins, a temporary cofferdam may be installed at the endpoint of the HDD starting point, where the SFEC conduit exits from the seabed. The cofferdam would be less than 600 m 1,969 ft offshore from the mean high water line MHWL, in 7.6 to 12.2 m 25 to 40 ft water depth, depending on the final siting point. The cofferdam, up to 22.9 m 75 ft by 7.7
m 25 ft, would serve as containment for the drilling returns during the HDD
installation to keep the excavation free of debris and silt. The cofferdam may be installed as either a sheet pile structure into the seafloor or a gravity cell structure placed on the seafloor using ballast weight. Installation of a gravity cell cofferdam would not result in incidental take of marine mammals and
is, therefore, not analyzed further in this document. Installation of the 19.5 m 64
ft long, 0.95 centimeters cm 0.375
inches in thick Z-type sheet pile cofferdam and drilling support would be conducted from an offshore barge anchored near the cofferdam.
If the potential cofferdam is installed using sheet pile, a vibratory hammer will be used to drive the sidewalls and endwalls into the seabed to a depth of approximately 1.8 m 6 ft; sections of the shoreside endwall will be driven to a depth of up to 9 m 30 ft to facilitate the HDD entering underneath the endwall. Cofferdam removal would consist of pile removal using a vibratory hammer, after HDD operations are complete and the conduit is installed see Table 1 for a summary of potential vibratory pile driving activity.
Vibratory hammering is accomplished by rapidly alternating 250 Hertz Hz forces to the pile. A system of counterrotating eccentric weights powered by hydraulic motors are designed such that horizontal vibrations cancel out, while vertical vibrations are transmitted into the pile. The vibrations produced cause liquefaction of the substrate surrounding the pile, enabling the pile to be driven into the ground using the weight of the pile plus the impact hammer. If the gravity cell installation technique is not practicable, South Fork Wind anticipates that any vibratory pile driving of sheet piles would occur for a total of 36 hrs 18 hrs for installation, 18
hrs for removal.
The source levels and source characteristics associated with vibratory driving would be generally similar to those produced through other concurrent use of vessels and related construction equipment. Any elevated noise levels produced through vibratory driving are expected to be of relatively short duration, and with low source level values. However, it is possible that if marine mammals are exposed to sound from vibratory pile driving, they may alert to the sound and potentially exhibit a behavioral response that rises to the level of take.
Installation of the Z-type sheet piles would occur primarily in daylight;
however, it is possible that vibratory pile driving may continue past sunset if required by the construction schedule.

TABLE 1SUMMARY OF PILE DRIVING ACTIVITIES FOR SFWF AND SFEC
Pile driving method
Pile size
Impact

11 m monopile

VerDate Sep<11>2014

19:54 Feb 04, 2021

Jkt 253001

Number of piles 16

PO 00000

Strikes/pile
Duration/pile
Standard pile: 4,500
Difficult pile: 8,000

Standard pile: 140 min
Difficult pile: 250 min

Frm 00006

Fmt 4701

Sfmt 4703

E:FRFM05FEN2.SGM

05FEN2

Number of piling days Standard scenario: 30.
Maximum scenario: 20.