United states department of interior bureau of ocean energy management



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2.4Borrow Areas


South Pelto Borrow Area

The South Pelto Borrow Area design is based on prior geophysical and geotechnical investigations in South Pelto Lease Blocks 13 and 14. Within the identified available borrow resource not utilized for Caminada Headland projects (BA-45 and BA-143), there are two (2) areas referred to as Expansion Area A and B, located on the southwest and northern borders of the BA-45 survey area, as shown on Figure 2-9. Area A is the larger of the two, and has a maximum length of 4,243 ft and maximum width of 3,478 ft. The surface area is approximately 190 acres. The sand thickness is up to 14 ft. Area B has a maximum length of 4,712 ft and maximum width of 807 ft, with a surface area is approximately 63 acres. The sand thickness is up to 18 ft. A 3-ft zone of allowable disturbance is included between the sand and the lower sediment horizons. The preliminary design cut volume is estimated to be 3.2 MCY and 1.5 MCY for Areas A and B, respectively. Preliminary Design cut volume estimates of suitable sediments within the South Pelto Borrow Areas A and B is approximately 4.7 MCY including a 1,000 ft offset from active and abandoned pipelines.

Material from this borrow area may be used for containment dike construction at East Timbalier Island, beach and dune construction at East Timbalier and West Belle Headland as well as the East Timbalier marsh platform construction.

Ship Shoal Borrow Area

The Ship Shoal Borrow Area is located in within the South Pelto Lease Blocks 12 and 13 on the east central portion of the Ship Shoal sand body on the Outer Continental Shelf (OCS). The Ship Shoal Borrow Area is located approximately 23 NM west-southwest of East Timbalier Island. Within the Ship Shoal Borrow Area, the seafloor elevations range from less than -27 ft to deeper than -34 ft NAVD88. The Ship Shoal Borrow Area is trapezoidal in form as shown on Figure 2-10. The dimensions vary with a maximum length of 9,600 ft and maximum width of 4,400 ft. The surface area is approximately 584 acres. The sand thickness is up to 17 ft. The average percent sand and grain size were computed to be 95% and 0.166 mm, respectively. A 3-ft zone of allowable disturbance is included between the sand and the lower sediment horizons. The Preliminary Design cut volume is estimated to be 6.5 MCY, including a 1,000 ft offset from active pipelines for Project construction.

Material from this borrow area may be used for containment dike construction at East Timbalier Island, beach and dune construction at East Timbalier and West Belle Headland as well as the East Timbalier marsh platform construction.

East Timbalier Marsh Borrow Area

The East Timbalier Marsh Borrow Area is located on the Little Pass Timbalier Ebb Tidal Delta approximately 9.1 NM west-southwest of East Timbalier Island, contiguous to the beach restoration borrow area utilized for the West Belle Pass Barrier Headland Project (TE-52). The East Timbalier Marsh Borrow Area is rectangular in form, as shown on Figure 2-11. The dimensions vary with a maximum length of 9,700 ft and maximum width of 2,300 ft. The surface area is approximately 441 acres. Water depths within the borrow area range from approximately 15 to 21 ft below NAVD88. The sediments ranged from silty sand varying in thickness between 2.1 to 6.6 feet overlying mixed sediments comprised of silt and clay. Subsamples of the surficial sediments had a mean grain size of 0.096 mm and averaged greater than 75% sand. The mixed sediments were generally described as very soft silty clay or clay with a high water content averaging greater than 50% which typically increased with depth. The Preliminary Design cut depth was set to -27 ft NAVD88 with a 3-ft limit of disturbance. The preliminary design cut volume is estimated to be 7.2 MCY.

Material from this borrow area may be used for the East Timbalier and West Belle Headland marsh platform construction.

West Belle Marsh Borrow Area

The West Belle Marsh Borrow Area is rectangular in form; however, it is separated into three subareas based on the previously excavation limits as shown on Figure 2-12. The West Belle Marsh Borrow area was previously analyzed for the West Belle Pass Barrier Headland Project (TE-52) and was estimated approximately 4 MCY of marsh material (silt and clay). The TE-52 Marsh Borrow Area was permitted and partially used to construction the TE-52 project but, was not completely exhausted in the construction of the project, leaving marsh borrow material available for cost-effective excavation. The West Belle Marsh Borrow Area is located approximately 2.5 NM south of the West Belle Headland. The overall dimensions have a maximum length of 3,050 ft and maximum width of 3,150 ft. The surface area is approximately 113 acres. The West Belle Marsh Borrow Area has marsh compatible sediment ranging in thickness from 10 to 20 feet. The Preliminary Design cut depths range from -31.0 feet to -50.0 feet NAVD88 along with a 3-foot limit of disturbance. The Preliminary Design cut volume is estimated to be 1.7 MCY.

Material from this borrow area may be used for the East Timbalier and West Belle Headland marsh platform construction.
Figure 2. South Pelto Borrow Area

Figure 2. Ship Shoal Borrow Area


Figure 2. East Timbalier Marsh Borrow Area

Figure 2. West Belle Headland Marsh Borrow Area


2.5Sediment Pump-Out Areas and Conveyance Corridors


Two (2) sediment pump-out areas were designed to convey the sand from the hopper dredges or scow barges to the Project templates, the East Timbalier Island Pump-Out Area (Figure 2-13) and the West Belle Pump-Out Area (Figure 2-14). Also, two (2) conveyance corridors were designed to connect the East Timbalier Marsh Borrow Area and the West Belle Marsh Borrow Area to the Project templates (Figures 2-15 and 2-14, respectively).

The East Timbalier Pump-Out Area selected for conveyance of the sand from the hopper dredges or scow barges to East Timbalier Island Restoration Area is located in Federal waters approximately 3.3 NM south of the Island. The combination of sailing distances and discharge pipeline lengths from the Ship Shoal and South Pelto Borrow Areas to the Restoration Area are approximately 23 NM and 20 NM, respectively. The East Timbalier Pump-Out Area is designated for direct pump-out only. No bottom dumping from hopper dredges or scow barges and re-suspension of sediment by cutterhead dredge will be allowed in this pump-out area. The East Timbalier Conveyance Corridor extends ~3.3 NM from the north end of the East Timbalier Pump-Out Area, north to the existing shoreline on the western end of the Island. The width of the East Timbalier Conveyance Corridor is 600 feet. A plan view of the East Timbalier Pump-Out Area and Conveyance Corridor is presented in Figure 2-13.

The West Belle Pump-Out Area selected for conveyance of the sand from the hopper dredges or scow barges to the West Belle Headland Restoration Area is located in State waters approximately 2.4 NM south of the Headland. The sailing distances from the Ship Shoal and South Pelto Borrow Areas to the Restoration Area are approximately 26 NM and 23 NM, respectively. This area was previously permitted in 2010 (MVN-2010-01801-WOO, P20100293) as a borrow area for the West Belle Pass Barrier Headland Restoration Project (TE-52). The West Belle Conveyance Corridor extends from the north end of the West Belle Pump-Out Area, north to the existing shoreline on the western end of the Headland. The width of the West Belle Conveyance Corridor is approximately 300 feet. The West Belle Conveyance Corridor corresponds to the previously permitted corridor for the TE-52 project and the conveyance corridor is also used for the West Belle Marsh Borrow Area. A plan view of the West Belle Pump-Out Area and Conveyance Corridor is presented in Figure 2-14.

The East Timbalier Marsh Conveyance Corridor was designed to convey mixed sediments from the East Timbalier Marsh Borrow Area to both of the Restoration Areas (Figure 2-15). The East Timbalier Marsh Conveyance Corridor to the East Timbalier Island Restoration Area extends from the northeast end of the East Timbalier Marsh Borrow Area, east to the intersection with the East Timbalier Conveyance Corridor. The width of the corridor is approximately 600 feet. The East Timbalier Marsh Conveyance Corridor also connects to the West Belle Headland Restoration Area extending from the northeast end of the East Timbalier Marsh Borrow Area, east past East Timbalier Island, and then north to the existing shoreline on the western end of the Headland. The width of the conveyance corridor varies from approximately 400 feet to 600 feet. A plan view of the East Timbalier Marsh Conveyance Corridor is presented in Figure 2-15.


Figure 2. East Timbalier Island Pump-Out Area

Figure 2. West Belle Pump-Out Area and Conveyance Corridor

Figure 2. East Timbalier Marsh Conveyance Corridor
Temporary Access Corridors

Temporary access corridors have been designed for construction access to both East Timbalier and West Belle areas.

The access channel to the East Timbalier Restoration Area (see Figure 2-2) will approach from the northwest within Timbalier Bay. The location and dimensions of the access channel was designed based on the existing water depths identified during the design survey (JCLS 2015), to avoid existing infrastructure, minimize pipeline crossings, and to connect to the existing channel directly behind East Timbalier Island. This existing channel is currently utilized and maintained by the local oil & gas operator and utilizing it to the maximum extent possible for construction access on this project, will reduce risk associated with removing sediment over buried pipelines.

Access channel segments requiring dredging would be cut to a maximum elevation of -9 feet NAVD88 and a maximum bottom width of 150 feet. Dredged material would be placed in temporary storage areas a minimum of 30 feet from the top of cut on both sides of the channel and have a maximum footprint width of 140 feet. For the temporary storage areas, the maximum height was set at +2.0 feet NAVD88, maximum crest width at 45 feet, and side slopes at 1V:6H. The total length of the access channel inclusive of the existing channel maintained by the oil and gas operations is approximately 8,500 feet. Once access to East Timbalier Island is no longer required via the dredged channel, backfilling operation shall commence and post-construction elevations shall be no higher than 0.5 feet above the pre-construction elevations.

Two access channels were designed for the West Belle Headland (Figure 2-4). The first option begins northwest of the Restoration Area and extends southeast approximately 10,800 feet to align with the exterior marsh containment dike borrow channel. Approximately 360,000 cubic yards (CY) of material may have to be dredged for this option. The second option begins in the GOM south of the Restoration Area, loops around the western end of the restoration template, connects to the first option, and then extends south to connect to the beach and dune fill at the western end of the containment dike. The length of this access option is approximately 13,400 feet. Approximately 448,000 CY of material may have to be dredged for this option. For both options, the access channel segments requiring dredging would be cut to a maximum elevation of -9.0 feet NAVD88 and a maximum bottom width of 150 feet. Dredged material would be placed in temporary storage areas a minimum of 30 feet from the top of cut on both sides of the channel and have a maximum footprint width of 140 feet. For the temporary storage areas, the maximum height was set at +2.0 feet NAVD88, maximum crest width at 45 feet, and side slopes at 1V:6H. Once access to the island is no longer required via the dredged channels, backfilling operation shall commence and post-construction elevations shall be no higher than 0.5 feet above the pre-construction elevations.

Sediment Mining and Conveyance to the Project Areas

Two (2) Borrow Areas containing beach compatible sand, the Ship Shoal and South Pelto Borrow Areas, are proposed for utilization for constructing the beach and dune fill templates. It is anticipated the preferred method of mining these two Borrow Areas and conveying the sand to the East Timbalier Island and West Belle Headland Restoration Areas will be either of the following two methods or both: trailing suction hopper dredges (hoppers) and/or hydraulic cutterhead dredges. The first method is by hopper dredge. The Hopper dredges are self-propelled ships that use suction to excavated sand through one or two drag arms that extend to the seafloor while underway. Sand is pumped into the hull (the “hopper) of the ship and when capacity is reached the dredge leaves the borrow area, heads to the pump-out location, and is hydraulically unloaded. Sea turtle relocation trawling (See Section 4.5.3) will be required for the hopper dredge method.

The second method involves the use of a cutterhead dredge. Cutterhead dredges utilize a rotary excavating bit to loosen the sediment. The bit or cutter is located on the end of an arm (the ladder) that is hinged off the forward end of the dredge. The ladder can be maneuvered vertically to control its depth and the dredge and ladder maneuvered laterally and fore/aft using anchors deployed off the forward quarters and stern. The loosened slurry is pumped up the ladder to a large suction pump in the dredge hull, which also pumps it ashore through a submerged pipeline, often aided by the booster pump. The cutterhead will excavate the sand and transfer it through a through a floating pipeline and barge mounted distribution system into scow barges within the borrow area. The full scows will be towed to the designated pump-out areas and sand hydraulically unloaded and pumped in slurry through a pipeline to the Restoration Areas. Table 2-1 below provides detail dredging considerations for various dredging operation scenarios.

Table 2. Dredging Equipment Operation Considerations



 

Hydraulic Cutterhead Dredge with Scow Barges

Hopper Dredge

Hydraulic Cutterhead Dredge Direct Pump

Anchoring, Spudding etc. by dredge and supporting equipment

A cutterhead dredge would have a 3 point anchor and a spider barge would have a 4 point anchor in the borrow area. An unloader would likely have a 4 point anchor in the pump out area.

A hopper dredge would likely have a 3 point anchor in the pump out area.

A cutterhead dredge would have a 3 point anchor.

Sea State Operational Restrictions

Approximately 4 ft

Approximately 6 ft

Approximately 6 ft

Speed at Which Dredge Travels While Excavating

Approximately 0.4 - 0.6 knots (Cutterhead Swing Rate)

Approximately 1.8 - 2.0 Knots

Approximately 0.4 - 0.6 knots (Cutterhead Swing Rate)

Number of Vessels or Equipment on Site at any time

A cutterhead dredge in the borrow area accompanied by a support tug and a survey boat. The number of scow barges would vary but, would estimate that 6 scow barges would each be accompanied by a tug boat from the borrow area to the pump out area. One unloader which would have daily visits by a supply boat. The unloader may have a quarters barge and other support barges for fuel and water.

One or two hoppers based on availability accompanied by a survey boat.

A cutterhead dredge in the borrow area accompanied by a survey boat. The cutterhead would be supported daily by a supply boat and monthly by a fuel vessel.

Daily Working Time in Borrow Area

Approximately 13-16 hours if there were 6 scow barges working

Approximately 5-6 hours per hopper

24/7 unless down for maintenance or weather.

Sediment delivery pipeline conveyance corridors have been delineated to connect the pump-out areas and the marsh borrow areas to the Restoration Areas. The conveyance corridors have been surveyed for depth, sonar targets, and magnetic anomalies (CPE, 2009 and OSI, 2016) and sited to avoid as many pipelines, production facilities, and cultural resource targets as possible. The conveyance corridors do not require any excavation for pipeline installation, as the weighted sediment discharge pipelines will be placed directly on the sea floor. Details on pipeline crossings, floating pipe to submerged pipe transitions, and USCG safety lights and signage will be addressed in Final Design.

Two (2) Borrow Areas containing sediment for marsh creation, the East Timbalier Marsh and West Belle Marsh Borrow Areas, are proposed for utilization for constructing the marsh platform. It is anticipated the preferred method of mining these marsh borrow areas and conveying the mixed sediments to the East Timbalier Island and West Belle Headland Restoration Areas for marsh creation is a hydraulic cutterhead dredge and booster pumps situated along a continuous pipeline that extends from the dredge to the Restoration Area. Sediment delivery pipeline conveyance corridors have been delineated to connect the marsh borrow areas to the Restoration Areas. The dredged material will be discharged into the Restoration Areas.



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