Technology and Construction

MPX has selected ABB to provide preliminary and indicative EPC cost estimates for the project. ABB is a world leader in HVDC transmission projects and has been part of the MPX development team since project inception. 

Click image to expand

DC Converter stations

A preliminary layout for the proposed ±320kV HVDC Light Voltage Source Converter stations in Haynesville, ME and Boston, MA has been provided by ABB and is pictured here. Except for the transformers, virtually all of the high voltage equipment is located indoors. This provides many advantages, including a smaller footprint, less visual and audible noise impact on surrounding communities and the environment, and improved protection of high voltage insulators against airborne pollutants. Each of the completed HVDC stations will have a footprint of approximately 4.4 acres. 

The 20 acres under option in Haynesville is more than sufficient for both the DC Converter and the 345kV Switch yard used to collect the domestic and international resources. The 5.5 acres in Massport are sufficient for the completed DC Converter Station, but temporary easements will be required during the construction phase, with potential construction laydown sites identified.

Click image to expand

HVDC underground cables

The MPX will utilize ABB HVDC Light polymeric insulated cables for the 115-mile terrestrial (underground) portion of the SL-ROW between Haynesville, ME and Searsport, ME. 

The HVDC Light design provides lightweight and environmentally friendly cable (no insulating fluids). The insulation system design consists of a semi-conducting polyethylene conductor screen, a main insulation layer and a semi-conducting polyethylene insulation screen. 

The cable is manufactured in a true triple-extrusion process in a vertical continuous vulcanization (“VCV”) line at ABB’s high voltage cable factory. 

underground cable installation

The HVDC Land Cable and AC Land Cable segments will be installed using a combination of construction and installation methods including:

  • Conventional cut-and-cover of the HVDC Land Cable (Direct Buried), 
  • Horizontal Directional Drill (HDD), and
  • Conventional cut-and-cover of concrete encased conduit (Duct Bank)

The choice of method will depend on location, mechanical loading considerations, safety factors, environmental impacts, and other applicable requirements. For example, HDD will be used where practicable to avoid direct impacts and potential disruption in wetland areas, recreation areas, and roadway/railway crossings. The HVDC Land Cables will be buried to a typical depth of 4 feet and to a minimum depth of 3 feet to the top of the cable along segments where any of the conventional cut-and-cover methods are used. The installation trench for the HVDC Land Cable will be approximately 3 feet wide and a maximum of 4'9" deep. Excavation will be performed with standard earthmoving machinery, including excavators and backhoes, and will be performed in accordance with applicable standards. Any excess soil or soil unsuitable for use as backfill will be removed off-site in accordance with applicable regulations.

Typical Direct Burial Profile - Click image to expand

For the vast majority of the Land Route, within the SL-ROW, where open trenches and splice boxes can be safely managed, and future mechanical loading is not of concern, the HVDC Land Cable will be Direct Buried. The trench excavation for the splice boxes will be approximately 8 feet wide at grade, with a 1:1 slope (The slope and width of excavation may vary due to geotechnical conditions and the terrain along the route). Speed shoring may be used in areas with unstable soil conditions. The HVDC Land Cables will be installed with roughly 4-feet of cover. They will be surrounded by a layer of compacted sand backfill and above the sand backfill there will be a twelve (12) millimeter board (HDPE stokboard) for mechanical protection and warning tape installed. The native material that is excavated out of the trench will be reinstalled on top of the sand layer and compacted in 8-inch to 12-inch lifts to meet compaction requirements. 

Duct Bank installation will be used to limit the length and time that trenches will be open for public safety reasons. This is especially important along public roadways, walkways, bike paths, etc. Duct Banks will be utilized when the cable is installed under roads, in parking lots, or in the roadway shoulders per the requirements of local DOT. The use of Duct Banks provides mechanical protection for the cable from vehicle loading. Furthermore, Duct Banks allow for easier access and less environmental disturbance in the event that a cable repair is necessary post installation.

Typical Duct Bank Profile - Click image to expand

A typical construction profile for Duct Bank installation in a roadway for the HVDC Land Cable is shown here. In roadway sections, saw cutting and removal of the existing pavement is required before excavation. After removal of the existing material, the 8-inch and 2-inch PVC conduits for the HVDC Land Cables will be installed in the trench. The Duct Bank will then be encased in 1,000 PSI concrete.

Native materials will be removed off site and a fluidized thermal material will be used to backfill the remainder of the trench based on DOT specifications and requirements. In areas outside the roadway, native fill may be used to backfill over the concrete encased conduit in place of the fluidized thermal backfill material and the top 8-inch section of concrete. 

Underground Cable Splicing

The HVDC Land Cables will require splices at specific intervals along the route. Underground splice boxes (shown below on the left) will be excavated or pre-cast concrete splice boxes installed to facilitate jointing of Land Cable sections together. The use of splice boxes limits the area of disturbance during installation; however, the use of a splice box may be necessary at certain locations where mechanical loading is a factor and future access may be necessary. The HVDC Land Cable will be pulled through the Duct Bank sections and spliced within the Transition Pits or Vaults. Splice pits will be backfilled with excavated soil and/or clean backfill and layered with appropriate protection and warning tape. 

Splice pits will be approximately 8 feet wide and 20 feet long and will house both cables. This is well within the 15' wide SL-ROW Lane 4. A stable base is installed and then a temporary metal splicing trailer is placed over the cables. The splices are assembled and placed on the ground. After completion, the splice container is removed and the splices are completely buried using compacted thermal sand. An HDPE stokboard is placed 2 feet above the splices (for mechanical protection) along with a warning tape during backfilling.

In Searsport, ME, the HVDC Land Cables will join the HVDC Submarine Cables within an underground Transition Vault (shown below on the right). Two Transition Vaults will be installed adjacently to house each of the two HVDC cable joints. 

Typical Splice Box - Click image to expand

Typical Splice Vault (Land Cable to Marine Cable Transition) - Click image to expand

HVDC Submarine Cables

The 200-mile Submarine portion of the MPX will also utilize ABB’s HVDC Light cable technology, which has slightly different characteristics from the underground cable. 

A continuous 200-mile submarine cable segment will be installed between transition splice vaults in Searsport, ME and Boston, MA. The maximum depth along the underwater route are not expected to exceed 300 feet (pending final routing) and, therefore, a single-armor design will be used. The HVDC Light provides lightweight and environmentally friendly cable (no insulating fluids). The insulation system design consists of a semi-conducting polyethylene conductor screen a main insulation layer and a semi-conducting polyethylene insulation screen. 

The cable is manufactured in a true triple-extrusion process in a vertical continuous vulcanization (“VCV”) line at ABB’s high voltage cable factory. 

Typical Submarine Profile - Click image to enlarge

submarine cable installation

A typical cross-section of the Submarine Cable installation is illustrated on the right. The Submarine Cable will be buried using jet plow embedment technology to a target depth of approximately 4-feet below the present bottom along the Submarine Cable Route.

A typical jetting device will be employed. Jet plow embedment methods for submarine cable installations are considered the most effective and least environmentally damaging compared to traditional mechanical dredging and trenching operations, and have been recently authorized for similar projects (e.g., Hudson Transmission Project, Bayonne Energy Center Project, and the Neptune Regional Transmission System) and used to successfully install corresponding cables within permitted water quality thresholds. This method of installation simultaneously achieves placement of the cable at the targeted depth and burial of the cable as fluidized sediment settles back into the trench, both with minimum bottom disturbance. 

The Submarine Cable will be delivered from the cable factory in continuous lengths aboard a purpose-built cable-laying vessel. In shallow water, where the cable transportation and installation vessel cannot access the route, a cable-lay barge will be utilized. The cable-laying vessels will be equipped with Dynamic Positioning to guide installation along a predetermined track and to maintain position as needed during Submarine Cable embedment.

Purpose-built Shallow Water Barge - Click image to expand

Purpose-built Cable Laying Vessel - Click image to expand

The Submarine Cable will be installed by a jet plow device that uses a process known as simultaneous lay and burial. The jet plow is a skid/pontoon-mounted device and has no propulsion system of its own. Instead, it depends on the cable vessel for propulsion. The Submarine Cable will be deployed from the cable-laying vessel in a bundled configuration to the jet plow device, and into the jetting blade. 

The jetting blade is fitted with hydraulic pressure nozzles, which direct pumped seawater downward and backwards, to fluidize the seabed sediments to create a “trench” approximately 18-24 inches wide such that the jet plow can advance in the direction of the cable laying and embed the cable in the seabed. The cable lay vessel is equipped with water pumps that take water from just below the ocean surface to provide high pressure seawater to the jet plow device via an umbilical. The hydraulic pressure nozzles create a direct downward and backward “swept flow” force inside the trench. This provides a down and back flow of re-suspended sediments within the trench, thereby “fluidizing” the in situ sediment column as it progresses along the predetermined Submarine Cable route such that the cable settles into the trench under its own weight to the planned depth of burial. The jet plow’s hydrodynamic forces do not work to produce an upward movement of sediment into the water column since the objective of this method is to maximize gravitational replacement of re- suspended sediments within the trench to bury or “embed” the cable as the jet plow progresses along its route. This method of laying and burying cable simultaneously will ensure the placement of the Submarine Cable at the target burial depth with minimal bottom disturbance and the majority of the fluidized sediment settling back into the trench.

Video produced by MPX partner Transmission Developers, Inc.

Submarine Cable embedment will begin in Searsport on the shallow water installation barge and proceed out of Penobscot Bay. The vessel will install the first section of cable until the end of that section is reached. At that point, the cable heads will be sealed and placed on the seabed and then marked with a buoy to enable recovery. The deep water installation vessel will be brought to the recovery point, the first section cable heads will be recovered and prepared for jointing operations. When jointing is complete the cable will be returned to the seabed and jet plow embedment of the second Submarine Cable section will continue to the Boston Landfall.

It is possible that the target burial depth may not be achieved at the field-installed joint locations. To protect the cable in these areas, concrete mattresses will be placed on the seabed at each field-installed joint location. The mattresses typically have a narrow profile (less than one-foot thick) and are expected to settle into the seabed after placement.

The Submarine Cable location and burial depth will be continuously recorded during installation for use in the preparation of as-built location plans, which will be provided to agencies and organizations as required for inclusion on future navigation charts and in easement agreements. 

Click image to expand

Underground HVAC Cable Installation

The Submarine portion of the route will make landfall on the eastern end of Massport Conley Terminal at the Transition Splice Vault to convert back from submarine to underground cable. From here, Duct Bank installation will be employed for approximately 0.1 miles from the Transition Splice Vault, along the Massport easement, to the southern DC Converter Station.

Finally, the 345kV AC lines (3) connecting the DC Converter station to ISO-NE will utilize an approximately 1.25-mile Duct Bank route to connect the southern DC Converter station to the existing 345kV K Street Substation, Boston, MA.