
Background
Baltimore Gas and Electric (BGE), the largest gas and electric utility provider for the state of Maryland, owns and operates among other assets, high-voltage power transmission grid system that supplies power to its many communities. The BGE high voltage transmission grid system in and around Baltimore, Maryland is shown in the figure below. The red and pink lines in the figure show the location of BGE’s high voltage powerline grid system.
The 2.5 miles portion of the existing line that crosses the Patapsco River, is located approximately 10 to 15-ft below the riverbed and has been over 50-years in service. In late 2010’s, this portion of line displayed signs of aging and deterioration. Given the vital nature of this portion of line to the overall resiliency of the transmission grid system, BGE planned to replace this part with new transmission line crossing. BGE started various alternative analysis for its replacement around the same time (late 2010) with due consideration for cost, design complexities, environmental impact, stake holder preferences, permitting complications, etc. In 2015 timeframe, the decision was reached that an overhead construction solution incorporating tall towers constructed within the Patapsco River was the preferred construction solution.
Challenges
The planned river crossing crosses the main shipping channel in the Patapsco River that enters the Baltimore harbor. The location map of the Baltimore harbor shows that many local port facilities owned and operated by Maryland Port Administration (MPA), Seagirt Marine Terminal (SMT), Dundalk Marine Terminal (DMT), Fairfield, and few others are situated in and around this area.
Many ship transits are therefore generated in and out of the harbor. A 2015 ship transit pathways data showed that there were nearly 15,000 trips made inside the shipping channels during that year alone. These ships constitute oil tankers, cargo vessels, cruise ships, bulk carriers, barges, etc. The ship transit data for the year 2015 is shown in the figure below. The gray line in the figure indicates a transit path of a vessel taken inside the Baltimore harbor in the year. Additionally, the red objects in the figure indicates the tower locations next to the shipping channel, and the red straight line indicates centerline location of the powerline crossing.
Given the proximity of the proposed towers to the main channel, potential for collision from a ship impact becomes a real concern.
Additionally, the general future trend of the ship industry shows that the ship sizes will continue to grow, which will require a large air clearance under the power line cables for ship passage, requiring super tall towers, over 400 feet in height located adjacent to the shipping channel.
Moreover, challenging soil condition which constitutes alluvial silt and soft clay deposits exists for a considerable depth below the riverbed rendering complexities for the design of foundations.
Given the Moffatt & Nichol’s experience with complex transportation, port and harbor, marine structures, and ship protection risk evaluation and structure design, their Baltimore Office was brought on-board in 2015 by the BGE team.






















Scope of Work and Alternative Analysis
Moffatt & Nichol design team, consisted of Structural, Coastal, and Geotechnical Engineers were tasked with ship collision risk evaluation, optimize the river crossing alignment and foundation locations to minimize risk of ship impact, develop ship impact protection and tower foundation structures alternatives that minimizes environmental impact and cost.
Between 2016 to 2018, Moffatt & Nichol working with the BGE team, the transmission line engineers, permitting agencies, and architects who were addressing the visual impacts, completed ship collision risk analysis and developed the structures alternatives based on cost, constructability, and permitting requirements.
The alternatives for the ship protection structures, which are the largest items planned for the project, included, pile supported concrete cap ring structures, multiple cellular coffer cell structures that would have been filled with sand inside and capped with a concrete cap, and an island structure that can stop
and ground a colliding ship. Due to the size of the ship and foundations that it would protect, the ship protection structures dimensionally were large, extending hundreds of feet in length and width directions. A few pictures showing the alternatives considered can be seen below.
Based on least cost, best constructability, and ease to permit, Pile Supported Concrete Ring structure for ship collision protection and pile supported concrete foundations for transmission towers were selected. Also, to minimize cost, environmental impact, and ship collision risk, five in-river tower foundation, Towers 2 to 6, three in-river ship collision protection structures (also, called as Vessel Collision Protection (VCP)), one for each Tower 3, 4 and 5, and three on land foundations were selected. Given the Towers 3 and 4 are located adjacent to the shipping channel, experiencing the most risk from a ship impact, incorporated the largest VCP structures. The plan and elevation of the selected crossing alignment is shown in the pictures below.
Towers 3 and 4 are the tallest of all towers, which are approximately 430-feet in height. The height of these towers was necessitated to provide an air draft clearance of over 220-feet below the high voltage power lines for ship traffic. The distance between Towers 3 and 4 were set at 2,200-feet, which pushed these two structures sufficiently far enough from the edges of the main shipping channel and assisted to reduce ship collision risk, consequently, reduced the design impact force. The 2,200-feet main span length provides a greater clearance for ship crossing compared to the Fransis Scott Key Bridge (or I-695 Key Bridge) that is situated 600-feet down stream of the key crossing. As can be seen in the figure above that the bridge consists of a steel truss superstructure with a main span length of 1,200-feet and an air draft clearance of 185-feet. A comparison of factoids between the two structures is shown in the table below.
Final Design
The final design of all Vessel Collision Protection (VCP) and tower foundation structures were carried out by Moffatt & Nichol design team between early 2018 to the end of 2019. Advanced Finite Element Analysis, soil structure interaction analysis was performed for all foundation structures. Same analysis approach was also used for VCP structures, but for those structures, pushover analysis was performed to determine their total energy absorption capability and displacement demand during a ship impact. The large demand during a ship impact scenario, and poor riverbed soil conditions required the use of twenty, 54-inch diameter steel pipe piles driven over 130-feet below riverbed for each of the Towers 3 and 4 VCP structures. The concrete pile cap that formed the ring structure for the VCPs were determined to be 14-feet wide, 7-feet deep with 5-feet tall wall extending above the concrete cap. The design demand also necessitated the use of #18 rebars, which are the largest available reinforcing for concrete. The foundation structures consisted of 42-inch diameter piles with concrete cap system.
Precast concrete members were utilized, and all cast-in-place concrete work was kept above water for constructability. The foundation and the VCP structures were outfitted with avian protection, navigation lights, fencing for security, and davit crane for material handling. The VCPs also were wrapped around with plastic lumber fendering, and Ultra High Molecular Weight Poly Urethane (UHMW-PE) sheeting to reduce frictional resistance of the surface during a ship impact scenario. The picture below shows a
typical Tower 3 and 4 foundation and VCP structure. Signed and sealed construction drawings and specifications were issued for construction in the end of 2019.
Construction
Request for proposal to competent marine constructors were issued in the end of first quarter of April of 2019. A 60% progress set of drawings and specifications were used for comparative cost and constructability evaluations by the marine contractors. McLean Contracting, a large and experienced marine contractor, local Baltimore area were selected for construction execution is fall of 2019. The selection was made based each short-listed contractor’s proposal based on cost, experience, technical approach, local knowledge, permitting experience, safety records, etc.
The test piling program for the project started in May 2020, production pile driving started in July of that year, concrete operation started soon thereafter. The test piling program constitutes statinamically tested pile and Pile Drivability Analyzer (PDA). Test piling program and installation of production piles started in yr-2020. Due to extensive local design experience of Moffatt & Nichol and detailed design and analysis during the final design phase, and comprehensive geotechnical investigation program and laboratory testing that were carried out in 2017, the installation of test and production piles proceeded, essentially as designed with minor modifications. A picture of a 54-inch pile installation can be seen below – The tallest towers and Towers 3 and 4 locations were installed in summer of 2021. The top sections of the towers were installed using a 400-feet tall lattice crawler crane supported by temporary trestle structures built for these operations. The tower sections were manually bolted by CW Write workers. CW Write were a sub-contractor working under McLean. Pictures showing tower erections are below.
The final concrete placement was for the Towers 3 & 4 VCP structures that occurred on March 10, 2022, and the substantial construction of the project was reached by McLean Contracting on April 22, 2022. During the last few months of construction, McLean completed installation of approximately 500-tons of cast-in-place reinforcing for each of the Towers 3 and 4 VCP structures, placed cast-place concrete, completed installation of structure fendering system, chain link fences, navigation and FAA light, and other miscellaneous hardware and items. Few key milestone dates are as follows:
- Construction Contract Award, February 2020
- Start of test pile program –
- Vibration monitoring piles installed – May 13, 2020
- Lateral Test Pile Program – May 27, 2020, at Tower 4 location
- PDA Test Pile installation at Tower – June 2, 2020, Tower 4 Foundation PDA.
- End of pile driving program – April 9, 2021, Tower 2 Dolphins
- Start of concrete pouring activities – October 23, 2020, Tower 6 Pile Plugs
- End of concrete pouring activities – March 15, 2022, Tower 4 VCP Fascia Wall
- Substantial Completion – April 22, 2022
- Circuit energization and operational – May 14, 2022
Owner: Baltimore Gas and Electric (BGE)
Engineers: Moffat & Nichol (Foundations and Ship Collision Protection structures)
Sargent & Lundy (Transmission Line Engineering)
Burns & McDonnel (Project Manager)
General Contractor: McLean Contracting Company