The New Bedford Marine Commerce Terminal is being constructed to meet the heavy lifting loads required for offshore wind energy development. The design combines cellular steel cofferdams with a pile-supported relieving platform. © CLE Engineering, Inc.
The New Bedford Marine Commerce Terminal is being built to exceptionally robust standards to meet the needs of offshore wind energy installations.
April 15, 2014—A new marine terminal under construction in New Bedford, Massachusetts, will be the first facility in the United States built especially to meet the high load demands of the offshore wind energy industry. With work under way on the Cape Wind project, and more offshore wind developers seeking leases from the U.S. Department of the Interior, developers expect the facility to serve a vital role.
The Massachusetts Clean Energy Center (MassCEC) is developing the New Bedford Marine Commerce Terminal immediately south of an existing terminal completed circa 1960. The new terminal site provides myriad benefits, including natural protection from hurricane storm surges, deep shipping channels, and ready rail and highway access. The site is also near areas that have been targeted for offshore wind energy development.
The project presented a formidable engineering challenge—massive cranes had to be able to perform lifts of up to 551 tons at a distance of up to 98 ft while located at the edge of a platform. Additionally, the facility had to accommodate these cranes as they traveled laterally across the entire 1,000 ft length of the terminal.
“The challenges associated with this project, including the need to build one of the highest-capacity terminals in the world, coupled with shallow bedrock that needed to be blasted and the contamination that needed to be excavated, required us to bring on a deep and talented team spanning many disciplines,” said Eric Hines, Ph.D. P.E., M.ASCE, a professor of engineering at Tufts University who serves as the senior advisor to MassCEC for the project, who wrote in response to queries from Civil Engineering online.
That design team includes Apex Companies, LLC, of Boston; GZA GeoEnvironmental, Inc., of Norwood, Massachusetts; and CLE Engineering, Inc., of Marion, Massachusetts. The team examined similar facilities in Europe—which leads the world in offshore wind energy development—and found that the terminals there required either lifts from special, hardened locations or lifts from specially designed jack-up barges outfitted with cranes. Such vessels would not be permitted to be used in the United States due to the limitations imposed by the Jones Act, which regulates commerce in U.S. waters.
“MassCEC looked to the European ports that had been retrofitted to accommodate the offshore wind industry and what they found was that in many cases, although the cranes could make the picks that are required for the different components, they required a hardened surface (such as concrete decking or lagging) and assistance from the water-side crane mounted on the jack-up barge,” says Susan Nilson, P.E., M.ASCE, the president of CLE Engineering.
“When speaking with potential users of the port facility, they identified the ability to make the picks right at the edge as an advantage for users, as well as being able to traverse the quay-side, to give them better flexibility and improve efficiency of operations,” Nilson says.
The team examined the load and weight distributions of two large cranes to develop design criteria that specified a uniform live load of 4,100 psf, as well as the concentrated loads from the six scenarios that involved the heaviest loads.
“We went through probably 25 iterations of different alternatives, costing them, and determining the pluses and minuses,” says John DeRugeris, P.E., a principal engineer with CLE Engineering. “There is not much—if anything—that has been built in the U.S. that even comes close to this kind of capacity.”
The team focused on four primary solutions: a combination of steel king piles and sheet plies; hollow, oblong concrete caissons; a marginal wharf with an inshore king-pile bulkhead; and cellular steel cofferdams.
“The site is all underlain by rock,” DeRugeris says. “That may sound like a good thing, because you have a lot of bearing capacity and that has its advantages. But the problem is most of the alternatives we came up with required heavy-duty piles, which would require socketing at a considerably higher cost than driven piles. The geotechnical conditions at the site—and most of New England—are difficult for driving piles and installing sheets because you have buried boulders and rocks to deal with.”
The team found that a cellular steel cofferdams with a pile-supported relieving platform was strongest and the most cost-effective solution for the project. The team developed a testing system to identify potential obstructions for the required pile driving before construction. The contractor was required to drive H piles to depth, spaced 2 ft apart on center, around the perimeter of the project.
“We had an engineer on board, confirming that the H pile was able to be driven to depth without any evidence of obstruction, tilting, or premature refusal,” Nilson says. “As a result, we created a very detailed subsurface mapping of the perimeter of all of the cells. We advised the contractor of locations where obstructions had been identified that needed to be removed before the cells were advanced.”
Another challenge was that the extreme loads of the terminal pushed the envelope of the interlocking steel sheets used to construct the cofferdam’s cells. Lateral soil pressure was reduced by pairing the cofferdams with a robust concrete platform and heavy concrete beams, DeRugeris says. The reinforced-concrete platform is approximately 2 ft thick and rests on concrete beams 4 ft thick and as much as 8 ft wide; the beams are supported by heavy piles.
“The slab is heavily reinforced. In fact, we had to build a model just to see that we could fit all the steel inside that we talked about,” DeRugeris says.
“It’s great seeing it come together,” DeRugeris adds. “It’s a beautiful project. All the planning that we did, the preliminary investigations and study certainly paid off. The work they have done so far has been gliding right along.”
The terminal is slated to be complete by the end of the year.