Off Site Build and Transport – A unique solution to a unique problem

Steven Thompson

PSEG’s new Sewaren 7, a 540 MW combined cycle generating facility in Woodbridge, New Jersey was put out to bid for mechanical contractors in August of last year. On many levels, it was a typical bid package: a gas fired combined cycle power plant consisting of a GE centerline package, a GE/Alstom designed C-Frame HRSG, a 20 cell ACC, and the balance of plant piping and equipment.

The project did, however, present two major challenges. First, the schedule was extremely aggressive – 11 months from turbine delivery to First Fire.

This was achievable, but would require a lot of careful upfront planning and coordination between contractors. Second, the site was extremely small, and elevated on an 8-foot-tall island.

The legacy plant at the site had sustained significant damage in Hurricane Sandy, and the new design called for the entire power block to be elevated on a sheet pile walled island to help protect it against future storm surges.

The severely restricted plot plan meant that the plant could be built in only one order, backing off the island as it was constructed, like painting your way out of a tight room. It also meant that multiple components could not be worked in tandem. For instance, the crane used to erect the turbine building would have to sit on the HRSG pad to do its work – meaning the HRSG construction couldn’t start until the building was complete. Similarly, the cranes to build out the ACC would have to sit on the pads of other balance of plant pieces of equipment, delaying their installation.

As the mechanical contractor, we ran scenario after scenario trying to fit so much work into such a tight schedule and a space – and we couldn’t do it.

The concept just didn’t seem viable. The schedule would either have to run out over 2 years, or we would need 600+ men on site – an unsafe and unmanageable number in such a tight spot, and also a number that the local union labor halls would struggle to provide. We needed to build outside the box – literally.

We turned to our historical experience in the power industry, and network of partners in the market, to propose a unique alternative: perform an off-site build of the Heat Recovery Steam Generator in one piece, modularize the Air-Cooled Condenser into fully built fan cells, and then barge the units to site while the rest of the plant was being built.

These would be the largest ACC units, and the largest HRSG, ever remotely built in the United States. Durr Mechanical proposed building the units at the Port of Coeymans, on the Hudson River, just South of Albany, and barging them 150 miles downriver to site in Woodbridge, NJ.

The Port of Coeymans, owned and managed by Carver Industries, is a unique facility: it is a full-service port providing marine logistics support, but also rents out waterfront space, with access to its docks and barges, to construction firms for off-site builds. We proposed dividing the ACC into 20 fully built fan cells.

The HRSG would be fully built out as a single 4,000-ton unit. PSEG accepted the proposal, and construction began.

Since both the ACC and the HRSG had been purchased in non-modularized formats, a number of engineering feasibility studies were performed. First, we needed to design temporary erection jigs for the ACC cells that would allow us to build an entire cell on the ground, and would also allow us to drive under it, mount it on a self-propelled motorized transport, and then allow a crane at the other end to pick the cell up in one piece to set on its support tower.

Then we needed to design a temporary transportable frame for the HRSG – this proved to be a much more complicated task.

We had to design and fabricate a steel structure that would temporarily replace an 8-foot-thick concrete foundation covering more than 7000 square feet. It would also have to be designed such that it could be driven under, mounted on a linked array of self-propelled motorized transports. It then needed to be welded to the deck of a barge, and resist wind and wave loads during transport.

Further studies had to be performed on how to stiffen the units to become self-supporting, and also how the units would act once floating on the water – attachment points and stiffeners were designed, wind and wave loadings calculated, tide surveys completed, barge and lashing surveys performed, and bridge height clearance certifications made. Both the Coast Guard and Army Corp of Engineers were engaged in the shipping logistics and permitting processes.

Construction at the Port of Coeymans began in January of 2017. The ACC cells were laid out in a grid fashion, with multiple cranes able to service multiple cells at once. They were built in sequence, with small crews moving in a water fall fashion from one cell to the next to maximize efficiencies and lessons learned.

Re-engineering of the units, including repositioning of the Steam Distribution Duct and Condensate Header Duct field welds to correspond to the modular boundaries, were carried out on the fly.

This created the need for an extremely integrated field engineering and project management team, with a team of representatives from the vendor and owner working with the contractor in the field full time.

The ACC cells were modularized into single and double fan units. They were welded out in their entirety, loaded onto self-propelled motorized transports, and driven down a 7% grade road towards the water, across the port docks and onto barges.

They were then barged down river, and offloaded at the installation site.

We were able to make regular deliveries, pre-scheduled months in advance, to tie in directly to the on-site construction sequence and scheduling.

At the same time, a separate construction crew began to assemble the HRSG. The HRSG C-frames were offloaded from overseas ships that had been redirected to the Port of Coeymans, and erected just as if it were happening on site, except that it was built on top of a mobile transport frame made up of over 250,000 pounds of steel and over 2 miles of welded connections. The GE/Alstom unit was made up of 10 C-Frames, the largest of which was over 700,000 lbs. In addition, the CO, SCR, and Duct Burner units were completely field assembled in places, forming a completed vessel 130 feet tall, 70 feet wide, and over 110 feet long.

Then, over 10,000 feet of piping was welded and hung from the unit, in addition to platforms, ladders, cable tray, and instrumentation. All of the internal welding was completed, all of the non-destructive examinations were performed, all of the pre-and-post weld heat treating was completed, all of the insulation was installed, and the inlet and outlet ducts were attached.

The HRSG transportation operation began in July of 2017. A combined self-propelled motorized transport array consisting of 138 separate axle units was linked together to form a single computer controlled unit.

This was then driven under the HRSG temporary lifting and transport deck, and the entire unit rolled over steel bridges and onto a 100′ x 400′ barge – one of the largest barges in the country.

The loading operation was so large and complex, that shipping along the Hudson River waterway was suspended during it.

The loading operation lasted approximately 13 hours – 8 high volume bilge pumps had to constantly compensate for the tide coming in and out at the same time the barge was being loaded with more and more weight. The entire unit was kept level throughout the whole operation to within a .5 percent tolerance.

It took another week of welding work to lash the unit down to the deck, before the HRSG was able to set sail down the Hudson River. The trip was covered by multiple news organizations, including USA Today, NBC Channel 4, and the Wall Street Journal.

The 150-mile journey took 36 hours, and the unit had to pass under 19 bridges – the lowest of which left us with just 9 feet to spare. It sailed past the Statue of Liberty and to the Woodbrige site, where we erected a 60-foot steel ramp bridge to drive the entire HRSG directly on to the site – without the use of a dock.

Once the lashings were cut, the HRSG was driven directly across the power block, onto its foundation, and lowered onto its anchor bolts. The process of tieing into the rest of the plant equipment began immediately.

During the 7 months of construction 150 miles away, the HRSG foundation pad was used by 2 separate large boom cranes – one to build the turbine building and one to build the balance of plant piping, pipe racks, and equipment.

It would have been impossible to even begin this work without the use of the HRSG pad as a staging area, so the concept was successful. The owner gained 7 months on the overall construction schedule, and we were able to move 300,000 manhours from a congested site to a fabrication shop environment 150 miles away. As a case study, we showed that performing off site builds for large pieces of power generation equipment, despite significant logistical challenges, is a viable avenue of schedule and cost surety.

It’s an answer to the trend of power projects in the Northeast to be built on ever smaller footprints and to ever tighter schedules.

The result – that Durr Mechanical safely and efficiently built and delivered the largest off-site power fabrication project ever attempted in the US, is one we expect to repeated on future projects.