Design by computer
Sargent & Lundy (S&L, www.sargentlundy.com) used its 3-D modeling system, PLADES, for the detailed design of the unit. S&L integrated equipment models from all of the major equipment vendors to develop the overall plant model. The model (Figure 8) served as the primary tool for walkthroughs, constructability reviews, interference checking, and intercompany communications. Burns & McDonnell (www.burnsmcd.com) served as the owner's engineer.
Scheduling work
The entire project, from notice to proceed (NTP) to substantial completion, took just 45 months to finish. Critical path procurement began immediately after the NTP was given to the team in September 2003. Long-lead components included boiler alloy parts from Sumitomo Metals, rotor forgings from Japan Steel Works, and boiler structural steel from Central Texas Iron Works (www.ctiw.com), all ordered by the end of 2003. Erection of the boiler's structural steel began in June 2004; the top girders were set in February 2005.
Initial site preparation began in September 2003 with the setting of pilings. Foundation work began in February 2004, and the turbine pedestal was completed about a year after receipt of the NTP.
Structural steel and ductwork delivery began in May 2004, using a temporary barge unloading facility (Figure 9) built on the Missouri River. It enabled modular shipments, including box sections of boiler ductwork. Boiler components began arriving in October 2004. The steam turbine and generator also were delivered via the Missouri River in May 2005; they were placed on their foundations by August.
Setting of the boiler's top girders, a major milestone for a sliding-pressure Benson boiler, occurred in February 2005. Once the girders were in place, proper boiler erection work began. Other mechanical equipment (condensers, coal mills, etc.) were installed soon afterward. The crossover coal conveyor between Units 3 and 4 was assembled on the ground and lifted into place in May 2005.
The boiler hydro test was completed in June 2006. First oil fire followed in November 2006, first steam flow in January 2007, and first coal fire in February 2007. The plant operated at 100% load as a prerequisite for Substantial Completion, which was achieved June 1, 2007. Plant shake-down operations and contract acceptance testing were continuing at press time.
Overcoming challenges
The tight project schedule necessitated the use of several modern construction processes. For example, HP and IP turbine installation time was reduced by putting the two turbines onto a single shaft before they left the factory.
Other advanced construction practices that Hitachi has pioneered—originally for boiling-water reaactors—are prefabricating large components and simulating erection of structures and components (see "Transfer ABWR construction techniques to U.S. shores," POWER, May 2007). Both techniques were applied on this project (Figure 10). Hitachi calls its process "simultaneous erection" of all boiler-related components such as ductwork, piping, and other items along with the structural steel. The process reduced installation time by eliminating the time-intensive placement of components after the steel was erected.
Construction learning curves also played a part in developing the project schedule. Over the past decade contractors have become expert in constructing "horizontal" power plants using the ubiquitous combined cycle. Supercritical steam plants, especially those using "simultaneous erection" techniques require a contractor to think "vertically" and in three dimensions. These are skills that require an investment of time and effort and can only be learned on the job.
Brownfield projects often pose design and construction problems when there are "surprises" during excavation, and the WSEC project was no different. In some instances, existing underground piping had to be partially excavated to establish exact coordinates for other design work in the area. Also, limited adjacent laydown space meant that component installation had to be staged further away. For this reason, the work of team members had to be closely coordinated to optimize the sequencing of design work and equipment deliveries.
For example, note in Figure 11 that Unit 4's air quality control equipment had to be oriented perpendicular to the boiler because of the locations of existing coal-handling facilities and because the water treatment plant is located within the track loop for coal unit trains. Construction access to these areas had to be coordinated with deliveries of coal to WSEC Units 1, 2, and 3.
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