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Stick-built plants are said to offer the lowest cost to fabricate and ship to site. And stick-built construction is often chosen if the shipping of modules has to occur within a small window of favorable weather conditions, or because of local politics or labor constraints.
Despite these limitations, there is a growing trend in Engineering, Procurement and Construction (EPC) to provide modular components for combined cycle plants as opposed to the traditional stick-built approach.
Many EPCs, such as Fluor Corporation, Gemma Power Systems, Vogt Power and Samsung C&T, as well as big OEMs, such as GE and Siemens, are leaning more towards modularizing some aspects of combined cycle facilities, i.e., the preconstruction of complete systems away from the job site that are then transported to the site.
“EPCs can provide modules which are a better economic investment than stick building something onsite,” said David Dunning, Group President for Business Development and Strategy at Fluor.
Gemma Power Systems employed modular construction at a recent project near Palm Springs, California, consisting of several LMS100 aeroderivative turbines. “We would have needed to conduct thousands of feet of welding for a very large exhaust system in a very windy environment,” said Thomas Mastronarde, Project Engineering Manager at Gemma Power Systems. “Our modules used a bolted assembly so no welding was required.” Other modular advantages include design standardization, ease of testing, faster project delivery and reduction of onsite labor.
Siemens also favors modular or “standardized” construction, but cautions that any effort to implement a cookie cutter approach to the entire design of a power plant would be challenging due to the immense variability in climate, market forces, politics, geography and preference.
Siemens began on the modular path in 2003 with a design for a modular pipe rack. “We had to build a pipe rack on time, on site in Florida after a hurricane when there were no workers available and we ended up paying a premium for labor,” said Juergen Diekmann, the company’s power plant designer. “After that, we saw the value of modularization as it reduces the cost and time of construction.”
To date, the company has produced 10 pipe-rack modules. One such project, he said, moved 68,000 man hours offsite. After the pipe-rack success, Siemens moved onto more modules for the air compressor, cooling skid, sampling container, fuel gas conditioning skid, raw water pump skid, fire pump house and demineralized water skid. As well as eliminating onsite labor, the benefits are said to include a reduction in scaffolding, painting, sandblasting and site congestion (Figure 1).
Figure 1: Modularization (dashed lines) can save time at every step of the EPC process versus stick built (solid lines)[/caption]
Figure 2: Siemens’ standardized Flex-Plant concept[/caption]
In terms of overall plant standardization, Diekmann is an advocate of a design-once approach. He suggested that reference plant designs are the way of the future, such as the Siemens Flex-Plant 10 and 30 (Figure 2). But that does not mean everyone gets the exact same elements. Instead, the plant can pick and choose among specific standard modules and sub-modules that make up the reference plant.
“Parallel engineering can then take place which generates a better workflow,” said Diekmann. “If you build pipe racks off site, workers can work on the HRSG onsite, and then you bring in the completed module and install it rapidly.”
On the planning side, greater standardization means the procurement process can begin much earlier and can shave months off the construction schedule. But he does not think all work should be done offsite. Certain elements, yes, while others are ongoing at the site. Local conditions determine which modules should be offloaded. But for every project, there is an optimum balance of onsite and offsite work (Figure 3).
Figure 3: An optimum amount of work can be moved offsite during construction[/caption]
“It is best to design once and fabricate many times, and also to design and procure in advance,” said Diekmann.
Health and safety results usually improve markedly through modularization and standardization, he added. And the OEM goes through a learning curve in fabrication once, rather than on every project as in a stick-building. Siemens second pipe rack, for instance, went much faster. The company has realized $3.6 million in savings using this methodology. “If you are looking for cost and scheduling gains, greater standardization is the way to go.”
The Avenza yard in Italy is where GE constructs massive modules for use in LNG and other oil & gas applications. Five systems were recently developed there for Australia’s Gorgon LNG field (Figure 4). GE is following that up with a further set of modules to be delivered to Petrofac Emirates for the Upper Zakum UZ750 field in Abu Dhabi.
Figure 4: GE’s Avenza yard where it constructs large oil & gas modules like this one destined for the Gorgon LNG field in Australia[/caption]
The scope of these systems is immense. Each module weighs more than 1,500 tons and is 44 meters long, 20 meters wide and 24 meters high. They house 43 MW GE 6B gas turbines and generators to supply power to oil production facilities, including pumps and gas compressors. According to GE, the value of modularization is that modules can be assembled, commissioned and tested ahead of shipment so they can be rapidly installed and brought online once they arrive.
GE is expanding the Avenza yard by adding more assembly and testing capabilities. By the end of 2014, the GE Module Construction Yard will cover 140,000 square meters, big enough to assemble 10 modules concurrently. “Plug-and-play solutions are solving some of the world’s most complex energy challenges in remote locations,” said Rafael Santana, CEO and president, Turbomachinery Solutions for GE Oil & Gas.
Modularization is also catching on in the Heat Recovery Steam Generator (HRSG) field. Standard design offerings by Vogt Power International (a Babcock Power company) incorporate modularization to reduce erection time and field-weld interconnections. If site access allows for increased shipping dimensions and higher shipping weight, an “Enhanced Constructability” HRSG methodology integrates main structural members with the heat transfer sections to further slash erection complexity and time (Figure 5).
Figure 5: A Vogt Power Module Steam Generator for a combined cycle plant. The structural steel is integrated into the heat transfer fintube module box[/caption]
“Vogt Power can offer stick built designs, but modularization is demanded in the typical markets we serve,” said Kelly Flannery, Chief Thermal Engineer at Vogt Power International.
By its nature, he said, a combined cycle plant does not lend itself very well to standard designs. Differences between OEM gas turbine and steam turbine performance characteristics, plant condenser design, and regional operating conditions (hot and cold climate and altitude) usually require that each combined cycle plant be custom designed.
Consider a gas turbine exhaust characteristic mated to an HRSG in 2x2x1 configuration with supplemental firing compared to the same gas turbine exhaust in a 1x1x1 configuration without supplemental firing. It is not possible for a single HRSG design to be optimized for efficiency and power output for each of these installations.
In some cases, Flannery concedes that a stick-built design may offer the lowest cost to fabricate and ship to site. But he is quick to add that this may not be the most economical path when field labor costs and longer erection times are factored in.
And there can be concerns over maintaining field weld quality and accurate inspection versus the surveillance available in shop fabrication. “A modularized design allows for fewer variables and greater opportunity for successful integration of the HRSG with the balance of plant piping terminal points,” said Flannery.
Modularization can also occur by shop assembling complex HRSG external piping arrangements, which include large bore valves, to the fullest extent possible. The main disadvantage is that extensive modularization requires that the HRSG vendor work closely with the EPC contractor to ensure seamless integration. This can be overcome using 3D modeling methods to ensure piping, instrument wiring, power cabling trays and process air supplies connect properly.
“The majority of Vogt Power projects behind large frame gas turbines (F-class or higher) and attached to large-scale regional electric grids tend to be custom built according to the specified operating range,” said Flannery. “Considering these projects are typically installed in locations with a relatively high cost of field labor, the designs incorporate a high degree of modularization.”
Vogt Power has had some success offering a standard HRSG design for combined cycle plants in multiple locations. These plants use small industrial or aeroderivative gas turbines and supply isolated power islands without interconnection to a regional electric grid. Standardization is further aided by the fact these plants are located in regions of Southeast Asia where the differences in ambient condition from one area to the next are of low magnitude.
Efforts to increase modularization will be an ongoing focus among HRSG vendors, said Flannery. However, as many projects being moved forward by utilities are those seeking the repowering of shuttered coal plants, this makes standardization a challenge. Variability in location and differing power output requirements will most likely result in unique HRSG designs for these combined cycle facilities, he said.
EPC contractor Samsung C&T Corp. has incorporated some standardization into plants it is constructing using Mitsubishi J-class and Siemens H-class turbines. “Standardization can be helpful in speeding and simplifying the construction process,” said Justin Zachary, Executive Vice President at Samsung C&T.
The most typical example of modularization is the HRSG, he noted, where creation of large preassembled modules minimizes the site work (especially welding at height, and brings savings in labor cost and schedule). In a similar fashion, turbomachinery OEMs try to standardize and package all equipment ancillaries (fuel, oil, water systems) to reduce field installation cost.
“However, modularization and standardization of power plant elements are more challenging,” said Zachary. “The layout of a power plant needs to be optimized for its specific site and there are many restrictions and requirements in terms of available land, noise, access roads, electrical interconnect and type of heat sink (cooling tower, ACC, one through).”
Additionally, many combined cycle arrangements comprise multiple gas turbines and HRSGs connected to a single steam turbine. One of the goals of plant arrangement is to minimize the length of high-temperature and high-pressure steam lines. If the design team is able to develop a detailed and accurate 3D model for the plant, the steam lines and their support can be pre-assembled and installed as a module with maximum dissimilar material welding done on the ground.
However such a setup does not permit any errors. A more straightforward arrangement for modularization is the single-shaft configuration, where the location of the steam turbine is fixed in relation to the HRSG. Based on 3D models, therefore, many OEMs have prepared standardized packages for this case that can be applied by the EPC contractor.
“Standardization and modularization are necessary tools for developing a competitive cost,” said Zachary. “However, their application without a thorough review of the specific site limitations, owner’s requirements and performance optimization might be a costly endeavor.”