Constructing elaborate and massive processing facilities, such as this, is simplified when the project is reduced to modules that are built off-site, then moved to location and assembled.

Modules may be assembled outdoors or inside (bottom), out of adverse weather.

Modules can be hauled by truck to their final assembly site over highways, but transportation regulations limit module size to 14 ft. wide by 16 ft. tall by 40 ft. long.

Each module represents a unique set of requirements.

The conversion of crude oil, thick and black from the ground, into products used in industrial, commercial and consumer applications, requires acres of structures that support reaction vessels and cookers, all linked by pipes, pumps, and cables that control the automated processes. Vertical space is also a factor; cracking towers can rise to several stories above the ground.

To an observer from outside the petroleum industry, it is difficult to imagine designing and constructing the labyrinthine path that takes oil through a refinery. The advent of computer-aided design and 3D CAD has made the designer's job easier over the past several decades, and the arrival of modular construction has provided improvements in the assembly process for construction crews.

Modular advantages
The fabrication and construction of petrochemical processing plants, and the modular construction of components for those plants, began more than 30 years ago, initiated by engineering firms such as Chicago Bridge & Iron Company N.V., today known as CB&I (www.cbi.com).

Instead of assembling all of the elements of a plant at the final operations site, the facility's structure is reduced to smaller elements that can be assembled indoors. A module leaves the fabrication plant nearly complete, with most of its support structure, pipe, instrument stands, electrical wiring, grating, fireproofing, insulation and other components built in and ready for operation, once connection is made to the plant's systems. Transported by truck, rail or ship to site, the modules then are assembled to other modules.

By performing most or all of the fabrication and assembly at an enclosed manufacturing facility, the modules, as well as the workers that put them together, are less at the mercy of environmental dynamics such as heat, wind, rain or freezing temperatures. Controlled working conditions allow a company to produce a higher-quality product — the metal is less prone to expansion and contraction from temperature fluctuations, and welding equipment runs on a steady voltage from the local power grid. Modularization also minimizes inclement weather delays and reduces costs since high-cost field work hours are transferred to the shop.

Fabricating modular processing facilities away from the petrochemical plant's final site reduces the impact of construction on the customer's site, a significant advantage when the installation site already is an operating plant. Modular construction also minimizes lay-down space, an important consideration when the field site is small or congested. In-air work is minimized, and foundation requirements often are simplified. Pipe often is laid out and assembled in a separate fabrication shop within the plant, then installed in the module.

Pre-fitting components onto a module permits correction of any fitting errors prior to shipment to a customer's plant, with less rework needing to be performed on-site. Procurement also is simplified, especially when the installation site is located where raw materials and equipment are expensive or difficult to obtain.

The labor component also is very critical, said Phil Baskerville, director of Ledcor Fabrication (www.ledcor.com/en/industrial/pipefabmodassembly) in Nisku, Alberta. "Building with modules reduces the need to maintain a skilled construction crew on or near site, which can cost $150 or more per day per worker just for housing," Baskerville said in an interview. Removing the need for highly skilled labor onsite offers an added advantage in areas where skilled labor is either costly or unavailable.

Assembling modules
Modules can vary widely in size. Ledcor builds modules that are 24 ft. wide, 25 ft. high, and as long as 120 ft. The company's products can weigh more than 600 tons. Ledcor builds its modules at a 46-acre all-weather, level-drained and graveled facility in Nisku. Specific modules are assembled in an enclosed space, if required, whenever weather-sensitive installations are encountered. Alternately, Jacobs Applied Technology (www.jacobs.com) builds petrochemical, pharmaceutical and specialty chemical plants in 25-ft. by 25-ft. modules that can weigh more than 600 tons, in a 400,000 square-foot building in Goose Creek, SC.

CB&I's fabrication facility in Tyler, Texas is remote from a rail line or ship dock. Transportation via truck limits modules to an average size of 14 ft. wide by 16 ft. tall by 40 ft. long. The company currently is setting up a new module fabrication facility in Beaumont, Texas, next to a waterway that also will permit construction of larger "supermodules" that can be transported by barge.

"Occasionally, we'll have a tall tower, column or vessel that will need to be cut and installed into two modules," said David Stine, quality assurance manager for CB&I in Tyler. "Once the modules are in the field and restacked, the vessel joint is welded closed."

Most modules are built complete with process support equipment, such as pumps or filters — not many modules are made with pipe only. In some installations, single modules are set on a foundation, are then joined by pipe to other modules elsewhere on the site.

Once a customer's site is evaluated and the engineers have rendered the modules as layout drawings, framing is set up on the shop floor. Module frames are joined with TIG and stick welding in installations where they are not bolted together. Flux-cored welding is used to assemble the primary support members, while lifting lugs are attached with stick welding. Lighter components are installed with short-arc, GMAW welds.

The next step is to set the equipment into place, and then pipe is installed from the equipment outwards. Assembling the pipe elements (or "spools") in an adjoining fabrication shop helps to reduce the time needed to produce the module, and adds to the savings of modular construction. At CB&I, pipe is welded primarily with GMAW, but TIG and stick welding also are used when dictated by the application. Heavy-walled pipes and small pressure vessels are joined with mechanized submerged arc welding.

Most pipes have welded, instead of flanged, joints where they are to mate to pipes outside the module. Fabrication tolerances in the shop are Piping and Fabrication Institute Standard PS1 on dimensional tolerances, typically ±¹/₈ in.

Once the pipes are installed onto the module, they are hydro-tested as part of the weld inspection, particularly any pipe that is to be painted and insulated before leaving the facility. When the module gets to the field and is connected to other pipes, its pipes are hydro-tested again.

Each module carries a complete record book. "We can track every weld made by every welder, any post-weld heat treatment records," said Stine.

A non-automated build
"No two modules are alike. This is not ‘cookie-cutter' manufacturing by any stretch of the imagination. The materials, the joint configurations, the positions are so demanding that it is pretty hard to get a set-up for steady production, which limits us to manual welding and semi-automatic weld processes," said Tom Landon, who is the manager of CB&I's corporate welding technology department in Plainfield, Ill.

"We had pursued automation in the past on the structural work and on the pipe assembly," said Stine. "We found that, when you're welding standard weight small pipe — 4 inch, 6 inch — and you need precision fit-ups, by the time you get a machine set up to do the job, a hand welder can have the job done. It almost ends up being more productive to hand weld."

"We build these modules from the ground up, so there's a lot of structural welding that goes into making the frames and support structures," said Landon. "The scope of welding runs the gamut, from heavy wall to sheet metal applications, with a variety of materials and code applications, ASME codes for the vessels, B31 for the pipe, AWS structural codes. We're working with Inconel, Monel, varieties of stainless (including alloy 20 and duplex), proprietary cast alloys (for hydrogen plants), and chrome alloys. Construction may call for post-weld heat treatment or not. And almost all on-module welding is performed out-of-position.

"Some ‘tall' modules that are to be installed vertically are built on their sides in the shop, so the workers have to adjust their mental perspective," said Stine.

With hydrogen's growth as a fuel source, modular construction will be in greater demand as a cost-and-time-saving measure. For example, a hydrogen plant at Chevron's El Segundo, Calif. refinery was replaced by Triad Engineers Limited (www.triadprojectcorporation.com) at a total installed cost of about 65 percent of the industry's usual cost.