How one of the largest pressure vessels in the world was made

This crude oil desalter, fabricated by McAbee, Tuscaloosa, Ala., is 175 ft. long and 14 ft. in diameter. It’s believed to be the biggest desalter in North America. At capacity it weighs in at 2 million lbs. Images: McAbee

In July 2019 McAbee, an industrial contractor and fabricator in Tuscaloosa, Ala., got a call to quote work to build a pressure vessel that would be 175 ft. long from the tip of one formed end to the other and would have an empty weight of 520,000 lbs. This desalter, which takes heated crude oil that is then mixed with water so that the water can dilute the chloride salts and other minerals in the crude, would be the longest, largest-diameter, and thickest-walled single vessel that McAbee had ever fabricated.

“Without being flippant, this job was just a little bigger than usual,” said Wade Bedsole, project manager, McAbee. “I wish every job went as well as that job did. That’s not necessarily how the big ones go, but it was a real pleasure to work on that project.”

Proper planning and a skilled workforce can make challenging work look routine, but that doesn’t mean the entire project didn’t have its moments of held breath—followed by exhales of relief. Building this desalter was a big deal for all involved.

The Beginning of the 11-month Process

McAbee has been in business since 1962. In addition to its heavy construction and plant maintenance business, the company has a robust fabrication business. It has a 144,000-sq.-ft. pipe fabrication shop, a 35,000-sq.-ft. pressure vessel shop, a sheet metal shop, a machine shop, and a 45-acre modular assembly yard on its campus near the Black Warrior River.

McAbee, which has more than 800 employees, has been building ASME-code pressure vessels for more than 40 years. The tank shop stays busy fabricating those types of pressure vessels, according to Bedsole, but the shop’s boilermakers and welders also work on other projects, such as API 620 and 650 storage tanks, when they come through.

McAbee was no stranger to large projects. It had fabricated vessels that were longer in length, larger in diameter, or greater in wall thickness than this desalter. The company can fabricate pressure vessels up to 23 ft. ID and 220 ft. long. It has fabricated vessels with wall thicknesses of 2 in. This one was a fabricating challenge because of its sheer size that dwarfed previous projects.

The desalter is 14 ft. ID and 168 ft. tangent to tangent, where the cylinder’s straight sides connect to the knuckles of the formed dish heads on the ends. The shell walls are 1.25 in. thick and constructed from SA516-70 normalized carbon steel plate.

The fabrication facility was large enough that working on the vessel under a roof was not an issue. Manipulating and moving the fabrication, however, was something that the team had to think about.

“We have a 25-, 28-, and two 15-ton overhead cranes in our shop and fabrication bay,” Bedsole said. “Now, that’s plenty of lifting power, but you can’t pick up this big rascal in one piece with any of those cranes. But we knew that from the get-go.”

That’s why the team—Bedsole, the shop manager, the foreman, and the men that would be working on the vessel—decided to tackle the job in sections. None of the three sections would be heavy enough to exceed their overhead crane capacity.

Because of limitations with overhead crane capacity, the desalter had to be fabricated in three sections that were then joined to create the one large pressure vessel.

The other thing the shop needed was a way to manipulate the entire vessel when final assembly was taking place. To help with that, Bedsole said the team purchased a set of 600-ton Ransome turning rolls for the project.

With those two material movement questions addressed, the project was ready to move forward. Codeware Compress software was used for the mechanical design of the system, and finite element analysis was done to help prove out the design. AutoCAD was used to produce all the fabrication drawings.

Each shell began as a 1.25- by 120- by 540-in. sheet of flat plate. The sections were plasma-cut to size and then formed on a plate roller to create 1.25- by 168- by 118-in. cans.

A core group of seven people performed a majority of the fabrication work. At times, however, the workforce grew to 15 as other people were freed up from other projects. Bedsole noted that even with the large project lasting months, the fabrication team was still able to meet deadlines on other jobs, one being a stainless steel reactor, for example.

Three separate crews handled fabrication on the three separate cans, with each can sitting on its own set of rollers. Four welders performed most of the joining work.

All longitudinal seam and circumferential seam welds were performed using submerged arc welding. Gas tungsten arc welding was used for root passes, and gas metal arc welding was used for items such as attachments. Shielded metal arc and flux-cored arc welding were used on a limited basis where specifications called for it.

All of the welds were 100% radiography, per code and customer-specified requirements. Given how the vessel was designed in terms of the amount of pressure it would be exposed to and the wall thickness, Bedsole said such extensive testing was to be expected. Hydrostatic, ultrasonic, and penetrant testing also were done where required.

“To be honest with you, there wasn’t a bad weld on the whole thing,” Bedsole said.

Bringing the three cans together was the next big challenge. Roundness checks were done to ensure proper fit-up, and location checks for internal and external attachments were needed to reduce the risk of misaligned piping and hanger systems when the final tank assembly was done. The three cans also had to be jacked up and leveled so that the new Ransome set of rollers could be set up and that proper fit-up could be done. (A firm that specialized in cold forming supplied the domed ends that sat on each end of the pressure vessel.)

“A big challenge was making the round seams line up, but also keep in mind that we had all of these internals that needed to line up as well because they all bolt up together,” Bedsole said.

With the desalter being as large as it was and with a last-minute change in the delivery plan, the moving process took seven days.

McAbee didn’t design the internals, but it fabricated them and had to install them. As a result, the team had to tweak the designs a bit to reflect the real world of fabricating. At every round seam they included a break so that a spool could be dropped in when final assembly was occurring.

“To give you an idea of what it looked like in there, we had 520 ft. of 10-in. pipe with 104 flange joints; 170 ft. of 3-in. pipe with a paltry 20 flange joints; and another 160 ft. of 2-in. pipe with another 20 flange joints,” Bedsole said. There also was a couple hundred feet of 3- by 3-in. and 4- by 4-in. angle for hangers for the electrostatic grid and just over 40 rigging clips.

With the addition of ladders and platforms, the desalter needed to be painted. Because of the size of the fabrication and the realization that a rain shower is never too far away during an Alabama spring, the painting took place in the fabrication shop. An enclosure was built around the entire completely assembled vessel, and when pretreatment was complete, a two-coat, high-temperature paint was applied. Bedsole added that the company’s new 600-ton set of rollers were removed before painting took place. Mammoet, the subcontractor that was responsible for moving the pressure vessel to its final destination, also assisted in removing the set of rollers and placing the vessel on hydro pads before the hydrotesting, blasting, painting, and insulating took place.

When all that was completed, it was time to move the vessel next door. Bedsole said that one of the perks of the job was that it was going to a refinery, a longtime McAbee customer that the company shared a fence with. There was not going to be any long-distance haul over country roads or on a barge. At least, it looked less complicated during the planning stage.

The Final Delivery

The desalter was built in such a way that when it was loaded up on the transporters, it would never have to be turned around. It would go from the fabrication shop to the nearby refinery and dropped off on its new piers. That was the plan, but plans are always subject to change.

As Bedsole described, these deliveries rarely are in the front of a plant in an open field. The customer needed to take an alternative route—straight through the facility.

“It was quite a challenge, but it wasn’t anything that we couldn’t handle,” Bedsole said.

Getting it out of the fabrication shop went as expected. Hydraulic jacks and cribbing were used to elevate the desalter and allow two transporters to back under it. It was lowered onto the transporters and driven to the customer’s site.

In the middle of the plant, they had to take the desalter off the transporters, lay it down on the track they had built under a pipe rack, jack it back up, put it back on the transporters, and finish taking it to its final home. Just to add to the stress of the project, the pipe rack that blocked the way was the main feedline to the plant.

The entire moving process took about seven days with a crew of nine people. Almost 11 months after the initial phone call was made to discuss the project, the desalter was sitting complete on its new piers.

“It was a big project, and it had some hurdles. But we planned our work and worked our plan,” Bedsole said. “It all went swimmingly, and everyone was happy with it.

“It was hard to beat, and I wish we had another just like it.”