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Refineries and gas plants along the U.S. Gulf Coast typically have tight footprints, putting space at a premium and making the crane size prohibitive due to lift radius. TF Companies (TF) has found that its window section resection (WSR) method is an excellent and cost-saving alternative to more traditional methods that involve complete removal of the top section of the column or tower (also called the 'lift down top half' approach).
Once thought to be a rather novel solution to plot problems, the TF team has utilized this in-situ method a number of times with great success. Although proper engineering and highly calculated preplanning are required, this method is very safe and the durations are comparable to the age-old process of severing the column entirely. The WSR also minimizes removal of internals as well as external piping and structure, except those in the immediate repair area. This makes the process an optimal solution when large quantities of pipe work are involved.
The WSR technique can also be utilized when the column/vessel is within a structure. TF has incorporated temporary trolley systems to move the shell sections in and out of structures safely.
Job scope of a recently completed turnaround
Below is a description of a recent deisobutaniser column resection project at a Texas Gulf Coast refinery. The column was externally corroded at the number 2 shell course (lower shell sections), and a complete 4 ft shell section had to be replaced to return the column to its original integrity.
Column description:
• Diameter: 11 ft 6 in. ID.
• Thickness at resection area: .755 in. minimum.
• Shell material: SA-515-70.
• Height: 126 ft 10 in. seam to seam.
• Original design NDE: Spot (85% joint efficiency).
Repair area included an external vacuum support ring and internal tray support rings including down comers. The FEA was performed modelling (two) window sections being cut out simultaneously and 360 apart from each other. The FEA analyzed the column considering wind and all vertical loads without any pressure in the system.
The FEA revealed that, for given loadings and the design configuration as described in the report, the maximum stresses in the vessel would be within code allowable stress limits, thus this cut window replacement procedure could be performed safely if the additional vertical and horizontal stiffeners were installed as prescribed in the final report.
Description/solution
The vessel plate replacement was to be done at 4 ft beginning at the 10 ft 7 in. elevation, from the bottom head seam. The plan was to remove two opposite facing windows simultaneously (each window was 4 ft high by 4 ft 8 in. wide). The cut windows were in shell course number 2 and each of the cut windows represented 45-degree angles as measured from the center of the vessel.
Figure 2.
The window resection method is used to avoid major projects such as this when the situation fits. By utilizing a window replacement method. major costs such as large cranes are avoided.
Since the vertical and wind loading was very high, a decision was made to introduce two vertical stiffeners attached to the stiffened top and bottom plates at each of the two cut windows. These vertical stiffeners were also restrained at the ends by two horizontal stiffeners that were welded at the top and bottom end of the stiffened plates. To ensure the column's stability, the complete internal support system was installed and welded out prior to cutting the first window sections.
The vessel thicknesses used in the calculations were those of a worst-case scenario. The top shell courses were considered as being full thickness and the bottom two courses were calculated as being fully corroded. The worst-case scenario was used because it is critical that the unit be properly reinforced to ensure that the vessel will not buckle or fail due to the windows in any way weakening the shell. Based on the FEA results, it was determined that the path forward was acceptable and within allowable stress limits for the given loadings and the cut window sizes.
Planning
TF devised a preliminary schedule and an original estimate to set the baseline. Close coordination with the customer's planning team led to the development of a revised schedule that included all tasks from cradle to grave. Subsequent meetings were held with all involved contractors so that tasks and schedule details could be refined.
Shop activities
The window sections were precut, rolled, prepped and all edges were NDE (PT) examined for any imperfections. The temporary supports were all fabricated and palleted for shipment to the jobsite. Replacement tray support rings and down comers were shop fabricated and palleted for shipment.
Preturnaround activities
Equipment and materials were mobilized and set up on site prior to the start of the turnaround. Prefabricated parts from TF’s Houston shop were inventoried. Numerous preturnaround meetings were held to ensure the plant personnel and participating contractors were in unison. One critical component to the project’s success was adequate scaffolding, which had to be set up at the proper elevations to facilitate the resection activities. Coordination with the insulation contractor occurred next, meaning that the TF team could help them layout where the cut lines would be, so insulation could be removed over the resection area.
Turnaround activities
Once the turnaround commenced, the column was chemically cleaned, blinded for entry and the external man ways were opened to allow the column to aspirate. After operations completed atmospheric testing of the confined space and it was cleared and permitted for confined space entry, TF craftspersons tunneled all of the existing trays and performed an initial inspection/discovery that revealed some tray hardware replacement would be required. However, tray hardware replacement was deferred until after the window sections replacement was performed.
Figure 3.
When reinstalling the trays and tray manways, a best practice is to take photos of the installation to ensure that they are installed properly and there is a record of the proper installation.
While the initial tunnelling and inspection was being performed, the insulation contractor removed the insulation covering the repair area itself, TF’s fitters laid out the actual cut lines for the windows. Plans and cut lines were inspected and accepted by the customer's engineering and inspection groups. The lower few trays and seal pans were then removed, and temporary supports were installed, welded out and received NDE (PT) to assure attachment weld integrity.
Next, the first two windows were cut and removed. The edges were beveled and NDE (PT) tested. The first two new replacement window shell sections were fit up using key plates and pins. Once inspection accepted the fit ups, they were tacked into place and the welding began on the external side. This was followed by back gouging of the internal side which was then 100% welded. After the first two windows were completely welded on the top and bottom sides, the next two adjacent windows were cut 360 degrees apart. These windows were fitted and welded in the same manner as the first two, but now it included two new vertical seams between the newly installed pieces. This process was repeated two more times until all eight segments were installed and completely welded. After the window section replacements were complete, new internal tray supports and the new external vacuum support ring were installed.
NDE of the new seams involved PT of the bevel preps, back gouged areas and final welds plus spot x-ray (RT) and some UT (Shearwave) to the completed seams. PT examinations were performed on the new internal tray supports and the external vacuum support ring as well. Additionally, areas where all of the temporary tacks and temporary attachments were located were ground smooth and also received PT examination.
After the window replacement process was complete and some additional nozzle and internal support modifications were performed at a higher elevation on the column, all of the welded modifications were post weld heat treated (PWHT). The PWHT procedure included lower temperatures and longer durations as allowed per code, thus not requiring expensive crane support.
Entry was made again so the trays and seal pans could be installed. This was followed by a complete final internal inspection before all of the internal man ways were closed. The column external man ways were then closed, and the vessel was filled with water for a complete vessel hydro test.
Issues/obstacles
No project is without its complexities and issues. This project was no exception. The start of the project was delayed for numerous shifts because of difficulties in getting the unit and vessel cleaned properly. This was followed by inclement weather that was uncommon to the Gulf Coast region even in the first quarter of the year. High winds, freezing temperatures and freezing rain made progress bitterly slow for a number of days and even cancelled shifts due to the conditions. Even the air hoses, air tools and water lines froze up a number of times. All this, coupled with other delays due to plant issues, made this critical path project even more intense.
High winds attributed to the few first aids incurred; two employees got dust in their eyes and had to have their eyes washed even though they were wearing the correct eyewear. Still, this project, which included extensive work on several other columns onsite, was OSHA recordable free and overall was a very safe project. The experienced staff and crew carried themselves well through the entire event, even with the unforeseen obstacles they faced daily.
Although the project had its setbacks, the customer was very pleased with TF’s staff and crew. The effort and dedication to a safe, quality end product prevailed and the project was completed safely and with high quality standards throughout.
This resection section method is a good alternative to standard lift and replace methods, with which most people in the industry are familiar. TF Companies has performed a number of these resection sections and find them to have many cost-saving benefits at any column elevation. This procedure has even been used to replace shell sections on horizontal drums.
For more information, visit www.tf-companies.com.
