Retrofit projects are often very time-consuming, both for the engineers who must take numerous field measurements to produce drawings and for the contractor that must fabricate each assembly on site. A more cost-effective approach is to begin with a highly accurate set of as-built 3D models produced by laser scanning technology.
Power plant retrofit or renovation projects can be tricky. Locating new equipment or structures at an existing site usually means preparing a design that will fit in a highly congested area in a way that will not clash with any existing structures, piping, or equipment. Nor can the added equipment or structure be located in areas that are inaccessible. In the past, designers have addressed this design complexity in one of two ways.
The first option was to take extensive field notes based on tape measurements. With extensive field notes taken by hand, the designer took responsibility for the complete design, particularly with respect to surrounding obstructions. The contractor would build to the drawings, and when (not if) problems were encountered during construction, the contractor would request a change order for more money and time to complete the project.
The second approach was to alert the contractor, in the drawings, that there were constraints that must be “field verified.” However, leaving key dimensions to be field verified requires the contractor to redesign the project, particularly structural steel and piping, because each piece must be hand-fit and welded in place. This approach will also significantly drive up field labor costs. Contractors, who are justifiably concerned about assuming additional cost and schedule risk, will price work accordingly.
LIDAR Sees All
Today, there is a better way. Light Detection and Ranging (LIDAR) is an optical remote-sensing technology that measures the distance to an object by illuminating the object using pulses from a laser to produce three-dimensional (3D) geometric information about the object. The term “laser scan” is frequently used instead of LIDAR.
LIDAR is particularly useful in power plant renovation projects because it brings the power plant design back to the office rather than requiring work in the field. The laser scan accurately locates existing pipes, conduit, equipment, structures, and other obstructions and appurtenances that affect the design of new structural framing, regardless of complexity, as will be described in the following case study.
In the office, the designer can take the point cloud produced by the LIDAR scanner and import it into a modeling program to produce 3D models of objects of interest. The designer is then able to model the new structures, equipment, and piping as required to work with existing as-built conditions. Clash conditions are easily detected and resolved in the model, not in the field. The model of the new and existing as-built conditions is then used to produce 2D design drawings suitable for steel fabricators.The assemblies can then be constructed from bolted, shop-fabricated steel members instead of field-cut welded members. The switch to using prefabricated pieces leads to more predictable installation costs and schedules, which means that projects can be competitively bid by regional contractors.
In sum, incorporating LIDAR into the design process saves time and money for engineering, makes field work safer, and reduces cost and schedule risk for the owner and contractor.
Limestone Mills Platform Case Study
LIDAR was used in the design of a series of platforms to access the limestone mills at a recently constructed fluidized bed combustion lignite plant in central Texas. Limestone is pulverized in the mills and injected into the furnace along with the crushed lignite. As the lignite burns, the limestone absorbs sulfur dioxide as part of the combustion process, which reduces the downstream sulfur dioxide content.
Moisture in the limestone can clog the feed chute of a limestone mill. The project began when plant operators requested platforms from which the feed chute at each mill could be cleaned. The platform also needed to be large enough to allow access around the outside of the mill housing to service a large access door.
The plant uses six limestone mills; the west mill is shown in Figure 1 in its “as found” configuration.
|1. Original equipment. This is one of the six limestone mills before platforms were constructed. Courtesy: JQ|
This was one of the first steel platform expansion projects to be constructed from bolted, shop-fabricated steel members instead of field-cut welded members at the plant. The limestone mill platforms were optimized to fit within the space and around the surrounding equipment, pipes, conduits, and valves. Neither the LIDAR scanning process nor the assembly of the platforms required an outage or interrupted any plant processes.
LIDAR, combined with 3D modeling, is a much improved variation of the traditional option one outlined earlier. The designer still takes responsibility for locating the existing pipe, equipment, and other details, but the design is based on more complete and accurate dimensions. In general, LIDAR scans are accurate to plus or minus 0.25 inch and are often more accurate than that. This substantially reduces the likelihood of a change order or construction delay due to interferences encountered in the field during steel erection.
The LIDAR Design Process
There are typically seven steps in the design process when using LIDAR. Below, we describe those steps and illustrate how LIDAR was successfully used in the platform design for this case study.
1. Perform the LIDAR High-Density Scans. Scanning is a line-of-sight process, so anything that cannot be seen from the vantage point of the scanner is not scanned. Moving the scanner to a new location and making another scan, where the object is visible, will fill in the resulting “shadows.” Scanning resolution is the density of points that can be measured and varies based on distance of the object from the scanner. An operator adjusts the resolution because resolution affects the speed of the scan. Typical resolution settings at 100 meters are: 20 x 20 cm, 10 x 10 cm, 5 x 5 cm (the most common), or 2 x 2 cm.
A “target” placed on the objects of interest determines the location and orientation of the scans relative to each other. Back in the office, the scans are registered relative to each other using the targets. As long as a scanner can “see” at least three targets in each scan, the scans can be registered to each other to obtain a point cloud of the volume of interest.
Figure 2 shows a LIDAR unit mounted on a standard surveyor’s tripod. The unit has two windows—one on the side and one on the top—through which it sends and receives the laser beam. LIDAR has an integral camera to take digital photos that are used to assign color to points from the scans. The handle can be removed to allow the scanner to scan directly above if the area above the scanner is included in the volume of interest.
|2. Easy targets. The laser scanner or LIDAR is mounted on a tripod adjacent to a plant motor with a temporary target—the white and blue sticker on the control panel cover—that is used to properly align, or “register” the scans. Courtesy: JQ|
Data describing the space near the mills was obtained with a LIDAR scanner in two days by technicians working safely away from operating equipment. Each scan takes 5 to 15 minutes, depending on the resolution and whether photos are taken. For this project, 25 scans were obtained for the six limestone mills.
2. Process the Scan Data. Back in office, the registered point cloud file is created and the file is colorized.
In the limestone mill project, once the scans were registered into a single point cloud, designers could measure existing conditions and check for interferences entirely in computer-assisted design (CAD) software, eliminating the need for several trips to the plant to record myriad field measurements. The point cloud captured encumbrances that were not recorded on as-built drawings and which interfered with the layout of the new platforms.
Figure 3 shows the raw, uncolorized scan of the limestone mill, and Figure 4 shows the same scan after photos from the scanner were used to assign colors to the points. The color makes it easier—in some cases much easier—for the designer to discern objects in the scan.
|3. Collected cloud data. This is the point cloud for the limestone mill shown in Figure 1—without color information from the integrated digital camera. Courtesy: JQ|
|4. Colorized cloud data. The point cloud shown in Figure 3 now includes color information provided by the integrated digital camera. Courtesy: JQ|
3. Import and Post-Process Data. Next, the point cloud file is imported into a 3D CAD file using third-party software. Then post-processing of the registered point cloud file takes place so that extraneous points are eliminated. The range of the scanner can be limited in the field, but limiting the range does not speed up the scan, so the scanner is usually left to scan everything.
The point cloud can also be divided into “levels” or specific areas of interest. This reduces the point cloud file size.
Finally, undesirable elements, such as scaffolding, vehicles, and people are removed.
4. Model Existing Equipment, Structures, Pipes, and Conduits. As the designer goes through the point cloud to lay out a new structural model, he or she can cut slices through the point cloud and eliminate some of the clutter. The designer can see the structural elements, railings, and equipment. As the designer zooms in with the point cloud file, objects become much clearer.
In addition to large pieces of equipment, the point cloud is used to model elements such as pipes, ducts, and conduits, including hangers, valves, concrete pedestals, and structural steel members. Even the flange width, beam depth, and flange thickness of existing structural members can be measured from the point cloud.
5. Lay Out and Design the New Structure(s). Once the 3D model of the as-built configuration is completed, the same 3D CAD file can be used to design the new elements, with particular attention paid to existing elements to avoid interferences. Most of the new structure is modeled with the point cloud turned off, but the designer occasionally toggles the point cloud back on to make sure the structure doesn’t clash with existing piping, structures, or equipment.
Figure 5 shows the 3D model of the new platforms and the point cloud of the existing plant. In general, every valve, instrument sensor or gauge, small-diameter tubing, and flexible electrical cabling is not modeled, but these items are accounted for when the structure is laid out around the items in the point cloud. Figure 6 shows cable trays and pipes that the new platforms were designed around. Figure 7 is a detailed view of the platforms’ 3D model, including a swinging hatch that affected the location of railing.
|5. Perfect fit. The 3D model of the new platforms is superimposed on existing equipment, structure, pipes, and electrical conduit produced from the colorized point cloud. Courtesy: JQ|
|6. Infinite views. The 3D model can be manipulated in any direction and at any zoom during design. Shown are the new platforms built around the model of the existing plant equipment generated from the point cloud. Courtesy: JQ|
|7. User input required. Plant operators were particularly interested in the elevated platform arrangement. This view of the 3D model shows all the necessary details, including the swinging access door on the mill. Courtesy: JQ|
6. Do a Reality Check. Provide views of the model to plant staff to review and confirm that the work will meet operational needs. In some projects, a virtual walk-through of the project is possible where equipment operators and maintainers can explore the design as they would after construction. Changes can be made in the software model much faster and cheaper than after the structural steel is installed.
7. Complete the Design Documentation. The final step is to cut sections through the model to start laying out the 2D drawings. After this point the production process is no different than the traditional 2D drawing production process, and the end result is a set of 2D construction drawings.
The comprehensiveness and detail of the information allowed the platform designers to quickly and accurately complete their work. With high-quality drawings, the fabricator was able to rapidly prefabricate parts. Simplifying the steel connections and reducing the amount of time the contractor was mobilized on site reduced the risk of impacting plant operations. There were no construction change orders on the project, and a last-minute addition to the scope of work that doubled the size of the platforms was designed in less than a week because no additional field measurements were required (Figure 8).
|8. Completed project. The completed platforms around the limestone mill that was shown in Figure 1. Courtesy: JQ|
Future Uses of 3D Models
Contractors have commented that when bidding on platforms projects based on LIDAR field measurements, they are confident that the drawings are going to accurately reflect the existing conditions and show the level of detail needed for the job. Lower contractor bid prices reflect the improved drawing quality. In fact, some contractors have taken the 3D CAD files created by the engineer and imported the files into detailing software to prepare shop drawings, further reducing the time required for manual re-entry of geometric and material information.
As more contractors adopt 3D modeling into their standard procedures, the 2D drawing creation step may no longer be necessary, because 3D models can then be turned over to the contractor to prepare shop drawings.
The 3D model of the completed project is also beneficial to the owner because the images are easier to understand by those without a technical or plant operations background. Plant engineers have commented on how the images from a 3D model, when shown to operators at the design phase, have elicited much more feedback than previously, leading to the optimum platform being constructed for the operator’s needs. An asset management engineer at the plant in the case study said, “The 3D models are terrific. We get a lot more feedback from operators compared to when we’d review drawings.”
The 3D models also serve as documentation of the final as-built configuration. And, after construction, the models from past projects have the added advantage of helping to visualize future projects in the same area.
— Jason Hart, PE (firstname.lastname@example.org), is the principal and John Bremer, PE is the engineering technical lead for JQ’s Industrial Facilities group.