By Gary R. Cannell, Fluor Enterprises, Inc., Nikhil Chaubey and Evan Arms, Fluor Nuclear Power
The United States commercial nuclear industry appears to be on the verge of resuming construction of new-build, nuclear power plants. This can be attributed to two primary drivers, increased energy demand and proliferation of greenhouse gases. It is estimated that U.S. energy needs will increase 26 percent by 2030 in order to satisfy projected commercial, residential and industrial growth. Because of concern over global warming and the impact greenhouse gases may play in that process, construction of CO2 emitting plants, fossil fuels, as a source of energy is being discouraged. Renewable energy sources, while environmentally friendly, cannot meet the demand, leaving nuclear power to fill the energy gap. For these reasons, many consider modern nuclear technology to be a viable and environmentally friendly source of energy that can help meet upcoming U.S. energy needs.
In preparation for the resurgence of commercial nuclear power plant construction, Fluor has renewed its ASME nuclear certifications; required for nuclear facility construction. In addition, Fluor has reviewed its historical work practices to identify areas where improved efficiencies might be realized in project cost and schedule.
One of these areas is the control and documentation of construction activities, via the work package process. In the past, this activity tended to be labor intensive and accounted for a significant part of the overall construction effort. In addition, because of the recent hiatus in U.S. nuclear construction, a shortage of trained nuclear workers exists. To improve on the "old way of doing business" and to address the current worker shortage, Fluor Construction has developed/prepared an electronic construction work package process that will require fewer trained workers, improve control of construction activities by reducing the potential for human error, and reduce the overall time and effort associated with preparation, handling, review and storage of paper associated with controlling and documenting construction activities.
Another area where Fluor sees opportunity for improved efficiencies is in the application of state-of-the-art welding equipment and processes. New and emerging technologies are being evaluated with the idea of automating as much of the welding as is feasible. Described here is a brief summary of the Fluor-prepared electronic work package system, a description of the development/qualification of a key activity within that system (an electronic welding program - Welding Module), and efforts to identify and implement state-of-the-art welding equipment and processes.
ELECTRONIC WORK PACAKGE SYSTEM SUMMARY
The overall work package process is referred to as Nucleus. It is computer-software based and electronically performs the following three main functions:
- Captures all necessary design information, needed to construct the system, structure or component.
- Evaluates design attributes, associated with the item to be constructed, against applicable code and construction practices to identify construction requirements.
- Documents construction activities, including work requirements, completion, sign-offs, acceptance, etc.
ELECTRONIC WELDING MODULE
Welding was the first of several construction activities (others include electrical, piping, etc.) to be addressed within the Nucleus system. The software prepared for each of the several activities is referred to as a module. Key subject matter experts in the areas of welding, nondestructive examination (NDE)/Inspection and software automation were assembled to develop and qualify the Welding Module.
Welding Module Development
The initial activity was to define the program objectives, i.e., what tasks (and associated requirements) should the Welding Module perform. A review of typical activities performed by construction welding identified the following key tasks:
- Welding Procedure Specification (WPS) – selection
- Postweld Heat Treatment (PWHT) – determination and temperature/time
- Preheat – determination and temperature
- Backpurge – determination
- NDE for pre-service examination – determination and method/extent/acceptance
- NDE for final weld examination – determination and method/extent/acceptance
- Welder Qualification – qualification status and maintenance of certification
Each of the above tasks was developed/qualified separately (referred to as sub-modules) and together comprise the Welding Module. It should be noted that the Welding Module scope is not comprehensive, i.e., not all of the work package requirements, associated with each task is generated by the sub-modules. For example, PWHT requirements for materials not expected to be part of the plant design, were not developed. In addition, requirements requiring development of complex logic, for activities performed infrequently or perhaps not at all, were also excluded. The development team wanted to keep this first version relatively simple to ensure successful implementation. The system however, can be easily modified to expand scope. In the mean time, requirements not generated by the Welding Module, will be manually entered into the work package by the field welding engineer. Figure 1 provides a flow diagram of how the module generates work package welding requirements.
The sub-modules evaluate just one weld at a time in generating welding requirements. For example, to determine if PWHT is required for Weld X (and if the answer is yes, what are the temperature and time values), the sub-module retrieves specific information (attributes) about Weld X from Nucleus. These attributes, such as the governing code and class for the component, material type, thickness, joint design, etc., are evaluated to determine PWHT requirements for Weld X. Determination is made by processing the attributes through a series of logic lines (See Table 1). Each logic line describes a unique set of weld attributes along with the resulting, code-specified PWHT requirements. Requirements are then written to the work package.
With the "old way" of doing business, Design Engineering would specify weld attributes on a drawing, piping schedule, specification, etc. The welding engineer would evaluate this information against applicable code rules and company practices to determine welding requirements. Requirements were then annotated to the work package. The Welding Module operates in a similar fashion, except that all weld information/attributes reside within the IMS system in electronic format. The sub-module essentially performs the role of the welding engineer in processing information, determining welding requirements and inputting requirements into the work package.
With regard to development, the greatest challenge encountered was the conversion of code rules into discrete, simple and unambiguous attributes that could be processed by the module. Again, using the PWHT sub-module as an example, the ASME Section III default position for PWHT is that all welds are to receive treatment, unless specifically exempted.
Exemption is granted based on a variety of variables, including component class, type of joint, material thickness, chemistry, etc. The difficulty arises from the codes use of terms that at times can be somewhat ambiguous and inconsistent in their application. For example, the term "attachment weld", depending on the code class, can mean different things with regard to weld requirements. Use of the terms "cladding, hard surfacing, buttering, buildup" may produce the same or different welding requirements, again, depending on how the particular code book uses the term. Such nuances may not be difficult for an experienced welding engineer to correctly interpret, but can be problematic for a computer-based system.
Each of the sub-modules was prepared by identifying all code and company practice attributes (information needed to determine welding requirements), reducing them to simple, "computer-friendly terms", and preparing sufficient logic lines to address welding conditions anticipated during construction. During the course of development, demonstration trials were conducted to gauge sub-module performance. In an effort to ensure integrity of the sub-modules, i.e., that all code and company practices are accurately reflected in the module requirements output, a formal system for validation was developed and performed.
Welding Module Validation
As noted above, demonstration trials were conducted to assess system performance and to verify that requirements output are in compliance with the various governing codes and standards, and company construction practices. In addition, a formal validation system was established which allows the system to recognize only those welds which have been validated or "pre-approved". Each weld anticipated to be performed in the field will have been run through the module and when verified (by a welding engineer) to produce the correct output (PWHT, Preheat, WPS, etc.), will become part of an "approved" data base of welds.
If the system is requested to process an un-verified weld, it will be flagged as "Unapproved" and will require manual processing by the field welding engineer. For example, let's assume the module is asked to process an un-approved weld calling for the use of ASME SA-53 piping material (not originally anticipated in the design). The module will provide the requested output data if all required attributes were input into the NPS-IMS system, but as noted above, an error flag will display, indicating manual processing required. In this case the field welding engineer will manually review the output data and if correct, will approve for work package input. This "new" weld will then be validated for system use and added to the approved data base of welds. If welds using ASME SA-53 piping material are subsequently required, the Welding Module will now automatically process the request and provide appropriate welding requirements to the work package.
Evaluation of New and Emerging Welding Technologies
Fluor sees opportunity for improved efficiencies through the identification and application of state-of-the-art welding equipment and processes. The following new and emerging technologies are being evaluated with the idea of automating as much of the welding as is feasible.
Welding Equipment and Processes
• Automatic Welding: There are several well-known advantages resulting from the use of automatic welding processes, including improved deposition and reduced defect rates. In addition to these, the ability to reduce the volume of deposited weld metal is realized through the use of "narrow-groove" joints, facilitated by the new automatic equipment. Weld end preps, for manual welding of heavy-wall pipe (1-1/4 inch and greater), typically use a compound bevel, where the first 5/8 inch (coming out from the root) consists of a 30 degree bevel, followed by a 10 degree bevel for the remainder of the joint. Narrow-groove joints, using today's automatic equipment, are designed with 1-5 degrees per side with no root gap. Significant reduction in weld time is realized through use of these new joints. In addition, weld joint distortion and base metal heat affected zone issues are minimized. Fluor intends to utilize automatic welding processes for the majority of welds where a precise fit can be reasonably expected and final NDE requires radiography.
Small-bore tube welding also lends itself well to the use of automatic equipment; manual welding of these joints can be challenging even for experienced welders. Automatic, autogenous GTAW welding can be expected to consistently produce X-ray quality welds. As noted in the Introduction, Fluor believes application of the new, automatic welding processes can help address one of the more significant industry challenges - shortage of trained welders.
- Automatic Welding Equipment Evaluation: Fluor has performed extensive, "hands on" evaluation of equipment from the leading equipment manufacturers. Evaluation included use of the equipment in simulated field conditions and weld joint positions using piping/tubing assemblies of material composition, sizes and thickness typical of nuclear power plant construction. The following attributes were key to this evaluation:
- Ease of use: Equipment selection was based not only on performance but also on ease of use. The pendant, the interface between the welding operator and the equipment, is the critical feature in terms of successfully using the equipment. A user-friendly pendant and consistency of use between the various welding machines was important to the evaluation.
- Ease of maintenance: Less can be more. Complexity was evaluated relative to "value added" when considering equipment repair and maintenance.
- Replacement parts: The availability (and cost) of parts that can be purchased off-the-shelf versus only from OEM suppliers was an important consideration.
- Equipment manufacturer technical support: One of the more important factors in selecting welding equipment involves the technical expertise and availability of the manufacturer's representative(s). It is likely that Fluor will rely heavily on the manufacturer at the start of operations for equipment support and consultation. In a very real sense, the equipment representatives will be part of the Fluor team in the early stages of construction. Therefore, technical expertise, availability and ability to work with the construction team, was a key consideration.
- Wave-Form Controlled Power Sources: Use of advanced waveform, semi-automatic welding processes can help address the lack of "skilled" welders in the current labor force. Processes such as surface tension transfer (STT) and regulated metal deposition (RMD) are designed to make it "easier' to make open-butt, root passes. The learning curve for these new processes is significantly reduced when compared to standard processes.
These new processes also allow the welding of full-penetration piping joints, in austenitic stainless steels, without the need for inert-gas backing (back purge). This creates substantial savings in the cost of welding gases and reduces the time associated with the making, fitting and removal of purge dams.
Miscellaneous Areas for Improved Efficiencies
Weld Joint Design: Construction codes and standards typically provide only general requirements for weld joint geometry, allowing the constructor some leeway with regard to specific joint details. As noted above, the automatic heavy-wall, narrow-groove joint designs offer improved efficiencies due to both increased wire deposition rates and reduction in weld metal volume.
Fluor is also looking at updating joint details for thin-wall (less than 1-1/4 inch), manually-welded applications where the standard is a V-groove with a 37.5 ± 2.5 degree bevel angle. Sufficient physical access to the root of the joint, to achieve proper fusion, is the historical basis for this standard. Testing of several "unconventional" joint geometries is being performed with the goal of reducing weld-metal volume for thin-wall joints. Details being considered include reduced bevel angle, root-area radius and root land dimensions. Ideally, such a joint would support both manual and automatic processes to provide flexibility for managing the welding activities.
Filler Material: Field issue of SMAW filler can present significant challenges regarding waste, material traceability, temporary storage (i.e. heated rod caddies), etc. Fluor is working with several suppliers to provide an option for smaller, more robust packaging and to provide traceability, by bar coding information directly to the filler. Additionally, both coated-rod and bare-wire lengths are being evaluated to better suit actual field-use conditions and reduce waste.
Fluor believes innovation of construction practices, such as that described above, will be required to successfully complete the next round of nuclear new-build plants. The electronic work package, use of new and emerging welding technologies, etc., are among the several activities Fluor is taking to prepare for and support U.S. commercial nuclear power plant construction.
The authors would like to thank and recognize the following Fluor employees for their efforts and contributions to the activities described in this paper: Richard Maurer, Construction Management; Mark Albertin, Construction Automation; Gene Parks, Construction Automation; Kevin Clark Automation Engineering.
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