We recently finished a project at MECS Engineering that reminded me of the importance of a transparent & well-defined design basis, a topic very close to my heart which I always talk over when I see a gap and hence this article.
Before starting any project, a critical and vital element is a well-defined design basis (also called design requirements or basis of design) in addition to a well-defined scope of work (SOW). If not appropriately defined and documented, it results in scope changes, delay in the project timeline, impact on the project budget, and sometimes a total disaster. A concise and complete design basis document makes the basis of a quality design product and a successful project.
Design basis documents the rationale, criteria, principles, assumptions, and constraints used for detailed engineering and the final design product. It is essentially a set of conditions, requirements, needs of the project & performance criteria, including the legal and code requirements. It applies to an SSC (system, structure, and component) of the complete facility or a modification in the existing facility. This article focuses on the engineering world, although the Design basis importance applies to almost every industry.
Various formats are followed by different organizations based on the type of industry. Sometimes it can be an extensive document covering every aspect of the project, from the initiation phase to the installation and commissioning phase, including the commissioning requirements. A caveat, though, is that one should not go into fine details, e.g., detailed calculations, construction details, etc. A project can also fail if the design basis document includes details meant to be part of the detailed engineering. These details are issued as separate documents.
If needed, there can be multiple design basis documents, e.g., a separate DB for Piping Design or Structural Design, where specific requirements can be added.
U.S. Department of Energy defines Design basis as "Information that identifies the specific functions to be performed by a structure, system, or component of a facility, and the specific values or range of values chosen for controlling parameters as reference bounds of Design. These values may be (1) restraints derived from generally accepted "state of the art" practices for achieving functional goals, or (2) requirements derived from analyses (based on calculations and/or experiments) of the effects of a postulated accident for which a structure, system, or component must meet its functional goals."
The Design Basis document is a dynamic document that can be updated as the project progresses. Typically, it starts at the definition or conceptual / feasibility stage of the project, focusing on the needs and assumptions to meet the project requirements, defining a narrative of the system description like the purpose of the system (or a modification) to meet the specified requirements. The design basis should also address the Constructability, Operability, Maintainability, and Safety (COMS) aspects. It is a crucial part and, if neglected, can even result in the project's cancellation during the execution stage. I have seen a couple of such cases. Every single component and line in the proposed design should have a thought process supporting it.
Most of the requirements, assumptions, and constraints should be captured during the conceptual phase; however, additional details may be needed during the preliminary stage, which should be added when known and the document revised. By the end of the Preliminary engineering phase, the Design Basis document should be rock solid with all required information for the detailed engineering. Any significant change during the detailed engineering phase would impact the project timeline, budget, and sometimes a total disaster.
Depending on the project's scope (minor OR major modification OR an entirely new facility), industry type, the design base document varies in detail. Nevertheless, the goal is to have all the required information as described above so that an independent team can deliver a final product without coming back to the author of the Design Basis document for any clarification (Ideal case).
Here is a generic partial list of information typically included in the DB.
Certain projects need special attention to the complete cycle of the project to include required design parameters. For example, one project MECS Engineering just completed involved the structural analysis of skid on a container that would be transported to various geographic locations after fabrication. In such cases, you need to include the wind and snow loads for all possible areas and pick the worst-case scenario as the design basis. Include the loads during the skid's lifting operation, applicable modes of transportation, acceleration values, etc.
There are opportunities to optimize the design by limiting the allowable loads in the design basis. e.g., there can be geographic locations where snow and wind loads are excessive, and if you try to cover every site as per the original scope of the project, the structure needed can be super heavy. In those cases, you can put a limit on allowable loads in your design basis and document it as a constraint. A separate design can be prepared to cover the very few locations with excessive loads instead of impacting the overall design applicable to all skids. These recommendations to the project director are made during the conceptual or preliminary stage and decisions captured in the design basis.
The term "Beyond Design Basis Events" became popular after the 2011 Fukushima Accident, particularly in the Nuclear industry. The tsunami and associated earthquake events were beyond the established design basis. These events can reduce or eliminate the margin of safety and result in catastrophic failure. The nuclear industry adopted mitigation strategies based on the lesson learned from the Fukushima accident.
This article covers only the high-level concet and requirements of the Design basis. There are many asects involved in a good design. For more details, lease contact us at MECS or author at arvin@mecsengineering.com
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