How to Write a Basis of Design

A basis of design (BOD) documents the assumptions, requirements, codes, and design criteria a regulated facility or system will be designed to — the reference the detailed design and later qualification trace back to. A clear BOD prevents the expensive rework that comes from undocumented or drifting design intent.

1. 1

   Capture requirements and design criteria

   Consolidate the requirements the design must satisfy and translate them into design criteria an engineer can work to: capacities, classifications, operating ranges, materials of construction, utility qualities and the environmental conditions each area must hold. The translation is the work. A requirement states what is needed; a design criterion states the number or specification that satisfies it, and the basis of design is where the reasoning between the two is recorded rather than lost.

2. 2

   Record codes, standards, and assumptions

   Record the codes, standards and regulatory expectations the design is built against, and — more importantly — the assumptions. Assumed occupancy, assumed batch frequency, assumed future expansion, assumed feed-water quality: each of these silently sizes something, and each becomes wrong at some point in the facility’s life. An assumption written down can be revisited when the premise changes; an assumption held in the designer’s head becomes an unexplained constraint years later.

3. 3

   Define quality-critical aspects

   Identify the aspects of the design that affect product quality, and say why. These are what determine where verification effort will go, so naming them here rather than at qualification is what makes a risk-based approach possible at all. The test of a good list is that a reviewer could disagree with it: aspects derived from a generic category list are a classification exercise, while aspects derived from this product and this process are an argument.

4. 4

   Align with the URS and C&Q strategy

   Reconcile the document against the user requirements and against the commissioning and qualification strategy, in both directions. Forward, every requirement should reach a design criterion. Backward, every criterion should trace to a requirement — and the ones that do not are either unrequested scope or requirements nobody wrote down. Both are cheap to resolve now and expensive to discover during qualification, when the trace is built for the first time.

5. 5

   Approve and control changes

   Approve the document and place it under change control, because from this point it is the reference the design is judged against. The common failure is that the basis of design is approved, the design then evolves through normal engineering decisions, and nobody updates it — so by qualification the reference document describes an earlier facility. Changes to it should be assessed for effect on the critical aspects, not merely recorded.