Why the Basis of Design (BoD) Document Matters in Process Engineering

In process engineering projects, the difference between smooth execution and constant firefighting often comes down to one document: the Basis of Design (BoD). It defines what will be designed, how it will be designed, and on which assumptions and criteria the entire plant design will stand.

For process engineers, the BoD is not just paperwork; it is the technical backbone that drives design decisions from pre-FEED all the way to commissioning.

What Is a Basis of Design in Process Engineering?

The Basis of Design (BoD) is a structured engineering document that lists the principles, requirements, criteria, assumptions, and reasoning on which design calculations and decisions are based.

It typically includes:

• Design philosophy and project objectives.

• Feed and product specifications.

• Operating and design conditions (pressure, temperature, flow, utilities).

• Key process configurations and selected technologies.

• Safety, HSE, and environmental requirements.

• Applicable codes, standards, and regulatory constraints.

• Design margins, assumptions, and known limitations.

In many oil and gas and process industry projects, the BoD is prepared primarily by process engineers with inputs from all disciplines and stakeholders. It then becomes the central reference that other deliverables (PFDs, P&IDs, equipment datasheets, control philosophy) are built on.

Why the BoD Matters for Process Engineers

1. Foundation for Process Engineering

From a process engineer’s perspective, the BoD captures the design intent in one place. It defines:

• Normal, minimum, and maximum operating conditions.

• Basis for process simulations and heat and mass balance.

• Required turndown, flexibility, and expansion philosophy.

• Utility balances and constraints.

This shared understanding avoids conflicting assumptions between process, piping, mechanical, electrical, instrumentation, and civil teams. When everyone works from the same document, interface issues reduce, and process safety and operability improve.

2. Role in the FEED Phase

During Front-End Engineering Design (FEED), the BoD is one of the most important deliverables. It helps to:

• Define the project scope and technical boundary conditions.

• Freeze major process options, technologies, and flow schemes.

• Document the design criteria used for sizing key equipment and systems.

• Capture assumptions used in cost estimates and project schedule.

A clear BoD at FEED stage enables more accurate estimates and aligns all stakeholders (owner, licensor, EPC, operations) on what will be delivered, reducing scope creep and change orders later.

3. Anchor for Detailed Design

In detailed engineering, every specification, datasheet, and drawing traces back to the Basis of Design. Engineers rely on it for:

• Equipment sizing and rating criteria.

• Material selection, corrosion allowances, and design temperatures.

• Relief system design, flare load basis, and HAZOP inputs.

• Control and safeguarding philosophy.

A well-written BoD increases efficiency, improves quality of deliverables, and ensures compliance with applicable codes and standards throughout the design.

How BoD Influences the Design Cycle

Process design is iterative: a single change in feed composition, pressure level, product specification, or utility availability can impact hydraulics, equipment sizing, flare loads, energy balance, and CAPEX/OPEX.

Because the BoD documents the assumptions and criteria behind these decisions, updating it when fundamentals change:

• Keeps all disciplines synchronized.

• Reduces unplanned rework and redesign.

• Makes the impact of changes transparent to management and stakeholders.

Industry experience shows that capturing requirements, assumptions, and constraints early and then refining the BoD through conceptual and preliminary phases significantly reduces cost and schedule overruns later.

Treating the BoD as a Living Document

The most common mistake is to treat the BoD as a one-time deliverable that is “filed and forgotten”. In reality, it should evolve with the project lifecycle.

Good practices include:

✅ Version control: Maintain clear revision history and change log.

✅ Integration: Cross-reference process simulations, PFDs, P&IDs, and equipment lists with BoD sections.

✅ Governance: Use the BoD as a checklist item in design reviews, HAZOPs, and model reviews.

✅ Communication: Share BoD updates formally with all affected disciplines and the client.

By doing this, the BoD becomes the single source of truth for design decisions, from early pre-FEED through start-up.

Example: A Simple BoD Use Case

Consider a gas processing unit where the original BoD assumes a specific H2S content and a particular export pressure. Later, new reservoir data increases H2S content and changes export pipeline pressure.

If the BoD is promptly updated:

• The process engineer re-runs simulations and updates absorber, regenerator, and equipment sizing.

• Mechanical and piping engineers adjust thickness and materials as per new conditions.

• Safety team reassesses relief loads and flare capacity.

Because the change is captured and communicated through the BoD, the project team avoids fragmented decisions and hidden mismatches that typically surface only during commissioning.