Turnarounds Are Decisions, Not Events
Turnarounds are not events to be managed. They are decisions — among the most consequential an industrial organization makes.
And the quality of that decision depends on one thing: whether you can see how your equipment is aging right now, not just at inspection time.
Most organizations rely on RBI reports, inspection snapshots, and calendar-based logic. But damage doesn't wait for inspections. In high-temperature systems, damage mechanisms like creep, fatigue, oxidation, and thermal cycling evolve continuously under stress and temperature. Equipment operating in the creep regime is degrading minute by minute — whether we observe it or not.
When we plan a turnaround based on averages or isolated data, we're not making a decision.
We're placing a bet.
And the house always wins.
Model-Based Damage Tracking Closes This Gap
Model-Based Damage Tracking (MBDT) closes this gap.
It is not a simulation. It is a formal method grounded in material science that reconstructs how damage accumulates over time using validated parametric models. It uses three inputs: operating history, material properties, and inspection results.
Three Core Inputs
- 1.Operating history from PI historian and DCS logs — temperature cycles, pressure profiles, run durations, upsets
- 2.Material properties from NIMS, supplier certifications, and published literature
- 3.Inspection results to validate and calibrate the model over time
How Model-Based Damage Tracking Works
Operating History
The operating history comes from PI historian and DCS logs — temperature cycles, pressure profiles, run durations, upsets. For each cycle, the model calculates the time-at-temperature above critical thresholds.
Temperature has a dramatic exponential effect on damage accumulation due to activation energy (Q) — the energy barrier atoms must overcome to participate in thermally activated processes such as diffusion, dislocation climb, and oxidation. The rate of these processes follows an Arrhenius relationship:
Rate ∝ exp(-Q/RT)
where Q is activation energy, R is the gas constant, and T is absolute temperature. This exponential dependence explains why even a moderate increase in metal temperature can substantially accelerate creep rates; the magnitude depends on the alloy, stress, temperature range, and active creep mechanism.
In some diffusion-controlled creep regimes, the apparent activation energy for creep can be comparable to the activation energy for lattice self-diffusion. The relationship depends on alloy, stress, temperature, microstructure, and the active creep mechanism. Changes in measured activation energy can indicate a mechanism transition; when that occurs, models calibrated for one regime may significantly mispredict behavior in another.
Material Properties and Parametric Models
Material properties come from NIMS, supplier certifications, and published literature. The method selects data sources according to their applicability, traceability, duration, statistical basis, and relevance to the component and damage mechanism. Applicable codes, component-specific qualified data, critically evaluated long-term databases, traceable supplier data, and peer-reviewed research may each be appropriate depending on the calculation.
MBDT employs multiple parametric models depending on the damage mechanism and material system. For high-temperature creep in low-alloy steels, the Larson-Miller parameter is one commonly used approach:
LMP = T · (CLM + log₁₀ tr)
Where:
- T = absolute temperature (in Kelvin or Rankine, depending on the unit system)
- CLM = Larson-Miller constant selected from data or curves applicable to the specific material
- tr = time to rupture in hours
Alternative parametric methods include Sherby-Dorn, Manson-Haferd, and Orr-Sherby-Dorn — each valid for specific material systems and temperature ranges. For fatigue, Coffin-Manson and Paris law models are used. For oxidation, parabolic rate laws apply. For corrosion, electrochemical kinetics or empirical corrosion rates are employed depending on the environment.
The choice of model depends on the mechanism, material, and available data. The key is that each model is grounded in physical principles and validated against experimental data — not arbitrary empiricism.
Damage Index Calculation
For each critical component, MBDT calculates a Damage Index (D) — a unitless, mechanism-specific indicator representing accumulated degradation or life consumption relative to a defined assessment basis. The calculation sheet documents all assumptions: source of inputs, temperature-dependent curves used, uncertainty ranges. D is calculated separately for each mechanism — such as creep, fatigue, or corrosion — using a defined assessment basis and acceptance criterion. Mechanisms are combined only when an applicable validated interaction method is available.
Damage Index Decision Bands
- D < 0.25→ Illustrative lower-band condition
- 0.25 ≤ D < 0.75→ Illustrative monitoring band
- D ≥ 0.75→ Illustrative intervention-review band
These bands are owner-defined decision indicators. Their applicability depends on the damage mechanism, component, assessment basis, uncertainty, and applicable acceptance criteria; they are not universal risk limits.
In high-temperature equipment, variations in refractory thickness or insulation condition directly affect local metal temperature. Even modest increases in local metal temperature can materially reduce rupture life; the magnitude must be calculated from applicable material data, stress, temperature, and exposure conditions. These differences are invisible to traditional RBI but decisive in practice.
MBDT Doesn't Replace RBI — It Completes It
Let's be honest: no one likes being told their RBI program is "incomplete." And rightly so — when done well, RBI is a powerful framework. But too often, what gets called "RBI" is just a spreadsheet with generic failure modes and average process conditions. It assumes steady-state operation, ignores thermal cycling, and treats time-dependent damage as a footnote.
That's not RBI. That's theater.
MBDT doesn't replace good RBI. It can supplement RBI by adding component-specific, time-history-based calculations for applicable damage mechanisms. It brings operating history and material-response models into the decision process, so the assessment considers not only what might fail, but also how degradation may evolve.
Once D Is Known, Everything Changes
Once D is known, everything changes.
You no longer ask: "What should we replace during the turnaround?"
You ask:
- •"When must we take the unit down?"
- •"Can we extend the cycle safely?"
- •"Which components define the critical path?"
The answer lies in identifying the component and mechanism with the most restrictive applicable acceptance criterion.
- •If a component approaches or exceeds its applicable decision criterion, perform the required engineering review and define the necessary action.
- •If the assessed components remain within their applicable criteria, evaluate whether an extension is justified through the established integrity-management process.
This shifts planning from guesswork to condition-driven execution.
MBDT Accounts for Uncertainty
But MBDT does not ignore uncertainty. It accounts for it.
- •Temperature uncertainty based on instrument calibration, sensor location, thermal gradients, and operating-data quality
- •Material-property uncertainty based on applicable test data, heat-to-heat variability, and model scatter
- •Thickness uncertainty based on the inspection method, calibration, coverage, and measurement repeatability
It performs sensitivity analysis to quantify impact. The effect of uncertainty must be evaluated against the applicable decision criterion for the relevant mechanism and component. Where results are close to an acceptance boundary, engineering judgment, additional data, or a more detailed assessment may be required.
Then, it links technical state to financial outcome.
A delay might add $2M in margin capture — but push two components past D = 0.80. Is it worth it?
Now you can calculate it.
Materials Aren't Perfectly Predictable — And That's Accounted For
Some might say: "But materials aren't that predictable." And they're right — if you treat steel like a single substance. But we don't. We know microstructure matters. Fine grains favor grain boundary diffusion. Precipitates pin dislocations. Certain grain-boundary precipitates or embrittling phases can promote intergranular damage under specific material and service conditions.
That's why the model doesn't assume idealized behavior. It incorporates these realities through conservative bounds and documented assumptions. When data is missing, it uses lower-bound properties. When inspection shows faster degradation than predicted, it revises the parameters — not because the model failed, but because now we know better.
It's not about being perfect. It's about being traceable.
MBDT Is a Living Process
MBDT is not a static report. It is a living process.
Every year, predicted damage accumulation is compared to actual field observations. When multiple inspections exist, actual damage rates are calculated and compared to projections. If the difference between observed and predicted condition exceeds a pre-established owner-defined review criterion, the assumptions, inputs, and model parameters are reassessed.
Inspection Methods
Inspection methods are standardized. Surface replication is performed after cleaning the component surface and applying a suitable replica medium. The replica is examined by optical microscopy at an appropriate magnification. Observed cavity density and morphology may be classified using the selected creep-damage classification system. If repeated replications show progression in observed damage, they provide evidence of damage evolution; quantitative remaining-life estimation requires a validated assessment that integrates representative examination results with operating history, material data, and the applicable damage model.
Post-Failure Analysis
For failed components, post-failure analysis validates or corrects the assumed mechanism. Root cause analysis asks: Was the failure mode predicted? Was D rising appropriately? Corrective actions follow: adjust material property data, revise operating envelope, change inspection frequency.
Industry experience is monitored. Peer company performance on similar equipment is tracked. Technical forums and user groups are followed. Improved models or parameters are adopted when published literature provides better basis.
This closes the loop.
The model learns. Confidence grows.
No One Trusts a Number Without Context
One thing we've learned: no one trusts a number without context. That's why every D value comes with its own biography — where the inputs came from, which assumptions were made, how uncertainty was handled. It's not just a result. It's an argument.
And like any good engineering argument, it's open to challenge.
MBDT Requires Structure
MBDT requires structure.
Organizational Roles
- •Asset Integrity Engineers calculate D and trend evolution
- •Inspection Coordinators execute NDT with documented coverage and flaw detection limits
- •Plant Management reviews monthly reports on high-damage equipment and aligns decisions with business strategy
Documentation Requirements
Documentation is mandatory. Each equipment item has an Equipment Data Sheet: ID, description, location, design conditions, original fabrication date, service start date, applicable code.
- •Operating History Summary tabulates nominal and upset conditions
- •Material Property Documentation includes grade, heat treatment, source reference, temperature-dependent curves
- •Damage Calculation Summary shows detailed sheets per mechanism, total D, assumptions, input sources
- •Inspection History records date, method, results, inspector qualification, measurement data, trend analysis, limitations
- •Risk Assessment and Recommendation specifies predicted remaining life with confidence interval, recommended action with justification
Turnaround Scope Summary
MBDT operates as a continuous review cycle, not a pre-turnaround exercise. Damage indices are updated and reviewed at an owner-defined interval based on criticality, degradation rate, operating changes, and data availability. This regular cadence ensures that equipment condition is continuously monitored and turnaround decisions are made with current data — not scrambled together weeks before shutdown.
Each review cycle produces a Turnaround Scope Summary Report with the following structure:
Section A: Executive Summary
Equipment status overview, recommended scope by system, critical path activities
Section B: Critical Equipment
Specific repairs required and engineering basis
Section C: High-Priority Equipment
Conditional scope, inspection plan, and success criteria
Section D: Equipment Eligible for Deferral Review
Next planned inspection date and documented basis for deferral
Section E: New Entries
Initial calculations and inspection plan
This regular review process eliminates the chaos of last-minute turnaround planning. By the time a turnaround date approaches, the scope is already known, justified, and integrated into long-term asset strategy. Training is maintained annually, and records confirm competency in MBDT responsibilities.
This Isn't Digital Hype — It's Engineering Discipline
It's true — some engineers still roll their eyes at "yet another digital initiative." But this isn't about dashboards or AI buzzwords. It's about something older and more reliable: a calculation sheet that tells you when to act, why, and what happens if you don't.
No drama. Just clarity.
MBDT is not digital hype. It is engineering discipline applied at scale.
It replaces betting with foresight.
It turns turnarounds from reactive interventions into controlled transitions.
The Only Sustainable Advantage Is Continuity
Because the only sustainable advantage in industrial operations is not speed — it is continuity.
- •Between data and decision
- •Between today's reading and tomorrow's failure
- •Between technical judgment and corporate outcome
When that continuity exists, you don't react.
You anticipate.
And that changes everything.
— Jorge Granada
Founder & Chief Architect, Knar Global LLC
Need Help With Turnaround Planning?
Knar Global specializes in implementing Model-Based Damage Tracking systems for high-temperature industrial assets. We help organizations move from reactive maintenance to condition-driven decision-making.
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