The integrity assessment of atmospheric storage tanks is one of the most critical processes within asset management in industrial plants. EEMUA 159, published by the Engineering Equipment and Materials Users Association, constitutes one of the primary technical reference guides for the inspection, integrity assessment, maintenance, and repair of flat-bottomed vertical steel atmospheric tanks.

EEMUA 159 is neither a design code in the same sense as API 650 nor a regulatory inspection code equivalent to API 653, but rather a technical guide for managing the integrity of these assets throughout their lifecycle. Its sixth edition consolidates decades of operational experience in refineries, storage terminals, and chemical plants with updated assessment, inspection, and maintenance methodologies.

Although both guides share the objective of preserving the integrity of atmospheric tanks in service, their philosophy and scope of application differ in relevant aspects. API 653 is a formal, more prescriptive code focusing specifically on the inspection, repair, alteration, and reconstruction of tanks designed in accordance with API 650, featuring standardized criteria and formulas to determine minimum thicknesses and inspection intervals. EEMUA 159, on the other hand, is a voluntary technical guide of broader scope that, in addition to inspection, covers the evaluation of degradation mechanisms, maintenance, and asset lifecycle management. It relies on an approach based on the tank’s knowledge, its condition, and its specific risk rather than on rigid rules. In practice, many organizations—especially those with operations in Europe or under British standards—use both guides complementarily, combining the prescriptive rigor of API 653 with the risk-oriented flexibility of EEMUA 159.

In this article, we analyze how EEMUA 159 structures integrity assessment, which deterioration mechanisms it prioritizes, and what tools it offers the integrity engineer to make informed decisions regarding inspection, repair, or replacement.

1. Scope and Philosophy of EEMUA 159

EEMUA 159 adopts an approach based on asset knowledge and condition assessment: each tank presents a specific integrity profile according to its stored product, operating history, degradation mechanisms, and foundation conditions.

Building upon this, the publication provides criteria to establish the scope and frequency of inspections using a risk- and condition-based approach, identifying relevant deterioration mechanisms and failure modes (corrosion, settlement, deformations, structural deterioration, fatigue) and defining an inspection and maintenance strategy tailored to the tank’s criticality.

One of the relevant novelties of the sixth edition is the incorporation of three tiers of integrity assessment of increasing complexity. These allow for selecting the appropriate depth of analysis based on the identified condition, preventing a localized condition from automatically implying repair or replacement without a deeper evaluation when justified. The guide is primarily oriented toward vertical cylindrical steel tanks, on-grade and flat-bottomed, and its application must consider the specific characteristics of each asset.

2. Risk-Based Integrity Management (RBI) and Reliability-Centred Maintenance (RCM)

One of the relevant aspects of EEMUA 159 is its integrity management approach based on Risk-Based Inspection (RBI) and Reliability-Centred Maintenance (RCM). Risk is evaluated by jointly considering the probability of failure and the associated consequences. The assessment must take into account relevant degradation mechanisms, the state and condition of the tank, inspection history, service conditions, prevention or mitigation measures, and the potential consequences of a failure.

The outcome of this evaluation can lead to different scopes and frequencies of inspection, in accordance with the tank’s criticality, identified degradation mechanisms, inspection history, service conditions, and available confidence level. The inspection strategy must be reviewed when relevant changes occur in operating conditions or in the asset’s state.

3. Deterioration Mechanisms: Corrosion and Settlement

Corrosion constitutes one of the most relevant degradation mechanisms in storage tanks, particularly in bottoms, where mechanisms can occur from both the product side and the soil side.

Corrosion rates can be determined from successive inspection data and used, together with applicable acceptance criteria, to estimate the future evolution of the component. The assessment must differentiate between generalized thickness loss and localized corrosion, taking into account the uncertainties associated with measurements and the evolution of the degradation mechanism. In the case of localized bottom corrosion, EEMUA 159 provides specific criteria for its assessment and for estimating remaining life.

Foundation settlement constitutes one of the critical aspects in a tank’s integrity assessment. EEMUA 159 contemplates different settlement and deformation patterns, including uniform, differential, and edge settlement, deformations between the center and the periphery, global tilting (planar tilt), and shell circumferential deformations.

When measurements approach or exceed applicable acceptance criteria, it may be necessary to perform a deeper evaluation to determine adequate operating, repair, or re-leveling conditions.

4. The Inspection and Maintenance Plan

Based on the integrity assessment and the risk management strategy, the results must be integrated into a specific inspection and maintenance plan for each tank. This plan must consider equipment history, identified degradation mechanisms, applicable inspection methods—including non-destructive testing, thickness measurement, visual inspection, and advanced techniques when appropriate—and the inspection intervals established based on the asset’s condition and criticality.

These activities must be performed by personnel with competence appropriate to their nature and complexity; EEMUA provides different competency levels for this purpose.

Current practice also incorporates advanced techniques (phased array ultrasonic testing, electromagnetic methods, robotic inspection, or unmanned aerial systems) when the degradation mechanism and assessment level justify it. Documental traceability of the entire process—inspection, assessment, maintenance, and repair—proves key in the face of audits, insurers, and regulatory bodies, constituting the objective basis for deciding on continued operation, repair, or replacement.

5. Best Practices and Common Pitfalls

At Arveng, experience supporting integrity assessment processes under EEMUA 159 allows for identifying recurrent errors: applying generic inspection intervals without justifying the actual risk of the tank; underestimating localized corrosion based solely on average rates; and failing to review the integrity strategy after significant service changes, such as a change in stored product—an aspect that EEMUA 159 contemplates within its condition- and risk-based approach.

As a best practice, we recommend integrating the results of each inspection into a digital historical record that allows analyzing degradation trends systematically, and involving geotechnical or structural engineering specialists early on when settlement approaches acceptance limits.

Conclusion

EEMUA 159 offers a broad and flexible framework for storage tank integrity management, complementary to API 653: while the latter establishes specific requirements for the inspection, repair, alteration, and reconstruction of certain tanks, EEMUA 159 provides broader guidance on degradation mechanisms, integrity assessment, and lifecycle management. Its strength lies in linking asset condition and risk with the inspection strategy, optimizing resources without compromising safety.

For the integrity engineer, mastering this methodology is, more than a regulatory requirement, an essential tool to make evidence-based decisions throughout the tank’s lifecycle.