A significant regulatory shift is underway for owner-operators of existing aboveground storage tanks in Washington State.  In July 2023, the Washington Department of Ecology enacted WAC 173-180, establishing new, rigorous requirements for seismic integrity.  This legislation is designed to mitigate the risk of spills resulting from seismic events, demanding a proactive approach to asset integrity management.  For facilities with existing storage tanks, understanding the technical nuances of this law is critical for ensuring compliance, safety, and operational continuity. 

This article provides a detailed overview of WAC 173-180 and discusses the core technical mandates, the required analysis methodologies, and the strategic advantages of early and thorough compliance.  Our Mechanical & Structural Engineering team is focused on understanding this regulation to provide practical recommendations that improve safety and mitigate risk from seismic activity. 

Core Requirements of WAC 173-180 

WAC 173-180 applies to existing Class 1 facilities and transfer piping systems.  The law sets a firm compliance deadline of July 2033, though this may be adjusted to align with a tank’s API 653 internal inspection schedule if the previous internal inspection occurred before July 2023.  Significant advancements in seismic engineering over the past few decades and the performance of storage tanks during major seismic events throughout the world have brought to light potentially significant vulnerabilities to critical storage tank assets, on top of the environmental and health concerns associated with potential major spills.  The primary objective is to ensure that existing infrastructure can withstand specified seismic loads and thereby prevent catastrophic failures. 

The API 650 Annex E Standard 

A central pillar of the new regulation is the mandatory use of API 650 Annex E (2020 version) for seismic analysis.  It is important to note that regulators have specified the allowable stresses defined in API 653, typically used for in-service tank assessments, are not acceptable for demonstrating seismic compliance under WAC 173-180. 

Instead, owner-operators must utilize the “new design” allowable stresses found within API 650.  This requirement effectively treats existing tanks as new builds for the purpose of seismic evaluation, necessitating a more rigorous analysis than a standard fitness-for-service (FFS) assessment.  The long-term benefit will be a consistent and high standard of safety across all applicable assets, regardless of their original design date. 

Understanding Seismic Use Groups (SUGs) 

Proper classification of storage tanks into seismic use groups (SUGs) is a fundamental first step.  Regulators expect most tanks at Class 1 facilities to fall into SUG 1 or SUG 2.  This classification dictates the importance factor used in the seismic calculations, directly influencing the design loads the tank must be proven to resist.  An accurate SUG determination is essential for a valid and defensible seismic analysis. 

The Engineering Behind Compliance: A Methodical Approach 

Compliance with WAC 173-180 begins with comprehensive data gathering and culminates in an advanced structural analysis.  

Site Seismic Hazard 

Many factors influence how the code-required seismic hazard for a site and particular tank is determined.  ASCE 7-16 is currently the minimum standard in Washington, while API 650 Annex E adopts factors from ASCE 7-10.  ASCE 7-22 was a major shakeup in seismic analysis wherein the familiar two-point response spectrum became a 22-point curve with varying site factors applied at all points (whereas only two site factors were previously applied).  Regulators are accepting ASCE 7-16 or later for WAC 173-180 compliance.  Appropriate seismic analysis requires an understanding of how these code versions differ, which is best to use, and how to adapt API 650 Annex E accordingly. 

Site-specific response spectra may provide benefit in some cases.  As low as 80% of the mapped spectral values can be taken if this is determined via analysis.  A thorough understanding of site soil conditions is requiredto form a response spectrum and for analysis using the mapped values.  It may be prudent to perform site geotechnical investigation and soil testing including shear wave velocity analysis so that a site soil class more advantageous than the default values may be used that would otherwise be required assumptions. 

Tank Seismic Analysis 

Equity Engineering performs seismic analysis per API 650 Annex E as required by WAC 173-180 using the TANK module within Equity Software’s® PlantManager® suite. 

This analysis considers two primary modes of excitation: 

1. Impulsive Mode: This represents the force generated by the tank shell and the lower portion of the contained liquid moving together as a single, rigid mass.  This mode generates the majority of seismic demand. 
2. Convective Mode: This represents the “sloshing” force generated by the upper portion of the liquid moving within the tank.  This convective action creates pressure on the tank walls, and its magnitude is a function of the tank’s geometry and the characteristics of the seismic event. 

A thorough analysis must accurately model both modes to determine the total demand placed on the structure.  Passing these code calculations with a reduced fill height is acceptable provided the site develops a plan to monitor and maintain this new maximum, which will be reviewed during annual inspections by the ecology department. 

While there are several more advanced techniques for assessing storage tanks subjected to seismic events, regulators are standing firm that only API 650 Annex E calculations are acceptable for qualifying the tank shell.  There are other options available for compliance that do not require meeting these code calculations, and regulators are open to advanced analysis approaches here, including finite element analysis (FEA). 

Protective Measures and Retrofit Solutions 

If the analysis indicates that a storage tank does not meet the requirements of API 650 Annex E (or the required reduced fill height is not economically feasible for the owner), WAC 173-180 requires the implementation of protective measures.  Our engineering and consulting services can design practical and effective solutions to bring assets into compliance.  

Approved measures include: 

• Flexible Piping Systems: Installing flexible connectors or performing analysis on existing pipes and tank nozzles to accommodate differential movement between the tank and attached piping, a common point of failure.  Regulators are in support of advanced analysis for piping and nozzles, and Equity Software’s SIMFLEX-IV piping flexibility analysis software is capable of quickly and efficiently assessing piping and tank nozzle compliance for these demands in one simple model. 
• Pile-Supported Foundations: For tanks on poor soil, deep foundations can provide the necessary stability to resist seismic loads.  While these are not a viable retrofit option, existing piles need supporting analysis demonstrating they are adequate for seismic loads if not already on file. 
• Mechanical Anchors: Upgrading or installing a robust anchoring system to secure the tank shell to the foundation, preventing sliding and overturning.  Similar to piling, supporting analysis is required to demonstrate the adequacy of existing anchorage.  Tank anchorage is evaluated for all load cases including seismic demands within Equity Software’s TANK module in PlantManager. 

It is important to remember that any proposed alternate seismic protective measures should be submitted to the Washington State Department of Ecology for approval prior to committing resources to the plan.  Regulators recommend a minimum lead time of 120 days to allow for review. 

The Strategic Value of Proactive Compliance 

While the 2033 deadline may seem distant, a proactive strategy offers substantial benefits beyond simply meeting a future date.  Waiting introduces unnecessary operational and financial risks. 

• Mitigate Catastrophic Risk: Early analysis and retrofitting directly reduce the risk of spills, environmental liability, and harm to personnel. 
• Informed Capital Planning: Understanding your facility’s compliance status now allows for the strategic allocation of capital over several budget cycles, avoiding a large, reactive expenditure near the deadline. 
• Minimize Operational Disruptions: Integrating necessary retrofits into scheduled maintenance and turnaround windows prevents costly emergency shutdowns. 

While WAC 173-180 is specific to Washington, it signals a trend toward more stringent seismic regulations in other regions.  Engaging Equity Engineering now allows you to “future-proof” your assets, ensuring you are prepared for upcoming legislation and can integrate necessary upgrades into long-term capital plans. 

Please fill in the form below to learn how Equity Engineering can support proactive seismic loading assessments of your existing storage tanks.  We will provide specialized consulting and engineering services to help your facility take the first step toward securing your facility and ensuring regulatory compliance.

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