Hydrotest exemption API 653 Brittle fracture assessement for storage tanks

1. Introduction

For the past few decades, API-653 has been the principal standard governing inspection, repair, and alteration of petroleum ASTs. Within API-653 “Major Repairs” include operations such as:

• installing a shell penetration larger than NPS 12 beneath the design liquid level

• installing a bottom penetration within 12 in. of the shell

• removing and replacing or adding a shell plate beneath the design liquid level where the longest dimension of the replacement plate exceeds 12 in.

• removing or replacing annular plate ring material where the longest dimension of the replacement plate exceeds 12 in.

• complete or partial (more than one-half of the weld thickness) removal and replacement of more than 12 in. of vertical weld joining shell plates or radial weld joining the annular plate ring

• a nonmetallic repair that contributes more than one-half the strength of the shell in an area more than 12 in. high.

• installing a new bottom

• NOTE Installation of a portion of a new bottom as described in 12.3.4.3 is not defined as a major repair.

• removing and replacing part of the weld attaching the shell to the bottom, or to the annular plate ring, in excess of the amounts listed in 12.3.3.5.1 a)

• jacking a tank shell.

Historically, API-653 required hydrotest after these major repairs. Hydrostatic testing is useful for leak checks, foundation settlement evaluation, and structural integrity confirmation. However, hydrotesting is not always practical as water may not be available, or water handling/disposal may be problematic.

2. Hydrotest Waiver Basis

API-653 Section 12.3.3 permits exemption from hydrotest under certain conditions.
Examples include:

• Stress in the repair area does not exceed 7000 psi

• Fall within the safe for use area in figure 5.2  of API 653 as shown below

• Robust option within 12.3.3 is the Fitness-For-Service evaluation using API-579.

3. Fitness-For-Service Concept

API-579 defines Fitness-For-Service as a quantitative engineering method that evaluates the structural integrity of equipment with flaws, deterioration, or damage.

Fracture Mechanics Method

The FFS method uses fracture mechanics. The fundamental idea:

• compute stresses at the repair weld (via FEA or code calculations)

• measure or estimate fracture toughness of welded HAZ (preferably via CTOD)

• calculate the critical defect size

If the measured flaw size using NDE is less than calculated critical flaw size, the hydrotest is not required.

4. Practical Notes

• Standard design stress equations may be overly conservative and may not capture localized stress peaks, therefore FEA is preferred.

• Charpy can be used if CTOD is not available, but CTOD on welded samples is more accurate.

• There is also option of determining fracture toughness without testing utilizing a conservative estimated based on indexing method (million dollar curve) in API 579.

5. Example Case (Condensed)

• 100 ft diameter × 40 ft height tank, SG 1.0

• repair at shell lower course / door sheet weld, tank bottom thickness and weld 0.25”

• Stress determined via FEA with refinement for tank bottom plate→ membrane stress 1 ksi and bending stress 33 ksi.  

• MDMT -5° F, fracture toughness using indexing method, 38.8 ksi in^0.5

• The assumed characterization of surface flaw depth as per API 579 section 9.3.6.3.a.2 states, a= min[t,c]. Thus, a=c is used for assumed flaw.

• Fracture mechanics calculated that the critical flaw size has depth of 1/16” and length 1/8”.   

• typical MT/PT acceptance limit ≈ 3/16”

• critical flaw size is smaller than NDE limits, however it can be detected by NDE.

→ Hydrotest not required if the flaw detected is less than 1/8”.

6.  Conclusion

API-653 allows tanks to be returned to service after major repairs without hydrotest if a documented engineering Fitness-For-Service evaluation (per API-579) demonstrates that the repair welds can tolerate flaws larger than NDE allowable limits. This fracture mechanics-based approach is rigorous, quantitative, and fully compliant when performed correctly.