API 20E: Breaking Down the Three Bolting Specification Levels (BSL-1, BSL-2and BSL-3)

In the high-risk world of petroleum and natural gas, even the smallest hardware components—like bolts and studs—play a critical role in ensuring equipment integrity, worker safety, and environmental protection. These fasteners are not simply off-the-shelf items; they must meet strict quality standards to perform reliably under extreme conditions.

To address this need, the American Petroleum Institute (API) introduced the API 20E standard, specifically for carbon and alloy steel bolting used in critical applications. A key feature of this standard is the introduction of three Bolting Specification Levels (BSLs)—BSL-1, BSL-2, and BSL-3—that classify bolting requirements based on the criticality of their intended service.

This article breaks down each BSL in detail, highlighting what makes them different and how engineers and procurement professionals choose the right level based on project needs.

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What Is API 20E?

Before diving into the BSLs, let’s briefly recap what API 20E covers.

API 20E is the “Specification for Alloy and Carbon Steel Bolting for Use in the Petroleum and Natural Gas Industries.” It defines requirements for material selection, manufacturing, testing, inspection, traceability, and documentation for bolts used in pressure-containing and pressure-controlling equipment.

Typical applications include:

• Wellhead assemblies
• Christmas trees
• Valves and flanges
• Pressure vessels
• Manifolds

These are often used in critical environments such as:

• High-pressure gas lines
• Offshore drilling rigs
• Subsea production systems
• Sour (H₂S) service environments

Due to varying levels of service criticality, API 20E categorizes bolting requirements into three Bolting Specification Levels (BSLs).

What Are BSLs?

Bolting Specification Levels (BSLs) represent tiers of quality and testing rigor. They ensure that the bolting used in a specific application meets the appropriate level of performance, reliability, and documentation based on its risk and service environment.

The three BSLs are:

• BSL-1: Basic level (least critical)
• BSL-2: Intermediate level (moderately critical)
• BSL-3: Highest level (most critical)

Each level specifies:

• Material and manufacturing controls
• Non-destructive examination (NDE) methods
• Mechanical and chemical testing
• Heat treatment
• Inspection and documentation requirements

Let’s explore each level in more detail.

BSL-1: Basic Level Bolting

Overview:

BSL-1 is intended for less critical service environments where the risk of bolt failure would not cause catastrophic damage or safety risks.

Key Features:

• Standard material and heat treatment requirements
• Minimal NDE (typically visual inspection only)
• Limited documentation (e.g., basic traceability and MTRs)
• Not required to have third-party inspection
• Less stringent manufacturing controls

Typical Applications:

• Non-pressure-containing structures
• Non-sour service components
• Utility connections where failure has minimal consequence

Pros:

• Cost-effective
• Faster manufacturing turnaround
• Suitable for lower-risk environments

Limitations:

• Not recommended for high-pressure or safety-critical installations
• Lower level of quality assurance

Who Should Use BSL-1:

• Manufacturers or engineers sourcing bolts for non-critical oil and gas infrastructure
• Operators in low-risk service conditions where bolt failure would not endanger safety or environment

BSL-2: Intermediate Level Bolting

Overview:

BSL-2 is for moderately critical service—environments where a bolting failure would cause significant equipment damage, production downtime, or safety risk but not catastrophic failure.

Key Features:

• Stricter material and process control than BSL-1
• Mandatory NDE such as Magnetic Particle Testing (MT) or Liquid Penetrant Testing (PT)
• Enhanced traceability and documentation
• More detailed mechanical testing, including tensile, hardness, and impact testing
• Heat treatment must be verified and documented
• Inspection requirements are higher, though third-party inspection may not always be mandatory

Typical Applications:

• Wellhead equipment
• Pressure-containing components not exposed to extreme conditions
• Surface production equipment
• Sour service under mild conditions

Pros:

• Balanced approach between quality assurance and cost
• Reduces risk of failure in important but not ultra-critical systems

Limitations:

• Not sufficient for subsea or high-pressure/high-temperature (HPHT) systems
• Longer lead time than BSL-1 due to additional testing

Who Should Use BSL-2:

• Projects with moderate risk levels
• OEMs and EPCs looking to meet operator specifications for general pressure systems

BSL-3: High-Criticality Bolting

Overview:

BSL-3 is the highest level of bolting specification under API 20E. It’s used in high-risk environments where bolting failure could result in loss of life, environmental disaster, or substantial economic impact.

Key Features:

• Extensive quality assurance and control
• Full NDE testing including MT, UT (Ultrasonic Testing), or PT
• Third-party inspection often required
• Complete traceability to raw materials, heat treatment, and production lots
• Heat treatment must be uniform and strictly controlled
• Advanced documentation, including full Material Test Reports (MTRs), certificates, and traceability logs

Typical Applications:

• Offshore and subsea production systems
• HPHT wells
• Critical valves, blowout preventers (BOPs), and control systems
• Equipment exposed to severe sour environments or cyclic fatigue loads

Pros:

• Maximum assurance of bolt performance and safety
• Reduces risk in the most critical environments
• Often required by major oil & gas operators for high-impact projects

Limitations:

• Highest cost
• Longer procurement timeline
• Requires certified and experienced manufacturers

Who Should Use BSL-3:

• EPCs working on deepwater, subsea, or HPHT wells
• Operators with zero-tolerance policies for failure
• Projects that require API monogrammed equipment

Comparison Table: BSL-1 vs. BSL-2 vs. BSL-3

Feature	BSL-1	BSL-2	BSL-3
Criticality	Low	Moderate	High
NDE Required	No	Yes (MT/PT)	Yes (MT, UT, etc.)
Traceability	Limited	Partial	Full (Heat lot to part)
Third-Party Inspection	Not required	Optional	Often required
Documentation	Basic MTR	Detailed MTR, test records	Full documentation package
Material Control	Basic	Intermediate	Strict, certified
Cost	Low	Medium	High
Application	Utility, structural	Surface wellhead, mild sour	Subsea, HPHT, critical safety

Choosing the Right BSL Level

Factors to Consider:

• Service Environment: High-pressure, sour, or subsea conditions usually require BSL-3.
• Risk Assessment: How severe would the consequence of bolt failure be?
• Regulatory/Contractual Requirements: Many operators and EPCs specify minimum BSL levels.
• Cost and Lead Time: Higher BSL levels take longer and cost more.

Practical Tip:

If in doubt, consult the API 6A or API 17D equipment specifications, which often reference bolting requirements. These will help determine the appropriate BSL based on the overall risk profile of your project.