Hot Tap & Linestop In-Service Welding Considerations

1. Introduction & Background.

In-service repairs provide cost-effective and environmentally friendly options for maintaining transport pipelines, associated piping infrastructure and process facilities to keep running for their design life. In-service welding is often required to:

• Repair a damaged pipeline or piping system
  • Corrosion or mechanical damage
• Install a branch connection to tie in a new customer or source of supply
• Economic & environmental incentives
  • Blow down vs. on-stream repair

A meticulously developed and qualified Weld Procedure Specification (WPS) and Procedure Qualification Record (PQR) is at the heart of consistently performing safe in-service welding on pressurized pipeline systems.

This paper focuses on some of the important aspects that has to be considered during the process.

2. Code requirements for safe in-service welding.

Decisions regarding in-service welding are taken only when alternative methods (like shutdown and purging) are liable to cause severe system disruption or significant contractual/regulatory penalties or environmental degradation. OSHA lockout / tag-out regulation, 29 CFR 1910.147 (a) (2) (iii) (B) provides exemption from application of standard when owner / employer demonstrates the following:

1. Continuity of service is essential,
2. A shutdown of the system is impractical,
3. Documented procedures are followed, and
4. Special equipment is used, which will provide sufficient protection for employees.

Problems and safety hazards encountered while welding on pressurized hydrocarbon pipelines, piping, or tanks are to be dealt with under the guidance of federal, state and local/municipal regulations, which also refer to relevant codes and standards. In-service pipeline welding should not be attempted by personnel lacking sufficient knowledge, experience, and expertise required under relevant codes, recommended practices, or standards developed for safe work execution. The reader is advised to consult the following to acquire adequate knowledge:

• API Std 510 – Pressure Vessel Inspection Code: Maintenance Inspection, Rating, Repair and Alteration
• API Std 570 – Piping Inspection Code: Inspection, Repair, Alteration and Rerating of In-Service Piping Systems
• API 1104 – Welding of Pipelines and Related Facilities
• API 1107 – Pipeline Maintenance Welding Practices
• API 2201 – Safe Hot Tapping Practices in the Petroleum and Petrochemical Industries
• ASME B 31.3 – Process Piping
• ASME B 31.4 – Pipeline transportation systems for liquids and slurries
• ASME B 31.8 – Gas Transmission Pipelines
• DOT 49 – Code of Federal Regulations Parts 190 – 199
• OSHA 29 – Code of Federal Regulations Part 1910 and Parts 1926.351 and 352 (gas pipelines welding)

3. Considerations for successful in-service welding.

The 3 primary concerns when performing in-service welding on a pressurized hydrocarbon pipeline are:

1. Risk of burn-through.
2. Hydrogen cracking.
3. Heat sink capacity of the pipeline at in-service welding location.

3.1. Burn-through.

Burn-through occurs when the unmelted pipe wall beneath the weld pool has insufficient elevated temperature strength to contain the pressurized product.  The following primary factors influences the risk of burn-through:

3.1.1. Pipe wall thickness

• Wall thickness is a primary factor in determining burn-through risk. The risk increases as the wall thickness decreases.
• Available data indicates that the risk of burn-through is negligible where the pipe wall is 0.250 inches or greater, assuming low hydrogen electrodes are used, and standard procedures are followed.
• In-service welding on thinner wall pipes will require a specially developed WPS & PQR, addressing limited heat input criteria.
• Before any in-service welding commences, it is critical that either UT or MPI is conducted in the weld area to determine the pipe wall thickness at the weld area as well as check for any possible laminar defects.

3.1.2. Effect of welding heat input

• Heat input is a primary factor in determining burn-through risk. The risk increases with an increase in heat input.
• This is a function of welding parameters and process.
• Welding heat input is a function of the following:
  • Welding current
  • Welding voltage
  • Travel speed

Each one of the 3 above functions are secondary factors in determining the burn-through risk.

• Heat input limits may conflict with hydrogen control measures
  • The minimum heat input required to avoid hydrogen cracking may be greater than the maximum heat input allowed to avoid burn-through.
  • Temper bead procedures can be used as an alternative approach.

3.1.3. Pipeline operating pressure

• For gas pipelines, pressure is a secondary factor in determining burn-through risk.
• Burn-through will occur if inner wall temperature is sufficiently high, even at low pressure.
• Pressure reduction formula:

• P = Maximum pipeline operating pressure
  • S = SMYS of pipe
  • F = Safety coefficient
  • D = Nominal diameter of pipeline
  • t = Wall thickness
  • c = allowance for heated metal loss of strength – API RP 2201 suggests 3/32 inch
    • Assumes a loss of strength around the entire circumference, which is not the case in reality.
• An increase in pipeline pressure, increases the risk of burn-through, but this is more applicable to thinner walled pipe, 0.125” or 0.188”

3.2. Hydrogen Cracking

For hydrogen cracking to occur, three conditions must be satisfied, simultaneously.  These 3 independent conditions are:

• High hydrogen levels in the weld
• The development of a crack susceptible micro-structure
• Tensile stresses acting on the weld

To prevent the occurrence of hydrogen cracking, at least one of the above three conditions must be eliminated or minimized.

The general consensus is that this type of defect is independent of the welder’s skill level, but more a function of a weld procedure defect.

3.2.1. High hydrogen levels in the weld

• Arc welding processes introduce hydrogen into the weld to some extent.
• Low hydrogen is generally considered as < 4 ml / 100 g
• The source of hydrogen could be from moisture absorbed from the pipe coating or contaminated surface, or from decomposition of covered electrodes.
• Limiting the amount of hydrogen entering the weld can be achieved by thoroughly cleaning the joints prior to welding and may include heating of the installed fitting with a propane torch prior to welding, and lastly, the use of low hydrogen electrodes.
  • Low hydrogen (EXX18) electrodes should be stored in hermetically sealed containers until the welder is ready to use them.
  • Once opened, these LH electrodes should be stored in an oven set at a temperature exceeding 212°F (100°C) if there is a chance of prolonged exposure to atmospheric conditions. Portable field ovens are ideally suited for this purpose.
  • Drying / baking exposed LH electrodes is certainly possible, but the effort and time spent on this endeavor will probably favor replacement of the exposed/contaminated electrodes.

3.2.2.  The development of a crack susceptible micro-structure

• Microstructure hardness is used as an indicator of crack susceptibility, with 350 H_v being the commonly used maximum allowable hardness level. This has to be checked as part of the PQR
• Microstructures make welds crack, not the hardness.
• Lower CE materials can crack at lower hardness levels
  • Microstructure could be 90% martensite with a CE = 0.06%
  • Martensite hardness is 545 Hv
  • Measured Hardness is 368 Hv
  • Low hardness but high cracking risk
• General rule of thumb on CE level is that it should be limited to 0.45% for pipelines of grade ≥ X42
• Rapid cooling rate
  • Studies have concluded that a difference in cooling rates appear to provide dramatic effects on the microhardness of steels, depending on the carbon content of that particular steel.
  • The microhardness increases with the increasing cooling rate and carbon content due to solid solution hardening and formation of the martensite phase

3.2.3. Tensile stresses acting on the weld

• External stress can be due to improper pipeline support or backfill.
• Stress concentrations can develop due to incorrect sleeve or fitting installation and welding sequence
  • Residual stress will develop as a result of the restraint imposed by the welded assembly and the thermal contraction of the weld as it cools.
  • Reduce the weld toe angle by reducing the gap between sleeves or fittings with the pipe on which it is being welded to. This is achieved by mechanical means or a technique known as weld metal buttering or simply a “buttering layer”. Using lower strength weld metal is an option too, but this may require a larger weld deposit.

3.2.4. Heat sink capacity of the pipeline

• Commonly known as the cooling effect.
• Factors influencing the welding cooling rates:
  • Pipeline content (gas or liquid)
    • As can be seen with the flow rates, it is important to know what the pipeline content is.
  • Flow rate
    • Content – Gas
      • No flow conditions are potentially hazardous.
      • In gas, high flow rates have rarely been found to negatively influence the weld quality and thus, maximum flow rates do not appear to be critical when executing an in-service weld.
    • Content – Liquids
      • No flow conditions can also be potentially hazardous.
      • Unlike gasses, high flow rates on a liquid pipeline will present severe challenges.
        • More heat input is required to ensure proper weld fusion.
        • High flow rates mean a rapid quenching of the weld (cooling rate), resulting in cracking.
        • Rule of thumb maximum flow rate is approximately 4 fps.
  • Pipe wall thickness
  • Temperature
    • Ambient
    • Pipe surface

References:

1. PRCI In-service welding – Practices and Procedures
2. Effect of cooling rate on hardness and microstructure of AISI 1020, AISI 1040 and AISI 1060 Steels – Adnan Çalik
3. API 1104 Welding of pipelines and related facilities
4. API RP 2201 Safe Hot Tapping Practices in the Petroleum and Petrochemical Industries
5. PRCI PR-82680771-5 Development of criteria/guidelines for welding onto in-service chemical pipelines.
6. PRCI L51548 Criteria for Hot Tap Welding.
7. Development of guidelines for the repair and hot tap welding on pressurized pipelines. Keifner, JF Fisher, RD Mishler HW
8. PRCI L52047 Pipeline Repair Manual