Pipeline Construction: Typical Construction Issues
Since 2007, the pipeline industry has been experiencing unparalleled growth driven by the need to satisfy the Nation's energy demand and bring new sources of supply to the market. As a result, PHMSA has stepped up the number of new pipeline construction inspections performed each year and our inspections of these new pipeline projects have discovered a number of issues that if left unresolved could have an impact on the long and short term integrity of the pipeline. PHMSA inspects pipeline construction to assure compliance with these requirements. Inspectors review operator-prepared construction procedures to verify that they conform to regulatory requirements. Inspectors then observe construction activities in the field to assure that they are conducted in accordance with the procedures.
When issues are identified by PHMSA inspectors, those concerns are brought to the attention of the pipeline constructor. Resolution of the concerns may involve procedure revisions, personnel training, modification to construction practices, or physical repairs to the pipeline, pipeline coating, or auxiliary pipeline features. PHMSA ensures that the issues are corrected prior to the pipeline being buried and prior to the pipeline commissioning through pre-commissioning integrity hydrostatic pressure tests and inline inspection tool (Smart Pig) runs.
|Hydrogen Assisted Cracking||Low Strength Pipe||Lowering/Installation in Ditch and Backfill|
|Excavation and Cover||Nondestructive Testing||Miscellaneous|
The applicable regulations addressing protective coating are §192.461 for gas pipelines and §195.559 for hazardous liquid pipelines. Additional discussion on coatings can be found at the Stakeholders Communications web site and the PHMSA presentation on Construction and Pipeline Coatings which was given in the April 23, 2009 workshop on new pipeline construction issues. Some specific considerations for coating applications during construction are provided below:
- The coating cutback can be too small or too large – The coating manufacturer's instructions on the coating cutback distance must be followed to ensure long-term bonding of the material to the pipe.
- Proper pipe surface preparation is a must. PHMSA inspection has seen instances where the surface was inadequately cleaned. The pipe should be sand blasted and then preheated as instructed by the coating manufacturer.
- Care must be taken in the application of coatings in the field. Shielding may be needed to avoid dust or rain water contamination. Following the coating application, v coating thickness must be verified to ensure the manufacturer's specifications have been met.
- Prior to backfilling, the entire pipeline should be visually inspected by trained and qualified inspectors.
- Inspection for coating holidays should use properly calibrated equipment. The inspection process must ensure that the proper voltage is used. Equipment calibration and use must be in accordance with the manufacturer’s specifications. Particular attention should be paid to identification of damage by weld bands.
PHMSA inspection has seen the following types of concerns when inspecting new pipeline construction:
|Coating - Fusion Bonded Epoxy Issues||Coating - Electronic Defect Detectors (Jeeping)|
|Coating - Melt Stick||Coating - Two Part Epoxy Issues|
The applicable regulations addressing bending are §192.313 for gas pipelines and §195.212 for hazardous liquid pipelines. Additional discussion on bending can be found at the Stakeholders Communications web site. Some specific concerns related to bending applications during construction are provided below:
- Pipe bend surface ripples out of tolerance. Regulations require that each bend must have a smooth contour and be free from buckling, cracks, or any other mechanical damage.
- Bending has occurred such that the pipe seam is not in the neutral axis. On pipe containing a longitudinal weld, the longitudinal weld must be as near as practicable to the neutral axis of the bend unless the bend is made with an internal bending mandrel; or the pipe is 12 inches (305 millimeters) or less in outside diameter or has a diameter to wall thickness ratio less than 70.
- Contractors installing new pipelines have had inadequate construction specifications and procedures for performing bending. Quality and detailed specifications and procedures must be in place prior to beginning any construction job.
- During pipeline construction project inspections, PHMSA has noted that the constructors have not followed required procedures for bending. All bending must be performed in strict accordance with federally prescribed standards and construction procedures to ensure integrity of the bend. Bend machine operators must be trained and qualified in the bending process.
The applicable regulations addressing welding are in Subpart E of Part 192 for gas pipelines and Subpart D of Part 195 for hazardous liquid pipelines. Additional discussion on welding can be found at the Stakeholders Communications web site. Some specific concerns identified related to welding applications during construction are provided below:
- Improperly qualified procedures or the use of wrong procedures. All welding procedures must be qualified and welding must be controlled to strict specifications. As part of the quality-assurance process, each welder must pass qualification tests to work on a particular pipeline job, and each weld procedure must be approved for use on that job in accordance with welding standards.
- PHMSA inspection has found that some constructors do not have welding procedures on site or are not following procedures.
- Part of the welding process is pre-heating of the pipe joint prior to beginning welding. Welders have not always ensured that the pre-heating requirements, established and documented in qualified welding procedures, are maintained. Improper pre-heating can lead to weld cracking after the completion of successful nondestructive testing of the weld. Refer to Hydrogen Assisted Cracking (HAC) for additional discussion.
PHMSA issued advisory bulletin ADB-10-03 to notify owners and operators of recently constructed large diameter natural gas pipeline and hazardous liquid pipeline systems of the potential for girth weld failures due to welding quality issues. Misalignment during welding of large diameter line pipe may cause in-service leaks and ruptures at pressures well below 72 percent specified minimum yield strength (SMYS). PHMSA has found pipe segments with:
- Line pipe weld misalignment,
- Improper bevel and wall thickness transitions,
- Out of roundness due to cut induction bends, and
- Other improper welding practices.
PHMSA inspection has seen the following types of concerns when inspecting new pipeline construction:
|Mechanized Welding||Manual Welding|
Recently there have been occurrences of through-wall cracks in welds that were discovered during the hydrotest phase of pipeline construction. These discoveries are extremely troubling as cracked welds are typically found and repaired or removed during the nondestructive testing and repair phase of pipeline construction. All of the failures have been found in high-strength pipe (X-70 or X-80) 20” or greater in diameter. Metallurgical investigations concluded hydrogen assisted cracking (HAC) was the failure mechanism.
Construction records associated with the leaks and additional investigations were reviewed and most of the time either ultrasonic inspection or radiography were completed the same day as the weld; no Non Destructive Testing (NDT) reports indicate cracks - this helps confirm Delayed HAC. In some cases the NDT was completed on the following day, here again no NDT reports indicate cracks. Ultrasonic testing (UT) was used on the mechanized projects and radiography was used on the manual welding projects.
Three factors must be present in the weld for HAC to occur; a source of hydrogen, a micro-structure susceptible to the effects of hydrogen, and stresses in the weld.
- Hydrogen is present in the coating of all E XX10 electrodes, used on many pipeline projects.
- There are always stresses present in the weld due to heating and cooling and the restrained geometry inherent in a pipeline weld. Higher stress levels can be present in repair welds, tie-in welds, transition welds, and welds with poor joint alignment.
- Solutions to avoid HAC include:
- Using a low-hydrogen welding process (GMAW – FCAW - E XX18 – E XX45) in a high strength pipeline weld
- If using cellulosic electrodes – Allowing sufficient time at temperature to allow any hydrogen to diffuse from the weld (higher preheat temperatures, preheat maintenance, minimum interpass temperature, avoid weld interruption)
- Minimizing installation stresses
- Optimizing weld metal electrode selection
For further information, refer to the PHMSA presentation on HAC PHMSA presentation on HAC which was given in the April 23, 2009 workshop on new pipeline construction issues.
The applicable regulations addressing installation of pipeline in a ditch (excavated trench) are §192.319 for gas pipelines and §§195.246 and 195.252 for hazardous liquid pipelines. Additional discussion on lowering and backfilling can be found at the Stakeholders Communications web site. Some specific concerns related to lowering and backfilling applications during construction are provided below:
- All pipe installed in a ditch must be installed in a manner that minimizes the introduction of secondary stresses and the possibility of damage to the pipe. Multiple sidebooms are typically required to avoid secondary stresses. A stress analysis should be performed in advance of lowering to determine how the pipe is to be lowered and to determine the side boom spacing. Sideboom spacing is to be in accordance with API 1104, Appendix A, ECA Stress Analysis. PHMSA inspection of new pipeline construction has noted instances where an inadequate number of sidebooms have been used to lower the pipe into the ditch.
- Care must be taken to protect the pipe and coating from sharp rocks and abrasion as the backfill is returned to the trench. In areas where the ground is rocky and coarse, the backfill material should be screened to remove rocks or the pipe can be covered with a material to protect it from sharp rocks and abrasion. Alternatively, clean fill may be brought in to cover the pipe. PHMSA inspection of new pipeline construction has noted instances where the fill was not screened to eliminate rock fill material.
- The pipeline coating must be inspected after lowering to identify any damage. Any damaged coating must be repaired prior to backfilling to avoid future corrosion concerns. PHMSA inspection of new pipeline construction has noted instances where coating damage would have gone unrepaired without the PHMSA inspector's discovery.
The applicable regulations addressing excavation and cover of pipeline are §192.327 for gas pipelines and §195.248 for hazardous liquid pipelines. Additional discussion on excavation and cover of pipeline can be found at the Stakeholders Communications web site. Some specific concerns related to excavation and cover during construction are provided below:
- Insufficient burial depth - The trenches must be deep enough to allow for an adequate amount of cover when the pipe is buried. Federal regulations require that transmission pipelines be buried at least 30 inches below the surface in rural areas and deeper in more populated areas. In addition, the pipeline must be buried deeper in some locations, such as at road crossings and crossings of bodies of water, and may be less in other locations such as consolidated rock.
- No One Call Notifications - Hitting underground utilities when are digging can cause injuries, even deaths, environmental damage and loss of critical infrastructure and services. [More].
- Inadequate use of rock shield, padding machines, or selective backfill
- Dents caused by placing pipe on rocks
- Construction crews damaging pipe and coating during installation in ditch and backfill and when installing river weights
Pipeline Construction: Nondestructive Testing
The applicable regulations addressing nondestructive testing are §192.243 for gas pipelines and §195.234 for hazardous liquid pipelines. Quality assurance ensures the quality of the ongoing welding operation. To do this, qualified technicians take X-rays of the pipe welds to ensure the completed welds meet federally prescribed quality standards. An X-ray technician processes the film in a small, portable darkroom at the site. If the technician detects any flaws, the weld is repaired or cut out, and a new weld is made. Another form of weld quality inspection employs ultrasonic testing (UT) technology. Some specific concerns related to nondestructive testing applications during construction are provided below:
- Essential wire or hole (used to verify the ability to identify and size defects) not visible on radiograph
- NDT records not adequate or up to date
- Poor radiographic technique - not meeting API 1104 requirements
- Film density is not within specification
- Incomplete qualification documents for technicians
- Inadequate interpretation of radiographic results
- Not meeting the minimum 10% NDT requirements
- Testing to achieve only minimum requirements of 192 or 195
The following is a summary listing of typical issues that have been identified by PHMSA inspections of new pipeline construction projects. Identified problems have primarily been due to a failure to implement existing industry standards, manufacturer's recommendations, and federal regulations. Some of these issues are discussed in more detail on other Pipeline Construction web pages, but are repeated here in order to provide a consolidated list.
Pipe and Miscellaneous Issues
Operation - Insufficient line markers
Inadequate Operator Qualification Documentation If Applicable
Post Construction Documentation
Stringing - Long seam alignment/orientation
Fusion Bonded Epoxy Issues
Electronic Defect Detectors (Jeeping)
Two Part Epoxy Issues