Pipeline repair
Special Pressure Containing Sleeve – Fig 1 to 4 above indicate how it is put on an In-service leaking pipeline

UFG is a technical issue and pipeline repair is the key to control UFG (Un-accounted For Gas) or LAUF (Lost And Un-accounted For) gas, provided metering is done accurately. SNGPL and SSGC have been in the habit of dealing with UFG as a political and security phenomena with OGRA always sympathetic towards their cause. These two companies have never considered lack of pipeline repair and maintenance as the main cause of their un-controllable UFG, besides metering and billing anomalies. This article serves to provide info for existing professionals in such utility companies to drive an initiative to compel their management to act more professionally in dealing with UFG.

 

Pipelines are efficient and environment friendly mode of oil and gas transportation provided they are operated under prudent industry practices and standards. Starting from wellheads high pressure steel pipelines handle multiphase fluids, slurry, crude, condensate and raw natural gas while on the downstream side they deliver petroleum products and pipeline quality gas to consumption points. On-shore pipelines constitute over-whelming majority of world pipeline installations. Repair of pipeline defects or failure due to circumferential stresses is a major consideration for pipeline maintenance managers.

Pipeline Damage – Metal Loss

 This know-how serves to benefit pipeline operators to know the cause and to take preventive actions before actual damage occurs to pipeline.

Bulk transport pipelines normally operate at high pressure and sometimes may also be subjected to high temperature. Metal loss occurs over time due to interaction with transport fluid and environment. Downstream pipelines normally do not experience internal pitting or corrosion but are exposed to external corrosive environment as they are buried underground. Internal metal loss in downstream pipelines may occur due to erosive velocities achieved by flowing fluid under low pressure conditions. On the other hand upstream pipelines (wellhead to gas processing plant or storage / refinery site) have greater risk of internal corrosion as they often carry high temperature produced fluids that contain corrosive substances like sulfur, mercaptans, organic acids, salts and acid gases – H2S, CO2.

Pipeline failures seldom occur under ordinarily applied engineering loads or stresses. Probability of pipeline failures significantly increase when engineering loads get amplified by residual stresses existing in the pipeline. These residual stresses get induced in pipeline structure as a result of fabrication processes or environmental interaction. Annealing or thermal stress relieving is not a practical option for pipelines, being dimensionally large assets. As such there are additional causes of mechanical damage to buried pipelines:

  • Stress corrosion cracking (SCC)
  • Sulfide Stress cracking (SSC)
  • Corrosion fatigue
  • Hydrogen induced cracking (HIC)
  • Accidental digging (by heavy construction or farming equipment)
  • Soil movements in geologically active areas.

 

Quantifying Pipeline Damage – Metal Loss

Once a pipeline is commissioned and operational, the integrity measures are primarily a combination of predictive and preventive maintenance strategies. These strategies can be successfully implemented through condition monitoring, validation of damages and prudent choices of repair techniques for different pipeline damage types – external as well as internal metal loss or damage. Pipeline Integrity Management Software also supports operators’ decision making related to prudent choice of repair technique.

Visual inspection, Inline inspection (ILI) using intelligent pigging and NDE through physical thickness testing provide the means for validation of threats to integrity of in-service pipelines. A threat that has been validated to be a risk to pipeline integrity has to be eliminated or rectified.

 

Pipeline Repair Techniques

Basic criteria for selection of a repair technique are rectification of damage at minimum cost as well as restoration of pipeline capability to designed operating conditions for remaining life i.e. prevention of damage propagation resulting in a future catastrophe. As such major classification of pipeline repair techniques is:

  1. In-service repair – for external damage or metal loss on pipelines
  • Minor damage on pipeline surface.
  • Significant damage to pipeline surface, where leakage has not initiated.
  • Significant damage to pipeline surface, where leakage has initiated.

 

  1. Isolated or off-service repair – for internal damage or metal loss on pipelines (also used for external damage or metal loss on pipelines when in-service repair is not possible)

And remember that pipeline repair is key to control UFG – cost incurred in repair is recovered many times in the form of UFG reduction.

1. In-Service Pipeline Repair Techniques

The in-service repairs are less costly because they do not require decommissioning of pipeline segment (including loss of gas / oil) and can be attempted with minimal disruption to normal pipeline operations. In-service repair techniques are deployed after carefully analysing requirement related to severity of damage to the pipeline, geographical location, operational constraints and pipeline configuration.

Such in-service pipeline repair techniques often involve welding on live pipeline – flowing hydrocarbon. Such welding should not be attempted by personnel who are lacking in sufficient knowledge, experience, and expertise required under relevant codes, recommended practices or standards developed for safe work execution. Following standards may be consulted to acquire and train workforce with adequate knowledge:

  • ASME B 31.8
  • ASME B 31.4
  • ASME B 31.3
  • API 1104
  • Relevant OSHA guidelines

 

SAFETY NOTE: Wrong execution of welding not only jeopardizes safety of worksite but may also severely compromise integrity of pipeline due to burn-through or material hardening. Operational pipelines are pressurized assets carrying oil / gas and have fire hazards when welding / fabrication jobs are carried out. Site supervisor must ensure to:

  • Carry out Job Safety Analysis (JSA)
  • Follow API RP 76 for Management of Change (MOC)
  • Make available required consumables / equipment and Proper PPE arrangement at site
  • Obtain Work Permit before job initiation from Authorized person
  • Follow job procedures including LOTO (Lock-Out-Tag-Out) when required

And remember that pipeline repair is key to control UFG – cost incurred in repair is recovered many times in the form of UFG reduction.

·        Grinding

Minor damages, welding anomalies or line pipe metallurgical discontinuities can be removed through grinding. Codes / standards do not provide any limitation on length of grinding span when it is used to remove metal that is < 10% of designed pipe wall thickness. Never attempt grinding where metal loss is more than 40% of designed pipe wall thickness. 

 

·        Weld Overlay / Weld Deposition Repair

Weld overlay is a pipeline repair technique where a suitable metal is deposited through welding to the pipe surface in the form of a layer. This not only improve pipe strength but also provide surface protection. Weld overlay is carried out by shielded metal arc welding, Metal Inert Gas (MIG) welding/Tungsten Inert Gas (TIG) welding, Submerged arc welding, CO2 welding and / or Plasma Transferred Arc (PTA) welding.

 

·        Welded Repair Patch

Repair patches can be a repair technique whereby metal plates are welded to cover non leaking defects (these are also used to repair leaking / spilling defects but in those cases welding are done after putting the pipeline out of service and removing hydrocarbons). Repair patches are not recommended for high pressure pipelines as they cannot hold high pressure.

 

·        Composite / Full-Encirclement / Welded Sleeve

Reinforcing the damaged pipeline section by putting on a sleeve is the most common repair technique. Sleeves are versatile in types and applications as follows:

 

  • TYPE-A: STRUCTURAL REINFORCEMENT SLEEVES

Full encirclement sleeves are normally used for non-leaking defects. Two sleeve halves are pressed and tightened around the damaged section of pipeline and firmly held in place with the help of pipe sleeve jack. Welding involved only for joining longitudinal ends of two half-sleeves together (butt welding) for complete encirclement grip or longitudinal end of one half-sleeve to the pre-crimped back-up strip (fillet welding) of second half-sleeve. Circumferential welding, when carried out as fillet weld with pipe metal provides some support against longitudinal structural stresses.

 

  • TYPE-B: PRESSURE CONTAINING SLEEVES

Full encirclement sleeves for pressure containment, when designed and installed carefully, result in restoring repaired pipeline yield strength to original SMYS (Specified Minimum Yield Strength). Two sleeve halves are pressed and tightened around the damaged section of pipeline and firmly held in place with the help of pipe sleeve jack. Welding involved only for joining longitudinal ends of two half-sleeves together (butt welding) for complete encirclement grip or longitudinal end of one half-sleeve to the pre-crimped back-up strip (fillet welding) of second half-sleeve. Circumferential welding is carried out as fillet weld with pipe metal and provides some support against longitudinal structural stresses.

 

 

  • SPECIAL PRESSURE CONTAINING SLEEVES

Special full encirclement sleeves are commercially available for ease in repairing leaking / spilling defects. These are pressure containing sleeves with a combination of bolting and welding. First the sleeve is placed on leaking or spilling defect and tightened around the pipe with the help of nut-bolts to stop leakage. These sleeves are provided with packing seals (longitudinal as well as circumferential) that helps in stopping leakage before welding is carried out. Once the sleeve is proved in position for no leakage occurrence, both halves are longitudinally welded to each other and circumferentially welded to the repaired pipe. In the end all the nuts are also seam welded to bolts as well as sleeve body to ensure durable pressure tight repair. 

 

 

·        Composite Sleeves / Composite Wrap

Composite sleeves or full encirclement composite wraps are the latest innovation with regards to pipeline defect repair where leakage or spill has not initiated. They are very advantageous for repairing contouring pipelines and those sections of pipelines that have fittings installed like small off-take or gauge point. These are capable of restoring damaged pipelines to withstand original MAOP with added advantage of providing pipe coating to guard against corrosion. Installation of composite sleeves is never recommended for damaged pipes where cracks have initiated.

And remember that pipeline repair is key to control UFG – cost incurred in repair is recovered many times in the form of UFG reduction.

2. Off-Service or Decommissioned Pipeline Repair Techniques

Off-service or decommissioned pipeline repair techniques are mostly used for rectification of internal damage to pipelines. However, occasions arise when external damages can be cost effectively repaired through partial decommissioning.

Although these techniques are expensive to implement but remember that pipeline repair is key to control UFG – cost incurred in repair is recovered many times in the form of UFG reduction.

·        Insert Sleeves

Most of the internal defects arise in pipelines at girth weld as remaining length of the pipe is often internally lined to guard against internal corrosion (especially when pipeline is carrying corrosive fluids). To prevent internal damage in the vicinity of girth weld insert sleeves are used. These sleeves provide cost effective solution to internal corrosion as they are easy to transport and install at site.

 

·        Hot Tapping / Bypass Operation

Some external defects require isolation of damaged portion of pipeline while keeping the remaining portion of pipeline in continued operations. For such applications Hot-Tapping is required upstream and downstream of the damaged portion to temporarily by-pass the damaged portion of the pipeline. Once the temporary by-pass pipeline is in operation, damaged portion of the pipeline can be isolated and decommissioned through insertion of plugging systems in live pipeline – commercially trade name of such operation is Stoppling by TDW. Stoppling is an operation where damaged portion of pipeline is isolated by welding Stopple-Split-Tees in close vicinity of the actual damage, upstream as well as downstream. Once welded and secured on pipeline, stopple tees are used to insert Train Plugging Systems inside the pipeline (both upstream and downstream of damaged pipe segment) to create an isolated pipe segment. These train plugging systems provide “Double block and bleed” protection against any leakage / spillage of fluid into the isolated pipe segment. Isolated segment is thus purged to be free of hydrocarbons. After additional purging of inert gas, any type of welded repair or cut-out operation can be done. Once the isolated segment is repaired and tested, train plugging system is retrieved and pipeline flow is restored through repaired segment.

 

·        Trenchless Pipe Replacement

Gas distribution pipelines have been operational in various metropolitan areas around the world for over 50 years. These pipelines often develop leakages because of internal or external corrosion and pipeline repair pose special challenges due to surrounding population.

There are now solutions available to repair such damaged pipelines with least disturbance to operations and surrounding urban life. Following ASTM standards define specifications for such PE piping and pipelines:

  • ASTM F-2896-11(2017) – Standard Specification for Reinforced PE Composite Pipe for Transport of Oil and Gas and Hazardous Liquids
  • ASTM F-2686-14 – Standard Specification for Glass Fiber Reinforced Thermoplastic Pipe
  • ASTM F-1974-09(2020) – Standard Specification for Metal Insert Fittings for PE / Aluminum / PE and Crosslinked PE / Aluminum / Crosslinked PE Composite Pressure Pipe
  • ASTM F-1281-17 – Standard Specification for Crosslinked PE / Aluminum / Crosslinked PE (PEX-Al-PEX) Pressure Pipe
  • ASTM F-1282-18 – Standard Specification for PE / Aluminum / PE (PE-Al-PE) Composite Pressure Pipe

It is found that companies like Smartpipe® have solutions that can restore corroded steel pipe segments to original pipeline rating through insertion of PE pipe within the degraded host steel pipeline, with minimum disruption to urban areas in terms of trenching or excavation. Such pipe can also be manufactured at site through installation of portable factory close to work site.

This pipeline repair technique requires pulling of long lengths of PE pipe into the original steel pipe through high-strength pulling tapes. Reduction in dia of original steel pipe is largely compensated by reduction of flow drag of PE insert thereby causing minimal reduction of pipeline flow capacity.

And remember that pipeline repair is key to control UFG – cost incurred in repair is recovered many times in the form of UFG reduction.