Tas Gas Networks
The Hobart Pipeline Catastrophe That Never Happened!!
The Hobart Steel Pipeline
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• Designed, constructed and operated in accordance with AS2885
• Constructed, tested, gauge plate pigged, and put into commission in 2004/5
• 24.1Km long from Bridgewater to New Town in Hobart
• ANSI #300 (MAOP 5.1MPa), SMYS 358MPa with a MOP of 3.2MPa due to wall thickness and no rupture requirement;
• 8”/200mm API 5L X52 grade linepipe of various wall thicknesses of 4.8mm – 10.0mm
• Location class of R2 and T1 with many sections being in CIC category
• Numerous special crossings, plus approx. 1/3 of length in rail reserve
• Many sections had a common 10Bar or 5Bar PE reticulation main laid in parallel, plus optic fibre cable
Pipeline Integrity Management from 2004
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• Minimum depth of burial >1200mm
• Post and plate and kerb markers
• Slabbing where deemed appropriate
• Pipeline vehicle and foot patrols (weekly)
• Survey accurate As Builts
• Dial Before You Dig and associated Engineering Assessments
• Free locations within 25M and free stand overs for works <5M from the pipeline, under a Permit To Work
• Gas Awareness Training across utilities and contractors
• Pipeline Planning Corridor legislated in Tasmania, making the Tas Gas a referral authority for Development Applications
Pipeline Cleaning and Pigging
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• Pipeline pigging restricted due to back-to-back 1.5D x 900
bends, and also low flow rates
• 2015/16 programme of foam pig cleaning and gauge plate runs started
• Results to be used in the 10 year Remaining Life Review process, and SMS’s
• Initial gauge plate runs started at 95% i.d. then down to 85% i.d. (Min i.d. 198.5mm, Max i.d. 209.5mm)
• Multiple runs, and re-runs, undertaken after checking valve positions, and previous hot tap locations for closure plate alignment
• Gauge plates alone tell of an issue but not the location
• Interest from the Office of the Technical Regulator
• Restricted operating pressure down from 2,500kPa to 2,000kPa
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Gauge Plate Inspection
85% of internal diameter gauge plate
To deflect meant a 30mm anomaly, if on thickest wall section of pipe
Gauge plate suggested at least a 40mm - 45mm intrusion, over 45 degrees (1/8th of pipe circumference)
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ROSEN RoGeo MD Geometry / Caliper Tool designed and run to measure the defects and locate the anomalies
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• Recovery of the missing parts was successful
• 2nd Run of the tool was also completed without incident
• The results downloaded and indicated a significant anomaly had been detected
• Data clearly showed the welds on the pipe, and therefore accurate location identified
• Location KP point 19,773M
• Indentation 18.2% of pipe internal diameter
Anomaly Location
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Pipeline
KP 19,778
• Middle of carriageway on Derwent Park Road, main arterial route to suburb of Glenorchy and Moonah
• On edge of rail track• Depth of pipeline 4.5M• Main commercial area
adjacent to Harvey Normans, Beacon Lighting, KD Warehouse & BOC Gases, etc
12.6 kW/m2 – likely to cause death if exposed for more than 30 seconds4.7 kW/m2 – likely to significant injury (second degree burns) if exposed for more than 30 seconds
Anomaly Investigation – Derwent Park Road
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• Part road closure organised and excavation /sucker truck used to exposed exact location
• A direct assessment completed 9th May 2017.
• Pipeline located at 4.5M depth (As expected)
• Visual assessment identified a yellow striped PEgas pipe (Tiger pipe) found directly over theanomaly!!! Had we scored an own goal!!!
• On site investigations determined the gas pipehad been used for a sewer main connecting twoman holes in Derwent Park Road.
• The sewer pipe was installed by horizontaldirectional drilling (HDD) and traced back toJanuary 2006!
• Sewer install for a new commercial developmenthad been rejected by Council as non-compliant!!
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• Decision made to leave damage undisturbed and plan a repair
• Road closure put in place affecting Metro buses, and 9,500 traffic movements on that road daily
• Wide Council, commercial businesses and public notification carried out
• Plant and equipment mobilized and excavations undertaken
• Flow stopping team arranged from inter-state
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Hobart weather in September, almost a year to the day, can be challenging!!
Coldest, and wettest, week for months and highest flow rates in the pipeline just to add to the challenge
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Due to severity a clamp was fitted to provide some external impact protection whilst works continued
Pipeline Repair
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Pipeline Internal Growth
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• On site UT tests indicate approx. 60% pipe wall loss• Large number of stress concentrators• Severe deformation• Exposed to high temperatures from drilling
operation
Defect Assessment
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Defect Assessment
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• FEA assessment undertaken• Planned destructive test to
correlate FEA results to actual test results
• The minimum design wall
thickness required for
pressure containment at
MAOP is
𝑡 =𝑃𝐷
2𝜎𝑦= 𝟏. 𝟗𝟑𝒎𝒎
• The pipeline defect was caused by the horizontal directional drilling of a sewer main.
• There were no records of any DBYD enquiries at the time of the incident January 2006.
• Pipeline Patrols did not note any activity. The drilling occurred on a Sunday.
• The Council did not refer the development to Tas Gas as required under the Pipeline Planning Corridor Regulations requirements.
• At the time of the incident, the pipeline was operating at a reduced pressure of 2,000 kPa, however it has been elevated in the past, and after 2006,to 3000 kPa for O&M activities.
• Tas Gas Networks identified the defect and repaired the pipeline in 2017 at a cost of almost $400,000. No cost recovery is likely.
• The defect had a reduced wall thickness of 1.878 mm and with the amplified stress concentration (at the dent ears), a pipeline failure was imminent!
• Tas Gas, with the Technical Regulator, have embarked on a comprehensive awareness programme.
Summary
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