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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