1. Workshop UN ECE Gerald Laheij | March 8 th 2006 Risk assessment on pipelines The Dutch approach

2. High pressure natural gas pipelines Total length: 12000 kilometre Main transport pipelines 18- 48 inch (66 bar) Average depth of cover 1.75 meter Regional transport pipelines 2-16 inch (40 bar) Average depth of cover 1.2 meter Workshop UN ECE

3. Prevention of accidents Risk reduced as much as is reasonably practicable through measures at the source Number of people exposed is reduced by a risk based zoning policy -Individual risk, creates distance between potential hazardous source and its surroundings -Societal risk, limits the population density around the potential hazardous source

4. Risk measures Individual risk Chance that an unprotected person residing permanently at a fixed location is killed as a result of an accident Presented as risk contours on a map Limit for dwellings and vulnerable destinations, guidance value for less vulnerable destinations (10 -6 per year) Workshop UN ECE

5. Risk measures Societal risk Chance that in a single accident a certain number of victims is exceeded Presented in a F-N curve Justification of new developments Guidance value, per km pipeline Workshop UN ECE

6. Standardised method Guidelines for quantitative risk assessment (QRA) are given in the Purple Book loss of containment events source term and dispersion exposure and damage calculation and presentation of results Guideline is reviewed, use of a single computer code Workshop UN ECE

7. Current building distances Building distances derived in 1984 Building distance depends on diameter, pressure of pipeline, and vulnerability of objects (4 – 60 meter) Construction (wall thickness) of new pipelines depends on area classification Building distances are currently under revision Workshop UN ECE

8. Loss of containment Full bore ruptures dominate the risk of natural gas pipelines External interference main cause of failure Failure frequency depends on depth of cover and wall thickness Workshop UN ECE

9. Source term and dispersion Pipeline rupture results in vertical jet which can ignite Source term, average release (first 20 seconds) Exposure dominated by effects of heat radiation Workshop UN ECE

10. Probability of ignition Casuistic available for rural areas, depends on diameter and pressure (5-80%) Influence build-up area not known Large scale experiments are planned

11. Consequences Workshop UN ECE Example for a 12 inch pipeline

12. Additional measures Building distances (1984) normative distance for new building developments, additional measures at source may be necessary Obligatory one-call system -Supervision of pipeline operator (dangerous substance) Concrete slabs and warning tapes Warning marks, camera surveillance For new pipelines depth of cover and wall thickness are chosen such that distance to individual risk contour of 10 -6 per year is less or equal to building distance of 1984. Workshop UN ECE