Risk Assessment for Ngong Ping Effluent Pipeline in the Proximity to Shek Pik Reservoir
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Risk Assessment for Ngong Ping Effluent Pipeline In the Proximity of Shek Pik Reservoir Final Risk Assessment Report Report Ref: R8111/03/1 Issue 3 Date: December 2002 AGREEMENT NO. CE 29/2001 OUTLYING ISLANDS SEWERAGE, STAGE 1, WORK PHASE I NGONG PING SEWAGE TREATMENT WORKS AND SEWERAGE Risk Assessment for Ngong Ping Effluent Pipeline in the Proximity to Shek Pik Reservoir Final Risk Assessment Report Prepared by BMT Asia Pacific Limited in association with Lloyd’s Register ARS R/8111/03/1 Issue 3, December 2002 Drainage Services Department BMT Asia Pacific Limited DOCUMENT CONTROL SHEET Client: Drainage Services Department Title: AGREEMENT NO. CE 29/2001 OUTLYING ISLANDS SEWERAGE, STAGE 1, WORK PHASE I NGONG PING SEWAGE TREATMENT WORKS AND SEWERAGE Risk Assessment for Ngong Ping Effluent Pipeline In the Proximity of Shek Pik Reservoir Final Risk Assessment Report Job No: 8111 Ref: R/8111/03/1 Issue 3 Version: Final Date: 4 December 2002 Prepared under the Management of: Signature: Name T. L. Yip Position Senior Marine Engineer Reviewed and Approved by: Signature: Name Norman Di Perno Position Managing Director Filename \\hkgnts19\civil\23400 - Secretary\Risk Assessment Report (BMT) for Effluent Export Pipe\Version 3 from BMT\Final Report.doc Distribution: DSD and forward distribution Page: 1 of 1 Risk Assessment for Ngong Ping Effluent Pipeline In the Proximity of Shek Pik Reservoir Final Risk Assessment Report CONTENTS 1 INTRODUCTION 1 1.1 Background 1 1.2 Objective 1 1.3 Scope of Work and Overview of Methodology 1 2 THE STUDY AREA 3 2.1 The Sewage Treatment Works and Pipeline 3 2.2 Safety and other Design Features 3 2.3 Other Study Area Information 4 3 HAZARD IDENTIFICATION 6 3.1 Introduction 6 3.2 Site Visit 6 3.3 Literature and Database Review 6 Guidelines for Natural Terrain Hazard Studies, GEO SPR 1/2002, April 2002 7 The Natural Terrain Landslide Study, Phases I and II, GEO Report No. 73, 1999 7 The Natural Terrain Landslide Inventory (NTLI) on the GEO website 8 F C Dai and C F Lee, Terrain Mapping of Landslide Susceptibility using a Geographical Information System: A Case Study. 2001 8 F C Dai, C F Lee and Y Y Ngai, Landslide Risk Assessment and Management: An Overview. 2001 9 F C Dai and C F Lee, Frequency-Volume Relation and Prediction of Rainfall- Induced Landslides. 2000. 9 Ductile Iron Pipe Research Association (DIPRA). External Corrosion and Protection of Ductile Iron Pipe. (Plus other references from their website) 9 Chlorine Chemistry Council. Are our Pipes Safe? October 1998. 9 Approaches to Chemical Safety Management in the Chemical Industry. Stan Grossel, Process Safety and Design, Inc. 10 Data from WSD on DI pipe failures 10 Data from DSD on DI and HDPE Pipe Failures 11 Miscellaneous International Data on Pipework, Valve and Joint Failures 11 3.4 Hazard Identification Workshop 14 Time and Venue 14 Workshop Participants 14 Methodology 15 Phases 15 System and Sub-systems 15 Question Categories 15 Workshop Findings 16 3.5 Failure Case Definition 19 4 FAILURE FREQUENCY ANALYSIS 20 R/8111/03/1 Issue 3, December 2002 i Drainage Services Department Risk Assessment for Ngong Ping Effluent Pipeline In the Proximity of Shek Pik Reservoir Final Risk Assessment Report 4.1 Introduction 20 4.2 Pipeline Failures due to Causes other than Landslides 20 Introduction 20 DI Pipe Leaks 20 Joint Leaks 21 Valve Leaks 21 4.3 Natural Terrain Landslide Risk Model 21 Introduction 21 Landslide Initiating Frequency 22 Event Tree Analysis 22 Application of the NTL Risk Model 24 NTL Risk Results 24 Calibration of the results 24 4.4 Summary of Initiating Event Failure Frequencies and Probability of Operation of Safety Systems 26 Summary of Failure Case Frequencies 26 Probability of Operation of Safety Systems 26 4.5 Event Tree Analysis and Summary of Outcome Frequencies 26 5 FAILURE RELEASE RATES AND QUANTITIES 28 6 RISK RESULTS AND ASSESSMENT 31 6.1 Introduction 31 6.2 Risk Assessment Criteria 31 6.3 Assessment 34 6.4 Sensitivity of the Results to Assumptions and Modelling 34 6.5 Risk Mitigation 35 Introduction 35 Potential Risk Mitigation Measures 35 Measure 1 - Fabricate 450m Critical Section from Continuous Welded Corrosion Resistant Material 36 Measure 2 – Uprate 1700m Section within LDGG 36 Measure 3 - Increase capacity of leak collection system to 100% of flow 37 Measure 4 - Provide adequate protection so that worst case landslide would not damage pipeline 37 Measure 5 - Treat water to standards so that it could be fed into a reservoir 37 Measures 1 and 4 Combined 37 Measures 3 and 4 Combined 37 Measures 1, 3 and 4 Combined 37 Summary 37 Conclusion 38 7 CONCLUSIONS AND RECOMMENDATIONS 40 7.1 Introduction 40 7.2 Risk Assessment of the Original Design 40 R/8111/03/1 Issue 3, December 2002 ii Drainage Services Department Risk Assessment for Ngong Ping Effluent Pipeline In the Proximity of Shek Pik Reservoir Final Risk Assessment Report 7.3 Recommendations for Risk Reduction 40 7.4 Conclusion 40 8 REFERENCES 42 LIST OF FIGURES Figure 1 Location of Sewage Treatment Works and Overall Pipeline Route Figure 2 Pipe Section Under Study Figure 3 The Leakage Collection System Figure 4 Outline Methodology Figure 5 Natural Terrain Landslides which have occurred on Slopes above Pipeline Figure 6 Hazard Identification Workshop Flowchart Figure 7 The Definition of Sub-slopes used in the Natural Terrain Landslide Risk Assessment Mode Figure 8 Concentration versus Distance from Discharge Figure 9 Event Tree for Landslide, External Effect and Damage by Others Figure 10 Event Tree for Pipe Break due to Internal or External Corrosion Figure 11 Event Tree for Pipe Leak . 15% of Flow due to Internal or External Corrosion Figure 12 Event Tree for Pipe Leak , 15% of Flow Figure 13 Cumulative Frequency of Leakage for Mitigated Case, Additional Cases I and II LIST OF TABLES Table 1 Unofficial Traffic Count along Keung Shan Road (3 Sept 2002, 4:30pm to 4:50pm) Table 2 Incidence of DI Fresh Water Pipe Leakage (WSD) Table 3 Incidence of DI Salt Water Pipe Leakage (WSD) Table 4 Incidence of DI Sewage Pipe Leakage (DSD) Table 5 Records of Natural Terrain Landslides on the Hillside above the Pipeline Table 6 Team Members Qualifications and Positions Table 7 Incidence and Frequency of DI Fresh Water Pipe Leakage (WSD) Table 8 Incidence and Frequency of DI Salt Water Pipe Leakage (WSD) Table 9 Incidence and Frequency of DI Sewage Pipe Leakage (DSD) Table 10 Geological Categories used in the NTL QRA Model Table 11 Natural Terrain Landslide Risk Model – Input Data for each Sub-slope Table 12 Natural Terrain Landslide Risk Model – Results for each Sub-slope R/8111/03/1 Issue 3, December 2002 iii Drainage Services Department Risk Assessment for Ngong Ping Effluent Pipeline In the Proximity of Shek Pik Reservoir Final Risk Assessment Report Table 13 Event Tree Outcome Frequencies Table 14 Released Quantity for each Failure Case with reduced Packet Size Table 15 Worksheet for Input and Output Data Table 16 Risk Guidelines Table 17 Risk Assessment Matrix Table 18 Risk Reduction Measures – Residual Risk and Assessment Table 19 Treated effluent Spill Frequencies with Measures 3 and 4 Table 20 Registered Slopes below the Pipe Section (East) Table 21 Registered Slopes above the Pipe Section (West) Table 22 Lookup Table for Terrain Component for Landslide Frequency and Probability of Long Runout and Mobile Factors Table 23 Lookup Table for Geological Category Landslide Frequency and Long Runout and Mobile Probability Factors Table 24 Lookup Table for Slope Angle Landslide Frequency Factor Table 25 Lookup Table for Vegetation Class Landslide Frequency and Long Runout and Mobile Probability Factors Table 26 Lookup Table for Elevation Landslide Frequency Factor Table 27 Lookup Table for Head Slope Angle Long Runout and Mobile Probability Table 28 Discussion of Practicality of Mitigation Measures Table 29 Leakage Quantities for Sensitivity Cases Table 30 Detailed Results for the Mitigated Case, and Additional Cases I and II LIST OF APPENDICES Appendix A Site Visit Photographs Appendix B Hazard Identification Workshop Logsheets Appendix C Details of Ownership of Man-made Slopes above and below the Pipeline Appendix D Natural Terrain Landslide Risk Model – Input Data, Even Trees and Lookup Tables Appendix E Pipeline Leakage Event Trees Appendix F Discussion of Practicality of Risk Mitigation Measures Appendix G Sensitivity Analysis Appendix H Comments and Responses R/8111/03/1 Issue 3, December 2002 iv Drainage Services Department Risk Assessment for Ngong Ping Effluent Pipeline In the Proximity of Shek Pik Reservoir Final Risk Assessment Report 1 INTRODUCTION 1.1 Background 1.1.1 As part of the Outlying Island Phase I Sewerage Work, a Sewage Treatment Works and the associated sewerage system will be built at Ngong Ping, serving the future developments in the Ngong Ping area on Lantau Island. The effluent from the proposed Ngong Ping Sewage Treatment Work, which will be treated to tertiary level, will be conveyed to the disposal point via the proposed effluent pipe. 1.1.2 The original proposal for effluent disposal was to have the treated effluent conveyed to and discharged at Tai O. During the detailed design phase, an alternative route was identified. This involves diverting the effluent for discharge into Tung Wan. For this alternative discharge option, a section of the effluent pipe (approx. 450 m in length) will be in close proximity to, and within the 100 m safety buffer zone of the Shek Pik Reservoir. This is highlighted in the attached plan (see Figure 1). 1.1.3 Given the close proximity of the effluent pipeline to the reservoir and the need to prevent possible pollution of Shek Pik Reservoir, WSD has requested a Risk Assessment to be carried out for the section of the proposed pipeline located within the 100m safety buffer zone (see Figure 2 and Figure 3).