Appendix 2A Major Accidents Hazards Report Appendices/Appendix 2...3.6.1 Natural Gas Pipeline 3-8...

Appendix 2A

Major Accidents Hazards

Report

Bord na Mona CCGT and OCGT Power Stations Co Offaly Mott MacDonald

Seveso Regulations - Major Accident Hazards Bord na Mona

246887/02/C - 11 December 2008/ P:\Manchester\Consultancy\Projects\246887 - US Offlay CCGT PS\09 Reports\246887_02_C_Major Accident Hazards Report.doc/LJM

Derrygreenagh

Rochfortbridge

Mullingar

Co. Westmeath

Bord na Mona CCGT and OCGT Power Stations Co Offaly

Seveso Regulations - Major Accident Hazards

December 2008

Mott MacDonald

Spring Bank House

33 Stamford Street

Altrincham

Cheshire

WA14 1ES

Tel : +44(0)161 926 4000

Fax : +44(0)161 926 4100



246887/02/C - 11 December 2008/

P:\Manchester\Consultancy\Projects\246887 - US Offlay CCGT PS\09 Reports\246887_02_C_Major Accident Hazards Report.doc/LJM

Bord na Mona CCGT and OCGT Power Stations Co Offaly

Seveso Regulations - Major Accident Hazards

Issue and Revision Record

Rev Date Originator

Checker

Approver

Description

A1 5

th Sep

2008

Lewis

Mitchell /

Martin Stone

Nigel

Harrison

Keith Mitchell

Internal Draft

B2 14

th Nov

2008

Martin Stone Nigel

Harrison

Keith Mitchell Revised for Approval

C 11

th Dec

2008

Martin Stone Nigel

Harrison

Keith Mitchell Incorporating BNM

comments

This document has been prepared for the titled project or named part thereof and should not be relied upon or used for any

other project without an independent check being carried out as to its suitability and prior written authority of Mott

MacDonald being obtained. Mott MacDonald accepts no responsibility or liability for the consequence of this document

being used for a purpose other than the purposes for which it was commissioned. Any person using or relying on the

document for such other purpose agrees, and will by such use or reliance be taken to confirm his agreement to indemnify

Mott MacDonald for all loss or damage resulting therefrom. Mott MacDonald accepts no responsibility or liability for this

document to any party other than the person by whom it was commissioned.

To the extent that this report is based on information supplied by other parties, Mott MacDonald accepts no liability for any

loss or damage suffered by the client, whether contractual or tortious, stemming from any conclusions based on data

supplied by parties other than Mott MacDonald and used by Mott MacDonald in preparing this report.



i 246887/02/C - 11 December 2008/i of iv


List of Contents Page

Summary S-1

Chapters and Appendices

1 Introduction 1-1

2 Identification of Potential Major Accident Hazards 2-1

2.1 Status of the Facility Design 2-1

2.2 Inventory of Dangerous Substances 2-1

2.3 Major Accident Hazards 2-2 2.3.1 HAZOP I Study 2-2

3 Consequences to the Public / Environment of Potential Major Accident Hazards 3-1

3.1 HSA Land Use Planning 3-1

3.2 Modelling the Consequences of Fires and Explosions 3-2 3.2.1 Pool Fires 3-2 3.2.2 Jet Fires 3-2 3.2.3 Flash Fires & Vapour Cloud Explosions 3-3

3.3 Diesel Bund Fire 3-3 3.3.1 Measures to prevent spillage of diesel into the bund 3-3 3.3.2 Consequences of a Bund Fire 3-4 3.3.3 Mitigation 3-4

3.4 Catastrophic Diesel Tank Failure 3-6 3.4.1 Measures to Prevent Catastrophic Failure 3-6 3.4.2 Consequences of Catastrophic Failure 3-6

3.5 Rupture of the Fuel Oil Transfer Line to the Turbine House 3-8 3.5.1 Measures to Prevent Failure 3-8 3.5.2 Consequences of Failure 3-8 3.5.3 Mitigation 3-8

3.6 Rupture of the Natural Gas Pipeline Outdoors (Jet Fire) 3-8 3.6.1 Natural Gas Pipeline 3-8 3.6.2 Measures to Prevent Failure 3-9 3.6.3 Frequency of Pipeline Failure 3-9 3.6.4 Consequences of Failure (Jet Fire) 3-10 3.6.5 Mitigation 3-10

3.7 Rupture of the Natural Gas Pipeline Outdoors (VCE & Flash Fire) 3-12 3.7.1 Natural Gas Pipeline 3-12 3.7.2 Measures to Prevent Failure 3-12 3.7.3 Frequency of Pipeline Failure 3-12 3.7.4 Consequences of Failure 3-13 3.7.5 Mitigation 3-14

3.8 Summary of Natural Gas Pipeline Rupture Events 3-17

3.9 Gas Release in the Turbine Halls (VCE) 3-17



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3.9.1 Measures to Prevent Failure 3-17 3.9.2 Consequences of Failure 3-18 3.9.3 Mitigation 3-18

3.10 Transformer Explosion 3-21 3.10.1 Measures to Prevent Failure 3-21 3.10.2 Consequences of Failure 3-21 3.10.3 Mitigation 3-21

3.11 Hazards from Other Substances 3-21 3.11.1 Hydrogen 3-21 3.11.2 Water Treatment 3-22 3.11.3 Boiler Water Treatment 3-22

3.12 External Hazards 3-22 3.12.1 Damage due to Vandalism 3-23 3.12.2 Aircraft Impact 3-23 3.12.3 Seismic Event 3-24 3.12.4 Fires Originating from Off Site Events 3-25

4 Hazards to Occupied Buildings 4-1

4.1 Occupancy Levels 4-1

4.2 Impact of MAH on Occupied Buildings 4-4 4.2.1 Design Considerations 4-4 4.2.2 Diesel Bund Fire 4-4 4.2.3 Jet Fires from Gas Pipelines 4-5 4.2.4 VCE External to the Turbine Halls 4-5 4.2.5 Flash Fires 4-5 4.2.6 Fire or Explosion with the Turbine Halls 4-5

4.3 Overall Risk to the Individual 4-6

5 Emergency and Contingency Arrangements 5-1

5.1 On Site 5-1 5.1.1 Water Supply 5-1 5.1.2 Fire Suppression Systems 5-1 5.1.3 Railway 5-2

5.2 External Arrangements 5-3

6 Conclusions 6-1

7 References 7-1

Appendix A Hazard Assessment Tables A-1

Appendix B PHAST Modelling Output B-1

B.1 Pool Fire Output B-1 B.1.1 Inner Zone (1.5 m/s, Cat F) B-1 B.1.2 Inner Zone (1.5 m/s, Cat D) B-2 B.1.3 Inner Zone (5.0 m/s, Cat D) B-2 B.1.4 Middle Zone (1.5 m/s, Cat F) B-3 B.1.5 Middle Zone (1.5 m/s, Cat D) B-3 B.1.6 Middle Zone (5.0 m/s, Cat D) B-4 B.1.7 Outer Zone (1.5 m/s, Cat F) B-4



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B.1.8 Outer Zone (1.5 m/s, Cat D) B-5 B.1.9 Outer Zone (5.0 m/s, Cat D) B-5

B.2 Jet Fire Output – 70 bar(g) Supply Line B-6 B.2.1 All Zones (NE direction) B-6 B.2.2 All Zones (SE direction) B-7 B.2.3 All Zones (SW direction) B-7 B.2.4 All Zones (NW direction) B-8

B.3 Rupture of the Natural Gas Pipeline Outdoors (VCE & Flash Fire) B-9 B.3.1 VCE Overpressure Drift Line (70 bar(g) incoming supply) B-9 B.3.2 Flash Fire Concentration Effect Zone (70 bar(g) incoming supply) B-10

B.4 VCE Gas Release inside the CCGT Building B-11

B.5 VCE Gas Release in OCGT Building B-11

Appendix C Excerpt from British Gas Research Paper C-1



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Glossary

Acronym Definition

1 ALARP As Low As Reasonable Practicable

2 BGN Bord Gáis Network

3 CCGT Combined Cycle Gas Turbine

4 COMAH Control of Major Accident Hazards

5 EPA Environmental Protection Agency

6 ESDV Emergency Shutdown Valve

7 HFL Higher Flammability Limit

8 HSA Health and Safety Authority

9 HSE Health & Safety Executive (UK)

10 IBC Intermediate Bulk Carrier

11 LEL Lower Explosive Limit

12 LFL Lower Flammability Limit

13 MAH Major Accident Hazards

14 NFPA National Fire Protection Association

15 OCGT Open Cycle Gas Turbine

16 PPE Personal Protective Equipment

17 SI Statutory Instrument

18 SIL Safety Integrity Level

19 VCE Vapour Cloud Explosion

20 WTP Water Treatment Plant



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Summary

Bord na Mona is in the process of submitting a planning application to construct a power plant

consisting of an Open Cycle Gas Turbine (OCGT) power plant and a Combined Cycle Gas Turbine

(CCGT) power plant at Derrygreenagh. The site is three miles from Rhode (County of Offaly) and

two miles from Rochfortbridge (County of Westmeath). The power station will be a lower tier

‘Seveso site’ under the Seveso Regulations (S1 74/2006). The power station will consist of a main

Combined Cycle Gas Turbine (CCGT) generation unit with a smaller Open Cycle Gas Turbine

(OCGT) generation unit for peak shaving. Both units will have facilities for firing with diesel. The

station will include boiler water and waste water treatment plant and the necessary administration and

maintenance facilities.

The Health and Safety Authority (HSA) is acting as the Central Competent Authority under the

Seveso Regulations. The Planning Authority, will request technical advice from the HSA on the

proposed development. In order that they can provide this advice the HSA will require information

on:

1. The potential Major Accident Hazards including an assessment of the extent and severity of

the consequences of such accidents; and

2. Demonstration that all necessary measures will be taken to limit the consequences of any

major accidents for people and the environment.

This report has been produced to provide this information.

The report details the dangerous substances that are to be stored on the site and identifies the Major

Accident Hazards (MAHs) that could occur. The following MAHs are described in detail with the

measures to be taken to prevent their occurrence and the mitigation available:

• Diesel Bund Fire;

• Catastrophic Diesel Tank Failure;

• Rupture of the Fuel Oil Transfer Line;

• Rupture of the Natural Gas Pipeline (Outdoors – Jet Fire);

• Release of Natural Gas (Outdoors - Flash Fire or VCE );

• Gas Release in the Turbine Building; and

• Transformers Explosion

For the worse case fire and explosion events the consequences have been modelled using the DNV

PHAST software and the Land Use Planning Zones plotted on the site location map.

It should be noted that very pessimistic modelling assumptions have been used. Even so, given that

the location of the site is well away from sensitive receptors, there is little risk to the general

population from the activities on the site.



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1 Introduction

S.I. No. 74 of 2006 (the Seveso/COMAH Regulations) implements in Ireland Council Directive

96/82/EC of 9th December 1996 on the control of major accident hazards involving dangerous

substances as amended by 2003/105/EC. Operators storing specified materials or materials with

properties specified in the Regulations in excess of the threshold quantities are subject to the

requirements of the Regulations and are referred to as ‘Seveso sites’.

Bord na Mona intends to submit a planning application to construct a power station at Derrygreenagh,

Co Offaly bordering Co Westmeath. The site for the proposed power station is a strategic location in

close proximity to a 220kV and 400kV electrical grid connection (at Derrygreenagh, Co. Offaly). It is

proposed that there will be two generating units located on the site. These are a flexible combined

cycle gas turbine unit (CCGT) of c. 430 MW and a reserve/peaking open cycle gas turbine unit

(OCGT) of c. 170 MW. The primary fuel source for the CCGT unit will be natural gas with diesel

stored onsite as a back up fuel as required by the Commission for Energy Regulation (CER). The

OCGT unit will be capable of dual firing, running on either natural gas or diesel. The proposed

development will also consist of all necessary ancillary structures and equipment to allow for the

efficient and safe running of the power plant. Further to this, it is proposed that the Bord na Móna

Power Generation business unit headquarters will be located at the site. The proposed power plant

will be a lower tier ‘Seveso Site’, specifically because of the requirement to store distillate fuel at the

site. Note that operators of lower tier ‘Seveso Sites’ are not required to prepare a Safety Report nor

prepare for an on-site emergency plan.

The Health and Safety Authority (HSA) is acting as the Central Competent Authority under the

Seveso Regulations. As such, it is empowered to issue land use planning advice with respect to

Seveso sites. The HSA bases its initial technical advice for a Seveso site on the calculated fire and

explosion consequences (a risk based approach is adopted for toxic releases). The planning authority

must seek technical advice from the HSA if a third party applies for planning permission for a

development:

1. In one of the specified development categories; and

2. Within the Consultation Distance of an existing Seveso site.

The proposed OCGT and CCGT are of types specified in the Planning and Development Regulations,

2001 (SI No 600 of 2001) for which the Planning Authority is obliged to seek technical advice from

the HSA. The proposed power station is not within the Consultation Distance of an existing Seveso

site. This report provides the following information in order to support the planning application:

1. An identification of all major accident hazards in the establishment in the context of S.I. No.

74 of 2006, including an assessment of the extent and severity of the consequences of such

accidents, specifically (i) the worst credible fire scenario involving diesel stock and its impact

on the nearest occupants to the proposed facility and (ii) the worst credible major accident to

the environment in the event of catastrophic failure of the diesel tank.



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2. Demonstration that all necessary measures will be taken to limit the consequences of any

major accidents for people and the environment in the context of S.I. No. 74 of 2006.

Specifically, good practice must be demonstrated where there is a reasonably foreseeable risk

of a major accident occurring in relation to bunding; tertiary containment in the event of bund

overtopping; firewater supply and retention; high level alarms; security and leak detection.

This report has been produced to address the requirements of the Seveso Regulations and provide

additional information as may be required as part of the planning process.



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2 Identification of Potential Major Accident Hazards

2.1 Status of the Facility Design

The design of the facility put forward for the purposes of the planning application is an outline design.

Bord na Mona has not yet selected the contractor to produce the detailed design and construct the

facility nor the manufacturer to supply the main items of equipment. There are differences in detail in

the design of OCGT and CCGT plants from different manufacturers.

This type of plant has a proven safety record both in Ireland (e.g. Huntstown Phases 1 and 2, Poolbeg,

Ringsend and Tynagh) and throughout the world. Further new CCGT installations of similar design

and output are proposed for Whitegate, Aghada and Toomes and these latter installations are due for

commissioning from 2009 to 2011.

2.2 Inventory of Dangerous Substances

The facility will store dangerous substances up to the amounts detailed in Table 2.1. The final figures

for the ‘as built’ may be slightly less depending on the final design chosen.

In addition there will be about 150m3 of transformer oils to BS 148 on site. However this material is

not classified as hazardous.



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Table 2.1: Inventory (maximum) of dangerous substances stored at the site

Proposed CCGT Power Plant at County Offaly

List of substances and preparations - Seveso Directive (S.I. No. 74 of 2006 European Communities [Control of Major Accident Hazards Involving Dangerous

Substances] Regulations 2006)

Substance CAS No Risk Phrases Classification

Physical

Form

Maximum

Quantity

(tonnes)

Lower Tier

Threshold

(tonnes)

Top Tier

Threshold

(tonnes)

Fraction of LT

Threshold

Fraction of TT

Threshold

Named substances

Distillate (Gas Oil) *

R40, R65,

R66, R51/53 Xn, N Liquid 12450 2500 25000 4.980 0.498

Natural Gas (not

stored) ** R12 F+ Gas n/a n/a n/a n/a n/a

Very Toxic

- 5 20 0.000 0.000

Toxic

50 200 0.000 0.000

Explosive

- 10 50 0.000 0.000

Extremely Flammable

Hydrogen (***) 1333-74-0 R12 F+ Gas 0.045 10 50 0.005 0.001

Highly Flammable

- 5000 50000 0.000 0.000

Flammable

- 5000 50000 0.000 0.000

Oxidising

- 50 200 0.000 0.000

Dangerous to the aquatic environment (very toxic to aquatic organisms)

Ammonium

Hydroxide (30%

concentration) 1336-21-6 R34/R50 C, N

Aqueous

Solution 2 100 200 0.020 0.010

Dangerous to the aquatic environment (toxic to aquatic organisms; may cause long term adverse effects in the aquatic environment)

Dilute

Carbohydrazide 497-18-7

R22-38-43,

R52/53 Xn

Aqueous

Solution 1 200 500 0.005 0.002

Seveso Totals: Very Toxic / Toxic 0.000 0.000

Explosive / Extremely Flammable / Highly Flammable / Flammable / Oxidising 0.005 0.001

Dangerous to the Environment 5.005 0.510

Other Substances

Sodium

Hydroxide 1310-73-2 R35 C Liquid 30 (n/a) (n/a)

Sulphuric Acid 7664-93-9 R35 C Liquid 25 (n/a) (n/a)

Tri-Sodium

Phosphate 7601-54-9 R34-36-38 C Solid 0.5 (n/a) (n/a)

Caustic Brine

(NaOH / NaCl

mix) 1310-73-2 R35 C Liquid 35 (n/a) (n/a)

(*) Various CAS numbers are used for Gas Oil depending on the exact compostion these include; 68334-30-5 and 68476-34-6

(***) 45 off K Class Cylinders Each cylinder 65kg empty, 66 kg full, l=1460mm,dia.=230mm,Press.=175barg. Vol.= 6.83Nm3

(**) Natural Gas is a mixture. The main constituents are Methane (74-82-8), Ethane (78-84-0), Propane (74-98-6), Nitrogen (7727-

37-9) and Carbon Dioxide (124-38-9)

2.3 Major Accident Hazards

The identification of Major Accident Hazards was undertaken based on a methodology developed by

Mott MacDonald for sites affected by the Seveso Regulations (Ref. 1). As part of this process a

HAZOP I study was undertaken, a brief summary of which is presented in section 2.3.1.

2.3.1 HAZOP I Study

HAZOP studies are a structured brainstorming technique involving the formal, systematic and critical

examination of the engineering intentions of a plant either at the design stage or as an audit operation

for an existing plant. The process leads to the identification of the causes of deviation (incorrect

information or system malfunction) and the consequences of each deviation to be discussed by a

suitably qualified and experienced study group.



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A HAZOP Level I study was undertaken on the Derrygreenagh facility. The HAZOP Level I analysis

is intended to identify hazards which are not necessarily localised, and which may have a wider sphere

of influence. Typically, such hazards can arise from the interaction of a particular system with its

external environment. For example, external events such as an earthquake, lightning strike or

maintenance activities may impact on a system to cause operational hazards that may need to be

addressed in terms of safety. The full results of the HAZOP study are presented in the HAZOP Report

(Ref. 2).

The potential Major Accident Hazards identified during the HAZOP study are presented in Appendix

A . For each of these potential hazards the preventative measures incorporated in the plant design

specification are listed as well as the design features and operational arrangements to reduce the

consequences of the hazards (mitigation). In addition, the external events with the potential to initiate

a Major Accident Hazard at the facility are considered in section 3.12.

The Major Accident Hazards with potentially the most serious consequences (referred to as

‘bounding’ accidents) are considered in more detail in section 3 of the report.



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3 Consequences to the Public / Environment of Potential Major Accident Hazards

3.1 HSA Land Use Planning

In general, the HSA bases its advice on consideration of the contours which coincide with the

following zones identified in Table 3.1:

Table 3.1: Land Use Planning Zones (fire)

Zone Heat Effect HSA Advice

Inner 1800 thermal dose units

(11.0 kWm-2

over 60

seconds)

Advise against residential, office and retail,

permit occasionally occupied developments e.g.

pump houses, transformer stations. Consult with

the HSA re. industrial developments

Middle 1000 thermal dose units

(7.0 kWm-2

)

Permit workplace development. Permit residential

densities from 28 to 90 persons/ha, density

increasing as risk decreases across the zone in

developed areas and 22 to 70 persons/ha in less

developed areas. Permit modest retail and

ancillary local services. Advise against shopping

centres, large scale retail outlets, undue

concentrations of restaurant / pub facilitates

Outer 500 thermal dose units

(4.0 kWm-2

)

No restrictions except for sensitive developments,

which would be subject to consultation if inside

the consultation range and should not be at a risk

greater than 0.3 x 10-6

/yr.

Sensitive developments include crèches, school,

hospitals and nursing homes. Locations of major

public assembly will be subject to individual

assessment

Table 3.2: Land Use Planning Zones (explosion)

Zone Overpressure Effect on people and buildings (UK HSE)

Inner 600 mbar Serious level of death

Middle 140 mbar Dangerous level (1% lethality)

Frame distortion of steel framed buildings

Outer 70 mbar Windows usually shattered (all sizes)



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3.2 Modelling the Consequences of Fires and Explosions

The model used was the DNV Technica software package PHAST v6.53.1. The package models fires

and explosions as discussed in the following section.

It should be noted that the results presented in this section, specifically the zoning diagrams based on

HSA policy, were produced based on the outputs from the PHAST model. Actual outputs from the

model are presented in Appendix B .

3.2.1 Pool Fires

For a catastrophic loss of containment from a tank, the model assumes that the contents of the tank are

lost almost instantaneously. Where the tank is bunded, a pool will form, and will spread as far as the

bund walls. If the liquid is ignited, a pool fire will result. Since the PHAST code only models circular

pool fires a series of circular pool fires centred at different points along the bund was configured as

part of the analysis. The surface area of each pool was representative of a pool fire equivalent to the

total surface area of the bund, including the footprint of the storage tank. In addition, the full range of

wind directions was combined to give an overall model of the consequences of a bund fire.

All scenarios were modelled under 3 weather conditions recommended by DNV (Table 3.3).

Table 3.3: Weather Conditions for Modelling

Wind speed (m/s) Pasquill (Atmospheric) Stability

Category

5 m/s D

1.5 m/s D

1.5 m/s F

3.2.2 Jet Fires

The PHAST model for jet fires was used. A substantial degree of pessimism was introduced in that

the jet fires were assumed to be projected horizontally across open land i.e. the protection offered by

buildings and equipment in terms of the propagation of the flame was discounted. In addition, the

hole size was assumed to be equal to the diameter of the pipe and pressure losses were discounted.

Note that the majority of the pipeline is to be run below ground so that only where the pipeline

emerges by the compressor building was modelled for the worst case horizontal release. All other

locations were assessed in terms of a vertical release in order to ensure that they were bounded by the

horizontal release at the compressor building.

The results produced, as with pool fires, combined the full range of wind directions and the weather

conditions as listed in Table 3.3.



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3.2.3 Flash Fires & Vapour Cloud Explosions

Flash fires were modelled based on the pipe source model in the PHAST package which produces a

flammable cloud footprint based on defined release parameters.

Vapour Cloud Explosions (VCEs) were modelled using the TNT equivalence method in the PHAST

model. This method equates the mass of flammable gas within the vapour cloud to a mass of TNT,

and applies a cube root scaling law to calculate the variance of the overpressure with distance from the

point of origin.

Note that both models assume that the release continues for 60 seconds following a rupture of the gas

pipeline and that the pressure is constant during this period. The gas supply system will have

Emergency Shutdown Valves (ESDVs) to cut off the flow in the event of a rupture.

Note also that, similarly to the modelling of jet fires, vertical releases only were modelled for

underground sections of the pipeline.

(i) VCE inside the Turbine House

As part of the modelling, it was assumed that the entire cloud of flammable gas was contained within

the volume of the turbine house (i.e. the effects of ventilation were discounted). The mass of

flammable gas was calculated based on the LEL of Methane and applied to a point source at the centre

of the building. Note that a factor of 50% was applied to the volume of the turbine house to account

for plant within the building. Overpressures were calculated assuming an unconfined explosion i.e.

the effect of the buildings and equipment in limiting the propagation of the overpressure was

discounted.

(ii) Flash fire / VCE due to rupture of the Natural Gas Pipeline (outdoors)

A substantial degree of pessimism is inherent in the modelling in that the release is assumed to project

horizontally across open land i.e. the protection offered by buildings and equipment was discounted

and the release was assumed to project with equal velocity in all directions. In addition, the hole size

was assumed to be equal to the diameter of the pipe and all pressure losses were discounted. The

majority of the pipeline is to be run below ground so that only the area where the pipeline emerges by

the compressor building was modelled as the worst case horizontal release. All other locations were

assessed in terms of a vertical release in order to ensure that they were bounded by the horizontal

release at the compressor building.

3.3 Diesel Bund Fire

3.3.1 Measures to prevent spillage of diesel into the bund

The following initiating events could potentially lead to leakage of diesel into the bund:

• Overfilling of the storage tanks;

• Corrosion of the floor of the tanks;



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• Water contamination leading to microbes in the product, which cause corrosion at the

water/oil interface; and / or

• Leakage from the pipework and fittings.

The tanks are to be new double bottomed tanks to BS EN 14015. They will be checked regularly for

water contamination and drains will be supplied to remove any water present. There will be a drain

between the two floors of the tank, which will allow checks for leakage to be undertaken regularly.

There will be electronic gauging of the tanks with remote readout and a ‘cat and mouse’ type gauge.

An independent high level alarm will be provided. The control system associated with the tanks will

also have a Safety Integrity Level of 1 (SIL 1).

The operators will check the tanks regularly. A cathodic protection system will be installed and

periodically an ultrasonic test of the bottom thickness of the tanks will be carried out.

It should be noted that the bund capacity is greater than 110% of the volume of one of the storage

tanks and the layout and design of the tank installation will comply with National Fire Protection

Association (NFPA) requirements.

3.3.2 Consequences of a Bund Fire

In the unlikely event of a major leak from the tanks to the bund it is unlikely that this will lead to a fire

as diesel has a flash point of greater than 55°C and is not classified as flammable. There will be no

sources of ignition within the bund.

In the remote event of the diesel being ignited a pool fire will result. The worst case pool fire in terms

of consequence assumes the loss of the entire contents of the tank into the bunded area, noting that the

bunded area exceeds 110% of the volume of a single tank. The consequences of such a fire, in terms

of the heat effects as applicable to the land use planning zones (Table 3.1) are presented in Figure 3.1.

3.3.3 Mitigation

The location of the tanks will comply with the minimum spacing recommendations as specified by

NFPA.

In addition, in the event of a fire cooling water will be applied to the tanks if required, in line with the

NFPA guidelines.



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Figure 3.1: Predicted land use planning zones based on a fuel oil pool fire in the bunded area



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3.4 Catastrophic Diesel Tank Failure

In the event of catastrophic failure of a storage tank, the full inventory of the tank would be rapidly

lost to the bunded area. The release dynamics during such an event would be such that a fraction of

the product may be lost over the bund wall, depending on a number of factors including the profile of

the bund wall and the ratio of the height of the liquid in the tank to the bund wall height.

3.4.1 Measures to Prevent Catastrophic Failure

The tanks will be double bottomed tanks to BS EN 14015. A schedule of inspection of the condition

of the tanks will be in place to ensure that the risk of mechanical failure is minimised. The inspections

will be undertaken in accordance with EEMUA 159:03. The tanks will be located greater than the

minimum recommended distance from the nearest hazard as specified in the NFPA guidelines.

It should be noted that recorded catastrophic tank failures are low temperature brittle failure events.

Recent work on this failure mode (Ref. 3) suggests that this type of failure of a tank designed to

modern standards in the weather conditions recorded in Ireland would be an extremely rare event.

It should be noted that the bund capacity is greater than 110% of the volume of a single storage tank.

3.4.2 Consequences of Catastrophic Failure

The following section presents an assessment of the fraction of liquid likely to overtop the bund based

on a calculation published in the Journal of Loss Prevention (Ref. 4).

(i) Overtopping Assessment

The amount of material which can overspill a bund wall following catastrophic tank failure is

dependent on several factors, such as the bund capacity, the bund design and the release dynamics

following the failure event.

However, it is mainly governed by the ratio of the height of liquid in the tank to the bund wall height

and can be calculated as shown in equation [1].

+

+=

H

rCx

H

hBxAQ ee loglog [1]

Where: Q = fraction of tank contents which overtops the bund wall

A = bund wall factor = 0.044 for a vertical wall

B = bund wall factor = -0.264 for a vertical wall

C = bund wall factor = -0.116 for a vertical wall

h = bund wall height



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H = liquid level in tank

r = distance from the centre of the tank to the bund wall

The bund at the proposed Offaly facility will be 128 x 40 m in plan and is designed to contain three

tanks, each of 30m diameter and designed to hold 5,000 m3 of product at a liquid level of 7.1 m. In

order to contain 110% of the volume of a single tank, the bund walls will need to be at least 1.5 m high

and will be vertical. The distance between the centre of the tanks and the bund wall will be 20 m at

the nearest point.

Application of equation [1] to the parameters above results in a value of 33.4% of the contents of the

tank overtopping the bund in the event of a catastrophic failure. This is equivalent to approximately

1,670 tonnes of product.

(ii) Tertiary Containment

The following is taken from an internal HSA guidance document:

“For example, the provision of tertiary containment and associated drainage systems to contain and

hold up to 110% of the maximum calculated overtopping fraction is considered by the Authority to be

an appropriate approach.”

In the event of a worst case catastrophic tank failure and subsequent overtopping event, the volume to

be contained by the tertiary containment is therefore equal to:

110% x 33.4% x 5,000 m3 = 1,837 m

3

It is proposed that the tertiary containment will consist of an impermeable area or apron – surrounded

by a suitable wall or dyke - surrounding the tank bund. The area of this apron will be 4,800 m2. The

above expected maximum overtopping volume would fill this area to a depth of 0.39 m. In order to

prevent escape of the oil from this tertiary containment area, we propose that the level of this area

should be 200 mm below the level of the surrounding ground, and that it should be surrounded by a

kerb 200 mm high. The access road to the north of the bund will pass through this area, and suitable

ramps will be provided so that road will pass over the kerb without interrupting its continuity.

Rainwater inside the tertiary containment area will be drained via a suitable oil interceptor to the main

storm water retention pit, which with a volume of at least 5,400m3 (TBC) will be capable of

containing the full volume of the tertiary containment area. Pumping facilities will be provided to

enable the drainage of the rainwater.

Finally, spill kits will be available on site and the staff trained to respond to spills.



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3.5 Rupture of the Fuel Oil Transfer Line to the Turbine House

3.5.1 Measures to Prevent Failure

The following initiating events could potentially lead to failure of the fuel oil line outside the bund:

• Vehicle impact with the line;

• Corrosion of the line; or

• Failure of joint or fitting.

The pipeline will be designed and tested to BS 13480, and will be maintained in accordance with the

contractors O&M manuals. The pipe will be run above ground on pipe supports with a high level

(5m) pipe-bridge across the railway and a suitable trench below the site road. Crash barriers and

safety signage will be provided.

3.5.2 Consequences of Failure

It should be noted that fuel oil is a secondary fuel of the CCGT and the primary fuel of the OCGT

However, since the OCGT plant will operate as a peaking plant with anticipated running of

approximately 200 - 500 hours per annum, the pipeline will only be used intermittently. The pipeline

will be checked for leakage whenever the secondary fuel is used. In the event of pipeline failure

during operations the feed pump, located close to the bunded area, will trip on loss of suction pressure.

There is also a local trip switch at the pump. In addition the power station protection systems will shut

down the boilers on fuel failure.

With the range of systems in place to detect a pipeline failure it is expected that the worse case loss of

oil in the event of a failure will be limited to the pipeline volume plus a volume less than the

equivalent of 5 minutes of pumping. In the event of pipeline failure when the fuel oil is not in use the

loss of oil will be limited to the volume of the pipeline.

3.5.3 Mitigation

Spill kits will be available on site and the staff trained to respond to spills.

3.6 Rupture of the Natural Gas Pipeline Outdoors (Jet Fire)

3.6.1 Natural Gas Pipeline

It should be noted that the route and size of the pipeline from the BGN network to the site will be

determined by An Bord Gáis. The pipeline will be underground for most of its distance and will only

be above ground close to the terminal points. For the purposes of the assessment it has been assumed

to be a 450mm diameter pipe operating at a maximum pressure of 70 bar(g) and at a temperature of

15°C entering the site from the east.



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The gas is preheated to about 40°C and compressed, if necessary, to 40 bar(g) to feed the CCGT

generator. The line from the compressors to the CCGT generator is assumed to be 300mm diameter.

The gas is also pumped to the OCGT generation plant in peak times when necessary at about 15°C and

compressed to 30 bar(g). The line from the compressors to the OCGT generator is assumed to be

200mm diameter.


The pipeline could fail as a result of:

• Over-pressurisation;

• Collision;

• Corrosion; or

• Leakage at a flange or fitting

The site pipeline joining the Bord Gáis pipeline will be designed and tested to BS EN 13480 and of

mostly welded construction. The pipeline from the compressors to the gas turbine generator will be of

316L stainless steel and be of all welded construction. The pipelines will be tested and subject to

NDE. There will be pressure relief valves provided to protect the pipeline against over pressurisation.

Industry standard measures will be introduced to limit, and where practical eliminate, the potential

sources of ignition.

3.6.3 Frequency of Pipeline Failure

From the Guidelines for Quantitative Risk Assessment (Purple Book), CPR18E, Committee for the

Prevention of Disasters, 1999 (Ref. 5), the frequency of a leak for pipes with a diameter greater than

150 mm is estimated as 5.0 x 10-7

m-1

y-1

. On this basis, Table 3.4 presents the estimated frequency of

pipeline failures for the site:

Table 3.4: Pipeline Failure Rates

Pipeline Length (estimate) Annual Pipe Failure

Rate

70 bar(g) (max) feed from

BGN

50 m 2.50 x 10-5

40 bar(g) supply to CCGT 85 m 4.25 x 10-5

30 bar(g) supply to OCGT 225 m 1.13 x 10-4

Note that all accidents modelled assume that the release continues for 60 seconds following failure and

that the pressure is constant during this period. The gas supply system will have ESDVs to cut off the

flow in the event of a rupture.



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3.6.4 Consequences of Failure (Jet Fire)

In the event of failure gas will be released from the pipeline. In order for a jet fire to occur the gas

would have to encounter an ignition source when in an appropriate concentration. The consequences

of jet fires for failures in the 70 bar(g) incoming gas supply, the 40 bar(g) feed to the CCGT generator

and the 30 bar(g) feed to the OCGT generator have been modelled.

Since the on site feed pipelines are underground, the worst case scenario was assumed to be a vertical

release, as opposed to the horizontal release modelled for the 70 bar incoming supply line at the

compressor house. As such, the majority of the radiated heat from failure of the underground feed

lines propagates vertically upwards and hence the associated hazard contours are less significant that

those resulting from failure of the supply line. Modelling demonstrated that the heat effects, as

applicable to the land use planning zones (Table 3.1), associated with vertical failure of the feed lines

are bounded by the failure of the 70 bar supply line.

Hence, the results of the jet fire modelling, in terms of the worst case heat effects as applicable to the

land use planning zones (Table 3.1) are presented in Figure 3.2 (70 bar(g) incoming gas supply). The

figure also presents nearby commercial and residential buildings within around 2 km of the site

boundary.

It should be noted that considerable effort will be made to remove potential sources of ignition from

the site, including ensuring that electrical equipment is rated for the appropriate environment. It is

therefore estimated that the probability of a gas release encountering a source of ignition in the

immediate vicinity of the pipe breach would be 0.3, giving the overall frequency of jet fires to be

about 7.5 x 10-6

for the 70 bar(g) pipeline, about 1.3 x 10-5

for the 40 bar(g) pipeline and about 3.4 x

10-5

for the 30 bar(g) pipeline.

There are no off-site commercial or residential buildings within the predicted accident envelopes and

the site operator has control of most of the land surrounding the facility. Therefore it is considered

that the probability of a member of the general public being killed by such an accident is less than 1%.

This gives an overall risk to the general public of the order of 5 x 10-7

per year, noting the inherent

pessimism in this figure given that vertical releases from the underground feed lines are not predicted

to present a risk off site.

3.6.5 Mitigation

In the event of a major leak from the gas pipelines there will be a large number of pressure and

temperature alarms on the compressors and generators which will warn of the event and shut down the

compressor and gas turbine generators. The gas supply can be isolated either by a manual valve on

site or by a remote operated valve on the BGN.

Sensitive equipment such as electrical equipment will be protected by suitable fixed fire suppression

equipment to NFPA guidelines. There will also be portable fire extinguishers provided.



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Figure 3.2: Predicted land use planning zones based on a jet fire in the incoming 70 bar(g) gas supply



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3.7 Rupture of the Natural Gas Pipeline Outdoors (VCE & Flash Fire)

It is considered that jet fires constitute the worst case failure outside the turbine house in terms of the

definition of land use planning zones. Specifically, it may be reasoned that VCEs should not occur

due to consideration of the location of the pipeline in line with factors used by the UK HSE’s Major

Hazard Assessment Unit in determining whether combustion of a vapour cloud is likely to result in a

VCE or a flash fire. The following provides a summary of the major reasons:

• The amount of vapour in the flammable range at any one time;

• The absence of strong ignition sources;

• Methane is a saturated hydrocarbon; and

• Open space, lack of congestion.

In such scenarios combustion takes place relatively slowly and there is no significant overpressure. It

is also generally assumed that the thermal effects are limited to people within the flame envelope and

that flash fires would have a negligible effect on plant and buildings due to the short duration of the

fire and the negligible overpressure created.

Nonetheless, since detailed plant design is not available in terms of the nature of the congestion

around the pipe route it is considered prudent to assess the consequences of a VCE, in terms of the

explosive limits applicable to the land use planning zones, at this stage. In addition the flash fire

effects are considered, in terms of the distance to half the lower flammability limit.

3.7.1 Natural Gas Pipeline

As per Jet Fire scenario, see section 3.6.1.



Note: It is reiterated that all pipework will be installed to the relevant standards and that similar

pipework is currently in use throughout the nation. In addition, the type of plant has a proven safety

record both in Ireland (e.g. Huntstown Phases 1 and 2, Poolbeg, Ringsend and Tynagh) and

throughout the world. Further new CCGT installations are also proposed for Whitegate and Aghada.

3.7.3 Frequency of Pipeline Failure




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(i) Vapour Cloud Explosion

The UK HSE’s Safety Report Assessment Guide for Methane Gas Holders (Ref. 6) states:

‘Explosion of an unconfined cloud of methane has been found to be virtually impossible. If gas

releases from pipelines or vessels operating at >1000kPa enter a semi-enclosed volume with

obstructions that aid flame acceleration, ignition may result in an explosion that produces a

dangerous side-on pressure at some distance from the seat of the explosion.’

This is consistent with research reported in the British Gas Paper (British Gas Research in the Field of

Safety and the Environment – Transactions of the Institution of Chemical Engineers, Vol 69, Part B,

Ref. 7) the relevant section of which is reproduced as Appendix C .

The analysis has conservatively assumed that a VCE could occur anywhere within the site boundary

but as the area surrounding the site is open fields there would not be the obstructions necessary to

produce overpressure effects and therefore a VCE was not modeled for these areas.

Similarly to the jet fire assessment (section 3.6), analysis has demonstrated that the hazard contours

associated with the failure of the underground feed pipelines are bounded by those associated with

failure of the [above ground] 70 bar supply line at the compressor house.

Hence, the results of the VCE modelling, in terms of the overpressure effects as applicable to the land

use planning zones are presented in Figure 3.3 (70 bar(g) incoming gas supply). The figure also

presents nearby commercial and residential buildings within a nominally 2 km radius of the site

boundary.

Note that the output represents the zones that may experience a given overpressure and not a single

explosion at the source of the failure.

The nature of the model is such that the dispersion cloud formed is allowed to ‘drift’ based on the

worst case wind conditions and the model assumes that ignition sources are present throughout the

‘drift zone’. The figures contained in Appendix B.3 show an example of ‘drift lines’ with the

subsequent explosion represented by the smaller contour lines.

It is considered that the probability of gas release encountering a source of ignition on-site but away

from the immediate vicinity of the pipe breach would be about 0.3 giving the overall frequency of

VCEs to be approximately 7.5 x 10-6

for the 70 bar(g) pipeline, 1.3 x 10-5

for the 40 bar(g) pipeline

and 3.4 x 10-5

for the 30 bar(g) pipeline.

Note that modelling has demonstrated that the dispersion cloud would be below the LEL were the

cloud to propagate such as to encounter potential sources of ignition at the switchyard to the southwest

of the site. As such this scenario has not been considered as part of the analysis.



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There are no off-site commercial or residential buildings within the predicted accident envelopes

therefore it is considered that the probability of a member of the general public being killed by such an

accident is less than 1%. This gives an overall risk to the general public of the order of 5 x 10-7

per

year noting the inherent pessimism in this figure given that vertical releases from the underground

feed lines are not predicted to present a risk off site.

(ii) Flash Fire

Where gas is dispersed to an open area and is then ignited a flash fire would occur. Similarly to the jet

fire assessment (section 3.6), analysis has demonstrated that the hazard contours associated with the

failure of the underground feed pipelines are bounded by those associated with failure of the [above

ground] 70 bar supply line at the compressor house. Hence the results of the flash fire modelling, in

terms of the overpressure distance to half the lower flammable limit are presented in Figure 3.4 (70

bar(g) incoming gas supply). The figure also presents nearby commercial and residential buildings

within a nominally 2 km radius of the site boundary. Flash fires were modelled based on the pipe

source model in the PHAST package which produces a flammable cloud footprint based on defined

release parameters.

Similarly to the VCE output, it should be noted that the output concentration area represents the zone

in which a cloud at 50% LFL concentration could occur and not a single cloud. The figures contained

in Appendix B.3 present examples of explosions which are represented by the smaller contour lines.

In considering an off-site flash fire scenario, it should be noted that the area surrounding the Power

Station is open fields and old peat workings, and there are no obvious sources of ignition (all the

existing structures on the site associated with the peat works will be removed as part of the re-

development of the site). Therefore the probability of ignition off-site, given the occurrence of a

pipeline failure (Table 3.4), is estimated as 0.1. This results in an overall frequency of off-site flash

fires of 2.5 x 10-6

for the 70 bar(g) pipeline, 4.3 x 10-6

for the 40 bar(g) pipeline and 1.1 x 10-5

for the

30 bar(g) pipeline.

There are no off-site commercial or residential buildings within the predicted accident envelopes

therefore it is considered that the probability of a member of the general public being killed by such an

accident is less than 1%. This gives an overall risk to the general public of the order of 2 x 10-7

per

year noting the inherent pessimism in this figure given that vertical releases from the underground

feed lines are not predicted to present a risk off site.

3.7.5 Mitigation

As per Jet Fire scenario, see section 3.6.5



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Figure 3.3: Predicted land use planning zones based on a VCE in the incoming 70 bar(g) (450mm diameter) gas supply



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Figure 3.4: Distance to 0.5 LFL (blue line) based on a flash fire in the incoming 70 bar(g) (450mm diameter) gas supply



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3.8 Summary of Natural Gas Pipeline Rupture Events

A summary of the risks to the general public from fires and explosions caused by pipeline failures is

presented in Table 3.5.

Table 3.5: Summary of fire & explosion risks to the general public caused by pipeline failures

Jet fire

(probability = 0.3)

VCE

(probability = 0.3)

Flash Fire

(probability = 0.1)

Pipeline

failure

frequency

(yr-1

) Freq (yr-1

) Risk (yr-1

) Freq (yr-1

) Risk (yr-1

) Freq (yr-1

) Risk (yr-1

)

Gas supply -

70 bar(g)

2.50E-05 7.50E-06 7.5E-08 7.50E-06 7.5E-08 2.50E-06 2.50E-08

Gas feed – 40

bar(g)

4.25E-05 1.30E-05 1.30E-07 1.30E-05 1.30E-07 4.25E-06 4.25E-08

Gas feed – 30

bar(g)

1.13E-04 3.40E-05 3.40E-07 3.40E-05 3.40E-07 1.13E-05 1.13E-07

Total 5.50E-07 5.50E-07 1.81E-07

Hence it can be seen that the total risk to the public is of the order of 1.3E-06 per annum noting the

inherent pessimism in this figure given that vertical releases from the underground feed lines are not

predicted to present a risk off site.

3.9 Gas Release in the Turbine Halls (VCE)


The gas system in the turbine building could fail as a result of:

• Over pressurisation;

• Collision/impact;

• Corrosion; or

• Leakage at a flange or fitting.

The pipeline to the gas turbine generator will be 316L stainless steel and will be of all welded

construction. The pipelines will be tested and subject to NDE. There will be pressure relief valves

provided to protect the system against over pressurisation. The gas turbine generator and associated

equipment will be installed and tested to the manufacturer’s standards and industry guidelines.



3-18 246887/02/C - 11 December 2008/3-18 of 26


Industry standard measures will be introduced to limit and where practical eliminate the potential

sources of ignition.


In the event of failure gas will be released into the building. If the gas encountered an ignition source

a fire, or in the worse case, a VCE could occur.

The results of the VCE modelling, in terms of the overpressure effects as applicable to the land use

planning zones (Table 3.2) are presented in Figure 3.5 and Figure 3.6.

3.9.3 Mitigation

There will be a full range of fire suppressions systems provided to NFPA guidelines including:

• Fixed gas injection systems for electrical plant;

• A water based foam system; and

• Portable fire extinguishers.



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Figure 3.5: Predicted land use planning zones based on a VCE in the CCGT house



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Figure 3.6: Predicted land use planning zones based on a VCE in the OCGT house



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3.10 Transformer Explosion


The site will have an oil filled step-up transformer to transform the voltage generated to grid voltage

and an auxiliary transformer to reduce the voltage for auxiliary power. These transformers, under

certain fault conditions can overheat and produce an explosion. Modern transformer design

incorporates a protection system and will shut down the transformer in the event of such overheating.

With these protection systems installed transformer explosion is an extremely rare event.


The consequence of a transformer explosion is bounded by the VCE in the Turbine Building (see

section 3.8).

3.10.3 Mitigation

The transformers are contained within reinforced concrete blast walls reducing the risk of ‘domino’

accidents.

The transformers will also be placed on foundations which will have a containment bund to collect any

oil spillage. On top of this bund will be a mesh with a 150 mm layer of stone chippings which will act

as a fire protection barrier between any collected oil and the associated transformer.

The transformers will also be provided with a water deluge system designed in accordance with NFPA

requirements. This deluge system will be activated upon detection of a fire on the associated

transformer.

3.11 Hazards from Other Substances

There are a number of other hazardous substances on site such as Hydrogen, Sodium Hydroxide,

Sulphuric Acid, Tri-Sodium Phosphate and Caustic Brine. These are stored in small quantities (Table

2.1) and do not require to be analysed under the Seveso regulations.

3.11.1 Hydrogen

Hydrogen is used in the generator cooling system and is delivered in cylinders to the site. It is

considered that risks from hydrogen fires or explosions would be bounded by those from natural gas.



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3.11.2 Water Treatment

The design of the water treatment plant has yet to be defined and hence a worst case, in terms of the

hazardous substances used, has been assumed. The worst case plant would be an ion exchange plant.

Sulphuric Acid, H2SO4 (concentration 96-98%), Sodium Hydroxide, NaOH, (concentration 47%), and

Caustic Brine (NaOH / NaCl mix), concentrations 5% NaOH, 24% NaCl are used in the ion exchange

water treatment process. These chemicals are delivered to the site by tanker and stored in storage

tanks of approximately 25,000 litres, 30,000 litres and 32,000 litres respectively, beside the Water

Treatment Plant (WTP). The WTP Chemical Storage tanks have a watertight bund that drains

manually to the WTP Neutralisation Tank where effluent is monitored and treated prior to discharge to

the Waste Water Treatment Tank. Demineralisation of the raw water is carried out inside the WTP

Plant Room.

3.11.3 Boiler Water Treatment

Ammonium Hydroxide and Carbohydrazide (an oxygen scavenging agent) are used for boiler water

treatment. Both chemicals are delivered to the site in 200 litre drums or IBC’s (1m3) and stored in the

Boiler Dosing Chemical Storage Room (BDCS). The BDCS has a watertight floor that drains to the

Process Water Effluent Pit, where effluent is monitored and treated prior to discharge to the Yellow

River. In addition, provision will be made for dosing with Tri-Sodium Phosphate. This may be

required from time to time depending on boiler water chemistry. The Tri-Sodium Phosphate will be

stored in a circa 150 litre tank.

Chemical treatment of the boiler feed water and boiler water is carried out inside the Dosing Room.

Each dosing agent has its own dedicated dosing tank (ca. 1.5 m3 capacity) with bund and dosing pump

train, complete with over pressure protection (vis-à-vis minimum flow return to dosing tank). When

replenishment of a dosing tank is required, the appropriate chemical drum will be transported from the

BDCS to the Boiler Dosing Room. The contents of the chemical drum will be pumped into the

appropriate dosing tank. Small bore pipework and fittings will be used and the dose rate carefully

controlled.

3.12 External Hazards

This section presents a review of external events with the potential to initiate hazards at the proposed

facility. In addition, the potential for domino effects, whereby an accident at the plant could initiate a

major accident at the other sites and vice versa, is also considered.

The following external events are considered as part of the analysis.

• Damage due to vandalism

• Aircraft impact

• Seismic event

• Off site fire



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3.12.1 Damage due to Vandalism

The possibility of members of the public encroaching onto site and causing damage such as to lead to

a MAH has been considered as part of the hazard analysis. Such an act of vandalism may involve the

breach of a storage tank leading to the leakage of product, or sufficient damage to a gas line such as to

cause rupture and subsequent leakage.

As part of the planned facility fencing and other measures such as CCTV will be installed to provide

security at the site. The site itself will be manned on a continuous basis with staff in the Control

Room continuously monitoring persons entering and leaving the site. The railway line entering the

site will be secured with gates to restrict unauthorised access. These gates will be controlled by the

movement of the train travelling along the track and will also be alarmed and monitored by CCTV.

In addition to the prevention measures identified above, it should be noted that, given the structure of

the tanks and pipes significant effort would be required to be expended by individuals to cause even a

minor leak.

In the event of attempting a breach of the diesel tank containment, the remote monitoring will provide

early warning. The tank level monitors and associated alarms will provide additional, immediate

notification to ensure that the incident would not escalate to a MAH, noting that the capacity of the

bunded areas is such that the entire contents of any one tank can be safely contained and removed

without any environmental impact.

In the event of such an act leading to the rupture of a natural gas line, there will be a large number of

pressure and temperature alarms on the compressors and generator which will warn of the event and

shut down the compressor and gas turbine generator. The gas supply can be isolated either by a

manual valve on site or by a remote operated valve on the BGN.

Hence, while the possibility of an act of vandalism cannot be discounted, it is not considered credible

that such an act could result in a MAH, particularly one for which the consequences would have a

greater off site impact than the other scenarios identified during the HAZID.

3.12.2 Aircraft Impact

The nearest major airfield to the Derrygreenagh facility is Dublin International Airport, which is

around 60km away. Nearer to the site there is a private airfield for light aircraft at Trim, which is over

30km away. The Irish Defence Forces Airbase at Baldonnel is about 50km from the site. Hence the

analysis of the frequency of an aircraft impact will be based primarily on the risk presented by Dublin

International Airport.



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Environmental Resources Management (ERM) Ireland Ltd was commissioned by the Department of

Transport and the Department of Environment, Heritage and Local Government to investigate Public

Safety Zones (PSZs) around Ireland’s three principal airports, namely Cork, Dublin and Shannon. The

aim of the PSZs is to protect people from the risk of aircraft impact by the use of land use planning

controls on developments in the vicinity of the airports. The outer zones define the boundary at which

the risk is considered to be less than 1 x 10-6

yr-1

. The proposed plant at Derrygreenagh is located a

considerable distance (circa 50km) from the outer PSZ, indicating that the individual risk of an aircraft

impact is less than 1 x 10-6

per year. While this does not directly represent the overall risk to a site, it

does indicate that the proposed plant is located in a low risk area well away from the main risk zone of

Dublin International Airport.

UK HSE Guidance on Technical Policy Lines to take for Predictive Assessors (Ref. 8), recommends

that, for sites outside high crash concentration areas and not close to airfields, then detailed calculation

of the frequency of an aircraft crash does not need to be performed. In this instance, background crash

rates should be reported.

Though background crash rates for Ireland have not been obtained, the following rate for England is

taken from the HSE AEA Technology Paper CRR 150/1997 (Ref. 8).

The total background crash rate for civil and military aircraft in England = 4.4 x 10-5

km-2

yr-1

.

Hence, assuming that the risk for Ireland is no worse than that for England, the total background crash

rate for civil and military aircraft in Ireland = 4.4 x 10-5

km-2

yr-1

.

Applying an estimate of the area of the site to this figure of 2.3 x 10-1

km2 results in a background

crash rate of 1.0 x 10-5

yr-1

, which demonstrates good agreement with the figure quoted for the outer

PSZs.

3.12.3 Seismic Event

As part of the NORA Marina Tank Farm Cork Safety Report (Ref. 9), the School of Cosmic Physics

(part of the Dublin Institute for Advanced Studies) was consulted regarding the risks posed by seismic

activity in Ireland. The School has had a seismic network in operation in Ireland since 1978, and

indicated that Ireland is seismically very stable and that there is nothing to suggest that this will

change in the coming millennia.

In addition, the Global Seismic Hazard Assessment Program (GSHAP; Giardini and Basham, 1993)

aims at promoting regionally coordinated and homogeneous seismic hazard evaluations and to produce

regionally harmonized seismic hazard maps. A paper published by GSHAP presenting the seismic

hazard assessment for Central, North and Northwest Europe states that Ireland is one of the countries

regarded as nearly aseismic.

This is illustrated in Figure 3.7, where it can be seem that seismic activity in Ireland is very low

compared to other parts of Europe. Hence, assessment of the possibility of a seismic event leading to

a MAH at the Derrygreenagh Power Station has demonstrated that this is not a credible event and as

such does not require further analysis.



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Figure 3.7: Seismic activity in Europe (source: GSHAP; Giardini and Basham, 1993)

3.12.4 Fires Originating from Off Site Events

There are no Seveso / COMAH sites or similar within the vicinity for which specific consideration of

the potential risk that they could present / initiate a Major Accident Hazard at the Derrygreenagh site

is required. In addition, the natural gas supply line to the site is subject to a separate safety assessment

by Bord Gais. Hence, consideration of off site fires is limited to potential ‘field’ fires in the vegetation

immediately surrounding the site and specifically the risk that they could initiate a Major Accident

Hazard at the Derrygreenagh facility.

The majority of the vegetation around the site is peat fields. It should be noted that the final landscape

mitigation will be determined after consultation with both HSA and Bord Gáis to confirm that the

vegetation would not pose a threat to the facility or the incoming gas pipeline.

Bog fires are typically caused from burning vegetation at the edge of bog areas - the main risk to the

island would be if a fire which started on the periphery of an adjoining bog area were transmitted

across the bog. There is a higher risk of a fire spreading in an area in active production, which will

typically have dry layers of peat on the surface, and intervening stockpiles, which will transmit fire

more efficiently. In this regard it should be noted that all of the adjoining peat lands to the site are out

of active production



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As the gas and diesel pipelines and valves are designed to resist fire, it is not considered credible that a

vegetation fire would be the direct cause of a pipeline failure. The main risk to the site from a fire

originating in the vegetation lies in the potential for the fire to act as an ignition source for materials

on site, namely natural gas and diesel. However, such an incident would require both the rupture of

the pipeline or storage tank, resulting in loss of containment in the direction and vicinity of the fire,

and the occurrence of the fire in the vegetation itself. Further analysis is not required since this is

considered an incredible event. In addition, it can be stated that the consequences of such an event are

covered by the analysis of pool fires (section 3.2.1), jet fires (section 3.2.2) and flash fires / VCEs

(section 3.2.3).



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4 Hazards to Occupied Buildings

This section presents an assessment of the risks from Major Accidents to persons on site, specifically

in terms of the occupied buildings. Note that all consequence analysis forming part of this assessment

is based on the PHAST modelling undertaken as part of section 3, Consequences to the Public /

Environment of Potential Major Accident Hazards.

4.1 Occupancy Levels

Table 4.1 presents the expected occupancy levels associated with each building that will be occupied

on site as part of normal working practice. Note that the figures in the table refer to staff only and do

not allow for visitors. The maximum number considers personnel on site to carry out maintenance and

overhauls; it is expected that these levels will be representative for around three weeks a year.

Occasionally there will be personnel in other buildings for inspection purposes.

Table 4.1: Occupancy Levels

Norrmal Occupancy

Day Night

Maximum

CCGT

Turbine Hall 1 1 10

GTG MCC Room 0 0 3

ACC MCC Room 0 0 3

Electrical Annex 0 0 3

Elec Annex Offices 2 0 7

Emergency Diesel Generator 0 0 2

Central Control Room 2 2 10

OCGT

Turbine Hall 2 1 10

Control Room 0 0 2

Common

Water Treatment Plant 0 0 3

Fire Pumphouse 0 0 3



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Norrmal Occupancy

Day Night

Maximum

Fuel Gas Heater Room 0 0 2

Gas Metering 0 0 2

Gas Plant Boiler House 0 0 2

Stores 2 0 5

Workshop / Stores / Canteen 2 1 20

Workshop 2 0 8

Administration Building 40 0 50

Switchyard Control Building 0 0 3

The operations staff will monitor the plant on a 24/7 basis that will see them circulating through all

parts of the plant on a semi-continuous basis. There will be three people on shift. The base of

operations is the control room, which is always manned with a min of one and typically two with the

third circulating the site doing routine checks and responding to control alarms.

The operations during the day will have a similar pattern, with the addition of maintenance staff (up to

9) carrying out routine maintenance and repair work. Some of the maintenance staff will be based in

the control room area (there is likely to be an electrical\instrumentation workshop there) and the

remainder in the workshop; there will be a stores supervisor based in the stores.

The location of the occupied buildings on site is presented in Figure 4.8.



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Figure 4.8: Location of occupied buildings

1

3

9

13

29

43

416

22

28

3433

36 38

55

39

15

Occupied building



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4.2 Impact of MAH on Occupied Buildings

4.2.1 Design Considerations

Apart from the turbine halls themselves, the occupied buildings are generally sited to the West of the

turbine halls i.e. in the event of a gas release the prevailing winds would tend to blow any gas clouds

away from these buildings, with other buildings standing between the occupied building; the exception

to this is the control room, which is sited above the electrical annex. The building with the highest

occupancy, the Administration building, is located at the North West corner of the site, well away

from sources of potential Major Accident Hazards.

The buildings have yet to be designed in detail but the turbine halls will be a steel framed structure

with steel cladding and the other buildings either reinforced concrete or brick and block construction.

This type of construction provides considerable resistance to external fires. The sighting of windows

and doors will take account of potential MAHs, in particular alternative exits will be provided from all

occupied buildings and windows will be suitably specified to reduce the risk of flying glass.

The building sited adjacent to, and South of, the main turbine hall, namely the electrical annex, will

have appropriate blast proof walls to provide protection to occupants. In addition, in the event of an

accident scenario arising in the turbine hall, a minimum of two means of escape to the South will be

provided; ventilation to the turbine hall will be sited such as not to impinge on the escape routes.

4.2.2 Diesel Bund Fire

A diesel bund fire is an extremely unlikely event. The consequence contours presented in Figure 3.1

demonstrate that the stores, mess room, workshop, electrical annex, central control room, main turbine

hall and part of the administration building are within the consequence contours of such an event.

However, it should be noted that the zones are based on land use planning considerations and there are

a number of further considerations that should be taken into account when assessing the safety of

personnel on site.

The nature of the scenario is such that the escalation to a MAH (i.e. the release and subsequent

ignition of a large amount of distillate) will require significant time to elapse, during which the

occupants of the buildings will have ample opportunity to exit the building. Should a MAH occur

while personnel are in the building, the building itself will provide significant thermal shielding and

escape routes to the North / West (i.e. via the opposite elevation to the fire) will be provided.

In terms of assessing the risk arising from this scenario to personnel on site, a frequency of loss of

containment from a tank of 3.86 x 10-3

/tank /year was assigned, based on HSE guidance1. Note that a

hole size factor was not included since any hole could potentially lead to this accident. Hence the

annual failure frequency for three tanks is 1.16 x 10-2

. Two factors were then applied to this figure,

the first being a factor of 0.01 based on probability of ignition of the fuel. An additional factor of 0.01

was applied given that personnel are extremely unlikely to be in the vicinity of any large fire, are

given ample time to escape and the afforded significant thermal shielding by the buildings. Hence the

risk to an individual of such an event is estimated at approximately 1.2 x 10-6

per year.

1 “Failure rates for Atmospheric Storage Tanks for Land Use Planning”, RAS/01/06, Health & Safety Laboratory



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4.2.3 Jet Fires from Gas Pipelines

The consequence contours from potential jet fire events are discussed in section 3.6.4. It would appear

fro

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