10.1.1 The overall objectives of the hazard assessment are to identify potential hazard concerns, the likely mitigation measures required and to report the findings.
10.2.1 The risk to the construction workers and road users due to chlorine usage at WSD Cheung Sha Water Treatment Works shall be assessed. The Applicant shall follow the criteria for evaluating hazard to life as stated in Annexes 4 and 22 of the TM EIAO in conducting hazard assessment and include the following in the assessment:
(i) Identification of all hazardous scenarios associated with the transport, storage and use of chlorine at the water treatment works, which may cause fatalities on Tung Chung Road during road construction and operation phases.
(ii) Execution of a Quantitative Risk Assessment expressing population risks in both individual and societal terms during road construction and operational phases.
(iii) Comparison of individual and societal risks with the Criteria for Evaluating Hazard to Life stipulated in Annex 4 of the TM; and
(iv) Identification and assessment of practicable and cost effective risk mitigation measures during road construction and operational phases.
10.2.2 The following study cases are evaluated and assessed:
¨ Construction Phase:
- Transient risk to construction workers only
¨ Operation Phase:
- Risk to the future new road population only
10.2.3 The risk levels for the population are assessed based on the year 2006 when the new road is undergoing construction completion and about to be operational. The future population is based on projected population estimates for the year 2021 which represents the ultimate case after 15 years of road operation.
10.2.4 It should be noted that the Cheung Sha WTW is not a PHI site and therefore no hazard assessment of this site has been conducted to date.
10.3.1
Background
10.3.1.1 A visit to the Cheung Sha WTW was conducted to ascertain details on the operation, process configuration, surrounding environment and possible population exposure. Details are provided below and photographs provided in Appendix J.
10.3.1.2 Cheung Sha WTW is located off the Tung Chung Road on South Lantau approximately 200m north of Cheung Sha Village. It is located about 100m above sea level on a gentle hill slope overlooking Cheung Sha Beach. The layout of the Cheung Sha WTW is shown in Figure 10.1, with its general location in South Lantau indicated in Figure 10.2. The access road to the site is approximately 150m long and runs along the side of a large catchwater. Between the catchwater and the road there is either fence or a kerb roughly 9 inches high for most of the length, but the kerb is missing for a short stretch of about 10m.
10.3.1.3 The WTW provides treated water for supply locally to Lantau Island and for fire hydrants along Tung Chung road. The sterilisation is done in 2 separate chlorination rooms and each has a chlorine store. The WTW uses and stores chlorine in 50kg cylinders. Chlorine consumption was advised to be 1 cylinder per 10-15 days, with maximum consumption at 1 cylinder per 3-5 days. An average consumption of 1 cylinder in 8 days was assumed.
10.3.1.4 For the local water supply, 1 duty and 1 standby chlorine cylinders were present in the chlorination room and 29 full and 5 empty cylinders were present in the store. 2 and 34 full cylinders respectively in the chlorination room and storage room will be modelled in the analysis. For the chlorination room of the fire hydrant supply, while the equipment could handle a maximum of 2 cylinders, only 1 cylinder was observed. 5 cylinders were present in storage, while a maximum of 8 may be present. All cylinders were modelled as full.
10.3.1.5 The chlorine stores are located at the chlorination rooms, at the centre of the WTW site and at the south-east corner, next to the service reservoir. The chlorine drum storage, draw off, evaporation and chlorination equipment is all located within the chlorine store.
10.3.1.6 All the cylinders were removed from the delivery racks and chained up in wall racks or the connected position. Each connected cylinder was on a weigh scale to indicate how full it was. On each connected cylinder, an outlet valve was screwed onto the cylinder, connected by a coil pipe (8mm) to a short length of fixed 20mm pipework then via a second coil pipe (8mm) to a multiport valve to select the duty cylinder. Fixed, 20mm pipework fed the chlorine to the vacuum regulator in the chlorinator. Since the draw off from a cylinder is gas, releases from this pipework would only be gaseous releases. The total length of pipework in each chlorination room was about 10m.
10.3.1.7 After the vacuum regulator the chlorine gas is at less than atmospheric pressure and does not pose any significant hazard. The chlorine gas passes the vacuum ejector and dissolves in water in high concentration but this solution is not considered to pose any significant hazard. The solution is then diluted into the final water of the WTW.
10.3.1.8 Chlorine cylinders are delivered to the works by truck. A truck can carry a maximum of 9 racks of 5 cylinders each. Consignments unloaded vary from 10 to 40 cylinders, so an average consignment will be assumed to be 25 cylinders. As a cylinder is assumed to last 8 days, a delivery would be made every 200 days. The truck does not spend any additional time on site (e.g. for paperwork) other than for unloading cylinders.
10.3.1.9 The racks are unloaded from the truck using the truck crane. Cylinders are then removed from the cage and manhandled for approximately 20m into the chlorine stores by rolling on one edge of the base. It is understood that the truck also makes deliveries to the Tai O WTW. The typical delivery to Tai O is 1 rack and so the truck leaves the site still carrying a few full cylinders.
10.3.2.1 There are numerous safety features of the chlorine facilities. A loud door open alarm was provided on the external doors to each room. It is necessary for the door to be opened for 15 to 30 seconds for the loading/unloading of each cylinder. Each room was provided with a low level extractor fan (12 inch diameter) and a high level inlet vent. Both were fitted with automatic louvres. A chlorine gas detector was provided in each room. On detection of 3ppm chlorine the louvres would be shut and the fan turned off automatically. It is understood that 1 or 2 false alarms occur per year, generally due to lightning storms.
10.3.2.2 It was observed the extractor fan in the fire hydrant chlorination room was running, but the fan in the local water supply room did not appear to be running, despite being set to automatic. It is possible that the door alarm triggered during the visit to the site had shut off the fan.
10.3.2.3 Both chlorination rooms were fitted with a 6 inch high kerb to prevent release of chlorine to outside the building if a chlorine spill occurred.
10.3.2.4 The quantity of chlorine stored at Cheung Sha WTW does not exceed the criterion of 10 tonnes of chlorine in 50kg cylinders or a single 1 tonne drum, and so the WTW is not classified as a PHI. Therefore no Consultation Zone is defined for Cheung Sha WTW. The areas which could be exposed to fatal effects from a chlorine release at the WTW include the construction site and proposed road, as shown in Figure 10.2.
10.4.1.1 The weather data for this study has been referred from the Hazard Assessment for Upgrading of Silvermine Bay WTW (ERM 1996). Table 10.1 and Table 10.2 show the probabilities for combination of wind speed, direction and Pasquil stability class used in the assessment. In line with the general geographical area covered by the reference report, DNV regards this weather data as relevant to South Lantau.
Table 10.1 Day Time Weather Probabilities
|
Wind |
Wind Speed and Stability Class |
Total |
|||||||
|
Direction |
B |
D |
F |
B |
E |
D |
D |
D |
|
|
|
0.5 m/s |
0.5 m/s |
1.0 m/s |
3.0 m/s |
3.0 m/s |
4.0 m/s |
7.0 m/s |
15.0 m/s |
|
|
N |
0.72 |
0.20 |
0.00 |
0.35 |
0.00 |
2.89 |
0.92 |
0.28 |
5.36 |
|
NNE |
0.50 |
0.25 |
0.00 |
0.27 |
0.00 |
4.88 |
1.18 |
0.52 |
7.60 |
|
NE |
1.09 |
0.38 |
0.00 |
0.27 |
0.00 |
2.77 |
0.37 |
0.16 |
5.04 |
|
ENE |
1.24 |
0.53 |
0.00 |
0.37 |
0.00 |
3.34 |
1.22 |
0.85 |
7.55 |
|
E |
0.68 |
0.43 |
0.00 |
0.61 |
0.00 |
7.84 |
5.78 |
6.95 |
22.29 |
|
ESE |
0.33 |
0.21 |
0.00 |
0.36 |
0.00 |
6.17 |
4.46 |
4.19 |
15.72 |
|
SE |
0.17 |
0.14 |
0.00 |
0.14 |
0.00 |
1.47 |
0.76 |
0.75 |
3.43 |
|
SSE |
0.18 |
0.11 |
0.00 |
0.13 |
0.00 |
0.65 |
0.30 |
0.30 |
1.67 |
|
S |
0.13 |
0.16 |
0.00 |
0.31 |
0.00 |
2.42 |
0.56 |
0.22 |
3.80 |
|
SSW |
0.14 |
0.17 |
0.00 |
0.26 |
0.00 |
2.80 |
0.41 |
0.09 |
3.87 |
|
SW |
0.14 |
0.12 |
0.00 |
0.18 |
0.00 |
1.24 |
0.31 |
0.09 |
2.08 |
|
WSW |
0.23 |
0.23 |
0.00 |
0.25 |
0.00 |
1.15 |
0.22 |
0.11 |
2.19 |
|
W |
0.61 |
0.31 |
0.00 |
1.56 |
0.00 |
1.67 |
0.08 |
0.05 |
4.28 |
|
WNW |
1.34 |
0.34 |
0.00 |
2.17 |
0.00 |
1.96 |
0.16 |
0.09 |
6.06 |
|
NW |
1.35 |
0.18 |
0.00 |
1.04 |
0.00 |
2.11 |
0.57 |
0.32 |
5.57 |
|
NNW |
0.78 |
0.18 |
0.00 |
0.25 |
0.00 |
1.50 |
0.54 |
0.28 |
3.53 |
|
Total |
9.63 |
3.94 |
0.00 |
8.52 |
0.00 |
44.86 |
17.84 |
15.25 |
100 |
Note: The numerical figures are the percentage of the time
that the wind speed, direction and stability class occurs. Letters A to F are
the Pasquil stability class of the weather. A indicates unstable conditions,
more common during the daytime, F indicates stable conditions, more common at
night.
Table 10.2 Night Time Weather Probabilities
|
Wind |
Wind Speed and Stability Class |
Total |
|||||||
|
Direction |
B |
D |
F |
B |
E |
D |
D |
D |
|
|
|
0.5 m/s |
0.5 m/s |
1.0 m/s |
3.0 m/s |
3.0 m/s |
4.0 m/s |
7.0 m/s |
15.0 m/s |
|
|
N |
0.00 |
0.13 |
0.85 |
0.00 |
0.76 |
1.94 |
0.62 |
0.19 |
4.49 |
|
NNE |
0.00 |
0.17 |
1.51 |
0.00 |
2.07 |
3.27 |
0.79 |
0.35 |
8.16 |
|
NE |
0.00 |
0.26 |
2.02 |
0.00 |
1.63 |
1.86 |
0.25 |
0.11 |
6.13 |
|
ENE |
0.00 |
0.35 |
2.85 |
0.00 |
1.96 |
2.24 |
0.82 |
0.57 |
8.79 |
|
E |
0.00 |
0.29 |
3.70 |
0.00 |
3.22 |
5.26 |
3.88 |
4.66 |
21.01 |
|
ESE |
0.00 |
0.14 |
2.85 |
0.00 |
3.14 |
4.14 |
2.99 |
2.81 |
16.07 |
|
SE |
0.00 |
0.09 |
1.62 |
0.00 |
0.79 |
0.98 |
0.51 |
0.50 |
4.49 |
|
SSE |
0.00 |
0.07 |
1.31 |
0.00 |
0.37 |
0.44 |
0.20 |
0.20 |
2.59 |
|
S |
0.00 |
0.11 |
1.85 |
0.00 |
1.33 |
1.62 |
0.38 |
0.15 |
5.44 |
|
SSW |
0.00 |
0.12 |
1.70 |
0.00 |
1.63 |
1.88 |
0.28 |
0.06 |
5.67 |
|
SW |
0.00 |
0.08 |
1.03 |
0.00 |
0.81 |
0.83 |
0.21 |
0.06 |
3.02 |
|
WSW |
0.00 |
0.15 |
1.22 |
0.00 |
0.59 |
0.77 |
0.15 |
0.07 |
2.95 |
|
W |
0.00 |
0.21 |
1.04 |
0.00 |
0.27 |
1.12 |
0.06 |
0.03 |
2.73 |
|
WNW |
0.00 |
0.23 |
0.89 |
0.00 |
0.25 |
1.31 |
0.10 |
0.06 |
2.84 |
|
NW |
0.00 |
0.12 |
0.54 |
0.00 |
0.34 |
1.42 |
0.38 |
0.21 |
3.01 |
|
NNW |
0.00 |
0.12 |
0.54 |
0.00 |
0.41 |
1.00 |
0.36 |
0.19 |
2.62 |
|
Total |
0.00 |
2.64 |
25.52 |
0.00 |
19.57 |
30.08 |
11.98 |
10.22 |
100 |
10.4.2.1 The population used for this study is considered both for the construction phase (year 2006) and the ultimate case (year 2021). Most of data was derived based on the Working Paper 3, "Traffic Forecasts", "Table 3.4: Population Figures Assumed for Traffic Modelling" covering the population assumption for Lantau obtained by Mouchel (Mouchel, 2001).
Construction
Phase
10.4.2.2 This new road runs along the hillside on the east of Cheung Sha Valley, continues eastwards to cross the catchwater and connects with South Lantau Road near the YWCA youth camp. Approximately 800m of the road falls within a 500m radius of the water treatment works. This section of road stretches from NNW to ENE of the water treatment works and the entire route section is located on higher ground than the water treatment works i.e. between 80m to 150m higher. The road has a steady gradient of 10% to 12%.
10.4.2.3 Construction work for the road is expected to end in year 2006. Data for the working population during construction was provided in correspondence with the Mouchel Project Manager (Mouchel, 2001). The main construction site will be in Tung Chung but an additional proposed construction site may be provided 800m to the east of the WTW, as shown in Figure 10.2. Neither construction site could be affected by releases of chlorine from the WTW. It is expected that approximately 10 gangs of 10 to 20 workers per gang will be spread along, on or very close to the route. Hence it is assumed that total of about 5 workers per 100m of new road would be present during the construction phase of the project (Table 10.3). Concerns about more workers temporarily working in a smaller area will be addressed as a sensitivity test of the results. This figure represents the conservative upper bound figure which includes both the construction and supervisory staff. The workers are present during the 12 hour day time and do not work at night time.
10.4.2.4 The alignment of the road and its work boundary are presented in Figures 2.1 to 2.13 and the scope of the landscape planting proposals in Appendix I. These figures show that the alignment in the vicinity of the WTW conforms to the contoured landscape of the hillside and involves minimal slope cutting or landscaping on the southern side of the alignment which is nearer to the WTW. The worker population is assumed to work within the proposed alignment, and they would not be required to work outside this boundary and hence will not be exposed to the risk of being closer to the WTW. The risk model has modelled the worker population being restricted along the proposed alignment.
Table 10.3 Construction Worker Data in 2006 (on new Tung Chung Road)
|
Location |
Time Period |
Number of Workers |
|
New
Tung Chung Road |
Day |
5
per 100m stretch |
|
|
Night |
(no
activity) |
10.4.2.5 The traffic data used throughout this study is provided by MVA Hong Kong Ltd. (MVA), reference (MVA, 2001). The data selection is based on the more conservative scenario where no TCRPZ permit system is enforced subsequent to the improvement to Tung Chung Road. Peak traffic data for the existing Tung Chung Road and the South Lantau Road during the construction phase was forecasted by MVA and is shown in Table 10.4. Assumptions were made to account for the average traffic for both the 12 hours day time and 12 hours night time.
Table 10.4 Vehicle Forecast for Construction Phase in 2006
|
Location |
Time Period |
Peak Number of
Vehicles / hr |
Average Number
of Vehicles / hr |
|
Existing Tung Chung Road |
Day |
180 |
120 |
|
|
Night |
N/A |
12 |
|
South Lantau Road |
Day |
180 |
120 |
|
|
Night |
N/A |
12 |
Assumptions
1)
Peak hours
assumed to be 4 hours out of the 12 hours of daytime, i.e. 2 hours in the
morning and 2 hours in the evening.
2)
Average no.
of vehicle is calculated based on 50% peak traffic for the non-peak hours
3)
Night time
average is assumed to be 10% of daytime average
0.4.2.1 The vehicle breakdown obtained from the same source, reference (MVA, 2001), indicates number of vehicle types per hour. Analysis of the data gave approximately 61% passenger car, including taxis, 32% goods vehicle and 7% bus. Table 10.5 below tabulates the calculation method to obtain an average of 4.22 people per vehicle. This number corresponds closely to the 4.6 people per vehicle based on the South Lantau Road taken from the Annual Traffic Census Statistics (DNV, 1999) albeit less than 10% variance.
Table 10.5 Calculation for Average Number of People Per Vehicle
|
Vehicle Type |
Fraction of Vehicle Type |
*Number of People / Vehicle |
Average Number of People / Vehicle |
|
Passenger
Car / Taxi |
0.61 |
3 |
1.83 |
|
Goods
Vehicle (LGV/HGV) |
0.32 |
2 |
0.64 |
|
Bus
/ Coach |
0.07 |
25 |
1.75 |
|
Total |
1.00 |
n/a |
4.22 |
Assumptions
·
The passenger
car / taxi each has 3 people, the goods vehicle each has 2 people while the bus
/ coach each carries 25 people.
0.4.2.2 The time exposure of the population travelling in moving vehicles is dependant on the speed of the vehicle. It is assumed that the average speed of the vehicles is 50km/hr, as shown in Table 10.6.
Table 10.6 Road Population for Construction Phase in 2006
|
Location |
Time Period |
Average Number of Vehicles / hr |
Vehicle Speed in km/hr |
Vehicle / km |
People / km |
|
Existing
Tung |
Day |
120 |
50 |
2.40 |
10.13 |
|
Chung
Road |
Night |
12.0 |
50 |
0.24 |
1.01 |
Operational Phase
0.4.2.3 The transient population for the ultimate case where the road traffic at the new Tung Chung Road reaches saturation was also estimated by MVA for year 2021 and is shown in Table 10.7. Population densities were calculated by assuming approximately 4.22 passengers per vehicle based on the above.
Table 10.7 Population Data for Operation Phase in 2021 (15 years after opening of new Tung Chung Road)
|
Location |
Time Period |
Average Number of Vehicles / hr |
Vehicle Speed in km/hr |
Vehicle / km |
People / km |
|
New
Tung Chung |
Day |
667 |
50 |
13.33 |
56.27 |
|
Road |
Night |
66.7 |
50 |
1.33 |
5.63 |
10.5.1.1 Although chlorine is used in low concentrations for disinfection of water, chlorine particularly if inhaled, is toxic. The hazard from the use of chlorine at WTWs arises from possible releases of chlorine gas into the atmosphere, which would then be blown with the wind and gradually disperse. This is referred to as the toxic cloud.
10.5.1.2 Chlorine (as pressurised liquid in 50kg cylinders) is delivered to the WTW by truck, unloaded from the truck within the building by crane and the chlorine is loaded into storage. As needed cylinders in the duty and standby positions are replaced by cylinders from storage, again using the crane. Groups of cylinders (draw off units) in the duty and standby positions are connected to the chlorination system. The chlorine first passes through a regulator which reduces the pressure of the chlorine to below atmospheric. The chlorinators are ejector type and operate at less than atmospheric pressure.
10.5.1.3 The hazard is therefore significant from when the truck arrives on the approach road (the battery limit of the WTW) to the regulator.
10.5.2
Recorded Chlorine Releases
in Hong Kong
10.5.2.1 The historical data available in Hong Kong, listed below, gives insight into the magnitude and frequency of the more common occurrences and is considered to realistically represent the smaller, internal events caused by human error and spontaneous failure. The Hazard Identification exercise has been based on the 6 previous Hong Kong incidents for chlorine releases, and supplemented with larger, internal and external releases cases from previous chlorine studies.
Drum Leak, Tai Po Tau, Hong Kong
10.5.2.2 A chlorine leak from a 1 tonne drum occurred on 8 June 1983 at
Tai Po Tau Water Treatment Works, Hong Kong.
Following changeover between drums, the copper coil connection failed. The operators were not carrying breathing
apparatus (BA). When BA sets were
obtained from the office, it was impossible to approach the leak because of the
pressure of the liquid release. The
emergency services were called 15 min after the start of the incident. The 9 personnel then evacuated the
works. The emergency services arrived
within 6 min, and entered the drum store wearing BA sets and turned off the
drum isolating valve 50 minutes after the start of the incident. A total of 725kg of chlorine had been
released. 6 of the operators were
treated in hospital for less than 24 hours.
The works are isolated and there were no off-site casualties.
Drum Leak, Tai Lam Chung, Hong Kong
10.5.2.3 11.2kg of chlorine leaked from a 1 tonne drum in the chlorine
store at Tai Lam Chung Pre-chlorination House in the New Territories on the 28
September 1990. The leak was repaired
on site by the chlorine supplier with the presence of Fire Services personnel
by tightening the gland of the drum isolating valve. There were no casualties in this incident.
Chlorine Charge Line Leak, Sha Tin, Hong
Kong
10.5.2.4 The incident occurred inside the chlorine transfer room of the Sha
Tin Water Treatment Works, the biggest treatment works in Hong Kong, on the 8
September 1992. Workers were charging
liquefied chlorine from 1 tonne drums to bulk chlorine vessels each with a
maximum holding capacity of 125 tonnes.
150kg of chlorine leaked from the liquid chlorine charging line. One staff at the treatment works together
with six residents living in the nearby staff quarters 200m away suffered
respiratory problems. The section of
the defective pipe was replaced after the incident and the frequency of
maintenance inspection was increased.
The chlorine plant and the chlorine bulk storage tank were
decommissioned at the end of 1992 and were replaced by a new chlorination house
with an absorption system. Seven
persons were injured, other than these injuries there was no other perceivable
impact.
Feed Pipe/Drum Connection Leak, Sheung
Shui, Hong Kong
10.5.2.5 2kg of chlorine leaked from the connection between the feeding pipe
and the chlorine drum at the Sheung Shui Water Treatment Works on 25 January
1993. Fire Services personnel were
informed and the leak was sealed up.
There were no casualties in this incident. A centralised ordering and inspection system was implemented for
the lead washers to ensure that they are supplied to good standards. No person was injured.
Valve Spindle Leak, Pak Yue Kong Swimming
Pool, Kowloon
10.5.2.6 Liquefied chlorine found leaking from the valve spindle of a 1 tonne
chlorine drum inside the chlorine store of the Pau Yue Kong Public Swimming
Pool on 18 May 1992. Fire Services
personnel eventually tightened the valve and stopped the chlorine gas from
further release. The released chlorine
escaped through the air vent of the chlorine store to the outside of the
swimming pool complex. A passer-by
suffered respiratory problems after inhaling toxic gas. One person was injured.
Leakage at Isolating Valve of Full Drum. Tai Lam Chung
Prechlorination House
10.5.2.7 In March 1994 leakage occurred at isolating valve of full chlorine
drum at Tai Lam Chung Pre-chlorination House.
The chlorine absorber was activated.
10.5.3
Types of Chlorine Releases
10.5.3.1 This study has divided potential releases into the following categories:
¨ internal releases;
¨ external releases;
¨ on the road;
¨ while unloading; and
¨ at the store, not contained due to collapse of building due to major external event such as earthquake.
Accidents within the
WTW Building
10.5.4.1 Accidents within the WTW building would lead to internal releases. Accident types would include:
¨ spontaneous failure (leak or rupture) of a cylinder, the cylinder valve or pipework, perhaps due to corrosion or lack of maintenance;
¨ dropping of containers during truck unloading or loading into the duty position;
¨ fire; and
¨
human error, leading to a leaking
connection.
Release Categories
10.5.4.2 Three release categories have been used to represent these previous
accidents:
¨ 50kg leaking through a small hole, equivalent to the diameter of the connecting pipe or a leaking valve (4.53mm);
¨
50kg released effectively instantly,
equivalent to a rupture of a cylinder or breaking off of the cylinder valve;
and
¨
100kg leaking through two small holes
(6.4mm).
10.5.4.3 These cases have been used as the basis for the accident rates in
the analysis.
Containment System Failures
10.5.4.4 If the containment system operates as designed then internal
releases will present a less severe offsite hazard. There is however potential for failure of the containment
system. The failure modes of this
system which are significant are:
¨
containment system not being activated
resulting in a release of chlorine through forced ventilation; and
¨
operation of containment system
resulting in a release of chlorine driven by the expansion of the release.
10.5.4.5 These are discussed in more detail in Section 10.6 below.
Accidents on the Road
10.5.5.1 There is a potential for a release of chlorine during the delivery
of chlorine cylinders to the store. The
scope of the study only considers such accidents within the site boundary. The accident cases which could occur are:
¨ roll-over;
¨ crushed at rear;
¨ crushed at side;
¨ vehicle fire (crash where other vehicle bursts into flames);
¨ tanker fire (crash where other vehicle is a tanker carrying large quantity of flammable DG and bursts into flames);
¨ spontaneous;
¨ load-shedding; and
¨ truck fire (fire on chlorine truck).
10.5.5.2 The failure cases considered are based on the Chlorine Transport
Study (DNV, 1997):
¨ brief, small gas leak, emergency response succeeds, 5kg, 0.01kg/s (SVCS);
¨ longer duration, small gas leak, emergency response fails, 50kg, 0.01kg/s (SVCF);
¨ small liquid leak, 50kg, 0.1kg/s (SLC);
¨ medium vapour leak, 50kg, 0.1kg/s (MVC);
¨ medium liquid leak, 50kg, 1kg/s (MLC);
¨ cylinder rupture, 50kg, instantaneous (RC); and
¨
multiple cylinder rupture, 500kg,
instantaneous (RCM2).
Accidents while
Unloading
10.5.5.3 The range of incidents which can occur are:
¨ dropped cylinder rack, large vapour release from one or five cylinders. It is considered that toppling of a cylinder during manhandling between the truck and the store would unlikely result in a leak;
¨ accident while manoeuvring, large vapour release from one or five cylinders;
¨ drive-away while unloading, large vapour release from one or five cylinders; and
¨ fire on truck while unloading, multiple cylinder rupture, 500kg.
Major External Impact
Events
10.5.5.4 Previous studies have considered a range of external events which
include:
¨ earthquake;
¨ subsidence;
¨ flooding or dam burst;
¨ aircraft crash;
¨ landslide; and
¨
lightning strike.
10.5.5.5 The WTW is constructed on a platform formed partly by cutting into the hillside to the north and partly by an embankment slope to the east.
10.5.5.6 The cut slope to the north is approximately 16m high. The chlorine stores are more than 40m from the crest of this cut slope, the debris from a slippage of this slope would not reach the chlorine store.
10.5.5.7 The embankment slope to the west is constructed to contain the service reservoir. It appears that the chlorination room and store for the pumping station could be within the area which could be undermined by slippage of this fill slope, which could in principle be caused by leakage from the storage reservoir. However, storage reservoirs are regularly inspected and subjected to leakage checks, and the angle of the underlying original ground level will provide for good drainage, even if the original ground is less permeable than the fill.
10.5.5.8 The first store and treatment room (for the “Treatment Works”) and the storage reservoir were built in the mid to late 80s. The chlorination room and store for the pumping station were a later addition. Since the formation of GEO / GCE in the mid to late 1970s, it has been required to design and construct fill slopes to Government standards. This includes consideration of possible failure modes or slip planes within the slope and adequate compaction of the fill material. The advent of computers has allowed virtually all slip planes to be considered in the design of a slope. Also, the slope and its as built drawings and other details will be inspected periodically by a competent engineer.
10.5.5.9 Two drainage lines run near to the WTW site. The one due north of the site drains into the catchwater to the north of the access road and could drain in either direction in case of blockage of the catchwater in one direction. A second drainage line runs to the east of the WTW through a culvert under the catchwater into a man made drainage channel running north to south bedside the WTW. The site plinth itself is constructed with a slight angle to allow water to naturally drain away and the slopes to the north will have appropriate drainage provisions, hence subsidence and flooding are thought to be unlikely. The following factors are also relevant:
10.5.5.10 Dam burst is not credible since there is no reservoir above the WTW.
10.5.5.11 The site is several km from Chek Lap Kok and not directly below the flight path and so aircraft crash is considered incredible.
10.5.5.12 Lightning strike could occur but is likely to result in electrical damage and not likely to directly result in a release.
10.5.5.13 The most significant of these is earthquake. Failure cases, shown in Table 10.8, have
been developed similar to those for the QRA of Tsuen Wan WTW, a similar WTW
using 50kg cylinders, (DNV, 1998).
Table 10.8 Summary of Failure Cases due to Earthquake
|
Reference |
Failure Case |
Released Mass
(kg) |
Hole diameter
(mm) |
Phase |
|
Eq1SVP |
1 cylinder valve in either chlorination room |
50 |
8 |
gas |
|
Eq1SPVM |
Piping from valves to vacuum regulator (4.53mm dia) |
50 |
4.53 |
gas |
|
Eq2SVP |
2 cylinder valves in either chlorination room |
100 |
11.28 |
gas |
|
Eq2SPVM |
2 pipe coils from valves to manifold (4.53mm dia) |
100 |
6.4 |
gas |
|
Eq1Rup |
Instantaneous Failure of 1 cylinder in either store |
50 |
rupture |
liquid |
|
Eq2Rup |
Instantaneous Failure of 2 cylinders in either store |
100 |
rupture |
liquid |
|
Eq34Rup |
Instantaneous Failure of 34 cylinders in Village Supply Store |
1700 |
rupture |
liquid |
|
Eq8Rup |
Instantaneous Failure of 8 cylinders in Hydrant Store |
400 |
rupture |
liquid |
Frequency of Release
within the Building
10.6.1.1 There have been 6 releases of chlorine in Hong Kong during the 14 years period 1983 to 1996. In 1995 4,270 tonnes of chlorine were used at WTWs in Hong Kong. Tuen Mun and Tsuen Wan WTW used 308 tonnes in 50kg cylinders. Thus 10,121 containers (3961 drums and 6160 cylinders) were used in 1995. Thus releases are expected at the frequency of:
6 / 14 / 10,121 = 4.23 x 10-5 per container per year
10.6.1.2 Of the 6 releases, 4 were in the 0 to 50kg category and 2 in the 50 to 250kg category. 95% of the 50kg releases are assumed to be small leaks and 5% to be ruptures. The base frequencies calculated are shown in column 5 of Table 10.9. The indoor release frequency is calculated by multiplying the base frequencies by the number of cylinders used per year, approximately 46 for the uprated Cheung Sha WTW. The indoor release frequencies are shown in the last column.
Table 10.9 Frequency Calculation
|
Maximum Released Mass (kg) |
Hole Diameter (mm) |
Proportion of Releases |
Number from the 6
Historical Releases |
Base Frequency (per Container year) |
Number of Cylinders Used
(per year) |
*Indoor Release Frequency (per year) |
|
50 |
4.53 |
0.95 |
4 |
2.68 x 10-5 |
45.625 |
1.22 x 10-3 |
|
50 |
Instant |
0.05 |
|
1.41 x 10-6 |
45.625 |
6.44 x 10-5 |
|
100 |
6.39 |
1 |
2 |
1.41 x 10-5 |
45.625 |
6.44 x 10-4 |
* Column 7 is given by the product of
Columns 5 and 6.
The Effect of the
Containment System
10.6.1.3 The containment system is designed to lessen the risk of releases of chlorine escaping from the building to the atmosphere. Forced ventilation releases to the atmosphere only occur if this system fails such that the ventilation fan continues to run, otherwise a release driven by the expansion of the liquid chlorine on release occurs. A standardised approach to the probability of failure of the containment system was discussed during a meeting between consultants and EPD on 18 July 1995. This suggested use of 1% probability of contain / absorb system failure, which is considered to be applicable to failure of the containment system (such that the ventilation fan remains on) in this study.
Frequency of Releases
to Atmosphere
10.6.1.4 Thus depending on whether the fan remains on or release is driven by the natural expansion of the chlorine liquid, the three failure cases identified in Table 10.9 produce 2 groups (forced ventilation and natural ventilation) of the same three failure cases, i.e. the 6 failure cases shown in Table 10.10.
Table 10.10 Frequencies of Release to Atmosphere for Indoor Releases
|
Reference |
Contain
and Absorb Failure Case |
Released
Mass (kg) |
Hole
Diameter (mm) |
Phase |
Base
Frequency (per year) |
Probability
of Containment Failure Case |
Frequency
of Release to Atmosphere (per year) |
|
SVSF |
Release, forced
ventilation |
50 |
4.53 |
Gas |
1.22 x 10-3 |
0.01 |
1.22 x 10-5 |
|
RupSF |
Release, forced
ventilation |
50 |
rupture |
Liquid |
6.44 x 10-5 |
0.01 |
6.44 x 10-7 |
|
LVSF |
Release, forced
ventilation |
100 |
6.39 |
Gas |
6.44 x 10-4 |
0.01 |
6.44 x 10-6 |
|
SVNV |
Release, natural
ventilation |
50 |
4.53 |
Gas |
1.22 x 10-3 |
0.99 |
1.21 x 10-3 |
|
RupNV |
Release, natural
ventilation |
50 |
rupture |
Liquid |
6.44 x 10-5 |
0.99 |
6.38 x 10-5 |
|
LVVF |
Release, natural
ventilation |
100 |
6.39 |
Gas |
6.44 x 10-4 |
0.99 |
6.38 x 10-4 |
10.6.1.5 Indoor release modelling has been carried out based on first principles, assuming that an indoor release mixes completely with the air in the chlorine store and it is the resultant mixture which is released to atmosphere. The concentrations and flow rates are calculated, forming the typical “saw-tooth” shape versus time, and averaged over a period of twice the duration of the release, see Table 10.11. The model assumes 95.2m3 store volume, the vent size is 300mm square and the velocity of forced ventilation is 2.5m/s. The equation for release concentration versus release flow rate is as follows:
V.dc/dt = vc
Where: V is the store volume (m3), c is the concentration in the air within the room and in the expelled air, v is the volumetric flow of air/chlorine mixture from the vent (m3/s) and t is the time (s).
Table 10.11 Release Rates and Concentrations to Atmosphere
|
Reference |
Contain
and Absorb Failure Case |
Store
Volume |
Indoor
release rate (kg/s) |
Released
mass (kg) |
Release
rate to atmosphere (kg/s) |
Concentration
of release to atmosphere |
|
SVSF |
Release, forced ventilation |
95.2 |
0.035 |
50 |
0.177 |
3.2% |
|
RupSF |
Release, forced ventilation |
95.2 |
instant |
50 |
Instant |
15.1% |
|
LVSF |
Release, forced ventilation |
95.2 |
0.069 |
100 |
0.177 |
15.6% |
|
SVNV |
Release, natural ventilation |
95.2 |
0.0349 |
50 |
0.012 |
10.2% |
|
RupNV |
Release, natural ventilation |
95.2 |
instant |
50 |
Instant |
15.1% |
|
LVVF |
Release, natural ventilation |
95.2 |
0.07 |
100 |
0.024 |
13.2% |
Failure Cases on the
Approach Road
10.6.2.1 The approach road between the WTW boundary gate and the chlorine store is 155m long. The chlorine truck delivers 5 racks of 5 cylinders (25 cylinders per delivery. The truck may leave the site carrying a part load. Thus the truck travels 0.57 km per year on site when loaded.
10.6.2.2 The base event frequencies from the recent QRA of the Transport of Chlorine in Hong Kong (DNV, 1997) have been used to calculate the release frequencies here, this is shown in Table 10.12.
Table 10.12 External Releases on the Approach Road
|
Ref. |
Failure
Case |
Released
Mass (kg) |
Hole
Diameter (mm) |
Phase |
Base
frequency (per km year) |
Distance
Travelled per year loaded (km) |
Frequency
of release to Atmosphere (per year) |
|
SVCS |
Small gas leak,
emergency response succeeds |
5 |
2.5 |
Gas |
8.90 x 10-10 |
0.57 |
5.04E-10 |
|
SVCF |
Small gas leak,
emergency response fails |
50 |
2.5 |
Gas |
8.90 x 10-10 |
0.57 |
5.04E-10 |
|
SLC |
Small liquid
leak |
50 |
2.5 |
Liquid |
2.90 x 10-7 |
0.57 |
1.64E-07 |
|
MVC |
Medium gas leak |
50 |
7.5 |
Gas |
8.20 x 10-9 |
0.57 |
4.64E-09 |
|
MLC |
Medium liquid
leak |
50 |
7.5 |
Liquid |
7.30 x 10-8 |
0.57 |
4.13E-08 |
|
RC |
Cylinder rupture |
50 |
Rupture |
Liquid |
1.60 x 10-8 |
0.57 |
9.05E-09 |
|
RCM2 |
Multiple rupture |
500 |
Rupture |
Liquid |
5.10 x 10-9 |
0.57 |
2.89E-09 |
Failures During
Unloading
10.6.2.3 The base frequencies used in the calculation of failure frequency during unloading are shown in Column 6 of Table 10.13 and are taken from the HA of Silvermine Bay WTW (DNV, 1999). A delivery is assumed every 200 days. The multipliers, shown in the last column of Table 10.13 for each accident case are calculated as follows:
¨ Dropped cylinder racks: in approximately 20% of dropped drums, there is a leak. 1% of these cases are assumed to result in large leaks from a single cylinder. 0.01% of cases are assumed to result in leaks from a whole rack. The remainder are small leaks, which will not have an offsite effect and are not considered further. Calculation of multipliers is as follows:
- UnLV: 365 / 8 / 5 x 0.01 x 0.2
- Un5LV: 365 / 8 / 5 x 0.0001 x 0.2
¨ Accident while positioning to unload: in 1% of cases there is a leak, 9% of these are large leaks, 1% leaks from all cylinders in a rack and the remainder are small leaks, which will not have an offsite effect and are not considered further. Calculation of multipliers is as follows:
- UnLV: 365 / 8 / 25 x 0.01 x 0.09
- Un5LV: 365 / 8 / 25 x 0.01 x 0.01
¨ Drive-away while still unloading: in 1% of cases there is a leak, 9% of these are large leaks, 1% leaks from all cylinders in a rack and the remainder are small leaks, which will not have an offsite effect and are not considered further. Calculation of multipliers is as follows:
- UnLV: 365 / 8 / 25 x 0.01 x 0.09
- Un5LV: 365 / 8 / 25 x 0.01 x 0.01
¨ Truck fire: the approach road is 155m long and the truck delivers 5 racks of 5 cylinders (25 cylinders per delivery). The truck may leave the site carrying a part load. Thus the truck travels 0.57km per year on site when loaded. Half of truck fires are considered to result during unloading and 10% are considered to result in 50% of cylinders with large leaks. Calculation of multipliers is as follows:
- UN22.5: 0.57 x 0.5 x 0.1
10.6.2.4 A summary of the calculation of the frequency of events during unloading is given in Table 10.13.
Table 10.13 Summary of Event Frequency during Unloading
|
Ref. |
Failure Case |
Released Mass (kg) |
Hole Diameter (mm) |
Phase |
Base frequency (per year) |
Multiplier (probability) |
Frequency of release to atmosphere (per
year) |
|
|
Dropped Cylinder Rack |
|
|
|
5.00E-06 |
1.83E-02 |
9.13E-08 |
|
|
Accident while positioning to unload |
|
|
|
5.00E-05 |
1.64E-03 |
8.21E-08 |
|
|
Driveaway while still unloading |
|
|
|
2.00E-05 |
1.64E-03 |
3.29E-08 |
|
UnLV |
Total |
50 |
8.00 |
Gas |
- |
- |
2.06E-07 |
|
|
Dropped Cylinder Rack |
|
|
|
5.00E-06 |
1.83E-04 |
9.13E-10 |
|
|
Accident while positioning to unload |
|
|
|
5.00E-05 |
1.83E-04 |
9.13E-09 |
|
|
Driveaway while still unloading |
|
|
|
2.00E-05 |
1.83E-04 |
3.65E-09 |
|
Un5LV |
Total |
250 |
5 x 8mm |
Gas |
- |
- |
1.37E-08 |
|
UN22.5 |
Truck Fire, 50% of cylinders leak |
1125 |
22.5 x 8mm |
Gas |
4.00E-09 |
0.03 |
1.13E-10 |
Earthquake
Failure Cases
10.6.2.5 The frequency of earthquake, Modified Mercali VIII, taken to be 1 in 100,000 years, has been apportioned between the failure cases as shown in Tables 10.14 and 10.15:
Table
10.14 Earthquake
Event Tree
|
|
Damage Item |
Damage Quantity |
Outcome |
|
|
|
|
|
|
|
|
One Cylinder,
valve leak |
One Chlorination
Cylinder Leaks |
|
|
|
0.25 |
0.125 |
|
|
|
|
|
|
|
|
Piping from valves
to vacuum regulator (4.53mm dia) |
Two Chlorination
Cylinders Leak |
|
|
|
0.25 |
0.125 |
|
|
Chlorination
Cylinder |
|
|
|
|
0.5 |
2 Cylinder valves
leak |
2 Cylinder valves
leak |
|
|
|
0.25 |
0.125 |
|
|
|
|
|
|
|
|
2 pipe coils from
valves to manifold (2 x 4.53mm dia) |
2 pipe coils from
valves to manifold (2 x 4.53mm dia) |
|
Earthquake |
|
0.25 |
0.125 |
|
|
|
|
|
|
|
|
One Cylinder
Rupture |
One Cylinder Rupture |
|
|
|
0.9 |
0.45 |
|
|
Storage Cylinder |
|
|
|
|
0.5 |
2 Cylinder Rupture |
2 Cylinder Rupture |
|
|
|
0.09 |
0.045 |
|
|
|
|
|
|
|
|
34 Cylinders
Rupture (Local Supply Store) |
34 Cylinders
Rupture (Local Supply Store) |
|
|
|
0.005 |
0.0025 |
|
|
|
|
|
|
|
|
8 Cylinders
Rupture (Hydrants Store) |
8 Cylinders
Rupture (Hydrants Store) |
|
|
|
0.005 |
0.0025 |
Table 10.15 Earthquake Failure Frequencies
|
Reference |
Failure Case |
Released Mass (kg) |
Hole Diameter (mm) |
Phase |
Release Rate to Atmosphere
(kg/s) |
Base Frequency (per year) |
Base Prob- ability |
Frequency of Release to
Atmosphere (per year) |
|
One Chlorination Set |
||||||||
|
Eq1SVP |
Cylinder valve |
50 |
8 |
gas |
0.13 |
1.00E-05 |
0.125 |
1.25E-06 |
|
Eq1SPVM |
Piping from valves
to vacuum regulator (4.53mm dia) |
50 |
4.53 |
gas |
0.035 |
1.00E-05 |
0.125 |
1.25E-06 |
|
Two Chlorination Sets |
||||||||
|
Eq2SVP |
2 Cylinder valves |
100 |
11.28 |
gas |
0.58 |
1.00E-05 |
0.125 |
1.25E-06 |
|
Eq2SPVM |
2 pipe coils from
valves to manifold (4.53mm dia) |
100 |
6.4 |
gas |
0.07 |
1.00E-05 |
0.125 |
1.25E-06 |
|
One Cylinder Rupture |
||||||||
|
Eq1Rup |
Instantaneous
Failure of one cylinder |
50 |
Rupture |
liquid |
Instant |
1.00E-05 |
0.45 |
4.50E-06 |
|
2 Cylinder Rupture |
||||||||
|
Eq2Rup |
Instantaneous
Failure of 2 cylinders |
100 |
Rupture |
liquid |
Instant |
1.00E-05 |
0.045 |
4.50E-07 |
|
34 Cylinders
Rupture (Local Supply Store) |
||||||||
|
Eq34Rup |
Instantaneous
Failure of 34 cylinders |
1700 |
Rupture |
liquid |
Instant |
1.00E-05 |
0.0025 |
2.50E-08 |
|
8 Cylinders
Rupture (Hydrants Store) |
||||||||
|
Eq8Rup |
Instantaneous
Failure of 8 cylinders |
400 |
Rupture |
liquid |
Instant |
1.00E-05 |
0.0025 |
2.50E-08 |
10.6.3
Summary
of Failure Frequencies
10.6.3.1 A summary of the failure frequencies used in the risk model is given in Table 10.16.
Table 10.16 Summary of Failure Frequencies
|
Internal Releases |
|
|
|
|
|
|
|
|
|
Reference |
Failure Case |
Released mass |
Hole diameter (mm) |
Phase |
Indoor release
rate (kg/s) |
Release rate to
atmosphere (kg/s) |
Concentation of
release to atmosphere |
Frequency of
release to atmosphere |
|
SVSF |
Release, forced
ventilation |
50 |
4.53 |
gas |
0.035 |
0.177 |
3.15% |
1.22E-05 |
|
RupSF |
Release, forced
ventilation |
50 |
rupture |
liquid |
instant |
Instant (16.9m^3) |
15.08% |
6.44E-07 |
|
LVSF |
Release, forced
ventilation |
100 |
6.39 |
gas |
0.069 |
0.177 |
15.64% |
6.44E-06 |
|
SVNV |
Release, natural
ventilation |
50 |
4.53 |
gas |
0.035 |
0.012 |
10.20% |
1.21E-03 |
|
RupNV |
Release, natural
ventilation |
50 |
rupture |
liquid |
instant |
Instant (16.9m^3) |
15.08% |
6.38E-05 |
|
LVVF |
Release, natural
ventilation |
100 |
6.39 |
gas |
0.07 |
0.024 |
13.20% |
6.38E-04 |
|
External Releases
while unloading |
|
|
|
|
|
|
|
|
|
UnLV |
Total |
50 |
8.00 |
gas |
- |
0.130 |
- |
2.06E-07 |
|
Un5LV |
Total |
250 |
5 x 8mm |
gas |
- |
0.650 |
- |
1.37E-08 |
|
UN22.5 |
Truck Fire, 50% of
cylinders leak |
1125 |
22.5 x 8mm |
gas |
- |
2.925 |
- |
1.13E-10 |
|
External Releases
on Road |
|
|
|
|
|
|
|
|
|
SVCS |
Small gas leak,
emergency response suceeds |
5 |
2.5 |
gas |
- |
0.0122 |
- |
5.04E-10 |
|
SVCF |
Small gas leak,
emergency response fails |
50 |
2.5 |
gas |
- |
0.0122 |
- |
5.04E-10 |
|
SLC |
small liquid leak |
50 |
2.5 |
liquid |
- |
0.0444 |
- |
1.64E-07 |
|
MVC |
medium gas leak |
50 |
7.5 |
gas |
- |
0.11 |
- |
4.64E-09 |
|
MLC |
medium liquid leak |
50 |
7.5 |
liquid |
- |
0.4 |
- |
4.13E-08 |
|
RC |
cylinder rupture |
50 |
rupture |
liquid |
- |
instant |
- |
9.05E-09 |
|
RCM2 |
multiple rupture |
500 |
rupture |
liquid |
- |
instant |
- |
2.89E-09 |
|
Earthquake Failure
Cases* |
|
|
|
|
|
|
|
|
|
Eq1SVP |
Cylinder valve |
50 |
8 |
gas |
- |
0.13 |
- |
1.25E-06 |
|
Eq1SPVM |
Piping from valves
to vacuum regulator (4.53mm dia) |
50 |
4.53 |
gas |
- |
0.035 |
- |
1.25E-06 |
|
Eq2SVP |
2 Cylinder valves |
100 |
11.28 |
gas |
- |
0.58 |
- |
1.25E-06 |
|
Eq2SPVM |
2 pipe coils from
valves to manifold (4.53mm dia) |
100 |
6.4 |
gas |
- |
0.07 |
- |
1.25E-06 |
|
Eq1Rup |
Instantaneous
Failure of one cylinder |
50 |
rupture |
liquid |
- |
instant |
- |
4.50E-06 |
|
Eq2Rup |
Instantaneous
Failure of 2 cylinders |
100 |
rupture |
liquid |
- |
instant |
- |
4.50E-07 |
|
Eq34Rup |
Instantaneous
Failure of 34 cylinders (Local Supply Store) |
1700 |
rupture |
liquid |
- |
instant |
- |
2.50E-08 |
|
Eq8Rup |
Instantaneous
Failure of 8 cylinders (Hydrants Store) |
400 |
rupture |
liquid |
- |
instant |
- |
2.50E-08 |
10.6.4
Variation
of Release Frequency With Time Period
10.6.3.2 The internal process related releases and the earthquake releases can occur at any time. The releases due to accidents on the approach road or unloading inside the chlorine store can only occur during working hours (on the days when the DG ferry runs). However, it has been found in previous studies, that the risk picture is usually dominated by the earthquake events and so the variation of release frequency with time period has not been modelled in detail.
10.7 Consequence and Impact Analysis
10.7.1.1 The models used to calculate the consequences of each of the failure cases are contained within the SAFETI Expert software package. This is considered to be a standard software tool for use in Hong Kong. The modelling approach used has been a flat terrain dense cloud model. Further detailed information on the modelling calculations is contained in the SAFETI Theory Manual.
10.7.1.2 The wind tunnel model in the 8 Water Treatment Works Reassessment Study (ERM 2000) showed that chlorine could travel uphill for a distance of up to 150m since the dilution of chlorine cloud with air reduces the density of the plume which tends towards passive dispersion. Based on the results of the 8 WTWs reassessment study, 150m height cut-off should be used in this study. The new route section and construction site located on ground up to 150m above the water treatment works are shown in Figure 10.2.
10.7.2
Risk Assessment Parameters
10.7.2.1 The assessment has used the standard DNV parameters for risk assessment of this type. Of particular significance are the assumptions relating the population distributions and the fraction of people killed in an event. In this assessment it has been assumed:
¨ 90% of outdoor population (e.g. construction workers and people in vehicles on the road) are modelled as exposed to the full outdoor toxic dose (the remaining 10% outdoor population successfully escapes, described in the 3rd bullet below) while no indoor population was modelled.
¨ transient populations (i.e. people in vehicles on the road) have been modelled as an average population density spread along the road and present at all times. Since the real life situation is groups of people passing along the road and only present for part of the time, the method of modelling will over-estimate the time related dose and hence the probability of death of those within the effect zone of a release, but may underestimate the actual maximum number of people who could be affected (e.g. light traffic composed mainly of busses). Overall the level of risk is considered to be an over-estimate and as the risk is found to be well within acceptable limits, this approximation does not change the conclusions of the study.
¨ escape and evacuation from the effected area have been modelled by the assumption above; namely the precept is that 90% of the outdoors population receiving the fatal toxic doses would be counted as fatality with the remaining 10% successfully escaping.
¨ the likelihood of becoming a fatality due to exposure of chlorine have been estimated using the standard Hong Kong probit equation:
P=-2.82 + 0.53 ln (C2t).
The time of exposure modelled by SAFETI in the Probit expression is exactly the time calculated for the cloud to pass by the person at risk.
10.7.2.2 The consequence results calculated by SAFETI are used directly within the integrated software. The intermediate consequence results are produced in the form of the concentration and dimensions of the cloud versus time. The consequence results for the two key large events involving a greater than 1 tonne chlorine release has been tabulated for various weather classes in Table 10.17 below.
Table 10.17 Consequence Results for > 1 tonne Chlorine Release
|
Failure Case Reference |
Probit Value |
Cloud Dimension |
Size of Chlorine Cloud at Probit Value
(m) |
|||||||
|
B |
D |
F |
B |
E |
D |
D |
D |
|||
|
0.5 m/s |
0.5 m/s |
1.0 m/s |
3.0 m/s |
3.0 m/s |
4.0 m/s |
7.0 m/s |
15.0 m/s |
|||
|
UN22.5 |
3.11 |
Length |
480 |
1,100 |
2,650 |
250 |
760 |
425 |
330 |
255 |
|
|
|
Depth |
65 |
64 |
35 |
30 |
29 |
27 |
21 |
15 |
|
|
6.64 |
Length |
90 |
90 |
100 |
80 |
85 |
80 |
70 |
65 |
|
|
|
Depth |
15 |
16 |
13 |
8 |
8 |
8 |
6 |
4 |
|
Eq34Rup |
3.11 |
Length |
300 |
420 |
975 |
320 |
505 |
425 |
500 |
700 |
|
|
|
Depth |
43 |
30 |
21 |
28 |
20 |
21 |
19 |
18 |
|
|
6.64 |
Length |
75 |
115 |
115 |
75 |
100 |
100 |
110 |
75 |
|
|
|
Depth |
7 |
8 |
5 |
4 |
3 |
3 |
4 |
8 |
The probit values of 3.11 and
6.64 relate to fatality levels of 3% and 95% respectively, and based on average
exposure time of 10 minutes.
10.8.1
Individual
Risks
10.8.1.1 The individual risk (IR) contours for the WTW are shown in Figure 10.3. The level of individual risk to affect the new Tung Chung Road is lower than 1 x 10-8 per year which is approximately 385m away at the point closest to the Cheung Sha WTW.
10.8.1.2 As the 1 x 10-5 per year individual risk from the Cheung Sha WTW is less than 70m from the site, it does not extends into any neighbouring permanent populated area.
Potential
Loss of Life
10.8.2.1
The
potential loss of life (PLL) values for the different cases in the Construction
Phase and Operation Phase are shown in Table 10.17.
Table 10.18 PLL for Construction and Operational Phases
|
Case |
PLL |
|
Construction Case |
|
|
1.
Transient risk to construction workers only. |
1.18E-9 |
|
Operation Case |
|
|
2.
Risk to the future new road population only. |
8.99E-9 |
10.8.2.2
The
risk during construction is assessed for 2006, for the construction workers and
the risk is negligible (Construction Case).
10.8.2.3
For
Operation Case, the risk to the new road users during year 2021 is also
negligible.
FN Curves
10.8.2.4 The FN curves for the Construction and Operation Phase are shown in Figure 10.4. The FN curve for the construction workers shows negligible risk where no fatality occurs with a frequency higher than 1 x 10-9 per year. The FN curve for the new Tung Chung Road user is very low, showing less than 2 fatalities occurring at a frequency of 1 x 10-9 per year. Since the FN curves lie within the acceptable region of the HKRGs, no mitigation measure is required.
10.8.2.5 Further, there is a scenario about more workers working in a smaller area and as a collective group they are exposed to risk from the WTW. As sensitivity test, the FN curve for the Construction Phase is factored up by multiplying the fatality impact 10 times. Whereas the assumption of 5 workers per 100m may be underestimated (Refer Section 10.4.2.3), the factor of 10 times worker fatality is selected to account for a conservative upper bound of the risk to the construction workers. This causes the FN curve for the Construction Phase to shift right, and as shown in Figure 10.4 this Conservative Upper Bound FN curve is still acceptable, lying very far below the acceptable limits of the HKRGs.
10.9.1 All predicted impacts are within acceptable levels and, thus, no mitigation measures are required and the residual impacts are acceptable. However, good working practice would require that the following measures be implemented:
¨ WSD should minimise the inventory at the WTW, particularly during the construction phase of the work, deliveries of chlorine should be made on a just in time basis.
¨ Workers within 200m of the WTW should receive information and training in relation to the hazards posed by the WTW.
¨ The contractor should provide appropriate personal protective equipment for the workers and consider the need for self contained breathing apparatus to protect against a chlorine release, but also smoke from hill fire.
¨ A means of providing a rapid, direct warning to construction workers should be provided (e.g. siren audible within 200m).
¨ The access route for the Tung Chung Road construction areas should be adequate for access by the emergency services.
¨ The emergency services should familiarise themselves with the emergency arrangements for the Tung Chung Road construction site, safety and site access arrangements.
¨ An emergency plan should be developed by the contractor for the workers in accordance with the recommendations above which should integrate with those of WSD and the emergency services.
10.10.1 The conclusions of the hazard assessment are as follows:
¨ The risk for the study area of the Cheung Sha WTW are calculated for both the individual risk and societal risk in terms of PLL and FN curves, and the results have been assessed for both the construction and operation phases (2006 and 2021).
¨ The individual risk levels for the route imposed by the Cheung Sha WTW are below the HKRG limit of 10-5 per year, and are acceptable.
¨ All the FN curves are acceptable and so no mitigation is expected to be necessary for both the construction and operation phases.
10.10.2 Based upon the above it can be concluded that the risk for the road in terms of both the individual risk and societal risk are low and acceptable and therefore, on risk grounds there are no constraints on the road development.
ERM, 1996. Hazard Assessment for Uprating of Silvermine Bay Treatment Works to 200 Mld. for WSD.
DNV, 1997. QRA report for The Risk Assessment of the Transport of Chlorine in Hong Kong for EPD.
DNV, 1998. HA of Rezoning in CZ of Tsuen Wan WTW for Confidential Client.
DNV,1999. Rev 2 Report on Lantau North-South Road Link between Tai Ho Wan and Mui Wo Hazard Assessment.
Mouchel, 2001. Fax correspondence; Extract of Working Paper 3 – Traffic Forecasts dated 2 November 2001.
MVA, 2001. Fax correspondence; Final Traffic Forecast dated 2 November 2001.
M.A. McBride, A.B. Reeves, M.D. Vanderheyden, C.J. Lea and X.X. Zhou; “Use of Advanced Technuques to Model the Dispersion of Chlorine in Complex Terrain”, Trans IchemE, Vol 79, Part B, March 2001.