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7.4 Design Description and Review |
Tables
7.1.1
Background
In view of the South East Kowloon
Development Project which consisted of a trunk road Central Kowloon Route (CKR)
and a new Cruise Terminal to be developed at the southern end of the former Kai
Tak Airport (KTA) runway, the existing twin 400mm submarine gas pipeline that
serves the gas supply from Ma Tau Kok (MTK) to North Point (NP) were requested
to be realigned by the Hong Kong SAR Government in order to cope with these
developments. The existing gas pigging stations at MTK and NP are required to
be re-located accordingly.
The purpose of the Project is to construct a
new gas transmission route from To Kwa Wan to North Point so as to replace the
existing route affected by the proposed Cruise Terminal at Kai Tak and the
proposed
The existing submarine gas pipelines from Ma
Tau Kok to North Point serve as the strategic gas mains for supplying town gas
to more than 300,000 customers on
The proposed new alignment for the gas
pipelines is shown in Figure 2.1.
7.1.2
Scope
of Study
The scope of the Hazard to Life Assessment
(HtLA) study includes the following:
¡
Twin
submarine gas pipelines (O.D. 457mm) across
¡
Two
land gas pipelines at To Kwa Wan and North Point respectively (O.D. 610 mm)
¡
Two gas
pigging stations for pigging operation (Refer to Appendix F3)
The approach and methodology of the Hazard
to Life Assessment study follow those described in the Project EIA Study Brief
(ESB-171/2007) Section 3.4.5. The HtLA report should be written to provide
hazard assessment information on the hazards associated with relevant aspects
of the construction and operation of the proposed submarine gas pipelines, the
gas pigging stations and associated facilities. The information should include
the following:
a.
description
and review of design of the submarine gas pipelines and the gas pigging
stations;
b.
identification
of potential hazards associated with these installations and evaluation of the
potential safety impacts to the public and the environment; and
c.
recommendations
on appropriate risk mitigation measures.
The two gas pigging stations, as associated facilities of the realigned
land / submarine pipelines, are not part of the EIA designated project. Assessment
of risk posed by the gas pigging stations are hence shown in Appendix
F3 of this report for reference.
The project is governed by the following key
legislation:
¡
Gas
Safety Ordinance, Gas Safety (Registration of Gas Supply Companies)
Regulations, Gas Safety (Supply) Regulations 1991
¡
Factories
and Industrial Undertakings Ordinance 1955
¡
Occupational
Safety and Health Ordinance 1997
¡
A risk ranking approach is adopted in the
study to qualitatively assess the risk associated with the Project. Risk
Ranking is a common methodology for making risk-based decision. In our study, a
5x5 risk matrix, which consists of consequences and likelihood, is adopted. The
risk is evaluated by its severity and the likelihood of happening. The details
of risk evaluation are given in Section 7.6.1.
7.4 Design Description and Review
7.4.1
Onshore
Gas Pipelines
On the To Kwa Wan side, the proposed land
pipes (O.D.
On the North Point side, the onshore landing
point will be located inside the North Point Police Station. A new section of
underground pipeline will connect it to the Gas Pigging Station, where it is
connected to the North Point Offtake Station via buried land pipeline across
The onshore gas pipelines are designed with reference
to the recommendations of Institution of Gas Engineers and Managers. The operating
pressure is about 7 barg. The land pipeline is coated internally with liquid
epoxy and externally with 3-layer epoxy-polyethylene. It will be protected by cathodic
protection system, and a minimum of soil cover of 1.2m is provided.
7.4.2
Submarine
Gas Pipelines
The proposed twin submarine gas pipelines
are of length 3.1km and O.D. 457mm, and will be laid across the
7.4.3
Nearby
Residential and Employment Population
The proposed gas pigging station on the
The proposed gas pigging station on the
The land use, description and location of
the population groups near the proposed gas pigging stations during operation
in Year 2012 are shown in Appendix F1 of this report for
reference.
7.4.4
Safety
Management
HKCG has a comprehensive operating
procedures and safety guidelines to safeguard its employees and public. This
section will briefly discuss the safety system under normal operating condition
and in emergency.
7.4.4.1
Normal
Operating Condition
HKCG has developed its in-house guideline and
sets forth the requirement for operation and maintenance of gas transportation
system to ensure any gas transportation above 400 kPa is in good condition.
Inspection of submarine pipeline will be
carried out by pigging operation about once every 10 years. Brief description
of the pigging operation together with illustration can be found in Appendix
F2.
7.4.4.2
Emergency
HKCG has a set of stringent in-house
guidelines and procedures recognized by the Government to deal with emergency.
In case of gas leakage, SCADA system will initiate the alarm for the Grid
Control Engineer immediate action and the Fire Services Department (FSD) will
be contacted immediately. The critical valve of the gas transmission pipeworks
can be remotely shut down if necessary. Should emergency team be required to be
dispatched, they will arrive of the scene within 30 minutes.
7.5.1
Overview
Towngas is a colourless and buoyant gas
under ambient condition. Odorant has been added to make it smell distinct for
easy gas leakage detection. It contains hydrogen and methane which makes it
flammable. It also contains carbon monoxide and carbon dioxide that make it
toxic and asphyxiated.
The proposed pipelines, gas pigging stations
and the associated facilities are not classified as potentially hazardous
installation (PHIs) under the HK Planning Standards and Guidelines. The major
hazard arises from these facilities is
mainly associated with the loss of containment events from the pigging
facilities and pipelines. This could occur as a result of the pipeline failure
and lead to release of town gas.
The major hazard arises from the proposed
pipelines is mainly associated with the loss of containment events. This could
occur as a result of the pipeline failure and lead to release of town gas.
The following section presents the hazard
scenarios identified for the risk assessment study. A structured and systematic
approach has been adopted to identify all the potential major hazards
associated with the pipelines.
7.5.2
Hazards
during Construction of the Underground Pipelines
Major hazards associated with construction
will mainly arise from accidental damage to the underground utilities during
excavation works when constructing the proposed underground pipelines. The
proposed underground pipeline alignment is situated at different locations from
the existing alignment. Detailed information on the underground utilities will
be collected prior to conducting any excavation work, in particular the
existing gas pipelines as required by Gas Safety Ordinance. In addition,
excavation work is well controlled in
The construction of the underground
pipelines will be carried out in accordance with the HKCG guidelines and the
relevant statutory regulations. The pipelines will not be filled up with any town
gas during construction. Thus, the likelihood of construction works causing any
damage to the existing pipeline or pipeline under construction and leading to a
town gas release scenario is very remote.
7.5.3
Hazards
during Construction of the Submarine Pipelines
During construction of the submarine
pipeline, the existing twin submarine pipelines are still in use. The major
hazard to life is the dredging works and the subsequent traffic arrangement
that may have potential damage to the existing submarine pipe and lead to a
release scenario.
According to the Marine Traffic Impact
Assessment Report under this project, some recommended risk control procedures
and mitigation measures are provided for developing the marine safety
management system which can enhance the safety of the submarine pipeline
construction. They are repeated in the
following sections:
7.5.3.1
Proper
General Traffic Management Measures
a.
Full
details of the schedule of operation will need to be provided to Marine
Department to enable the appropriate Marine Department Notices to be drawn up
and promulgate. It is also advised that the contractor to provide full
construction details to Marine Police during all stage when marine work is undertaking;
b.
The
schedule of operations will need to demonstrate that all relevant Marine
Department requirements are to be complied with as will the International and
local requirements for the marking with the yellow marker buoy (Reference:
Marine Department Notice No.29 of 1998) and the lighting of the works craft;
c.
Deployment
of guard boats (minimum 2) for Works within fairways;
d.
The
Contractors should maintain communication with the Vessel Traffic Centre and
the Operations Centre of Harbour Patrol Section (HPS) of Marine Department at
all times. Marine VHF should be equipped in working craft for effective
communications; and
e.
Operations
should be curtailed to the practicable minimal scale if the environment is not
favourable, i.e. visibility falls below one nautical mile, and Tropical Cyclone
Warning Signal No. 3 or Black Rainstorm Warning is hoisted.
7.5.3.2
Minimisation
of Works Activity Footprint – Dredging and Backfilling
a.
Given
the limitation of dredgers in the Works area, the key opportunities to improve
safety are to reduce dredged volume (hence shorter duration of Works),
therefore, optimization of the trench sizes design could allow sufficient
protection of gas pipe with minimum dredging volume. In order to optimise the
trench, tests such as geotechnical centrifuge model testing of armour
protection could be conducted. The final
design must balance construction Works duration against the key nature of the
pipeline – a key gas supply corridor to
b.
The
backfilling operations will be similar (although of shorter duration) to those
during construction as a Grab Dredger is expected to be used to place armouring
rock. As such these two operations share
identical marine impacts and mitigation measures.
7.5.3.3
Safety
Provision during Dredging and Backfilling
a.
The
Works area along the submarine pipeline should be demarcated with marker buoys
and boundary floats to prevent vessel entering this working zone. Yellow marker
buoys fitted with yellow flashing lights should be used to mark the locations
of the anchoring points around the dredging vessels.
b.
Deployment
of guard boats (2 numbers for all periods when Works are set within a
designated Fairway). All vessels should be marked with the words “Guard”, in
English and Chinese, with characters of 1m in height, and set a minimum 1m
above deck level.
c.
All
vessels marking, signals and lights should comply with the COLREGS and local
signals adopted.
d.
The
Contractor should ensure the diameter of Works for all dredging or backfilling
Works craft operations does not exceed 100m.
e.
Only
one spoil removal / rock delivery barge may attend the dredging Works vessel at
any time.
f.
No
spoil removal / rock delivery barges may wait or be anchored within a fairway.
7.5.3.4
Liaison
with relevant Government Departments before and during Construction Stage
a.
The
Contractor should liaise with local cruise operators and Hong Kong Pilots to
advise them of the timing and location of all Works.
b.
If the
Works craft are set within an area of any fairway that may impede the passage
of a cruise liner or other large ocean-going vessel, the Works craft must, at
the request of Marine Department, be removed from the work site 30 minutes
prior to the anticipated passage.
7.5.3.5
Requirements
during the Submarine Pipe Pulling
a.
All
pipe pull wires and pipe pull heads deployed during pipe pull operations shall
be closely monitored.
With the implementation of the above
mitigation measures, the risk of indirect damage to the existing submarine
pipeline during construction phase will be minimized. Besides, the existing
submarine pipeline is also protected by armour rock and is about 500m apart
except near the North Point landing site, direct damage to the gas pipe is not
anticipated.
7.5.4
Hazardous
Scenarios from Pipelines
7.5.4.1
External
Causes
Natural Hazards
Natural hazards such as earthquake, storms,
typhoon, subsidence and tsunami may cause potential damage to the pipelines and
lead to loss of containment. The section will discuss the credibility of loss
of containment due to these natural hazards with respect to
Earthquake
Hong Kong is situated on the southern coast
of mainland
Subsidence / Landslides
Excessive subsidence may lead to failure of
the structure and ultimately loss of containment scenario. However, subsidence
is usually slow in movement and such movement can be observed and remedial
action can be taken in time. Besides, the land where the proposed pipelines
will be located was reclaimed long time ago. Soil condition is considered
rather stable; also there are no hilly regions nearby. The probability of town
gas leak due to subsidence or landslides is considered to be remote.
Severe Environmental Event
Loss of containment due to severe
environmental event such as typhoon or tsunami (large scale tidal wave) is not
possible as the proposed pipelines are located underground or inside the
seabed.
Third Party Damage
Third party damage (TPD) on land pipes
includes the damage to pipes due to drilling, hammering and excavation works,
etc. In fact, the potential of TPD damage to the land pipes depends on the
surrounding environment. Pipe wall thickness, buried depth, concrete cover and
design factor will all have influence to whether a TPD would result in pipe
damage.
In
Anchor Drop / Drag
For subsea pipe, anchorage drop / drag from
other vessels could lead to pipe damage. The level of damage would depend on
the size of the anchor and the subsea pipeline protection. Anchor drag may hook
onto the subsea pipe and cause damage during retrieval.
It should be noted that the proposed subsea
pipelines are laid across the Victoria Habour. It will be shown in the Nautical
Chart and Mariners will be aware of the pipeline and will not drop anchor
within its vicinity. In addition, it will be buried at least 3m below the
seabed and with armour rock protection. This reduces the possibility and the
likelihood of the damage caused by routine anchorage. Anchor dropping in the
fairway is considered a credible scenario since emergency anchor dropping may
occur due to poor weather condition or human error and it will be considered in
this assessment.
Vessel Sinking, Fishing and Dredging
Activities
Vessel sinking usually involves collision of
vessels or is caused by typhoon. The frequency of occurrence for vessel sinking
in collision depends on traffic density and frequency. Vessel sinking may lead
to impact on the proposed submarine gas pipe. Amongst fishing activities, bottom
trawling is the most likely fishing method that causes damage to the proposed
gas pipe. Moreover, the gas pipe may be damaged by cutting heads in dredging
operations. Damage to the gas pipe can also be caused by anchors for
restraining the dredger.
7.5.4.2
Internal
Causes
Corrosion
Internal corrosion could occur due to the
presence of moisture and contaminants in the gas. The proposed pipeline will
carry dry town gas which is not expected to have any moisture. The pipe will be
coated with appropriate coating materials internally and externally to prevent
corrosion. Regular inspection and maintenance will be carried out for gas pipes
to ensure the pipe integrity.
Material Defect
Material defect of pipeline including
welding failure and mechanical strength failure could occur for various
reasons. Selection of pipe materials will follow international standard to
ensure material quality. Welding of pipe materials will be subject to 100%
non-destructive tests which can ensure all the welds are acceptable. The proposed
pipeline will be subject to strength test and leak test after construction
which would prevent any mechanical failure due to material defects. However,
historical data showed that material failure leading to loss of containment is
credible and it will be included in our assessment.
Construction
Defect
Construction defect for pipeline include
defects during welding and defects during coating. Under HKCG’s requirements,
all underground and submarine pipelines, as well as most of the piping at gas
pigging stations (except at instrument connections for valves, pig traps etc.)
have to be welded. HKCG has rigorous procedures to ensure the integrity of the
welding parts. All the weldings in HKCG pipelines have to comply with
international welding standards. Prior to the commission of work, the welding
procedures submitted by the contractor has to be verified by HKCG, and test
welds have to be carried out the destructive testing for qualification. The
welding works have to be carried out by qualified welders. Finally, all the
welded parts will be checked using X-ray test to ensure the welding quality.
The underground pipe, aboveground pipe and
submarine gas pipelines will have appropriate coating for protection.
All the pipe joints will achieve the coating
quality as good as the pipeline as according to the design standards.
7.6.1
Risk
Evaluation
A risk ranking approach adopted in other
Publics Works Programme in accordance with technical circulars of Development
Bureau [4] is adopted to evaluate the potential risk associated to
the public with the proposed new gas pigging station at To Kwa Wan. A 5x5 risk
matrix (Table 7.1)
is utilized to evaluate the risk. The risk evaluation criteria for consequence
and likelihood are depicted in Table 7.2 and Table 7.3 respectively.
Table 7.1: 5x5 Risk Assessment Matrix
|
|
|
Consequences |
||||
|
|
|
Insignificant |
Minor |
Moderate |
Major |
Catastrophic |
|
Probability
(Likelihood) |
Rare |
Low |
Low |
Low |
Medium |
Medium |
|
Unlikely |
Low |
Low |
Medium |
Medium |
High |
|
|
Possible |
Low |
Medium |
Medium |
High |
High |
|
|
Likely |
Medium |
Medium |
High |
High |
Very High |
|
|
Frequent |
Medium |
High |
High |
Very High |
Extreme |
|
Table 7.2: Consequence Evaluation Criteria
|
Descriptor |
Description of Consequences |
|
Insignificant |
No injury |
|
Minor |
Minor injury |
|
Moderate |
A number of injury / hospitalization |
|
Major |
Extensive injuries / long term treatment |
|
Catastrophic |
Fatality |
Table 7.3: Likelihood Evaluation Criteria
|
Descriptor |
Description of Frequency |
|
Rare |
Event may occur in exceptional circumstances (Can be assumed not to occur over the life time of the facility |
|
Unlikely |
Event is unlikely to occur, but it is possible during the life time of the facility |
|
Possible |
Event could occur during life time of the facility |
|
Likely |
Event likely to occur once or more during the life time of the facility |
|
Frequent / Almost Certain |
Event occurs many times during the life time of the facility |
7.6.2
Risk
Ranking for the Underground Pipelines
7.6.2.1
During
Construction
As discussed in Section
7.6.2.2
During
Operation
Risk mitigation
measures have already been adopted in the design of the underground pipe to
lower the risk level. Utility mapping will be conducted to identify any
underground utility along the proposed pipeline alignment before any
construction work commences. Drawings of the existing gas pipelines must be
obtained prior to any excavation as required by the Hong Kong Gas Safety
Ordinance.
The qualitative risk assessment of failure
of the underground pipelines based on different failure causes is shown in Table 7.4 according
to the risk rating criteria in Table 7.1. The likelihood of a loss of containment scenario
is based on the causes considered in Section
Table 7.4: Risk Assessment of Different Failure Causes of the Underground Pipelines during Operation
|
Scenario |
Likelihood of Occurrence |
Consequence to Public |
Risk Level |
|
External Causes |
|||
|
Natural Event |
|||
|
· Earthquake |
Rare |
Moderate |
Low |
|
· Subsidence / Landslides |
Rare |
Moderate |
Low |
|
· Severe Environmental Event |
Rare |
Moderate |
Low |
|
Third Party Damage |
Rare |
Moderate |
Low |
|
Internal Causes |
|||
|
Corrosion |
Unlikely |
Minor |
Low |
|
Material Defect |
Rare |
Moderate |
Low |
7.6.3
Risk
Ranking for the Submarine Pipelines
7.6.3.1
During
Construction
As discussed in Section
1.
Proper
general traffic management measures.
2.
Minimisation
of works activity footprint – dredging and backfilling.
3.
Safety
provision during dredging and backfilling.
4.
Liaison
with relevant Government Departments before and during construction stage.
5.
Requirements
during the submarine pipe pulling.
With the implementation of the above
mitigation measures, the risk during the construction phase is expected to be
low.
7.6.3.2
During
Operation
Risk mitigation measures to prevent the
damage of submarine pipeline during operation will be adopted. They are listed
as follows:
1.
The
submarine gas pipeline will be covered by armour rock, damage from marine
vessels could be prevented.
2.
After
the submarine pipeline is installed and will be tested to the design standards.
3.
According
to HKCG's past record, pigging operation will normally be carried out once in
every ten years.
The qualitative risk assessment of failure
of the submarine pipe based on different failure causes is shown in Table 7.5 according
to the risk rating criteria in Table 7.1. The likelihood of a loss of containment scenario
is based on the causes considered in Section
7.5.4. According to the risk matrix, for any high risk item, further risk
mitigation measures should be considered as necessary to reduce the risk; for
moderate risk item, further risk mitigation measures should be considered to
reduce the risk to as low as reasonably practicable; and low risk item, further
risk mitigation measures are not considered necessary since the risk is
considered broadly acceptable.
Table 7.5: Risk Assessment of Different Failure Causes of the Submarine Pipelines during Operation
|
Scenario |
Likelihood of Occurrence |
Consequence to Public |
Risk Level |
|
|
External Causes |
||||
|
Natural Event |
|
|||
|
· Earthquake |
Rare |
Minor |
Low |
|
|
· Severe Environmental Event |
Rare |
Minor |
Low |
|
|
Third Party Damage |
Rare |
Minor |
Low |
|
|
Anchor Drop/ Drag |
Unlikely |
Minor |
Low |
|
|
Vessel Sinking |
Unlikely |
Minor |
Low |
|
|
Fishing |
Rare |
Minor |
Low |
|
|
Dredging Activity |
Unlikely |
Minor |
Low |
|
|
Internal Causes |
||||
|
Corrosion |
Unlikely |
Insignificant |
Low |
|
|
Material Defect |
Rare |
Minor |
Low |
|
The risk of the proposed gas pigging
stations, the underground and submarine pipelines has been assessed
qualitatively and found at low risk level. Apart from qualitative assessment, a
quantitative risk assessment has been conducted by HKCG, and both the
individual risk and societal risk results for the submarine gas pipelines and
the two gas pigging stations have been found “Acceptable” as per risk
guidelines in
A risk assessment study has been conducted
for the relocation of the existing submarine gas pipeline and the associated
facilities. The assessment has considered various failure causes for towngas
leakage. Based on the evaluation of potential safety impacts, the risk
associated with the proposed realigned gas facilities is considered low.
Notwithstanding the low level of risk, risk minimisation measures have been
incorporated into the design to further lower the risk and safeguard population
in vicinity.
Though it can be concluded that there are no
insurmountable risks associated with the construction and operation of the
proposed gas pigging stations based on the risk ranking analysis, a
quantitative risk assessment has been conducted by HKCG. Both the individual risk and societal risk
results for the submarine gas pipelines and the two gas pigging stations have
been found “Acceptable” as per risk guidelines in