2. project description - epd.gov.hk · existing seawall at ma tau kok (mtk) and a gas pigging...
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Installation of Submarine Gas Pipelines and Associated Facilities from To Kwa Wan to North Point for Former Kai Tak Airport Development Consultancy Services for Feasibility Study and Detailed Design Environmental Impact Assessment Report
2.1 Project Background
An existing twin 400mm diameter submarine gas pipelines across the Victoria Harbour were constructed by
Hong Kong China Gas Company Limited in the early 1970s to supply town gas from Kowloon to the Hong
Kong Island. The existing twin 400mm diameter submarine gas pipelines are currently aligned 200m west
of and parallel to the Former Kai Tak Airport (KTA) runway between a gas offtake and pigging station at the
existing seawall at Ma Tau Kok (MTK) and a gas pigging station at North Point (NP). The submarine gas
pipelines are buried in a trench beneath the seabed and protected with rockfill. Owing to the proposed
Cruise Terminal Development (CTD) and Central Kowloon Route (CKR) projects, the existing twin 400mm
diameter submarine gas pipelines were requested to be diverted.
Mott MacDonald Hong Kong Ltd. was appointed by the Hong Kong and China Gas Company Limited to
carry out the Feasibility Study and Detailed Design including the Environmental Impact Assessment for the
proposed Installation of Submarine Gas Pipelines and Associated Facilities from Ma Tau Kok to North
Point for the Former Kai Tak Airport Development. As there is already a pair of existing submarine gas
pipelines from Ma Tau Kok to North Point for supplying town gas to the Hong Kong Island, the project title
was changed to “Installation of Submarine Gas Pipelines and Associated Facilities from To Kwa Wan to
North Point for the Former Kai Tak Airport Development (hereinafter called “the Project”) since August
2008.
2.2 Project Alignment
The Project comprises construction of a new gas pipeline network from To Kwa Wan to North Point so as
to replace the existing one affected by the proposed Cruise Terminal dredging works adjacent to the
existing Kai Tak runway and the proposed Central Kowloon Route crossing the Kowloon Bay at Ma Tau
Kok. The gas pipeline network will consist of a twin submarine gas pipeline across the Victoria Harbour,
two new pigging stations and associated land sections. The route of the proposed gas pipeline network is
shown in Figure 1.1. The proposed Project is to construct and operate a twin submarine gas pipeline, two
new pigging stations and associated land sections.
The Project will ensure that the replacement of the existing gas facilities from To Kwa Wan to North Point
are completed in 2014 and the new facilities are in service before the de-commissioning of the existing
facilities.
The Project commenced with the identification of key issues and constraints within the Study Area. The
engineering constraints at the Victoria Harbour include To Kwa Wan and Kwun Tong Typhoon Shelters,
Government Mooring Buoys, Eastern Quarantine and Immigration Anchorage, Hung Hom and Eastern
Fairways, To Kwa Wan Sewage Outfall, North Point Sewage Outfall, Kwun Tong Sewage Outfall and
Island Eastern Corridor. After the identification of constraints at the Victoria Harbour, the landing locations
and landlines options were set. The final submarine pipelines alignment, landing locations and the landline
routes for existing network connection selected were formed by different evaluations in the feasibility study,
liaison with different Government Departments and consultation to interested parties. The project scope
2. Project Description
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Installation of Submarine Gas Pipelines and Associated Facilities from To Kwa Wan to North Point for Former Kai Tak Airport Development Consultancy Services for Feasibility Study and Detailed Design Environmental Impact Assessment Report
also include the assessment of marine and land traffic, land application, planning of construction method
and detailed engineering design.
The harbour area concerned is currently the To Kwa Wan Typhoon Shelter, government mooring area,
Eastern Fairway and the Eastern Quarantine and Immigration Anchorage Area.
The landfall site at To Kwa Wan will be on previously reclaimed land. The pipeline at To Kwa Wan will run
through part of the previously reclaimed land after landing. The use of landing point at To Kwa Wan has
been zoned as other specific used (“OU”) according to the latest information from Planning Department.
The landfall site at North Point will be on previously reclaimed land. The pipeline at North Point will connect
to existing gas pipeline at Java Road after landing. The use of landing point at North Point is within North
Point Police Station.
2.3 Scope of the Project
The scope of the proposed Project comprises the following:
� twin submarine gas pipelines across the Victoria Harbour from To Kwa Wan to North Point (a
designated project under EIA Ordinance);
� two land gas pipelines at To Kwa Wan and North Point respectively (non designated project under EIA
Ordinance); and
� two pigging stations for pigging operation at To Kwa Wan and North Point respectively (non designated
project under EIA Ordinance).
The “submarine gas pipeline” component of the Project is classified as Designated Project under item H.2
of Part I of Schedule 2 of the Environmental Impact Assessment Ordinance (EIAO) (Cap. 499). The
dredging operation associated with the formation of trench for installation of submarine gas pipelines is
classified as Designated Project under item C.12 (b) of Part I of Schedule 2 of the Environmental Impact
Assessment Ordinance (Cap. 499) as it is less than 100m from a seawater intake point. An Environmental
Permit (EP) issued under the EIAO is required for the construction and operation of the designated project.
An application for an EIA study brief under section 5(1) of the EIAO was submitted by the Hong Kong and
China Gas Company Limited on 28 September 2007 with a Project Profile (No. PP-328/2007). The EPD
issued an EIA Study Brief (No. ESB-171/2007) on 8 November 2007, detailing the requirements for
carrying out and reporting the EIA study.
As the proposed To Kwa Wan pigging station is located within an area zoned “Other Specified Uses”
annotated “Sewage Treatment Plant”(“OU(STP)”) on the Draft Hung Hom Outline Zoning Plan (OZP) No.
S/K9/23, planning permission from the Town Planning Board under Section 16 of the Town Planning
Ordinance had been applied according to the Notes of the OZP as utility installation not ancillary to the
specified use in “OU(STP)” zone requires planning permission.
As the proposed North Point pigging station is located within an area zoned “G/IC” and “Road” on the North
Point OZP, planning permission from the Town Planning Board under Section 16 of the Town Planning
Ordinance had been applied according to items 7 and 8 of the Notes of OZP as the proposed North Point
pigging station in Road zone requires planning permission.
Approval of the Section 16 Applications for the two pigging stations from Town Planning Board were
granted on 19 March 2010. Assessment of environmental impact for the pigging stations were incorporated
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Installation of Submarine Gas Pipelines and Associated Facilities from To Kwa Wan to North Point for Former Kai Tak Airport Development Consultancy Services for Feasibility Study and Detailed Design Environmental Impact Assessment Report
in the approved planning permission applications for the two pigging stations prepared under Section 16 of
the Town Planning Ordinance and therefore are not covered in details in the EIA report.
The decommissioning/ removal works of the existing cross-harbour submarine pipelines and gas pigging
stations are not included in the scope of the proposed Project.
2.4 Relationship between the existing cross-harbour gas pipelines and pigging stations and the proposed gas pipelines and pigging stations
As part of the Kai Tak Development (KTD) as shown in Figure 2.1, a Cruise Terminal is proposed to be
constructed at the southern end of the former KTA runway. For the provision of the safe berthing and
manoeuvring of the vessels at the Cruise Terminal, the foreshore and seabed at the southern side of the
former KTA runway would generally be dredged down to about 12m below Hong Kong Chart Datum, with
the berthing area of about 50m wide adjoining the edge structures of the Cruise Terminal to be dredged
further down to about -13mPD below Hong Kong Chart Datum and surfaced with scour protection layer.
The second berth of Cruise Terminal will be ready for berthing medium-sized cruise vessels in 2014. After
relocation of the submarine gas mains, the second berth will be able to accommodate mega cruise vessels
in 2015/2016.
The proposed Central Kowloon Route (CKR) as shown in Figure 2.1 would connect West Kowloon with the
proposed SEKD Development and the road network in Kowloon Bay. To avoid reclamation, the proposed
CKR would be built underneath the seabed at the portion in Kowloon Bay between Kowloon City Ferry Pier
and former KTA runway in the form of an immersed tube. The construction of CKR was scheduled for
completion in 2016.
As the proposed Cruise Terminal dredging works adjacent to the existing Kai Tak runway and Central
Kowloon Route crossing Kowloon Bay at Ma Tau Kok would have conflict with the existing twin 400mm
diameter steel submarine gas pipelines aligned 200m west of and parallel to the Former Kai Tak Airport
(KTA) runway, at the request of the HKSAR Government, the existing submarine gas pipelines together
with the pigging stations at To Kwa Wan and North Point needs to be relocated to facilitate the above two
projects. Prior to the abandonment of existing submarine gas pipelines in which decommissioning would
take place in 2014, installation of new submarine gas pipelines and construction of new gas pigging
stations are required to maintain the supply of town gas to the Hong Kong Island.
2.5 Do-nothing Scenario
The existing twin 400mm dia. submarine gas pipelines from Ma Tau Kok to North Point were laid below the
existing seabed. They were crucial for the supply of town gas to the Hong Kong Island. The proposed
submarine gas pipelines and pigging stations are essential to maintain the gas supply from Kowloon to the
Hong Kong Island as conflict between the proposed Cruise Terminal dredging works and CKR projects and
the existing twin 400mm diameter steel submarine gas pipelines aligned 200m west of and parallel to the
Former Kai Tak Airport (KTA) runway between a pigging station at the existing seawall at MTK and a
pigging station at NP means diversion of the existing submarine gas pipelines to allow gas to supply from
Kowloon to the Hong Kong Island is inevitable. Without the project, the town gas supply to the Hong Kong
Island will be interrupted.
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2.6 Consideration of Alternative Alignment Options
Eight options were considered for the proposed submarine gas pipelines across the Victoria Harbour. The
alignments considered are shown in Figure 2.4.
The selection of the proposed submarine gas pipelines alignment is subject to many constraints.
Firstly, conflict with the major installations and underground structures within the study area which are
considered to be physical constraints to the alignment of the proposed submarine gas pipelines have to be
avoided. These include:
� To Kwa Wan and Kwun Tong Typhoon Shelters;
� Small Craft Mooring Buoys;
� Government Mooring Buoys;
� Eastern Quarantine and Immigration Anchorage;
� Hung Hom and Eastern Fairways;
� To Kwa Wan, North Point and Kwun Tong Sewage Outfall; and
� Island Eastern Corridor.
Secondly, the potential environmental impacts of the project should be avoided or minimised. These
include impacts to water quality, sediment management, coral communities and marine archaeology.
The landing points of the submarine gas pipelines should avoid foundations/pier structures of the Island
Eastern Corridor and utilities and infrastructure along the coast of Hong Kong Island East and South East
Kowloon. The pigging stations locations and land gas pipelines alignment should avoid existing planting
area and trees near the coast of Hong Kong Island East and South East Kowloon.
The submarine gas pipelines are proposed to be laid across the Victoria Harbour at a minimum depth of
approximately 4 to 9 m below the existing seabed level and to interface with pigging stations and land gas
pipelines at the landing points in To Kwa Wan and North Point. The proposed alignment should keep a
minimum separation of 50m as far as practicable from the existing or planned marine installations.
As shown in Figure 2.1, the proposed submarine gas pipelines are bounded by To Kwa Wan and North
Point Sewage Outfall and the Eastern Quarantine and Immigration Anchorage. The existing To Kwa Wan
Typhoon Shelter, sewage outfall for To Kwa Wan Sewage Treatment Works, seawater intake for Quarry
Bay Salt Water Pumping Station, foundations/pier structures of Island Eastern Corridor while the proposed
pigging stations and land gas pipelines are bounded by proposed extension of To Kwa Wan Sewage
Treatment Works and proposed extension of Man Hong Street Playground. Horizontal alignment of the
proposed submarine gas pipelines is shown in Figure 2.3. Location and layout plans, sections and
elevations with dimensions in appropriate scales of the proposed To Kwa Wan and North Point pigging
stations are shown in Figures 2.9 to 2.12.
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By considering the constraints discussed above, the most feasible submarine gas pipelines alignment and
landing points are as follows:
For the submarine portion, the shortest and the most feasible route for this portion is an alignment between
the dictated landing points at the open space near Hoi Shum Park and at the waterfront area next to North
Point Police Station in a manner as shown in Figure 2.1 such that the impact on water quality, coral
communities, sediment management and marine archaeology be minimized and impact on underwater
infrastructure and marine traffic is minimal.
For To Kwa Wan, the most feasible landing point is at the area to the north of existing DSD To Kwa Wan
Sewage Treatment Works and then connected to a new pigging station located at adjacent to the future Hoi
Sam Park extension close to the existing Sewage Treatment Plant.
For North Point, the most feasible landing point inside the North Point Police Station and then connected to
a new pigging station located at the same area.
2.7 Consideration of Alternative Pigging Stations Locations/Designs
2.7.1 Locating the pigging station wholly or partly underground
An underground or recessed design of pigging station could possibly mitigate operation phase landscape
impacts and release more space on ground level for enjoyment of the public. However, an underground
pigging station would pose additional and thus undue risk to the public, by virtue of entailing a confined
space which facilitates the accumulation of any explosive gas as well as additional equipment that is
subject to failure. An underground design of pigging station would require cut and cover construction of a
large confined space to house a large underground plant room which would lead to dust, noise and
landscape impacts on the sensitive receivers during construction phase. Due to the increase in risk and
from the gas safety point of view, the underground design of pigging station is not acceptable. Apart from
an increase in risk, the reliability of the gas supply to the entire Hong Kong Island via the pigging station is
affected due to the increased risk of flooding, fire and explosion.
Similarly, a recessed pigging station (the top of the station is covered by removable gratings so that a
ventilated environment could be maintained) could also possibly mitigate landscape impacts. However, any
gas release may pass through the ground level which would pose undue risk to the persons on the top of
the recessed station. On the other hand, the persons on the top of the recessed station would be a
potential threat to the station itself as they can introduce ignition sources such as cigarette butts to the
station easily. A recessed design of pigging station would require cut and cover construction of a large
space to house a large underground plant room which would lead to dust, noise and visual disturbance on
the sensitive receivers during construction phase. The issue on flooding is more serious than an
underground pigging station as the grating would not detain any surface or rainwater from going into the
recessed pigging station. The reliability of gas supply is affected due to the increased risk of flooding and
accidental damage by persons on top of the pigging station. The recessed design of pigging station is
therefore unacceptable.
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2.7.2 Locating the pigging station outside the existing/planned “Open Space” zones
as shown on the concerned Outline Zoning Plan
2.7.2.1 Proposed To Kwa Wan Pigging Station
Thirteen options were considered for the selection of site on the Kowloon side for the construction of new
pigging station. The locations of site considered are shown in Figures 2.5 and 2.6.
Figure 2.5 To Kwa Wan Pigging Station Options 1 to 7
Option 3 (Open space)
Option 1 (Newport Centre)
Option 2 (Grand
Waterfront)
Option 4 (Proposed
extension of Hoi Sum
Option 6
(Preliminary Treatment
Plant)
Option 5 (No existing
building or structure)
Option 7 (Hilder Centre, Harbour centre Blk 2 and Pier)
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Figure 2.6 To Kwa Wan Pigging Station Options 8 to13
The selection of site for the To Kwa Wan pigging station is subject to many constraints. Firstly, Hung Hom
and To Kwa Wan areas are mostly developed, a lot of existing buildings were constructed in the concerned
area (Options 1, 2, 6, 7, 8, 9, 11, 12 and 13). Secondly, the proposed pigging station site should avoid
encroach on existing or proposed “Open Space” as classified in the Outline Zoning Plan (Options 3, 4, 5
and 10), for example, King Wan Street Park, Land next to the Vehicle Inspection Centre, Area for Hoi Sam
Park Extension. Finally, the only feasible site identified for the construction of To Kwa Wan pigging station
was the site adjacent to the existing Preliminary Treatment Plant (Option 5) which is classified as “Other
Specified Use (OU)” while the others are all zoned “Open Space” or occupied by existing buildings. As the
proposed To Kwa Wan pigging station does not require substantial site formation works and demolition of
existing buildings, it’s environmental nuisance (e.g. dust, noise and landscape impact) to the general public
would be minimized during construction and operation stage of the pigging station. A waterfront promenade
of at least 25m wide would be available next to the proposed To Kwa Wan pigging station.
2.7.2.2 Proposed North Point Pigging Station
Seventeen options were considered for the selection of site on the Hong Kong side for the construction of
new pigging station. The locations of site considered are shown in Figures 2.7 and 2.8 below.
Option 9 (Laguna Verde)
Option 10 (Tai Wan Shan
Park)
Option 11 (Tai Wan Shan
Swimming Pool)
Option 12 (Harbourfront
Landmark Tower)
Option 13 (Harbour Plaza Hong Kong & One Harbour Front)
Option 8 (Fisherman’s
Wharf)
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Figure 2.7 North Point Pigging Stations (Options 1-9)
Option 1 Pier
Option 4 North Point
Estate
Option 5 Bus
Terminus
Option 6 Vehicle
Ferry Pier
Option 7 North Point
Vehicle Ferry Pier Playground
Option 3 Pier
Option 2 Bus
Terminus
Option 8 K. Wah Centre
Option 9 ICAC
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Figure 2.8 North Point Pigging Station (Options 10-17)
The selection of site for the North Point pigging station is subject to lots of constraints along the coastal
area of Hong Kong Island. Seventeen options were considered but only option 14 was considered as
feasible for the proposed North Point pigging station. Firstly, there are existing buildings/structures which
are currently in use for options 1 to 3, 5, 6, 8, 9, 11 to 13, and 16. Option 4 was zoned as residential area
and is not suitable for construction of North Point pigging station. Options 7, 10, 15, and 17 were zoned as
open space which was planned for the development of recreational uses serving the public and local
resident. In particular, Option 10 was proposed in 2007 but was not supported by relevant government
departments due to shortfall of local open space in North Point. A temporary waterfront promenade
alongside the seafront (Option 17) was also proposed by the Eastern District Council and was scheduled
for construction in 2010. Construction of North Point pigging station in these open spaces would not be
supported by relevant government departments and the public. Finally, the only feasible site identified for
the construction of North Point pigging station was the site inside the compound of the North Point Police
Station (Option 14) where the existing North Point pigging station was located which is classified as
“Government, Institute and Community” and “Road”. The location of proposed North Point pigging station
has been agreed with Hong Kong Police Force. As the North Point pigging station would be inside the
compound of existing North Point Police Station, it’s environmental nuisance (e.g. dust, noise and
landscape impact) to the general public would be minimized during construction and operation stage of the
pigging station. The valuable open space within the district would not be affected.
Option 12 North Point Fire Station
Option 13 North Point Government
Office
Option 14 Eastern District
Headquarter and North
Point Police Station
Option 11 Kodax House
Option 10 Open Space
Option 16 WSD Land Allocation for Quarry Bay S.W.
P/S
Option 15 Open Space
Option 17 Open Space
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2.7.3 Connection to and expansion of the existing pigging station in the vicinity
2.7.3.1 Existing Ma Tau Kok Pigging Station
In order to maintain the gas supply to the Hong Kong Island, the new submarine gas pipelines should be
built and commissioned before the decommissioning of the existing submarine gas pipelines. Therefore,
the existing gas pigging station, which forms an integral part of the existing submarine gas pipelines,
should always be maintained in operation during the operations of the existing submarine gas pipelines.
As there is no vacant land inside the existing pigging station for the construction of a new pigging station,
re-use of the existing pigging station requires modification of the existing pigging station which would
involve construction works inside the existing pigging station for connection of new gas pipelines to the
existing gas pipelines. However, these modification/connection works inside the existing pigging station
might involve ground excavation, welding of gas pipes and lifting of heavy materials in the vicinity of live
gas pipelines. It may pose safety hazards causing disruption to the existing gas facilities and impose risk
for affecting the town gas supply to the Hong Kong Island. It is not acceptable to HKCG for bearing the risk
of interrupting the town gas supply to the Hong Kong Island due to any construction works inside the
existing gas pigging station.
In addition, the proposed immersed tube tunnel in the Central Kowloon Route project would have conflict
with the existing gas pipelines near the existing MTK pigging station. The purpose of laying new submarine
gas pipelines is to avoid the conflict with the CKR project. If the new submarine gas pipelines are
connected to the existing pigging station, they would inevitably have interface with the CKR project and the
purpose of the project cannot be achieved.
In view of the above, it is considered not feasible to reuse the existing pigging station.
2.7.3.2 Existing North Point Pigging Station
There is an existing pigging station inside the North Point Police Station (NPPS) compound and it has been
operating for years. Re-use of the existing pigging station could possibly minimise dust, noise and waste
impacts on the sensitive receivers during construction phase. Re-use of the existing NP pigging would
involve construction works inside the existing pigging station such as connection of new gas pipelines to
the existing gas pipelines. As there is no vacant land inside the existing NP pigging station for the
construction of new pigging station, modification of the existing NP pigging station would be required.
However, these modification works, which may involve ground excavation, welding of gas pipes and lifting
of heavy materials in the vicinity of live gas pipelines, may cause disruption to the existing gas facilities
and impose risk for affecting the town gas supply to the Hong Kong Island. It is not acceptable to HKCG
for bearing the risk of interrupting the town gas supply to the Hong Kong Island due to any construction
works inside the existing NP pigging station.
2.8 Consideration of Alternative Construction Methods
The methods commonly used to install submarine gas pipelines include “jetting (Trenching)”, “jetting
(injection)”, “horizontal directional drilling”, and dredging to form the trench followed by “float and sink”,
“bottom pull” or “lay barge” followed by backfilling to protect the pipeline. For this project the jetting
technique is not feasible as described in Section 2.8.1.
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Jetting installation involves the use of water jets to loosen the seabed material sufficiently to allow the pipe
to be pulled into the seabed with significant disturbance to the seabed. Horizontal directional drilling
involves taking the pipelines directly from the start to end point by underground drilling with no surface
disturbance being necessary. Dredging involves the removal of marine sediments from the seabed to form
the trench, into which the submarine gas pipelines are laid by possible methods including Bottom Pull, Lay
Barge or the Float and Sink Method. Backfill material would be placed on top to protect the pipeline and
minimize the cross section of dredging and backfilling works. A typical cross section of the submarine gas
pipelines is provided in Figure 2.2. Design of the cross section and the resulting amount of marine
sediments to be dredged from the seabed to form the trench will be depending on the required minimum
cover as described in Section 2.8.3 and the existing seabed level for the Bottom Pull, Lay Barge or the
Float and Sink Method adopted for submarine gas pipelines installation.
An analysis of different construction methods and techniques to minimise impacts on water quality, marine
ecology, fisheries and waste was carried out. Details of the analysis are presented below.
2.8.1 Jetting Installation
2.8.1.1 Jetting (Trenching)
Jetting involves the uses of pressurized water jets to break-up, remove or liquefy the soil from under the
submarine pipeline and allow it to settle at the design level below the seabed. Jetting is a technique by
which the pipeline is pre-installed (laid) on the seabed before jetting is carried out.
The jet machine ejects plumes of fluidised soil out of the eductors on the sides of the machine in the bottom
portion of the water column.
Based on the ground investigation and laboratory testing data, the seabed material within the works area of
the project consist of about 30% of contaminated sediment. As the contamination level of the sediment
along the dredging area at Victoria Harbour was high in terms of heavy metals such as Cu, Ni, Zn, Pb, Hg
and high molecular weight Polycyclic Aromatic Hydrocarbons (PAHs) in which contaminated sediment
(Type 3) was located near the To Kwa Wan Typhoon Shelters and contaminated sediment (Type 2) was
located near both proposed pigging stations at North Point and To Kwa Wan which is extended up to about
the middle of proposed gas pipelines in Victoria Harbour. Based on the ground investigation the sediment
immediately below the seabed comprises mostly very soft slurry deposit which are soft, fine, dark grey and
black, clay/silt (contaminated marine deposit), the disturbance to the contaminated marine sediment must
be minimized. Jetting is not feasible for such marine sediment properties under the seabed. The jet
machine would eject plumes of fluidised soft fine slurry, clay and silt out of the sides of the machine and
these plumes of fluidised marine sediment would become entrained in the water column along the tidal flow
and cause adverse impacts on water quality and ecological sensitive receivers in the vicinity.
The identified very soft clay would require a slope gradient of about 1 in 3 on the sides of the trench for
stability. Additional and repeated passes would be required to achieve the required trench width and depths.
This would increase the volume of contaminated marine sediment being ploughed up and thus increase the
spread of highly contaminated substances. This also indicates that the jetting method would not be
effective in the soft mud and slurry ground conditions. Furthermore, the contaminated slurry on the sides of
the trench at the top layer of the seabed would tend to collapse into the trench formed by jetting with steep
gradient and the required armour rock protection cover cannot be achieved accordingly.
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The jetting method would result in extensive pollution in heavily contaminated ground. The jetting force
would cause transportation/movement of contaminated sediment and fine particle of contaminants from its
original locations to other parts of the Victoria Harbour which would be unacceptable and should be
avoided. The jetting method is thus not feasible in the project area.
2.8.1.2 Jetting (Injection)
The jetting injection method is similar to the jet trenching in that it is a post lay technique. The difference is
that the marine sediments are not excavated from the trench once they are fluidised. Instead the jetting
machine comprises two swords to cut into the marine sediments and then fluidise the marine sediments in
front of the machine and between the two swords. The pipelines are positioned between the two swords
and settle down into the space of the fluidised marine sediment. This technique may require more than
three passes to lay the pipelines at the required depth of 3m. The practicality and environment impacts of
jetting (injection) are similar to the Jetting (Trenching) method as described above.
2.8.2 Horizontal Directional Drilling
Horizontal Directional Drilling (HDD) is a method which takes the pipeline directly from start to end point by
underground drilling with no surface disturbance being necessary. A pilot hole will be drilled with fluid
pumping down the drill pipes for lubricating and stabilising the walls of the drillhole. After the full length pilot
hole is complete, the drill is replaced with a reamer which is pulled back or pushed forward in several
passes to enlarge the pilot hole to the required size. On completion of reaming, lengths of pipe are joined
and pulled from one side of the landing point to the other using a bonded pull. Finally, the gap between the
enlarged hole and the pipe string is grouted for fixing the pipe in position.
The potential for impacts to water quality from HDD is lower than dredging as sediments on the seabed
would not be disturbed. However, the drilling fluid would require treatment prior to discharge. As the risk
and difficulty for recovery in the event of jamming is considered to be very high, a large temporary
workspace is required for setting up a horizontal drilling rig and its ancillary equipment which is not
available at the land portion at North Point and there is potential conflict between the foundation of the
Island Eastern Corridor and the HDD operation as installation of protective measures to the pier foundation
is not feasible for the HDD operation and thus the risk of affecting the pier foundation can not be avoided,
this method is considered not a feasible option.
2.8.3 Trench Excavation
2.8.3.1 Dredging
For submarine utility installations, dredging involves the removal of marine sediments from the seabed to
form the trench, into which the pipelines are laid. Many dredging techniques, such as grab dredging, cutter
suction and trailer suction dredging are available and chosen depending on the prevailing engineering,
environmental and risks conditions e.g. shear strength of marine deposits, marine traffic impact etc. As the
submarine gas pipelines would be located across a number of Government Mooring Buoys, the Eastern
Quarantine and Immigration Anchorage and the Hung Hum and Eastern Fairways, grab dredging is
selected, as cutter suction and trailer suction dredging which requires a working area of over 150m in width
will result in unacceptable impact on marine traffic and are thus not feasible. Dredging by suction dredging
will also produce more marine sediment by volume (due to high water content) when compared with grab
dredging. Grab dredging is therefore the best practicable and feasible method to minimize dredging and
dumping requirements and demand for fill sources.
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The proposed submarine gas pipelines should be laid at a minimum depth of 3m incorporating a rock
armour protection layer or at a minimum depth of 5m should the trench be back-filled with original seabed
material in accordance with the requirement of CEDD/Port Works. Trench of minimum 3m covered with
armour rock would be adopted in the design in order to provide adequate protection to the pipelines and to
ensure safety. At the fairway, the designed minimum cover is at least 4m, with rock armour protection. The
dredging quantity is kept to a minimum of about 300,000m3 while the filling level at the seabed would match
to the existing seabed level. Hydrographical surveys of seabed levels would be conducted before and after
construction to ensure the original seabed level is restored.
Dredging can be a comparatively fast way to construct a submarine gas pipelines and is necessary in
areas where extra pipeline protection is required (e.g. rock armour protection). The excavated
contaminated sediments would require disposal off-site at a designated disposal ground.
2.8.4 Submarine Gas Pipelines Installation
2.8.4.1 Float and Sink Method
In the float and sink method, a pre-dredging survey would be undertaken to establish the mean seabed
level and determine the required trench level. Pipes would be welded together into a long pipeline that
would be floated and moored along the seafront. The long pipeline would then be towed and positioned
across the fairway, EQIA, government mooring zone, positioned above the dredged trench and than sunk
into position. Backfilling of the trench with armour rock would be carried out. This method is a common and
successful method, but it causes considerable impact to the marine traffic as the pipeline needs to be
floated across the fairway and a number of marine facilities and sunk into place. It is therefore not
considered suitable for the sections of the submarine gas pipelines across the fairway given the high
volume of marine traffic using the existing Hung Hom and Eastern Fairways.
2.8.4.2 Bottom Pull Method
In the bottom pull method, pipelines are typically pull out from shore along the seabed and into a pre-
dredged trench. Pipes are welded together in an onshore yard and a winch barge is moored/stationary
offshore. A pull-cable from the winch barge is brought to shore and attached to the first pipe string, which is
then pulled out to sea and into the pre-dredged trench. Careful control of the buoyancy of the pipelines and
the pulling forces ensure that the end of the pipelines are pulled along the seabed, rather than embedding
itself into the seabed. Once the first pipe string has been pulled out to sea, a second pipeline string is
attached to the end of the first pipe string and the second pull begins. This operation continues until the
pipelines reaches the winch barge. Backfilling of the trench with armour rock after pipe pulling would be
carried out. This method is suitable for pipelines with comparatively straight horizontal profile. This method
also poses less impact to marine traffic, which is a significant advantage over the “Float and Sink” method.
2.8.4.3 Lay Barge Method
In the lay barge method, while the work barge moves along the pipeline, the pipes are progressively added
to form a string, which are hung in a catenary from at the back of the barge, and are gradually lowered into
the pre-dredged trench. Due to limited capacity of work barge, additional marine plants are required to
transport pipes from the shore to the work barge throughout the mainlaying operation. Similar to the Float
and Sink method, the lay barge method would introduce intolerable marine traffic impact due to its long
suspended pipeline at sea during the installation. This method is considered not a favourable option for
pipeline crossing fairways.
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2.8.5 Best Practical Method
By comparing the pros and cons of the various construction methods as mentioned in Section 2.8.1 to
2.8.4, grab dredging followed by pipe laying i.e. “Bottom Pull” method across the fairway, “Float and Sink”
method near the landing point and either “Bottom Pull” or “Lay Barge” for other sections of the submarine
gas pipelines followed by protection of the submarine gas pipelines by backfilling are the most practical
construction method for the installation of the proposed submarine gas pipelines.
Based on the ground investigation and laboratory testing data, the seabed material within the works area of
the project consist of about 30% of contaminated sediment. The contamination level of the sediment along
the dredging area at Victoria Harbour was high in terms of heavy metals such as Cu, Ni, Zn, Pb, Hg and
high molecular weight Polycyclic Aromatic Hydrocarbons (PAHs) in which contaminated sediment (Type 3)
was located near the To Kwa Wan Typhoon Shelters and contaminated sediment (Type 2) was located
near both proposed pigging stations at North Point and To Kwa Wan which is extended up to about the
middle of proposed gas pipelines in Victoria Harbour. Based on ground investigation the sediment
immediately below the seabed comprises mostly very soft slurry deposit which are soft, fine, dark grey and
black, clay/silt (contaminated marine deposit), therefore the disturbance to the contaminated marine
sediment must be minimized. Jetting is not feasible for such marine sediment properties under the seabed.
The jet machine would eject plumes of fluidised soft fine slurry, clay and silt out of the sides of the machine
and these plumes of fluidised marine sediment would become entrained in the water column along the tidal
flow and cause adverse impacts on water quality and ecological sensitive receivers in the vicinity.
The identified very soft clay would require a slope gradient of about 1 in 3 on the sides of the trench for
stability. Additional and repeated passes would be required to achieve the required trench width and depths.
This would increase the volume of contaminated marine sediment being ploughed up and thus increase the
spread of highly contaminated substances. This also indicates that the jetting method would not be
effective in the soft mud and slurry ground conditions. Furthermore, the contaminated slurry on the sides of
the trench at the top layer of the seabed would tend to collapse into the trench formed by jetting with steep
gradient and the required armour rock protection cover cannot be achieved accordingly. The jetting method
would result in extensive pollution in heavily contaminated ground. The jetting force would cause
transportation/movement of contaminated sediment and fine particle of contaminants from its original
locations to other parts of the Victoria Harbour which would be unacceptable and should be avoided. The
jetting method is thus not feasible in the project area.
The proposed submarine gas pipelines should be laid at a minimum depth of 3m incorporating a rock
armour protection layer or at a minimum depth of 5m should the trench be back-filled with original seabed
material in accordance with the requirement of CEDD/Port Works. Trench of minimum 3m covered with
armour rock would be adopted in the design in order to provide adequate protection to the pipelines and to
ensure safety. At the fairway, the designed minimum cover is at least 4m, with rock armour protection. The
dredging quantity is kept to a minimum of about 300,000m3 while the filling level at the seabed would match
to the existing seabed level. Hydrographical surveys of seabed levels would be conducted before and after
construction to ensure the original seabed level is restored.
The assessment results, recommendations and conclusions have been addressed in this EIA report based
on the proposed construction techniques or methods.
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2.8.6 Installation Configuration
The proposed twin submarine pipelines can be installed in separate trenches or bundled together in one
common trench. The engineering and environmental implications of these two options are examined. From
an engineering perspective a bundle lay would result in a shorter overall construction period. The bundle
lay was considered, qualitatively, to result in less severe disturbances to the marine environment as it will
cause less direct and indirect disturbance to marine sediments and hence to water quality, marine ecology
and fisheries resources.
2.8.7 Sequencing and Timing
The issue of timing and sequencing has been analysed as part of the water quality impact assessment in
Section 3. Modelling has examined the impacts on water quality of undertaking the work in either the dry or
wet season. For the proposed dredging works, both seasons have been examined to be acceptable in the
sense that water quality, marine ecology and fisheries criteria are complied with.
2.9 Selection of the Preferred Option
The discussions presented in Sections 2.6 and 2.7 have examined the rationale behind the selection of the
preferred alignment and pigging stations locations, the preferred construction method and the issue of
timing. The environmental and physical constraints have been presented along with the preferred
alignment and pigging stations locations for the Project. The submarine gas pipelines alignment avoids
direct impacts to the coral areas while the pigging stations avoids the use of open space and direct impacts
to plantings and trees. The alignment presented on Figure 2.1, therefore, represents the preferred
alignment for the gas pipelines and pigging stations taking into account ecological, water quality and
marine traffic constraints.
By comparing the pros and cons of the various construction methods, the preferred construction method is
grab dredging followed by pipe laying. i.e. “Bottom Pull” method across the fairway, “Float and Sink”
method near the landing point and either “Bottom Pull” or “Lay Barge” for other sections of the submarine
gas pipelines followed by protection of the submarine gas pipelines by backfilling of armour rock.
This proposed alignment and construction methods for the gas pipelines have been studied in detail as part
of this EIA Report. The selection of this position was taken after a holistic review of the environmental
constraints (corals, plantings and trees), physical constraints (navigation channel), the results of the water
quality modelling and noise assessment exercise and the form and appearance of the proposed above
ground structures with visual mitigation measures.
2.10 Tentative Project Programme
The construction of the proposed Project is scheduled to commence in January 2012 for completion by
June 2014. The tentative project programme is given in Appendix A1.
2.11 Concurrent Projects
Concurrent projects with likely interaction with this Project are identified below and shown in Appendix A2.
The status of these concurrent projects is based on available information at the time of the submission of
this Report. It should be noted that the implementation of individual projects would be subject to the
ongoing review by relevant project proponents.
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Concurrent projects which would be implemented in the vicinity of the Project include:
� Dredging Works for Proposed Cruise Terminal at Kai Tak
� Wan Chai Development Phase II and Central-Wan Chai Bypass
� Shatin Central Link
� Trunk Road T2
� Providing Sufficient Water Depth for Kwai Tsing Container Basin and its Approach Channel
� Central Kowloon Route
Twin 400mm diameter steel submarine gas pipelines are currently aligned 235m west of and parallel to the
former Kai Tak Airport runway. It is noted that there is a potential for decommissioning/removal works of
these existing cross harbour gas pipelines. However, the decommissioning/removal program is still subject
to confirmation. It is not expected the marine works related to the decommissioning/removal for these
existing cross harbour gas pipelines would commence prior to commissioning of the new submarine gas
pipelines in 2012 as the new submarine gas pipelines and associated facilities must be in service before
the decommissioning/ removal of the existing cross harbour gas pipelines and associated facilities.
Potential cumulative impacts with the proposed submarine gas pipelines and associated facilities are
therefore not expected.