2. project description - epd.gov.hk · existing seawall at ma tau kok (mtk) and a gas pigging...

237926/ENL/ENL/33/D June 2010 P:\Hong Kong\INF\Projects2\237926 Twin 400 Gas Main\Environmental\MM Report\EIA\Final\2nd\EIA_Rpt_RevD_Ch 2.doc

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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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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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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




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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.

2. project description - epd.gov.hk · existing seawall at ma tau kok (mtk) and a gas pigging...

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