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2.
PROJECT DESCRIPTION
2.1
Background
2.1.1
The project is entitled “Improvement of Fresh Water Supply
to Cheung Chau” and is referred hereafter to as “the Project”.
2.1.2
Cheung Chau, with a population of
around 23,000, is currently supplied with treated water from Silver Mine Bay
Water Treatment Works on Lantau via two submarine water mains, 10” (about 250
mm) and 500 mm in diameter, across Adamasta Channel. Treated water is normally
provided by the 500 mm diameter main, with the 10” main serving as emergency
backup.
2.1.3
Laid
in 1963, the existing 10” submarine water main providing partial emergency back
up fresh water supply to Cheung Chau is approaching the end of its design life
of 50 years. Repairs would be
uneconomical and take a long time given the difficulty of the task and the
condition of the main. To improve the reliability of water supply to Cheung
Chau, it is necessary to strengthen the emergency back up by replacing the
existing 10” submarine water main with a new 500 mm diameter submarine water
main.
2.2
Location and Description of
the Project
2.2.1
The Project is located in the southeastern coast of Chi Ma Wan
Peninsula at South Lantau, in the northwestern coast of Cheung
Chau near Tai Kwai Wan and the Adamasta Channel.
2.2.2
The
Project is to construct and operate a new submarine water main across Adamasta
Channel from Lantau to Cheung Chau to replace the existing 10” submarine water
main, which is serving as emergency back up, to improve the reliability of
water supply to Cheung Chau. The Project will comprise the following:
(i)
Laying of a submarine water main of approximately 1,400
m in length and 500 mm in diameter across Adamasta Channel;
(ii)
Construction of landfall and associated works within Lantau South
Country Park,
Lantau Island; and
(iii)
Construction of
landfall and associated works near Tai Kwai Wan, Cheung Chau.
2.2.3
The location of the Project and the
associated works are shown in Figures 2.1 – 2.3. Details of the proposed landfall
locations are shown in Figures 2.4 – 2.5.
General Description of the Project Area
Cheung Chau Landfall
2.2.4
Cheung Chau is situated south-east of
Lantau and according to the 2001 Census has a population of 23,000. Cheung Chau
is the most densely populated outlying island. It is dumbbell-shaped with
vegetated knolls in the north and south and a narrow strip of flat land in the
central part which is bounded by a typhoon shelter to the west and Tung Wan to
the east. The central lowland contains the development core where most of the
existing village areas, commercial uses and major community facilities are
concentrated.
2.2.5
The northern part of the island is
hilly and largely undeveloped. A private low-rise residential development, a rural
public housing estate and infrastructure facilities serving the whole island
(such as refuse transfer station, sewage treatment works, abattoir and fresh
water pumping station) are the key features at the north-western promontory.
2.2.6
Regular licensed passenger ferry
service is the major means of public transportation for Cheung Chau. Ferry
services to Central, Mui Wo, Chi Ma Wan and Peng Chau are provided. Cheung Chau
enjoys a car-free environment with only a limited number of village vehicles (Plate 5), fire engines,
police vehicles and ambulance vehicles serving local needs.
2.2.7
The proposed landfall on Cheung Chau
would be constructed at a low headland in the southern end of Tai Kwai Wan (Plate 2), which is a sandy
bay situated at the northwestern coast of Cheung Chau
(Figure 2.2).
The site is currently occupied by two WSD’s works compound (Plates 3 & 4). The
villages of Tai Kwai Wan San Tsuen and Siu Kwai Wan are located close to the
proposed landfall site.
2.2.8
The proposed landfall and works area
on Cheung Chau fall on areas zoned “Government, Institution or Community”
(G/IC) on the approved Cheung Chau Outline Zoning Plan (OZP) No. S/I-CC/4.
2.2.9
After making landfall, the proposed
land-based main will be laid along Cheung
Kwai Road (Plate
3) and will be connected to the existing water main near Cheung
Chau Fresh Water Pumping Station (Plate
8).
Lantau Landfall
2.2.10
The proposed landfall site on Lantau
would be located at Ha So Pai, in the southeastern corner of Chi Ma Wan
Peninsula, Lantau (Figure 2.3).
It is a remote coastal bay at the southern boundary of Lantau South
Country Park.
The proposed landfall and temporary works area is located on a rocky shore (Plate 1) and falls on an
area zoned “Country
Park” on the approved
South Lantau OZP No. S/SLC/15. There are no villages or development in the
vicinity of the site except the landfall and associated waterworks of the existing
10” and 500 mm submarine mains supplying fresh water from Lantau to Cheung
Chau.
2.2.11
After making landfall, the proposed
land-based main will be connected to the nearby existing exposed main on Lantau
(Plate 7).
2.2.12
The site is not accessible by
vehicles, but can be reached by hiking the Lantau Trail via Chi Ma Wan or using
commercially-hired boats.
The Adamasta Channel
2.2.13
The proposed submarine main would be
laid from Cheung Chau to Ha So Pai of Lantau across the Adamasta Channel. The
Adamasta Channel, with water depth ranging from 5 – 10 m, is a narrow passage
between Chi Ma Wan Peninsula of Lantau Island
and Cheung Chau. The proposed submarine water main will be laid underneath the
seabed to avoid accidental damage by ship anchors, trawling and other
activities.
2.2.14
Beside the two existing 500 mm and
10” submarine fresh water mains, twelve submarine cables consisting of three
33kV and nine 132kV electricity supply circuits were laid across the Adamasta
Channel.
2.2.15
The Adamasta Channel is the main
traffic route for high speed ferry services between Hong Kong and Macau, several outlying island ferry services and river
trade vessels (Plate 6).
2.3
Need for the Project
Existing Conditions
2.3.1
Currently, potable water supply in Cheung Chau is provided by
the Silver Mine Bay Water Treatment Works in Lantau. After treatment, treated
fresh water gravitates from Silver Mine Bay Service Reservoir on Lantau to
Cheung Chau Fresh Water Pumping Station via two land mains and two submarine
mains. Water is then pumped from Cheung Chau Fresh Water Pumping Station to
three existing service reservoirs on Cheung Chau where it gravitates via
distribution mains to consumers on the island.
2.3.2
The land mains in Lantau were laid in the fifties and are
currently being rehabilitated/replaced under WSD’s Contract No. 2/WSD/07 –
“Replacement and Rehabilitation of Water Mains, Stage 2 – Mains on Lantau
Island East”, which is scheduled for completion in 2010. The two submarine
mains across Adamasta Channel, 10” and 500 mm in diameter, were laid in 1963
and 1986 respectively. Treated water is normally provided by the 500 mm
diameter main, with the 10” main serving as emergency back-up so as to maintain
water supply to Cheung Chau.
2.3.3
With the 10” submarine main in service for over 46 years and
approaching the end of its service life, repairs would be uneconomical and take
a long time given the difficulty of the task and the condition of the main.
Moreover, the supply through this 10” submarine main alone can only partially
meet the demand of Cheung Chau for treated water. To improve the reliability of
water supply to Cheung Chau, it is necessary to strengthen the emergency
back-up by replacing the existing 10” submarine water main with a new 500 mm
diameter submarine water main.
Need for the Project
2.3.4
The need for the Project has evolved from the requirement to
provide a secure water supply to Cheung Chau. Specifically, it has been determined
that the existing water main serving as emergency backup is approaching its
design life of 50 years. There will be increasing risk of having one water main
under maintenance while another main has to be taken out of service without
warning.
2.3.5
Being the only water supply to Cheung Chau, it is necessary
to lay a new submarine water main from Lantau to Cheung Chau to maintain the
reliability of water supply to Cheung Chau.
2.3.6
The maximum mean daily demand for Cheung Chau is about 9,500
m3/day in 2013 and beyond. The 10” submarine main alone (carrying
capacity 7,600 m3/day), is not sufficient to meet the projected
fresh water and flushing water demand. For replacing this 10” submarine main,
it is assessed that the proposed main, running at roughly the same alignment,
should be 500 mm in diameter so that it alone can cope with the projected water
demand for Cheung Chau.
Purpose and Objective of the Project
2.3.7
The purpose of the Project is to construct and operate a new submarine water main across
Adamasta Channel from Lantau to Cheung Chau to replace the existing submarine
water main, which is serving as emergency back up, to improve the reliability
of water supply to Cheung Chau.
Scenarios With and Without Project
Without Project
2.3.8
The existing 10” submarine water main in Cheung Chau was laid
almost 50 years ago and is approaching the end of its serviceable life.
Consequences of not proceeding with the Project will cause serious water supply
interruption, disruption of everyday life and businesses in Cheung Chau leading
to adverse inconvenience to the public and tourists in the event the current
water main is taken out of service unexpectedly.
With Project
2.3.9
The proposed Project will provide a secure and reliable water
supply to Cheung Chau.
Justification and Benefits of the Project
2.3.10
This Project is in line with the WSD’s objective to plan and
develop reliable and efficient treated water supply and distribution systems
for the territory.
2.3.11
The estimated population expected to be directly benefited by
the Project will be the residents of Cheung Chau of about 23,000 as well as the
tourists visiting Cheung Chau.
2.4
Consideration of Alternative
Options of Water Supply, Water Main
Alignments, Landfall Locations and Construction Methods
2.4.1
Various alternatives of improving the fresh water supply to
Cheung Chau, water main alignments, landfall locations as well as the
construction methods have been investigated to identify feasible options for
further detailed evaluation.
Alternatives for Improvement of Fresh Water Supply to Cheung
Chau
2.4.2
A number of alternatives for improving the adequacy and
reliability of water supply to Cheung Chau have been identified. These
included:
·
laying new water mains from Lantau to Cheung Chau;
·
constructing a desalination plant at Cheung Chau; and
·
rehabilitating the existing water main.
2.4.3
Other alternatives such as new water mains directly from
Lamma Island, Hong Kong Island or Kowloon to Cheung Chau are considered to be
not practical given the expected large distance separation, the associated
adverse environmental impacts and the high construction costs, therefore are
not considered further.
Laying New Water Mains from
Lantau to Cheung Chau
2.4.4
Currently, potable water supply in Cheung Chau is provided by
the Silver Mine Bay Water Treatment Works (WTW) in Lantau. Treated fresh water
gravitates from Silver Mine Bay Service Reservoir to Cheung Chau via two land
mains and two submarine mains (near Ha So Pai at Chi Ma Wan peninsula).
Currently the land mains are being rehabilitated/replaced under WSD’s Contract
No. 2/WSD/07 – “Replacement and
Rehabilitation of Water Mains, Stage 2 – Mains on Lantau Island East”, for
completion in 2010.
2.4.5
Possible options for laying water mains from Lantau to Cheung
Chau include new mains from the Chi Ma Wan peninsula coast and new mains
directly from Silver Mine Bay Water Treatment Works.
2.4.6
Both the current 500 mm and 10” water mains cross the
Adamasta Channel from Ha So Pai at the southern part of Chi Ma Wan peninsula to
Tai Kwai Wan in Cheung Chau. The Adamasta Channel is about 1.4 km wide and
feasible options include laying a new 500 mm diameter submarine fresh water
main or two smaller submarine fresh water mains in stages using conventional
open trench or tunneling methods.
2.4.7
Silver Mine Bay WTW is over 6 km from Cheung Chau. Due to
this large distance, either tunnel boring or submarine laying will be
necessary. Potential constraints on tunneling works near WTW include risk and
hazard from chlorine facilities at the WTW, spoil management and high
construction cost. Tunnel boring is also unsuitable for small diameter
pipeline, thus will not be considered further. The other feasible option is to
lay a new 10 km long pipeline directly connecting Silver Mine Bay WTW to Cheung
Chau. The main constraints are environmental, cost and marine traffic issues.
Desalination Plant in Cheung
Chau
2.4.8
An alternative to constructing new pipeline to Cheung Chau
would be to construct a reverse osmosis desalination plant on Cheung Chau. The reverse
osmosis (RO) process uses pressure to force pure water through semi-permeable
membranes that inhibit the passage of salts.
2.4.9
The most important limitations to the use of RO desalination
plant are the cost, high energy consumption, ultimate disposal of waste streams
and land availability. Based on the estimated water supply demand of 9,500 m3/day
for Cheung Chau, a permanent site of approximately 0.5 ha would be required. As
shown in Figure 2.6,
there are very little possible sites of suitable size in Cheung Chau for the
desalination plant.
Rehabilitating the Existing
10” Water Main
2.4.10
The existing 10” (~250 mm) submarine water main could be rehabilitated
by using slip lining, swagelining or cure in place pipe which entails inserting
a new pipe inside the old pipe and pushing and/or pulling it into place to
extend the serviceable life of the pipe.
2.4.11
Such option can preclude the need of laying new submarine
main. The completed main will, however, have a significantly smaller internal
diameter (about 150 mm) than the original pipe (about 250 mm). This main alone
would not be adequate to provide the necessary water supply to Cheung Chau.
2.4.12
Moreover, there will be high operational risk in the event
the 500 mm main is taken out of service unexpectedly while the 10” main (which
serve as emergency backup) is being rehabilitated leading unacceptable water
supply interruption to Cheung Chau.
Alternatives for Water Main Alignment
2.4.13
The length of the main from Chi Ma Wan to Cheung Chau (about
1,400 m) is within the bounds of possibility for HDD. As shown in Figure 2.6,
the alignment for Cheung Chau is constrained by natural coastlines (most of
which have been zoned as CPA), Tai Kwai Wan Archaeological Site and existing
submarine pipelines. The factors governing the water main alignments between
Chi Ma Wan and Cheung Chau are presented in Figures 2 & 3 of
Appendix 2.
2.4.14
It is recommended that the water main alignments should
follow the existing alignment from Ha So Pai at Chi Ma Wan peninsula to Tai
Kwai Wan at Cheung Chau as it is generally shorter and can be more easily
connected to the existing water main. For Cheung Chau, this alignment is close
to the Cheung Chau Fresh Water Pumping Station and can be benefited by shorter
length of land-main. There are also suitable landfall locations (evaluated in
the next section below) for this alignment.
2.4.15
The alignment of the water main at Lantau is constrained by
the nearby natural coastline and Lantau
South Country
Park (Figure 2.6).
To minimize potential impacts to Lantau South Country Park and natural
coastline, it is preferable to have the alignment near the existing exposed
land-main currently being rehabilitated to facilitate easy connection such that
works can be minimized as far as possible. The coastal area near the exposed
main is already partly disturbed by previous waterworks and therefore is more
suitable.
Alternatives for Landfall Location
2.4.16
As shown in Figure 2.6, the southern part of Chi Ma Wan
peninsula is located within the Lantau
South Country
Park. The coastline
consists of natural rocky and sandy shores. To minimize potential impacts to
Lantau South Country Park and natural coastline, it is preferable to locate the
landfall location close to the existing land-based main currently being
rehabilitated to reduce the works necessary for pipe connection. The proposed
landfall location at Lantau is located in a coastal area which has already been
partly disturbed by previous waterworks.
2.4.17
As for Cheung Chau (Figure 2.6), the northern portion is natural
shoreline zoned as Coastal Protection Area (CPA) while the central portion is all
built-up area. There are sandy shores near Tai Kwai Wan which has also been
zoned as CPA. Part of the sandy shore is also within the Tai Kwai Wan
Archaeological Site. The only feasible landfall location of suitable size for
works site is the headland at Tai Kwai Wan which is zoned as G/IC and has been
reserved for waterworks. The site is currently used as temporary works
compound. As the site has already been formed and highly disturbed, the
associated ecological impact is considered to be negligible. The site is also
close to the Cheung Chau Fresh Water Pumping Station, thus a shorter land-based
main for connection to the existing main.
2.4.18
For HDD works, a sufficiently sized flat area is required to
accommodate the drill rig and supporting equipment. In addition, the entry
point location also requires good access as well as stable ground conditions to
support heavy equipment. The headland at Tai Kwai Wan in Cheung Chau is the
only suitable landfall site.
2.4.19
Another important consideration for the launching site is the
availability of water supply for the drilling fluid and flushing of the water
main. The site at Cheung Chau is located near water supply and has available
flat area for the necessary storage and treatment.
Alternative Construction Methods and Sequences of Works
2.4.20
The common methods for constructing submarine pipelines
include the conventional trenching methods and horizontal directional drilling
method. Tunnel boring is generally for large diameter pipeline (1m or above)
and is therefore considered not suitable for this case.
Conventional Trenching
Methods
2.4.21
Conventional trenching involve forming a trench in the sea
bed, installing the pipe and subsequent backfilling. There are a variety of
techniques to form a trench and number of ways of installing the pipe (see Figure 4 of Appendix 2).
2.4.22
Trenching may be done by dredging or jetting and
the pipe can be installed by lay barge, bottom pull, or float and lower
methods. After the pipe has been placed in position, the trench can be
backfilled by bottom dump, tremie, or other methods. An alternative is to
simply lay the pipe on the sea bed with or without additional protection.
2.4.23
Grab or Suction Dredger. Dredging involves
the removal of marine sediment from the seabed to form a trench, into which the
pipeline is laid. Dredging by grab
dredger is commonly used in Hong Kong because
it uses relatively light and readily available equipment and is low cost. The
rate of dredging using a grab dredger is relatively low and leakage of dredged
material will have an impact on the quality of the surrounding water. Suction
dredging is faster, more environmentally friendly but higher in cost.
2.4.24
Jetting (Trenching). This uses water
jets to break-up, remove or liquefy the soil from under a marine pipeline
allowing the pipeline to settle to an elevation below the seabed. Jetting is a
post-trenching technique, which means the pipeline is laid on the seabed before
jetting is carried out. The marine sediment in front of the machine
is fluidised to allow the pipeline to settle into the sediment. The sediment to
be fluidized may be suspended in the water column and thus have an impact on
the water quality.
2.4.25
Jetting
(Injection). The injection
process is an improved post-trenching jetting method. The jetting machine uses two swords to cut
into the sediment and then fluidises the marine sediment in front of the
machine and between the two swords. The pipeline is positioned between the two
swords and hence settles down into the fluidised sediment. Depending on the
strength of the sediment it may require several passes of the machine to lower
the pipeline to the designated elevation. The technique limits the pipeline
protection layer within the width of the machine. Submarine pipelines requiring
a wider protection layer against damage from dragged anchors may require extra
trench dredging.
2.4.26
Lay Barge Pipe Installation. Sections of the
pipe are welded together on board a barge and the continuous length of welded
pipe is allowed to descend from the barge into the trench. The barge moves slowly along the pipe route
as new sections of pipe are welded together. The lay barge requires a
considerable distance ahead and behind to allow the pipeline to descend into
the trench. This can cause considerable problems in areas of busy marine
traffic and the method is probably not suitable for short crossings.
2.4.27
Bottom Pull Method of Pipe Installation. Pipes are assembled into a complete string
or strings in which the first string is pulled into the trench by a winch on
the opposite side (or on a barge) and
subsequent strings are jointed sequentially to the trailing end by welding as
the pipeline is advanced along its alignment. This method typically applies to
steel pipes, which are installed empty and then filled with water. The pipes have a concrete weight coat to make
them just sink when empty and this fine balance of weight and buoyancy helps keep
pulling forces low. Where there is not
enough space, the pipes can be joined and pulled pipe by pipe or, if required,
space can be formed by temporary reclamation or from a temporary jetty or a
moored flat top barge with launching ramp and welding facilities. The pipe will be towed along the sea bed from
the launching site to the receiving site.
2.4.28
Float and Lower. The pipe string
is welded together and towed floating to the installation location. It is then lowered into position onto the sea
bed by filling it or by filling attached buoyancy tanks.
2.4.29
Conventional trenching methods are not preferable as it has
the potential to cause deterioration in water quality with subsequent secondary
impacts on marine ecology and in particular to fisheries. This is because
the dredging activities will disturb the seabed and the water column,
dispersing the sediment which will spread in to the wider water body increasing
suspended solid (SS) concentrations and depleting the dissolved oxygen (DO).
The sediment may be contaminated and this will impact on nearby sensitive
receivers such as fish and other marine organisms. During subsequent
deposition, the sediment may smother plants and animals living on the sea bed.
Horizontal Directional
Drilling Method
2.4.30
Horizontal
Directional Drilling (HDD)
involves drilling a pilot hole, typically 100 to 150 mm in diameter, and then
progressively enlarging the hole, using reaming tools in steps of 200 to 300
mm, until the required diameter is achieved (1.3 to 1.5 times the outside diameter
of the pipe). Reaming is either done in the reverse direction to the pilot
boring or in the same direction (forward reaming). Once the reamed hole has
been fully formed, the pipeline is pulled and/or pushed into the reamed hole.
HDD has developed over the past 20 years and drill lengths in excess of 2,000 m
and diameters of 1,500 mm or even larger have now been installed - though not
in this combination. The combination of length and diameter for this project is
therefore well within the bounds of possibility for HDD but will nevertheless
require careful engineering and qualified contractors. The drilling process utilizes
drilling fluid (bentonite and/or polymer) to remove the cuttings from the bore
and in soft ground to stabilize the hole during construction.
2.4.31
Construction
of any submarine pipe line will have environmental impacts but compared with
conventional trenched methods, the environmental impacts associated with HDD
are likely to be significantly less as the pipes are installed at considerable
depths below the seabed. Impacts will typically be limited to the entry and
exit portals and along the alignment of any pipe strings during construction.
2.4.32
A
particular concern with HDD is the potential for inadvertent loss of drilling
fluid either through break out through the overlying soils or at either end of
the drill path. However, such impact can be mitigated by proper drilling fluid
management and provision of adequate drilling fluid recovery, containment and
treatment systems.
2.4.33
A
detailed comparison of HDD and conventional laying method is presented in Figure 5 of Appendix 2.
Alternative Options Considered
2.4.34
From the above discussed alternatives, the following options
are evaluated further in Table 2.1.
Option 1 – construct a new 500 mm
diameter submarine fresh water main from Chi Ma Wan peninsula to Cheung Chau
using HDD method.
Option 2 – construct two smaller
diameter (about 300 mm) submarine fresh water mains in stages from Chi Ma Wan
to Cheung Chau.
Option 3 – construct a submarine
fresh water main directly from Silver Mine Bay Water Treatment Works to Cheung
Chau using conventional trenching method.
Option 4 – construct a reverse
osmosis desalination plant in Cheung Chau.
Option 5 – rehabilitate the existing
10” water main to prolong its serviceable life.
2.4.35
The environmental benefits and dis-benefits of each option
have been evaluated in particular to the potential impacts to water quality,
marine ecology and fisheries.
2.4.36
Other factors or constraints such as construction, long term
operational and maintenance, marine traffic, cost issues have also been
considered in the evaluation.
Recommended Option
2.4.37
Current evaluation indicates that Option 1 – a single 500mm
diameter polyethylene (PE) pipe within a steel casing from Chi Ma Wan peninsula
to Cheung Chau installed by Horizontal Directional Drilling (HDD) as the
recommended option for the alignment across Adamasta Channel. After making
landfall, the land-based water main (500mm diameter ductile iron (DI) pipe)
will be laid underground using conventional open cut method.
2.4.38
The technology and process of HDD is briefly described below.
Horizontal Directional Drilling (HDD)
Introduction
2.4.39
Horizontal Directional Drilling (HDD) is a form
of trenchless technology. It is a technique for installing pipes or utility
lines below ground using a surface-mounted drill rig that launches and places a
drill string at a shallow angle to the surface and has tracking and steering
capabilities. The equipment and procedures are intended to minimize surface
damage, restoration requirements, and disruption of vehicular or maritime
traffic with little or no interruption of other existing lines or services*.
* ASTM
Standard F1962-05 Standard Guide for Use of Maxi-horizontal Directional
Drilling for Placement of Polyethylene Pipe or Conduit under Obstacles,
Including River Crossings.
Advantages of HDD
2.4.40
The main advantages of HDD are:
·
The technology offers maximum depth of cover
under the obstacle, thereby, affording maximum protection and minimizing
maintenance cost.
·
Traffic is not interrupted, as most of the work
is confined to both works area only.
·
HDD has a predictable and short construction
schedule.
·
Needs little or no excavation and hence little
or no restoration is required.
·
In many cases, HDD is less expensive than other
methods.
·
Has least environmental impacts.
·
Reduces public inconvenience from road /
navigation fairway closure.
2.4.41
The main disadvantages or limitations of HDD
are:
·
The feasibility of HDD is dictated by the length
of the bore hole to be drilled, the diameter of the pipe string, as well as the
subsurface soil / bedrock conditions.
·
It requires a suitably sized working area to
accommodate drilling rig, pipes and other auxiliary equipment.
·
Constraint by site geometry and topography.
·
Potential water quality impacts from spillage or
escape (frac-out) of drilling fluids.
·
Requires specialized equipment and contractor in
undertaken the works.
Process
2.4.42
The method comprises a three stage process
wherein first stage drills a pilot hole on the designed path and the second
stage enlarges the hole by passing a larger cutting tool known as reamer. The
third stage places the product or casing pipe in the enlarged hole. The
elements of a HDD installation are:
·
A rig, which provides the physical means –
thrust and torque, to open the hole and pull in the product.
·
A transmitter / receiver system for tracking the
location of the bore.
·
The down-hole equipment – drill pipe, drill
bits, and reamers, which converts the physical properties of the rig to open
the hole and pull in the product.
·
The drilling fluid, which serves to stabilize
the hole, cool the down-hole equipment, and remove the cuttings from the hole.
·
The drilling fluid delivery, recovery and
containment systems, made up of tanks, mixing systems, pumps and, when
recycling fluids, a system of screens, filters, shakers, cones, etc. to
separate cuttings from the fluid so that it can be reused.
2.4.43
The typical arrangement and procedures for
undertaking a HDD project are described below.
* HDD
Consortium (2001) Horizontal Directional Drilling, Good Practices Guidelines.
Equipment Set-up
2.4.44
The drill rig is off-loaded and positioned over
the bore centerline a sufficient distance behind the entry point to allow for
the carriage height and entry angle, such that the drill bit will enter the
ground at the correct location and angle. Depending on the rig size and entry
angle, the distance may be 1 to 6 m. The entry angle is usually between 8 and
16 degrees. To make the drill string entry easier, a small pit is usually
excavated over the entry point. The pit facilitates entry of the bit at the
proper angle and helps contain the drilling fluid. Another pit is usually
excavated at the exit point to facilitate containment of drilling fluids once
the pilot bore reaches the exit point.
Drilling Fluid
2.4.45
Drilling fluid is composed of a carrier fluid
(water) and drilling fluid additives (bentonite and/or polymers). Bentonite is
a naturally occurring clay mineral (montmorillinite) that forms a mud when
mixed with water.
2.4.46
The functions of drilling fluids used in HDD
are:
·
Transporting drilling cuttings to the surface by
suspending and carrying them in the fluid stream flowing in the annulus between
the bore wall and the drill pipe/product.
·
Cleaning off build-up on drill bits or reamer
cutters by directing fluid streams at the cutters.
·
Cooling the downhole tools and electronic
equipment.
·
Lubricating to reduce the friction between the
drill pipe/product and the bore wall.
·
Stabilizing the bore and exerting a positive
hydrostatic pressure against the bore wall.
2.4.47
Mixing systems are used to mix the proper
drilling fluid additives with water to create a drilling fluid mixture suitable
for the local geological conditions.
2.4.48
Holding (frac) tanks for pre-mixed volumes of
drilling fluid are used mainly during large diameter and mud motor
applications. They are also used to temporarily store drilling fluids prior to
disposal.
2.4.49
Cleaning systems are used for removal of
cuttings and recycling of drilling fluids.
2.4.50
The recycling process begins with a mud pump
delivering drilling fluid under high pressure from the holding tank to the
drill head nozzle. Drill cuttings are then transported with the drilling fluid
into a mud tank for temporary storage. A submersible pump delivers the drilling
fluid and slurry with cuttings to a shaker. The shaker helps to separate large
cuttings for washing, storage and subsequent removal. Lighter density liquid is
then transferred to desander/desilter where finer solids are separated for
storage and removal. After removal of cuttings, the drilling fluid is pumped to
the holding tank ready for conditioning and reuse. Wastewater generated during
the recycling process, from the cleaning systems along with wastewater from
wheel washing facility and site runoff will be treated (e.g. sedimentation) and
will be reuse for washing within the site or re-circulated back to the bore
hole. No discharge of wastewater during the drilling fluid treatment and
recycling process is expected.
Pilot Bore
2.4.51
Prior to drilling, the drilling fluids should be
mixed and sufficient quantities should be available to complete the pilot bore,
pre-reaming, and reaming passes.
2.4.52
The pilot bore is drilled along the planned
alignment from entry to exit. Another pit is usually excavated at the exit
point to facilitate containment of drilling fluids and entry of the pipe during
pullback operations.
2.4.53
The type of drill bit used will vary depending
on ground conditions and contractor preferences.
2.4.54
The drill head is tracked by monitoring an
electromagnetic signal transmitted from the transmitter mounted in the drill
head to the receiver at the surface via a wireline or wireless non-walkover
system. The drill locator determines drill head location and calculated depth,
drill head inclination, or tool face angle, and orientation of the slanted
face. The drill locator provides the tracking information to the driller or
provides instructions to the drill rig operator for steering.
Reaming / Hole Enlargement
2.4.55
If the pilot bore is to be enlarged, a reamer
hole opener is attached to the drill pipe and the drill pipe is pressurized to
ensure the jets are open. The reamer is then rotated and pulled (or pushed in
some cases) back through the pilot bore to enlarge the bore in one or more
reaming passes. The number of reaming passes depends on the diameter of the
product compared to the diameter of the pilot bore, ground conditions and
contractor preferences.
2.4.56
Reaming can be accomplished by pulling or
pushing the reaming tool through the hole. The final bore diameter must be
larger than the product diameter to reduce frictional pullback loads and to
facilitate flow of drilling fluids around the product. As a rule of thumb, the
final bore diameter should be 1.5 times the diameter of the product.
Pipe Layout, Fabrication and Testing
2.4.57
The product is usually prepared for installation
while the bore is being reamed.
Pullback
2.4.58
During pullback, a pulling head is attached to
the product. A swivel is installed between the reamer and the pulling head to
prevent rotation of the product.
Connections
2.4.59
The new product is finally connected to existing
pipes.
Demobilization, Site Cleanup and Restoration
2.4.60
After the product is installed and
satisfactorily tested, the entry and exit pits are cleaned of drilling fluids
and cuttings and backfilled with native soil or selected backfill. Bentonite
slurry is generally reconditioned and reused wherever practicable. In the case
of this Project, the drilling fluid (bentonite) will be reuse in the borehole
throughout the drilling process. Spent bentonite will be dewatered and
transported offsite to public fill reception facilities for final disposal
after completion of drilling works.
Site Area
2.4.61
The recommended works area requirement for large
scale HDD works is typically about 45 m x 75 m.
Preferred HDD Works for this Project
2.4.62
Marine geophysical survey and ground investigation survey
were conducted to study the existing geological data and to assess the
subsurface conditions and characteristics likely to be encountered during the
drilling process. As shown in Figure 2.7, the underground conditions of the drill
path are considered to be geotechnically feasible for HDD. HDD technique,
consisting of entirely land-based drilling works, will be used to lay the
submarine water main across Adamasta Channel within bedrock layer. No dredging
or marine works is required.
2.4.63
The length and diameter of the proposed main is
also within the bounds of possibility for HDD. The bore diameter of 900 mm is
recommended as the final bore size. To avoid / minimize adverse environmental
impacts to Lantau
South Country
Park, it is recommended
that the main staging area and the launching site be confined to Cheung Chau.
The proposed site at Tai Kwai Wan in Cheung Chau is of sufficient size for
large scale HDD works. In order to avoid welding together a long pipe string on
the Lantau side, the pipe should be welded on the Cheung Chau side and pulled
in by using a winch located on Lantau side and/or by pushing pipe in from Cheung
Chau side, or both. The recommended procedures are described below (Figure 2.8):
i.
The pilot hole will be drilled from Cheung Chau
side to within about 50 m of exit.
ii.
The bore will then be enlarged in stages to the
required diameter using forward reaming.
iii.
All drilling fluid recycling equipment will be
on Cheung Chau.
iv.
The reamed hole will then be totally cleaned by
flushing with water until the returns are totally clear.
v.
The last section will be drilled using water
only instead of drilling fluid so that any fluid exiting from the Lantau side
of the bore will be just water.
vi.
The pipe will be welded up in sections as it is
pushed in from Cheung Chau side or pulled from the Lantau side using a winch,
or both.
2.4.64
A schematic layout of the proposed launching site at Cheung
Chau is shown in Figure 2.4.
The work site has been adjusted to retain a number of trees. The site is about
2,340 m2 and can be separated into clean and dirty working areas.
The clean area will be used to accommodate the site offices and storage of
water pipes. The dirty area will be the main area for the drilling works
consisting of the drilling rig, entry pit, drill pipes storage, holding tanks,
drilling fluid recovery, recycling and containment systems, stockpiling area
and wastewater treatment facilities. Concrete bund will be constructed around
the entire dirty area to contain and divert polluted site runoff to wastewater
treatment tank for treatment. Concrete bund will also be provided around the
entry pit to provide additional tier of containment to prevent spillage of
drilling fluid.
2.4.65
A schematic layout of the reception site at Lantau is shown
in Figure 2.5. A small exit pit of 2 m x 3 m will be
excavated during the final boring process with water as the drilling fluid.
Concrete bund will be provided around the exit pit to prevent spillage of
drilling fluid. A temporary working platform (8 m x 4 m) using steel decking or
wooden scaffolding will need be to be constructed to provide a suitable working
area in retrieving the water main and connection. Such temporary working
platform can be easily erected and dismantled without the need for site
formation hence minimizing impact to the surrounding rocky shore habitat.
2.4.66
It is estimated that approximately 980 m3 of
drilling fluid (bentonite) will be required for the Project. Most of the
drilling fluid will be below ground level within the bore, while portion of the
drilling fluid will be stored in the holding tank and mud tank above ground in
Cheung Chau.
Sterilization of Water Main Prior to Commissioning
2.4.67
All new fresh water mains must be cleaned and
sterilized before being put into operation. Typically, water mains are
sterilized by chlorination. The purpose of chlorination is to disinfect the
water main, resulting in an absence of coliforms as confirmed by laboratory
analysis, before they are placed in service. Chlorine solutions are normally
used as the sterilizing agent.
2.4.68
The cleaning and sterilization procedures of
fresh water mains according to the General
Specification for Civil Engineering Works Volume 2, 2006 Edition (Section
22.73) and WSD Departmental Instruction No. 805: Mainlaying - Cleaning and Sterilization of Fresh Water Mains are
listed below. Notwithstanding the procedures below, the Contractor should also
adhere to the Project contract document with regards to sterilization of the
water main.
1.
All extraneous materials should be removed
before the fresh water pipeline is laid and connected.
2.
The pipeline should be filled slowly with water
and tested to the required pressure.
3.
After the pipeline has been successfully
pressure tested, it should be cleaned internally and flushed through with
potable water. For long length of small diameter mains which are inaccessible
for cleaning (size below DN600), swabbing should be carried out to remove dirt
and materials inadvertently left in the pipeline.
4.
The pipeline should then be completed filled
with water that has been dosed with a homogeneous solution of sterilising
chemicals (e.g. chloride of lime) such that the final concentration of free
chlorine in the water is at least 30 ppm. The water should be left in the
pipeline for at least 24 hours.
5.
After the 24-hour period, the pipeline should be
drained down and the sterilizing water should be flushed out using potable
water until the remaining chlorine level in the pipeline is less than 1 ppm.
Bacteriological and chemical samples should be taken and submitted to the
Waterworks Chemist for analysis in accordance with the contract requirements.
6.
To minimize or avoid any possibility of
contamination, sterilization of the pipeline should take place not more than 7
days before putting it into operation.
2.4.69
Approximately 590 m3 of water will be
required to clean and flush the completed water main. Any solids flushed out
will be settled out through the sedimentation tank. Approximately 295 m3
of chlorinated water will be used to sterilize the water main.
2.4.70
If high chlorinated water reaches an aquatic
system, it has the potential to kill fish and other aquatic organism because of
the chlorine concentration. One mode of action is likely through damaging the
gills, thus preventing the fish from breathing. Therefore, it is necessary to
dechlorinate the water in order to make it safe for discharge to the
environment.
2.4.71
Currently, sodium bisulfite, sodium sulfite and
sodium thiosulfate are most frequently used for dechlorination. Sodium
thiosulfate is preferred since it is less hazardous and consumes less oxygen
than sodium bisulfite and sodium sulfite. The Contractor is required to submit
for the Engineer approval, details of the processes and chemicals including
dosage to be used in the chlorination and dechlorination of the sterilizing
water.
2.4.72
The sterilization water must be treated to the
relevant discharge requirement stipulated in “Technical Memorandum on Standards
for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal
Waters” (TM-DSS) before discharging. With the implementation of appropriate
treatment such as dechlorination to control water discharge from sterilization
of water main, adverse impact on water quality is not expected. Monitoring of
the total residual chlorine concentration at the discharge point is
recommended.
Site Access
2.4.73
The remote coastal bay of the proposed landfall location in
Lantau is located within the Lantau
South Country
Park. Neither road nor
pier is found within or in the vicinity of the site. Heavily vegetated natural
terrain was found behind the works area. A small access track via Cheung Sha
Wan can be found along the alignment of the existing land-based water main. To
avoid potential environmental impacts on Lantau South
Country Park,
no haul road will be constructed. Small commercial boats or helicopter may be
the transportation to send the workers, construction equipment and materials to
the site.
2.4.74
Access to Cheung Chau will rely on marine transportation.
Construction equipment and materials could be offloaded along the Pak She Praya Road
and transported via village vehicles along Cheung Kwai Road to the site or directly
offloaded at the marine frontage of the Cheung Chau works area subject to approval
from relevant authorities.
2.5
Implementation Programme
2.5.1
The design phase of the Project is scheduled to
commence in late 2010 to late 2011. The implementation programme for the
proposed works is anticipated to commence in early 2012 for completion in early
2014 for approximately 24 months. Detailed construction programme is not
available at this current stage.
2.6
Project Interfaces /
Concurrent Projects
2.6.1
According to information provided by various works
department, only the project below is
likely to have potential cumulative impact with this Project.
·
Agreement No. CE 31/2007 (DS) – Outlying Islands
Sewerage, Stage 2 Upgrading of Cheung Chau and Tai O Sewage Collection,
Treatment and Disposal Facilities (PWP Item 354DS).
2.6.2
This DSD project consists of construction of village sewers
in Tai Kwai Wan San Tsuen and other unsewered villages in Cheung Chau. The
tentative programme is 2013 – 2017. A short section of about 30 m of the sewer
alignment is located where the land-based water main will be laid and connected
to the existing land main. Close liaison will be undertaken with the project
proponent of the interfacing project to avoid concurrent works as far as
possible. Alternatively, feasibility of entrusting the works or sharing a
common excavated trench during laying of the water main and sewer will be
explored to minimize the nuisance to the public from repeated excavation. By
avoiding concurrent works and given the short section of the interfacing works
and the small scale works involved, adverse cumulative environmental impact is
not expected.
2.6.3
Notwithstanding, continuous liaison will be
conducted with all works department and utilities companies to ensure adequate
phasing with each concurrent projects is taken into consideration, if
interfacing does eventually occurs.
2.7
Scope of Works during
Operational Phase
2.7.1
The completed water main is only to convey treated water from
Lantau to Cheung Chau. No maintenance of the water main is necessary during the
operational phase.
2.8
Continuous Public
Involvement
2.8.1
The process of Continuous Public Involvement (CPI) through
public consultation have been undertaken during the EIA study.
2.8.2
The stakeholders included relevant Green Groups, Rural
Committees, village representatives, Island District Council and local
fishermen / trade association have been or are being consulted.
2.8.3
The preliminary design was reviewed and revised according to
the views and comments received. The main comments and recommendations are
summarized in Table 2.2 below.
Table 2.2
Summary of Public Consultation
|
Relevant
Parties
|
Process
|
Main
Comments / Recommendations
|
Responses
/ Outcomes
|
|
Green Groups
· Conservancy Association (CA)
· Friends of the Earth (FoE)
· Green Lantau Association (GLA)
· Green Power (GP)
· Hong Kong Bird Watching Society (HKBWS)
· Kadoorie Farm and Botanic Garden
(KFBG)
· World Wide Fund for Nature Hong Kong (WWF)
|
Consultation paper (9 January 2009)
Meeting (8 May 2009) attended by
KFBG and WWF
|
· Express concern on site practices
and site runoff
· Recommend using existing boulder to
shield the view of the exposed water main at Lantau to minimize visual impact
· Noting the works are mostly on
disturbed land, they expressed no particular concern on the ecological
impact.
|
· Propose two tiers of containment to
contain and divert polluted site runoff away from the sea
· Propose tanks of adequate number
and sufficient capacity to contain and treat site runoff
· Propose using boulders and rocks to
shield view of the exposed water main
|
|
Island District Council member (Ms.
K.C. Lee, MH) and local mariculture/fishermen association
|
Meeting (28 August 2009)
|
· Fishermen association recalled
unpleasant experience from previous dredging / reclamation works and expressed
concern about polluted marine water impacting the nearby fish culture zone
· Suggest having monitoring team to
check the water quality and to ensure the mitigation measures are implemented
properly.
· To have close liaison with the
fishermen
· No adverse objections in principle
to the Project
|
· The Project will adopt HDD, no
dredging or marine works will be required.
· Propose two tiers of containment to
contain and divert polluted site runoff away from the sea
· Propose tanks of adequate number
and sufficient capacity to contain and treat site runoff before discharge
· Implement water quality monitoring
and audit programme to ensure no adverse impact to marine waters during
construction
|
|
Island District Council (Tourism, Agriculture, Fisheries & Environmental
Hygiene Committee)
|
Meeting (21 September 2009)
|
· Some member raised question about
potential noise during construction.
· All members supported the Project
|
· Adequate measures including use of quiet
plant, erecting temporary noise barrier and good site practices have been
recommended in the EIA. Noise monitoring is proposed to ensure no adverse
noise impact will arise during construction.
|
2.8.4
Overall, no adverse comments were received from Green Groups,
mariculture/fishermen association and District Council.
2.9
Illustrative Materials for
Landscaping Measures
2.9.1
According to Item 3.4.7 of the EIA study brief,
“The Applicant shall include in the EIA
report the design proposals, covering appearance, façade treatment, colour
scheme and texture of materials used, etc., of the landfalls and the
landscaping design measures at the landfalls locations. Annotated illustrative
materials such as computer-generated photomontages, oblique aerial photographs,
photographs, plans, elevation and section drawings shall be used as appropriate
to illustrate the visual/aesthetic effects of the landfalls.”
Cheung Chau
2.9.2
Based on the preliminary design, the landfall at Cheung Chau
will be an underground chamber, approximately 2 m x 3 m, to accommodate an
isolation valve and an air releasing valve. The proposed submarine main will be
connected to the chamber at the landfall 2 to 3 metres below ground level. The
underground chamber will be made of reinforced concrete. The chamber will be
covered with precast concrete slab flushed with the ground level and ductile
iron covers for the openings. The remaining parts of the landfall will be
reinstated to its original condition after construction. It should be noted
that the landfall location is currently zoned as G/IC and reserved for several
waterworks sites.
2.9.3
After making landfall at Cheung Chau, the
proposed water main will be laid along Cheung Kwai Road and connect to the
existing main near Cheung Chau Pumping Station. The proposed main (500 mm
diameter ductile iron water pipe) will be entirely underground. The works area
will be reinstated to its original condition and ground profile after
construction (i.e. concrete footpath).
2.9.4
The existing view of the landfall location and
land main at Cheung Chau and the photomontage showing the completed water main
is shown in Figure 2.9.
Lantau
2.9.5
The landfall at Lantau will be an underground chamber,
approximately 2m x 3m, to accommodate an isolation valve and air release valve.
The proposed submarine main will be connected to the chamber at the landfall 2
to 3 metres below ground level. The underground chamber will be made of
reinforced concrete. The slab will be flushed with ground level as far as
possible, with precast concrete slab covers and ductile iron covers for the
openings.
2.9.6
After making landfall at Lantau, the proposed
water main will connect to the existing exposed land main currently being
rehabilitated. The proposed main will be 500 mm diameter ductile iron pipe.
2.9.7
To minimize potential visual impact of the
chamber and exposed main, natural materials such as boulders / rocks sourced
from nearby area will be used to shield and to blend in with the surrounding
coastal environment. The locations where the boulders / rocks will be collected
and placed as well as the method statement should be proposed by the Contractor
for verification by the Environmental Team Leader and approval by the Engineer
to ensure no adverse ecological or visual impacts will arise from the removal
and placement.
2.9.8
In general, the boulders / rocks to be removed
should preferably be located above the high water mark to avoid impact on
intertidal organisms. No vegetation should be disturbed during collection. The
size should be in the range of 200mm – 400mm for safe and easy handling
withoutThe existing view of the landfall location and land main at Lantau and
the photomontage showing the completed water main is shown in Figure 2.10.
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