In accordance with Clause 3.3 of the EIA Study Brief (ESB-208/2009), the following section presents a consideration of
the alternatives for the Project.
The section has been divided into a discussion of the following:
·
Consideration
of Alternative GRS Locations;
·
Consideration
of Alternative Construction Methods for the Reclamation;
·
Consideration
of Alternative Installation Techniques for the Submarine Gas Pipelines; and
·
Consideration
of Works Sequences.
Based on the above considerations, the Environmental
Impact Assessment (EIA) of the preferred scenario is presented in subsequent
sections.
2.1
Consideration of
Alternative GRS Locations
This section presents considerations of the potential
locations of the GRSs that have been assessed as part
of this Project. It represents the
outcomes of a preliminary screening study undertaken by CAPCO to investigate
the engineering and technical feasibility of constructing and operating a GRS
at selected potential locations.
The assessment considers both the benefits / difficulties of the
construction and operation of the GRS as well as some of the associated
potential environmental impacts.
2.1.1
Potential Locations
Four potential locations of the GRS were considered
in CAPCO’s preliminary screening study:
·
Site
A on the western side of the West Ash Lagoon;
·
Site
B on a reclamation adjacent to the existing GRS compound at BPPS;
·
Site
C on an area east of the existing GRS;
·
Site
D within the existing GRS compound at BPPS.
Locations of these sites are depicted in Figure
2.1. A summary description
of each location is provided in Table 2.1.
Table 2.1 Description
of Potential GRS Locations
|
Potential Location |
Description |
|
Site A |
Site A
comprises a strip of the western side of the West Ash Lagoon, which is near
the existing GRS and presently accessible from the power station site via the
GRS eastern boundary road. The
main vehicular access is via Lung Kwu Tan road,
which also provides access to the WENT Landfill site. |
|
Site B |
Site B
comprises a new reclamation adjacent to the existing GRS that would be an
extension of the existing reclamation on which the existing GRS stands. The site is presently accessible via
the existing power station |
|
Site C |
Site C comprises
an area at the north eastern end of the existing GRS that would be an
extension of the existing compound on which the existing GRS stands. Much of the site area is presently
natural hillside that would have to be cut to create a flat area for the
GRS. The site is presently
accessible via the existing power station |
|
Site D |
The existing
GRS features three areas that do not presently have any equipment or piping
installed. Additionally, the slug
catcher is redundant and could be removed, creating more space area on the
south side of the pipe rack. Work
previously done for the HKLNG project showed there is sufficient space for
one additional pipeline terminal and GRS. The site is presently accessible from
the power station site via the existing power station |
2.1.2
Evaluation Criteria
The evaluation criteria adopted in the preliminary
screening study of GRS locations included:
·
Site location:
consideration is given to factors including pipeline shore crossing, fuel gas
header route, constructability, operability and impact on existing operations;
·
Available
space for GRS facilities;
·
GRS
facilities layout options and flexibility;
·
Capacity
for future expansion; and
·
Potential
concerns/ constraints, such as safety, risk and health issues, environmental
issues, financial impact, schedule impact and commercial/ regulatory concerns,
etc.
2.1.3
Evaluation Outcome
Evaluation outcomes are summarised in Annex
2A-1. The preliminary
screening study indicated the following key issues:
·
Site A is not preferred due to
uncertainties in land acquisition and permitting / gazettal issues hence insurmountable
schedule impact: the
construction of a GRS at this site will involve the decommissioning of the West
Ash Lagoon (or part thereof) which in itself is a Designated Project under the EIAO and will require an EIA of
extensive scope. Inclusion of such
assessment in the present EIA is anticipated to lead to a delay of at least 12
months, which cannot be accommodated in the target schedule of replacement gas
supply in 2012. In addition, it is
known that the HKSAR Government wishes to resume the land occupied by the West
Ash Lagoon for use by the
·
Site C is not preferred due to the need
for vegetation and slope cutting for site formation: this site is presently on undeveloped land and the
construction of a GRS at this site will involve the need to conduct an
environmental assessment covering different technical issues to this EIA, e.g.
one that considers aspects such as terrestrial ecology, land excavation and
associated waste management.
Acquisition of the land could be a complex process because of its
current natural state. Also site
formation would necessitate clearance of all vegetation and aggressive slope
cutting, which are less favourable from an environmental perspective and cannot
be accommodated in the target schedule of replacement gas supply in 2012.
Sites D and B therefore represent the preferred
options. Given the space
requirement of two proposed GRSs, none of these two
sites is ideal for accommodating both GRSs, and thus
it is recommended to accommodate one GRS at each site. This would also reduce the extent of
seabed reclamation at Site B to the maximum practicable extent since only one
GRS will be located on newly reclaimed land.
Of the two options under Site B, Site B1 is preferred
over Site B2 since it is situated exactly on the location proposed in the HKLNG
EIA ([1])
for which an Environmental
Permit has already been granted and hence may represent few potential
uncertainties (i.e. from an environmental, engineering and permitting
perspective).
Sites
D and B1 are thus regarded as the preferred scenario for locating the GRSs in this EIA.
2.2
Consideration of
Alternative Construction Methods for the Reclamation Works
In accordance with Clause 3.3.1 of the EIA Study Brief (ESB-208/2009), this section presents the consideration of
alternative construction methods for the reclamation required for one of the
GRS.
The assessment has been conducted to investigate
potential methods and plant for the construction of the proposed
reclamation. The objective of the
assessment is to identify the preferred alternative to avoid the likelihood of
unacceptable adverse environmental impacts.
On the basis of these requirements, the following
alternative methods have been considered for the construction of key
facilities:
·
Reclamation:
no-reclamation alternative and partially- vs
fully-dredged method; and
·
Seawalls:
dredged (with vertical vs sloping seawall) and
non-dredged methods.
Regarding the onsite GRS facilities since they are
highly specialised and will be constructed to best industry standard, alternatives
for construction will not be discussed.
2.2.1
No–reclamation Alternative
The no-reclamation alternative for the GRS site, i.e.
piling, is discussed in
Consequently, this alternative is not considered
further in the remainder of this Report.
|
Consideration |
Piling to create a decked structure –
Details |
|
Environmental |
Unmitigated
piling activities produce elevated underwater sound levels which may affect
Chinese White Dolphins.
Mitigation measures such as the use of bubble curtains / bubble
jackets and seasonal work closure periods may be expected to be required
pending further analysis. There would be
potential aesthetic concerns of a high profile visual structure. |
|
Engineering |
The GRS
facility may be supported on a deck structure founded on a large number of piles
socketted into bed rock. It will involve a significantly large
number of piles (assuming that about 80 nos. of piles are required at 8x8m
grid over the proposed 50m x100m area).
A key constraint is that the soffit of the deck structure will need to
be placed a few metres above the highest tide level resulting in the GRS
facilities at least 5-10m above the adjacent ground level. |
|
Safety |
Piled structure
poses potential safety concerns for gas installations due to ground movement
or accidental collision by marine traffic. |
|
Schedule |
The piling
works for the decked structure could take up to 10 months and mitigation
measures such as no piling during peak calving season for dolphins could significantly
delay the replacement gas schedule in 2012 (assuming construction at 1
pile/week by 2 rigs). |
|
Conclusion |
Potentially feasible option with further detailed
investigation required. |
2.2.2
Reclamation
A small-scale reclamation will be required to form
new land as the platform area for the GRS.
One of the methods to construct the reclamation area is to dredge away
all soft seabed materials beneath the seawall and under the entire reclamation
area. This option is referred to as
a ‘Fully-dredged Construction Method’.
Recently in
The
partially-dredged method is regarded as the preferred method for the
construction of the reclamation to accommodate one GRS.
2.2.3
Seawalls
Permanent vertical seawalls will be constructed
around the seaward boundaries of the proposed reclamation to protect it from
wave and tidal action. Vertical
seawalls are preferred over sloping seawalls as they involve a smaller
environmental footprint for the marine works area (a reduction of 0.38 ha, Table 2.2). As a consequence of the small footprint
less dredging will be required (a reduction of about 26,000 m3, Table 2.2) which will consequently have
benefits in reducing the scale of potential waste management, water quality and
marine ecological impacts. The
total length of vertical seawalls to be built along the reclamation is approximately
200 m.
Table 2.2 Comparison
of Dredging and Filling Requirements for Vertical and Sloping Seawalls
|
|
Vertical Seawall |
Sloping Seawall |
|
Reclamation
Area |
0.5 ha |
0.5 ha |
|
Affected Seabed
Footprint Area |
1.35 ha |
1.73 ha |
|
Dredging of
Marine Mud (Bulk Volume) |
0.156 Mm3 |
0.182 Mm3 |
|
Total Fill
Required |
0.170 Mm3 |
0.222 Mm3 |
The design of the seawall should achieve a minimum Factor
of Safety to ensure stability against slope failure and provide adequate
bearing capacity to support the seawall without significant settlement. Both dredged and non-dredged options
with ground improvement methods have been considered.
A dredged method, i.e. removal of the soft material
beneath the seawall, will ensure the stability requirement is met without
significant settlement. However, in
order to minimise dredging of marine deposit, the feasibility of non-dredged option with ground improvement measure for
the seawall was considered. Ground
improvement measures such as the use of vertical band drains and surcharge were
considered to be inadequate as they cannot improve the shear strength of marine
deposits to ensure the seawall stability.
A preliminary investigation of alternative ground improvement methods
has been conducted and presented below.
The first alternative makes use of Sand Compaction
Pile (SCP) as the seawall foundation.
SCP is considered to be one of the effective ground improvement methods
for seawall structure on the soft marine deposit. This is because SCP can increase the
shear strength of ground by installing well compacted sand piles in the ground
and stabilises the seawall structure.
There is, however, a lack of track record in the application of SCP in
Hong Kong, although the use of SCP as the seawall foundation has been adopted
in reclamation projects in
An important issue of SCP is the up-heaving of seabed
after installation of SCP. In
shallow waters, the up-heaved seabed may affect the operation of the SCP barges
as well as other vessels. With the
consideration of up-heaving, the seabed level should be -6 mPD
or below to allow adequate water depth to ensure the proper operation of SCP
barge without being affected by the up-heaving of the seabed. However, within the proposed reclamation
seabed level varies between -2 mPD to -5 mPD which is not sufficient to operate SCP barge. Also given the seawall length is
relatively short, the overall cost-effectiveness for the use of SCP is
considered to be low. With the above
constraints, SCP is not considered a feasible option.
The second alternative makes use of ground
improvement technique, such as Deep Cement Mixing (DCM), to enhance the
strength of the marine deposits before filling up for the seawall. In DCM, the soft soil is mixed in situ with an appropriate additive,
typically cement or lime, using an auger or other mixing device. No spoil removal is required. A similar technique called Deep Cement
Method has been developed in
The third alternative requires a long counter fill on
the seaward side of the seawall to provide toe stability against slip failure
during construction. The use of
this method is, however, considered to be unsuitable for this Project as it is
likely to lead to significant ongoing settlement of the seawall after the GRS
is in operation.
On the basis of the above, none of the alternative
methods is preferred over the conventional method of dredging beneath the
seawall.
The
conventional method of carrying out dredging of the marine deposits before filling
up for the seawall is recommended as the preferred method for the construction
of the seawalls for the GRS reclamation.
2.3
Consideration of
Alternative Installation Techniques for the Submarine Gas Pipelines
The two proposed submarine pipelines will be
installed in two separate seabed trenches to the necessary burial depths. Installation of the submarine pipelines
would involve either pre-trenching (i.e. dredging a trench and then laying the
pipeline in the trench) or post-trenching (i.e. direct lay and burial of the
pipeline typically using a jetting machine). The alternatives considered for
installation of the submarine gas pipelines are:
·
Grab
dredging;
·
Trailing
suction hopper dredger (TSHD) dredging; and
·
Jetting.
Other non-dredged alternatives for submarine pipeline
installation are discussed in
Box 2 Other
Non-dredged Alternatives for Installation of Submarine Gas Pipelines
|
Consideration |
Ploughing |
Horizontal Directional Drilling (HDD) |
|
Engineering |
Feasibility of
ploughing will depend on sediment characteristics and seabed stability/
conditions, since the plough (machine) is a very heavy mechanical object and
is not suitable for unstable seabed.
In particular ploughing is not suitable for soft marine clay found in
the Project Area, since the plough sinks into the mud and the trench cannot
be maintained open as the plough usually cuts a steeper slope. Ploughing cannot provide sufficient
depth for pipeline burial for armour rock protection, and there does not seem
to be any precedence of the use of ploughing for trenching to a depth of 3 m. |
Sea to Sea HDD
is an extremely high technology with high risk. Feasibility will be determined by
sediment and geotechnical characteristics. The soft marine clay seabed of the
Project Area may not be suitable as it will be very difficult to maintain an
open hole. Failure caused of the
HDD due to drill hole collapse may cause the loss of the entire pipe string. In addition, the pipeline size with
large diameter (> = 32”) will make this HDD operation extremely difficult,
and the distance for which the HDD can construct/ operate is quite short (~ 2
km). Also future pipeline repair
or inspection will be impossible.
HDD cannot be adopted solely for the shore end with a subsea tie in as
the geotechnical conditions at the BPPS landfall do not appear to be
suitable. A temporary reclamation
would be required to allow access for the HDD rig due to space constraints at
BPPS. |
|
Environmental |
A non-dredged
option which does not generate dredged material. The spread of suspended
sediment generated by the plough is expected to be less than for the jetting
machine. Compliance with WQO
would need to be demonstrated.
Safety concerns may be the overriding issue as the pipeline may not be
able to be ploughed to the required safe depth. |
A non-dredged
option which does not generate dredged material. Water quality impacts are also
expected to be minimal. However
spoil from drilling will need to be disposed of which lead to potential waste
management concern. |
|
Schedule |
Similar to the
working rate of a jetting machine.
However a full-scale trial and detailed site survey will be required
to establish its engineering and environmental viability prior to its
adoption in |
A full-scale
trial and detailed site survey will be required to establish its engineering
and environmental viability prior to its adoption in |
|
Conclusion |
Not considered as a feasible option |
Not considered as a feasible option |
Dredging will be employed at the landing sites of the
pipelines, due to the proximity of these locations to the shoreline requiring
accurate removal of potential marine muds and rock
fill. In addition, dredging and
backfilling with a combination of gravel and rock armour will be required when
the pipelines cross fairways and other specific locations in order to provide
adequate protection from third party damage.
There are two common dredging plants that are
employed for the removal of marine sediments in
Alternatively, along the pipeline routes, there is the
potential to employ jetting in order to trench these facilities to the required
depths. A description of the
jetting method is also presented below.
2.3.1
Grab Dredgers
A grab dredger comprises a rectangular pontoon on
which is mounted a revolving crane equipped with a grab. The dredging operation consists of
lowering the grab to the bottom, closing the grab, raising the filled grab to
the surface and discharging the contents into a barge. Grab dredgers are usually held in
position while working by anchors and moorings but some have a spud or pile,
which can be dropped onto the bottom while the dredger is operating.
Grab dredgers may release sediment into suspension by
the following mechanisms:
·
Impact
of the grab on the seabed as it is lowered;
·
Washing
of sediment off the outside of the grab as it is raised through the water
column and when it is lowered again after being emptied;
·
Leakage
of water from the grab as it is hauled above the water surface;
·
Spillage
of sediment from over-full grabs;
·
Loss
from grabs which cannot be fully closed due to the presence of debris;
·
Release
by splashing when loading barges by careless, inaccurate methods;
·
Disturbance
of the seabed as the closed grab is removed.
During the transport of dredged materials, sediment
may be lost through leakage from barges.
However, dredging permits in
Sediment is also lost to the water column when
discharging material at disposal sites.
The amount that is lost depends on a large number of factors including
material characteristics, the speed and manner in which it is discharged from
the vessel, and the characteristics of the disposal sites.
2.3.2
Trailing Suction Hopper Dredgers
A Trailing Suction Hopper Dredger (TSHD) is designed
to use a suction mouth at the end of a long tremie
pipe. As the barge moves, the
suction hopper trails along and sucks up the soft seabed sediments. During dredging the drag head will sink
below the level of the surrounding seabed and the seabed sediments will be
extracted from the base of the trench formed by the passage of the drag
head.
The main source of sediment release is the bulldozing
effect of the drag head when it is immersed in the mud. This mechanism means that sediment is
generally lost to suspension very close to the level of the surrounding
seabed. Due to the relatively large
volumes of material removed during dredging the instantaneous loss of sediment
during dredging (in terms of kg per second) can be higher than grab dredging
even though the net total amount of mud lost to suspension over an entire
dredging programme is lower than more conventional methods.
During dredging marine sediments are pumped into the
vessel’s hopper. Once the hopper is
loaded the dredging operation will be stopped and the vessel will sail to a
designated disposal area. A TSHD is
usually positioned by dynamic positioning, thus they have no anchor wires. In comparison to grab dredgers, TSHDs generally have a higher production rate.
2.3.3
Jetting
The jetting machine will either be self-propelled or
be towed by barge. The
self-propelled machine has wheels resting on the pipeline and uses the pipe for
traction. Stability is achieved
with the use of buoyancy aids. A
‘Non-conventional’ jetting machine may be utilised, as it does not use air to
assist with discharge of the sediment.
This results in less adverse effect on the water quality of the
surrounding areas.
From the soil data, a nozzle configuration that best
suits the in situ soil
characteristics will be determined.
The method is based on fluidising the muds
allowing the pipe to sink to the chosen depth.
During the installation of the submarine utilities
using jetting technology, it would be expected that seabed sediment would be
released close to the seabed and will settle out relatively quickly. The sediment would, therefore, only be
in suspension for a short period of time and as such, the potential for impacts
to occur, such as through the exertion of the oxygen demand on the receiving
waters, will be limited.
The significant environmental advantage of the use of
jetting in pipeline installation works is that the method avoids the need to
remove material from the seabed and therefore avoids the need for off-site
disposal.
2.3.4
Consideration of Installation Technique
An evaluation of the three techniques presented above
was undertaken to evaluate their engineering feasibility, schedule implications
and overall environmental performance, in particular their environmental
acceptability in terms of dredged material management and water quality
impacts. Outcomes of the evaluation
are presented below.
Environmental Considerations
The impact on water quality for grab dredging, TSHD
dredging and jetting options was examined for the northwestern
waters in
As for the potential impact on dredged material
management, unlike grab or TSHD dredging, the jetting machine does not generate
any mud requiring offsite disposal.
It is important to recognise that existing capacity for contaminated
sediment disposal is very limited.
A reduction of dredged material arisings is
seen as highly advantageous to both the project and to the Government’s
management of the limited capacity for contaminated mud disposal. The potential merit of the jetting
method is thus considered as critical to the overall environmental
acceptability of this Project.
Engineering Considerations
Along the shore approach and some sections of the
proposed pipeline alignment, the water depths are too shallow (sometimes as
shallow as -3 mPD only) to utilise a TSHD which
typically has a draft of at least 5 m.
For the section of pipelines in deeper waters, i.e. along Urmston Road
at about -15 mPD, it is considered possible to use
TSHD, though the cost-effectiveness and practicality of utilising a TSHD just
for this short section (less than 1.8 km) is low ([3]).
In contrast, the use of the jetting method along the
Grab dredging is widely adopted for pipeline
pre-trenching in
Schedule Considerations
Trenching by a jetting machine offers a schedule
benefit as the working rate by a jetting machine is much higher than that of a
grab dredger, thus allowing the works to be completed in a shorter
timeframe. Dredging by a TSHD also
presents similar schedule benefits since the dredging speed is also higher than
that of a grab dredger. Overall,
when the same daily working time is considered, grab dredging will result in
the longest construction programme among the three proposed methods but is
acceptable to the project schedule.
Summary of Key Considerations
·
A
reduction of dredged material arisings is paramount
to the overall environmental acceptability of this Project, hence the use of
jetting is preferred and the majority of the pipeline route is planned to be
installed by jetting. An added
benefit is that the pipeline installation works may be completed in a shorter
timeframe than using grab dredging only.
·
Due
to insurmountable engineering constraints the Urmston Road Crossing pipeline
section cannot be installed by jetting and this pipeline section will be
installed by dredging method.
·
For
dredging methods, whilst it is considered that grab dredging and dredging by a
TSHD would offer similar overall environmental performance and acceptability, grab
dredging will have a higher cost-effectiveness than dredging by a TSHD. Therefore grab dredging is the preferred
method for pipeline installation along the Shore Approach and Urmston Road
Crossing pipeline section.
Taking
the above into consideration it was concluded that although from an
environmental perspective all three options appear to be suitable, grab
dredging and the jetting method were considered to be the more practicable
options.
2.4
Consideration of
Works Sequences
The EIAO-TM specifies the priorities for
addressing impacts is avoidance and minimization. This philosophy was referred to in
designing the marine works construction programme by reducing the overall
duration of marine works. This has
been seen by leading local marine mammal specialists (Würsig,
Dredging works in
In order to achieve the master construction programme
for the Project all other activities associated with the pipeline installation,
i.e. pipe-laying and the placement of armour rock protection will operate over
24 hour periods and throughout the year.
Neither of these works activities cause adverse impacts to the marine
environment as discussed in Section 3.3.4.
In addition to the above the Project is planned to be
conducted in two phases: the First Phase for one pipeline and the co-located
GRS and the Second Phase for the other pipeline plus the GRS on
reclamation. The phasing is
discussed in further detail in Section 3. Such a Phased Construction approach is a
result of an optimisation of the Project’s implementation schedule to resolve
potential concerns regarding dredged material management.
2.5
Selection of
Preferred Scenario
For the GRS location the preferred
scenario/alternative to be taken forward to the EIA stage is Site D (Co-located
GRS) with Site B1 (GRS on B1 Reclamation) Layout. Full details of the components of the
preferred scenario are detailed in Section
3 of this EIA Report.
The preferred construction method of the GRS reclamation
to be taken forward is the partially-dredged method, while for the submarine
gas pipelines the installation methods to be considered are grab dredging and
jetting in
·
The
use of jetting for certain sections of the pipelines alignment will reduce the
volumes of dredged material substantially from 0.428 Mm3 to 0.253 Mm3
(bulk volume) per pipeline;
·
The
adoption of jetting will shorten the period for marine construction works and
hence reduce the severity of impacts to marine ecological resources;
·
A
reduction in the seabed areal extent of the reclamation, as one of the new GRSs will be located on existing land within BPPS;
·
Avoidance
of potential impacts on terrestrial ecology as vegetation clearance and slope
cutting is avoided;
·
A
reduction in the seabed footprint area by 0.38 ha through the use of vertical
instead of sloping seawall for the reclamation, hence reducing the dredging
volumes by about 26,000 m3; and
·
A
reduction in dredging volumes through siting one GRS
on existing land and through selection of reclamation design and construction
methodology, hence reducing off site impacts during disposal of dredged muds.