5
PROJECT DESCRIPTION
This section of the EIA Report presents the details
of the proposed development against which environmental impacts have been
addressed. The construction,
operation and maintenance phases are described in terms of the likely component
options and their installation and use.
In summary, the key components of the project include the following:
·
The
construction of around 35 nos. of 2.3 to 3.6MW class wind turbine units including
seabed works required for foundation emplacement. Should 3.6MW class wind turbine be
selected, the number of wind turbines would be reduced to around 28 to 30 in
order to maintain the wind farm capacity of around 100MW.
·
The
installation of interconnecting submarine electricity cables between turbine
units, to the offshore substation and to grid.
·
Construction
of an offshore substation. There
may, however, be an option for the offshore substation to be replaced by an
onshore one subject to detailed engineering design.
·
Development
of an onshore lay down area and quayside for material storage and pre-assembly
works.
·
Development
of an offshore wind monitoring mast.
The components for the wind farm will not be procured
until the Detailed Design Phase, which will commence once the EIA has been
approved. Therefore a description
of the likely components and their installation is provided in this EIA Report
together with any alternatives (if necessary) and corresponding methods of
installation. The assessment of
environmental impacts will correspond to the component or process option giving
rise to the maximum perceived impact.
The preferred scenario/alternatives for
development of the wind farm and installation of the submarine and terrestrial
cables to be taken forward in this EIA are described in Section 4. On the basis
of the alternatives selection, the layout plan for the proposed development is
shown in Figure 5.1.
The turbine layout and cable route shown in Figure 5.1 are
indicative only and will be confirmed during the Detailed Design Phase of the
Project.
The key elements of the Project are as
follows:
·
Wind
turbine construction;
·
Offshore
substation construction;
·
Offshore
wind monitoring mast construction;
·
Submarine
cable installation; and
·
Onshore
cable installation.
A general summary of each of these key elements
is presented followed by a description of the key construction and operational
activities.
5.2.1
Wind Turbine Design and Construction
Foundation Design & Ancillary Features
Foundations
Foundations are required to support turbine
towers, nacelles and blades and also to provide a platform above sea level for
ongoing maintenance access. In
addition foundations will also need to be constructed for the offshore wind
monitoring mast and offshore substation.
Three types of wind turbine foundation have been considered for this
Project and these are discussed in Section
4 of this EIA Report. Of those
options assessed only the tripod / tetrapod and monopile foundations were considered to be technically
feasible and environmentally acceptable with proven mitigation. The monopile
foundation has been identified as potentially having the worst case scenario
impacts given the relatively larger footprint and also the potential need to
incorporate scour protection material.
This foundation is therefore taken forward for the EIA. The monopile
for this size of turbine is anticipated to have a diameter of 5 to 7 m which
will lead to a physical footprint of approximately 38.5 m2 with a
pile wall thickness of approximately 80 mm. However, impacts associated with the use
of tripod / tetrapod foundations are covered by the
assessment of monopile foundations. As the wind monitoring mast will be
constructed much earlier than the rest of the wind farm equipment, deployment
of special duty plant from overseas, which are not commonly used for marine
structures in Hong Kong, such as large jack up (with long legs) vessels and,
heavy duty hammer, dedicated for monopile foundation
and wind turbine installations solely for the purpose of wind monitoring mast monopile foundation construction would be unfeasible. It is, therefore, expected that adoption
of monopile foundation for the wind monitoring mast
similar to that for the wind turbines will be highly unlikely. Alternative foundation design for the wind
monitoring mast will therefore be adopted.
Based on the preliminary engineering design conducted by HK Electric’s
consultant, the foundation of the wind monitoring mast will be made up of a
lattice of 8 piles, each expected to be of 1.6m diameter (See Section 4).
Turbine Ancillary Features
Each turbine foundation will have a platform, which
is required for maintenance access (see Section
4). This platform will be
lifted into place using a crane barge.
As discussed in more detail below, the wind turbines
will be inter-connected by submarine cables to provide both power and telemetry
links (see Figure 5.1).
Provision is therefore made for the entry and protection of the
cables. The cables are most likely to
be installed in a “J-tube” arrangement, a steel tube of approximately 250-350
mm diameter attached to the side of the turbine support structure extending
from above the high tide level to the seabed. Each structure will have between two and
five J-tubes. The cable entry and
protection will be pre-installed at the quayside.
Scour Protection
As discussed in Section 4, rock scour protection will be constructed at the base of
the monopile foundation. It is assumed that this scour protection
will have an overall width of 30m and length of 30 m and overall area of 900 m2. The height of the scour protection is
expected to be 0.5 m above the seabed.
It is proposed that stability calculations will be undertaken during
pre-contract engineering to ensure stability of the material and that suitable
rock sizes be identified during the Detailed Design.
Wind Turbines
Wind Farm and Wind Turbine
Dimensions
In order to prevent wake loss, the preliminary
separation distance of the turbines for the EIA is 650m East-West and 360 m
North-South. However, this layout
is subject to further detailed engineering design and consideration of impacts,
e.g. visual impacts (see Section 11). Preliminary dimensions are not expected
to exceed a tip height of +125mPD.
In the event the wind turbine model with a maximum rotor diameter of
111m be adopted, the maximum tip height would be +136mPD. The area of development for the turbine
site is approximately 6 km2 (see Figure 5.1). Figure
5.2 shows a typical offshore wind turbine structure.
Figure 5.2 Offshore
Wind Turbine
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The diameter of the tower base will be approximately
5 - 6 m, tapering to approximately 4 m at the hub. The tower of each turbine will be made of
tubular steel, and the blades of composite material, which consists of fibre
glass and epoxy resin. Each blade
is protected from lightning damage by means of a receptor system. The turbines will each have three
blades.
Operation parameters
The turbines will have a rotational speed
of between 9 and 19 revolutions per minute (rpm) and have weights of between
220 - 480 tonnes. The wind turbines
will generate power when the wind speed at the hub height is between 2 - 5 ms-1
(metres per second) and will have a full generation potential at wind speeds of
about 14 ms-1. Once
erected, the turbines will operate automatically using remote diagnostics and
control systems. These systems will
require ongoing maintenance during the operational period whenever
necessary. The turbines will also
be serviced at regular intervals following the manufacturer’s
requirements.
The turbines will have a “fail-safe”
operation in that if wind speeds greater than 25 ms-1 occur for
extended periods the turbines will shut down automatically. The maximum tolerable wind speeds for
the structural integrity of the turbines is approximately 70 ms-1.
All rotors will rotate clockwise when viewed from the
windward direction. This will
ensure that there will be some conformity in how the wind farm is seen from
Visually Sensitive Receiver’s (see Section
11).
Colour
The final decision on the colour of the turbine mast
and blades will be made during the Detailed Design Phase. However, it is currently assumed to be
the “standard” colour for offshore wind turbines, a semi-matt pale grey colour
RAL 7035. Marking requirements for
aviation and navigation are discussed separately below.
Corrosion Protection
Corrosion protection on the steel structure will be
achieved by a combination of a protective paint coating and installation of
sacrificial anodes on the submarine structure. The anodes are standard products for
offshore structures and are welded onto the steel structures. The anodes typically consist of zinc and
aluminium, and are connected to the structure via doubler plates to ensure the
integrity of the primary structure is maintained in the unlikely failure of an
anode connection. The number and
size of anodes would be confirmed during the Detailed Design phase.
Lighting and Marking
Minimum marking requirements for offshore wind farms
are as follows (see Figure 5.3),
which has been agreed with the Civil Aviation Department ([1]):
For those turbines at the periphery of the
wind farm:
·
The
blade tips and the top of the nacelle and the part of the Supporting tower
corresponding to the marked portion of the blade tip when the blade is pointing
downwards should be marked in orange; and
For all other turbines:
·
Only the
blade tips and the top of the nacelle should be marked in orange.
The minimum extent of markings on blade and tower is
that each band should have a width approximately one-seventh of the longest
dimension of the wind turbine, i.e. its tip height (see Figure 5.3).
Figure 5.3 Marking
and Aviation Lighting Requirements for Offshore Wind Farms
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Aviation lights will be required for the offshore
structures. For structures at the
periphery of the wind farm, the offshore substation and monitoring mast,
lighting will be required at the highest practical point on the nacelle and on
an intermediate point on the supporting tower. This should consist of low intensity
continuous red lights. In addition
for turbines within the wind farms site, only the highest practical point on
the supporting tower should be lighted in low intensity steady red light ([2]).
In addition, there will also be a need for navigation
marking and lighting, which should comply with the International Association of
Marine Aids to Navigation and Lighthouse Authorities (IALA) Recommendation
0-117 of May 2000 ([3]).
In general, there will be a need for navigation lights placed on each
corner of the wind farm development, and mid-way along each side of the wind
farm. The corner lights will be
yellow flashing Morse ‘U’ code lights (5 second interval) visible for 5
nautical miles (9.3 km), located at least +12mPD with radar reflectors situated
beside them. The intermediate (mid-way)
lights will flash at 2.5 seconds and will be visible for 2 nautical miles (3.7
km). There will need to be two
lights on each lit turbine so that the light is visible through 360°. Each tower should also be marked
by a unique number or code, clearly visible from a position mid-way between
adjacent towers such that any vessel in difficulty can easily report its exact
position within the wind farm and that any emergency service vessel can locate
a casualty with equal ease.
During construction, the offshore working area will
need to be established and marked in accordance with Marine Department Notice No. 23 (2009). A safety / exclusion zone of 500 m will
be closed to all vessels around the area of works. The purpose of this area
will be to protect the safety of construction plant and personnel and also
third parties who may wish to navigate through this area. This safety zone will cover the whole
wind farm area, but the extent of the safety zone will change as per the
rolling construction programme.
Oils and Fluids
Each of the turbines will contain lubricants and
hydraulic oils (nominally 100 l of gearbox oil, 250 l of hydraulic oil, 20 l of
motor oil, 2,500 l of transformer oil and potentially limited quantities of
coolant depending on design). Discharges
of contaminants from wind turbines or other installations (e.g. offshore
sub-stations) are anticipated to be extremely unlikely due to oils and fluids
from gearboxes, hydraulics and pitch drive and yaw drive systems being
mechanically contained ([4]) ([5]).
Noise Emission
The wind turbines will generate aerodynamic and
mechanical ambient noise during their construction and operation. The major
ambient noise sources are expected to come from crane operation, vessel
movement and percussive piling during the construction phase and from the wind
turbines during the operational phase.
Aerodynamic noise generated during operation of the
turbines is broad-band in nature, relatively unobtrusive and is dependent upon
wind speeds prevalent at the site of development. Studies from offshore wind farms in the
Mechanical noise during operation can be successfully
controlled at the design stage of the turbine, using advanced gearbox design and
anti-vibration techniques. The
present generation of turbines considered for this Project incorporate design
features which ensure that such tonal noise emissions are not considered
significant.
Based on a total of 35 wind turbines to be installed for
the Project and a sound power
level of 109.4 dB(A)
for each wind turbine at a wind speed of 9 ms-1 at 10m above ground
level, which is typical of 3.0MW wind turbines, the nearest sensitive receivers at Lamma Island and Cheung Chau
would likely experience a noise level of about 35 dB(A) and 38 dB(A),
respectively (estimated in accordance with the procedures outlined in the ISO
9613-1 ([6])
and ISO 9613-2 ([7])) (Annex 5A).
The operational noise levels anticipated at the nearest sensitive
receivers are well within the specified noise criterion (ie
43 dB(A) and 45 dB(A) for the sensitive receivers at
The site
is a significant distance from the nearest sensitive receivers (NSR), with Yung
Shue Wan and Lo So Shing
being at least 4 km away. In view of the considerable separation distances
between the NSRs and the site, the noise generated during the construction and
operational phases is not expected to be a concern. In accordance with the EIA Study Brief ESB-151/2006, quantitative construction
and operational noise impact assessments are therefore not required. However,
underwater sound generation
is discussed with respect to impacts on marine ecology in Section 9 and fisheries in Section
10.
The normal
working hours of the contractor will be between 0700 and 1900 hours from Monday
to Saturday (except public holidays).
Should percussive piling and any construction works during evening and
night works between 1900 and 0700 hours or on public holidays (including
Sunday) be required, the contractor should submit a Construction Noise Permit
(CNP) application and the application would be assessed by the Noise Control
Authority. Conditions stipulated in
CNP should be strictly followed.
Wind Farm Layout
As discussed in
Section 4, in order to inform the EIA an interim geometric design is
considered as being indicative of how the wind turbines will be arranged with
two distinct circuits within the turbine array. This is shown in Figure 5.1.
5.2.2
Offshore Substation
For the purposes of this EIA, the base
case design of the wind farm includes for an offshore sub-station to transform
the voltage of the electricity generated at the wind turbine to a high voltage
suitable for transmission of power ashore.
22kV, 33kV or other voltage rating according to the proprietary design
of wind turbine manufacturer’s will be used for the wind farm internal grid and
connection to the offshore substation where the electricity voltage will be
stepped up to 132 kV for transmission to HK Electric’s grid at the Lamma Power Station Extension. The substation is expected to be a steel
platform with an area of about 200 m2 and a height of > +20mPD. The offshore sub-station platforms will
have boat access points to assist operations and maintenance or emergency
evacuation. A model of a typical
offshore substation is shown in Figure
5.4 below.
Figure 5.4 Indicative
Model of an Offshore Substation at Horns Rev Offshore Wind Farm (Demark) ([8])
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It should be noted, however, that as mentioned in Section 5.1 there may be an option for
the offshore substation to be replaced by an onshore one subject to detailed
engineering design. For this case,
a network of six cables (22kV, 33kV or other voltage rating according to the
proprietary design of wind turbine manufacturers) from the wind farm internal
grid will be connected to HK Electric’s grid at the Lamma
Power Station Extension.
5.2.3
Offshore Wind Monitoring Mast
A monitoring mast will be required to measure wind,
wave and current information for operational purpose. The structure will consist of a steel
lattice tower erected on top of a foundation (see Section 4, and Figure 4.10
for the indicative foundation design). Anemometry equipment will be installed
on the mast and wave and current sensors installed on the foundation
structure. A model of a typical
offshore monitoring mast is shown in Figure
5.5 below.
Figure 5.5 Indicative
Monitoring Mast
5.3
Offshore Wind
Farm Construction
5.3.1
General Construction Sequence
The general construction sequence is presented
below. It should be noted that this
sequence may be subject to refinement and potentially further study at the
Detailed Design Phase.
1.
Site
preparation will compose the creation of a delineated Lay Down area.
2.
Support
vessels for foundation construction (monitoring mast, offshore substation and
turbines) will need to be mobilised to the site.
3.
The
offshore monitoring mast will be constructed before the construction of the
wind turbines so that data can be collected at the site.
4.
Foundations
will be constructed for the offshore monitoring mast first at the area shown in
Figure 5.1.
5.
If
required, scour protection will be installed at the base of the monitoring mast
foundation prior to installation of the foundation.
6.
The
Offshore monitoring mast will be delivered to the quayside at Lamma Power Station Extension with a lay down area located
adjacent to the quay. This will
then be transferred offshore so that it can be fixed to the foundation
structure.
7.
Once
approval is obtained for marine works, dredging and jetting will be undertaken
to install the submarine cables. It
is possible that this could occur in parallel to the construction of
foundations and/or turbine installations.
8.
Foundations
will be constructed for each turbine site and substation at the locations shown
in Figure 5.1.
9.
Again,
if required, scour protection will be installed at the base of each turbine
foundation and the offshore substation prior to installation of the foundation.
10. The turbine components will be delivered
to the quayside at Lamma Power Station Extension with
a lay down area located adjacent to the quay. The turbine mast sections (number yet to
be determined) will be pre-mounted in a vertical position onshore and left in
this position until being transferred offshore once foundations are in
place.
11. Turbine and substation components will be
transferred from the Lamma Power Station Extension
for fixing to the foundation support structure.
12. Dredged material would be removed to
approved disposal/storage sites by barge.
13. Once submarine cables have been installed and
landed through the seawall, the cable will be installed to the Switching
Station on the Lamma Power Station Extension.
A more detailed description of the
construction activities is presented below.
5.3.2
Marine Works
Marine works associated with the offshore wind farm
development will be divided into the following works:
·
Installation
of key wind farm components; and
·
Submarine
cable circuit and crossings.
A description of the works associated with
each of the above activities is presented below. It should be noted that the information
included here is taken from preliminary design and will be subject to further
study at the Detailed Design stage.
Installation of Key Wind Farm Components
Foundations
The percussive piling method will be used for the
construction of the wind turbine, wind monitoring mast and offshore substation
foundations (see Section 4 for a
comparison of alternative installation methods).
This technique involves the driving a hollow steel
pile into the seabed, relying on the frictional properties of the seabed
sediments and the underlying in-situ
materials for support. The
installation of a driven pile will also take place from a jack-up barge or from
a floating barge. Once at the
turbine location, the jack-up barge will jack its legs down onto the seabed to
create a stable platform. The
jack-up barge or the floating platform will have 1 - 2 mounted marine cranes, a
piling frame adjustable to different degrees, and a hammer. A support jack-up barge or floating
barge, support barge, tug, safety vessel and personnel transfer vessel may also
be required.
Once the platform is stable or when the floating
barge has been anchored, the pile is driven into the seabed. Pile driving will commence with low
energy impulses until the pile is stable (free standing). An alternative method is to vibrate the
pile downwards until the free standing state is achieved. The pile is then driven until the target
penetration is achieved.
Since the pile is driven, the top of the pile can
become distorted during the driving process, and a
transition piece may be required to make the connection with the wind turbine
tower. This transition piece is
generally fabricated from steel, and is subsequently attached to the pile head
using grout.
Potential impacts of underwater sounds and the need
or otherwise for mitigation measures is discussed are discussed in the Marine Ecological Assessment (Section 9 of this EIA Report).
Scour Protection
If scour protection is required, this is likely to
comprise a rock structure at the base of the foundation. Suitable graded rock will be loaded onto
a rock-dumping vessel at the Lamma Power Station
Extension and transferred to the foundation site. Scour protection will be established
after installing the foundations.
After the scour protection is constructed a further layer of cover
stones will be placed around the foundation.
Rock dumping is based on the use of typical Hong Kong
Lighters (1,800 – 3,000T), configured to place rocks using grabs. These units have the capacity to place
2,400/3,600 T/day of graded rock.
It is likely that the Contractor will manipulate the numbers of units
working in any area depending upon the available equipment at any time and on
its actual progress vs. that planned.
Turbine Installation
Approach
The turbine components will be delivered to the
quayside at Lamma Power Station Extension with a lay
down area located adjacent to the quay.
The turbine mast sections (number yet to be determined) will be
pre-mounted in a vertical position onshore and left in this position until
being transferred offshore. On
another part of the quay, the nacelles will be mounted on transportation frames
and nose cones attached. Two of the
three blades will then be fixed to cones.
Once assembled, the turbine masts will be transferred
to an installation vessel using the vessel’s onboard crane and onshore trailing
crane. Once the masts are secured
onboard, the nacelles will be loaded onto the vessel, followed by the remaining
blades. For marine installation,
support vessels will again be required, which will comprise similar vessels to
that required for foundation construction.
Once all components are secured, they are transferred
to the site ready for installation.
Upon arrival to site, the installation vessel will approach the intended
location for each single turbine.
It will then position itself next to the foundation within a distance of
20 to 25 m. After positioning the
vessel, the legs will be lowered to the seabed and the vessel will jack itself
to a stable position.
The tower will then be installed first followed by
the nacelle, taking approximately fours hours to complete per turbine. The third blade is then attached to the
nose cone. The final phase of the turbine installation is the establishment of
the electrical and mechanical connections.
Grouting will be required to fix transition pieces
(i.e. fixing the foundation to the turbine). As per grouting proposals for
foundations, the grout will either be mixed in large tanks aboard the jack-up
platform, or mixed ashore and transported to site. The grout is also likely to be pumped
through a series of grout tubes previously installed in the pile, so that the
grout is introduced directly between the pile and the walls of the transition
piece. The level of grout in each
tube is monitored during pumping by a grout probe unit so that the flow can be
switched off once the required levels have been reached.
Offshore Substation and
Monitoring Mast Installation
The wind monitoring mast is comprised of 8 nos. of
1.6m diameter steel tubular piles fixed into seabed in which each pile
individually can be considered as a small monopile. An indicative drawing of the wind
monitoring mast design is presented in Figure
4.10. The mast pile will be
installed at an above sea level of approx. 18 mPD on
a concrete deck platform on-top of the underwater steel substructure. Percussive piling using a hydraulic
hammer from a piling barge will be used to install the tubular piles. A description of the works associated
with marine piling is presented above.
The offshore substation is expected to be
a steel platform also set on tubular piles with an area of about 200 m2
and a height of > +20mPD. The
substation platforms will have boat access points to assist operations and
maintenance or emergency evacuation.
Percussive piling using a hydraulic hammer from a piling barge will be
used to install the tubular piles.
A description of the works associated with marine piling is presented
above.
Submarine Cable Installation
As discussed in Section 4, submarine power cables are required to connect the wind
farm to the electricity distribution system (see Figure 5.1). These cables will also comprise fibre
optic communication links for wind farm control processes. Curved pipe ducts will be provided at
the foundation of the wind turbines and Offshore Substation for submarine cable
landing. The provisions would be
included in the civil foundation for wind turbine and offshore substation.
Figure 5.6 shows the proposed layout of existing
submarine cables, the proposed cable circuit for this Project and the location
of crossing points. The selection
of installation method and sequencing activities for the cable circuit has been
discussed in Section 4. Grab dredging is the preferred
installation technique at nearshore area to the Lamma Power Station Extension and jetting is preferred
elsewhere.
Seawall Removal and Reinstatement
In order to connect the submarine cable to
land, the existing rubble mounted seawall at the west shore of the Lamma Power Station Extension will be exposed for
installation of a duct bank (2 m internal steel slipway with concrete
lining). Approximately 2,145 m3
of existing seawall will be removed and reinstated as part of the
works. All removed seawall material
will be reused to reinstate the seawall back to the existing condition. Figure 5.7 shows the proposed
area of seawall disturbance. Figure
5.8 provides a cross section showing the area of seawall removal.
In the event that an onshore substation is
selected during the detailed design, approximately 3,400 m3 of
existing seawall material would be removed and reinstated as part of the
works. All removed seawall material
will be reused to reinstate the seawall back to the existing condition.
Dredging
Grab dredgers will be utilised in the nearshore cable landing area to construct a short
underwater trench. For submarine
utility installations, dredging involves the removal of marine sediments from
the seabed to form the trench, into which the cable is laid.
For the base case design, ie
offshore substation, the cable trench will be trapezium shape with bottom width
of 5 m. The upper width shall be 8
to 12 m and the trench depth of 1.5 – 3.5 m deep. It is assumed that the trench length
will be a maximum of 100 m. Figure
5.9 shows the proposed nearshore dredging
works. The expected maximum amount
of sediment to be dredged is 3,000 m3.
For the alternative design, ie
onshore substation, the cable trench will be trapezium shape with bottom width
of 9 m. The upper width shall be 12
to 16 m and the trench depth of 1.5 – 3.5 m deep. It is assumed that the trench length
will be a maximum of 300 m. The
expected maximum amount of sediment to be dredged is 13,125 m3.
The Water Quality Impact Assessment (Section 6) has examined the effects of
dredging on water quality and should be referred to for further details.
Jetting
Offshore of the
grab dredging area, cable installation will be undertaken using jetting
methods. The maximum burial depth
for the base case design, ie offshore substation,
will be 5 m and have a cross-sectional area of 0.75 m² (0.5 x 5 m x 0.3
m).
Based
on a total cable trench of 17.3 km (13 km for the internal turbine array and
4.3 km from the offshore substation to the grab dredging area), the approximate
volume of sediment to be disturbed will be approximately 13,000 m3. The internal turbine array cables will
be laid in a loop within the trench.
This will mean that the overall cable length will be approximately 35
km, but the length of seabed disturbance will be only 17.3 km.
For the onshore
substation design, a total cable trench of 48.3 km (14.6 km for the internal
turbine array and 33.7 km for the six cables to the grab dredging area), the
approximate volume of sediment to be disturbed will be approximately 37,000 m3.
The submarine cable laying can start either from Lamma Extension or from the Offshore Substation
[hereinafter named as Point (A)].
Initially the submarine cables will be laid out from the laying barge
and buoys will be attached to the cable at about 1m intervals to allow the
cable to float on the sea surface.
The cable is then pulled to Point (A) by means of a winch.
After pulling to Point A, submarine cables will be
set in the plough of the burying machine which is outfitted with the following
typical equipment:
·
A
towing wire rope;
·
A
cable for power supply to the water pump cable;
·
A
control cable for monitoring the working conditions of the burying machine;
·
One
high pressure jet delivery hose; and
·
A
hydraulic hose for control of the nozzle frame.
Cable guide equipment will be attached to the towing
wire rope between the burying machine and the laying barge, so that the
submarine cables are supported and guided into the burying machine. The burying
machine will then be lowered into the sea using a crane. By winding the anchor wire ropes, the
cable laying barge will move forward and tow the burying machine at the same
time.
As the burying machine advances, a small trench will
be ploughed by fluidizing the seabed using water jetting method and the
cable(s) will be laid onto the trench simultaneously. Only a small amount of sediment will be
disturbed at the seabed and the majority will subsequently settle over the
cables.
Jetting speeds have been taken as 360 m
hr-1 for cable circuit installation. This rate relates to typical practices
by contractors in
Upon reaching the opposite landing point [hereinafter
named as Point (B)], the submarine cables will be detached from the burying
machine and laid out from the cable laying barge to form a loop line on the sea
surface. Buoys will be attached to
the cables in the same manner as at Point A. The cable will be pulled to Point B by
winch and then lowered to the seabed by detaching the buoys one by one.
Within the turbine site area, typically an underwater
trench shall be formed at the Offshore Substation and at each wind turbine for
22kV, 33kV or other voltage rating according to the proprietary design of wind
turbine manufacturer’s cable landing.
The Water Quality Impact Assessment (Section 6) has examined the effects of
jetting on water quality and should be referred to for further details.
Cable Crossings
The new 22kV, 33kV or other voltage rating according
to the proprietary design of wind turbine manufacturer’s and 132k V submarine
cables will have to cross over some existing submarine communication
cables.
The submarine cable will be laid by simultaneously
laying and burial method. Before
cable crossing, a short section of cable trench (~30 m) will be formed at each
side of the crossing point. During
cable crossing, the burial machine will be temporarily lifted up and the
submarine cable will be laid directly on the seabed/trench. After cable laying, the cable trench
will be backfilled with marine sand.
The typical crossing method is to lower the existing
communication cable by water jet method to about 3~5m depth, depending on their
burial depth and available slack length.
The submarine cable will then be laid and buried at 2~3m at the cross
over point. Since portion of the
submarine cable at the cable crossing point will be at shallow burial depth (e.g.
3 m for water depth > -18mCD and 5 m for water depth < -18mCD). Figure 5.10 shows the
typical design details for cable crossings that may be adopted for this
Project.
A number of alternatives have been considered to
provide protection to the above-bed (surface laid) cable at the pipeline
crossing point should it be required, including the use of rock armour,
pre-cast concrete mattresses or grout filled bags known as ‘fill in situ’
mattresses. Should cable protection
be required the preferred method of installation will comprise the development
of a Reinforced Concrete (RC) cover at the crossing or other locations, which
will be precast and delivered to site or be formed in situ.
In the worst case the existing submarine cable cannot
be lowered down and laid at seabed surface, the following typical cable
crossing method will be adopted:
·
A 0.3
m thick concrete mattress (about 30 m long) or 0.3 - 0.5 m thick aggregates
will be laid on top of the existing cable to provide a partition at the
crossing point;
·
The
submarine cable will be laid on top of the concrete mattress;
·
After
cable laying, for the cable portion above seabed level, a 0.5m thick fill in-situ concrete mattress will be
installed on top of the cables for cable protection and to present a smooth top
profile that does to hamper fishing gear. The highest point of the concrete
mattress will be 0.8 m above seabed level; and
·
For
the cable portion below the seabed with a shallow burial depth, RC covers will
be installed.
5.3.3
Land Based Works
The land based works would be expected as follows:
·
Cable
connection to grid
·
Development
of a Laydown Area
Each of these proposed works are discussed
below.
Cable Connection to Grid
The submarine cable will be landed at the west side
of Lamma Extension and then connected to Lamma Extension 275 kV Switching Station by 132 kV land
cable, 132/275 kV step up transformer and 275 kV cable via preformed RC cable
troughs. Similar to conventional
trenching work on land, a 1 m wide x 1 m depth x 250 m long RC cable trough
will be constructed from the submarine cable landing point to the 275 kV
Switching Station Compound. Figure 5.11
shows the location of the onshore cable route. The trench will be 1 m wide x 1 m depth
x 250 m long.
Figure 5.11 Preliminary
onshore cable route
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|
Laydown Area
It is proposed that the Lamma
Power Station Extension quayside will be used for the Laydown
area and pre-assembly area during the construction phase (see Figure
5.12).
5.4
Operation and Maintenance of the Offshore Wind Farm
Components
There will be an ongoing requirement to maintain the wind
turbines, monitoring mast and offshore substation during their design lifetime,
which is expected to be 20-25 years.
The wind turbines will be configured so that they
operate with a minimum of supervision.
The turbines are monitored and controlled by microprocessors installed
within the turbine tower. This
system detects faults, and if necessary, the turbine is automatically shut down
for safety purposes.
All information relating to on-site conditions,
turbine status and generated output will be held within a central Supervisory
Control and Data Acquisition (SCADA) system linked to each individual turbine
microprocessor. This will be
controlled by an operational base at the Lamma Power
Station, which will provide the remote control of individual turbines (or a
number of turbines) should it be needed.
The wind farm will be serviced as per the
manufacturer’s requirements. It is anticipated that ongoing maintenance will be
required as necessary. Inspections
of support structures and submarine cables will also be performed regularly as
will ad hoc visits for surveillance.
Should inspections show that cables have been un-earthed, then these
will be re-buried using jetting techniques as discussed above. Maintenance crew will use vessels to
access the turbines via the platforms already constructed. There will be an inbuilt crainage system within the turbine nacelle, which allows
heavy equipment to be lowered to sea level should major work be needed.
An operational safety zone of 50 m radius will be in
force from the substation, turbine and monitoring mast. This will apply to non-Project vessels
(excluding fishery vessels) throughout the operational period regardless of
other exclusion arrangements. No
fishing activity or anchoring will be allowed within the wind turbine array or
within 500 m of any turbine, offshore substation or offshore monitoring mast
(the impacts of such an exclusion have been discussed in Section 10). It is expected
that during maintenance work exclusion from access to the wind farm is likely
to be required in accordance with Marine
Department Notice No. 23 (2009).
Decommissioning is a term describing all the stages
of the process implemented when an installation approaches the end of its
useful life. The process can
generally be categorized into three key phases as follows (see Figure 5.13):
·
Pre-decommissioning
activities: includes the detailed planning (development of the decommissioning
plan) and approval procedures;
·
Decommissioning
activities: removal and re-use, recycling, leaving in-situ, or disposal of all, or part, of the installation; and
·
Post-decommissioning
activities: site survey, close-out report and field monitoring as necessary.
Figure 5.13 Stages
of Decommissioning
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|
The activities involved during decommissioning and
abandonment phase include decommissioning of the wind farm facilities. The facilities will, however, only be decommissioned when it is no
longer economical to operate the facility.
Decommissioning Plan
The decommissioning plan will realistically only be
developed during the latter stages of the production life of the facilities. The assessment of environmental impacts
associated with the decommissioning will need to be conducted once the
decommissioning plan is finalized and the marine works/ operations are defined.
A detailed decommissioning plan will be developed taking
into account of the most cost effective and best practicable methods, legal
requirements and industry practices at the time of field abandonment. To ensure that due consideration is
given to all the relevant issues it is recommended that a detailed evaluation
of decommissioning options is carried out.
The evaluation should consider environmental issues in conjunction with
technical, safety and cost implications to establish the best practicable
environmental options (BPEO) for the decommissioning of the wind farm. A risk assessment will also be conducted
to ensure that nothing which could be constituted as a hazard for other users
of the area or for the environment in general will be left at the site. The site will be left in a safe and
environmentally acceptable condition.
In addition to any
Typically, offshore wind farms are designed with an
appreciation of decommissioning in mind.
In general terms, the turbines are designed so that they can be cut and
lifted off in steps which are in reverse of the installation procedure. The piles are typically cut flush to the
seabed to leave no obstruction following abandonment.
At the end of the project activities, the decommissioning
shall be treated as a standalone project.
The following points should be considered during planning: cessation
plan, installation cold phase, cleaning operations, hazardous products
elimination, pieces of equipment and facilities which will be dismantled and
removed, environmental surveys and impact assessment studies. On this basis, the assessment of impacts
related to decommissioning are not presented herein, but will be conducted once
the detailed decommissioning plan has been developed.
Table 5.1 presents the summary of the Project
details.
Table 5.1 Summary
of Project Description
|
Detail |
Preliminary Design
Information |
|
Wind farm site area |
600 ha |
|
Submarine cable route trench (inter-array) |
Approximately 13 km |
|
Submarine cable route trench (offshore sub station
to landing point) |
Approximately 4.3 km |
|
Submarine cable route length (inter-array loop) |
Approximately 26 km |
|
Submarine cable route length (offshore sub station
to landing point loop) |
Approximately 9 km |
|
Grab dredging volume |
3000 m3 |
|
Jetting area/volume |
13,000 m3 |
|
Turbine foundation footprint area |
38.5 m2 |
|
Scour protection footprint area |
900 m2 |
|
Volume of grab dredging arisings for disposal |
3000 m3 |
|
Volume of excavated C& D material |
Seawall = 2,145 m3 |
|
|
Onshore cable trench = 250 m3 |
|
Volume of excavated C&D material for disposal |
0 m3 |
|
Volume of grout per turbine |
70 m3 |
|
Lay down area |
2.73 ha |
The preliminary programme for the Detailed
Design and Construction Phases is presented in Figure 5.14.
Figure 5.14 Construction
Programme
At present the identified potential
concurrent projects are the marine dumping activities near South Cheung Chau.
Consideration will be given to potential cumulative impacts wherever
appropriate in the EIA.