12.1 For both Aberdeen and Ap Lei Chau project sites, potential damages to the Potential Hazardous Installations (PHIs), namely the Hongkong & China Gas Company’s Gas Holder (PHI No. H4) (hereinafter referred to as “HKCG Depot”) and the Shell LPG Transit Depot/Bulk Domestic Supply (PHI No. H5) (hereinafter referred to as “Shell Depot”). can be avoided with implementation of safety measures and close monitoring procedures. Operation of these PHIs would not be disturbed and gas supply would not be disrupted provided that vibration and ground settlement caused by construction works can be controlled within allowable limits.
12.2
Ground settlement is to be monitored by a series of
geotechnical instrumentations. A series of ground settlement markers and
piezometers are to be installed in the vicinity of the proposed tunnel and near
the HKCG Depot. Monitoring of groundwater and settlement will form an important
part of the strategy to confirm the effectiveness of the mitigation measures
and to adjust the inflow limits if necessary. In order to cater for unforeseen
problems encountered during the construction stage, additional monitoring and
instrumentation system would be installed in the vicinity of the HKCG Depot at
a later stage by the Contractor before the commencement of construction works.
Typical allowable limits for well maintained buildings and historic buildings
are 25mm and 5mm respectively. Allowable limit of 13mm is proposed and the
actual allowable limit is subject to further requirements of Civil Engineering
and Development Department (CEDD) and relevant authorities. Gas facilities
should be able to withstand ground settlement (including differential
settlement) within the allowable limit.
12.3
Monitoring plan should include setting up monitoring
points at fixed locations adjacent to more sensitive structures which have
lower vibration tolerance. The monitoring would be carried out at ground level
with sensors mounted on small concrete bases embedding into undisturbed ground.
Each sensor would be able to record ground vibration. For some particularly
sensitive structure within the Shell Depot such as LPG compound, sensor would
be located at the structure to ensure the vibration limit at the structure is
not exceeded. Allowable limit of 5mm/s PPV was proposed to HKCG. The same
criteria would be also applied to the Shell Depot.
12.4
It was advised, during the HAZOP meeting, that no gas
leakage detection alarm is installed in the Shell Depot. It is recommended to
install gas detector/ alarming system to provide warning to Shell operator and
Construction Site staff in case of gas leakage. This provides an early warning
for operation staff and construction staff in case of gas release. Specific
emergency procedures should be developed, together with Shell Depot Operation
Team, and documented in an emergency plan to cater for gas leakage scenarios
before the construction stage.
12.5
The following measures are recommended as “Best
Practice” for DSD to implement during construction stage.
(i) Emergency evacuation procedures should be formulated and DSD should ensure all workers on site should be familiar with these procedures as well as the route to escape in case of gas release incident occur. Diagrams showing the escape routes to a safe place should be posted in the site notice boards and at the entrance/exit of site;
(i) The emergency procedures should specify means of providing a rapid and direct warning (e.g. Siren and Flashing Light) to construction workers in the event of gas release in the gas facilities; and
(ii) The construction site officer of DSD should establish a communication channel with the gas operation personnel during construction stage. In case of any incidents in the gas facilities that need site evacuation, operation staff of gas facilities should advise the DSD’s site officer to evacuate the construction workers; and
(iii) Induction Training should be provided to any staff before working at the both work site; and
(iv) Periodic drills should be coordinated and conducted to ensure all construction staffs are familiar with the evacuation procedures. Upon completion of the drills, a review on every step taken should be conducted to identify area of improvement.